Comparative Study of the Flow and Thermal Performance of Liquid-Cooling Parallel-Flow and Counter-Flow Double-Layer Wavy Microchannel Heat Sinks

Applications of microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be an alternative cooling technology to enhance the heat transfer, and are successfully applied in heat exchangers. In this article, three kinds of liquid-cooling double-layer microchannel heat sinks, such as a rectangular straight microchannel heat sink, a parallel-flow wavy microchannel heat sink, and a counter-flow double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude and volumetric flow ratio on heat transfer, pressure drop, and thermal resistance are also observed. Results show that the counter-flow double-layer wavy microchannel heat sink is superior at a larger flow rate, and a more uniform temperature rise is achieved. For a slightly larger flow rate, the parallel flow layout shows better performance. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu₀)/(f/f₀) and (Nu/Nu₀)/(f/f₀)^1/3, are applied and similar results are obtained.     

A Finite Volume Analysis of Thermoelectric Generators

ABSTRACT

The electric power produced by a thermoelectric generator (TEG) is strongly influenced by the applied heat sink. While a TEG is aimed at harvesting waste heat, the optimization of the efficiency of the heat sink is a key task for the design of waste heat recovery systems implementing TEG. A TEG model is proposed and implemented in an open source toolbox for field operation and manipulation (OpenFOAM) for the purpose of performing optimizations of the heat sink, using a commercially available TEG as basis. This model includes the multi-physics thermoelectric coupled effects. Conservation principles of energy and current are considered simultaneously. This includes the thermal and electric conduction, Seebeck effect, Peltier effect, Thomson effect, and Joule heating. Particular attention is given to a proper modeling of the boundary conditions. The thermoelectric model is implemented in such a way that it can readily be combined with other physical models in OpenFOAM. The model is validated by comparing the predictions to analytical results, measurements as well as the simulation data of other authors.     

Numerical study of a solar thermoelectric generator with nanofluids based microcooling system

Abstract

Nanofluids have been recently gaining ever-increasing attention in solar thermoelectric applications due to their promising potentials as heat transfer fluids. This research investigates numerically the performance of a thermoelectric generator (TEG) that is cooled by Al₂O₃/water nanofluid flows in zigzag microchannel heat sinks (ZMCHS). The one-way fluid–structure interaction (FSI) tool was used to couple the thermal-electric and fluid flow tools in ANSYS 15.0. The present study focused on the effects of heat flux (2–50?kW/m²), laminar Reynolds number (5–1500), inlet flow temperature (293–303?K) and the nanoparticle concentration (1–6%) on the output electric power and the efficiency of the TEG module. The applied heat flux limitations and its relation to the thermal limitations of thermoelectric materials were considered. The results indicated that the increase of heat flux increased the output power and the efficiency of TEG. Higher Reynolds numbers (Re > 400), inlet temperature and nanofluid concentration had an insignificant impact on the TEG performance.     

An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O3/water nanofluid

Experiments were conducted to investigate forced convective cooling performance of a copper microchannel heat sink with Al₂O₃/water nanofluid as the coolant. The microchannel heat sink fabricated consists of 25 parallel rectangular microchannels of length 50 mm with a cross-sectional area of 283 μm in width by 800 μm in height for each microchannel. Hydraulic and thermal performances of the nanofluid-cooled microchannel heat sink have been assessed from the results obtained for the friction factor, the pumping power, the averaged heat transfer coefficient, the thermal resistance, and the maximum wall temperature, with the Reynolds number ranging from 226 to 1676. Results show that the nanofluid-cooled heat sink outperforms the water-cooled one, having significantly higher average heat transfer coefficient and thereby markedly lower thermal resistance and wall temperature at high pumping power, in particular. Despite the marked increase in dynamic viscosity due to dispersing the alumina nanoparticles in water, the friction factor for the nanofluid-cooled heat sink was found slightly increased only.     

