Highest Performance Evaluation Criterion Research Articles

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.

Investigation of Thermal-Flow Characteristics of Pipes with Helical Micro-Fins of Variable Height

The results of numerical investigations of heat transfer and pressure drops in a channel with 30° helical micro-fins are presented. The main aim of the analysis is to examine the influence of the height of the micro-fins on the heat-flow characteristics of the channel. For the tested pipe with a diameter of 12 mm, the micro-fin height varies within the range of 0.05–0.40 mm (with 0.05 mm steps), which is equal to 0.4–3.3% of its diameter. The analysis was performed for a turbulent flow, within the range of Reynolds numbers 10,000–100,000. The working fluid is water with an average temperature of 298 K. For each tested geometry, the characteristics of the friction factor f(Re) and the Nusselt number Nu(Re) are shown in the graphs. The highest values of Nusselt numbers and friction factors were obtained for pipes with the micro-fins H = 0.30 mm and H = 0.35 mm. A large discrepancy is observed in the friction factors f(Re) calculated from the theoretical relationships (for the irregular relative roughness values shown in the Moody diagram) and those obtained from the simulations (for pipes with regular roughness formed by micro-fins). The PEC (Performance Evaluation Criteria) heat transfer efficiency analysis of the geometries under study is also presented, taking into account the criterion of the same pumping power. The highest PEC values, reaching 1.25, are obtained for micro-fins with a height of 0.30 mm and 0.35 mm and with Reynolds numbers above 40,000. In general, for all tested geometries and for large Reynolds numbers (above 20,000), the PEC coefficient reaches values greater than 1, while for lower Reynolds numbers (less than 20,000), its values are less than 1.

Performance enhancement and comprehensive experimental comparative study of cold plate cooling of electronic servers using different configurations of mini-channels flow

A comprehensive experimental investigation of water cold plate with different flow configurations for electronic cooling was conducted. Serpentine, parallel and wavy channels cold plate were designed to study the hydrothermal performance of water cold plate for electronic servers cooling. Thermal resistance, pressure drop, Nusselt number, servers’ temperatures and performance evaluation criterion were measured, evaluated and compared to evaluate the hydrothermal performance of the different configurations. The results showed that the serpentine channels cold plate has a better thermal performance than both parallel and wavy channels cold plate. The serpentine channels cold plate yields a lower thermal resistance by (6.02–10.5%), (2.4–8.7%) and a higher average Nusselt number by (7.9–13.9%), (6.3–13.7%) compared to parallel and wavy channels cold plate, respectively. Also, the results indicated that the serpentine channels cold plate has a higher pressure drop and pumping power compared to parallel and wavy channels cold plates. Similarly, the results showed that the wavy channel has better thermal performance than parallel channels due to the larger flow path and the rotational flow occurred. The I-Type inlet and outlet configuration used in the wavy channel cold plate provide a lower pressure drop compared to parallel channels however the large flow path. The performance evaluation criterion showed that the wavy channels cold plate has a higher performance evaluation criterion than serpentine channels due to its lower pressure drop.

Heat Transfer Enhancement of Nanofluid Flow in a Tube Equipped with Rotating Twisted Tape Inserts: A Two-Phase Approach

Al2O3–water nanofluids along with stationary and rotating twisted tape inserts are used to increase the rate of heat transfer in a plain tube. The simulations are conducted through varying the design parameters including angular velocity of twisted tape, Reynolds number and nanofluid volume concentration. It is found that inserting a twisted tape inside a tube substantially increases the heat transfer coefficient and friction factor compared to the plain tube. Compared to the stationary twisted tape, the rotating twisted tape exhibits a great potential to modify the average Nusselt number by about 32.8–39.6% at Reynolds number of 250, depending on the angular velocity. This is attributed to the formation of conical tornado-shape structures in the flow pattern, causing more effective mixing in the flow. By increasing the Reynolds number, the enhancement in the average Nusselt number increases in stationary and decreases in rotating configurations compared with the plain tube. To assess the tradeoff between heat transfer enhancement and pressure loss penalty, the performance evaluation criterion (PEC) is calculated. The results suggest that the highest PEC is obtained at Reynolds number of 250, nanofluid volume concentration of 3% and the highest studied angular velocity.

