Heat Transfer Enhancement Factor Research Articles

Experimental study of the hydrothermal characteristics of copper tube in the presence of the multi-string and multi-strip magnetic turbulator

Electromagnetic vibration (EMV) is a novel active method to enhance heat transfer in heat exchangers. This method installs a magnetic turbulator vibrating at a specific frequency inside a tube. Recent studies have used a single turbulator inside the tube. This paper is the first exploration of the effect of using multiple turbulator on hydrothermal parameters. In this regard, a turbulator with strip or string geometries with three different arrangements of double, triple, and quadruple turbulators was placed inside the tube to investigate its effect on heat transfer and pressure drop. Experiments were performed for the Reynolds number range from 500 to 1500. The results indicated the significant effect of the geometry and the magnetic turbulator arrangement on hydrothermal parameters. The highest heat transfer and thermal enhancement factor (TEF) were observed in a quadruple strip turbulator. In the presence of this turbulator, heat transfer was up to 3.02 times that of the plain tube, and TEF was 2.01.

Experimental investigation of the effects of metal oxides and nucleating agents on nano-emulsions heat transfer performance in mini-channels

Recently, thermal conductivity, even and stable suspension of particles, and agglomeration of particles have been studied comprehensively. These properties are especially important parameters for nanofluids in small-size heat transfer devices like mini-channels, which are prone to block due to the small size of the channels. In the present study, a method was proposed to increase heat transfer using composite nano-phase change emulsions (NPCEs) slurries, utilizing the latent heat of PCM during melting. In this experimental study, a series of composite nano-phase change emulsions (NPCEs) with high thermal conductivity, high thermal storage, and low supercooling were developed and prepared from hexadecane, octadecane, hexadecanol, octadecanol, and nano-Al2O3. Heat transfer experiments were conducted in a mini-channels using the prepared NPCEs as working fluids. The effects of the nanoparticle concentration, NPCE type, and flow rate on multiple parameters, including the heat transfer coefficient, flow resistance, and heat transfer performance of composite NPCEs were investigated. As a result, the heat transfer experiments demonstrated that the proposed NPCEs as heat transfer fluids could enhance the heat transfer performance in mini-channels in comparison with deionized water. Compared with deionized water, the convective heat transfer coefficient was increased by a maximum of 1.28 %, 4.07 %, and 4.19 % with an octadecane concentration of 5 wt%, 10 wt%, and 20 wt%, respectively. The nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3 achieved a maximum increase of 7.01 % in the heat transfer enhancement factor. In addition, the nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3, in addition to 1.25 wt% hybrid nucleating agents allowed a maximum increase of 26.49 % in the heat transfer enhancement factor. The obtained results and performed analyses demonstrate that metal particle-composite alcohol hybrid nanofluid has high stability, high thermal conductivity, low subcooling, high durability, and a convenient preparation method, and is an appropriate choice as a working fluid, especially in small-scale heat transfer devices.

Effect of meniscus curvature on phase-change performance during capillary-enhanced filmwise condensation in porous media

Sustainably enhancing condensation heat transfer performance is a major challenge in thermal management and energy systems, since typical condensation enhancement methods (i.e., dropwise condensation with low surface energy coatings) have limited lifetime/durability, restricted compatibility with working fluids, and sustainability concerns due to the coating composition (e.g., fluorinated compounds). The robust and scalable capillary-enhanced filmwise condensation mode presented in this work demonstrates high heat transfer coefficients for water and low surface tension liquids condensing in a porous wick. Thin porous wicks offer the highest enhancements in heat transfer, however such thin porous wicks have thickness-dependent permeability, and the effective liquid thickness of the wick depends on the shape of the liquid-vapor interface. In this study, we leverage a spatially-discretized porous media model to characterize the effect of the wick thickness on condensation heat transfer performance. The model uses a spatially-varying permeability that depends on the local liquid-vapor interface shape/curvature and the resulting effective wick thickness. We apply this model to investigate the correlation between the heat transfer enhancement and various geometric factors, which enables the design of optimal porous structures for relevant phase-change application. We also predict favorable enhancement in condensation performance with a few common hydrocarbon and fluorocarbon fluid refrigerants. This study provides fundamental insight into the effects of the shape of the liquid-vapor interface on the phase-change performance in the capillary-enhanced filmwise condensation mode.

