Thermal Entropy Generation Rate Research Articles

Investigation of thermal performance and entropy generation rate of evacuated tube collector solar air heater with inserted baffles and metal foam: A CFD approach

This study investigates the impact of metal foam embedding into evacuated tube collector solar air heaters with inserted baffle (IBMF-ETC-SAH) on the device thermal performance using a computational fluid dynamics (CFD) approach. A three-dimensional model is employed to analyze the system under steady-state conditions. The key finding of this study is that the utilization of metal foam leads to a substantial increase in thermal performance, with an average improvement of approximately 300 %. The investigation examines the influence of metal foam porosity and pores per inch (PPI) on the performance of the IBMF-ETC-SAH across Reynolds numbers ranging from 6000 to 18,000. The results indicate that increasing the PPI from 20 to 40 at a constant porosity of 0.93 has negligible effects on device performance. However, the pressure drop increases by 48 % for a PPI of 40 compared to a PPI of 20. Furthermore, reducing the porosity from 0.95 to 0.9 leads to a 7 % enhancement in thermal performance. The IBMF-ETC-SAH exhibits a significant decrease in the total global entropy generation rate (EGR) compared to the device without metal foam. The maximum value of total EGR for the IB-ETC-SAH case is 2.32 times higher than the corresponding values for the IBMF-ETC-SAH case.

Use of machine learning in predicting heat transfer and entropy generation in a flat plate solar collector with twisted tape turbulator and ferrofluid under the influence of an external uniform magnetic field: A numerical study

The flat-plate solar collector is the most common and widely used model of solar collectors. This model of solar collectors is more popular than other solar collectors due to its easy installation and lower cost. It is possible to further reduce the cost and size of these systems by improving their performance. For this purpose, simultaneous application of external uniform magnetic field (MF), water-Fe3O4 ferrofluid, and twisted tape turbulator along with changing the cross-section of the serpentine absorber tube of a collector is proposed in the present research. The outlet temperature of working fluid, pressure drop of working fluid, the rate of thermal energy transferred to the flowing fluid in the collector along with the entropy production rate in the system due to fluid friction and heat transfer were considered as the performance parameters of the system. The effect of cross-section of absorber tube (circular, square, triangular), Reynolds number (Re = 500, 1000, 1500 and 2000), pitch distance of turbulator (δ = 150 mm, 200 mm and 250 mm), ferrofluid volume fraction (σ = 0, 1 %, 2 % and 4 %) and magnetic field intensity (0, 600, 900 and 1200 Gauss) on these parameters was investigated numerically using the finite volume method. In the investigations conducted in the absence of MF, collectors equipped with circular and triangular absorber tubes showed the best and worst thermal performance, respectively, while the lowest and highest total entropy production rate was associated with collectors equipped with circular and rectangular tubes, respectively. Moreover, the escalation in σ from 0 % to 4 % and under the MF effect showed an increase of 0.14 %, 8.62 %, 0.45 %, and 4.55 % in the outlet temperature, pressure drop, useful heat, and thermal entropy generation rate, respectively. While the frictional entropy generation diminished by 8.73 %. In addition, the intensification of MF intensity from 0 G to 1200 G led to enhance the outlet temperature, useful heat, and pressure drop, by almost 0.0014 %, 3.96 %, and 0.013 %, respectively at three different σ values. While total entropy generation rate reduces by 0.195 % as MF intensity increases from 0 G to 1200G. Meanwhile, the results of neural network modeling were presented as two second-order polynomial functions to predict the total entropy generation rate and useful heat under the MF effects.