Analysis of microchannel heat sinks for electronics cooling

This paper presents an analytical and numerical study on the heat transfer characteristics of forced convection across a microchannel heat sink. Two analytical approaches are used: the porous medium model and the fin approach. In the porous medium approach, the modified Darcy equation for the fluid and the two-equation model for heat transfer between the solid and fluid phases are employed. Firstly, the effects of channel aspect ratio (α_s) and effective thermal conductivity ratio (k?) on the overall Nusselt number of the heat sink are studied in detail. The predictions from the two approaches both show that the overall Nusselt number (Nu) increases as α_s is increased and decreases with increasing k?. However, the results also reveal that there exists significant difference between the two approaches for both the temperature distributions and overall Nusselt numbers, and the discrepancy becomes larger as either α_s or k? is increased. It is suggested that this discrepancy can be attributed to the indispensable assumption of uniform fluid temperature in the direction normal to the coolant flow invoked in the fin approach. The effect of porosity (ε) on the thermal performance of the microchannel is subsequently examined. It is found that whereas the porous medium model predicts the existence of an optimal porosity for the microchannel heat sink, the fin approach predicts that the heat transfer capability of the heat sink increases monotonically with the porosity. The effect of turbulent heat transfer within the microchannel is next studied, and it is found that turbulent heat transfer results in a decreased optimal porosity in comparison with that for the laminar flow. A new concept of microchannel cooling in combination with microheat pipes is proposed, and the enhancement in heat transfer due to the heat pipes is estimated. Finally, two-dimensional numerical calculations are conducted for both constant heat flux and constant wall temperature conditions to check the accuracy of analytical solutions and to examine the effect of different boundary conditions on the overall heat transfer.     

The characteristics of convective heat transfer in microchannel heat sinks using Al2O3 and TiO2 nanofluids

The present study aims to provide an overall analysis about nanofluids flowing through microchannel heat sinks. Al₂O₃ and TiO₂ nanofluids based on deionized water with particle volume fractions of 0%, 0.1%, 0.5%, 1.0% were prepared by the two-step dispersion method. Nonionic surfactant polyvinylpyrrolidone (PVP) was added into the nanofluids to avoid particle aggregation and enhance stability. An ImageIR 3350 was used to get the temperature distribution on the substrate of microchannel heat sinks. The results reveal that the thermal conductivity and dynamic viscosity of Al₂O₃ and TiO₂ nanofluids are both improved with the increase of particle volume fraction. Compared with a rectangular microchannel heat sink, the performance of heat transfer in fan-shaped microchannel heat sink is more strengthened using Al₂O₃ nanofluids. The thermal motion of nanoparticles could promote the interruption of laminar flow and intensify the heat transfer between fluids and channel walls. The cyclical change with a fixed period on equivalent diameter could also help destroy the boundary layers.     

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm² was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.     

Optimization design of microchannel heat sink geometry for high power laser mirror

Microchannel heat sink for high power laser mirror with water cooling was analyzed as a function of microchannel geometry and operation parameters. A comparative analysis of the thermal deformation on the mirror surface without cooling and that with cooling revealed that the maximal thermal deformation on the mirror surface could decrease from about 0.115 μm to around 0.040 μm under the laser power of 200 W/cm² by using microchannel heat sink designed. In order to enhance the performance of microchannel heat sink, the effects of channel width, channel depth, fin width, mirror thickness and cooling region were investigated. The results indicated that the heat transfer performance of the microchannel heat sink could be further improved by narrow and deep channel, narrow fin, thin mirror and large cooling region.     

An experimental investigation on the thermal efficiency of fractal tree-like microchannel nets

In this paper, a fractal tree-like microchannel net heat sink (20 mm × 20 mm × 1.4 mm) for cooling of electronic chips was fabricated on a silicon wafers by advanced MEMS technology. The length, width and height of the entrance microchannel were 10 mm, 800 μm and 25 μm, respectively. The fractal dimension D and the circulation number m of the fractal tree-like microchannel net were 2 and 4, respectively. It is confirmed experimentally that the thermal efficiency (defined as heat transfer rate per unit power required) of such a fractal tree-like microchannel heat sink is much higher than that of the traditional parallel microchannel heat sink for the same heat transfer rate, the same temperature difference and the same inlet velocity.     

