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  • Experimental investigation of effect of different tube row-numbers, fin pitches and operating conditions on thermal and hydraulic performances of louvered and wavy finned heat exchangers
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-21
    Abdulkerim Okbaz; Ali Pınarbaşı; Ali Bahadır Olcay

    Heat exchangers are used in air conditioners, heat pumps, marine, land and air vehicles, refrigeration systems, thermal and nuclear power plants, etc. Increasing heat transfer capacity of a heat exchanger means that the volume of the heat exchanger and the material used will be reduced. Besides, effects of some geometric parameters on heat transfer and pressure drop are more complex depending on the fin structure in fin and tube heat exchangers. In this study, three of the most dominant parameters affecting the thermal-hydraulic performance of a finned and tube heat exchanger were experimentally investigated. These are fin-type (louvered and wavy fins), fin pitch and number of tube-rows. The intermittent geometric structures of louvered fins break growing of the boundary layer and reduce its thickness yielding heat transfer enhancement. On the other hand, wavy fins cause an increase in the heat transfer area due to its large flow length and create instabilities in the flow due to flow separations increasing the heat transfer coefficient. In the present study, specifically five louvered finned and three wavy finned and round tube heat exchanger prototypes were manufactured. Heat transfer and pressure drop experiments of these heat exchangers were performed at a wind tunnel in a conditioned room. Heat transfer and pressure drop characteristics were presented as heat transfer coefficient ho, Stanton number St, Nusselt number Nu, dimensionless pressure drop coefficient Cp, Colburn-j factor, Fanning friction factor f, jlouver/jwavy, flouver/fwavy, j/f1/3 ratios and JF factor. The results were examined from the point of heat transfer and pressure drop mechanisms of louvered and wavy fins for the different number of tube-rows, fin pitches and air inlet velocities. It is found that Colburn-j factors and Fanning friction f factors of the LFRTHXs are higher than those of the WFRTHXs for all the studied cases. Colburn-j factors of the LFRTHXs are higher by 16.4–6.9%, 28.5–18.3% and 25–11.7% than those of the WFRTHXs for the cases of two tube-rows, three tube-rows and four tube-rows, respectively. On the other hand, pressure drops of the LFRTHXs are significantly higher than those of WFRTHXs. However, the thermal-hydraulic performances of the LFRTHXs are still higher than that of WFRTHXs. The thermal-hydraulic performance criteria j/f1/3 ratios of the LFRTHXs are higher by 9.6–4.1%, 22.1–16% and 16.8–7.4% than those of the WFRTHXs for the cases of two tube-rows, three tube-rows and four tube-rows, respectively.

    更新日期:2020-01-22
  • Measuring the in-plane thermal diffusivity of moving samples using laser spot lock-in thermography
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-21
    M. Colom; A. Bedoya; A. Mendioroz; A. Salazar

    In this work we deal with samples that move at constant speed and are illuminated by a modulated and focused laser beam. We have obtained a general expression for the surface temperature of these moving samples: it is valid not only for opaque and thermally thick materials, but also for thermally thin and semitransparent samples. Moreover, heat losses by convection and radiation are taken into account in the model. Numerical calculations indicate that the temperature (amplitude and phase) profiles in the directions parallel and perpendicular to the sample motion are straight lines with respect to the distance to the laser spot. The slopes of these straight lines depend on sample speed, modulation frequency and in-plane thermal diffusivity of the sample. Provided the two first experimental parameters are known, the in-plane thermal diffusivity can be retrieved in a simple manner. Measurements performed on materials covering a wide range of thermal diffusivity values, from insulators to good thermal conductors, confirm the validity of these linear methods.

    更新日期:2020-01-22
  • 更新日期:2020-01-21
  • Effect of temperature modulation on natural convection in a horizontal porous annulus
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-20
    Jabrane Belabid; Karam Allali

    The effect of temperature modulation on natural convection in a horizontal porous annulus is investigated in this paper. The porous medium is confined between the inner and outer cylinders of the annulus which is subjected to time-dependent temperature. It is assumed that the time-dependent modulation is periodic with frequency σ and amplitude λ. The model consists of the heat equation and the equations of motion under the Darcy law. The derived problem with the stream function-temperature formulation is solved numerically using the alternating direction implicit method. It is shown that a stabilizing effect can be gained for appropriate values of the frequency σ and the amplitude λ.

    更新日期:2020-01-21
  • Experimental study and numerical simulation of thermal hydraulic characteristics of a finned oval tube at different fin configurations
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-12
    Alireza Hashem-ol-Hosseini; Mehran Akbarpour Ghazani; Mohsen Davazdah Emami

    Finned tube heat exchangers are among the most common types of heat exchangers that are used in engineering applications. Most of the tubes used in finned-tube heat exchangers are of circular type, but the oval tubes have more desirable features as they impose lower pressure drops with approximately the same amount of heat transfer. In this paper, a finned oval tube is studied experimentally and numerically to obtain temperature fields around the tube at different locations. Moreover, several 3D numerical simulations are performed to study the effects of fin length, fin spacing and diameter ratio of the tube cross section. Based on the obtained results, fin spacing of 1 mm is the optimum at all tube aspect ratios and fin lengths when Reynolds number is low while at high Reynolds numbers, fin spacing of 3 or 4 mm is the better choice. As to the fin length considerations, for a short fin, a spacing of 3 or 4 mm is the best choice at high Reynolds numbers. For larger fin lengths, sensitivity to fin spacing is less but a fin spacing of 3 or 4 is preferable at high Reynolds numbers.

    更新日期:2020-01-13
  • Phase change heat transfer in a rectangular enclosure as a function of inclination and fin placement
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-13
    Dominic Groulx; Pascal H. Biwole; Maha Bhouri

    In this paper, melting of a phase change material (PCM) inside a rectangular enclosure, possibly finned and inclined, is studied numerically. The application of this work is related to the temperature control of a finned PV panel filled with PCM and installed at different tilt angles. The studied system is modeled as a 2D rectangular enclosure filled with PCM (RT25) and packed between two aluminum plates, where the front side is exposed to a constant heat flux of 1000 W/m2 for 2 h. Four geometries were considered including a non-finned PCM enclosure, a PCM enclosure with one centered full-width fin, one half-width fin attached to the front plate, and one half-width fin attached to the back plate. Results have shown that the most efficient thermal management of the PV-PCM panel is obtained when the PCM enclosure is equipped with a full-width fin simultaneously attached to the front and back plates. With such a PV panel design, the PCM melting is dominated by natural convection heat transfer from both sides of the PCM enclosure at an early stage, with added heat losses from the back plate to the external environment. Accordingly, low values of the front and back plates temperatures can be maintained during a stabilization time of 80 min as long as the tilt angle is varied from 0° to 75° from the vertical. The efficient temperature control resulting from the full-width fin geometry is mainly related to the high overall heat transfer coefficient obtained during the whole melting process.

    更新日期:2020-01-13
  • Effect of wing-pitch ratio of double-sided delta-wing tape insert on the improvement of convective heat transfer
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-13
    Agung Tri Wijayanta; Indri Yaningsih; Wibawa Endra Juwana; Muhammad Aziz; Takahiko Miyazaki

    After publishing an article in which the impact of wing-width ratio of double-sided delta-wing tape (T-W) inserts on thermal-hydraulic performance [Wijayanta et al., Appl. Therm. Eng. 145 (2018) 27–37] were investigated, we have extended our work to modified enhanced heat transfer area design for wing-pitch ratio (P/W) acting on the geometric features of delta-wing tape inserts. There are a limited number of studies in this area regarding the wing-pitch ratio. In the current study, T-W inserts with P/W of 1.18, 1.47, and 1.65 were manufactured and evaluated to improve single-phase convective heat transfer, under the conditions of a Reynolds number between 5,300 and 14,500, with water used as the working fluid. T-W inserts with a P/W of 1.18 offer the highest average Nusselt number, with an increase of approximately 177% compared to a plain tube. However, the friction factor is 11.6 times greater compared to a plain tube, showing that the friction loss is more significant with T-W inserts. In addition, T-W inserts with a P/W of 1.18 produce the greatest thermal performance factor of 1.15. Generally, Nusselt number, friction factor, and thermal performance factor of the heat exchanger increase following the decrease of P/W of the T-W inserts. In this study, the empirical correlations have been modeled using the obtained experimental data.