A machine learning approach and numerical investigation for intelligent forecasting of entropy generation rate inside a turbulator-inserted solar collector tube

The irreversibility rate analysis on the solar thermal collectors can be used to determine the regions in the geometry with high intensification of the thermal and viscous entropy generation rates (S˙th and S˙fr), thereby improving the geometry. Although the numerical analysis of the hydrothermal characteristics of the solar thermal collector has been widely studied, the entropy generation rate inside a turbulator insert absorber tube is less understood. In addition, there is still the lack of an accurate predictive model for the estimation of the total entropy generation rate inside the turbulator insert absorber tube. This study deals with the determination of S˙th and S˙fr of water flow inside a tube considering three twisted tape inserts with pitch distances (H) of 50 mm, 100 mm, and 150 mm as well as the Re numbers of 500, 1000, and 1500. The results showed that the escalation of H from 50 mm to 150 mm at Re numbers of 500, 1000, and 1500, S˙fr reduces by 15.40%, 24.50%, and 29.67%, respectively. Also, a 90% increase in S˙fr was observed as Re intensified from 500 to 1500. In addition, the increment in Re from 500 to 1500 reduces by 43%, 41%, and 34% at H values of 50 mm, 100 mm, and 150 mm, respectively. Besides, the increase in H from 50 mm to 150 mm escalates S˙th b by 55%, 63%, and 60%, respectively, for Res of 500, 1000, and 1500. Meanwhile, the entropy generation contour plots revealed that the rate of irreversibilities near the blade and tube wall is high indicating a turbulator geometry modification is needed for the lower entropy generation rates. Moreover, the results of the artificial neural network model showed the high accuracy of the introduced correlation for the estimation of the total entropy generation rate in terms of Re and H.

Second-law analysis of nanofluid-based photovoltaic/thermal system modeling and forecasting model based on artificial neural network

The PVT solar collectors can produce the thermal energy and power in a same frame. The improvement of the PVT's efficiency leads to reducing the system size and capital costs. To this end, this paper studied the irreversibilities of the Al2O3Cu/water hybrid nanofluid (NF) in a PVT solar collector considering two single and double serpentine channels (SS and DS). The influences of Re and nanoparticle concentration (φ) on the thermal and frictional entropy generation rates (S˙th and S˙fr) were investigated and the thermal, electrical and overall exergy efficiencies (ψth, ψe, ψov) of the PVT with SS and DS channels were compared and discussed. Based on the results, the DS channel exhibited S˙fr of almost 75 % lower than SS channel due to lower nanofluid inlet velocities and velocity gradients. In addition, S˙th for the DS channel is nearly 65 % lower and 26 % higher than that for the SS channel at Re numbers of 500 and 2000, respectively. Besides, the Re escalation from 500 to 2000 intensifies S˙fr by almost 94 % at different φs in the SS and DS channels. The increase in φ from 0 % to 1 % escalates S˙fr by almost 99.98 % times for two configuration regardless of the Re number. ψth of the DD channel is nearly 14.5 % and 12.77 % higher than that of the SS channel at Res of 500 and 2000, respectively. Besides, ψe of the PVT with the DS channel is 2.36 % higher than that with SS channel at Re=500 at four studied φs. Moreover, the maximum ψe for the PVT with the DS and SS were obtained as 22.29 % and 21.28 %, respectively, which are associated with Re=1500 and φ=0.25 %. Additionally, a predictive model was presented to determine the total entropy generation rate based in the Re and φ as the inputs.

The parametric optimization of a pin-fin heatsink in the presence of ultrasonic vibration based on the second law of thermodynamic

The goal of this numerical study is to investigate the effect of ultrasound waves on the entropy generation aspects in a water-cooled pin-fin heatsink. The effect of the number and locations of ultrasound transducers (32 different scenarios), Re number (500–2000) and frequency of ultrasound waves (F = 15–30 kHz) are investigated on the thermal and frictional entropy generation rates. It was revealed that different configurations with the transducers on two, three, and four walls entail a low thermal entropy generation rate, however, most of which have a high frictional entropy generation rate with the exceptions for case#27 and two other scenarios that bring a moderate frictional entropy generation rate. Therefore, case#27 was selected as the optimum configuration from the second thermodynamic law points of view. For the studied range of Re, the thermal entropy generation ratefor the optimum case is 21–47% lower than that for the base case at F = 30 kHz. While the frictional entropy generation rate for the optimum case escalates to 11–247 folds of the base case for Re numbers of 500–2000. In addition, the escalation of frequency from 15 to 25 kHz reduced the thermal and frictional entropy generation rates by 5.31% and 40.22%, respectively.