Effect of tip clearance on the cooling performance of a microchannel heat sink

In this paper, the effect of tip clearance on the cooling performance of the microchannel heat sink is presented under the fixed pumping power condition. The thermal resistance of a microchannel heat sink is defined for evaluating its cooling performance. The effect of tip clearance is numerically investigated by increasing tip clearance from zero under the fixed pumping power condition. From the numerical results, the optimized tip clearance is determined, for which the thermal resistance has a minimum value. Finally, we show that the presence of tip clearance can improve the cooling performance of a microchannel heat sink when tip clearance is smaller than a channel width.     

Optimum thermal design of microchannel heat sinks by the simulated annealing method

By adopting the simulated annealing method, a three-dimensional numerical simulation is executed to minimize the thermal resistance of the microchannel heat sink corresponding to the optimum specification under the fixed flow power. The depths of the microchannel heat sink in this study are fixed at either 1 cm or 2 cm. Based on the theory of the fully developed flow, the pressure drop between the inlet and exit in each single channel can be analytically derived if the flow power and the associated specification of the microchannel heat sink are fixed in advance. Then, this pressure drop will be used as the input condition to calculate the temperature distribution of the microchannel heat sink. For the first part of the optimum analysis, the fin width, and channel width are chosen as the design variables to find their optimum sizes. As to the second part of the present analysis, three design variables including channel height, fin width and channel width are individually prescribed as a suitable range to search for their optimum geometric configuration when the other specifications of the microchannel heat sink are fixed as 24 different cases.     

Thermoelectric conversion of waste heat from IC engine‐driven vehicles: A review of its application,issues, and solutions

Thermoelectric generator (TEG) is a promising thermoelectric (TE) conversion technology to effectively recover and convert waste heat from vehicle exhaust into useful energy, ie, electricity. Exhaust TEG (ETEG) is a system that is incorporated into the exhaust manifold of a vehicle. Exhaust TEG comprises of a heat exchanger, TEG modules, heat sink, and power conditioning unit. The present work reviews different vehicular ETEGs based on engine type, engine‐rated power, type and number of TEG module, efficiency of ETEG and TEG, exhaust and coolant temperature, and power output of ETEG . In addition to these, the technical issues faced in these ETEGs are addressed under 2 categories, viz., primary (TEG with low ZT TE material and inefficient heat exchanger and heat sink) and secondary issues (low operating temperature TEG modules and installation position of ETEG). In addition to it, effects of vibration and thermal cycling of exhaust system on TEG modules that may arise in ETEG are also discussed. A review of preventive solutions to the issues is also presented. Finally, the economic aspects of an ETEG are also discussed. The review highlights the need of commercialization of TE materials with ZT > 2, high‐temperature operating range, and segmented TEG modules in large volumes so that their practice can be extended in vehicular applications. Heat exchanger modeling using computational fluid dynamics and interfacing with heat transfer theory is essential to maintain temperature uniformity across the TEG modules. Installation of ETEG in the exhaust pipe should be such that it does not affect the performance of the engine. It is also realized that sturdy TEG modules should be developed for long‐term operation to prevent degradation due to mechanical vibration and thermal cycling of the vehicle. Further, ETEG is economically beneficial in vehicles such as trucks owing to availability of high thermal energy in their exhaust stream.     

