Effect of radiation and Navier slip boundary of Walters’ liquid B flow over a stretching sheet in a porous media Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-20 U.S. Mahabaleshwar, Ioannis E. Sarris, Giulio Lorenzini
This paper investigates the steady-state momentum and radiation heat transfer flow of a viscoelastic fluid in a porous media in the presence of a linear Navier slip boundary condition. The velocity of the fluid over the linear stretching sheet is varied linearly with the axial distance while a Walters’ liquid-B model is assumed for the viscosity. A similarity transformation reduces the Navier–Stokes equations to a set of partial differential equations that are converted into ordinary differential equations and solved analytically for the velocity. Moreover, heat is balanced between a temperature dependent heat source and radiation and leads to a differential equation with variable coefficients. The temperature equation is transformed to a confluent hypergeometric differential equation using the Rosseland approximation for the radiation and solved analytically. Results are discussed for two boundary conditions of the sheet, the prescribed surface temperature and the wall heat flux. Parameters like the Reynolds number, the viscoelastic parameter and the boundary slip parameter are found to determine the flow field. In addition, the Prandtl number, the radiation number, the wall temperature and the heat source/sink parameters are found to control the temperature distribution inside the stretching sheet.
Optimization possibility of beam scanning for electron beam welding: Physics understanding and parameters selection criteria Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-19 Manlelan Luo, Renzhi Hu, Tongtong Liu, Bing Wu, Shengyong Pang
Scanning electron beam welding (SEBW) is a very important process for welding of thick plates in aerospace, aeronautical and power industries. However, since the invention of this process, the scanning parameters are typically selected through time consuming and costly trial and error. No existing mathematical criterion can be used to select the optimal parameters due to lack of physical understanding of the welding process. In this study, we developed a three-dimensional mathematical model of SEBW capable of modeling the coupled keyhole and weld pool dynamics for the first time, and used it to understand the underlying physics of the welding process and explore the process optimization criterion of circular mode beam scanning by combining welding experiments and modeling. We showed that beam scanning may not always stabilize the keyhole and weld pool, and may not improve the final weld quality of electron beam welding. However, beam scanning can modulate the movement of high temperature positions on keyhole wall, and contribute to a better uniformity of weld pool dynamics behind the keyhole. For circular beam scanning, we proved that low frequency scanning may lead to more welding defects such as porosity, spiking, and spatters because it increases the tendency of keyhole oscillations as compared to the no scanning case. High frequency scanning could stabilize the keyhole to a certain degree and modulate the fluid flow of the weld pool to make it more regular. Additionally, the scanning radius should be neither too small nor too large. Too small radius may lead to more defects, and too large radius can decrease the penetration depth significantly. A dual direction energy uniformity (DDEU) criterion was proposed to select the scanning parameters by considering the energy uniformity degree in the welding direction and the transverse direction. It was demonstrated that process parameters including beam scanning frequency and radius can be successfully optimized using the proposed criterion.
Role of multiple discrete heaters to minimize entropy generation during natural convection in fluid filled square and triangular enclosures Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-19 Debayan Das, Leo Lukose, Tanmay Basak
The discrete heating strategy has been identified as an energy efficient method. The current work is aimed at achieving a thermally efficient triangular-design 1 (regular isosceles triangle), triangular-design 2 (inverted isosceles triangle) and square enclosures based on the entropy generation studies involving strategic positioning of the double heaters along each side wall (case 1: larger heater in lower half and smaller heater in central half, case 2: larger heater in central half and smaller heater in lower half, case 3: two heaters of identical lengths located at central and lower halves) for Pr=0.015 and 7.2 involving Ra=103–105. The numerical results of the cases 1–3 have been further compared with the case involving single heater along each side wall (case 0). Galerkin finite element method is implemented for the accurate evaluation of the entropy generation terms based on elemental basis set. Cases 1–3 exhibit lower entropy generation and higher heat transfer rates in the convection dominant regime (Ra=105) compared to the case 0. Overall, case 3 is concluded to be optimal based on higher rate of heat transfer and lower entropy generation.
Thermal transition and its evaluation of liquid hydrogen cavitating flow in a wide range of free-stream conditions Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-19 Tairan Chen, Hui Chen, Biao Huang, Wendong Liang, Le Xiang, Guoyu Wang
The objectives of this paper are to (1) validate the present numerical modeling framework for liquid hydrogen cavitating flow, (2) investigate the dynamic evolution of liquid hydrogen cavitating flows in a wide range of free-stream conditions and (3) propose a thermal parameter to evaluate and predict the transition process of two typical cavitation dynamics in thermo-fluids. The dynamic evolutions of liquid hydrogen cavitating flows in a wide range of free-stream conditions (T∞ = 14–33 K, U∞ = 63.9 m/s, 90 m/s, σ∞ = 0.38, 0.8, 1.2) are numerically investigated. General agreements are obtained between the numerical results and the experimental measurements, including the pressure distribution, temperature drop as well as the cavity structures. The results show that the cavitation behaviors near the triple point are similar to cavitating flows without thermodynamic effects. Two typical cavitation dynamics named the quasi-isothermal mode and the thermo-sensitive mode in liquid hydrogen are observed under the same cavitation number and flow velocity. When the free-stream T∞ is below 30 K (TN ≤ 0.85), as the temperature increases, the cavity area increases to the maximum around the thermal transition temperature and then decreases when the cavitation dynamics transits from the quasi-isothermal mode to the thermo-sensitive mode. Further analysis indicates that the thermal transition temperature for liquid hydrogen is approximately at T∞ = 17 K and the transition temperature slightly increases with the increasing free-stream velocity and cavitation number. When the free-stream T∞ is above 30 K (TN > 0.85), the thermodynamic effects significantly decrease due to the suddenly changed physical properties. And then, the cavitation dynamics turns from the thermo-sensitive mode to the quasi-isothermal mode with the increasing temperature. The modified C-factor including both the thermodynamic effects and the effects of turbulent pressure fluctuations proposed in this paper could quantitatively evaluate and predict dynamics transition from the quasi-isothermal mode to the thermo-sensitive mode in thermos-fluids cavitating flows. It could be utilized as a parameter to design the operating conditions for thermos-fluids apparatus or system to avoid the maximum cavitation aggressiveness around the thermal transition temperature.
Numerical modeling for alumina nanofluid magnetohydrodynamic convective heat transfer in a permeable medium using Darcy law Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 M. Sheikholeslami, S.A. Shehzad, Zhixiong Li, Ahmad Shafee
CVFEM is employed in this article to model alumina nanofluid magnetohydrodynamic flow through a permeable enclosure. Influences of Hartmann number, buoyancy, radiation parameters on nanofluid treatment were displayed. Viscosity and thermal conductivity of alumina are predicted considering Brownian motion and shape factor impacts. Results are displayed that Lorentz forces boosts the conduction mechanism. Nu ave enhances with reduce of Ha . Augmenting radiation parameter makes thermal boundary layer to be thinner.
Analyses of coupled steady heat transfer processes with entropy generation minimization and entransy theory Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 XueTao Cheng, XinGang Liang
The entropy generation minimization and the entransy theory are widely used in the optimization of heat transfer. In this paper, the two theories are applied to coupled steady heat transfer systems. The two different definitions of radiative entransy flux are discussed. It is found that the extremum principle of entransy dissipation for coupled heat transfer systems can be obtained only when temperature is treated as the driving force of radiative heat transfer and the definition of entransy flux for radiative heat transfer is the same as that in conductive and convective heat transfer. Taking temperature as the driving force of radiative heat transfer, we have analyzed three coupled heat transfer examples, which are the one-dimensional coupled conductive and radiative heat transfer, the coupled convective and radiative heat transfer and the coupled conductive, convective and radiative heat transfer. The results show that the extremum principle of entransy dissipation always leads to the best system performance, while the entropy generation minimization does not always. The definition of radiative entransy flux in which the blackbody emissive power is treated as the driving force is also used for the three examples. However, the results show that neither the extremum radiative entransy dissipation rate nor the extremum conductive/convective entransy dissipation rate results in the best system performance. Therefore, this definition is not suitable for coupled heat transfer systems.
Phase state and velocity measurements with high temporal and spatial resolution during melting of n-octadecane in a rectangular enclosure with two heated vertical sides Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 J. Vogel, D. Bauer
A novel validation experiment for the melting of a phase change material is presented. The goal is to measure phase state and velocities with high accuracy and resolution. The geometry and boundary conditions of the test section are the most generic found in latent heat storage systems: a phase change material is contained in a rectangular enclosure, where it is isothermally heated from two opposing vertical side walls. The enclosure has a height of 105 mm and a width and depth of 50 mm. The bottom, front and back sides are solid transparent walls and on top is a thin layer of air. Near-adiabatic boundary conditions are realized at the non-heated sides with a surrounding insulated air-filled chamber and an actively controlled trace heating system. In this study n-octadecane is used as the phase change material. During melting, the liquid phase fraction is measured with a shadowgraph technique and velocities due to natural convection in the liquid phase are measured with particle image velocimetry (PIV). Interior and boundary temperatures are measured with thermocouples to control and analyze boundary effects. A thorough error estimation is done for all the measured quantities. The main result is a comprehensive dataset of liquid phase fractions and velocities with high spatial and temporal resolutions. The liquid phase fraction is additionally measured for three different driving temperature differences and a scaling by dimensionless numbers is performed. This results in a correlation function for the liquid phase fraction that predicts similar melting processes and is valuable in system design and optimization.
