Nanoparticles for improving the efficiency of heat recovery unit involving entropy generation analysis

Highlights

• New thermal unit as heat recovery system to augment the available energy is studied.

• Gas flows inside inner pipe and nanofluid flows in annulus region.

• To involve such range of Re, K-ɛ approach has been utilized.

• Growth of these factors makes S_gen,f to increase which is unfavorable effect.

• S_gen,th reduces by 35.97% with rise of n when Re=2000.

Abstract

The present article aims to suggest new thermal unit as heat recovery system to augment the available energy. Hot gas flows inside the inner pipe and the operating fluid in annulus region is nanofluid (CuO-water). Helical tape with various revolution number (N) was employed to augment the fluctuations of fluid and reduce the boundary layer thickness. Range of Reynolds number (Re=4000 to 20,000) indicates turbulent regime. To involve such range of Re, K-ɛ approach has been utilized and its validity was reported. Achieving to greater useful energy happens if the irreversibility of system minimized. This function has two terms: frictional term (S_gen,f) and thermal term (S_gen,th). The thermal term is higher than frictional term and it should be considered as main source of irreversibility. With augment of rotational speed as a result of higher values of Re and N, velocity gradient goes up but thinner boundary layer will be generated. So, with rise of such parameters, temperature gradient declines and residence time enhances which results in better mixing of fluid. So, two scrutinized factors have favorite effect on S_gen,th and considering greater values of them leads to lower thermal irreversibility. Also, growth of these factors makes S_gen,f to increase which is unfavorable effect. As N augments from 3 to 5, the value of S_gen,f enhances about 47% but S_gen,th reduces about 42.47% when Re=5000. Also, with selecting higher N from 3 to 7 when Re=2000, frictional entropy augments about 128.95% while S_gen,th reduces by 35.97%. S_gen,th faces 39.14% decrement if Re goes up when N = 3 while with same change, the value of S_gen,f augments to 7.27 times greater value.

Introduction

Heat transfer increment methods are comprehensively employed in numerous engineering uses such as recovery procedure, shell-and-tube heat exchanger, refrigeration mechanisms, chemical process plants, etc [1]. The main variables in decreasing the cost and the size of thermal unit are principally the pressure loss and thermal factor. A growth in heat transfer factor results in another benefit of decreasing entropy generation. Various modern systems were defined and developed for growing the thermal efficiency in heat exchangers by passive and active techniques. Second technique is the approach that requires the extra power resource like jet implement, mechanical aids, and etc. Passive technique will not need a power resource and can be employed as: swirl stream systems, tube insert, twisted tape (TT), surface coating, etc. Using TT in a tube [2] is considered as effective increment method and with considering modification for TT (such as employing alternate-axes and center wings, peripherally-cut tapes) better performance can be achieved. In comparison to a conventional TT, heat transfer increment by revised TT is obtained by better liquid mixing. Nonetheless, a development in performance is obtained at a cost of grew friction. The most favorable TT is the one yield wonderful Nu with minimum growth in pressure drop.

One of the most utilized increment techniques of heat transfer rate is twisted tape inserts growing both fluid friction and convective heat transfer in the stream area. They lead to turbulence and then develop the swirl stream. Furthermore, geometric shape of twisted tape inserts disturbs the boundary layer, leading to better rate of heat transfer. Nevertheless, augmentation of fluid friction can take tolls on the overall increment ratio. The performance of pipe including TT inserts relies on twist and pitch ratios. In recent analyses, many scientists have done numerical and empirical analyses to specify the optimal shape in accordance with the twist and pitch ratios [3]. Using nanotechnology in classic thermal systems results in the generation of an innovative category of fluids, called nanofluid [4]. Since typical fluids such as oil and water illustrate inappropriate heat transfer features, nanofluid has been defined which are constituted by suspending nanoparticles in the typical heat transfer fluids. The developments in nanofluid have led to the improvement of a classification of fluid called nanomaterial. Wongcharee and Eiamsa-Ard [5] empirically analyzed impact of H₂O- CuO nanomaterial and wavy tube facilitated with TT on thermal efficiency. Based on their findings, at the similar working conditions, friction factor, thermal efficiency and heat transfer rate related to the simultaneous use of copper/H₂O nanomaterial and twisted tape are greater in comparison to those related to the individual methods. Obviously, the rate of heat transfer grows with growing copper oxide/H₂O nanomaterial concentration and reducing twist ratio. The maximum thermal efficiency coefficient of 1.57 has been found in counter arrangement at the Re of 6200. The efficiency of a CuO/H₂O nanomaterial inside a U-shaped minitube was examined by Vinodhan et al. [6] who expressed that the best productivity was achieved employing the 0.05% nanomaterial. Based on their obtained results, the productivity of the considered nanomaterial in the U-shaped duct was increased at an optimum nanopowder fraction. Empirically, the performance of a pipe including perforated triple TT was analyzed by Bhuiya et al. [7] and demonstrated that a soar in Nu was obtained with augmentation in Darcy factor. A considerable increment in performance was achieved with perforated TT insert at fixed blower power. The authors concluded that perforated TT leads to thermal increment performance of 1.13 to 1.5. Numerically, the fluid stream features of multiple TT inserted in a wavy duct for heat recovery usage was analyzed by Hong et al. [8] and displayed that the alone utilization of the sinusoidal rib tube leads to an increment of around 27%−40% and the grown friction fall of about 49%−75% greater than in the baseline of spirally corrugated tube. Empirically, a solar collector including circular geometry was assessed by Moravej et al. [9] and proved that the performance grows with growth in H₂O stream rate and the solar flux. The productivity of considered unit was larger than case of rectangular one due to existence of secondary streams. Numerically, the stream and performance of heat sink was examined by Izadi et al. [10] who examined that growing the concentration of nanopowder grew the rate of heat transfer. A ribbed sheet inside solar energy mechanisms to analyze fluid stream and heat transfer features of different nanomaterial was employed by Zheng et al. [11]. Their results expressed that both pressure fall and heat transfer increment for nanomaterials illustrate considerable augmentation. The copper oxide/H₂O and Fe₃O₄/H₂O nanomaterials illustrate the worse and the best thermal efficiency of the considered heat exchanger. Selimefendigil and Öztop [12] analyzed the mixed convective inside a nanomaterial-accumulated lid driven tank including heat generation. Based on their outcomes, the amount of elastic modulus and nanopowder concentration grow the Nu.