Numerical investigation of the effect of perforation inclination angle on the performance of a perforated pin-fin heatsink using two-phase mixture model

The numerical analysis of water/silver nanofluid (NF) flow in a perforated pin-fin heatsink (PFHS) is performed considering the nanoparticle concentration (φ) of 1% and seven different perforation inclination angles (γ). The configuration with the best hydrothermal performance and the lowest entropy generation rate at different values of Reynolds number (Re) is determined using the two-phase mixture technique. According to the results, the γs of 45º and -45º exhibit the highest heat transfer coefficient (h) and the lowest thermal resistance factor and mean CPU temperature at four studied Res of 500, 1000, 1500, and 2000. Consequently, the highest hydrothermal performance evaluation criterion (PEC) was obtained for the heatsink with γ of 45º followed by γ of -45º at Re of 500; the increase in Re reduced the PEC due to the pressure drop. PEC of the heatsink with γ of 45º was almost 11.06–16.63% higher than that with γ of 0º. In addition, γ of -15º and 45º provided the lowest frictional and thermal entropy generation rates (S˙fr and S˙th) at Re of 1500 and then 2000. S˙fr and S˙th for γ of 45º were 1401% and 97% lower than those for γ of 0º at Re=1500.

Entropy generation analysis of turbulent flow in conical tubes with dimples: a numerical study

Minimizing the entropy generation rate is one of the key performance indicators for enhancing the thermal design of heat exchangers. This paper introduces a comprehensive numerical entropy generation analysis of turbulent water flow inside—newly proposed—conical tubes with dimples subjected to a constant heat flux. The effect of different tube diameter ratios (DR = 1, 1.5, 2, 3, and 5) and flow modes (convergent and divergent tube configurations) on the thermal, viscous, and total entropy generation rates is investigated within Reynolds number (Re) range of 3 × 103–40 × 103 using ANSYS-Fluent package. Realizable k-ε (RKE) turbulence model is adopted in this study. A well-validated 3D model was adopted to estimate the dimensionless indices: Bejan number (Be), enhanced entropy generation ratio (Ns,en), and the irreversibility distribution ratio ({phi }_{mathrm{s}}) to characterize the entropy generation performance and to compare conical dimpled tubes to smooth ones. The results showed that total entropy production rate values for dimpled tube geometries are lower than those for the corresponding smooth ones, especially for convergent dimpled tubes. Convergent dimpled tubes with DR in the range of 1.5–3 achieved the lowest total entropy production values over the whole Re range, with an average value of 0.20 W K−1, as compared to an average value of 0.34 W K−1 for the smooth configurations. The average Ns,en values for dimpled convergent tubes with DR = 1.5–3 are 0.46 and 0.80 at Re = 3000 and 40,000, with reductions of 50.54% and 3.61% at both Re values, respectively. The study also showed that the entropy generation analysis could provide an effective tool to highlight the optimal design of tube heat exchangers based on the minimum entropy generation and the enhanced entropy generation ratio.

Effect of inclination angle on the thermal-hydraulic characteristics and entropy generation of Al2O3–water nanofluid for in-tube turbulent flow

PurposeInclination angle has been reported to have an enhancing effect on the thermal-hydraulic characteristics and entropy of some thermal systems. Therefore, this paper aims to numerically investigate the effects of inclination angle, volume concentration and Reynolds number on the thermal and hydraulic characteristics and entropy generation rates of water-based Al2O3 nanofluids through a smooth circular aluminum pipe in a turbulent flow.Design/methodology/approachA constant heat flux of 2,000 Watts is applied to the circular surface of the tube. Reynolds number is varied between 4,000 and 20,000 for different volume concentrations of alumina nanoparticles of 0.5%, 1.0% and 2.0% for tube inclination angles of ±90o, ±60o, ±45o, ±30o and 0o, respectively. The simulation is performed in an ANSYS Fluent environment using the realizable kinetic energy–epsilon turbulent model.FindingsResults show that +45o tube orientation possesses the largest thermal deviations of 0.006% for 0.5% and 1.0% vol. concentrations for Reynolds numbers 4,000 and 12,000. −45o gives a maximum pressure deviation of −0.06% for the same condition. The heat transfer coefficient and pressure drop give maximum deviations of −0.35% and −0.39%, respectively, for 2.0% vol. concentration for Reynolds number of 20,000 and angle ±90o. A 95%–99.8% and 95%–98% increase in the heat transfer and total entropy generation rates, respectively, is observed for 2.0% volume concentration as tube orientation changes from the horizontal position upward or downward.Originality/valueResearch investigating the effect of inclination angle on thermal-hydraulic performance and entropy generation rates in-tube turbulent flow of nanofluid is very scarce in the literature.