Enhancement of loop heat pipe performance with the application of micro/nano hybrid structures Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 Xueli Wang, Jinjia Wei, Yueping Deng, Zan Wu, Bengt Sundén
To further improve the flat-type loop heat pipe (LHP) performance, this study evaluates the practical potential of use of highly enhanced boiling structures. It is found that in our proposed new heat pipe (NHP) system, the working fluid from the evaporator outlet to the condenser inlet is in a liquid–vapor two phase flow, which is different from the classical LHP theory. A new P-T diagram is developed to better understand the thermal and hydraulic process during the NHP steady operation. In this study, by using the laser ablation technique two different types of micro- and nanoscale hybrid structures are synthesized on the boiling pool substrate. It is indicated that the formed valleys with a larger opening width play an important role in more effectively improving the bubble nucleation and bubble growth at the micrometer sites, which can subsequently lead to an increased number of active nucleation sites. The best loop performance is obtained with the micro-cone structured substrate at a heat load of 140 W, at which the maximum boiling pool heat transfer coefficient of 42.17 kW/m2·K is achieved. Compared with the polishing Cu substrate, it is enhanced by 110%. When maintaining the boiling pool temperature lower than 85 °C, the proposed new heat pipe system can tolerate a maximum heat flux of 35.12 W/cm2, which is larger than that of the most conventional LHPs with methanol as the working fluid.
A mechanistic approach to developing two phase flow pattern transition maps for two-phase dielectric fluids subject to high voltage polarization Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 S. Nangle-Smith, J.S. Cotton
High voltage electric fields can be used as an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids, with the major component of this enhancement being attributed to phase redistribution and mixing in the system due to electric body forces. Many performance prediction tools for convective boiling and condensation rely on flow pattern maps to identify the flow regime for use with flow pattern specific heat transfer and pressure drop correlations , . In this paper, a mechanistic approach to developing two phase flow pattern maps for two-phase dielectric fluids subject to high voltage fields using a fully contained, equation-based electric Froude number approach is presented and compared against the experimental flow pattern data for EHD convective boiling & condensation available in the literature and from the current experimental study. Despite the development of newer diabatic, semi-mechanistic flow pattern models for free-field heat exchanger performance analysis, it is recommended that the Steiner equations for flow pattern transition be used as a basis for the mapping when additional physics such as EHD is coupled, as the empirical fits in newer semi-mechanistic maps are unreliable when additional physics is present.
Thermal performance of LHSU for electronics under steady and transient operations modes Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 Hafiz Muhammad Ali, Adeel Arshad, Muhammad Mansoor Janjua, Wajahat Baig, Uzair Sajjad
This experimental study investigates the effect of passive cooling of electronic devices using a latent heat storage unit (LHSU) filled with n-eicosane as phase change material (PCM),under steady-state and transient heating operations. A pin–fin heat sink of square configuration, made of aluminum, pin-fins acted as a thermal conductivity enhancers (TCEs), is used to augment the rate of heat transfer through n-eicosane which has low thermal conductivity. A range of input heating loads from 1.2 – 2.8 kW / m 2 with an interval of 0.4 kW / m 2 are applied at the base of LHSU, filled at three different volumetric fractions of n-eicosane, to quantify the steady-state and transient heat transfer characteristics for different operation modes for reliable passive cooling of electronics. The results are reported under two phases i.e. steady-state and transient heating, and thermal performance of LHSU is elucidated in terms of enhancement in operation time, enhancement ratio, effect of PCM amount and various usage modes under different heating loads.
Thermal cloak with adaptive heat source to proactively manipulate temperature field in heat conduction process Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 J. Guo, Z.G. Qu
Thermal cloaks can thermally hide an object without disturbing background thermal field. The coordinate transformation method for designing conventional thermal cloaks face some challenges that the physical properties of metamaterials are required to be anisotropic and inhomogeneous. Furthermore, the coordinate transformation is not applicable for complex geometrical conditions. In this study, a proactive thermal cloak is proposed. The cloak contains two concentric annular zones: one with high thermal conductivity and another with adaptive heat source. The cloaked zone is surrounded by the high thermal conductivity zone. The high thermal conductivity zone keeps the object in the cloaked zone from thermally being affected by outside zones. The heat source zone adjusts the background thermal field without any distortion. The thermal cloak with an analytical heat source distribution is presented for the 2D and 3D background thermal fields with uniform gradient. The adaptive heat source in the thermal cloak is designed by inverse problem theory of a numerical method for the 2D arbitrary background thermal field. This proactive thermal cloak can avoid the issue of anisotropic physical properties for the conventional thermal cloak designed with coordinate transformation method. The heat source thermal cloak with relatively easy implementation can manipulate temperature field proactively without contact. This thermal cloak is applicable for arbitrary background thermal fields and complex geometrical conditions by a numerical design approach. The proposed thermal cloak opens a new strategy to manipulate the thermal field with adaptive heat source.
Flow visualization study of partially filled channel with aluminium foam block Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 Fadhilah Shikh Anuar, Iman Ashtiani Abdi, Kamel Hooman
This experimental study investigated fluid dynamics of a channel partially filled with a metal foam block with various pore densities; 5, 10 and 30 PPI. The effects of foam heights, in a range of 0.004 – 0.050 m, and inlet velocities, from 3.9 to 12.5 m/s, on the flow field were investigated using Particle Image Velocimetry (PIV). For a more comprehensive analysis of the problem, detailed flow patterns inside the porous foam were visualized by applying a thermal imaging method. Results show that there are different flow behaviours in the partially filled channel depending on the key parameters. A boundary layer is formed on the horizontal fluid/foam interface, which is much more noticeable and thicker in high blockage ratio cases. The low-pore density foam allows the flow to pass through its porous structure while the higher pore density foam imposes more restrictions on the flow pushing the fluid away from the foam into non-porous region on the top of the foam block before reaching the foam end.
Boiling performance on surfaces with capillary-length-spaced one- and two-dimensional laser-textured patterns Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Jure Voglar, Peter Gregorčič, Matevž Zupančič, Iztok Golobič
This investigation used laser-processed 25-μm-thick stainless steel foils as heaters in pool boiling experiments under subcooled and saturated conditions at atmospheric pressure. Boling surfaces were modified by a nanosecond fiber laser. In most cases, laser-textured parts on boiling surfaces were spaced apart by a capillary length of water (2.5 mm) and had different shapes and arrangements. Multi-scale micro-cavities (with diameters ranging from 0.2 to 10 μm) on the laser-textured areas of the surfaces provided potential active nucleation sites. The highest heat flux measured before the burnout was observed on the fully treated sample; this heat flux was a factor of 3.7 greater than that of the untreated sample. The sample with hexagonally arranged textured circular shapes with a diameter of 2.0 mm provided a more than 4-fold higher heat transfer coefficient compared to the untreated sample. All of the laser-textured boiling surfaces showed enhanced pool boiling heat transfer performance in comparison to the untreated surface. The optimal spacing between the laser-textured regions was experimentally found to be equal to the capillary length of the working fluid. Our results demonstrate that laser texturing has strong potential for producing patterned surfaces for engineering applications of boiling heat transfer.
Dropwise condensation on superhydrophobic nanostructure surface, part II: Mathematical model Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Jian Xie, Jinliang Xu, Wei Shang, Kai Zhang
Inspired by our finding that nano-grass-surface (NGS) with smaller droplets has poorer condensation heat transfer performance than smooth-single-molecule-layer surface (SSML) and the long-term operation of NGS deteriorates heat transfer to approach limit values, the secrets of droplets interacting with nano-pillars structure are explored. A comprehensive dropwise condensation model is developed. The contact angles are treated to show correct trend with respect to Cassie, partial Wenzel and Wenzel morphologies. A mixed droplet detachment model is developed to consider the coalescence-induced-jumping, rolling and sliding modes simultaneously. The maximum drop radius is rmax = min(rmax,jump, rmax,roll, rmax,slide), where rmax,jump, rmax,roll and rmax,slide are maximum drop radii in jumping, rolling and sliding modes, respectively. The number of drop nucleation sites on NGS is f times of that on SSML, where f is the surface roughness. Our model predictions match the measured heat transfer data well. It is concluded that the dropwise condensation is the outcome of a series of positive and negative effects by introducing the nanostructure. The increased droplet population density and number of drop nucleation sites are the positive contributions, while the decreased single drop heat transfer rate and additional nano-porous thermal resistance are the negative contributions. The densely populated nano-pillars structure has the largest capability to enhance heat transfer. The Heterogeneous hydrophilicity/hydrophobicity surface limits droplet sweeping distance within neighboring hydrophilic dots, to avoid that stone rolls on the lawn to spoil the grasses, the heterogeneous surface is recommended to resist nanostructure failure for long-term operation.
