Research on the shell-side thermal performances of heat exchangers with helical tube coils

This paper presents the results of experimental research on shell-side heat transfer coefficient concerning 3 heat exchangers with helical coils. Measurements were carried in laboratory and the following correlation was found to be adequate Nu = 0.50 ? Re^0.55 ? Pr ^1/3 ? (η/η_w)^0.14 where Re and Nu are based on shell-side hydraulic diameter.     

Optimum thermal design of triangular,trapezoidal and rectangular grooved microchannel heat sinks

A three-dimensional numerical simulation is conducted to investigate the effect of geometrical parameters on laminar water flow and forced convection heat transfer characteristics in grooved microchannel heat sink (GMCHS). Four geometry variables which are; the depth, tip length, pitch and orientation of the cavities are taken into account in order to optimize the aluminum heat sink design. These geometric parameters could change the cavity shape from triangular to trapezoidal and then to rectangular shape. The governing and energy equations are solved using the finite volume method (FVM). The performance of GMCHS is evaluated in terms of Nusselt number ratio, thermal/hydraulic performance (JF) and isotherm and streamlines contours. The results showed that the trapezoidal groove with groove tip length ratio of δ = 0.5, groove depth ratio β = 0.4, groove pitch ratio of ψ = 3.334, grooves orientation ratio of ζ = 0.00 and Re = 100 is the optimum thermal design for GMCHS with Nusselt number enhancement of 51.59% and friction factor improvement of 2.35%.     

Experimental investigation of thermal model of parallel flow microchannel heat exchangers subjected to external heat flux

This communication documents the experimental investigation of the theoretical model for predicting the thermal performance of parallel flow microchannel heat exchangers subjected to external heat flux. The thermal model investigated in this communication is that previously developed by the authors of this communication; Mathew and Hegab [B. Mathew, H. Hegab, Application of effectiveness-NTU relationship to parallel flowmicrochannel heat exchangers subjected to external heat transfer, International Journal of Thermal Sciences 31 (2010) 76–85]. The validity of the theoretical model with respect to microchannel profile, hydraulic diameter, heat capacity ratio and degree of external heat transfer is checked. The microchannel profiles investigated are trapezoidal and triangular with hydraulic diameter of 278.5 and 279.5 μm, respectively. The influence of hydraulic diameter is analyzed using trapezoidal microchannels with hydraulic diameters of 231 and 278.5 μm. Experiments are conducted for heat capacity ratios of unity and 0.5 using the heat exchanger employing the trapezoidal microchannel with hydraulic diameter of 278.5 μm for purposes of validating the model. Experiments are done for all heat exchangers for two different levels of external heat transfer; 15% and 30% of the maximum possible heat transfer. Irrespective of the parameter that is investigated the experimental data are found to perfectly match with the theoretical predictions thereby validating the thermal model investigated in this communication.     

Fluid flow and heat transfer characteristics of cross-cut heat sinks

In this study, effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition are experimentally studied. To find effects of the length, position, and number of cross-cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm were fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<P_p < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. Based on experimental results, the friction factor and Nusselt number correlations for heat sinks with a cross-cut are suggested. Using the proposed correlations, thermal performances of cross-cut heat sinks are compared to those of optimized plate-fin and square pin-fin heat sinks under the constant pumping power condition. This comparison yields a contour map that suggests an optimum type of heat sink under the constraint of the fixed pumping power and fixed heat sink volume. The contour map shows that an optimized cross-cut heat sink outperforms optimized plate-fin and square pin-fin heat sinks when 0.04 < log L1 < 1.     

Experimental study of forced convection heat transfer from a cam shaped tube in cross flows

An experimental investigation has been conducted to clarify forced convection heat transfer characteristic and flow behavior of an isothermal cam shaped tube in cross flow. The range of angle of attack and Reynolds number based on an equivalent circular tube are within 0° < α < 180° and 1.5 × 10⁴ < Re_eq < 2.7 × 10⁴, respectively.The results show that the mean heat transfer coefficient is a maximum at about α = 90° over the whole range of the Reynolds numbers. It is found that thermal hydraulic performance of the cam shaped tube is larger than that of a circular tube with the same surface area except for α = 90° and 120°. Furthermore, the effect of the diameter of the cam shaped tube upon the thermal hydraulic performance is discussed.     