    更新日期:2020-01-13
  • Augmentation and diminution of non-Boussinesq effects due to non-Newtonian power-law behavior in natural convection
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-11
    Jacob Koshy Mulamootil; Sukanta Kumar Dash

    This study investigates the extent and significance of augmentation and diminution of non-Oberbeck–Boussinesq (NOB) effects due to power-law rheology in natural convection. The relative significance of each temperature dependent thermophysical property of a non-Newtonian power-law fluid was first evaluated using an order of magnitude analysis. The limiting criteria were derived for a class of fluids having an Arrhenius-type thermodependent consistency index, and a constant power-law index. Significant fluid property dependencies were identified, incorporated into the conservation equations, and solved numerically. The benchmark problem of natural convection around an isothermal vertical flat plate, immersed in a quiescent power-law fluid, was re-investigated from the viewpoint of NOB effects. It was found that power-law rheology significantly augmented or diminished the acceleration of flow caused by NOB effects. Compared to the Newtonian case, strong shear thinning behavior more than doubled the NOB acceleration of the flow field, and shear thickening inhibited the acceleration by nearly half. Numerical solutions facilitated the visualization of velocity and temperature distributions, and thus provided insights to the underlying physics. It is demonstrated that the cumulative effect of the power-law behavior and temperature dependence of properties is more than a mere superposition of the individual effects. Important contributions from the present work include modifications to the OB approximation’s limiting criteria applicable to power-law fluids, insights into the effect of temperature dependent properties on the flow field and consequent heat transfer, and their correspondence to the results of the order of magnitude analysis. The present investigation of the underlying physics of augmentation and diminution facilitate a better understanding for future studies of NOB effects combined with non-Newtonian behavior.

    更新日期:2020-01-13
  • Numerical investigation on heat transfer of water spray cooling from single-phase to nucleate boiling region
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-10
    Chang Cai; Hong Liu; Ming Jia; Hongchao Yin; Rong Xie; Peiliang Yan

    In the present study, spray cooling heat transfer characteristics were numerically investigated under the heat flux ranging from 50 to 170 W/cm2. Effect of nozzle height and spray pressure was also studied in order to reveal the microscopic mechanisms of the heat transfer enhancement from single-phase to nucleate boiling regions. A two-phase flow heat transfer model was adopted based on the Euler-Lagrangian approach. The dependence of thermo-physical properties of fluid on the temperature was also taken into account. The comparison between simulated wall temperature and experimental data demonstrates a satisfactory agreement. The results show that the heat transfer coefficient increases monotonously with the heat flux. In the nucleate boiling region, the wall film is thinner with a smaller velocity compared with those in the single-phase region. Spray cooling heat transfer is enhanced evidently as the nozzle height decreases and the spray pressure increases. The most important feature which separates the present study from the literature is that the numerical simulation in this paper presents some detailed microscopic characteristics, e.g. the wall film thickness and velocity, which play an important role in spray cooling heat transfer.

    更新日期:2020-01-11
  • Approximate Bayesian computation applied to the identification of thermal damage of biological tissues due to laser irradiation
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-09
    Bruna R. Loiola; Helcio R.B. Orlande; George S. Dulikravich

    This paper deals with the solution of an inverse bioheat transfer problem, by using Approximate Bayesian Computation (ABC). A Sequential Monte Carlo (SMC) method is applied for simultaneous model selection and model calibration (estimation of the model parameters) by using synthetic measurements. Two competing models are considered in the analysis of the thermal damage of biological tissues. The results show that the ABC-SMC algorithm provides accurate results for the model selection and estimation of the thermal damage model parameters.

    更新日期:2020-01-09
  • Numerical investigation of heat transfer from a plane surface due to turbulent annular swirling jet impingement
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-09
    Farhana Afroz; Muhammad A.R. Sharif

    The heat transfer characteristic of an annular turbulent swirling jet impingement on a heated plane surface is investigated numerically. The annular jet configuration causes instabilities and fluctuations in the flow. Depending on the combinations of different parameters, the annular jet impingement may have positive or negative effects on the heat transfer from the impingement surface. Swirl, on the other hand, introduces vorticity and fluid mixing in an impinging jet, which is desirable in some applications. The axisymmetric two-dimensional flow domain on a radial-axial plane is considered in the numerical model. Both non-swirling and swirling jet impingement is studied. The numerical computations are performed using the ANSYS Fluent CFD code. The Realizable k-ε turbulence model with enhanced wall treatment is used in the computation. The computational process is validated against other published data on similar flow configuration for non-swirling annular impinging jets. The flow and geometric parameters are the jet exit Reynolds number, Re (5000 to 25,000), the swirl strength Sw (0–0.77), the jet exit to the impingement surface distance, H (0.5–4.0), and the moderate blockage ratio of the annular jet, BR (0.4–0.6). The thermal-hydraulic field in the domain is computed for various combinations of these parameters. The effects of these parameters on the Nusselt number distribution on the impingement surface are analyzed. For short separation distance (H = 0.5), the swirling motion positively affects the overall heat transfer, and the average Nu is increased as high as 8% for certain combinations of Re, Sw, and BR; compared to the non-swirling annular jet impingement. For higher separation distances, the average and peak Nusselt number is initially reduced and then increased with increasing swirl strength.

    更新日期:2020-01-09
  • Heat transfer performance evaluation of a novel concept of the core-hole heat exchanger for polymer foaming process
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-08
    Waqas Mughal; Chunling Xin; Jiangnan Li; Masroor Abro; Zhaoping Yang; K.H. Solangi; Yadong He

    The tandem extrusion method is quite famous for polymer foaming production in up-scale. In this method, two extruders coupled together for different functionality. The primary extruder used for melting, homogenizing the foaming agent in the melt. The secondary extruder used for reducing the melt temperature to the required foaming temperature limit. In this study, propose newly developed core-hole heat exchanger by replacing secondary extruder as an alternative solution. The heat transfer performance of the core-hole structure investigated in this work. According to heat transfer performance mainly focus on heat transfer rate, temperature distribution, local heat transfer coefficient, Nusselt number, and overall heat transfer coefficient in this work. The experiments were carried out on 180 °C, 190 °C, and 200 °C inlet temperature, whereas the flow rate was 08 to 20 kg/h with 4 unit of the interval. Non-homogeneous temperature distribution resulted in 08 kg/h and 180 °C of the inlet temperature, but overall average temperature approximately similar to surface temperature. Good mix ability and the uniform temperature achieved on 12 and 16 kg/h in all range of temperatures in this study. Whereas, 20 kg/h of mass flow rate reduces the temperature distribution quality of the melt. Because of the higher temperature difference and flow rate, higher heat transfer rate achieved on 200 °C and 20 kg/h. Convective heat transfer coefficient has been increased concerning flow rate and local point temperature difference. In this work, it investigated that 190 °C of inlet temperature gives a higher overall heat transfer coefficient on 16 and 20 kg/h of mass flow rate.

    更新日期:2020-01-08
  • Modelling the stability of Marangoni convection in a layer of nanofluid
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-06
    A.A. Abdullah; N.M. Alraiqib; K.A. Lindsay

    A linear stability analysis is undertaken for the onset of Marangoni convection in a horizontal layer of a nanofluid heated from below. The model employed for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The lower boundary of the layer is assumed to be a rigid surface at fixed temperature while the top boundary is assumed to be a non-deformable free surface cooled by convection to an exterior region at a fixed temperature. The lower boundary of the layer is assumed to be impenetrable to nanoparticles with their distribution being determined from a conservation condition. Material properties of the nanofluid are modelled by the non-constant constitutive expressions developed by Kanafer and Vafai based on experimental evidence. The steady state solution across the layer is shown to be well approximated by a linear distribution of temperature and an exponential distribution of nanoparticle volume fraction. Constitutive properties are assumed to be non-constant functions of temperature and the volume fraction of nanoparticles. New behavior is introduced which in turn leads to significantly different stability boundaries from those predicted by historical analyses.

    更新日期:2020-01-07
  • Evaporation of a sessile oil drop in the Wenzel-like regime
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-06
    Dorra Khilifi; Walid Foudhil; Souad Harmand; Sadok Ben Jabrallah

    In this work, we present an experimental study of the evaporation of oil drops deposited on both textured and smooth silicon substrates at two different temperatures (20 °C and 270 °C). We show that the sessile drops take a hexagonal form and are linked by an oil film (a droplet sitting on a mixture of solid and liquid). This wetting regime represents the Wenzel-like regime. We are particularly interested in the propagation of the oil film on the textured surface over time. The effect of the surface fraction of the micro-textures ∅ and the temperature of the substrate Th on the film propagation were also studied. We reveal that the spreading length of the oil drops increases as ∅ decreases and Th increases. We also demonstrate that the textured surface favors oil drop evaporation.

    更新日期:2020-01-07
  • Experimental and numerical investigation of air-side forced convection on wire-on-tube condensers
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-06
    Alişan Gönül; Özden Ağra; Ş. Özgür Atayılmaz; Hakan Demir; M. Kemal Sevindir; İsmail Teke

    In this study, wire-on-tube condensers used frequently in industrial cooling applications have been investigated experimentally and numerically, which are exposed to forced convection. Five different coils with different geometric properties were studied in the experimental study. The numerical study has been validated by the experimental data obtained through these coils. After validation, the effect of wire diameter (Dw), tube diameter (Dt), wire pitch (Sw) and tube pitch (St) have been investigated parametrically for five different air velocities (0.5, 1.0, 1.5, 2.0 and 2.5 m/s). The parametric study was performed for the variation for wire diameter (1.2 mm–2.0 mm), tube diameter (4.2 mm–7.2 mm), wire pitch (5.5 mm–9.5 mm) and the tube pitch (20 mm–50 mm) to determine their effects on wire-on-tube condensers in terms of heat transfer. It is observed that the increase of the wire diameter decreased the heat transfer coefficient on the wire by 10–12% while the heat transfer coefficient on the tube increased by 10–15%. It is determined that the effect of increasing tube diameters on the convection coefficient on the wire is very low. When the distance between the wires is increased from 5.5 mm to 9.5 mm, the average convection coefficient on both tubes and wires is decreased by 5%. When the distance between the tubes is increased from 20 mm to 50 mm, it is concluded that there is a decrease in the convection heat transfer coefficient on both the tube and the wires by approximately 7–8% depending on the velocities. As a result, the correlations are proposed to determine the amount of heat transfer generated over both the wire and the tube. The proposed correlations yield accurate results in the error range of 10% with CFD results and %15 with experimental results.

