Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.     

The role of microchannel geometry selection on heat transfer enhancement in heat sinks: A review

Extensive research has been carried out by researchers for improving the thermal efficiency of the microchannel. There are various types of methodologies that have been proposed by authors for different geometry and fluid flow. The use of microchannel in the miniature heat exchangers and microchannel heat sink (MCHS) have taken the science of heat transfer to an another level for which the field of electronic device cooling, aerospace applications, automobile sectors, biomedical engineering, and chemical engineering sectors are being keen toward further development of the technology. Since 3 decades, the microchannel has been tested numerically, experimentally, and analytically for establishing the theories of hydraulic and thermal efficiency during fluid flow. Improper geometry selection of microchannel may lead to carry various losses such as pressure drop, friction factor, wall shear stress, and temperature jump. Available investigations and results have been reviewed immensely in this paper to give a clear prospective for further research in selecting a proper channel geometry.     

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.     

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.     

Microscale heat transfer enhancement using thermal boundary layer redeveloping concept

We demonstrated a new silicon microchannel heat sink, composing of parallel longitudinal microchannels and several transverse microchannels, which separate the whole flow length into several independent zones, in which the thermal boundary layer is in developing. The redeveloping flow is repeated for all of the independent zones thus the overall heat transfer is greatly enhanced. Meanwhile, the pressure drops are decreased compared with the conventional microchannel heat sink. Both benefits of enhanced heat transfer and decreased pressure drop ensure the possibility to use “larger” hydraulic diameter of the microchannels so that less pumping power is needed, which are attractive for high heat flux chip cooling. The above idea fulfilled in microscale is verified by a set of experiments. The local chip temperature and Nusselt numbers are obtained using a high resolution Infrared Radiator Imaging system. Preliminary explanation is given on the decreased pressure drop while enhancing heat transfer. The dimensionless control parameter that guides the new heat sink design and the prospective of the new heat sink are discussed.     

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.     

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.     

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.     

Thermal-fluid flow in parallel boards with heat generating blocks

A method is developed to numerically investigate thermal-fluid flow behavior in a bundle of parallel boards with heat producing blocks. The system simulates cooling passages in a stack of electronic circuit boards with heat generating chips. At a low Reynolds number flow, a developing flow may achieve a fully developed flow state at certain block number from the entrance. Thermal conductivity of the board and thermal contact resistance between the chip and board has a considerable impact on thermal performance. The fluid flow and heat transfer performance in this channel flow is similar to that in ribbed channel flow.     

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.     

Cooling performance of a microchannel heat sink with nanofluids

In this paper, the cooling performance of a microchannel heat sink with nanoparticle–fluid suspensions (“nanofluids”) is numerically investigated. By using a theoretical model of thermal conductivity of nanofluids that accounts for the fundamental role of Brownian motion, we investigate the temperature contours and thermal resistance of a microchannel heat sink with nanofluids such as 6 nm copper-in-water and 2 nm diamond-in-water. The results show that the cooling performance of a microchannel heat sink with water-based nanofluids containing diamond (1 vol.%, 2 nm) at the fixed pumping power of 2.25 W is enhanced by about 10% compared with that of a microchannel heat sink with water. Nanofluids reduce both the thermal resistance and the temperature difference between the heated microchannel wall and the coolant. Finally, the potential of deploying a combined microchannel heat sink with nanofluids as the next generation cooling devices for removing ultra-high heat flux is shown.     

Flow boiling in constructal tree-shaped minichannel network

Flow boiling in constructal tree-shaped minichannel network with an inlet diameter of 4 mm is numerically investigated using a one-dimensional model, taking into consideration the minor losses at junctions. The pumping power requirement, pressure drop, temperature uniformity and coefficient of performance of the constructal tree-shaped minichannel network are all evaluated and compared with those of the corresponding traditional serpentine channel, and the fluid stream undergoes a phase change from saturated liquid to saturated vapor. The effects of the length dimension and top view area (i.e. the path length) on saturated gas–liquid two-phase flow boiling heat transfer in tree-shaped minichannel networks are all analyzed and discussed. The results indicated that, the tree-shaped network configured with length dimension of two is able to maximum flow access; the path length plays a significant role in the determination of flow boiling in tree-shaped minichannel networks. In particular, compared to the traditional serpentine channel, flow boiling in constructal tree-shaped minichannnel network possesses less pressure drop, lower pumping power requirement, better temperature uniformity and higher coefficient of performance (COP).     

