Highest Performance Evaluation Criterion Research Articles

Experimental investigation of the effects of graphene nano particle added water fluid on heat transfer in a smooth tube with double inserts

In this study, experimental studies were carried out to increase the performance of heat exchangers. The effects of two techniques (inserts and nanofluid) on thermohydraulic performance was investigated simultaneously. The twisted tape and the helically wire coil were originally created. The two separation ratios (0.1 and 0.05), the eight twisted tape pitch ratios (1.9/2.3/2.7/3 and 2.1/2.4/2.8/3.2), the four coil wire pitch ratios (0.8/1.2/1.5/1.6) and three weight fractions (0.5/0.75/1 %) for graphene-water nanofluids were used with the Reynolds number ranging from 5000 to 26,000. Experiments in which a coiled wire coil and twisted tape are used together have been found to increase both the heat transfer rate and the friction factor compared to experiments using distilled water in an empty tube. According to the results of all experiments, it was found that the highest heat transfer rate and friction factor were obtained with the model containing graphene-water fluid used at a weight fraction of 1 %. The highest performance evaluation criteria is found as 2.5 b y using the graphene-water mixture model with a wall separation ratio of 0.05, a twisted tape ratio of 2.1, a wire coil ratio of 0.8 and a 1 % weight fraction among all combinations.

Investigating the Impact of Flow Profile on Heat Transfer in Nanofluid Flow: A Numerical Study

Assessing the profitability of an energy system requires careful consideration of various factors, including fluid characteristics, geometry shape, and operating conditions. This study investigates the influence of sinusoidal rib shapes, with different space ratios (e/b) ranging from 0 to 1, on heat transfer in nanofluid flow. The channel's upper surface is subjected to a uniform heat flux, employing Al2O3 nanofluid as the working fluid and varying Reynolds numbers from 5000 to 20000. Additionally, the effect of aluminum nanoparticle volume fraction, ranging from 0 to 6%, is analyzed. Simulation results indicate that the performance of the corrugated surface in the channel is significantly influenced by rib shapes and their geometrical parameters. The highest Performance Evaluation Criteria (PEC) index is achieved for ribs with a space ratio (e/b) of 0 at Reynolds number of 5000 and a volume fraction of 6% nanoparticles. Furthermore, the average Nusselt number shows an increasing trend with higher particle volume fraction and Reynolds numbers.

Thermohydraulic and Irreversibility Analyses of Swirl Flows Utilizing Distorted Radial Fins: Entropy Generation and Entransy Dissipation Evaluation

The current investigation analyses the convective heat transfer performance, entropy generation, and entransy evaluation of swirl flows generated by distorted radial fins (DRF). As swirl flows and various vortical structures induce large temperature gradients, second law and entransy analyses are necessary to thoroughly evaluate their true thermodynamic influence on heat transfer enhancement. The results indicated that due to the influence of swirl flows and vortices, all angles of the DRF were capable of inducing intense fluid mixing, thinner thermal boundary layers, and turbulent eddies. It was found that overaggressive swirl flows may hinder local heat transfer performances, by enclosing low-velocity heated fluids within the thermal boundary layers. However, as these overaggressive swirl flows and strong vortices propagate downstream, beneficial fluid mixing was eventuated, favouring heat transport over large regions. In terms of thermal performances, the maximum heat transfer enhancement was exhibited by the α=45° DRF, improving the Nusselt numbers up to 59.3%. Accordingly, the highest performance evaluation criterion (PEC) of 1.269 was obtained by the α=45° DRF at the Reynolds number 2389, attributed to the centrifugal effects of the swirl flows. Optimal entropy generation numbers were also exhibited by the α=45° DRF at the highest studied Reynolds number, reducing total entropy generation by 36.81%. Lower entransy thermal resistances were also accredited to greater DRF angles due to the intense swirl effects. In essence, the study concludes that the effects of swirl flows and vortices significantly enhance heat transfer, whilst reducing both entropy generation and entransy dissipation rates, leading to optimal thermal performances.