Effects of the configuration of the delta winglet longitudinal vortex generators and channel height on flow and heat transfer in minichannels

Minichannels with the delta winglet longitudinal vortex generators (LVGs) is commonly used in the heat exchanger. However, there are few studies on the configuration of LVGs on the opposite walls and the large aspect ratio of the channel in the existing literature. In this paper, five LVGs configurations including arrangement on the opposite walls are considered, and four channel heights (1, 2, 4 and 8 mm) are considered to achieve a wide coverage of the channel aspect ratio (0.125–1). In the Reynolds number range of 3000 to 18,000, the effects of LVGs configuration and channel height on flow and heat transfer are obtained by evaluating parameters such as friction factor, Nusselt number, thermal performance factor (TPF), field synergy and frictional entropy generation rate and analyzing the vortex structure displayed by Ω method. The results show that the LVGs configuration and channel height mainly affect the main vortex behavior downstream of LVGs, but have little effect on the vortex structure near LVGs. The Mixed configuration at H = 1 mm and the CFU1 configuration at H = 2 mm have better heat transfer enhancement performance, and their Nusselt numbers are increased by 28–35% and 27–33%, respectively, compared with smooth channels. In addition, the configuration selection aiming at high overall performance under different conditions is given.

Effect of open-ring pin fin arrangement on the thermal performance and entropy generation of a heat sink cooled by biologically synthesized silver-water nanofluid

Thermal performance of a heat sink having open-ring pin fins (ORPF) and cooled by biologically synthesized silver-water nanofluid is explored in terms of first and second laws of thermodynamics within the present work by focusing on the arrangement of ORPFs, namely the in-line and staggered ones. Influences of four different volumetric fractions of nanoparticles (φ=0–1%) and four different Reynolds numbers (Re=500–2000) on the thermal performance and entropy generation behavior of the nanofluid are numerically evaluated. Outcomes revealed that the convective heat transfer coefficient is improved by up to 71% and 19.3% in the in-line, and 65% and 24.7% in the staggered ORPF configurations, respectively, due to the increment in Re and addition of nanoparticles (at Re=500). Moreover, a decline in the pumping power is noticed by adding φ=1% of nanoparticles at Re=1500, compared to base fluid. The pumping power, on the other hand, is increased by 10.5% at Re=2000 and φ=0% when staggered configuration is considered instead of in-line one. Thermal and frictional entropy generation rates are remarkably reduced by incorporation of nanoparticles. In addition, using staggered ORPF arrangement instead of the in-line one can decrease the frictional and thermal entropy generation rates by up to 17% and 4.4%, respectively, at Re=2000.

The effect of inlet velocity profile on entropy generation and hydrothermal performance of a pin-fin heatsink with biologically prepared silver/water nanofluid coolant: Two-phase mixture model

The prediction of heat transfer characteristics numerically depends on several factors such as the numerical method, the boundary and operating conditions, etc. The present paper investigated the influence of using the uniform and non-uniform inlet velocity profiles on the heat transfer coefficient (h), pumping power (W˙p), temperature uniformity (θ), and thermal and frictional entropy generation rates (S˙th and S˙fr) in a pin-fin heatsink with silver/water nanofluid (NF) coolant. The numerical study is conducted utilizing the two-phase mixture scheme. The results proved that h is almost the same for two inlet profiles with the maximum deviation of 0.2%. θ and W˙p, however, for uniform profile is nearly 2.52–1.86% and 15–26% higher than those for the other case at Reynolds numbers of 500–2000, respectively, indicating a lower temperature uniformity and higher power consumption for uniform velocity profile. The figure of merit was obtained as 1.19–1.36 at Re numbers of 500–2000; indicating the better hydrothermal performance of the non-uniform inlet velocity profile over the other case. In addition, S˙th was almost the same for both cases due to same mean temperature and temperature gradients. However, S˙fr for non-uniform profiles was 4.41–8.84% lower than that for other profile at Reynolds numbers of 500–2000.