Multi-parameters optimization for microchannel heat sink using inverse problem method

This work describes an inverse problem method to optimize the geometric design for microchannel heat sinks using a novel multi-parameter optimization approach, which integrates the simplified conjugate-gradient scheme and a fully developing three-dimensional heat transfer and flow model. Overall thermal resistance is the objective function to be minimized with number of channels, N, channel aspect ratio, α, and the ratio of channel width to pitch, β, as search variables. With a constant bottom area (10 mm × 10 mm), constant heat flux applied to the heat sink bottom surface (100 W cm^?2), and constant pumping power (0.05 W), the optimal design values are N = 71, α = 8.24, and β = 0.6, with a minimum overall thermal resistance of 0.144 K W^?1. Increasing pumping power reduces overall thermal resistance of the optimal design; however, the design’s effectiveness declines significantly under high pumping power. The N and α values in the optimal design increase and β decreases as pumping power increases.     

An experimental and numerical study of forced convection in a microchannel with negligible axial heat conduction

Experiments are conducted for laminar forced convection of water in a microchannel under partially-heated and fully-heated conditions on one wall with negligible axial heat conduction. The microchannel had a trapezoidal cross-sectional shape, with a hydraulic diameter of 155 μm and a heating length of 30 mm. Three-dimensional numerical simulations, based on the Navier–Stokes equations and energy equation, are obtained for forced convection of water in this microchannel under the same experimental conditions. It is found that the numerical predictions of wall temperatures and local Nusselt numbers are in good agreement with experimental data. This confirms that classical Navier–Stokes and energy equations are valid for the modeling of convection in a microchannel having a hydraulic diameter as small as 155 μm. For a microchannel with the same cross-sectional shape with one-wall heated and a heating length of 100 mm, numerical results show that the thermal entrance length is given by z = 0.15RePrD_h, with the fully-developed Nusselt number approaching a constant value of 4.00.     

Regularities of flow and heat transfer at the surface of transversely finned tubes

A great number of experimental investigations allowing one to reveal the physical mechanism of processes responsible for their thermal and hydraulic performance are carried out in attempt to solve problems of updating constructions and methods of thermal design of heating surfaces of transversely finned tubes widespread in power engineering. Results of flow visualization and investigation of pressure fields and local heat transfer at the fin surface over the Reynolds number range Re = (1.0 ? 6.6) · 10⁴ are presented for the case of a wide variation of geometric characteristics of finned tubes and parameters of their arrangement in a bundle. Regularities substantially changing the existing concept of transfer processes in the interfin space and in the wake behind a finned tube are revealed. It is found that the flow behavior and the distribution of local heat transfer coefficients over the fin surface change significantly at the fin height-to-finned tube diameter h/d approximately equal to 0.4. The results obtained are generalized in the form of the patterns of flow and heat transfer at the finned tube surface, including seven characteristic regions and four types of flow separation.     

Experimental and numerical investigation of low melting point metal based PCM heat sink with internal fins

In this paper, low melting point metal (LMPM), eutectic alloy Bi_31.6In_48.8Sn_19.6 (E-BiInSn), was adopted as phase change material for potential thermal management applications. First, E-BiInSn was prepared and its main thermophysical properties were characterized. Then, transient thermal performances of E-BiInSn based heat sinks with internal crossed fins were tested, in comparison with that of organic PCM (octadecanol) which has close melting point. Three types of heat sink structures which have different number of internal fins were studied. Three heating conditions were applied, namely 80 W (2.2 W/cm²), 200 W (5.6 W/cm²) and 320 W (8.9 W/cm²). For all of the cases, E-BiInSn exhibited much superior thermal performance than that of octadecanol. Furthermore, cyclic test of the E-BiInSn heat sink was carried out, which showed good repeatability and stability, and without supercooling. Finally, a simplified 3D conjugate numerical model was developed to simulate the melting process of LMPM heat sink, which showed good agreement with the experimental results. This simplified model would be much useful in practical thermal design and optimization of LMPM heat sink, for that it would significantly save the computational time consumption.     

Fluid flow and heat transfer in vascularized cooling plates

In this study, the hydrodynamic and thermal characteristics of new vascular designs for the volumetric bathing of the smart structures were investigated numerically by addressing three-dimensional continuity, momentum, and energy conservation as a conjugate heat flow phenomenon. The numerical work covered the Reynolds number range of 50–2000, cooling channels volume fraction of 0.02, pressure drop range of 20–2 × 10⁵ Pa, and six flow configurations: first, second, and third constructal structures with optimized hydraulic diameters and non-optimized hydraulic diameter for each system size 10 × 10, 20 × 20, and 50 × 50, respectively. The numerical results show that the optimized structure of cooling plates could enhance heat transfer significantly and decrease pumping power dramatically compared with the traditional channels. The difference in thermal resistance performance between optimized and non-optimized structures was found to increase and manifests itself clearly as the system size increased. The channel configurations of the first and second constructs are competitive in non-optimized configurations, whereas the best architecture was the third construct across all working conditions in non-optimized configurations.     