Parametric analysis of different Al2O3 nanoparticle shapes and expansion angles for sudden expanded tube regarding the first law of thermodynamics

The thermo-hydraulic performance of Al2O3/H2O nanofluid with different nanoparticle shapes flowing in a sudden expansion tube with variable sudden expansion inclination angles and elliptical dimpled fins with different diameters were numerically investigated. Investigation of variable sudden expansion inclination angles, elliptic dimpled fins, and different nanoparticle shapes together reveals the novelty of this study. The main purpose of this study is to analyse the effect of nanofluid particle shapes, sudden expansion inclination angles, and elliptical dimpled fin on thermo-hydraulic performance for sudden expansion tube. The platelet, cylindrical, and blade nanoparticle shapes of Al2O3 nanoparticle (φ = 1.0 %) were separately mixed into base fluid to obtain working fluid. Numerical studies were carried out under laminar flow regime (500 ≤ Re ≤ 2000). Furthermore, the sudden expansion tube was assumed to have inclination angles with α = 30°, 45°, 60°, and 90°. The results presented that the highest Performance Evaluation Criterion is obtained for the case of DT6 using Al2O3/H2O with platelet nanoparticle shape at Re = 2000. Besides, the highest Nusselt number and Performance Evaluation Criterion were realized at the inclination angle of 45°. The increment rate of Nusselt number and Performance Evaluation Criterion at α = 45° were determined as 8.75 % and 10.52 % compared to α = 30°, respectively. Moreover, elliptical dimpled fins with sized as a = 6 mm and b = 12 mm presented the highest thermo-hydraulic performance, and this condition showed an increment of 153.9 % compared to case of a = 2 mm and b = 4 mm.

Numerical simulation study of flow and heat transfer characteristics of the spiral ribbed tube with elliptical dimples

In this study, a new type of spiral rib-reinforced heat transfer tube with elliptical dimples (referred to as SRTED) is proposed. Numerical simulations were performed to investigate the effect of different geometric parameters on the flow and heat transfer characteristics. The field synergy theory was adopted to reveal the enhanced heat transfer mechanism. The results demonstrated that the flow velocity reaches the maximum at the smallest cross-sectional area in the tube; besides, it tends to decrease with the increase of the cross-sectional area in the tube. The distribution trend of the local heat transfer coefficient (h) of SRTED has a periodic pattern; the average value is 1.41 times that of the plain tube and 1.13 times that of the dimpled tube. In the calculated Re range, the Nu/Nu0 of SRTED varies from 2.84 to 4.46, and f/f 0 varies from 8.98 to 23.51. The overall thermal performance evaluation criterion (PEC) value varies from 1.18 to 1.78, with higher PEC at low flow rates.

Numerical investigation of the effect of cross-section on the hydrothermal and irreversibility features of water/Fe3O4 ferrofluid flow inside a twisted tube in the presence of an external magnetic field effect

This paper studied the heat transfer and entropy generation rate of water-Fe3O4 magnetic nanofluid flow inside three twisted tubes with square, triangular, and elliptical cross-sections at the absence and presence of a magnetic field (MF) effect for Reynolds number (Re) range of 400–800, pitch distance (Ps) range of 25–75 mm as well as the nanoparticle concentration (φ) range of 1%, 2%, and 4%. Based on the results, the increase in Re from 400 to 800 escalated convective heat transfer coefficient (h) by 33.98% (or 4.66%), 23.97% (or 18.46%) and 31.36% (or 20.91%) in the square, triangular, and elliptical twisted tubes, respectively, under the absence (or presence) of the MF. At P=50 mm and φ=2%, the MF improved h by 21–45%, 21–26%, and 0–16% within the Re range of 400–800 for the square, triangular, and elliptical twisted tubes, respectively. Nearly 60% and 50% pressure drop observed as Re escalated from 400 to 800 in the absence and presence of the MF, respectively. The highest performance evaluation criterion (PEC) (i.e. 1.45) and the lowest PEC (i.e. 0.91) were obtained for the square twisted tube at Re=400 and elliptical tube at Re= 800, respectively. The highest and lowest PEC of the square twisted tube (i.e. 1.88 and 1.45) at Re=400 were observed for P=50 mm and φs of 4% and 2%, respectively. In the presence of the MF effect, nearly 37–48% (or 32–35%) increase in the S˙fr (or S˙th) were obtained at Ps of 25–75 mm against the cases without the MF effect.