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.     

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACT

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm² with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.     

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.     

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.     

Modulating fluid rheology and confinement toward augmenting the performance of a double layered microchannel heat sink

The improvement of the cooling performance of liquid-cooled microchannel heat sinks used for densely packed electronic circuits is sorted via passive techniques like tuning substrate or coolant properties. We propose a design for enhancing heat sink performance by simulataneously modifying the channel geometry and tuning the fluid rheology. By modeling the coolant as a power law fluid, its rheological behavior is varied ranging from shear-thinning to shear-thickening, alongside Newtonian fluid. We introduced tapering to the middle wall that separates the bottom and top channels of a double layered microchannel heat sink (DL-MCHS), causing both channels to converge. This convergence not only increases the flow velocity within the downstream microchannel but also reduces the apparent viscosity of the shear-thinning fluid being subjected to shear, resulting in enhanced thermal and hydraulic performance. We analyze the results from both the first and the second law of thermodynamics context, demonstrating that a tapered DL-MCHS with shear-thinning fluid outperforms a straight partition wall DL-MCHS with Newtonian coolant. However, we also discovered that extreme tapering compromises thermodynamic viability, but by fine-tuning the extent of tapering, we inferred that a DL-MCHS with shear-thinning fluid can become viable with little compromise in the thermal performance.     

Liquid cooling in the mini-rectangular fin heat sink with and without thermoelectric for CPU

In the present study, the liquid cooling in the mini-rectangular fin heat sink with and without thermoelectric for CPU is studied. Six mini-rectangular fin heat sinks with two different material types and three different channel widths are fabricated from the copper or aluminum with the length, the width and the base thickness of 37, 37, 5 mm, respectively. The de-ionized water is used as coolant. Effects of channel width, coolant flow rate, material type of heat sink and run condition of PC on the CPU temperature are considered. The liquid cooling in mini-rectangular fin heat sink with thermoelectric is compared with the other cooling techniques. The thermoelectric has a significant effect on the CPU cooling of PC. However, energy consumption is also increased. The results of this study are expected to lead to guidelines that will allow the design of the cooling system with improved heat transfer performance of the electronic equipments.     

Three dimensional numerical analyses and optimization of offset strip-fin microchannel heat sinks

This paper presents a numerical study on laminar forced convection of water in offset strip-fin microchannels network heat sinks for microelectronic cooling. A 3-dimensional mathematical model, consisting of N–S equations and energy conservation equation, with the conjugate heat transfer between the heat sink base and liquid coolant taken into consideration is solved numerically. The heat transfer and fluid flow characteristics in offset strip-fin microchannels heat sinks are analyzed and the heat transfer enhancement mechanism is discussed. Effects of geometric size of strip-fin on the heat sink performance are investigated. It is found that there is an optimal strip-fin size to minimize the pressure drop or pumping power on the constraint condition of maximum wall temperature, and this optimal size depends on the input heat flux and the maximum wall temperature. The results of this paper are helpful to the design and optimization of offset strip-fin microchannel heat sinks for microelectronic cooling.     

Optimization of Dimples in Microchannel Heat Sink with Impinging Jets-Part B: the Influences of Dimple Height and Arrangement

The combination of a microchannel heat sink with impinging jets and dimples(MHSIJD) can effectively improve the flow and heat transfer performance on the cooling surface of electronic devices with very high heat fluxes. Based on the previous work by analysing the effect of dimple radius on the overall performance of MHSIJD, the effects of dimple height and arrangement were numerically analysed. The velocity distribution, pressure drop, and thermal performance of MHSIJD under various dimple heights and arrangements were presented. The results showed that: MHSIJD with higher dimples had better overall performance with dimple radius being fixed; creating a mismatch between the impinging hole and dimple can solve the issue caused by the drift phenomenon; the mismatch between the impinging hole and dimple did not exhibit better overall performance than a well-matched design.