Nanofluid multi-morphology effect on dual-fluid sinusoidal-wavy grooved absorber tube parabolic trough solar collector performances enhancement based on experimental data

In the present research, forced convection heat transfer features of EG-water base boehmite alumina nanofluid in a dual-fluid Parabolic Trough Solar Collector (PTSC) equipped with corrugated/grooved absorber tube is studied. The control volume method using the SIMPLE algorithm is employed to solve the three-dimensional governing equations numerically. Optimization is carried out to access the optimum nanoparticle morphology considering the highest Performance Evaluation Criteria (PEC). Obtained results indicated that employing the multi-morphology nanofluids can lead to higher PEC. It is shown that the PEC of nanofluid containing cylindrical 20%:80% brick portion is the best among all the studied cases at the investigated Reynolds number. And this, although it is ranked the 2nd in terms of higher Nusselt number and lower friction factor. Although the nanofluid having cylindrical 100%:0% brick portion has the highest average Nusselt number and the nanofluid having cylindrical 0%:100% brick portion has the lowest friction factor. The energy efficiency value of PTSC using multi-morphology boehmite alumina/EG-water nanofluid having a volume fraction of 4% and cylindrical 20%:80% brick portion of non-spherical nanoparticle diameter with Reynolds number of 4000 is the highest value among all the other studied cases, and it is equal to 91.31%.

Effect of Aspect Ratio on Overall Thermal Performance of Forced Convective Heat Transfer Utilizing Turbulent Nanofluid Flow

AbstractThis study is concerned with studying the performance of SiO2–water nanofluid flow through a three-dimensional straight mini-channel with different values of aspect ratio (AR) of (0.5, 1.0, and 1.6) and a fixed hydraulic diameter under a uniform heat flux. The governing equations are developed and solved numerically using the finite volume method for a single-phase flow with standard Kappa-Epsilon (ҡ–ɛ) turbulence model via a user-defined function (UDF) over the Reynolds number (Re) range of (10,000–35,000). Numerical results indicated that the average Nusselt number ratio increases as the Reynolds number and volume concentration of the nanoparticles increase for all values of the channel aspect ratio. The results indicated that the maximum enhancement of the heat transfer coefficient (benefit) achieved is 94.69% at AR = 0.5, along with the lowest increase of pressure drop (penalty) of 13.1%. The highest performance evaluation criterion (PEC) of 1.64 is found at AR = 0.5, Re = 35,000, and 5% concentration.

The effects of variable porosity and cell size on the thermal performance of functionally-graded foams

The thermal performance of several engineering devices, such as heat exchangers, volumetric solar receivers and thermal energy storage systems, is improved by open-cell metal or ceramic foams. Among them functionally-graded foams, through which morphological characteristics are variable, look promising. Heat transfer and pressure drop in a functionally-graded foam, with a uniform heat flux entering one of its sides, are investigated numerically in this paper. Porosity and cell size variable in the direction of the entering heat flux according to different power-law functions are considered; their maximum and minimum values are constrained at the opposite upper and lower sides of the foam. Governing equations, written with reference to a Representative Elementary Volume (REV) of the foam, are solved with a Local Thermal Non Equilibrium (LTNE) model by means of a finite element scheme; the code is validated with experimental data from the literature. The thermal performance of the foam is expressed by a Performance Evaluation Criterion (PEC), referred to the average morphological characteristics of foams with the same surface area. Nusselt numbers, friction factors and Performance Evaluation Criteria (PECs), for different power-laws, are predicted. A 38% higher PEC than in foams with uniform porosity is found in foams with variable-porosity while a 14% larger PEC is exhibited in foams with variable cell size. A 42% increase in PEC is found in foams that account for both variable porosity and cell size.