Heat transfer during condensing droplet coalescence Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-18 Sanjay Adhikari, Alexander S. Rattner
Dropwise condensation can yield heat fluxes up to an order of magnitude higher than filmwise condensation. Coalescence is the primary mode of growth for condensing droplets above a small threshold size (e.g., radius r > 2 μm for water at 1 atm), but no prior studies have quantitatively assessed heat transfer during coalescence. Previous models of dropwise condensation have generally described coalescence as an instantaneous event, with a step reduction in heat transfer rate. However, coalescence and recovery of a quasi-steady droplet temperature profile requires a finite time, during which the direct droplet condensation heat transfer rate gradually decays. Additionally, during this period, the droplet may oscillate, repeatedly clearing the surrounding surface and resulting in high overall heat fluxes. This study employs Volume-of-Fluid (VOF) simulations to quantitatively assess these two transient heat transfer processes during droplet coalescence. It is shown that the direct mechanism of gradual heat transfer decay can be represented by a decaying exponential function with a time constant τ . Simulations are performed to determine τ ( r 1 , Rt ) for ( 1 μ m ⩽ r 1 ⩽ 25 μ m; 1 ⩽ Rt ⩽ 4 ) where r 1 is the radius of the smaller droplet and Rt is the radius ratio between the two merging droplets. For water at atmospheric pressure this spans the range of droplet sizes through which most of the heat transfer occurs on a surface ( ∼ 80 % ). A simple correlation is proposed for τ ( r 1 , Rt ) for the studied droplet size range, fluid properties, and surface conditions. These simulations are also employed to determine the order of magnitude of heat transfer enhancement due to repeated clearing of the surrounding surface as droplets coalesce. Findings can inform improved models of dropwise condensation that more accurately predict transient heat transfer during coalescence events.
Steady-state and transient solutions to drop evaporation in a finite domain: Alternative benchmarks to the d2 law Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Ashish Pathak, Mehdi Raessi
The d2 law, commonly used to assess the accuracy of computational solvers, corresponds to evaporation under steady-state conditions and in an infinite gaseous domain. In the present work, we propose new alternative benchmarks that relax those constraints. The benchmarks are developed for a spherical as well as a circular drop. First, steady-state analytical relations are derived that are applicable to a finite domain. A radially-symmetric transient model is then proposed that can provide transient benchmark solutions in finite domains. Adaptive mesh-refinement and body-fitted mesh are used for numerical solution and to ensure highly accurate transient results. The proposed steady-state and transient benchmarks are examined in several evaporation problems, and the evolution of drop diameter, temperature and vapor mass fraction fields are analyzed. In one such problem, we report a secondary transient phenomenon that appears towards the end of drop lifetime. The benchmarks proposed in this work are straightforward to be implemented for systematic validation of computational solvers.
Evaluation of anisotropic tangential conduction in printed-circuit-board heated-thin-foil heat flux sensors Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 A.F.M. Torre, A. Ianiro, S. Discetti, G.M. Carlomagno
The effect of the tangential conduction contribution in a thermally anisotropic heated-thin-foil heat flux sensor is examined. A parameter to assess the degree of importance of tangential conduction in the sensor is defined and evaluated in order to justify the need of tangential conduction corrections. Printed circuit boards (PCBs) are typical examples of sensors with anisotropic thermal conduction properties, due to the different conductance values in the directions either parallel or orthogonal to the copper tracks which are placed onto a fiberglass substrate. A parametric study on PCBs with different tracks coverage fraction and copper-to-fiberglass heat conductance ratio is carried out. A revised heated-thin-foil formulation, including a correction for anisotropic thermal properties of the PCB, is experimentally tested. The selected thermo-fluid-dynamic test case is the convective heat transfer of a normally impinging round jet for which axisymmetric maps of the Nusselt number are expected. The anisotropic tangential conduction results in non-axisymmetric temperature distributions. Consequently, if anisotropy is not properly accounted for, non-axisymmetric Nusselt number maps are obtained. The anisotropic conduction effects are shown to be weakly sensitive to the copper tracks coverage fraction while strongly dependent on a parameter called degree of anisotropy, which accounts for the copper-to-fiberglass heat-conductance ratio. Anisotropic conduction effects are found to be almost negligible in PCBs with low values of the degree of anisotropy and/or low values of the tangential conduction degree of importance. Accounting for the anisotropic tangential conduction in the heated-thin-foil formulation allows minimizing the differences between the Nusselt number profiles measured in the directions parallel and orthogonal to the copper tracks. For all the considered PCBs, differences after correction are found to be below the measurement uncertainty expected for an equivalent thermally-isotropic sensor. The proposed approach also allows reducing below the measurement uncertainty the spread between the Nusselt number values measured with all the considered printed circuit boards, providing measurements practically independent of the sensor characteristics.
Thermal boundary conditions to simulate friction layers and coatings at sliding contacts Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Oleksii Nosko
A brief review of the thermal boundary conditions specified at sliding interfaces was performed. New thermal boundary conditions were derived aimed at solving problems of sliding with account of surface layers representing friction layers and tribological coatings. Based on the assumption of linear temperature distributions in the surface layers, the proposed conditions enable one to simplify simulations by eliminating the surface layers from consideration and merging thermally the bulk bodies. A problem of non-stationary heat conduction in two sliding semispaces with the proposed conditions at their interface was solved analytically. The particular cases of the solution were shown to coincide with the exact temperature expressions obtained for different well-known boundary conditions. The temperature influence of the basic parameters was analysed in dimensionless form on example of a decelerative sliding between a semispace covered with a surface layer and a semispace of constant temperature. An estimation of the error introduced by the proposed conditions was done. It was found that the error is below 1% for a wide class of friction layers and coatings met in practice.
Evaluation of multi-objective inverse heat conduction problem based on particle swarm optimization algorithm, normal distribution and finite element method Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Xiaowei Wang, Huiping Li, Lianfang He, Zhichao Li
The boundary heat transfer coefficient (BHTC) and thermal conductivity are the most essential thermal physical parameters, which have a significant effect on the calculation accuracy of physical fields in the numerical simulation. To simultaneously estimate the temperature-dependent BHTC and thermal conductivity by solving the inverse heat conduction problem (IHCP), a hybrid method (ZPSO) based on particle swarm optimization algorithm (PSO), normal distribution method and finite element method (FEM) is presented, where normal distribution method representing the transient solution space is used to improve the convergence speed. A two-dimensional direct heat conduction problem (DHCP) with the temperature-dependent thermal-physical parameters is simulated based on FEM. Some temperature curves at certain position of FEM model are attained in the simulation. The temperature-dependent BHTC and thermal conductivity are simultaneously evaluated by PSO–FEM and ZPSO–FEM according to the temperature curves attained in DHCP. Comparing the temperature-dependent thermal-physical parameters attained in the IHCP to that used in the DHCP, the results show that the BHTC and thermal conductivity evaluated by the ZPSO are well consistent with those used in the DHCP. The evaluation results show that the convergence of the hybrid method is well, and the convergence speed is accelerated by the ZPSO.
Numerical inverse method for calculating heat flux in temperature-sensitive-coating measurement on a finite base Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Zemin Cai, Tianshu Liu, Javier Montefort
The one-dimensional unsteady heat transfer of a polymer layer on a finite base is studied. A numerical inverse method is developed to calculate heat flux from surface-temperature images obtained using temperature-sensitive coating particularly in high-enthalpy wind tunnels. It is found that the effect of the finite thickness of the base is significant as time increases such that the semi-infinite base assumption made in many methods is no longer accurate in heat-flux calculation. The method developed in this work can take into account the effects of the finite thickness of a base and the temperature-dependencies of the thermal properties of the materials. This method has been validated through simulations and processing the experimental temperature-sensitive-paint (TSP) images.
Numerical investigation of heat transfer and flow inner tube with periodically cosine oscillation Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Lei Rui, Hanzhong Tao
In the present study, both heat transfer and pressure drop was studied numerically for static round tube and cosine oscillating tube with finite volume method. The effect of dimensionless amplitude and frequency on heat transfer and pressure drop was also investigated for Reynolds number changing from 6170 to 14,000. Under the influence of cosine oscillation, the heat transfer and pressure drop is periodically changing and the frequency for heat transfer and pressure is half of that for cosine oscillation motion. Heat transfer was enhanced by cosine oscillation and increased by 1.9% compared to static tube for Re = 6140. Meanwhile, heat transfer increased by 35% as dimensionless amplitude increased from 1 to 10. While cosine rotation also caused larger pressure drop and the pressure drop for cosine oscillating tube is 3.01% that of static tube. The pressure drop is proportional to dimensionless amplitude and frequency. However, heat transfer first reduce and then increase with increase in frequency.
On the analytical modelling of the initial ice growth in a supercooled liquid droplet Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 K. Schulte, B. Weigand
Ice formation in metastable, super-cooled droplets, which are frequently found in the atmosphere, influences the appearance and characteristics of atmospheric clouds significantly, for example regarding precipitation. Its numerical investigation can provide deep insight into the underlying physical mechanisms and supports the deduction of models that describe these processes on the microscale; those models are required for a description of the macrophysical system. However, even the processes on the microscale span about four orders of magnitude. A semi-analytical sub-scale model based on similarity solutions is thus deduced in order to narrow the gap between the different scales describing the initially spherical ice growth in a super-cooled droplet, which can be reduced to a radially symmetric, but highly non-linear Stefan-type problem. All relevant physical effects, e.g. the reduction of the melting temperature, the expansion of the water phase due to the decrease of density upon solidification and high degrees of supercooling, are taken into account in contrast to classical approaches. The maximum relative error in terms of the freezing time, which is given explicitly as well as the temperature fields, is less than 10% at a degree of supercooling of 35 K and decreases rapidly as the ambient temperature increases.