An Experimental Study of Thermohydraulic Performance of Solar Air Heater Having Multiple Open Trapezoidal Rib Roughnesses

ABSTRACT In the present experimental study, heat transfer enhancement and friction factor penalty of a solar air heater (SAH) with newly designed artificial roughness attached to the lower side of the absorber plate have been investigated. Multiple open trapezoidal ribs artificial roughness has been designed to take the advantages of secondary flows concerning heat transfer enhancement. Multiple open trapezoidal ribs roughness has been designed considering the following parameters: relative limb length (L/w), angle of attack (α), relative rib height (e/Dh), relative width ratio (W/w), and relative roughness pitch (p/e) having their respective ranges of 0.2–1.0, 0.023–0.054, 6–12, 6, and 60°. The other operating parameter encompassed in the study is the flow Reynolds number, which varied from 2000–16000. The results of the experiment shows that highest augmentation in the Nusselt number (Nu) is observed to be 6.05 times higher with respect to the smooth absorber plate at p/e = 10, L/w = 0.8 and e/Dh = 0.043. In addition, thermohydraulic performance has been evaluated, which is found to be in the range of 3.24–3.79.

Ultrasonically-enhanced convective heat transfer: Evidence of a relationship with the thermal boundary layer

The influence of ultrasound on convective heat transfer depending on the flow regime is investigated in this study, using an experimental setup consisting of a rectangular channel with a heating plate on one side and an ultrasonic transducer opposite it on the other. In order to understand the physical phenomena taking place, this work was based on two approaches. The first one focuses on hydrodynamic analysis using Particle Image Velocimetry, focusing specifically on the influence of ultrasound on velocity fields. The second approach aims to determine the convective heat transfer coefficient and Nusselt number, by measuring fluid and heating plate temperatures. Experimental results show that 25 kHz ultrasound allows heat transfer to be enhanced over the Reynolds number range studied, from a laminar regime (Re = 890) to a turbulent regime (Re = 14500), as the acoustic cavitation induce disturbances within the thermal boundary layer. The heat transfer enhancement factor decreases as the Reynolds number increases, until an asymptote is reached in the turbulent regime. For 2 MHz ultrasound, the acoustic streaming generated allows to improve convective effects within the flow, consequently enhancing heat transfer. The heat transfer enhancement factor decreases as the Reynolds number increases up to 7500, at which point 2 MHz ultrasound no longer induces heat transfer enhancement. Finally, ultrasonically induced heat transfer enhancement has been analyzed in terms of the initial thermal boundary layer thickness for silent conditions. The overall results of this study demonstrate the existence of a strong relationship between heat transfer enhancement and initial thermal boundary layer thickness. The thicker the silent initial thermal boundary layer, the greater the heat transfer enhancement.

Predicting heat transfer and flow features of supercritical CO2 in printed circuit heat exchangers with novel wavy minichannels

Printed circuit heat exchangers (PCHEs) are unrivaled choices for heat exchange purposes in power cycles working with natural refrigerants such as carbon dioxide (CO2). Minichannels play a major role in heat transferring in these devices. They take up more than 95% of PCHEs volume; therefore, increasing the performance or compactness of PCHEs will be realized, provided that novel designs are adopted for the minichannels. In this study, the investigation of non-uniform wavy minichannels applying supercritical carbon dioxide (SCO2) as working fluid is carried out. To fulfill defined goals, a 3D conjugated model is developed to analyze the performance of novel geometries in both cooling and heating modes by considering the variations of SCO2's thermophysical properties. The results demonstrate that the proposed structures for the wavy minichannels create an upsurge in the heat transfer coefficient under the given operating conditions. It is explored that replacing the conventional wavy minichannel with the cases having short wavelengths at the upstream can increase the heat transfer enhancement factor from 36.1 to 51.1% in the cooling mode and from 40.8 to 56.5% in the heating mode. Furthermore, the larger the wave amplitudes at the upstream are, the more superiority it takes. Compared to all the proposed structures, the case with the largest wave amplitude and the smallest wavelength at the upstream attains the best overall performance. This structure of wavy minichannel can be utilized to manufacture highly efficient PCHEs with the capability of improving the overall performance index more than 1.5 times compared to the conventional wavy minichannel.

