Studies on Liquid Flow and Heat Transfer for Cooling of Integrated Circuits Using Microchannels

Abstract

The past couple of decades has witnessed a rapid progress in the applications of Integrated Circuits (ICs) forcing the size of these circuits to decrease drastically with high demand on increased operating speeds and package densities. These factors have lead to high die temperatures which are detrimental to circuit performance and reliability. A need has therefore arisen for new and innovative techniques for the development of embedded cooling solutions, IC-level integration of thermal sensors and heat sinks, and systematic synthesis techniques for ICs that contain embedded heat dissipation mechanisms. Microchannels and minichannels based heat sinks, which provide large surface to volume ratio, are the obvious choice for this task as they provide a large heat transfer surface area per unit fluid flow volume. The large surface to volume ratio leads to a high rate of heat transfer, making these micro devices as excellent cooling systems. Microchannels provide an efficient means to remove heat from a surface but pose certain challenges in bringing the fresh coolant to the heated surface and returning it to the cooling system before the coolant reaches the stringent temperature limits; however, this can be achieved at the cost of higher pressure drop per unit length. This problem is interlinked with the performance issues of the systems being cooled, which dictate a lowering of the surface temperature to increase reliability, speed of the processor units, or other system considerations. Various researchers have studied the option of using liquid flow based microchannel heat sink (MCHS) as a solution for thermal management. An MCHS has an inlet plenum, from where the fluid is supplied to an array of micro-channels, and an outlet plenum, where it is collected after being made to pass through the microchannels. In order to meet specific requirements for various applications, the fluid inlet and outlet of the microchannel can be arranged in a number of ways. Owing to the small size of the channel dimensions, the entrance and exit effects will significantly affect the heat transfer characteristics of the flow field in the channels. In order to understand the performance of the heat sink as a single unit, it is necessary to study the effect of inlet and outlet flow arrangements on heat transfer characteristics of the microchannels. A detailed literature review indicates that most of the research works available have focused on heat transfer and fluid flow analysis for the flow field along the direction of the fluid within the microchannels. However, to the best knowledge of the author, no detailed experimental investigation has been carried out to date to study the effect of flow arrangements and size of the inlet and outlet plenums on the performance of MCHSs. These studies also need to be researched in detail under various heat input and different sizes of inlet and outlet plenums under different flow arrangements to suit practical demands in cooling applications of electronic devices. With these points as objectives in the present dissertation work, an attempt is made to understand the effect of different flow arrangements on the performance of MCHS. As the different flow arrangements are expected to affect the fluid flow and heat transfer characteristics within the heat sink, it is important to investigate the effect of inlet/outlet plenum flow arrangements to gain further knowledge on the performance of MCHS. It is also observed that flow variations at the entrance and exit regions of the microchannel need to be understood fully for efficient design of MCHSs. As part of the present dissertation work, microchannels with different dimensions were fabricated to study the effect of channel aspect ratio and plenum aspect ratio on the thermal performance of MCHSs. Though channel aspect ratio is a well established parameter in microchannels research, a new aspect ratio called the plenum aspect ratio has been proposed in the present work to understand the influence of plenum dimensions on convective heat transfer within the MCHS. In the present research work, three different types of flow arrangements namely P-type, U-type and S-type were selected, where P stands for perpendicular, U stands for U-shape and S stands for serpent shape entry and exit of the liquid flow. In order to evaluate the thermal performance of the MCHS, experimental test runs were carried out according to the test matrix made with three flow arrangements, P-, U- and S-types; three heat input values, 125 W, 225 W and 375 W; six Reynolds numbers, 224.3, 336.5, 448.7, 673.0, 897.4 and 1121.7; three inlet and outlet plenum aspect ratios 2.5, 3.0 and 3.75, and two channel aspect ratios 4.72 and 7.57 giving rise to a total of 324 experimental trial runs. Based on the experimental data obtained heat transfer coefficient, Nusselt number and pressure drops were computed. Correlations for Nusselt number have also been developed, which can be readily used for estimating the convective heat transfer for a given MCHS. The experimental analysis is also supported by CFD simulation carried out with the help of Solidworks Flow Simulation software, in order to obtain a detailed temperature and pressure distributions within the MCHS which otherwise will be very difficult to obtain by experimental method also. Experimental analysis shows that the reduction in channel width (increase in aspect ratio, defined as depth to width of channel) for the studied cases has shown about 126 to 165% increase in Nusselt number, whereas increase in plenum length (reduction in plenum aspect ratio defined as width to length of plenum) has resulted in 18 to 26% increase in Nusselt number. For a given flow arrangement, the Nusselt number increases when the plenum aspect ratio is reduced and at a given range of Reynolds number, with increase in plenum aspect ratio there is decrease in the pressure drop. The minimum pressure drop is for P-type flow arrangement followed by U-type and S-type flow arrangements. As the channel aspect ratio is increased, the fluid velocity through the microchannels and the wall shear stress within the microchannels increase, resulting in higher pressure drop. The maximum pressure drop occurs at the higher channel aspect ratio, ARc=7.57 and the lowest plenum aspect ratio, ARp=2.5 for S-type flow arrangement, whereas the minimum pressure drop is noted for the case of P-type flow arrangement for ARp=3.75. The results of the CFD analysis of the complete MCHS unit carried out for various heat inputs, flow arrangements, plenum sizes and microchannel widths were found to be in line with the results of the experimental analysis. The heat transfer coefficients computed by CFD analysis were observed to be 2 to 19% higher as compared to the experimental values for the entire range of flow arrangements and heat inputs.