Experimental study on forced convection heat transfer of a nanofluid in a heat exchanger filled partially porous material

Heat transfer of MgO-EG nanofluid flowing in a double tube heat exchanger with an inner tube containing a partial porous material under laminar and turbulent flows was investigated experimentally. The MgO nanoparticles with 20 nm of size were added to the EG to made the working fluid with a concentration of 0–1 vol%. The copper metal foam was used to create a partial porous medium in the inner tube. The Nusselt number and friction factor were reported as a function of the Reynolds number, using the measurement of the temperatures and pressure drop of flows inside the tube. The accuracy of the empirical results was also validated using the presented theoretical formulas in the literature. The results showed a considerable increase in the Nusselt number by employing the nanofluid and porous media. Using porous media resulted in an increase in the effectiveness of nanoparticles in improving heat transfer. The results indicated a significant increase in the performance evaluation criteria (PEC) parameter. The highest PEC was 1.19 for 1.0% concentration of nanoparticles. The effect of porous material porosity on the EPC was also investigated. The results showed that by decreasing the porosity, the PEC increased. The Nusselt number was very sensitive to porosity variations. Therefore, using partial porous media is suggested to improve the thermal performance of heat exchanger in industrial applications. New correlations are proposed to predict the Nusselt number and friction factor for empty tubes and tubes containing partial porous media.

Experimental investigation of hydrothermal characteristics for flow within a circular tube equipped with twisted conical strip inserts under different alignments

This study attempts to experimentally investigate the thermohydraulic characteristics inside a tube fitted with the staggered and non-staggered twisted conical strip inserts. The inserts are built via stereolithography 3D printer. The influences of geometrical attributes such as alignment type (i.e., staggered or non-staggered) and the pitch ratio as well as the Reynolds number (Re) on the Nusselt number (Nu) and friction factor (f) are examined. Because of flow mixing and vortex creation in the flow path, the tube fitted with the conical inserts has greater Nu and f in comparison with the smooth tube. The Nu and f have larger amounts in the tube having the non-staggered inserts in comparison with that with the staggered ones. With the pitch ratio elevation, the Nu and f diminish, while the elevation in the Re decreases the f and enhances the Nu. Thus, the highest Nusselt number and pressure drop belong to the non-staggered insert with pitch ratio of 2.5 and Reynolds number of 2250 with amounts of 15.15 and 795 Pa, respectively. The highest improvement of the Nusselt number in comparison with the plain tube occurs at Reynolds number of 350 for the non-staggered insert with pitch ratio of 2.5, which is recorded about 133.8%. Besides, the performance evaluation criterion (PEC) is higher than 1 at Re numbers of 820 and less than 820, which shows the proper merit of applying both the staggered and non-staggered inserts in this Re span. For small Re, the non-staggered inserts lead to larger PEC than the staggered ones, while an opposite outcome is observed for the high Reynolds numbers. The highest PEC is about 1.56, which belongs to the non-staggered alignment with pitch ratio of 5 and Reynolds number of 350.

Experimental investigation of the heat transfer coefficient and pressure drop of R1234yf during flow condensation in helically coiled tubes