Conjugate heat transfer in fractal-shaped microchannel network heat sink for integrated microelectronic cooling application

The hydrodynamic and thermal characteristics of fractal-shaped microchannel network heat sinks are investigated numerically by solving three-dimensional N–S equations and energy equation, taking into consideration the conjugate heat transfer in microchannel walls. It is found that due to the structural limitation of right-angled fractal-shaped microchannel network, hotspots may appear on the bottom wall of the heat sink where the microchannels are sparsely distributed. With slight modifications in the fractal-shaped structure of microchannels network, great improvements on hydrodynamic and thermal performance of heat sink can be achieved. A comparison of the performance of modified fractal-shaped microchannel network heat sink with parallel microchannels heat sink is also conducted numerically based on the same heat sink dimensions. It is found that the modified fractal-shaped microchannel network is much better in terms of thermal resistance and temperature uniformity under the conditions of the same pressure drop or pumping power. Therefore, the modified fractal-shaped microchannel network heat sink appears promising to be used for microelectronic cooling in the future.     

Constructal design and thermal analysis of microchannel heat sinks with multistage bifurcations in single-phase liquid flow

Based on constructal theory, five different cases with multistage bifurcations are designed as well as one case without bifurcations, and the corresponding laminar fluid flow and thermal performance have been investigated numerically. All laminar fluid flow and heat transfer results are obtained using computation fluid dynamics, and a uniform wall heat flux thermal boundary condition is applied all heated surfaces. The inlet velocity ranges from 0.66 m/s to 1.6 m/s with the corresponding Reynolds number ranging from 230 to 560. The pressure, velocity, temperature distributions and averaged Nusselt number are presented. The overall thermal resistances versus inlet Reynolds number or pumping power are evaluated and compared for the six microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with multistage bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sink with a long bifurcation length in the first stage (Case 1A) is superior. The usage of multistage bifurcated plates in microchannel heat sink can reduce the overall thermal resistance and make the temperature of the heated surface more uniform (Case 3). It is suggested that proper design of the multistage bifurcations could be employed to improve the overall thermal performance of microchannel heat sinks and the maximum number of stages of bifurcations is recommended to be two. The study complements and extends previous works.     

Optimization of Dimples in Microchannel Heat Sink with Impinging Jets——Part A: Mathematical Model and the Influence of Dimple Radius

With increasing heat fluxes caused by electronic components, dimples have attracted wide attention by researchers and have been applied to microchannel heat sink in modern advanced cooling technologies. In this work, the combination of dimples, impinging jets and microchannel heat sink was proposed to improve the heat transfer performance on a cooling surface with a constant heat flux 500 W/cm~2. A mathematical model was advanced for numerically analyzing the fluid flow and heat transfer characteristics of a microchannel heat sink with impinging jets and dimples(MHSIJD), and the velocity distribution, pressure drop, and thermal performance of MHSIJD were analyzed by varying the radii of dimples. The results showed that the combination of dimples and MHSIJ can achieve excellent heat transfer performance; for the MHSIJD model in this work, the maximum and average temperatures can be as low as 320 K and 305 K, respectively when mass flow rate is 30 g/s; when dimple radius is larger than 0.195 mm, both the heat transfer coefficient and the overall performance h/ΔP of MHSIJD are higher than those of MHSIJ.     

Effects of fuel cell vehicle waste heat temperatures and cruising speeds on the outputs of a thermoelectric generator energy recovery module

A thermoelectric generator (TEG) module is designed to harvest low grade waste heat from a 2 kW fuel cell vehicle and improve its energy utilization. The module integrates a TEG cell with a heat pipe and a finned heat sink. A numerical model is developed based on an experiment setup where the fuel cell temperature is 45–60 °C while the cruise speed is 25 kmh^?1. The numerical model is validated with less than 5% deviation. Extended cases are simulated for series and parallel power train configuration under changes to the waste heat temperature and vehicle speeds to evaluate the power and heat recovery ratio. A single TEG cell output between 2 and 3 W is achievable even at low grade heat. The parallel drive generates 50% more power than the series drive at 100 kmh^?1 speed. A 2% heat recovery is theoretically achievable for a 16 cell module assembly.     