Effects of trapezoidal and twisted trapezoidal tapes on turbulent heat transfer in tubes

Trapezoidal and twisted trapezoidal tape elements which are used for heat transfer enhancement in tubes are experimentally investigated. The effects of the tapes on heat transfer, pressure drop, and overall performance factor are analyzed. Three different thread heights are used in the experiments, which are obtained by dividing the tape width by 4, 6, and 10. Pitch length is kept constant for the trapezoidal tapes, while the constant twisting ratio is used for the twisted trapezoidal ones. Tapes were held in the tube with no wall contact by stretching mechanically. Experiments were conducted under constant heat flux boundary condition and for Reynolds number interval of 5000–25,000, targeting turbulent flow at which overall performance factor values reported in literature are relatively small comparing to laminar flow. The experimental results show that trapezoidal tape elements enhance heat transfer and increase pressure drop with changing rates depending on thread heights. Overall performance factor in the tested interval is detected over unity for two twisted trapezoidal inserts, making them significant alternatives comparing to the literature. The best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height, while Nu is 90.52 and friction factor is 0.12. A maximum Nu of 118.46 is achieved with the proposed twisted trapezoidal tape, which is 1.9 times higher than the plain tube case and 1.57 times higher than the conventional twisted tape case. Since the twisted trapezoidal tapes perform better than the plain trapezoidal ones, it is concluded that the combination of the twisted and trapezoidal forms may be favorable in terms of heat transfer enhancement and overall performance factor regarding combination of swirl flow effect and periodical flow disturbance effect.

A Comprehensive Review and Comparative Study of Various Rib Configurations Used in Rectangular Duct for Heat Transfer Enhancement

This research study provides a comprehensive review of various heat transfer enhancement techniques that are being used by various researchers for enhancing heat transfer rate and overall thermal performance factor of the heat exchangers. Based on the present review study, it can be concluded that intensive study has been carried out by various researchers to determine the effect of various rib geometries on the heat transfer enhancement and friction factor characteristics of rectangular duct. The enhancement in the heat transfer rate is obtained by introducing the blockages or restrictions in the fluid flow path by providing various rib turbulator configurations. This research paper puts highlights on the various heat transfer enhancement techniques are used for increasing overall thermal performance factor in the heat exchangers. Introduction of ribs as flow disturber results not only into increase in heat transfer surface area but also into increased turbulence, recirculation of working media, viscous effect and secondary flow that results into swirl. The interruptions or blockages created by ribs provided in the flow channel results into increased pressure drop and disruptions in boundary layer development. Provision of ribs also causes secondary flow which ensures enhanced thermal contact between working fluid and surfaces of the flow channel. This results into increase in heat transfer coefficient and thermal performance of heat exchanger respectively.

Numerical analysis on heat transfer enhancement of Al2O3 nanofluid in the flattened conically coiled tubes

AbstractEnhancement in the heat transfer of the working fluid flowing inside the tube is achieved by modification of tubes and changing the direction of fluid flow. For this purpose, the straight tube can be changed into a conical coil tube, thereby, heat transfer is increased due to the centrifugal effect caused by the curvature of the tube. Furthermore, the circular cross‐section of the tube is modified into an oblong shape for more heat transfer enhancement. In this study, the numerical heat transfer and pressure drop characteristics of Al2O3–water nanofluids in the flattened conically coiled tube are studied. The effect of the aspect ratio, cone angle, coil pitch, and solid volume fraction of nanofluids on heat transfer and pressure drop are examined. It is found that the heat transfer coefficient and pressure drop increase with the rise of aspect ratio, Reynolds number, and solid volume fraction of nanofluid, while it decreases with the increment of cone angles and coil pitches of conical coil tube. Furthermore, the effect of aspect ratio on the heat transfer enhancement factor, pressure drop ratio, and thermal performance index for Al2O3–water nanofluid are presented.