Investigation of thermal performance of microchannel heat sink with triangular ribs in the transverse microchambers

A numerical investigation is conducted to predict the thermal and hydraulic performances of the microchannel heat sink (MCHS) with different geometric parameters of triangular rib in the transverse microchamber. The parametric variables of width, length and height of the triangular rib are studied to find optimum design. The flow structure and characteristics of the interrupted MCHS are interpreted in details. The dimensionless ratios of average Nusselt number, friction factor and thermal enhancement factor are evaluated. It is found that the heat transfer rate is increasing with the increase of rib width and height, but decreasing with the increase of rib length. The boundary layer interruption and redevelopment effects introduced by the triangular rib are discussed. The results of thermal enhancement factor reveals an optimum geometrical parameters for the triangular rib with width = 100 μm, length = 400 μm and height = 120 μm for about Reynolds number of 500, yielding 43% enhancement relative to non-interrupted rectangular MCHS at equal pumping power. The results of mean Nusselt number ratio reveal an optimum enhancement of 56% relative to non-interrupted MCHS.     

Investigation of natural convection induced outer side heat transfer rate of coiled-tube heat exchangers

Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ ε [0.035, 0.082]) and a wide range of flow parameters (60 <= T_tank <= 90, T_in = 19 and 60 <= T_in <= 90, T_tank = 20, 4000 <= Re <= 45000) have been examined to broaden the validity of the results gained from this research. A fluid-to-fluid boundary condition has been applied in the numerical calculations to create the most realistic flow configurations. Validity of the numerical calculations has been tested by experiments available in the open literature. Calculated results of the inner side heat transfer rate have also been compared to existing empirical formulas and experimental results to test the validity of the numerical computation in an independent way from the outer side validation of common helical tube heat exchangers. Water has been chosen to the working fluid inside and outside of the coiled tube (3 < Pr < 7). Outer side heat transfer rate along the helical tube axis has been investigated to get information about the performance of the heat transport process at different location of the helical tube. It was found that the outer side heat transfer rate is slightly dependent on the inner flow rate of any helical tube in case of increasing temperature differences between the tank working fluid temperature and the coil inlet temperature. A stable thermal boundary layer has been found along the axial direction of the tube.In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades.     

Numerical study of pin-fin heat sink with un-uniform fin height design

This study presents the numerical simulation of the heat sink with an un-uniform fin height with a confined impingement cooling. The governing equations are discretized by using a control-volume-based finite-difference method with a power-law scheme on an orthogonal non-uniform staggered grid. The coupling of the velocity and the pressure terms of momentum equations are solved by the SIMPLEC algorithm. The well-known k–ε two-equations turbulence model is employed to describe the turbulent structure and behavior. The parameters include the Reynolds number (Re = 15,000 and Re = 25,000) and 12 un-uniform fin height designs (Type-b to Type-m). The objective of this study is to examine the effects of the fin shape of the heat sink on the thermal performance. It is found that the junction temperature can be reduced by increasing the fin height near the center of the heat sink. The results also show that there is a potential for optimizing the un-uniform fin height design.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Constructal cooling channels for micro-channel heat sinks

This paper documents the geometric optimisation of a three-dimensional micro-channel heat sink. The objective is to minimise the peak temperature from the walls to the coolant fluid. The optimisation is performed numerically by using the finite volume method. The numerical simulation was carried out on a unit cell with volume ranging from 0.1 mm³ to 0.9 mm³ and pressure drop between 10 kPa and 75 kPa. The axial length of the micro-channel heat sink was fixed at 10 mm. The cross-sectional area of the micro-channel heat sink is free to morph with respect to the degree of freedoms provided by the aspect ratio and the solid volume fraction. The effect of the total solid volume fraction and the pressure drop on the aspect ratio, channel hydraulic diameter and peak temperature is investigated. The numerical results show that the degrees of freedom have a strong effect on the peak temperature and the maximum thermal conductance. The optimal geometric characteristics obtained numerically (the aspect ratio and the optimal channel shape (hydraulic diameter)) are reported and compared with those obtained from approximate relationships using scale analysis. The predicted trends are found to be in good agreement with the numerical results.     