An innovative approach of alternating magnetic field diversified with different wave types and magnet positions for ferrofluid flow in dimpled tube

The purpose of the present experimental study is to investigate the forced magneto-convection of the Fe3O4/H2O ferrofluid flowing along smooth and dimpled tubes implemented with alternating magnetic field (B = 0.16 T and f = 5 Hz). All experiments were carried out for the working fluids pure water and ferrofluid with 1.0% volume fraction in the laminar flow regime (1131 ≤ Re ≤ 2102). Also, an original perspective of this study is to use of changing magnetic field wave types (Sinus, Triangle, and Square) and positions at different axial distances (x/D = 20, x/D = 40, and x/D = 60). Findings shed light on that the highest convective heat transfer ratio was realized at x/D = 20 location and square wave type for all cases. The average Nusselt number of the dimpled tube subjected to square wave type applied at x/D = 20 is increased by 58.13% compared to smooth tube without magnetic field and using H2O as working fluid. Moreover, when the friction factor was compared for the same conditions, the increment rate has been seen by 85.73%. Although Nusselt number is reached to its highest level using square wave type, the highest Performance Evaluation Criteria is detected around 1.45 using dimpled tube subjected to alternating magnetic field with triangle wave type applied at x/D = 20 for Re = 1131.

Experimental investigation and performance comparison of Fe3O4/water and CoFe2O4/ water ferrofluids in presence of a magnetic field in a cooling system

BackgroundCooling has a great effect on improving equipment performance. One of the effective ways to increase heat transfer is the use of a magnetic field (MF) and magnetic nanoparticles (NPs). Therefore, the type of NPs will be very effective. MethodsThe performance of a cooling system using water/Fe3O4 and water/CoFe2O4 ferrofluids (FFs) under the MF was investigated and compared with water. The effects of different Reynolds numbers (200 to 1000), the MF intensity (0, 400, and 600 G), and a concentration of 0.5 vol% were investigated on heat transfer coefficient, Nusselt number (Nu), friction factor, and performance evaluation criterion (PEC). Significant FindingsSubstituting FF instead of water increased the system's cooling performance so that at Re=400, without MF, Nu of CoFe2O4/water and Fe3O4/water FFs in comparison to water was increased by 10%, and 5.5%, respectively. So, without MF, CoFe2O4 NPs were more effective on heat transfer than the Fe3O4 NPs. In the presence of MF, Fe3O4 NPs were more effective than the CoFe2O4 NPs. So that the Nu of Fe3O4/water compared to water increases by 15% and 21%, respectively using the MF of 400 and 600 G. Generally, for all investigated cases, the highest PEC was obtained at Re=400.

Assessing heat transfer and nanofluid laminar flow in the curved micro-mixers by adopting two-phase model