    更新日期:2020-01-07
  • Ignition of polymers under exponential heat flux considering both surface and in-depth absorptions
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2020-01-06
    Junhui Gong; Chunjie Zhai; Lizhong Yang; Zhirong Wang

    Analytical models addressing ignition of solids under constant heat flux have been developed in previous studies utilizing surface or in-depth absorption or combination of them. When encountering time-dependent heat flux, the majority of the studies focused on polynomial heat flux and surface absorption assumption. However, in-depth absorption also should be taken into account under time-dependent heat flux in analytical models especially for infrared translucent solids. In this work, an analytical model aiming at revealing the ignition mechanism of translucent polymers under exponential time-increasing heat flux is established considering both surface and in-depth absorptions. Critical temperature is employed as ignition criterion. Four typical non-charring polymers, polymethyl methacrylate (PMMA), polyoxymethylene (POM), polyamide 6 (PA 6) and polypropylene (PP), are utilized as the reference materials, and a numerical solver is employed to validate the analytical model. The results show that the developed analytical model provides accurate predictions of surface temperature and ignition time. Surface heat loss by convection and reradiation has little effect on surface temperature, ignition time and critical energy, but it affects the ignition heat flux greatly. Thermal penetration depth differs from the one under constant heat flux, and it gets smaller as the surface heat loss is considered. The ignition time, thermal penetration depth and critical energy decrease as the heat flux increasing rate gets larger. Meanwhile, the ignition heat flux for in-depth absorption is higher than that for surface absorption, and both increase with heat flux increasing rate. Furthermore, the linearity between ignition time and the squared critical energy, proposed in constant and linear heat flux scenarios, is also found valid under this exponential heat flux condition.

    更新日期:2020-01-07
  • The one-dimensional Stefan problem with non-Fourier heat conduction
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-26
    Marc Calvo-Schwarzwälder; Timothy G. Myers; Matthew G. Hennessy

    We investigate the one-dimensional growth of a solid into a liquid bath, starting from a small crystal, using the Guyer-Krumhansl and Maxwell-Cattaneo models of heat conduction. By breaking the solidification process into the relevant time regimes we are able to reduce the problem to a system of two coupled ordinary differential equations describing the evolution of the solid-liquid interface and the heat flux. The reduced formulation is in good agreement with numerical simulations. In the case of silicon, differences between classical and non-classical solidification kinetics are relatively small, but larger deviations can be observed in the evolution in time of the heat flux through the growing solid. From this study we conclude that the heat flux provides more information about the presence of non-classical modes of heat transport during phase-change processes.

    更新日期:2019-12-27
  • A novel optimum constructal fork-shaped fin array design for simultaneous heat and mass transfer application in a space-constrained situation
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-27
    Saheera Azmi Hazarika; Tuhin Deshmukhya; Dipankar Bhanja; Sujit Nath

    The present study deals with the design and analysis of an array of constructal fork-shaped fins adhered to a circular tube and operating under fully wet conditions. Fork-shaped fin arrays with two and three numbers of branches are considered in the current work. The mass transfer process is calculated by using a cubic relation between the humidity ratio of saturated air and the corresponding fin surface temperature. The governing equations are highly non-linear and hence they are solved by using a semi-analytical technique called Homotopy Perturbation method. The optimisation is done by maximising the net heat transfer rate of the fin array and un-finned surface and by imposing certain constraints. The constraints are taken such that the radial space limitations fin material limitations, as well as the minimum fin gap considerations, are taken into account. As the present problem involves a large number of design parameters as well as the interrelated constraints, a bio-inspired metaheuristic algorithm called the Firefly Algorithm has been employed for obtaining the optimum condition. The analysis has been performed for different operating conditions and the results have been compared with the corresponding rectangular fin array. From the study, it has been seen that the heat transfer rate from the optimum fork-shaped fin array with two branches is higher than that from the optimum rectangular fin array. However, in a few cases, it has been found that there is marginal difference in heat transfer rate between the optimum fork-shaped fin array with two branches and the rectangular fin array but the total length of each fin for the former case is found to be smaller and hence fork-shaped fin array with two branches would be a better selection than the rectangular fin array. However, increasing the number of branches of the fork-shaped fins from two to three does not provide any benefit in terms of either lower fin length or higher heat transfer rate.

    更新日期:2019-12-27
  • Temperature uniformity in convective leaf vein-shaped fluid microchannels for phased array antenna cooling
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-26
    Hui Tan; Kuan Zong; Pingan Du

    This paper describes numerically and experimentally how the phased array antenna which is subjected to a discrete heat load can be kept under the uniform temperature by the topology design of microchannels. First, a model of microchannel with two discrete heat sources is established and the result presents that temperature gradient between chips in series connection cannot be eliminated in the typical straight channels. Then the multi-scale leaf vein-shaped microchannel network including the first order assembly (FOA), second order(SOA), and the third order (TOA) are proposed, and the result shows that TOA yields the lowest chip temperature and the best temperate uniformity evaluated by the standard deviation of temperature (SDT). The number of microchannels in SOA and TOA are subsequently optimized with the criterion of minimum SDT. Finally, the optimal SOA and TOA are made by 3D printing technique and the orthogonal test is carried out to investigate the effect of inlet fluid temperature, volumetric flow rate and heat flux. The experiment result testifies that TOA has better temperature uniformity than SOA with the decrease of inlet fluid temperature and the increases of volumetric flow rate, and the difference between them increases with the increasing heat flux. Therefore, TOA is recommend for the improvement of temperature uniformity with the ever-increasing heat flux.

    更新日期:2019-12-27
  • Heat exchange of an evaporating water droplet in a high-temperature environment
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-26
    G.V. Kuznetsov; P.A. Strizhak; R.S. Volkov
    更新日期:2019-12-27
  • Development and validation of an inverse method for identification of thermal characteristics of a short laser pulse
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-27
    Piotr Łapka; Karol Pietrak; Małgorzata Kujawińska; Marcin Malesa

    The paper presents development and validation of an inverse method for the identification of thermal characteristics of a short super-Gaussian laser pulse interacting with a metal sample. The method was applied to find unknown power of the laser pulse, dimensionless shape parameter of the super-Gaussian function describing the spatial energy distribution of the beam as well as beginning and end times of its interaction with the heated body. The proposed inverse method is based on the Levenberg-Marquardt technique and utilizes temporal and spatial distributions of temperature on the rear surface of the sample, i.e., the opposite to the irradiated one. The temperature profiles were registered by a high-speed IR camera. During the experiments some of the laser beam parameters, i.e., the power, the laser beam spatial profile, beginning and end times of the exposition as well as thermophysical and optical parameters of the aluminum sample were known. Therefore, the measured data were used for both validation of the numerical model which described thermal interaction of a laser pulse with the sample as well as the assessment of correctness and accuracy of the inverse method. At the first step, numerical model of the forward problem, i.e., heat transfer in the aluminum sample irritated by the laser pulse, was developed and validated based on experimentally-determined temperature distributions. The validation was performed for both single and multiple laser pulses. The numerical model of the forward problem was implemented in the commercial software ANSYS Fluent. Then an inverse algorithm was developed and implemented with the aid of the GNU Octave environment. Subsequently, series of numerical tests were carried out. During these numerical simulations, sensitivity analysis as well as initial calibration and verification of the developed algorithm were performed. Parallel to the modeling tasks, the experimental stand was built and series of experiments were performed which allowed to assess the performance of the method using physical temperature data. Performed investigations showed that the problem is ill-conditioned. Nevertheless, relatively good accuracy of the retrieval has been obtained. It was revealed that the sensitivity of the objective function to the end time of the laser pulse is relatively low and that two of the parameters affect measured temperatures in a similar way. These two properties contributed to ill-conditioning of the inverse problem. Dependence of inverse problem solutions on the initial guess has been observed and methods allowing to minimize its influence have been identified. The accuracy of the method was affected by relatively low temporal resolution of the IR camera (500 Hz, with the exposure time approximately from 0.2 to 1 ms). Despite aforementioned problems, the method was abled to retrieve unknown laser pulse parameters with 20–25% accuracy.