Predicting entropy generation of a hybrid nanofluid in microchannel heat sink with porous fins integrated with high concentration photovoltaic module using artificial neural networks

This paper evaluates the characteristic of second law of thermodynamic, including Bejan number and entropy generation for hybrid nanofluid containing graphene-silver nanofluid through a MCHS whit porous fins. Finite-volume technique is utilized to solve the governing equations. To simulate the problem, different porous medium thicknesses, nanoparticle concentrations, and inlet mass flow rates are used while the heat flux remains constant. The minimum values of the frictional and thermal entropy generation are 5 × 10−4 and 6.25 × 10−2, while the maximum values are 3.2 × 10−4 and 9.75 × 10−2. With increasing nanoparticle concentration up to 0.06% wt at constant porous thickness tp=200 µm, frictional entropy generation rises up by 3 × 10−5 and heat transfer rate go up while, thermal entropy generation decreases by 1.5 × 10−2. In addition, by doubling the input mass flow rate and reaching 0.02% at constant nanoparticle concentration (0.06%), thermal entropy generation decreases by 2 × 10−2 while the frictional entropy generation increases by 2.4 × 10−4. The minimum magnitude of Bejan number is 0.994. This show that the irreversibility is derived significantly from thermal entropy generation rate. Finally, an artificial neural network is employed to obtain a model for entropy generation.

The entropy generation analysis and optimization of a water/silver nanofluid flow inside a photovoltaic thermal collector considering plain, ribbed, and porous-ribbed absorber tubes

BackgroundThe application of three (a) plain, (b) ribbed and (c) porous-ribbed absorber tube in a PVT solar collector was numerically investigated from the second law of thermodynamic points of view. MethodsThe 3-D laminar forced convection numerical analysis and optimization were performed for Res of 500–2000 and considering water/silver NF with the nanoparticle concentration ratios of 0–2%. Significant findingsThe results showed that the lowest thermal entropy generation rate (S˙th) is associated with the porous-ribbed configuration which is nearly 323% (or 253%) and 1211% (or 1059%) lower as compared to the plain (or ribbed) absorber tube. The moderate frictional entropy generation rate (S˙fr) was obtained for the porous-ribbed configuration which was 23.45% higher and 14.50% lower than that for the plain and ribbed tubes, respectively. The thermal exergy efficiency for the PVT with the porous-ribbed tube was nearly 50% and 46% higher than that with the plain and ribbed absorber tubes, respectively. However, the electrical exergy efficiency of the PVT with the porous-ribbed tube were superior to the ribbed and plain tubes only at low Res of 500 and 1000, while a reverse trend was obtained for the higher Res.

Thermal performance improvement of a heat-sink using metal foams for better energy storage systems

Newly, energy storage systems and harvesting energy equipment are modern and important topics in engineering problems, but they still face the efficiency problem. Using the advantages of micro-scale flow and metal porous materials can be applied to improve the performance of these systems. So, the present investigation provides a numerical study on a heatsink employing various arrangements of metal foams inside a heat sink (in the direction and against the fluid flow). The influences of diverse fluid flow characteristics, metal foams' permeability and thermophysical properties of the coolant (using CuO-MgO/water nanofluids) on the thermal efficiency of the heatsink are examined. Then, the variations of convective heat transfer coefficient and pressure loss for all variables are investigated, and performance evaluation criteria are defined to distinguish the simultaneous impacts of improving heat exchange against incrementing the pressure loss. It indicates the metal foams located in the direction of the flow perform better than those located against the flow, which increases the performance evaluation criteria (PEC) of the heatsink between 1.18 and 1.55 depending upon the permeability of porous media. Also, using the nanofluids in the presence and absence of metal foams decreases the heatsink performance. Investigating the impacts of volumetric flow rate illustrates that thermal performance increases by increasing the flow rate. In addition, outcomes reveal that although adding metal foams and nano-additives and increasing volumetric flow rate enhance the frictional entropy generation rate, the total amount of entropy generation rate decreases due to more contribution of the thermal entropy generation rate, that reduces by those variables.