The condensing flow of R1234yf in a smooth horizontal helically coiled tube is studied experimentally to achieve the heat transfer coefficient and frictional pressure gradient. The helical coil diameter is 95.3 mm with a constant pitch of 16.7 mm and is made by coiling a flexible smooth tube with an internal diameter of 8.2 mm. The effect of vapor quality, mass velocity and saturation temperature on the two-phase heat transfer coefficient and frictional pressure gradient are investigated. The experiments are carried out at saturation temperatures of 20 °C and 30 °C for the mass velocity ranging from 95 kg m−2 s−1 to 285 kg m−2 s−1 and vapor quality from 0.1 to 0.8. In addition, the experimental results of the helically coiled tube are compared with those achieved for a straight smooth tube at the same operating conditions to evaluate the effect of coiling on the condensation heat transfer and frictional pressure gradient. The results suggest that the average heat transfer coefficients of the helically coiled tube are 60–120% higher than those of the straight tubes with the penalty of 46–108% for the frictional pressure gradient. Considering both thermal and hydraulic performances, the Performance Evaluation Criteria (PEC) is defined. The values of PEC are varied between 0.91 and 1.52 while the highest PECs are observed for the intermediate mass velocity of 190 kg m−2 s−1. New correlations to estimate the heat transfer coefficient and frictional pressure gradient are suggested based on the experimental results of the current study.

Effect of Reynolds asymmetry and use of porous media in the counterflow double-pipe heat exchanger for passive heat transfer enhancement

In order to enhance the heat transfer in a double-pipe counterflow heat exchanger, the use of porous media and nanofluid is analyzed. The effect of complete filling of one or both channels of the heat exchanger with porous metal foams on the heat transfer and pumping power has been studied, considering laminar flow with various Reynolds numbers (Re = 100 to 2000) and porous media (Da = 0.1 to 10−4). As a novelty, in order to select proper porous media with an appropriate Reynolds number in each channel, this study focused on the coupling of Re asymmetry and porous medium properties used in the inner and outer channels of the heat exchanger. Flow through porous media is simulated by the non-Darcy law and two-phase mixture model used for the nanofluid flow. Results are presented and investigated in terms of the effectiveness (e-NTU method) and the performance evaluation criterion (PEC). It is shown that the effectiveness could only depict the thermal performance, while the PEC reflects the influences of the porous media on both the pumping power and the heat transfer. Use of porous media in both channels (case D) has led to the highest effectiveness (between 0.6 and 1). In addition, PEC study reveals that the optimal Re values exist for flow in each one of the channels and that case D has the highest PEC (more than 4). To maximize the PEC, for the cases with only one porous channel (B and C), the channel which does not include porous media should have the highest Re, while a low or moderate Re should be selected for the porous channel. However, for case D, Rei should have its highest value, while Reo has an optimal value in the range of 500 to 1000.

Numerical investigation of the effect of water/Al2O3 nanofluid on heat transfer in trapezoidal, sinusoidal and stepped microchannels

PurposeThe purpose of this study is three-dimensional flow and heat transfer investigation of water/Al2O3 nanofluid inside a microchannel with different cross-sections in two-phase mode.Design/methodology/approachThe effect of microchannel walls geometry (trapezoidal, sinusoidal and stepped microchannels) on flow characteristics and also changing circular cross section to trapezoidal cross section in laminar flow at Reynolds numbers of 50, 100, 300 and 600 were investigated. In this study, two-phase water/Al2O3 nanofluid is simulated by the mixture model, and the effect of volume fraction of nanoparticles on performance evaluation criterion (PEC) is studied. The accuracy of obtained results was compared with the experimental and numerical results of other similar papers.FindingsResults show that in flow at lower Reynolds numbers, sinusoidal walls create a pressure drop in pure water flow which improves heat transfer to obtain PEC < 1. However, in sinusoidal and stepped microchannel with higher Reynolds numbers, PEC > 1. Results showed that the stepped microchannel had higher pressure drop, better thermal performance and higher PEC than other microchannels.Originality/valueReview of previous studies showed that existing papers have not compared and investigated nanofluid in a two-phase mode in inhomogeneous circular, stepped and sinusoidal cross and trapezoidal cross-sections by considering the effect of changing channel shape, which is the aim of the present paper.