Experimental study of turbulent flow heat transfer and pressure loss over surfaces with dense micro-depth dimples under viscous sublayer

Presented are experimentally-measured heat transfer and pressure loss characteristics for dimpled surfaces, placed along one surface of a channel, with different ratios of dimple depth to channel height δ/H, and for Reynolds numbers ranging from 10,000 to 70,000. With the same relative dimple spacing to dimple print diameter, and the same ratio of dimple depth to dimple print diameter of δ/d = 0.20, experimental results for arrays of spherical indentation dimples with micro depths of δ = 0.6 and 1.0 mm (δ/H = 0.03 and 0.05) are compared with thermal characteristics for arrays of larger dimples with δ = 4.0 mm and δ/H = 0.20. Local surface heat transfer distributions are measured using transient liquid crystal thermography. Additional steady-state experiments are also employed to provide overall heat transfer enhancement and friction factor augmentation values. Results indicate that Nusselt number ratios and friction factor ratios of a turbulent flow in the channel are dependent upon the ratio of dimple micro depth to channel height, which are different findings from previous studies with larger-depth dimples in the cooling channels. As such, the present data for micro-depth dimples with ratios of dimple depth to channel height δ/H of 0.03 and 0.05 are firstly provided. The importance of a critical Reynolds number is demonstrated by the results, which is determined when the ratio of viscous sublayer layer thickness to the dimple depth is equal to unity, and has been ignored in the previous dimpled channel heat transfer design. Nusselt number enhancements for the micro dimples are generally lower at lower Reynolds numbers, but increase as the Reynolds number becomes larger and approaches the critical Reynolds number. Nusselt number enhancements are then approximately invariant with Reynolds number, when Reynolds number then exceeds the critical value.

Enhanced heat transfer mechanism and flow topology of a channel contained with semi-circular hinged V-shaped baffles

The current work presents a numerical analysis of heat transfer mechanism and flow topology of channels with V-shaped baffles (VB) and semi-circular hinged V-shaped baffles (SH-VB). The vital purpose of applying SH-VB roughened channels is to induce combined vortex and impinging jets flows and reduce the pressure drop in the channel as compared to the use of VB. The effects of geometric parameters: pitch ratios (PR = 0.5, 1.0, 1.5 and 2.0), blockage ratios (BR = 0.1, 0.15 and 0.2), semi-circular hinge angles (β = 5°, 10° and 15°) were studied in the turbulent region (3000 ≤ Re ≤ 19,000). The study was performed at constant aspect ratio (AR) of 10.0 and V-baffle attack angle of α = 45°. The numerical results showed that the decrease of PR and increase of BR resulted in an increase of heat transfer enhancement and pressure drop. The maximum heat transfer enhancement of 5.57 times over the smooth rectangular channel was found at PR = 0.5, BR = 0.2, β = 10°, and Re = 3000. The numerical results also indicated that SH-VB gave higher heat transfer and thermal enhancement factors (TEF) and lower pressure losses than VB. SH-VB provided higher Nu/Nu0, lower f/f0 and higher TEF than VB by around 1.73–8.98%, 5.41–26.84% and 4.51–15.22%, respectively depending on Re, BR, PR and β. The optimum parameters of SH-VB which gave the maximum TEF at 2.6, were BR = 0.10, PR = 0.5 and β = 10° at Re = 3000.

Structural Optimization of Self-Supporting Rectangular Converging-Diverging Tube Heat Exchanger

A three-dimensional numerical investigation of turbulent heat transfer and fluid flow characteristics of the new heat exchanger and self-support of a rectangular converging-diverging (SS-RCD) tube bundle heat exchanger with different inserts was performed. The values of the Reynolds number varied from 27,900 to 41,900. The baseline case (without an insert) was compared with two enhanced configurations: one circular hole in the baffle plate (one-circle case) and a rectangular hole in the baffle plate (one-rectangle case). Compared with the baseline case, the airside Nusselt number (Nu) of the enhanced cases improved by 39.6~48.0% and 36.2~40.2% and had an associated friction factor (f) penalty increase of 53.9–66.7% and 60.7–77.8%, respectively. The baseline case was compared with three enhanced configurations: one-circle case, two-circle case, and three-circle case baffle plate. Compared with the baseline case, Nu of the enhanced cases improved by 39.6–48.0%, 36.2–45.4%, and 35.0–44.2%, with f penalty increases of 53.9–66.7%, 44.9–60.0%, and 43.8–60.0%, respectively. The overall performance was conducted by heat transfer enhancement factor (η). It was found that the one circle case obtained the best overall performance. The numerical results were analyzed from the viewpoint of the field synergy principle. It was found that the reduction in the average intersection angle between the velocity vector and the temperature gradient (θ) was one of the essential factors influencing heat transfer performance.