Micromixers are devices used for mixing fluids at the microscale level and are used for a variety of applications, including chemical reactions, mass transport, and heat transfer processes. In heat transfer processes, micromixers can enhance the heat transfer rate by creating a more homogeneous fluid mixture. In this research, the flow and heat transfer of a nanofluid are simulated in four different geometry of micromixers. The objective is to investigate which one has a better performance compared to a smooth microchannel with regard to the performance evaluation criterion (PEC), which proposes an evaluation with regard to the trade-off between heat transfer and pressure drop. For this purpose, the Ag/water nanofluid is simulated in micromixers of different geometries at Re numbers ranging from 200 to 800 and solid volume fractions (SVF) of 0, 2, and 4% using a finite volume method. The mixture model is adopted to simulate the nanofluid as a two-phase fluid, which is subjected to a laminar flow regime. The micromixers’ walls are exposed to constant heat flux. The effect of increasing the Re number and the SVF is assessed and compared in all geometries, and the results indicate that an increase in both these figures increases the PEC considerably. Moreover, the PEC for each case is assessed and compared, and the best and worst geometry in terms of heat transfer as well as pressure drop are presented. Specifically, in the best case with the highest PEC at the Re = 800 and SVF = 4%, the Nu number is increased by 2.3 fold while its pressure drop is increased by 1.42 fold compared to a smooth microchannel.

Heat Transfer Investigation of Laminar Flow Mixed Convection of Nanofluids in a Uniformly Heated Horizontal Annulus: A Combination of Theoretical-Based and Experimental-Based Models of Thermal Conductivity and Viscosity

This present study is intended for a CFD analysis of hydrodynamic and thermal characteristics of water-based fluid containing TiO2 or CuO nanoparticles flowing in laminar regime in a 3D uniformly heated horizontal annulus utilizing several. Four distinct models have been developed using various combinations (A, B, C and D) of the available theorical-based and experimental-based thermal conductivity and viscosity correlations. A CFD-Fortran code based on the finite volume technique was elaborated for the numerical solution of the mathematical model of the problem. The implications of Grashof number, volume fraction, and type of nanoparticle on isovelocity, isotherms, mean and wall temperatures, Nusselt number, heat transfer coefficient, pressure drop, and thermal performance evaluation criteria are explored using these different models. The results demonstrate that the Nusselt number and heat transfer coefficient of all developed models improve with the addition of nanoparticles. For 2% of nanoparticles’ concentration, the largest enhancement was reached for model D by about 23.5% with respect to the based liquid, while the smallest enhancement was obtained for model B by about 1.16%. The highest Performance Evaluation Criteria (PEC) are attained by employing model D by about 1.263, followed by model C by about 1.074.

Experimental investigation of the effect of mechanical vibration and rotating magnetic field on the hydrothermal performance of water-Fe3O4 ferrofluid inside a rifled tube

Vibration can enhance the hydrothermal performance by disturbing the thermal boundary layer. Also, the magnetic field increases the ferrofluid mixing, thereby enhancing the heat transfer rate. In this study, an experimental analysis of ferrofluid flow inside a rifled tube under the vibration and rotational magnetic field (RMF) effects was conducted by considering different Reynolds numbers (Re), nanoparticle concentrations (φ), and rifled tube pitches (P). In the first stage, the effect of Re and φ on the hydrothermal performance of the system in the absence of the vibration and RMF was explored. In the second stage, the effect of vibration on the performance evaluation criterion (PEC) of the system was investigated. Finally, the RMF effect was considered. Based on the results, the system with P = 5 mm showed the highest PEC in all experiments. The highest PEC without the vibration and RMF effects was obtained as 1.62 for P = 5 mm and φ = 0% at Re = 2000. The highest PEC under the vibration effect (1.28) was also found for Re = 2000 but at φ of 2%, when the highest vibration acceleration (5 m/s2) was applied. Among the RMFs examined, the RMF with the counter clock-wise along with the counter clock-wise fluid flow inside the rifled tube resulted in the highest PEC of 1.62. RMF improved the PEC of the system from 1.28 to 1.62, corresponding to a 21.32% increase, under the vibration.