    更新日期:2019-12-27
  • Heat transport in 2D van der Waals heterostructures: An analytical modeling approach
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-27
    M. Sadegh Alborzi; A. Rajabpour; A. Montazeri

    Synthesis of multi-layered nanostructures has led to better management of the heat transfer at the nanoscale. There are several parameters that affect this phenomenon, among them, geometrical features of the layers and their interconnections should be deeply analyzed. In this paper, heat transfer in a two-dimensional van der Waals heterostructure consisting of a finite length layer coated on top of a longer under-layer is investigated based on a newly developed analytical model. Accordingly, the in-plane and the cross-plane heat fluxes are thoroughly studied along with the temperature profile within the layers. It is demonstrated that by increasing the overlaying length, the thermal transport capability of the system enhances, and the temperature jump between the layers diminishes. Additionally, the results reveal that by increasing the cross-plane to in-plane thermal conductivity ratio, the total heat flow enhances in the heterostructure. Moreover, for the heterolayers with different types of materials, influence of the thermal conductivity of the finite top layer on the overall heat transport is systematically examined. The results of this study can be practically used in the thermal design of two-dimensional van der Waals heterostructures.

    更新日期:2019-12-27
  • Forced convection in non-circular tubes with non-linear viscoelastic fluids including viscous dissipation
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-26
    Mario F. Letelier; Dennis A. Siginer; Cristian Barrera; Amaru González; M'hamed Boutaous

    The steady, laminar, non-isothermal fully developed flow of a class of non-linear viscoelastic fluids in tubes of arbitrary contour is analyzed under constant wall heat flux including viscous dissipation. Equations of motion and energy are solved analytically and velocity and temperature fields are determined through an asymptotic approach in terms of the Weissenberg number coupled with the shape factor method a one-to-one and continuous mapping taking the circular boundary into a large, continuous spectrum of non-circular tube contours. The analysis developed is general and covers all members of the family of constitutive models considered as well as a large array of non-circular tubes. The case of tubes with circular and triangular contours are discussed as specific examples for various numerical combinations of the Weissenberg, Reynolds, Péclet and Brinkman numbers and the Nusselt number variation is computed for fluids abiding by the Modified Phan-Thien-Tanner (MPTT) and Simplified Phan-Thien-Tanner (SPTT) models. Newtonian velocity and temperature fields in a large spectrum of non-circular tubes are recovered at the lowest order of the asymptotic analysis. Through a matching procedure we also extend the computation of the Nusselt number in round tubes to any desired value of the Weissenberg number.

    更新日期:2019-12-27
  • Large-Eddy Simulation of turbulent thermal flow mixing in a vertical T-Junction configuration
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Cenk Evrim; Eckart Laurien

    Turbulent mixing of warm and cold water streams within a vertical T-junction configuration of a nuclear power plant piping system causes near-wall temperature fluctuations which may lead to High-Cycle Thermal Fatigue (HCTF) failure of the wall material. To predict frequency and amplitude of such temperature fluctuations accurately the method of Large-Eddy Simulation (LES) in the numerical simulation code OpenFOAM is used. As an experimental test case, the turbulent mixing experiment of Kamide et al. [1], where a vertical branch pipe with a cold water stream is connected from below to a horizontal main pipe with a warm water stream to form a vertical T-junction configuration. From the various observed flow patterns, the temperature and velocity data of the ‘wall jet’ are chosen as a reference because it is known to have the highest potential for thermal fatigue. Incomplete mixing and an instability associated with large turbulent structures near the junction region lead to significant temperature fluctuations close to the wall before a vertical thermal stratification further downstream stabilizes the flow. The highest near-wall fluctuation amplitudes are 26% of the temperature difference between the streams. A spectral peak occurs at a Strouhal number of 0.2. The results of the simulations demonstrate, that the mean velocity and temperature profiles of the experiments are well captured, whereas the root-mean-square (RMS) temperature fluctuations deviate from the measurements in some cases, in particular when coarse grids are used. In order to find a lower limit for the required spatial resolution three different numerical meshes with a variable number of cells up to 28 × 106 are investigated. The results demonstrate that with a sufficient mesh resolution the velocity and temperature distributions as well as the spectral peak can be simulated with good agreement to the experimental data.

    更新日期:2019-12-26
  • Numerical analysis of natural convection between a heated cube and its spherical enclosure
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Hedia Welhezi; Nader Ben-Cheikh; Brahim Ben-Beya

    Three-dimensional numerical simulations were conducted for the natural convection phenomena which occurs between an inner hot body and its outer enclosure. The physical model considered here is that a body of cubical shape is located at the center of an isothermal cooled spherical enclosure. Therefore, the fluid flow inside the enclosure results from the temperature difference between the cooled spherical enclosure and the heated cube. The governing equations are solved using a second-order accurate finite volume approach on a staggered grid system and multi-grid acceleration. Three different fluids, an air (Pr = 0.71), a water (Pr = 6.2) and the other a dielectric liquid (Pr = 25) are employed encompassing descriptive Rayleigh numbers Ra that range three orders of magnitude from 104 to 107. The conducted benchmark study leads to excellent accordance with previous findings. Detailed three-dimensional flow and thermal structures in the enclosure were analyzed using the distribution of iso-contours of temperature, iso-surfaces of the standard velocity vector and streamtraces for different Rayleigh numbers. The variation of the local and the surface-averaged Nusselt numbers at the inner hot cube wall are also presented to exhibit the overall heat transfer characteristics inside the enclosure. At the end, monomial correlations are presented for the quantification of the heat transfer that emanates from the heated cube and the spherical enclosure in harmony with the various Rayleigh number. It was found that the thermal and flow fields eventually reach steady state for Rayleigh numbers ranging from 104 to 107. Results indicate also that the heat transfer is increasing significantly by increasing Rayleigh numbers and optimal heat transfer rate is obtained for high Rayleigh number set to 107.

    更新日期:2019-12-26
  • Thermal diffusivity measurement of insulating materials at high temperature with a four-layer (4L) method
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Yves Jannot; Alain Degiovanni; Vincent Schick; Johann Meulemans

    This article presents a temperature-temperature thermal characterization method for the measurement of the thermal diffusivity of insulating materials at high temperature. This novel method, noted 4L, is a transient absolute measurement method based on the estimation of the transfer function at the center of a symmetrical stack composed of two specimen of an insulating material sandwiched between two conductive metallic plates. Two different direct 1D semi-analytical models were developed. The first one considers purely conductive opaque materials and the second one takes into account the coupling between radiative and conductive heat transfer modes for purely scattering semi-transparent materials. The first model was used to estimate the thermal diffusivity of two different opaque insulating materials (calcium silicate boards) up to 800 °C with accuracy better than 10%. The second model was used to estimate the thermal diffusivity of a semi-transparent insulating material (ceramic foam) up to 1000 °C with an accuracy of approximately 10%. The second model was used to identify significant estimation errors occurring if a purely conductive model is used for semi-transparent materials. The conducto-radiative model was also used to estimate an average value of the mean extinction coefficient of the semi-transparent material.

    更新日期:2019-12-26
  • Thermal radiation model for the buoyancy-controlled diffusion plumes from rectangular fire sources
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Xianjia Huang; Xunjia Zhuo; Tao Huang; Chaoliang Xing; Chihonn Cheng; Wanki Chow

    This study investigated the thermal radiation from a rectangular fire source with different aspect ratios. Fire experiments with burners of the same surface area but different aspect ratios were conducted. The flame was split into several sub-flames as the aspect ratio of burner increased. Compared with the axis-symmetric fire source, the flame split in the rectangular fire source led to a change in the thermal radiation mechanism. Thus, a multi-cuboid flame model was developed to estimate the radiant heat flux from a rectangular fire source. The number of sub-cuboid flames considered in the proposed model depended on the aspect ratio of the rectangular fire source. Compared with experimental results and data available in the literature, the standard error of estimation by the developed model was 0.505, indicating that the proposed model outperformed the cuboid flame and point source models found in the literature. This developed model has better prediction for the thermal radiation from rectangular fire source with large aspect ratio. Based on sensitivity analysis of model parameter, the ratio of the aspect ratio of the burner to the number of sub-cuboid flames could be considered as a constant. Thus, this proposed model can be applied to risk assessment of rectangular fire.

    更新日期:2019-12-26
  • Impact of surface tension and viscosity on falling film thickness in multi-effect desalination (MED) horizontal tube evaporator
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Furqan Tahir; Abdelnasser Mabrouk; Muammer Koç

    Falling film evaporators are extensively used in many applications because of their higher heat and mass transfer coefficient over low temperature difference. Falling film thickness, which depends on liquid spray density and thermophysical properties, is an important parameter in determining heat and mass transfer performance as it represents thermal resistance. In multi-effect desalination (MED) evaporators, the thermophysical properties such as surface tension and viscosity change due to operating temperature differences, i.e. higher in the first evaporator/effect and lower in the last evaporator/effect. The surface tension variation with temperature is often neglected in most of the film thickness characterization and modeling studies. Therefore, it is needed to analyze surface tension effects distinctly on film thickness. In this study, a 2D CFD model was developed by implementing volume of fluid (VOF) multiphase model to distinguish liquid and gas phases. The effects of viscosity and surface tension were analyzed separately, and it was found that with constant surface tension, viscosity effects account for 66.1% variation in film thickness for operating temperature range of 85 °C–5 °C. However accounting both viscosity and surface tension dependence, the results showed 72% increment in the film thickness. In addition, CFD results exhibited lower conduction thermal resistance of 0.2 m2 K/W at 85 °C against 0.4 m2 K/W at 5 °C, which reflects better evaporator performance at higher temperature. Hence, more focus and detailed analysis are recommended given in increasing the first effect temperature from 65 °C to 85 °C as it would improve thermal performance due to lower thermal resistance rather than decreasing last effect temperature from 40 °C to 5 °C as the thermal resistance would increase.