Effects of regular triangular prisms on thermal and hydraulic characteristics in a minichannel heat sink

Micro/mini-channel heat sinks (MCHSs) are currently projected as an advance cooling solution for heat dissipation in the high heat flux devices. Triangular prism (TP) has a special effect on forced-convection heat transfer due to the specificity of its flow separation. To further improve the thermal performance in MCHS, this study explores the effects of regular TPs on thermal and hydraulic characteristics in a minichannel heat sink under inlet Reynolds number (Re) of 221–884 using numerical simulation. The orientation arrangement and height (Hp = 0.5–2 mm) of TP, as well as the offset value (S = 0–1.2 mm) between two adjacent TPs in the width direction of the minichannel are selected as studied parameters. The results show that the minichannel with backward triangular prism (MC-BTP, apex of TP points to outlet) performs the best heating wall temperature uniformity, highest Nusselt number (Nu) and largest friction coefficient (f). The MC-BTP also presents the greatest comprehensive performance evaluation criterion (PEC) at most of operation condition, especially at high Re. For MC-BTP, with the increase in Hp, the f increases, but the Nu and PEC increase at first and then decrease; moreover, increasing S results in the f, Nu and PEC declining. Overall, the MC-BTP with Hp = 1 mm and S = 0 yields highest PEC at most of operation condition. The friction coefficient ratio, heat transfer enhancement factor and PEC in this minichannel are in ranges of 1.45–2.22, 1.39–2.33 and 1.23–1.79, respectively. Also the maximum PEC value (1.79) is obtained at Re = 884.

Effects of central cut on performance intensification of counter-flow integral heat sinks

To enhance the thermal management of different miniature systems, a counter-current integral minichannel heat sink with a central cut is proposed and investigated through a numerical approach. Six different lengths of the central cut are adopted, and their performances are estimated at various Reynolds numbers ranging from 100 to 900. Obtained results depict that, not only does the central cut provides fluid exchange between adjacent minichannels, but it also leads to generating several vortices. These consequences lead to breaking down and redeveloping the boundary layers. Drawn conclusions imply that short length of the central cut (a = 5 mm) does not bring any noticeable improvement, but by adopting medium values for this factor (a = 10 and 15, and 20 mm), the performance index is remarkably improved up to 69.2%. Outcomes reveal that there is an optimal central cut length, i.e., a = 15 mm, at which the heat transfer enhancement factor adopts the highest value (Nu ratio = 1.92), and also the junction surface achieves the best temperature uniformity (∆Tmax.b = 2.08 K). However, with the further increase of the central cut length, the thermal performance declines, and the junction surface temperature gets an increasing trend.

Multi-objective optimization of thermal-hydraulic performance in a microchannel heat sink with offset ribs using the fuzzy grey approach

The effect of four geometrical parameters (width of the offset ribs, depth of the offset ribs, incline angle of the offset ribs, and distance between the adjacent offset ribs) on heat transfer enhancement, friction loss, and performance evaluation criteria is analyzed on the basis of the orthogonal experimental design (OED) method and fuzzy grey relational analysis (FGRA). Results of FGRA reveals that depth is the most influential factor in heat transfer enhancement, width affects the friction loss more than others, whereas incline angle is the most significant factor in the performance evaluation criteria (PEC) of microchannel heat sink (MHS). A series of S-shaped staggered triangular offset ribs (SSTOR) is proposed to strengthen the performance of MHS based on the results of FGRA. The MHSs with novel offset ribs are studied at different Reynolds numbers from 100 to 1000 to find the optimal offset ribs. The average Nusselt number, pressure drop, PEC, and maximum temperature at the bottom wall of the MHS with five different novel offset ribs are discussed. The result shows that the optimal offset rib structure, SSTOR-45, can significantly improve the behavior of MHS with PEC = 1.36 at Re = 1000. Furthermore, this work offers an excellent reference for MHS design in the electronic package heat dissipation field.