Effect of internal heat generation/absorption on Rayleigh-Bénard convection in water well-dispersed with nanoparticles or carbon nanotubes Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 C. Kanchana, Yi Zhao
Linear and weakly nonlinear stability analyses of Rayleigh-Bénard convection with internal heat generation/absorption in water-based nanoliquids is studied analytically in the paper using the generalized Buongiorno two-phase model. The Boussinesq approximation and small-scale convective motion are assumed to be valid. By considering a minimal-mode Fourier representation, we arrived at an analytically intractable fifth-order, autonomous, generalized Lorenz model. Results on linear stability (direct and pitchfork bifurcations) and subcritical instability (inverse bifurcation) are obtained using the generalized Buongiorno two-phase model. The generalized Lorenz model with quadratic nonlinearities is then reduced to an analytically tractable Ginzburg-Landau equation with cubic nonlinearity. The analytical solution of this equation is used to quantify heat transport in terms of the Nusselt number. The contribution of nanoparticles/nanotubes in advancement of onset of convection and hence in the enhancement of heat transport by water is explained. The results of the enclosure problem are extracted from those of the Rayleigh-Bénard convection problem. Tall, square and shallow enclosures are considered in the study by varying aspect ratio. The study reveals that among the three enclosures, tall enclosure transports maximum heat. The influence of heat generation/absorption is shown to augment/inhibit onset of convection and enhance/diminish heat transport. A mechanism of improving heat-removal in water-based cooling systems through the use of nanoparticles/nanotubes is proposed. Several limiting cases of the study are presented.
Churning losses analysis on the thermal-hydraulic model of a high-speed electro-hydrostatic actuator pump Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Junhui Zhang, Ying Li, Bing Xu, Xia Chen, Min Pan
The speed of Electro-Hydrostatic Actuator (EHA) pump can recently reach a maximum of 20,000 rpm due to its high power density requirements. The thermodynamics of EHA pumps become increasingly important because the oil in the casing is used to cooling the high speed electric motor and solving the “hot spots” problem in the EHA system. In order to realize the boundary condition for the motor cooling at high speeds, more investigations should be given to the churning losses in terms of the large centrifugal forces of piston/cylinder assemblies and high fluid kinetic energy in high-speed conditions. This paper analyzes the influence of churning losses on the thermal-hydraulic model. Using the control volume method, heat transfer analysis of the piston pump is presented and the heat flow inside the piston pump produced by churning losses is described in details. The steady-state leakage temperatures of the high-speed EHA pump are simulated and validated through experiments. The experimental results show that the churning power losses need to be considered on the thermal-hydraulic model to ensure good performance for the prediction of the leakage temperature in EHA pump.
A hybrid phonon Monte Carlo-diffusion method for ballistic-diffusive heat conduction in nano- and micro- structures Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Han-Ling Li, Yu-Chao Hua, Bing-Yang Cao
The existing phonon Monte Carlo (MC) for ballistic-diffusive heat conduction are limited to small and simple structures owing to the huge time cost following with the increasing scale. This article presents a new hybrid phonon Monte Carlo-diffusion method for ballistic-diffusive heat conduction, which successfully characterizes the ballistic effect with significantly reducing the computational cost. Based on the idea that the phonon-boundary scattering mainly affects the regions adjacent to the boundaries when the system is considerably large, the whole system is divided into three zones: the boundary MC zone and the middle diffusion zone, between which is the overlap zone. By using an alternating method and setting virtual phonon bath or specular reflection as the boundary condition for the MC zones, the results of the phonon tracing MC and diffusion equation can be coupled and converge at the overlap zone. To verify, the cross-plane and in-plane film heat conduction, where slip boundary conditions are the major characteristics of the ballistic-diffusive regime, are simulated by the hybrid method as well as standard phonon tracing MC which works as a benchmark. It is found that the hybrid method can accurately predict the distributions of temperature and heat flux in the system with nearly the same precision as the phonon tracing MC while the computation time can reduce up to 90%, validating its potential use for larger and more complex structures.
Understanding of the thermo-hydrodynamic coupling in a micro pulsating heat pipe Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Aejung Yoon, Sung Jin Kim
In this study, experimental and theoretical investigations are performed to identify the thermo-hydrodynamic characteristics of a micro pulsating heat pipe (MPHP). Specifically, the relationship between the heat input and the vapor distribution observed in the MPHP is revealed. A silicon-based MPHP with five turns and a hydraulic diameter of 667 μm is fabricated using MEMS techniques. Experiments are performed, using ethanol as a working fluid at a filling ratio of 55%, in a vertical orientation with a bottom-heating mode. Flow visualization is conducted together with a temperature measurement. In the MPHP, two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the position of one meniscus (up-header) is always located higher than that of the other (down-comer) and is linearly increased with an increasing heat input. At a critical value of the heat input, the position of the up-header meniscus reaches its upper limit and cannot be increased further. At this upper limit, the thermal performance of the MPHP reaches its maximum and cannot be increased further. This suggests that physically the (asymmetric) vapor distribution is the key factor that determines the heat transport capability of the MPHP. To theoretically explain the relationship between the heat input and the vapor distribution in the MPHP, a model for the asymmetric vapor distribution is developed. Based on the model, a correlation for predicting the positions of vapor menisci is proposed. Finally, the proposed correlation is shown to be useful for predicting the heat input at which the MPHP attains its maximum thermal performance.
Instability mechanisms for thermocapillary flow in an annular pool heated from inner wall Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Hao Liu, Zhong Zeng, Linmao Yin, Long Qiao, Liangqi Zhang
The linear stability analysis of thermocapillary flow in an annular pool heated from inner wall was performed by using spectral element method. The physical instability mechanisms for different Prandtl numbers (Pr) ranging from 0.001 to 1.4 were studied by energy analysis under critical mode. We observed three instability types: (i) the Hopf (oscillatory) bifurcation with wavenumber k = 18 for Pr < 0.02, and the corresponding neutral mode is amplified due to the basic shear flow; (ii) the stationary instability with k = 3 or k = 2 for 0.02 < Pr < 0.6; (iii) the Hopf bifurcation again with k = 3 for 0.6 < Pr < 1.4. In both Prandtl number ranges of 0.02 < Pr < 0.6 and 0.6 < Pr < 1.4, thermocapillary force induced by the disturbance temperature field drives primarily the flow instability.
Modeling of pressure drop in two-phase flow of mono-sized spherical particle beds Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Jin Ho Park, Hyun Sun Park, Mooneon Lee, Kiyofumi Moriyama
To reduce the uncertainty in predicting pressure drop for two-phase flow through spherical particle beds, especially composed of small-sized particles that adversely affect the cooling, an experimental study was performed. A series of experiments were performed to measure the pressure drop of upward two-phase flow through well-packed spherical particle beds with 2, 3.5, and 5 mm diameter particles under isothermal conditions varying the gas flow rates with no water inflow at the bottom. Based on the experimental results and the results of our previous study related to the modification of Ergun constants, a new improved model for prediction of the two-phase flow pressure drop through porous media was proposed. The adequacy of the proposed model was verified by comparison with various existing experimental data for particle diameters of 3.18–6.35 mm and the superficial air velocity of 0–0.8 m/s.
Simulation and experiment on supercooled sessile water droplet freezing with special attention to supercooling and volume expansion effects Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Xuan Zhang, Xin Liu, Xiaomin Wu, Jingchun Min
The freezing process of supercooled water droplets on cold plates is studied experimentally and numerically. A numerical model that considers both the supercooling effect on the physical properties and the volume expansion effect on the droplet freezing profile is established to simulate the droplet freezing behaviors using the Solidification/Melting model. Experiments are conducted on both hydrophilic and hydrophobic surfaces for 5, 10, 20, 30 and 40 μL supercooled water droplets. The droplet freezing behaviors including the freezing front movement and freezing time are observed using the image recognition technology. The evolution of the freezing front calculated by the numerical model agrees better with the experimental observation than the traditional model that ignores the supercooling effect or uses the initial droplet profile. The model reduces the deviation of freezing time from about 30% to about 10% for both hydrophilic and hydrophobic surfaces. With use of the average value of the freezing times yielded by the numerical and theoretical models, a correlation is developed for predicting the freezing time. It can correlate more than 90% of the simulation data and all of the experimental points within ±25%. As the cold plate temperature, droplet volume and contact angle increase, the freezing time increases, with the plate temperature and contact angle possessing a more significant influence than the droplet volume on the freezing process.
A continuum thermomechanical model of in vivo electrosurgical heating of hydrated soft biological tissues Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Wafaa Karaki, Rahul, Carlos A. Lopez, Diana-Andra Borca-Tasciuc, Suvranu De
Radio-frequency (RF) heating of soft biological tissues during electrosurgical procedures is a fast process that involves phase change through evaporation and transport of intra- and extra-cellular water, and where variations in physical properties with temperature and water content play significant role. Accurately predicting and capturing these effects would improve the modeling of temperature change in the tissue allowing the development of improved instrument design and better understanding of tissue damage and necrosis. Previous models based on the Pennes’ bioheat model neglect both evaporation and transport or consider evaporation through numerical correlations, however, do not account for changes in physical properties due to mass transport or phase change, nor capture the pressure increase due to evaporation within the tissue. While a porous media approach can capture the effects of evaporation, transport, pressure and changes in physical properties, the model assumes free diffusion of liquid and gas without a careful examination of assumptions on transport parameters in intact tissue resulting in significant under prediction of temperature. These different approaches have therefore been associated with errors in temperature prediction exceeding 20% when compared to experiments due to inaccuracies in capturing the effects of evaporation losses and transport. Here, we present a model of RF heating of hydrated soft tissue based on mixture theory where the multiphase nature of tissue is captured within a continuum thermomechanics framework, simultaneously considering the transport, deformation and phase change losses due to evaporation that occur during electrosurgical heating. The model predictions are validated against data obtained for in vivo ablation of porcine liver tissue at various power settings of the electrosurgical unit. The model is able to match the mean experimental temperature data with sharp gradients in the vicinity of the electrode during rapid low and high power ablation procedures with errors less than 7.9%. Additionally, the model is able to capture fast vaporization losses and the corresponding increase in pressure due to vapor buildup which have a significant effect on temperature prediction beyond 100 °C.