Fan et al. [13] analyzed the hydraulic features in sinusoidal tubes including various turbulators under different magnetic fields. They found that a great nanomaterial concentration, perforated tabulator and magnetic field can present superior efficiency. The thermal efficiency inside a pipe including twisted ring turbulator with various pitch and width ratios has been exhibited by Thianpong et al. [14]. They expressed that frictional resistance goes up when the hole diameter ratio was reduced. The greatest thermal increment performance of 1.24 was derived of the pipe including twisted ring at the pitch and width ratios of 1.0 and 0.05. FVM used by Bozorg et al. [15] who examined the efficiency of a parabolic collector including oil/alumina oxide as operating liquid. Researchers demonstrated that when Re and volume fraction of the nanopowder grow, heat transfer factor, thermal performance and pressure fall grow. Nonetheless, the growth in inlet temperature resulted to the decrement in Nu, thermal performance and pressure fall. The effects of nanomaterial utilization in cooling rate were surveyed by Cuce et al. [16]. Regarding to their achievements, the best increment in indoor temperature variation among the first and the ultimate sate of the cooling procedure is obtained for 1% alumina oxide at the ambient temperature of 30°C including 26% for without load conditions. El-Maghlany et al. [17] analyzed the employing an in-house CFD for an efficient cooling of pipe. Based on their findings, the assisting stream increased the heat transfer rate in comparison to opposing stream. Furthermore, the nanopowders have a positive effect on Nu over the whole range of Richardon number; nonetheless, the nanopowders concentration should be soared to augment the Nu. An innovative technique for convective mode in a double backward facing step with mixed impacts of oriented magnetic field was presented by Selimefendigil and Öztop [18]. The cylinder rotation, location and arrangement have been found to change hydro-thermal efficiency whereas mean Nu is increased with greater Hartmann number and Re. The existence of the upper vortex placement led to greater deflection of the main flow toward the hot bottom wall, leading to greater Nu. Tarighaleslami et al. [19] tested the inclusion of nanomaterial as effective method and they reported that the air-side unit restricts the effect of employing for the air-liquid units. The CPU cooling procedure with implement of porous media with nanomaterial under the impact of MHD was simulated by Izadi et al. [20]. They stated that the growth of Eckert number and aspect ratio can reduce heat transfer efficiency.

Very few articles have been scrutinized on thermal units for utilizing energy of exhaust gas and evaluation of useful energy. Besides, review of previous articles illustrates that the heat exchangers combined with gasoline engine was investigated rarely. Also, employing the nanoparticles inside the base fluid to obtain more useful energy in heat recovery unit was not scrutinized yet. Therefore, the chief purpose of current context is investigating thermal behavior of heat recovery unit including double pipe unit. The fluid inside the annulus region was nanofluid consist of mixture of CuO-water. Exhaust gas flows within the inner tube and turbulent region exist in both regions. To reach more disruptions of flows in annuls zone, helical tape were installed. With insert of such device, the flow pattern changes and stronger swirl flow appears. To find the available energy of unit, entropy generation within both zones were analyzed. In next sections, the modeling procedure was presented and best turbulent model was selected during validation part. After presentation of explanations about utilized grid, outputs in term of temperature and irreversibility have been examined.