Dual Synthetic Jet Actuator and Its Applications—Part IV: Analysis of Heat Dissipation and Entropy Generation of Liquid Cooling with Dual Synthetic Jet Actuator

Increasing heat flux restricts the development of the miniaturization of electronic devices. There is an urgent need for a heat dissipation method that will efficiently cool the chip. This paper presents a novel liquid cooling device based on dual synthetic jets actuator (DSJA) technology. The characteristics of the temperature and velocity field of the device are numerically studied by a three-dimensional coupled heat transfer model. The entropy generation rate caused by heat transfer and fluid friction was studied to analyze the effective work loss and irreversibility of the heat transfer process. When the DSJA is turned on, the temperature of the heat source with a heat flux of 200 W/cm2 is 73.07 ∘C, and the maximum velocity is 24.32 m/s. Compared with the condition when the the DSJA is closed, the temperature decreases by 25.15 ∘C, and the velocity increases by nearly 20 m/s. At this time, the total inlet flow is 1.26 L/min. The larger frictional entropy generation is mainly distributed near the inlet and outlet of the channel and the jet orifice. The higher the velocity is, the more obvious the frictional entropy generation is. Due to the large temperature gradient, there is a large thermal entropy generation rate at the fluid–solid interface.

The application of two-phase model to assess the nanofluid entropy generation in serpentine and double-serpentine heatsinks

The frictional and thermal entropy generation rates (S˙fr and S˙th) of two heatsinks with serpentine (Conf. A) and double serpentine (Conf. B) channels with water/silver Nano fluid (NF) were numerically investigated. The results demonstrated that the augmentation of nanoparticle concentration (φ) from 0% to 1% leads to decrease S˙th by 6.36% and 3.51% in Conf. A and Conf. B, respectively, for Re=500. The escalation of Re from 500 to 2000 increases this percentages to 9.93% and 7.51%, respectively. Also, the increase of Re from 500 to 2000 at a constant φ reduces S˙th by 63% and 68% in Conf. A and Conf. B, respectively, due to enhance the heat dissipation. Moreover, the increasing of φ from 0% to 1% decreases S˙fr by 12.41% in both configurations and at four studied Res due to decreasing the NF velocity. Besides, the escalation of Re from 500-2000 intensifies S˙fr nearly by 62% and 67% in Conf. A and Conf. B, respectively. The entropy contour plots reveal that S˙fr intensifies near the turn regions of the serpentine channels. Also, S˙th is intensifies at the entrance of heatsink channels due to the high temperature gradient at these points. Conversely, S˙fr escalates at the exit of the channels due to intensification of the vortexes formed at the turn parts and along the serpentine channel.

Entropy analysis and parametric optimization on the Nano fluid flow and heat transfer of a pin-fin heatsink with the splitter using the two-phase mixture model

The nanofluid (NF) flow rate and concentration (φ) in a pin-fin heat sink with the splitter (WS) were numerically investigated and the optimum Re and φ values were determined by applying a parametric optimization analysis on the thermal and frictional entropy generation rates (S˙th and S˙fr). To this end, the discretization of the governing equation was done by the second-order upwind scheme, the finite volume method. The results were compared with those obtained for the pin-fin heatsink without the splitter (WOS). Based on the findings, the intensification of Re and φ reduces S˙th by almost 43-45% and 10-13%, respectively, for both WS and WOS heatsinks. An insignificant difference between S˙th of two configurations was obtained (0.37-2.93%). Although S˙fr diminishes by 12% as φ intensifies from 0 to 1%, the escalation of Re from 500 to 2000 intensifies S˙fr by 2312% and 4618% in WS and WOS heatsinks, respectively. In addition, S˙fr of WS heatsink is nearly 43-48% lower than that WOS heatsink due to using splitters. The absence of the splitter in the WOS heatsink leads to a delay in flow separation from the pin-fin bodies and deteriorates the heat dissipation, thereby increasing the CPU temperature and S˙fr intensification.