A modified Lie-group shooting method for multi-dimensional backward heat conduction problems under long time span Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Yung-Wei Chen
This paper proposes a modified Lie-group shooting method to solve multi-dimensional backward heat conduction problems under long time spans. The backward heat conduction problem is renowned for being ill posed because the solutions are generally unstable and highly dependent on the given data. For dealing with those problems, the Lie-group shooting method is one of the most powerful tools to find the unknown initial condition for the backward heat conduction problems in the time domain. In previous studies, the Lie-group shooting method uses the time and spatial semi-discretization technique to change the integration direction of numerical schemes and then increase the time span. However, the conversional Lie-group shooting method cannot get to the core of divergence problems for the backward heat conduction problems, especially the increased computational time. The main reason is that a real single-parameter Lie-group element occurs at zero, and a generalized midpoint Lie-group element is not equivalent to the single-parameter Lie-group element in the Minkowski space. Hence, to overcome the above problems, the relationship of the initial condition, the final condition and a real single-parameter r is assessed. According to the constraint condition of the initial and final condition, a real single-parameter r depends on the time span to maintain the numerical convergence. Again, in order to preserve the same Lie-group property in the time direction, the high-order Lie-group scheme based on the generalized linear group in Euclidean space is introduced, which concurrently satisfies the constraint of the cone structure, the Lie-group and the Lie algebra at each time step. The accuracy and efficiency are validated, even under noisy measurement data, by comparing the estimation results with existing literature.
Mixed convective vertically upward flow past side-by-side square cylinders at incidence Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Chirag G. Patel, Sandip Sarkar, Sandip Kumar Saha
Fluid dynamics and heat transfer behaviour are studied past side-by-side square cylinders at incidence in vertical flow arrangement numerically considering laminar, Newtonian, steady, incompressible and two-dimensional flow using Finite Volume Method based ANSYS Fluent solver. Using air (Pr = 0.7) as the working fluid, computations are performed at a representative value of Re = 100. The angle of incidence (α) is varied from 0 ° to 45 ° in the step of 5 ° , whereas, the lateral distance between the cylinders is kept constant. To consider all possibilities, cylinders are rotated either clockwise or counter-clockwise, simultaneously or individually, which generates thirty-four different orientations. Buoyancy assisting phenomenon is created by changing the Richardson number (Ri) from 0 to 1 in step of 0.25. Due to the variation of the angle of incidence and its orientations, vortex generation is observed in the buoyancy assisting case at a critical value of Ri. To study heat transfer characteristics, time average and the local Nusselt number are analysed. For a fixed lateral distance between the cylinders, maximum heat transfer is found to occur at an incident angle of 45 ° . Other important parameters, such as drag coefficient (CD), lift coefficient (CL), Strouhal number (St) are also studied.
Nanofluid unsteady heat transfer in a porous energy storage enclosure in existence of Lorentz forces Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Zhixiong Li, M. Sheikholeslami, M. Samandari, Ahmad Shafee
In current article, nanofluid time dependent heat transfer under the influence of Lorentz forces during discharging process is simulated by means of FEM. In order to overcome the limitation of PCM, NEPCM has been utilized. CuO and water are employed as nanoparticles and PCM. Brownian motion role is taken into consideration to estimate nanofluid characteristics. Graphs are illustrated as isotherm, solid fraction and stream line contours. Results reveal that discharging rate improves with augment of Lorentz forces. As Hartmann number augments, solid fraction profile will be converged in lower time. Dispersing nanoparticles has significant impact on phase change front.
Entropy generation of electromagnetohydrodynamic (EMHD) flow in a curved rectangular microchannel Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Yongbo Liu, Yongjun Jian, Wenchang Tan
The entropy generation analysis of electromagnetohydrodynamic (EMHD) flow of Newtonian fluids through a curved rectangular microchannel is performed in this study. Under the assumption of thermally fully developed and the condition of constant wall heat flux, the distributions of velocity and temperature are derived analytically and numerically, which are utilized to compute the entropy generation rate. Analytical solutions of the velocity are contrasted with the numerical and experimental solutions and the agreements are excellent. The results show that the flow and the temperature depend on the strength of the electric field (S), magnetic field (Ha), aspect ratio of the rectangular cross section (α), curvature ratio (δ), peclet number (Pe) and viscous dissipation (Br). Then the entropy generation rates are investigated under the appropriate nondimensional parameters. The results show that the local entropy generation has a decreasing trend from the wall towards the centerline of the microchannel. Moreover, the entropy generation rate increases with the increase of S and Br but decreases with Pe and α. Finally, the entropy generation rate increases with the increase of Ha when Ha is small, and reaches a constant as further increase of Ha. The present endeavor can be utilized to design the efficient thermal micro-equipment.
Effect of solid-to-fluid conductivity ratio on mixed convection and entropy generation of a nanofluid in a lid-driven enclosure with a thick wavy wall Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 S.K. Pal, S. Bhattacharyya, I. Pop
A numerical study on the conjugate heat transfer by mixed convection of a Cu-water nanofluid and conduction in a solid region is conducted in an enclosure with a thick wavy heated wall. The upper lid of the enclosure is made to slide horizontally at a constant speed, along with that the condition of heated outer boundary of the thick bottom wall leads to a mixed convection within the enclosure. The impact of the wavy fluid-solid interface, solid-to-fluid thermal conductivity ratio and nanoparticle volume fraction on the heat transfer characteristics is analyzed for different choice of the Richardson number. The computational domain is transformed into an orthogonal co-ordinate system. The transformed governing equations along with the specified boundary conditions are solved through a finite volume method for a wide range of Richardson number, nanoparticle volume fraction, wave amplitude, wave number and wall-to-fluid conductivity ratio for different Reynolds number. Results show that the heat transfer rate increases substantially due to the inclusion of nanoparticles. Heat transfer rate varies due to the variation of the solid-to-fluid conductivity ratio, amplitude and wave number of the wavy wall. The impact of the wavy surface is stronger when the solid conductivity is in the order of the conductivity of the fluid. The Bejan number and the entropy generation are determined to analyze the thermodynamic optimization of the conjugate mixed convection.
Line by line based band identification for non-gray gas modeling with a banded approach Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Hadi Bordbar, Timo Hyppänen
The banded approach or box model is a method to include the non-grayness of combustion gases in radiation heat transfer calculations. However, the determination of the correct limits for the bands and the effective band absorption coefficients is still something of a black art. In this study, the line by line (LBL) spectral absorption coefficient profile has been implemented to obtain the effective number of bands, and bands’ limits for pure H2O, pure CO2 and a H2O-CO2 gas mixture. A mathematical technique has been used to smooth the LBL profiles of pure gases in atmospheric pressure in order to be used for identifying the gray bands. The optimization for selecting the bands is done by analyzing the radiative heat transfer in several one-dimensional benchmarks. After obtaining the optimal band dividing scheme, a set of correlations for the pressure based gray band absorption coefficient of pure gases is found by integrating the line by line spectral absorption coefficient weighted by the corresponding black body intensity along the bands. In contrary to the previous similar works, by using the LBL data for pressure based gray absorption coefficient of the bands, the current correlations are independent of gas concentration and path length. The present approach could successfully support the non-gray walls. The method has been validated using several benchmarks and exhibited comparable accuracy with other available models.
Experimental investigation of geyser boiling in a two-phase closed loop thermosyphon with high filling ratios Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Yun Liu, Zhigang Li, Yuhua Li, Seolha Kim, Yuyan Jiang
The geyser boiling instability in a two-phase closed loop thermosyphon (TPCLT) is experimentally investigated through flow visualization and simultaneous measurement of pressure and temperature fluctuations. Wide ranges of filling ratios of R134a fluid from 90% to 53%, and heat flux from 20 W cm−2 to 220 W cm−2 are examined. The Power Spectrum Density (PSD) method is applied to analyze the periodicity of geyser boiling, and the parameter Standard Deviation (SD) is used to characterize the oscillating amplitude. The effects of heat flux and filling ratio on the geyser boiling occurrence and the oscillation characteristics are discussed in detail. The results show that, the flow regimes experience the consecutive variation of single-phase flow, bubbly flow, churn flow, bubbly flow, and single-phase flow within each geyser boiling cycle, leading to the fluctuation of flow and heat transfer characteristics. The geyser boiling is more liable to occur in the conditions of higher filling ratio and moderate heat flux. The initiating heat flux for the onset of geyser boiling decreases with the increase of filling ratio, but the range of heat flux for the geyser boiling occurrence is narrow under the high filling ratio conditions. The frequency of geyser boiling increases with the increase of heat flux for a certain filling ratio. With the increase of filling ratio, the oscillating frequency firstly decreases and then increases. The minimum oscillating frequency occurs at the combination of filling ratio of 76% and heat flux of 90 W cm−2 under the experimental conditions in this work. Both the fluctuation amplitude of pressure and temperature increase with the increase of heat flux, while decrease with the increase of filling ratio. Compared to R134a under the same filling ratio of 76%, the water has higher heat flux for geyser boiling occurrence, smaller oscillating amplitude of pressure, and lower oscillating frequency due to the difference in thermophysical properties.