The first and second law analyses of thermodynamics for CoFe2O4/H2O flow in a sudden expansion tube inserted elliptical dimpled fins

In this paper, thermo-hydraulic performance and entropy generation of the sudden expansion tube with elliptical dimpled fins (DFs) at different directions was computationally performed at different Reynolds (Re) numbers ranging between 100≤Re≤2000 using cobalt ferrite/H2O (CoFe2O4/H2O) nanofluid (NF) (0.0≤φ≤ 2.0). The novelty of this study is to investigate the effect of the elliptical DF and its arrays in the sudden expansion tube in terms of thermodynamics laws (energy and entropy generations). While the sudden expansion ratio (D2/D1) is 2.5, elliptical dimple width (a) and height (b) show variations between 4 and 8 mm. In addition, the distances among elliptical DFs (P) are taken as P = 10, 15, and 20 mm while the diameter (D1) and length (L1) of the inlet tube are D1=8 mm and L1=375 mm. The diameter (D2) and length (L2) of the tube with the sudden expanding and elliptical DFs are D2=20 mm and L2=1125 mm, respectively. The study comprehensively exhibits results of the thermo-hydraulic performance and entropy generation based on numerical data. The results demonstrated that the highest heat transfer rate is recorded by the case of DT3, and DT3 enhances the convective heat transfer rate by 221%, and increases the pressure drop by 17.38% at Re=2000 compared to smooth tube (ST). It was observed that as the Re increases, the performance evaluation criteria (PEC) gradually increases too, and the highest PEC has been obtained for DT3 as 2.11 at Re=2000. The entropy generation (EnG) was also evaluated in terms of both geometric designs and volumetric concentrations. Moreover, the best total EnG was found around 17% for DT3 at the Re=1000 and φ=2.0% compared to ST using H2O. Consequence of the applied analyses, the use of elliptic DFs in a sudden expansion tube enhances the convective heat transfer rate and decreases total EnG.

Thermo-Hydraulic Performance of Pin-Fins in Wavy and Straight Configurations.

Pin-fins configurations have been investigated recently for different engineering applications and, in particular, for a cooling turbine. In the present study, we investigated the performance of three different pin-fins configurations: pin-fins forming a wavy mini-channel, pin-fins forming a straight mini-channel, and a mini-channel without pin-fins considering water as the working fluid. The full Navier–Stokes equations and the energy equation are solved numerically using the finite element technique. Different flow rates are studied, represented by the Reynolds number in the laminar flow regime. The thermo-hydraulic performance of the three configurations is determined by examining the Nusselt number, the pressure drop, and the performance evaluation criterion. Results revealed that pin-fins forming a wavy mini-channel exhibited the highest Nusselt number, the lowest pressure drop, and the highest performance evaluation criterion. This finding is valid for any Reynolds number under investigation.

The developing flow characteristics of water - ethylene glycol mixture based Fe3O4 nanofluids in eccentric annular ducts in low temperature applications

• The effect of nanofluids’ lower viscosity values on heat transfer was investigated. • Annular flow propagation of water-ethylene glycol based Fe 3 O 4 nanofluid was studied. • Maximum heat transfer enhancement was obtained for highest concentration. • Addition of the nanoparticles resulted in delay to reach steady-state conditions. • The performance evaluation criteria, pressure drop and exergy analysis were studied. Natural circulation loops with double pipe heat exchangers at heating and cooling ends have a potential to be used in the refrigeration systems as an alternative to suction line heat exchangers. The heat transfer capability of such natural circulation loops depends on the geometrical parameters as well as thermophysical properties of the working fluid. This study aims to investigate the effect of water-ethylene glycol mixture based Fe 3 O 4 nanofluids (0.01, 0.05 and 0.1 vol.%) on the annular flow propagation and heat transfer in the annuli of double pipe heat exchanger at low pressure side of the refrigeration cycle. In addition to increased non-dimensional velocity values due to the lower viscosity and higher non-dimensional temperature values with expanded temperature gradient, improved heat transfer by nanofluids shows that they can be used as secondary heat transfer fluids at low-pressure side in refrigeration systems. Although the maximum transferred (13.6% improvement compared to base fluid) heat observed for the highest concentration, the nanofluids with smallest concentration has the minimum pressure drop value (25% reduction compared to base fluid) and the highest performance evaluation criteria (PEC) value (PEC = 1.08) with tiny increase in exergy destruction (1.45% compared to base fluid).