Thermal-hydraulic performance of mist/compressed humid air two-phase flow in an airfoil channel recuperator

The introducing water mist into the cold channel of recuperator was adopted to improve the thermal-hydraulic performance of compressed humid air in a megawatt grade humid air turbine cycle. The influences of mist content, air pressure and droplet diameter on the thermal-hydraulic performance of the humid air were numerically simulated. The results showed that when the quantity of mist increased from 0 to 16%, the pressure drop decreases by 24.35% while the Colburn factor increases by 8%. The method can effectively mediate the contradiction between heat transfer and resistance. However, the variation of air pressure had little effect on the thermal-hydraulic performance. Besides, the best heat transfer performance can be realized when the inlet droplet diameters are between 5 and 10 μm. A new heat transfer enhancement factor of mist content was proposed, and the correlations of the Nusselt number and friction factor were obtained. When enhancement factor increases from 0 to 0.16, the Nusselt number increases by 141.5%, and the friction factor decreases by 86.4%. The method of injecting mist can improve the thermal-hydraulic performance and reduce the volume of recuperator, which is further beneficial to reduce the cost and promote the thermal efficiency in the practical application of humid air turbine cycle. It has high application value in the future commercial application of humid air turbine cycle. The results and correlations can provide model and guidance for the design and application of recuperator and humid air turbine cycle.

Optimization design of helical micro fin tubes based on exergy destruction minimization principle

• Increased heat exchange area and the secondary flow are the primary sources of heat transfer enhancement. • The local exergy destruction rates are related to the equivalent heat transfer coefficient and the unit pressure drop. • Optimal design tends to have a larger number of starts and a lower micro fin height. • The exergy destruction minimization principle is proved to be still applicable in the case of the periodic model and fully developed turbulence. The helical micro fin tubes (HFT) are commonly used in various double pipe heat exchangers because of the excellent processing and anti-fouling performance. It is of great significance to further improve the overall efficiency of the HFT so as to diminish energy consumption. In this work, the heat transfer and flow characteristics of the HFT are studied by numerical simulation. The results show that the heat transfer enhancement factors of the HFT are the secondary flow generated near the wall and the increase of the heat exchange area. In addition, the effects of the geometrical parameters on thermal–hydraulic performance are studied at Re = 36,636. It is found that the micro fin height ( e ), the helical angle ( φ ), and the number of starts ( Ns ) have a significant impact on the overall performance, and there is a strong mutual coupling between them. According to the parametric analysis, the HFT with a low micro fin height and a large number of starts is considered to be a better geometrical type. Finally, in order to select (or design) the HFT quickly under the specific working conditions, based on the exergy destruction minimization principle, the geometrical parameters are optimized by using the artificial neural network and genetic algorithm. An optimal solution ( e = 0.23 mm, φ = 36.1°, and Ns = 66) is selected from the Pareto front by the TOPSIS method. The results indicate that the optimal solution has a sensible balance between the exergy destruction caused by heat transfer and fluid flow. Besides, it has a better thermal–hydraulic performance as well ( PEC = 1.73). This work fills the gap of heat transfer and the geometrical optimization study of HFT based on the second law of thermodynamics and provides strong evidence that the exergy destruction minimization principle is still applicable in the case of the periodic model and fully developed turbulence. We hope that it will be contributed to the structural design of the HFT.

Combined effects of heat input and clamping pressure on the thermal performance of phase change thermal interface materials for electronics cooling applications