Development of correlations for effective thermal conductivity of a tetrakaidecahedra structure in presence of combined conduction and radiation heat transfer Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Vipul M. Patel, Miguel A.A. Mendes, Prabal Talukdar, Subhashis Ray
The variations in the total effective thermal conductivity ( k eff , t ) of a tetrakaidecahedra unit cell structure as functions of porosity ( ϕ ), thermal conductivity of the solid phase ( k s ) and the average temperature of the medium ( T avg ), in the presence of combined conduction and radiation heat transfer, are presented in this article. For this purpose, the governing energy conservation equation is numerically solved using the blocked-off region approach based on the finite volume method. In addition, the variations in the radiative properties of the structure as functions of surface reflectivity ( ρ s ), pore density ( PPC ) and ϕ are investigated, for which, a pure radiation heat transfer based numerical model is developed and used. From the detailed numerical simulations, three different correlations for k eff , t are proposed. Correlation 1 is developed by fitting the raw simulated data, although its form does not respect some of the limiting conditions. Particularly for k s < 5 W / mK and in the absence of thermal radiation, it under-predicts the effective thermal conductivity due to pure heat conduction ( k eff , PC ). Correlation 2, on the other hand, satisfies all possible limiting conditions, although it requires one additional simulation or correlation for k eff , PC . Finally, correlation 3 is obtained by superposing the effective thermal conductivities due to pure radiation ( k eff , R ) and k eff , PC , while introducing an adjustable coefficient in order to account for the coupling between them. From the investigation on radiative properties, it is observed that the extinction coefficient increases with the decrease in ϕ and with the increase in PPC as well as ρ s and hence k eff , t as well as k eff , R is expected to decrease for these conditions.
Influence of heat exchangers blockage ratio on the performance of thermoacoustic refrigerator Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Kheira Nehar Belaid, Omar Hireche
In this paper, a numerical study is carried out to assess the effect of heat exchangers plates inter-spacing on the performance of a thermoacoustic refrigerator (TAR). Considering that typically most of the heat loss occurs near the heat exchangers, a great emphasis will be placed on the space between the latter and the stack. Several geometric arrangements of heat exchangers plates inter-spacing were tested. The numerical method used is based on two-dimensional low Mach number model. This model assumes that the length of the assembled heat exchangers and stack is less than the wavelength. The results are in agreement with those provided by the DeltaEC software. It can also be observed that the larger the heat exchangers plates inter-spacing, the greater the cooling impact through the thermoacoustic effect.
A comparative study of experimental flow boiling heat transfer and pressure drop characteristics in porous-wall microchannel heat sink Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Y.T. Jia, G.D. Xia, L.X. Zong, D.D. Ma, Y.X. Tang
In this work, we proposed a porous-wall (PW) microchannel heat sink, in which micro pin fin arrays were fabricated on sidewalls of rectangular microchannels by MEMS (Microelectrical Mechanical System) technique. High speed flow visualizations were performed simultaneously with heat transfer and pressure drop measurements to investigate the flow boiling characteristics of PW microchannel heat sink. Conventional rectangular (Rec) microchannel heat sink was also explored together as a comparison. Experiments were carried out with pure acetone liquid at inlet temperature of 30 °C, mass flux from 255 kg/(m2·s) to 843 kg/(m2·s), heat flux from 4 W/cm2 to 110 W/cm2 and the maximum vapor quality at the outlet of the channel was 0.88. Experimental results demonstrated that the PW microchannels reduce wall superheat of onset of nucleate boiling (ONB) and improve critical heat flux (CHF) compared to the Rec microchannels. Moreover, the PW microchannels show significant heat transfer enhancement, pressure drop reduction and mitigation of two-phase flow instability. The porous walls provide numerous nucleation sites and the intensive pin fins arrangements introduce significant wicking effect to maintain the liquid rewetting, which contribute to the above notable flow boiling enhancement.
Influence of wettability due to laser-texturing on critical heat flux in vertical flow boiling Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Joseph L. Bottini, Vineet Kumar, Sabrina Hammouti, David Ruzic, Caleb S. Brooks
The critical heat flux (CHF) marks the upper limit of safe operation of heat transfer systems that utilize two-phase boiling heat transfer. In a heat-flux-controlled system, exceeding the CHF results in rapid temperature excursions which can be catastrophic for system components. Recent studies have focused on the influence of surface wettability on the departure from nucleate boiling (DNB) through surface modifications and coatings, though many of these studies are limited to pool boiling systems. In this study, the surface wettability influence is studied on the boiling curves and specifically the point of DNB. A femtosecond laser is used to texture the surface to change the wettability from hydrophilic to hydrophobic. A parametric study is performed with mass flux, pressure, and inlet subcooling in a vertical rectangular channel that is heated from one side. CHF excursions are triggered under various system conditions and are compared with existing models. For the experimental conditions considered, the hydrophobic surface showed delayed onset of nucleate boiling compared to the hydrophilic surface, shifting the boiling curves to higher wall superheat. The hydrophobic surface also showed significantly lower CHF for the same system conditions and less sensitivity to changes in subcooling.
Investigation of bubble departure diameter in horizontal and vertical subcooled flow boiling Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Jingyu Du, Chenru Zhao, Hanliang Bo
According to the force balance analysis on a bubble, the bubble departure diameter in horizontal subcooled flow boiling is mainly influenced by quasi-steady drag force, surface tension force and bubble growth force while an additional force, buoyancy force, plays an important role in vertical flow boiling. In this paper, the effects of these forces can be concluded by a series of dimensionless parameters including density ratio of vapor and liquid, Prandtl number, Jacob number and bubble Reynolds number. Based on the different forces, two different characteristic lengths are adopted to non-dimensionlize bubble departure diameters in horizontal flow boiling and vertical flow boiling, respectively. Finally, the semi-empirical correlations for bubble departure diameters in both horizontal and vertical subcooled flow boiling are proposed in this paper based on the force balance analysis and available experimental data from literature. The predicted results using the present correlations agree fairly well with the experimentally measured values with a mean relative error of 19.72%.
A translucent honeycomb solar collector and thermal storage module for building façades Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Peter W. Egolf, Nicolas Amacker, Gregory Gottschalk, Gilles Courret, Arsene Noume, Kolumban Hutter
An innovative new translucent honeycomb solar collector and thermal energy storage module has been designed. The honeycomb module contains two different kinds of channels, namely empty ones and those that are filled with a Phase Change Material (PCM). The latter are sealed at the front and back side and, therefore, form chambers. Solar radiation enters the empty channels and is transmitted forward into them. The absorbed energy fraction at the side walls of these empty chambers leads to a melting of the PCM in their neighboring filled chambers, where then thermal energy is stored as latent heat. A heat transfer fluid, usually air, crosses the empty channels by forced convection and additionally charges or discharges the storage modules. Such elements are ideal to equip “intelligent” building façades in passive houses or then to form thermal storage elements in decentralized air-conditioning systems by integration into the façade of e.g. an office building. This first article presents the innovative solar system and gives new results on simplified two-dimensional temporal-spatial physical modeling, numerical simulation of charging and discharging modes and of a combined mode for a typical practice case. The content of a second planned article on system optimization and full system performance, including the description of thermal behavior of system/building combinations is roughly sketched in the section ‘Conclusions and Outlook’.
Convective stagnation point flow of a MHD non-Newtonian nanofluid towards a stretching plate Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Xi-Yan Tian, Ben-Wen Li, Zhang-Mao Hu
A numerical study is carried out for the magnetohydrodynamics (MHD) convective stagnation point flow of a incompressible nanofluid towards a stretching surface. The effects of Brownian motion and thermophoresis are incorporated for the characteristic of the nanofluid. The transformed dimensionless governing equations are solved directly by the collocation spectral method (CSM). Graphical results are presented and analyzed for non-dimensional velocities, temperature and nanoparticle concentration. The effects of Prandtl number, magnetic field, viscous dissipation, heat source/sink with suction/injection, Lewis number, thermophoresis parameters and Brownian motion parameter, etc., on flow, thermal and concentration boundary layer as well as on skin-friction coefficient, local Nusselt number and Sherwood number are obtained and discussed. The two-dimensional irregular characteristics have been found in the velocity and temperature distributions in the flow and normal directions.
Heat transfer enhancement on a surface of impinging jet by increasing entrainment using air-augmented duct Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 C. Nuntadusit, M. Wae-hayee, N. Kaewchoothong
Flow and heat transfer characteristics of impinging jet from pipe nozzle with air-augmented duct were experimentally and numerically investigated. The effects of air-augmented duct geometry on heat transfer enhancement were concerned. The experimental parameters included a diameter (D) and a length (L) of air-augmented duct in the range of D = 2d, 3.3d, 6d, and L = 2d, 4d, 6d where d was the inner diameter of main pipe nozzle at 17.2 mm. The distance from air-augmented duct outlet to impingement surface (S) at S = 2d, 4d and 6d were considered. The conventional impinging jet was also studied to compare the results with the case of an air-augmented duct. The result comparison was based on constant jet mass flow rate by fixing the jet Reynolds number of conventional pipe at Re = 20,000. The temperature distributions on the impingement surface were measured by using a thermal infrared camera, and profiles of velocity and turbulence intensity of the jet were measured by using hot-wire anemometer. The 3-D numerical simulation with SST k-ω turbulence model was also applied to reveal the flow characteristics. The results show that the heat transfer rate on the impingement surface for the case of an air-augmented duct in conditions of 2d ≤ D ≤ 4d and L = 2d is noticeably higher than the case of conventional impinging jets due to increasing air entrainment. The heat transfer rate for the case of D = 6d, L = 2d at S = 2d, is the largest by getting 25.42% higher compared to a conventional impinging jet.