    更新日期:2019-12-26
  • Capillarity enhancement of micro heat pipes using grooves with variable apex angle
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-25
    Manjinder Singh

    We present a novel methodology to enhance the capillarity and thus the heat transfer capacity of a polygonal micro heat pipe by axially varying the apex angle of grooves. To analyse the effect of axial variation of apex angle on thermal performance of micro heat pipes, we use a quasi-one-dimensional mathematical model that takes into account the axial variation of apex angle. We analyse micro heat pipes with linear variation of apex angle and uniform apex angle. Numerical results of our model show that a linear increase in the apex angle from the evaporator to the condenser section can increase the heat transport capacity of the micro heat pipe by more than 55% compared to micro heat pipe with uniform apex angle. We describe how the axial variation in apex angle leads to enhanced capillarity in a micro heat pipe. We show that axial variation of apex angle redistributes the liquid mass from the condenser section of the micro heat pipe to the evaporator section resulting in a reduction of liquid pressure drop. Lastly, we mathematically optimise the axial variation of apex angle to obtain 100% enhancement in the maximum heat transport capacity in comparison to micro heat pipe with uniform apex angle.

    更新日期:2019-12-26
  • A model for the simulation of the chill block melt spinning (CBMS) process using OpenFOAM®
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-23
    Marcelo Barone; Francisco Barceló; Marcelo Pagnola; Axel Larreteguy; Andrés G. Marrugo; Jairo Useche

    This work shows the results of a numerical model developed to simulate the CBMS technique for the production of the Fe78Si9B13 metallic magnetic ribbons for application in electronics. The model proposes a numerical approximation to a Vogel-Fulcher-Tammann (VFT) expression as a method in the solidification process. This approximation is introduced into the “compressibleInterFoam” routine, included in the OpenFOAM® suite, originally developed for the simulation of two immiscible, non-isothermal and compressible fluids. This routine solves, the phase fraction transport using the Volume of Fluids (VOF) approach. The boundary conditions imposed in the model were experimentally validated by digital image analysis with a high-speed camera at 5602 fps for the determination of the temperature profiles. The phase change is represented as a growth of several orders of magnitude of the alloy viscosity (μ) as the temperature (T) decreases, reaching solidification around the crystallization temperature (Tg). Also, we establish the condition of initial stability of CBMS process (R > 1.5) for Peclet numbers close to 400, and the validity up to limits of rotation in the wheel close to 40 m s−1. The proposed methodology is validated with previous work. Encouraging results show that the solution of the CBMS process can be adequately simulated with the proposed approach.

    更新日期:2019-12-25
  • Experimental study on flow boiling heat transfer characteristics of R134a, R245fa and R134a/R245fa mixture at high saturation temperatures
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-21
    Qiang Guo; Minxia Li; Xiangdong Tian

    The flow boiling heat transfer characteristics of R245fa, R134a and non-azeotropic mixture R134a/R245fa (0.33/0.67 by mass) at high saturation temperatures were investigated in a horizontal tube with 10 mm I.D. Effects of high saturation temperatures on heat transfer coefficients of R134a, R245fa and the mixture were investigated from 55 °C to 95 °C. Heat flux was controlled from 6 to 24 kW/m2 and mass flux ranged from 100 to 300 kg/m2.s. The results show the heat transfer coefficient of the mixture (R134a/R245fa with 33%/67% by mass) is much lower than that of pure R134a and is close to that of pure R245fa. At high saturation temperatures, mass flow rate has less influence on heat transfer coefficient of R245fa and the mixture at a large heat flux. The heat transfer coefficients of three working fluids increase slightly when the saturated temperatures change within 10 °C. The pressure drops of pure R245fa and the binary mixture were studied and the pressure drop of the mixture (R134a/R245fa with 33%/67% by mass) is a little higher than that of R245fa. A modified heat transfer correlation considering the mixing effect of binary mixture is proposed for the mixture of R245fa/R134a, and the mean absolute deviation is within 20%. A modified two phase frictional factor for the mixture is proposed and the deviation of the mixture using modified two phase frictional factor is between −6% and +22%.

    更新日期:2019-12-21
  • Electric field effect on the heat transfer enhancement in a vertical rectangular microgrooves heat sink
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-20
    Jinchen Tang; Xuegong Hu; Yingying Yu

    The EHD pumping is an attractive solution to insufficient liquid supply and dryout occurrence in open microgrooves heat sink and micro heat pipes. It offers the advantages of active control of capillary pumping capacity. In this study, effects of applied non-uniform electric field on evaporation heat transfer enhancement in a vertical rectangular microgrooves heat sink are investigated experimentally. The pin-plate electrodes pair is used to produce the non-uniform electric field. A series of experiments of parameters have been considered, including heat flux, interelectrode spacing and microgrooves dimension are conducted under various applied DC applied voltages. Results show that the application of pin electrode can promote the capillary wetting length, decrease the wall temperature of microgrooves and enhance the heat transfer performances effectively.

    更新日期:2019-12-20
  • Analysis of heat transfer and flow characteristics in typical cambered ducts
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-20
    Xilong Zhang; Yongliang Zhang; Zunmin Liu; Jiang Liu

    The heat transfer and flow characteristics of the air-water cross flow over cambered ducts were experimental and numerical investigated. The sequence of their Core Volume Goodness Factor (CVGF) is cosinoidal, parabolic, circular, trapezoidal and rectangular ducts successively from superior to inferior. Cambered ducts have more uniform temperature difference distribution than the equal cross section duct, and it has the minimum temperature difference in inlet and outlet of the cosinoidal duct. With the optimal overall heat transfer performance, the cosinoidal duct is superior to that of the rectangular duct by 7.3%–28.1%. In the cosinoidal duct, the smaller the amplitude is, the better the heat transfer performance is. The thickness of the thermal and velocity boundary layers adjacent to the wall surface decreases constantly with increased Reynolds number. In the near wall region, n = 5um, the main heat transfer area is the peak and middle areas, but with weaker heat transfer performance in the trough region. Although gradually expanding cambered duct slows down the flow velocity, the structure form decreases the pressure drop loss during the flow process. While improving the convective heat exchange capability of the upstream, the heat transfer area of the downstream is also improved to boost the overall heat transfer performance.

    更新日期:2019-12-20
  • Comparison of different approaches to evaluate the equivalent thermal diffusivity of building walls under dynamic conditions
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-20
    S. Corasaniti; M. Potenza; P. Coppa; G. Bovesecchi

    In the present work, three different methodologies to determine the equivalent thermal diffusivity of an opaque wall, part of a building envelope, were compared. The thermal behaviour of a typical wall of a recently built building has been recorded; the internal and external temperatures were measured for one month, during day and night, with one acquisition every 10 min. The experimental data were processed by means of three analytical models, getting as a result the effective thermal diffusivity of the wall. The first model used is the semi-infinite region, the second a symmetric slab with periodic surface temperatures and the third the ISO 13786:2007 rule. All models produce different values if thermal diffusivity is evaluated from the ratio of the external and internal amplitudes or from their phase shifts. For the first model the α values obtained are respectively (1.512·± 0.016)·10−6 m2 s−1 and (2.813 ± 0.004)·10−6 m2 s−1; for the second (1.741 ± 0.150)·10−6 m2 s−1 and (2.603 ± 0.007)·10−6 m2 s−1, and (1.880 ± 0.017PCNM)·10−6 m2 s−1 and (2.168 ± 0.003)·10−6 m2 s−1 for the third. This difference is likely due to the lack of correspondence of the wall models to its real structure. When a non-linear regression is applied to data, a unique value is obtained, intermediate between the two, i.e. (2.544 ± 0.021)·10−6 m2 s−1 (first model), (1.854 ± 0.010)·10−6 m2 s−1 (second model) and (1.988 ± 0.012)·10−6 m2 s−1 (third model). Both the difference between the two values of thermal diffusivity and the root mean square obtained from the nonlinear-least square regression asses that the best results come from the application of the ISO 13786 rule.