This work deals with experiments on the thermal performance of phase change materials (PCMs) used as thermal interface materials (TIMs) between a heat pipe cooling system and an electronic component. A test rig is conceived and realized so that experiments can be carried out under various heat input powers, clamping pressures, and cooling temperatures. Three commercial PCMs (Hi-flow 105, Hi-flow 200, and Hi-flow 625), having different thicknesses, thermal conductivities, and phase change temperatures, were tested. A heat transfer enhancement factor is introduced to compare the contact thermal resistance obtained without PCMs to that obtained with PCMs. The experimental results show that the thermal performances obtained by Hi-flow 105 and Hi-flow 625 are affected greatly by the heat input power and they are less sensitive to the clamping pressure. The maximum heat transfer enhancements obtained by these two PCMs, which increase with the heat input power, are 75 % and 55 % for Hi-flow 105 and Hi-flow 625, respectively for a heat input power of 115 W. However, with Hi-flow 200, the thermal performances are sensitive to heat input power, and the clamping pressure for heat input powers lower than 100 W. The heat transfer enhancements obtained by Hi-flow 200, which decrease with the heat input and the clamping pressure, are lower than those obtained with Hi-flow 105 and Hi-flow 625 PCMs whatever the heat input power and the clamping pressure, and they do not exceed 25 %. Thermomechanical simulations were carried out to analyze the combined effects of the heat input load, the temperature level, and the clamping pressure. The comparison between the results issued from the thermomechanical model and those obtained from the experiments which are relative to axial temperature distribution along the heating and cooling blocks shows a good agreement that is demonstrated by statistical indicators. An analytical model is developed to calculate the contact thermal resistance between two rough surfaces including or not PCMs. The comparison between the analytical results and those issued from the literature and the present study shows a good agreement.

Phase Change Process inside Toroidal Tube Heat Exchanger with Internal Fins

Inadequate melting of phase change material (PCM) in concentric tube and shell and tube heat exchangers appeal motivations for innovative latent heat thermal energy storage (LHTES) systems. In the current research, a novel application of toroidal tubes inside the LHTES system for charging of PCM is presented. The proposed heat exchanger geometry is numerically investigated with the help of enthalpy – porosity approach. Toroidal tubes are assumed to be filled with PCM (i.e., coconut oil) and fabricated inside cylinder accommodating the heat transfer fluid (HTF). To establish and accelerate homogenous melting, flat thin fins are attached inside the toroidal tubes, and transient simulations are performed. The system performance is investigated for variations of (i) fin numbers (Nf), (ii) fin length (L0), and (iii) fin orientations (Fo) in terms of (i) melt fraction (MF), (ii) average PCM temperature, (iii) energy stored by PCM, and (iv) Nusselt number (Nu). Simulation results indicate that fin orientation stands out as the dominant factor for heat transfer enhancement compared to fin numbers and fin length. In the toroidal tube LHTES system, it is found that even though fin numbers and fin length are identical, correctly oriented fins (i.e., case 8) can provide 50.52% more rate of energy storage than a system with random fin orientation (i.e., case 5). Moreover, the optimum fin orientation (i.e., case 8) can (i) save 65.28% time for 100% melting and (ii) provide 168% more rate of energy storage than a system without fins. Based on the melting progression pattern, the current study encourages the use of multiple PCMs having dissimilar melting point temperatures with fins.

Numerical Investigation of the Heat Transfer Characteristics of R290 Flow Boiling in Corrugated Tubes with Different Internal Corrugated Structures

The heat transfer and pressure drop characteristics of R290 flow boiling in a corrugated tube were investigated through computational fluid dynamics (CFD) in this study. We established a model of flow boiling in a corrugated tube with different corrugated structures (rectangular and circular corrugations) and validated the model using the Liu–Winterton and Xu–Fang empirical equations. The heat transfer coefficient (HTC) and pressure drop were obtained at a mass flow rate of 0.04–0.2 kg/s and a water inlet temperature of 310–330 K. The results show that the HTC and the drop in the pressure of the corrugated tubes both obviously increased compared with a smooth tube as the mass flow rate increased. The HTC decreased for the three tubes as the water inlet temperature increased, while the drop in pressure slightly increased for the three tubes. Moreover, the corrugated structure was found to significantly enhance the heat transfer; the heat transfer enhancement factor (E1) of the corrugated tube with the rectangular corrugations and the corrugated tube with the circular corrugations was 2.01–2.36 and 1.67–1.98, respectively. The efficiency index (I) for both the rectangular corrugated pipe and the circular corrugated pipe was greater than 1 (1.05–1.24 and 1.13–1.29, respectively). The application of corrugated tubes with round and rectangular corrugations can reduce the heat transfer area required for the exchange of heat and, thus, reduce the cost.