Experimental study of the heat transfer coefficient and pressure drop of R1234yf condensing flow in flattened smooth tubes

The present study investigates the heat transfer coefficient and pressure drop of refrigerant R1234yf condensing flow in flattened tubes. Three tubes with internal heights of 6.2, 5.1 and 2.8 mm (aspect ratios of 1.5, 2 and 4), made by flattening 8.2 mm inner diameter round tubes, were used. The tests were conducted at a mass velocity range of 95–380 kg m−2 s−1 and vapor quality range of 0.15–0.8. The effects of tubes flattening, mass velocity and vapor quality on heat transfer coefficient and pressure drop were studied. The results indicated that using flattened tubes increases the heat transfer coefficient and pressure drop. However, at low mass velocities, flattening does not have a significant effect on the heat transfer coefficient. Based on the results, two correlations were proposed to predict the heat transfer coefficient and pressure drop in flattened tubes by modifying the existing correlations in the literature. The performance of flattened tubes was evaluated by defining the performance evaluation criteria (PEC), considering both heat transfer enhancement and pressure drop increase at different working conditions. The highest PEC was observed for the tube with an aspect ratio of 4 at the mass velocity of 380 kg m−2 s−1.

3D numerical analysis of atmospheric-aerosol/carbon-black nanofluid flow within a solar air heater located in Shiraz, Iran

Purpose In this study, the effects of using corrugated absorber plate (instead of flat plate) and also using aerosol/carbon-black nanofluid (instead of air) on heat transfer and turbulent flow characteristics in solar collectors were numerically investigated. Design/methodology/approach The 3D continuity, momentum and energy equation were solved by finite volume and SIMPLE algorithm. As a result, the corrugated absorber plate was inspected in the case of triangle, rectangle and sinuous with the wave length of 1 mm and wave amplitude of 3 mm in turbulent flow regime and Reynolds number between 2,500 and 4,000. Choosing the proper geometry was carried out based on the best performance evaluation criteria (PEC) and increasing the air temperature from collector inlet to outlet. Findings The results revealed that for all times of the year the highest PEC was obtained for corrugated Sinusoidal model; however, the highest temperature increase from inlet to outlet was obtained for rectangular corrugated model. In addition, the results indicated that in sinusoidal model, the nanoparticles volume fractions increase leads to heat performance coefficient increase and the best heat performance conditions were attained in volume fraction of 0.1 per cent and Reynolds number of 4,000 for both six months period. In model with rectangular corrugated plate, usage of nanofluid in all range of Reynolds numbers leads to reduction of outlet temperature. Originality/value The effect of some nanoparticles on heat transfer using thermal– hydraulic performances in heat exchangers has been assessed, but the effects of atmospheric aerosol-based nanofluid using carbon-black nanoparticles (CBNPs) on the heat transfer in corrugated heat sink solar collectors by 3D numerical modeling has not been yet investigated. In present study, usage of CBNPs with different volume fractions in range of 0 to 0.1 per cent in turbulent regime of fluid flow is analyzed. Furthermore, in this paper, besides the effects of using CBNPs, a solar absorber located in Shiraz, as one of the best solar irradiation receiver cities in Iran is evaluated.

Exergy and thermal assessment of a Novel system utilizing flat plate collector with the application of nanofluid in porous media at a constant magnetic field