IMPACT OF TWISTED DUCTS WITH DIFFERENT TWIST RATIOS ON HEAT TRANSFER AND FLUID CHARACTERISTICS OF NiO/WATER NANOFLUID FLOW UNDER MAGNETIC FIELD EFFECT

Laminar forced convection of NiO/water nanofluid in a twisted square duct has been investigated numerically under the effect of an external magnetic field (B = 0 G, 450 G, and 550 G) in the Reynolds number range of 500 ≤ Re ≤ 2000. Four different twist ratios (D/L = 0.0, 1.0, 1.5, and 2.0) of the square duct have also been examined. The nanoparticle volume fractions (NPVF) of NiO/water nanofluid have been selected between 0.6 vol.% and 2.5 vol.%. The effects of the magnitude of the magnetic field (MF), twist ratio, Reynolds number, and nanoparticle volume fraction on thermohydraulic performance have been examined by using the homogeneous model in the numerical analyses. The results of the numerical computations have been reported with average Nusselt number Nu, pressure loss, average Darcy friction factor f, and performance evaluation criterion (PEC). The highest heat transfer increment by 20% has been achieved at D/L = 2.0 with the highest nanoparticle volume fraction, namely, 2.5 vol.% NiO/water nanofluid, compared to the case of distilled water (DW) flowing in a plain duct (PD). In addition, it was determined that the magnetic field effect increases the convective heat transfer in the twisted duct with D/L = 2.0 up to 35% compared to the cases in the absence of a magnetic field at D/L = 2.0. Among all cases, the highest PEC has been obtained with 2.5 vol.% NiO/water nanofluid flowing in the twisted duct with the twist ratio D/L = 2.0.

A Novel Approach of Heat Rate Enhancement in Rectangular Channels with Thin Porous Layer at the Channel Walls

Heat transfer enhancement is a topic of great interest nowadays due to its different applications in industries. A porous material also known as metallic foam plays a major role in heat enhancement at the expense of pressure drop. The flow in channels demonstrates the usefulness of this technology in heat extraction. In our current study, a porous strip attached to the walls of the channels is proposed as an alternative for heat enhancement. The thickness of the porous strip was varied for different Reynolds numbers. By maintaining a laminar regime and using water as a fluid, we determined an optimum thickness of porous material leading to the highest performance evaluation criterion. In our current study, with the aspect ratio being the porous strip thickness over the channel width, an aspect ratio of 0.2 is found to be the alternative. A 40% increase in heat enhancement is detected in the presence of a porous strip when compared to a clear channel case for a Reynolds number equal to 200, which improves further as the Reynolds number increases accordingly.

Experimental assessment on convection heat transfer characteristics of aqueous magnetite ferrofluid in a rifled tube under a rotating magnetic field

This work is devoted to the experimental evaluation of the impact of a rotating magnetic field (RMF) on the thermal-hydraulic characteristics of laminar forced convection of aqueous magnetite ferrofluid through a rifled tube. The RMF consists of two small permanent magnets that revolve around the rifled tube. Considering the clockwise and counterclockwise directions for the rotation of each magnets, four different arrangements were considered for the RMF. The results were determined for different values of Reynolds number (Re), nanoparticle concentration (φ) and rifling pitch (P). Nusselt number (Nu), friction factor and Performance Evaluation Criterion (PEC) were considered as performance parameters. The analysis of the PEC parameter showed that from an overall hydrothermal point of view, the use of rifled tube and RMF is desirable. In the absence of RMF, the highest PEC was equal to 1.813, which occurred in the case of Re=2000, φ=0% and P=5 mm. In addition, among the cases with RMF, the maximum PEC was equal to 1.601, which belonged to the case of Re=2000, φ=2% and P=5 mm.