Prediction of the maximum heat transfer capability of two-phase heat spreaders – Experimental validation

The present paper is devoted to an experimental study to determine the thermal behaviour of a two-phase heat spreader (TPHS) with micro-grooves. The proposed application is the cooling of fuel cell systems. This TPHS aims at reducing the volume of actual cooling systems and to homogenize the temperature in the hearth of fuel cells. The TPHS is flat with a wide evaporating area (190 × 90 mm²) compared to the condenser area (30 × 90 mm²). It has been tested with three working fluids: water, methanol and n-pentane. Experimental results obtained with methanol show a temperature difference lower than 1.6 K on the entire evaporator area for a heat transfer rate equal to 85 W and a working temperature equal to 70 °C. The TPHS has been tested in both horizontal and vertical favourable orientation (thermosyphon orientation). The temperature field is similar in both cases for heat transfer rates lower than 155 W. In horizontal orientation, a confocal microscope is used to measure the meniscus curvature radius along the grooves. A two-phase flow model allowing the calculation of the meniscus radius, the liquid and vapour pressures and the liquid and vapour velocities along the TPHS is developed. The comparison between experimental and model results shows the good ability of the numerical model to predict the meniscus curvature radii from which the maximum heat transfer capability of the TPHS is depending.     

Numerical study of the heat sink with un-uniform fin width designs

The thermal performances of the heat sink with un-uniform fin width designs with an impingement cooling were investigated numerically. The governing equations are discretized by using a control-volume-based finite-difference method with a power-law scheme on an orthogonal non-uniform staggered grid. The coupling of the velocity and the pressure terms of momentum equations are solved by the SIMPLEC algorithm. The well-known k ? ε two-equations turbulence model is employed to describe the turbulent structure and behavior. The parameters include the five Reynolds number (Re = 5000–25000), three fin heights (H = 35, 40, 45 mm), and five fin width designs (Type-1–Type-5). The objective of this study is to examine the effects of the fin shape of the heat sink on the thermal performance. The results show that the Nusselt number increases with the Reynolds number. The increment of the Nusselt number decreases gradually with the increasing Reynolds number. Furthermore, the effects of fin dimensions on the Nusselt number at high Reynolds numbers are more significant than that at low Reynolds numbers. It is also found that there is potential for optimizing the un-uniform fin width design.     

Numerical study of transient conjugate heat transfer of a turbulent impinging jet

This study presents the numerical study of transient conjugate heat transfer in a high turbulence air jet impinging over a flat circular disk. The numerical simulation of transient, two-dimensional cylindrical coordinate, turbulent flow and heat transfer is adopted to test the accuracy of the theoretical model. The turbulent governing equations are resolved by the control-volume based finite-difference method with a power-low scheme, and the well-known low-Re κ–ω turbulence model to describe the turbulent structure. The SIMPLE algorithm is adopted to solve the pressure–velocity coupling. The parameters studied include turbulent flow Reynolds number (Re = 16,100–29,600), heated temperature of a circular disk (T_h = 373 K) or heat flux (q″ = 63–189 kW/m²), and orifice to heat-source spacing (H/D = 4–10). The numerical results of the transient impinging process indicate that the jet Reynolds number has a significant effect on the hydrodynamics and heat transfer, particularly in the stagnation region of an impinging jet. High turbulence values lead to greater heat transfer coefficients in the stagnation region and cause a bypass of the laminar-to-turbulent transition region in the wall jet region. Induced turbulence from the environment around the jet also influences the variation of the stagnation heat transfer. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Development of heat and mass transfer model for condensation

Divergence and the interfacial temperature deviation are the two main problems in condensation simulation with the Lee model. Based on the heat transfer analysis at the vapor-liquid interface, a correlation is revealed describing the relationship between interfacial temperature deviation and the model parameters, q_i ≈ (T_sat − T_i)(Ak_v)^0.5 where A = h_fgrρ_v / T_sat. With this correlation, the determination of the condensation frequency r is no longer empirical. Furthermore, the correlation indicates that the thermal conductivity of vapor plays an important role. Accordingly, an improved model is proposed amplifying the thermal conductivity of vapor in the phase interaction region. The model is verified with the Nusselt problem and the impacts of the model parameters are discussed and compared with the original Lee model. It is shown that the interfacial temperature deviation is reduced by the amplified thermal conductivity of vapor. The convergence is maintained by increasing both A and k_v synchronously. Verification is also obtained on the forced convection condensation of R134a. The correlation predicts a temperature deviation at 0.1 K and the numerical result successfully reaches 0.12 K.