    更新日期:2019-12-20
  • Shape optimisation of air-cooled finned-tube heat exchangers
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-20
    H. Nemati; M.A. Moghimi; P. Sapin; C.N. Markides

    The use of annular fins in air-cooled heat exchangers is a well-known solution, commonly used in air-conditioning and heat-recovery systems, for enhancing the air-side heat transfer. Although associated with additional material and manufacturing costs, custom-designed finned-tube heat exchangers can be cost-effective. In this article, the shape of the annular fins in a multi-row air heat exchanger is optimised in order to enhance performance without incurring a manufacturing cost penalty. The air-side heat transfer, pressure drop and entropy generation in a regular, four-row heat exchanger are predicted using a steady-state turbulent CFD model and validated against experimental data. The validated simulation tool is then used to perform model-based optimisation of the fin shapes. The originality of the proposed approach lies in optimising the shape of each fin row individually, resulting in a non-homogenous custom bundle of tubes. Evidence of this local-optimisation potential is first provided by a short preliminary study, followed by four distinct optimisation studies (with four distinct objective functions), aimed at addressing the major problems faced by designers. Response-surface methods – namely, NLPQL for single-objective and MOGA for multi-objective optimisations – are used to determine the optimum configuration for each optimisation strategy. It is shown that elliptical annular-shaped fins minimise the pressure drop and entropy generation, while circular-shaped fins at the entrance region (i.e., first row) can be employed to maximise heat transfer. The results also show that, for the scenario in which the total heat transfer rate is maximised and the pressure drop minimised, the pressure drop is reduced by up to 31%, the fin weight is reduced up to 23%, with as little as a 14% decrease in the total air-side heat transfer, relative to the case in which all the fins across the tube bundle are circular. Moreover, in all optimised cases, the entropy generation rate is also reduced, which shows a thermodynamic improvement in tube bundle performance.

    更新日期:2019-12-20
  • Heat removal enhancement in a channel with a single or an array of metallic foam obstacles
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-18
    Arman Haghighi; Ahmed Albojamal; Kambiz Vafai

    A single or an array of metallic porous structures of various geometries are introduced on the inside surface of a partially heated wall of a channel with air or water flow. Forced convective heat transfer rate from the heated wall to the fluid, as well as the average pressure drop along the channel are numerically studied. Effects of the aluminum porous block's structural parameters such as porosity (ϵ), Darcy number (Da) and Forchheimer coefficient (F) are taken into account. The optimum geometrical conditions of the system is extensively explored considering parameters such as blocks' height, width and spacing between them. Our results demonstrate great potential for metal foams to perform as heat sinks and pave the way for their further implementation in applications such as solar thermal collectors and electronic cooling.

    更新日期:2019-12-19
  • Film cooling performance measurement over a flat plate for a single row of holes embedded in an inclined trench
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-17
    Sanjay D. Barahate; R.P. Vedula

    Film cooling heat transfer coefficient and effectiveness measurements downstream of a single row of holes embedded in an inclined trench injecting secondary fluid into the mainstream over a flat surface are reported here. The well-studied trench geometry is one where the walls of the trench are perpendicular and film injection holes are at an angle to the mainstream flow direction. In the current investigation, the trench is inclined at an angle to the mainstream and the injection holes are normal to the flow direction. Coolant injection in the form of a uniform stream at an angle to the mainstream is likely with this configuration. The trench inclination is kept constant at 35° and three hole pitch to diameter ratios equal to 3, 4.5 and 6 are studied for the blowing ratio varying between 0.3 and 2.5. The injection hole length to diameter ratios equal to 1.8 and 5.5 were studied and the influence on the net heat flux reduction ratio was found to be very small. The local effectiveness and the heat transfer coefficient values are presented and the variations in the cross-stream direction are observed to be very small. The effectiveness values for the current angular trench are higher whereas the heat transfer coefficients are smaller compared to the normal trench at higher blowing ratios. This results in increased Net Heat Flux Reduction ratio (NHFR) values for the current configuration which are observed to remain high at large downstream locations also.

    更新日期:2019-12-18
  • Natural convection heat transfer from linearly, circularly and parabolically bent plates: A study of shape effect
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-17
    Shibajee Behera; Swastik Acharya; Sukanta K. Dash

    In the present study natural convection heat transfer from three differently bent shapes (linear, circular, and parabolic) has been investigated numerically for laminar flow in the range of Ra from 103 to 106. The simulations are carried out for different orientations by varying X/L from 1 to 0.3 keeping the surface area same for all the shapes. The present numerical study is able to capture a complete and very interesting picture of temperature plume and flow field over the bent plates. From the numerical results, it has been observed that heat loss from an upward bent plate is more compared to a downward bent plate for all the shapes at same Ra. Parabolically bent plate is most effective in losing heat compared to a linearly and circularly bent plate of same area. There is an optimum X/L for maximum heat loss for bent plate; further bending the plate reduces heat transfer slightly. The effects of parameters like X/L, Ra on average Nu from the top and bottom surface, total heat transfer, skin friction coefficient, temperature plume, and flow field are shown in the study which would enrich the literature archive tremendously.

    更新日期:2019-12-18
  • Experimental investigation on natural convective heat loss of cylindrical cavity with/without a quartz window
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-16
    Lan Xiao; Xiao-Dong Cai; Shuang-Ying Wu

    This paper presents an experimental setup for the study of the natural convective heat loss characteristics of a cylindrical cavity with/without a quartz window. The thermal performance such as wall temperature and heat loss of two types of cavity receivers are studied by changing the tilt angles and heated states of the walls, then the effect of quartz window on the heat loss of the cavity is highlighted. The average temperature of cavity with a quartz window is respectively 68.61 °C, 79.36 °C, 48.22 °C higher than same cases without a quartz window (corresponding tilt angle is −90°, −30° and 45°). The temperature difference is within 10 °C for quartz window cavity and 40 °C for no quartz window cavity when q = 700 W/m2 and all surfaces are heated. More than 70% cases occur that the natural convective heat loss of no quartz window cavity is greater than that of the quartz window cavity. The experimental results show that the quartz window can significantly increase the operating temperature of the cavity and weaken the influence of tilt angle. The quartz window plays an important role in the balance of cavity temperature and reduction of natural convective heat loss. The empirical correlations of predicting the radiant and natural convective Nusselt numbers of quartz window cavity considering tilt angle and heat flux have also been obtained.

    更新日期:2019-12-17
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  • Levitation mechanism of impacting nanodroplet on heated wall
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-13
    Hirofumi Tabe; Kazumichi Kobayashi; Hisao Yaguchi; Hiroyuki Fujii; Masao Watanabe

    Levitation of a droplet after impacting on a high-temperature wall is well known as the Leidenfrost effect. In this study, we conducted a molecular dynamics (MD) simulation to elucidate the mechanism of nanodroplet levitation and to investigate the relation between the levitation mechanism and liquid–solid intermolecular force. We found that the impacting nanodroplet levitated from its edge on the heated wall. Analyses on velocity fields inside the droplet indicated that an upward velocity at the edge of the nanodroplet was caused by an internal flow. The high wall temperature or strong liquid–solid intermolecular force caused an intensive evaporation near the three-phase contact line before levitation occurred, and this intensive evaporation yielded a large internal flow within the nanodroplet.

    更新日期:2019-12-13
  • Contact melting of a rectangular block with temperature-dependent properties
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-12
    V. Cregan; J. Williams; T.G. Myers

    We consider a model for the contact melting of a block of phase change material on a flat, heated surface. The block and melt have linear temperature-dependent thermal conductivity and viscosity. The model consists of heat equations for the solid and liquid temperatures, the Navier–Stokes equations in the liquid melt layer, a Stefan condition at the solid–liquid interface and a force balance between the weight of the solid and the countering pressure in the liquid. The heat balance integral method is used to obtain an approximate solution for the solid temperature. We demonstrate that in the case of n-octadecane the inclusion of temperature-dependent effects slows down the melting process. Finally, we vary the parameters in the linear expressions for the conductivities and viscosities to understand the behaviour of the system.

    更新日期:2019-12-13
  • Investigation of flow structure and heat transfer enhancement in rectangular channels with dimples and protrusions using large eddy simulation
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-11
    Ming Li, Xin Chen, Xinjian Ruan

    Large-eddy simulations are used to investigate flow structures and heat-transfer enhancements in rectangular channels with dimples and protrusions as Reynolds number varies from 5600 to 22,000. Roughness elements adopt sharp edges and are arranged inline, the ratios of their depths or heights to their respective print diameters are 0.1, and the ratio of channel height to dimple print diameter is 1.0. Comparisons are carried out between effects of Reynolds number and the gap between roughness elements, and the data presented include thermal performances, flow structures, local Nusselt numbers, local spatial correlation coefficients, characteristic frequencies, and turbulent kinetic energy. Results show that thermal performances can be increased considerably by either increasing Reynolds number or decreasing the gap ratio, but the type of flow structure is only related to Reynolds number. And inline arrangements for dimples and protrusions in many ways are equivalent to deepening the depth of the dimple, especially along the centerline of the arrangement, such that they can achieve higher thermal performances than a denser arrangement of dimples with a corresponding depth ratio. Meanwhile, sampling grids are deployed in the spacings between roughness elements to obtain local spatial correlation coefficients and characteristic frequencies. Frequent passing of vortical structures from roughness elements results in local minima of spatial correlation coefficients and protruding characteristic frequencies, and results show that contours of peak Nusselt numbers are in good correlation with them and thus directly resulting from these vortical structures and their enhancing effects. And it is found that the asymmetry in Nusselt number contours in the wake of the dimple is resulted from the combined influences of sizes of primary passages for vortical structures from dimples, shedding frequencies, and the turbulent kinetic energy carried by these vortical structures.