Differential heat and mass transfer rate influences on the activation efficiency of laminar flow condensation particle counters Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Jikku M. Thomas, Xiaoshuang Chen, Anne Maißer, Christopher J. Hogan
Laminar flow condensation particle counters (CPCs) are uniquely sensitive detectors for aerosol particles in the nanometer size range (i.e. below 10 nm in size they can have single particle sensitivity). Their operation hinges upon the creation of supersaturation of a working fluid; particles exposed to supersaturated vapor grow by condensation to optically detectable sizes. The degree of supersaturation is fully controlled via differential rates of heat transfer and working fluid vapor mass transfer. Because of the Kelvin relationship governed vapor pressure of small particles, in all CPCs there is a critical size/cut-size (diameter), and particles smaller than this size do not grow and are not detected efficiently. While efforts have been made to control the CPC activation efficiency (i.e. the fraction of particles detected as a function of size), prior studies have not examined how differential heat and mass transfer in CPCs are governed by changes in gas composition. Here, we measure and model CPC activation efficiencies (with 1-butanol as the working fluid) in mixtures of gases of disparate thermophysical properties, namely helium and molecular nitrogen. Our experiments show that the activation efficiency of smaller particles (i.e. below 8 nm in the tested CPC) can be increased by adding a modest amount of helium to the aerosol (mole fractions near 0.20). This is expected based upon the increased Lewis number brought about by Helium addition, and supported by predictions of CPC activation efficiency based upon thermophysical property variable models of coupled heat, mass, and momentum transfer within the CPC condenser region. Interestingly, we find that when operating with a constant precision orifice diameter (choked flow), the activation efficiency for a given sub-10 nm particle diameter first increases with increasing Helium mole fraction and then decreases as the Helium mole fraction increases beyond 0.67. In comparison, experiments with constant mass transfer Peclet number (Pem = 77) show an increase in CPC activation efficiency up to a helium mole fraction of 0.67, but then the activation efficiency decreases more modestly beyond this helium mole fraction. We attribute these contrasting results to the increased flowrate through the instrument under constant orifice diameter conditions, which affects the performance of the CPC saturator. Finally, through modeling we show that the ability to enhance the activation efficiency of a CPC via a modest amount of helium addition is general, and can be applied with other heavy working fluids. The results presented in this study elucidate the importance of gas composition and Lewis number controlled differential heat and mass transfer rates on the performance of condensation based nanoparticle detectors.
Condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger for high-temperature heat pumps Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Junyub Lim, Kang Sub Song, Dongwoo Kim, DongChan Lee, Yongchan Kim
In this study, the condensation heat transfer characteristics of R245fa in a shell and plate heat exchanger (SPHE) are measured and analyzed by varying the mean vapor quality from 0.16 to 0.86, mass flux from 16.0 to 45.0 kg m−2 s−1, heat flux from 1.3 to 9.0 kW m−2, and saturation temperature from 90 to 120 °C for high-temperature heat pumps (HTHPs). The condensation heat transfer coefficient and frictional pressure drop are shown to increase with increasing mean vapor quality and mass flux and with decreasing saturation temperature. The condensation heat transfer coefficient of R245fa in SPHE is on average 5.9% lower than that in the brazed plate heat exchanger (BPHE) with a similar effective heat transfer area. However, the average two-phase frictional pressure drop of R245fa in SPHE is 16.7% lower than that in BPHE. Empirical correlations for the condensation heat transfer coefficient and two-phase frictional pressure drop in SPHE are developed with mean absolute errors of 8.2% and 9.4%, respectively.
Square array of air-cooled condensers to improve thermo-flow performances under windy conditions Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Ruonan Jin, Xiaoru Yang, Lijun Yang, Xiaoze Du, Yongping Yang
The thermo-flow performances of air-cooled condensers (ACCs) get deteriorated severely under windy conditions for the inherent defects of rectangular array of condenser cells. In this work, a square array of ACCs is proposed to restrain the unfavorable effects of ambient winds. By means of CFD methods and experimental validation, the air variable fields, mass flow rates and total heat rejections of the ACCs with two types of arrays at different wind speeds and directions are obtained and analyzed. The results show that the reverse flow in the upwind condenser cells and hot plume recirculation of the peripheral condenser cells are both weakened for the proposed square array under windy conditions. Accordingly, the mass flow rate and total heat rejection increase a lot especially in the wind direction of −90° with a heat rejection improvement of 25.21% at the wind speed of 16 m/s, showing that the thermo-flow performances of ACCs get improved thanks to the square array of ACCs. In the absence of ambient winds, the total heat rejection of ACCs in the square array is almost the same as the conventional array.
Assessment of nonequilibrium air-chemistry models on species formation in hypersonic shock layer Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Qinglin Niu, Zhichao Yuan, Shikui Dong, Heping Tan
The present study aims to assess the performance of chemical rate models for species formation in the reacting shock layer flows over the blunt-cone hypersonic vehicles. Three 11-species nonequilibrium chemical models for air, Gupta 90, Park 93 and Ozawa’s modified models, are assessed. Two controlling temperature expressions (T0.5Tv0.5 and T0.7Tv0.3) for dissociation reactions are taken into account in these chemical kinetic models. To further examine the performance of these models for predictions of nonequilibrium effects and species formations in the shock layer, two typical flight cases are adopted: (1) the NO formation in reacting flows over the Bow-shock Ultraviolent (BSUV) vehicle at Mach number 17.7, and (2) the electron formation over the Radio Attenuation Measurements (RAM) C-II vehicle at Mach number 23.9 and 25.9. Firstly, comparisons of interested parameters between the computed results in different rate models and available reference data are carried out. Secondly, the reasons for the difference of species formations in these models are discussed. Results show that both the chemical rate model and the weight factor of the controlling temperature have a distinctive influence on species concentration and distribution in the shock layer. The weight factor determines the level of the vibrational-electronic temperature and the reaction release heat in nonequilibrium processes. With the increasing of the weight factor, the NO concentration increases and the electron density decreases in the same rate model. The spectral integration within the wavelengths of 205–255 nm shows that the prediction accuracy of the Park-0.5 and Ozawa-0.7 models is relatively high. Numerical results also indicate that the Ozawa-0.7 model may be an all-round model to predict electron formation in the shock layer.
Modeling and parametric study of the ultrasonic atomization regeneration of desiccant solution Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Wei Li, Yue Pan, Ye Yao, Mengcheng Dong
Desiccant solution regeneration is an important task of the desiccant solution air conditioning system. The regeneration temperature of conventional packed-bed regenerator is high, which consumes a lot of thermal energy to regenerate the desiccant solution and prohibits the potential use of some low-grade and renewable energy as heat source. The ultrasonic atomization technology may be an effective way to reduce the regeneration temperature. In this paper, an ultrasonic atomization desiccant solution regenerator (UADR) was designed and studied. A theoretical model was developed to predict the heat and mass transfer characteristics of the UADR. The model was experimentally validated and used to investigate the influence of inlet parameters (e.g., air temperature, air humidity, solution concentration, solution temperature, gas-liquid ratio, droplet diameter and droplet jet velocity) on the outlet parameters and regeneration performance of the UADR. The results manifest that the contact area between the air and the desiccant solution droplets has great effect on the regeneration performance, and there exists an optimum droplet diameter for the best regeneration performance. In comparison with packed-bed regenerator, the regeneration temperature of UADR can drop from 3.1 to 6.6 °C.
An experimental investigation of flow boiling heat transfer coefficient and pressure drop of R410A in various minichannel multiport tubes Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Nguyen Ba Chien, Kwang-Il Choi, Jong-Taek Oh, Honggi Cho
The present work demonstrates the two-phase flow boiling heat transfer coefficient and pressure drop of R410A inside various minichannel multiport tubes. The experimental investigation was performed in four tube types with the hydraulic diameters of 1.16, 1.14, 1.07 and 0.96 mm and the number of parallel channels are 7, 11, 16 and 9, respectively. The data were conducted with the heat fluxes of 3 and 6 kW m−2, the mass flux ranging from 50 to 500 kg m−2 s−1, the saturation temperature of 6 °C and the vapor quality up to 1.0. The heat transfer coefficient of R410A was found to be affected by the heat flux, mass flux, vapor quality and the geometry of channel. The frictional pressure drop gradient was also affected by the mass flux and vapor quality but independent with the variation of heat flux. In addition, the present data were compared with various heat transfer coefficient correlations and pressure drop models in literature. Finally, new correlation of heat transfer coefficient was proposed based on the present experimental data.
Lorentz forces effect on NEPCM heat transfer during solidification in a porous energy storage system Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 M. Sheikholeslami, Zhixiong Li, Ahmad Shafee
In this article, Lorentz forces impact on Nano enhanced PCM heat transfer during discharging process is simulated in a storage porous unit by means of computational method namely FEM. Lorentz forces and nanoparticles are utilized to accelerate this unsteady phenomenon. Impacts of volume fraction of NEPCM, Hartmann and Rayleigh numbers on discharging process were illustrated. Results reveal that employing Lorentz forces can augment the discharging rate. Discharging time decreases by dispersing nanoparticles.