A Honeycomb Microchannel Cooling System for Microelectronics Cooling

A honeycomb porous microchannel cooling system for electronics cooling was proposed in this article. The design, fabrication, and test system configuration of the microchannel heat sink were summarized. Preliminary experimental investigation was conducted to understand the characteristics of heat transfer and cooling performance under steady single-phase flow. In the experiments, a brass microchannel heat sink was attached to a test heater with 8 cm² area. The experimental results show that the cooling system is able to remove 18.2 W/cm² of heat flux under 2.4 W pumping power, while the junction wall temperature is 48.3°C at the room temperature of 26°C. Extensive experiments in various operation conditions and parameters for the present cooling system were also conducted. The experimental results show that the present cooling system is able to perform heat dissipation well.     

Modeling and simulation on the mass flow distribution in microchannel heat sinks with non-uniform heat flux conditions

This paper describes modeling and numerical simulation on the mass flow distribution in microchannel heat sink, which is a promising device for cooling miniature electronic systems. The microchannel heat sinks in this study consist of headers, multiple fluidic channels and port holes, all of which influence flow distribution in the multiple channels. This study focuses on design of the header with non-uniform heating conditions over the channel area. To investigate the effect of non-uniform heat flux, three different non-uniform heat flux conditions were applied. The simulation work has been carried out to find optimal header geometry for two-phase flow in the microchannel heat sinks. The header geometry was expressed in mathematical terms by defining a geometric parameter of header shape, n. For the optimal design of microchannel heat sinks, absolute average deviation and root mean squared deviation of the flow distribution under various header shapes have been calculated as well as pressure drop. The results show that mass flow rate distribution tends to be less changed among microchannels over a certain value of n.     

Channel cross section effect on heat transfer performance of oblique finned microchannel heat sink

In the present work, the effect of channel cross section on the heat transfer performance of an oblique finned micro-channel heat sink was investigated. Water and Al₂O₃/water nanofluid of volume fraction 0.25% were used as a coolant. The oblique finned microchannels are designed with three channel cross-sections namely square, semicircle and trapezoidal. The primary work of this paper is to study the heat transfer and hydrodynamic characteristics in the oblique finned microchannel. The experimental setup and procedure are validated using water as coolant in a micro-channel heat sink. Heat transfer and flow characteristics are examined for three cross-sections of varying mass flux. The trapezoidal channel cross-section increases the considerable heat transfer rate improvement for both water and nanofluid by 3.133% and 5.878% compared to square and semicircle cross section. Also, the pressure drop is higher in the trapezoidal cross-section over the square and semicircle cross section. This is due to increase in friction loss of trapezoidal cross section. The results indicate that trapezoidal cross-section oblique finned micro-channel is more suitable for heat transfer in the electronic cooling application.     

Numerical simulation of nanostructured thermoelectric generator considering surface to surrounding convection

Thermoelectric systems (TE) can directly convert heat to electricity and vice-versa by using semiconductor materials. Therefore, coupling between heat transfer and electric field potential is important to predict the performance of thermoelectric generator (TEG) systems. This paper develops a general two-dimensional numerical model of a TEG system using nanostructured thermoelectric semiconductor materials. A TEG with p-type nanostructured material of Bismuth Antimony Telluride (BiSbTe) and n-type Bismuth Telluride (Bi₂Te₃) with 0.1 vol.% Silicon Carbide (SiC) nanoparticles is considered for performance evaluations. Coupled TE equations with temperature dependant transport properties are used after incorporating Fourier heat conduction, Joule heating, Seebeck effect, Peltier effect, and Thomson effect. The effects of temperature difference between the hot and cold junctions and surface to surrounding convective on different output parameters (e.g., thermal and electric fields, power generation, thermal efficiency, and current) are studied. Selected results obtained from current numerical analysis are compared with the results obtained from analytical model available in the literature. There is a good agreement between the numerical and analytical results. The numerical results show that as temperature difference increases output power and amount of current generated increase. Moreover, it is quite apparent that convective boundary condition deteriorates the performance of TEG.