    更新日期:2019-12-11
  • Numerical study of heat transfer and solute distribution in hybrid laser-MIG welding
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-10
    Xuyang Chen, Gang Yu, Xiuli He, Shaoxia Li, Zhiyong Li

    In the quest for the transport mechanism in the molten pool during hybrid laser-MIG welding of aluminum alloy, an improved three-dimensional numerical model is developed. A modified model for laser heat source is utilized to investigate the energy absorption mechanism in keyhole. Some driving forces are considered to simulate the fluid flow, such as electromagnetic force, surface tension and buoyancy. The effects of arc pressure and droplet impact are taken into account to track the free surface. Several dimensionless numbers are utilized to analyze the relative importance of driving forces. The temperature field, liquid velocity field and magnesium and zinc distribution are numerically and experimentally studied. Results shows that the laser beam create a great impression on the heat transfer, fluid flow, solute distribution and weld bead geometry. In MIG welding, there is an insufficient mixing zone at the front of the pool, while the solute distribution in hybrid laser-MIG welding is observed more uniform. Magnesium and zinc are found concentrated in lower and upper part of the molten pool, respectively. The mathematical model is well validated by the experimental observations, and the calculated element distribution agrees well with the experimental measurements. Furthermore, the improved model provides an effective method for parametric optimization to improve the properties of hybrid laser-MIG welding joints.

    更新日期:2019-12-11
  • Adaptive interface treatment for aerothermal coupling using a Discontinuous Galerkin method
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-10
    Minh Nguyen, Marc Errera, Odile Labbé, Rocco Moretti, Florent Renac, Vincent Couaillier

    This paper presents the application of a discontinuous Galerkin method to conjugate heat transfer problems using a Dirichlet-Robin interface treatment. The use of optimal coefficients derived from a Godunov-Ryabenkii stability analysis is adapted to the discontinuous Galerkin discretization. The stability and convergence of different coupling coefficients are explored for fluid-structure interactions of varying strength. The effects of increasing the order of the polynomial approximation are examined. It was found that for weak fluid-structure interactions, the optimal coefficients provide stable and quickly converged results. However, for moderate and strong interactions, relaxation coefficients that are larger than optimal must be used to stabilize the process. Because the coupling coefficient was adapted to the polynomial order of approximation, increasing the order of the polynomial was not found to destabilize conjugate heat transfer processes using adaptive coefficients. Finally, at the end of the paper, a validation vs empirical correlations is presented.

    更新日期:2019-12-11
  • First approach for modelling the physical foaming of tannin-based thermoset foams
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-09
    Z. Marie, V. Nicolas, A. Celzard, V. Fierro

    In the present paper, the foaming process of biosourced thermoset foams was considered at the lab scale with the aim of creating a numerical model able to simulate, for the first time, their manufacturing process. A multi-physical model was thus generated, having as driving force of the expansion the inner gas pressure of the foam, calculated from mass, heat and mechanical balances. In addition, and this is another originality of our work, the study was implemented in a 2D axisymmetric mobile mesh. The simulation was not stopped just after the rise of temperature and the related material expansion but, instead, a complete simulation of the whole process was offered, i.e., including cooling. Simulated and experimental data such as temperature changes and foam growth were compared, and a fair agreement was observed, suggesting the relevance of our approach.

    更新日期:2019-12-11
  • Application of improved decentralized fuzzy inference methods for estimating the thermal boundary condition of participating medium
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-09
    Shuangcheng Sun, Guangjun Wang, Hong Chen

    The decentralized fuzzy inference method (DFIM) was applied to estimate the time-dependent heat flux of 1D participating medium. The direct problem concerned on coupled radiation and conduction heat transfer in the medium was solved by the finite volume method and discrete ordinate method. The simulated boundary temperature was served as input for the inverse analysis. The inverse problem was formulated as an optimization approach. Three improved decentralized fuzzy inference methods (IDFIMs) were developed to accelerate the convergence rate and enhance the estimation accuracy. Five kinds of time-dependent heat fluxes were considered to test the performance of the present inverse technique. No prior information on the functional forms of the unknown boundary conditions was needed for the inverse analysis. All retrieval results showed that the incident heat flux of participating medium can be accurately estimated by DFIMs. The proposed IDFIMs achieved better performance than the original DFIM in terms of computational accuracy and efficiency. Moreover, a comparison between the IDFIM and other optimization techniques was conducted. The proposed IDFIM was proved to be more efficient and accurate than conjugate gradient method, Levenberg-Marquardt method, stochastic particle swarm optimization algorithm and genetic algorithm.

    更新日期:2019-12-11
  • The effect of droplets thermophysical properties on turbulent heat transfer in a swirling separated mist flow
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-09
    Maksim Pakhomov, Viktor Terekhov

    The numerical investigation of the effect of the swirl number and the thermophysical properties of water, ethanol, and acetone droplets on the particle scattering, turbulence modification and heat transfer in a droplet-laden flow is carried out. The set of 3D steady-state Reynolds-averaged Navier–Stokes (RANS) equations for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. The flow swirling significantly shortens the length of the recirculation zone (up to two times in comparison with non-swirling flow at swirl number S = 0.5) for all types of the studied liquids. The highest value of heat transfer rate is obtained for the ethanol droplets (up to 10%), and the lowest one is obtained for the acetone droplets (up to 50%), both in comparison with water droplets. The swirling of the turbulent mist flow causes heat transfer enhancement (more than 1.5 times in comparison with the non-swirling droplet-laden flow). In the swirling flow, the value of the volume fraction of the dispersed phase has a maximum in the axial region of the pipe, and further in the direction of the wall its value is very small. The effect of heat transfer enhancement weakens with increasing Reynolds number of the flow. A region without droplets appears in the wall zone of the swirling two-phase flow, and the region is largest for acetone droplets.

    更新日期:2019-12-09
  • Experimental investigation of evaporation dynamic of sessile droplets in pure vapor environment with low pressures
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-09
    Shuang Ye, Li Zhang, Chun-Mei Wu, You-Rong Li, Qiu-Sheng Liu
    更新日期:2019-12-09
  • Finite volume method for multiphase flows with radiation and phase change
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-05
    A. Hasečić, S. Muzaferija, I. Demirdžić

    A mathematical model which can describe flow and phase change of a number of phases at high temperatures is presented. It combines an interface capturing multiphase model, the P–1 radiation model, and a melting/solidification model. The resulting equations are solved by employing the finite volume discretization, a segregated solution procedure and the SIMPLE algorithm. The melting/solidification model is a finite rate model which in the limiting case behaves like a thermodynamic equilibrium model and it can also be used in situations where the phase change occurs within a range of temperatures as well as for problems where the phase change occurs at a constant temperature. The method is verified on a number of problems. The results obtained show a good agreement with exact solutions or results which can be found in literature.

    更新日期:2019-12-05
  • A new process for the determination of the Soret coefficient of a binary mixture under microgravity
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-03
    Abdelkader Mojtabi

    In the presence of a gravity field or under microgravity, pure thermo-diffusion leads to very weak species separation in binary mixtures. To increase the species separation in the presence of gravity, many authors use thermo-gravitational diffusion in vertical columns (TGC). For a given binary mixture, the species separation between the top and the bottom of these columns depends on the temperature difference, ΔT, imposed between the two vertical walls facing each other, and the thickness, H, between these two walls (annular or parallelepipedic column). These studies show that, for a fixed temperature difference, the species separation is optimal for a thickness, Hopt, much smaller than one millimetre. The species separation decreases sharply when the thickness H decreases with respect to this optimum value. It decreases progressively as H increases with respect to Hopt. In addition, for mixtures with a negative thermo-diffusion coefficient, the heaviest component migrates towards the upper part of the column and the lightest one towards the lower part. The loss of stability of the configuration thus obtained leads to a brutal homogeneity of the binary solution. The objective of this study in microgravity was to increase the optimum of species separation. For this purpose, the binary fluid motion was provided by uniform velocities imposed on the two walls of the cavity facing each other. This forced flow led to species separation between the two motionless walls of the cavity. In this case, the fluid motion generated in the cavity was not dependent on the imposed temperature difference, ΔT contrarily to the case of thermogravitational column. Under these conditions and for a given column of thickness H, there are three independent control parameters: ΔT and the two velocities of the walls facing each other. Using the parallel flow approximation for a cell of large aspect ratio, the velocity, temperature and mass fraction fields within the cavity were determined analytically. Thus the parameters leading to optimal species separation were calculated. The analytical results were corroborated by direct numerical simulations. The present paper thus proposes a new process for the determination of the Soret coefficient, the thermodiffusion coefficient and mass-diffusion coefficient.