Experimental and numerical investigation on the role of holes arrangement on the heat transfer in impingement/effusion cooling schemes Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Antonio Andreini, Lorenzo Cocchi, Bruno Facchini, Lorenzo Mazzei, Alessio Picchi
In the present work, two different impingement/effusion geometries have been investigated, both having staggered hole configuration and an equal number of impingement and effusion holes. The first geometry, which is designed in case of low coolant availability, has impingement hole pitch-to-diameter ratios of 10.5 in both orthogonal directions, a jet-to-target plate spacing of 6.5 hole diameters, with effusion holes inclined of 20° with respect to the target surface. The second geometry, which is designed in case of high coolant availability, has impingement hole pitch-to-diameter ratios of 3.0, a jet-to-target plate spacing of 2.5 diameters and normal effusion holes. For each geometry, two relative arrangements between the impingement and effusion holes have been investigated, as well as various Reynolds numbers for the sparser geometry. The experimental investigation has been performed by applying a transient technique, using narrow band thermochromic liquid crystals (TLCs) for surface temperature measurement. A CFD analysis has also been performed in order to support interpretation of the results. Results show unique heat transfer patterns for every investigated geometry. Weak jet-jet interactions have been recorded for the sparser array geometry, while intense secondary peaks and a complex heat transfer pattern are observed for the denser one, which is also strongly influenced by the presence and position of effusion holes. For both the geometries, effusion holes increase heat transfer with respect to impingement-only, which can be mainly attributed to a reduction in flow recirculation for the sparser geometry and to the suppression of spent coolant flow for the denser one.
Theoretical and experimental study of a membrane-based microfluidics for loading and unloading of cryoprotective agents Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Xiaoming Zhou, Xin M. Liang, Ji Wang, Pingan Du, Dayong Gao
Cells are routinely cryopreserved for various applications. Successful cryopreservation requires the loading of cryoprotective agents (CPAs) to prevent cells from cryoinjury during freezing and thawing, as well as the unloading of CPAs before use since CPAs are often toxic. Moreover, the CPA loading/unloading processes are usually effortful and error-prone. In this study, a membrane-based microfluidic device is introduced to aid the CPA loading and unloading processes. The device consists of two helical microfluidic channels with a sandwiched layer of microfiltration membrane. CPA exchange across the membrane via diffusion and filtration is realized when cell suspension and replacement fluid flow counter-currently in the channels. CPA concentration in the cell suspension can be regulated in a well-controlled, progressive manner. A theoretical model has been developed to better characterize the dynamic change and distribution of the CPA concentration, as well as the cell volume response, with respect to various mass transfer parameters within the device during CPA loading and unloading steps. Physical experiments have been conducted to validate the proposed model. Furthermore, operating conditions of the device have been optimized under the guidance of the established model and different devices with various sizes have been fabricated to evaluate their performance characteristics with respect to different levels of throughput. The optimized condition enables the loading of DMSO (from 0% to 10%, v/v) at a rate of 180 ml/h and unloading (from 10% to 1%, v/v) at 5 ml/h with a single pass through the initial prototype. A second enlarged device further improves the DMSO unloading rate to 180 ml/h with the washing solution consumption rate at 225 ml/h. The higher throughput and reduced washing solution requirement demonstrate the significant advancement in efficiency with the presented approach. Lastly, the system is easy to operate and the process is robust, thus provides a reliable solution for loading/unloading of CPAs for various sized cell suspensions.
Analytic solution of Guyer-Krumhansl equation for laser flash experiments Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 R. Kovács
The existence of non-Fourier heat conduction is known for a long time in small and low temperature systems. The deviation from Fourier’s law has been found at room temperature in heterogeneous materials like rocks and metal foams (Both et al., 2016; Ván et al., 2017). These experiments emphasized that the so-called Guyer-Krumhansl equation is adequate for modeling complex materials. In this paper an analytic solution of Guyer-Krumhansl equation is presented considering boundary conditions from laser flash experiment. The solutions are validated with the help of a numerical code (Kovács et al., 2015) developed for generalized heat equations.
Response surface analysis of the dimensionless heat and mass transfer parameters of Medium Density Fiberboard Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Abdelkrim Trabelsi, Zakaria Slimani, Joseph Virgone
The development of predictive models to simulate heat and mass transfers in building envelopes is crucial for better energetic and environmental design of buildings. The models in the literature are based on heat and mass balance equations. Many of these use constant or briefly described input data. They use a dimensional formulation where the mechanisms involved are difficult to analyze. This article presents a dimensional analysis of Heat, Air and Moisture (HAM) transfer model where the thermophysical properties of the materials are taken as a function of the state variables describing the system. The analysis of the magnitude of the dimensionless numbers of the model and their response surface led to the estimation of the mechanisms governing HAM transfers for Medium Density Fiberboard. The methodology presented in this study can be very useful in the design phase since it allows the direct comparison of different families of wall elements.
Suppression of the condensational growth of droplets of a levitating cluster using the modulation of the laser heating power Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Alexander A. Fedorets, Nurken E. Aktaev, Leonid A. Dombrovsky
The relatively stable clusters of regularly positioned small droplets formed over the locally heated water surface have been studied starting from the early paper by the first author. The life of a cluster appeared to be not long because of growing of droplets due to condensation of steam and the final coalescence of large droplets with the substrate layer of water. However, further laboratory studies of biochemical processes in single droplets will be possible only in the case of their longer life. One way to stabilize the cluster suggested recently by the authors is an external infrared heating which prevents growing of droplets. The present paper is focused on another way to stabilize the cluster. This is an expected rebuilding of droplet clusters at periodic changes in power of the heating laser beam. The frequency of these oscillations is more important than their amplitude because of the resonance nature of the phenomenon. The experiments showed that in the simplest variant of alternating constant power values with the same duration of heating, the droplet growth rate is approximately twice reduced with a power modulation period of about 0.9 s. It means that such a “rejuvenescence” procedure can be used at laboratory conditions to prolong the life of levitating droplet clusters.
Thermal illusion with twinborn-like heat signatures Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Shuling Zhou, Run Hu, Xiaobing Luo
Thermal illusion device, which can change an arbitrary object into another one when observed from an infrared vision, recently has attracted considerable attention amongst a series of innovative thermal phenomena by resorting to the unique thermal metamaterial. To enhance the illusion deceptiveness, current thermal illusion devices mostly change the location or shape of the target, but seldom deal with the number of heat signatures. In this paper, we develop a general formula to enhance the illusion deceptiveness by splitting the thermal target to appear like thermal bilocation with twinborn distinct heat signatures. The exhaustive design process based on transformation thermodynamics is rigorously introduced in order to realize thermal bilocation. Simulative and experimental results agree well with each other and validate the conception of thermal bilocation. Additionally, the influence of three structure parameters on the bilocation effects has been considered for further optimization. Our device can thermally camouflage the original target with twinborn signatures at different locations, improving the deceptiveness greatly compared with only moving or reshaping. The present thermal bilocation not only can fortify the performance of thermal camouflage, but also can stimulate the extension of illusion thermodynamics to further physical applications.
Numerical and experimental investigation of the heat exchanger with trapezoidal baffle Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Xin Gu, Yuankun Luo, Xiaochao Xiong, Ke Wang, Yongqing Wang
Periodic whole cross-section computation models of shutter baffle heat exchanger and twisty flow heat exchanger were established respectively. The heat transfer coefficient, flow resistance and thermal performance in the shell side are numerically studied. Compared with that of the shutter baffle heat exchanger, the results show that the heat transfer coefficient of twisty flow heat exchanger is improved by 7.3–10.2%, pressure drop decreased by 18.5–21%, and the thermal performance factor TEF enhanced by 14.9–19.2%. The correctness and accuracy of simulation method and results are confirmed by experiments. In addition, the influence of inclination angle of trapezoidal baffle, baffles width, baffles pitch and number of baffles on the heat transfer performance are studied. The results show that the inclination angle of trapezoidal baffles and baffles pitch have a significant effect on heat transfer performance, the effect of baffle width has a secondary effect, and the number of baffles has a less significant effect. The results of this paper provide a new scheme for the structural improvement of the shell and tube heat exchanger, and provide the references for further performance optimization of the twisty flow heat exchanger.
Effects of hole shape on impingement jet array heat transfer with small-scale, target surface triangle roughness Int. J. Heat Mass Transf. (IF 3.891) Pub Date : 2018-07-14 Patrick McInturff, Masaaki Suzuki, Phil Ligrani, Chiyuki Nakamata, Dae Hee Lee
Considered is the addition of special surface roughness patterns to impingement target surfaces to improve the effectiveness and surface heat transfer augmentation levels of impingement cooling. Three different impingement hole shapes are employed within a jet array: circle, racetrack, and triangle. These are designed so that all three hole shapes have the same hydraulic diameter. Different sizes, different shapes, and different distributions of surface roughness elements are utilized, including arrays of small triangle roughness, with different roughness heights, employed with and without the addition of large pin roughness elements. Data are presented for impingement jet Reynolds numbers of 900, 1500, 5000, and 11,000. Overall, the racetrack hole configuration generally provides approximately the same heat transfer augmentation as the circle hole configuration, with slightly better performance, under some conditions. The triangle hole configuration provides lower heat transfer augmentation, compared to both the circle hole and racetrack hole configurations. The addition of small triangle roughness to the target plates shows an increase in heat transfer augmentation, compared to the baseline target plate, for both the circle and racetrack hole configurations. For the triangle hole configuration, the addition of small triangle roughness generally shows negligible or negative improvement, compared to the baseline target plate. Also illustrated is significantly different Reynolds number dependence for impingement cooling, as hole shape changes from circle to racetrack, and triangle.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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