Evaluation of Multiple Semi-Twisted Tape Inserts in a Heat Exchanger Pipe Using Al2O3 Nanofluid.

The hydrothermal performance of multiple semi-twisted tape inserts inside a heat exchanger pipe is numerically examined in three-dimensions. This study aims to find the optimum case for having the highest heat transfer enhancement with the lowest friction factor using nanofluid (Al2O3/water). A performance evaluation criterion (PEC) is defined to characterize the performance based on both friction factor and heat transfer. It was found that increasing the number of semi-twisted tapes increases the number of swirl flow streams and leads to an enhancement in the local Nusselt number as well as the friction factor. The average Nusselt number increases from 15.13 to 28.42 and the average friction factor enhances from 0.022 to 0.052 by increasing the number of the semi-twisted tapes from 0 to 4 for the Reynolds number of 1000 for the base fluid. By using four semi-twisted tapes, the average Nusselt number increases from 12.5 to 28.5, while the friction factor reduces from 0.155 to 0.052 when the Reynolds number increases from 250 to 1000 for the base fluid. For the Reynolds number of 1000, the increase in nanofluid concentration from 0 to 3% improves the average Nusselt number and friction factor by 6.41% and 2.29%, respectively. The highest PEC is equal to 1.66 and belongs to the Reynolds number of 750 using four semi-twisted tape inserts with 3% nanoparticles. This work offers instructions to model an advanced design of twisted tape integrated with tubes using multiple semi-twisted tapes, which helps to provide a higher amount of energy demand for solar applications.

CFD simulation of the impact of tip clearance on the hydrothermal performance and entropy generation of a water-cooled pin-fin heat sink

In this paper, the 3D numerical calculations of the water flow inside a pin-fin heat sink (PFHS) to determine the influence of tip clearance on the hydrothermal behavior and frictional & thermal irreversibilities of the fluid flow. The numerical simulations were conducted for five different fin heights (ξ = 1.5 mm to 2.5 mm) considering four different Reynolds numbers (Re=500, 1000, 1500, and 2000). The results demonstrated that the highest heat transfer coefficient is associated with ξ = 2.25 mm due to more uniform vorticity distributions, while the minimum central processing unit (CPU) average temperature is obtained for ξ = 2.5 mm due to the higher heat transfer surface area. The minimum thermal and frictional entropy generation rates are associated with PFHS with ξ = 1.5 mm. However, the PFHS with ξ = 2.25 mm has the highest Performance Evaluation Criterion (PEC) of nearly equal to 1.28 as compared to the other ξs. Besides, the fluid flow from the pin-fin base to the top and vice versa leads to create the vortices especially at the entrance of the PFHSs with tip clearance.

Numerical investigation of the flow and heat transfer in tube with a concave generator

The installation of a vortex generator (VG) on the heat transfer surface is a common enhanced heat transfer technology. The vortex generator is used to induce the vortex to destroy the flow boundary layer, reduce the heat transfer resistance, and improve the efficiency of the heat exchange equipment. In the present study, a numerical investigation is performed to compare the heat transfer capacity of concave VG, convex VG and trapezoidal VG. Since the position of the vortex affects the heat transfer and the vortex generated by concave VG is closer to the wall, concave VG has a more reasonable heat transfer capacity. Flow characteristics and heat transfer enhancement of concave VGs with 65 different geometric parameters have been analyzed in a rectangular channel under steady flow conditions. It is concluded that the concave VG enhances the heat transfer and reduces the resistance loss. It should be indicated that the highest performance evaluation criteria (PEC) and the increment of PEC are 1.93 and 93%, respectively. Moreover, Nusselt number (Nu) has a strong correlation with volume averaged turbulent dissipation rate and wall shear stress and the correlation coefficient is 0.935.