A Novel system is introduced which utilizes Fe3O4/Water nanofluid in porous media at a constant magnetic field in a Flat Plate Collector (FPC) absorber tubes. The Fe3O4/Water nanofluid was used in 1% and 2% weight concentration flowing inside porous media with 0.8 porosity at constant magnetic fields of 200 and 400 G. The Novel system is studied by exergy and thermal analysis means and is compared to a Rival system which is a regular FPC that uses water as working fluid and porous foam inside its tubes. A new evaluation approach has been adopted which assesses the thermal effectiveness, exergy, and energy efficiencies in a variety of working conditions. Furthermore, a novel parameter, which is regarded as Feasibility Indicator Parameter (FIP), is introduced which represents the performance of the solar collector. The results reveal that the Novel system average Nusselt number is 1.36 times the average Nusselt number of the Rival system. Moreover, the overall heat loss coefficient is reduced to 3.149 W/m2K in comparison to 3.4 W/m2K in the Rival FPC. Additionally, the thermal efficiency of the Novel system has been demonstrated to rise to 83.97% compared with 83.63% efficiency in the Rival system. The absorbed heat flux, feasibility factor or Performance Evaluation Criterion (PEC), and the heat transfer coefficient enhancement of the Novel system are 1.015, 1.32, and 1.52 concerning the Rival system, respectively. Furthermore, the novel assessing parameter, FIP value is higher than unity for the Novel system with 200 G constant magnetic field and 1% and 2% wt nanoparticle. However, the required pumping power in the Novel system increases considerably. Hence, the relative exergy efficiency of the Novel system to the Rival system is below the value one. Although the Rival FPC system has a higher exergy efficiency than the Novel system, however, the Novel system provides better thermal performance and higher PEC and FIP than the Rival system. Thus, it can be used wherever the availability of space and higher thermal efficiency are crucial.

Performance assessment of an innovative exhaust air energy recovery system based on the PV/T-assisted thermal wheel

This research work aims to assess the performance of an innovative exhaust air energy recovery system consisting of a building integrated photovoltaic/thermal (BIPV/T) and a thermal wheel (TW). This BIPV/T-TW system has two main operating modes, namely winter and summer. The exhaust air is used for pre-heating and pre-cooling the ambient fresh air through a TW in the winter and summer modes of operation, respectively. Besides, the pre-heated fresh air (in the winter mode) and the heated exhaust air (in the summer mode) is used for reducing the temperature of PV panels and consequently improving their electrical efficiency. The electrical and thermal performance of the system is calculated and compared with those of the conventional BIPV/T and TW systems. Besides, a performance evaluation criterion (PEC) is defined in this study to investigate the overall performance of the studied systems. The results shows that the BIPV/T-TW system has higher PEC than the BIPV/T and TW systems. Furthermore, the effect of various important parameters on the yearly average PEC of the BIPV/T-TW system is analyzed. The economic assessment of BIPV/T-TW represents that the return time of investment according to simple payback (SP) and net present value (NPV) is a reasonable value.

Turbulent thermal, fluid flow and thermodynamic characteristics in a plain tube fitted with overlapped multiple twisted tapes

Turbulent thermal–hydraulic characteristics in a plain tube by using overlapped multiple twisted tapes (MTTs) with counter large/small combinations (C-L/S-C) were experimentally investigated. Reynolds number (Re) ranged from 5800 to 19200, tape number changed from 3 to 5 and overlapped twisted ratios (P1:P2) were 0.74 to 2.97. By using air as working fluid, heat transfer tests were performed under the constant heat flux conditions and pressure drop experiments were conducted at the isothermal conditions. Nusselt number (Nu) and friction factor (f), overall thermal performance evaluation criterion (PEC), entropy generation and entransy dissipation in the MTTs were compared. The results show that Nu and f increase with increasing tape number and decreasing overlapped twisted ratio. The experimental results demonstrates that both increasing tape number and decreasing overlapped twisted ratio result in the reduction of entropy generation due to heat transfer and the rise of entropy generation due to friction resistance. It is found that the indicator of equivalent temperature difference, reflecting entransy dissipation and heat transfer ability, has the same sequence to that of the heat transfer behaviors. In the scope of this study, both the maximum Nu and the largest increased f are obtained in the 5MTTs with values respectively about 98.4% higher and 9.13 times than that in the plain tube while the highest PEC is found in the 3MTTs with P1:P2=0.74 with a value of being about 1.08. The least total entropy generation is gained by the 3MTTs with P1:P2=0.74 at Re=17090 and the smallest entransy dissipation per unit energy is achieved by the 5MTTs under Re=17294. Furthermore, thermal–hydraulic empirical correlations with deviations of ±5% are developed and comparisons with previous studies are also conducted.