Multi-criterion optimization of a biologically-produced nanofluid flow inside tubes fitted with coaxial twisted tape based on first and second law viewpoints

The current study deals with a numerical simulation of heat transfer inside enhanced tubes having the coaxial double-twisted tape in the presence of an eco-friendly nanofluid containing graphene nanoplatelets (GNPs). The results unveil that when the particle concentration ( ϕ ) escalates, the thermal entropy generation rate ( S ̇ h ) tends to decline, whereas the heat transfer coefficient enhances. It was found that around 21.6% reduction in the S ̇ h is attained by a decrease of the twisted ratio ( ω ) from 3.5 to 2.5 at the concentration of 0.075% and Reynolds number of 10,000. Adding more GNPs to the water yields an increment in either the pressure drop or the frictional entropy generation. The findings refer to this issue that a multi-criterion optimization to find the best optimum state is required. A genetic algorithm-based optimization with the entropy generation rate, heat transfer, and pressure loss results in the optimum values at Re = 16,995.4, ϕ = 0.093%, and ω = 3.41. • Employing an eco-friendly nanofluid containing graphene nanosheets. • Second law analysis of a novel coaxial double twisted tape inside tubes. • Smaller twisted ratio causes further decrement in total entropy generation rate. • The higher Reynolds numbers weaken the positive effect of the nanofluid. • Multi-criterion optimization is done to determine the best optimum state.

Extensive investigation of the fluid inlet/outlet position effects on the performance of micro pin-fin heatsink through simulation

ABSTRACT One of the most important and simple ways to improve the performance of heatsinks is to make small changes in their designs. In this work, considering a reference structure for a pin-fin heatsink, different modes are examined through simulation by shifting the location of input/output currents. The operating fluid is water, and by raising its Reynolds number (Re) number in the laminar flow range (500–2000), functional parameters such as convective heat transfer coefficient (h), distribution of temperature in fluid/heatsink, thermal resistance, performance evaluation criterion value as well as the thermal and frictional entropy generation rates are evaluated. The significant point of the results was that, the geometry in which the inlet and outlet streams were perpendicular to the top plate and on either side of the diameter, and the geometry in which the input and output were placed on two opposite sides (rectangular widths) showed the highest efficiencies. In contrast, the placement of the input/output currents in one direction of the side plate had the lowest performance. Although boosting the inlet flow velocity increased heat transfer, it augmented the production of thermal and frictional entropy (especially frictional) as well as raised the pumping power. The highest PEC value obtained was equal to 1.131, which belongs to the geometry in which the input and output were placed on two opposite sides (rectangular widths) and occurred at Re = 1000.

Loss Analysis of Cavity Leakage Flow in a Compressor Cascade

Abstract The cavity flow of shrouded stators is quite complex and not yet completely understood, especially for its loss characteristics. In this paper, a matrix of numerical simulations was designed and conducted to investigate the loss characteristics of the cavity flow in a compressor cascade, with a wide range of incidence, two flow coefficients, and three seal clearances. The effect of the skewed boundary layer was introduced to simulate the practical compressor environment as far as possible. The results show that the total pressure loss coefficient is unable to evaluate the cavity leakage loss correctly since it ignores the temperature rise. With the reduction of the flow coefficient, the entropy loss coefficient and thermal entropy generation rate tend to increase, while the change of the viscous entropy generation rate is dependent on the incidence since the high tangential velocity deteriorates the flow of the pressure side at large negative incidence but significantly improves the flow of the suction side as the incidence increases. In addition, there is a significant difference in loss sensitivity to the seal clearance at various incidences. One of the major reasons is the change of pressure difference between the downstream and upstream. And the other major reason is the distribution difference of the high-loss fluid caused by the variation in the passage vortex. These key findings are able to provide insights leading to reducing the compressor loss due to the cavity leakage flow.