    更新日期:2019-12-04
  • Heat transfer enhancement of wavy fin-and-tube heat exchangers via innovative compound designs
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-12-03
    Ali Sadeghianjahromi, Saeid Kheradmand, Hossain Nemati, Chi-Chuan Wang

    The present study examines some novel improvements in association with the airside performance of wavy fin-and-tube heat exchangers. Effects of waffle height, adding slits on wavy fins and vortex generators on the thermal resistance of wavy fin-and-tube heat exchangers are investigated. Increasing waffle height, imposing slits on the wavy fin, or adding vortex generators can provide 6–10% reduction in thermal resistance subject to constraints of the same pumping power. Yet, the compound design including all these three features together is even more powerful to achieve a 16% reduction of thermal resistance. Increasing waffle height will decrease thermal resistance, but the accompanied pressure drop is even higher. Hence, it is highly recommended that the waffle height should be less than the fin pitch or at a corrugation angle being less than 15°. Punching slits on the top of wavy fin surface can result in 6–7% decrease in thermal resistance. Effect of increasing the width of slits on heat transfer characteristics is negligible. Conversely, increasing the length of slits is much more effective. It can be seen from the results that slits with a length of 12 mm and a width of 1 mm yield 8–10% reduction in thermal resistance.

    更新日期:2019-12-04
  • Numerical investigation of impingement heat transfer on smooth and roughened surfaces in a high-pressure turbine inner casing
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-29
    Fujuan Tong, Wenxuan Gou, Zhenan Zhao, Wenjing Gao, Honglin Li, Lei Li

    The multiple impingement jet system in a high-pressure turbine inner casing has been studied numerically. Four target surface configurations, i.e., smooth, cambered rib, square and round pin-fin are investigated, respectively. Three different boundary conditions (i.e., the maximum, medium and minimum scheme) are set based on the real turbine operating condition. The numerical validation reveals that the selected computational method can provide a good prediction of the impingement heat transfer on both the smooth and roughened surface structures. The results indicate that the roughness elements can significantly improve the heat transfer characteristics of the multiple impingement jet system. And the cambered rib surface displays the best enhancement of the impingement cooling effect. For the turbine inner casing with different target surfaces, local/average heat transfer parameters and the flow structure are obtained and compared under the same boundary condition, and such process has been repeated under three selected boundary conditions. All of the roughened surfaces show the outstanding cooling effect than the smooth surface. Especially, with the cambered rib configuration, the average temperature of the turbine inner casing domain can be decreased by 20 K, and the average Nusselt number can be increased by up to 62.6% than that on the smooth surface. The little temperature difference also demonstrates that the cambered rib configuration can promote the cooling uniformity and decrease the thermal stress in the turbine inner casing. Moreover, the analysis of the flow structure also illustrates that the cambered rib configuration can effectively reduce the crossflow and adjacent jet interaction, which promotes the turbulent mixing and augments the impingement heat transfer. The proposed structure can be used to improve the cooling effect of the turbine inner casing and is expected as the potential design reference for the turbine engine in the future.

    更新日期:2019-11-30
  • The impact of square shape perforations on the enhanced heat transfer from fins: Experimental and numerical study
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-29
    Thamir K. Ibrahim, Ahmed T. Al-Sammarraie, Manar S.M. Al-Jethelah, Wadhah H. Al-Doori, Mohammad Reza Salimpour, Hai Tao

    Heat sinks are widely adopted as heat transfer boosters for their role in promoting heat transfer surface area. To enhance the thermal behaviour of heat sinks under forced convection heat transfer, perforated fins are utilized. In the present investigation, the effect of utilizing the square perforation technique has been experimentally and numerically investigated. The study was conducted for different numbers of perforations, i.e. 4, 6, 9, 12, and 15; heat supplied rates of 1730, 2200, 2680, and 3150 W; and airflow velocities of 0.4, 0.7, 1.1, 1.4, and 1.8 m/s. The present numerical results were validated against the present experimental results and available data in the literature, and excellent agreements were found. Our results indicated that the perforated fin with square perforations has shown a reduction in the fin temperature up to 16 °C compared to the non–perforated one. Moreover, by increasing the number of perforations, higher heat rates were dissipated because of the rise in the flow turbulence level and the reduction in the thermal resistance between the fin surface and its surroundings. In addition, the existence of perforations reduced the friction factor around the fin and sequentially reducing the required flow pumping power, which adds another merit to the aforementioned perforation technique. Using square perforations showed a 5.9% reduction in average friction factor compared to circular perforations.

    更新日期:2019-11-30
  • Investigating the effect of water nano-droplets injection into the convergent-divergent nozzle inlet on the wet steam flow using entropy generation analysis
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-30
    Mohammad Ali Faghih Aliabadi, Ali Jahangiri, Iman Khazaee, Esmail Lakzian

    In many industrial pieces of equipment, such as ejectors, thermo-compressors, supersonic separators, etc., in which the condensation phenomenon occurs spontaneously, steam enters the nozzle in a saturated condition. Liquid droplets are likely to be present in the saturated vapor. In the present paper, the effect of the existence of a number (9 cases) of liquid droplets (with a fixed size of 20 nm in all cases) on the saturation steam has been studied. The results demonstrate that with the increase in the number of droplets at the nozzle inlet; pressure, temperature, liquid mass fraction, mass-generated rate and entropy increase in the convergent part. However, the growth of droplets and Mach number decrease. Furthermore, due to steam condensation and latent heat transfers to steam, the degree of supercooling, the supersaturation ratio and the flow rate at the nozzle inlet are decreased, the intensity of condensation shock is reduced, and shock occurs with delays. With an increase in the number of droplets, condensation shock and the nucleation phenomenon will not occur. The amount of entropy production decreases with the surge in the number of droplets at first, and then begins to increase. The results demonstrate that in the presence of 3 × 10+14 droplets, compared to the absence of water droplets at the nozzle inlet, generated entropy, liquid mass fraction and the mass flow rate will decrease by 9.4%, 12.5%, and 1.7%, respectively.

    更新日期:2019-11-30
  • Empirical derivation of a Nusselt number in a thermally stratified enclosure
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-30
    Izumi Hagiwara, A. Tokuhiro, P.G. Oduor

    A hydrophilic gelatin suspension possesses thermal properties that may affect the ratio of convective to conductive heat transfer across a boundary. This ratio, also known as Nusselt's number, can represent laminar or turbulent flow regimes. In this study, thermal transitioning empirical values were determined using a gelatin as a polymer gel surrogate. The crux of this study was to (a) quantify heat transfer properties of the gelatin as a suspended semi-solid and (b) understand thermal profiles with respect to particle size and loading. We can adduce the existence of a thermally induced: lower rise velocity, stratified Nusselt number, and a boundary layer.

    更新日期:2019-11-30
  • Defect depth retrieval method based on nonlinear transformation for pulsed thermographic inspection
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-28
    Meiling Wang, Bin Gao, Tongle Wu, Bozhen Hu, Li Liu

    The carbon fiber reinforced polymer (CFRP) has been widely used in the aerospace field. During its utilization under the severe environment, CFRP is prone to defects, including impacts, debonds, delaminations, and cracks. Optical pulsed thermography (OPT) nondestructive testing has been used for qualitative and quantitative analysis of such defects. This paper proposes the nonlinear transfer model of peak contrast time analysis to determine defect depth by using OPT technology. The mechanism of the nonlinear relationship between peak contrast time and defect depth is demonstrated and validated by experiments. To effectively predict the defect depth, gaussianization transform is modeled as a nonlinear conversion in defects depth determination. The results of the experiments have indicated that the proposed method has significantly enhanced the accuracy in depth determination.

    更新日期:2019-11-29
  • Manipulation of bubble migration through thermal capillary effect under variable buoyancy
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-28
    Yang Ma, Yongpan Cheng, Yang Shen, Jinliang Xu, Yi Sui
    更新日期:2019-11-29
  • Influence of flow non-uniformity on the dynamic behaviour of three-fluid cross-flow compact heat exchanger
    Int. J. Therm. Sci. (IF 3.488) Pub Date : 2019-11-28
    Harpreet Kaur Aasi, Manish Mishra

    The flow uniformity at any cross-section along the flow is highly influenced due to different practical limitations such as improper header/distributor design, manufacturing defect etc. Hence, the compact three-fluid heat exchanger with cross-flow arrangement is numerically investigated with non-uniform flow at the entrance of central fluid for four feasible flow arrangements (C1,2,3,4) for turbulent flow regime. The experiments are conducted to validate the numerical solutions obtained from Implicit Finite Difference scheme. The dynamic response of the cross-flow three-fluid heat exchanger is analysed with step and sinusoidal perturbation at the entry and both the cooling and heating efficacy of the heat exchanger is discussed. The model is made more realistic by incorporating the dispersive axial heat conduction effect within the fluids as well as at the inlet section of heat exchanger (Dankwert's boundary condition) and wall longitudinal heat conduction in the conducting walls. Four cases of flow non-uniformity (P, Q, R and S), which resemble the most realistic and possible case of non-uniform flow in the heat exchangers are selected for present study. It is learnt that case P and case Q offers the maximum and the minimum augmentation or deterioration in the efficacy of heat exchanger when non-uniform flow exists only in central fluid. The impact of flow non-uniformity on the performance of heat exchanger is more significant in turbulent flow regime than laminar. The analysis is further extended for the flow non-uniformity in all the three fluid streams simultaneously and for certain combinational case of U (uniform flow), P and Q. It is found that flow non-uniformity drastically affects the heat exchanger's efficacy and the upshot of axial dispersion causes more deterioration in the thermal performance than that by longitudinal heat conduction.

    更新日期:2019-11-28
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