Studies on thermal hydraulic performance enhancement methods of microchannel heat sink

Abstract

Advancement in technology has led to the development of microscale heat transfer devices, these devices in compact size offer high heat transfer coefficient. Microchannel heat sinks (MCHS’s) are currently projected as a twenty-first-century cooling solution. Increasing the efficiency of MCHS’s is vital to ensure the integrity, long life, and wide applicability of these little miniatures. The present work's objective is to find out different ways (design modification) by which the thermal-hydraulic characteristics of conventional MCHS are improved within the acceptable pressure drop penalty. All the studies are carried out numerically using ANSYS FLUENT software. A comprehensive review of various passive and active techniques used for heat transfer enhancement such as pinfins, flow disruptions, surface roughness, channel curvature, re-entrant obstructions, secondary flows, fluid additives, and flow pulsation, etc., are carried out along with pressure drop variations associated with it. The chapter also reports the summary of various flow maldistribution mitigation tecniques and design optimization studies related to MCHS. Initially, heat transfer enhancement in a microchannel using an extended surface has been carried out. Rectangular microchannel and cylindrical microfins are used for the study. Three different configurations of extended surface microchannel; Case I (upstream finned microchannel), Case II (downstream finned microchannel), and Case III (complete finned microchannel), are compared with the plain rectangular microchannel. It is found that the heat transfer performance of Case I is better than Case II. Case I even perform better than Case III at a low Reynolds number. Average surface temperature is also significantly reduced in the case of extended surface microchannels. Optimization of extended surface microchannel has also been successively carried out following univariate search method for the number of fins, pitch, diameter, and height of fins. The average heat transfer enhancement observed in the optimized case is around 160 %, with an acceptable pressure drop penalty. Non-uniform performance of the microchannel heat sink's parallel channels is one of the main limitations aggravating many undesired influences in both the single-phase and the two-phase heat transfer in the microchannel heat sink. The flow rate non-uniformities (termed as flowmaldistribution) among a stack of microchannels connected through the inlet/outlet plenums of a microchannel heat sink (MCHS) are one of the main hindrances associated with effective and efficient operations. It induces many undesirable effects, including accentuation of lateral and flow-direction non-uniformities in surface temperatures (for uniform heat-flux loads). This can lead to feedback induced lateral heat-flow in the electronics underneath the MCHS, which is to be avoided. A new flow maldistribution mitigation technique – involving the splitting the single inlet port to the inlet manifold (conventional) into two separate inlet ports – is proposed, and results are compared with the conventional method. The proposed scheme helps in reducing the flow maldistribution problem. The new scheme is tested against both horizontal and vertical inlet of fluid into the manifolds. For the horizontal inlet of fluid into the manifold, the case when of two ports in the front of the inlet manifold, the reduction is about 26.2%, and for the case of one suitably placed port on each side of the inlet manifold, the flowmaldistribution is reduced by about 68.5% and, in addition, MCHS efficiency (defined as heat carrying capacity per unit pumping power) is improved by 7.7%. In case of vertical inlet of fluid into the manifold, the maximum reduction in flowmaldistribution is 55.0%. As observed from the flowmaldistribution mitigation study outcomes, the mitigation of non-uniformity in the mass flow rate does not significantly mitigate the non-uniformity in heat flux transferred from the interface of parallel microchannels. The poor heat transfer (adiabatic/natural convection) boundary condition is primarily responsible for heat flux non-uniformity. A novel design of parallel microchannels heat sink is proposed, which is obtained by decreasing non-uniformity of mass flow rate and heat transfer existing between parallel channels of the microchannel heat sink. The existing variable-width approach removes the non-uniformity of the mass flow rate, and heat transfer non-uniformity among parallel channels is solved by the newly developed slanted microchannel provision. Evolved design normalizes flow non-uniformity by 95.5% and heat transfer non-uniformity by 97.5% compared to the conventional design microchannel heat sink. The proposed design's two major benefits are cooled (4.2 K lower than the conventional design) and uniform (3.1 K lesser than the conventional design) base surface temperature. It is also found that the proposed work even facilitated in the reduction of average base temperature (1.3 K lower than the conventional design). The maximum improvement in Nusselt number is 4.07%; the proposed design also extends benefits at off-design conditions even. Later on, to obtain the best design of conventional MCHS, which have the least thermal resistance and lowest pumping power simultaneously, a study on multi-objective optimization of thermal resistance and pumping power of rectangular MCHS is carried out. The objective functions are optimized with the help of a fast Non-dominated Sorting Genetic Algorithm or NSGA-II. Sorted design results are compared with the numerically simulated results. The optimized design in the current study gives thermal resistance of 0.1167 °C/W and pumping power 0.061 W. Keywords: microchannel, single-phase flow, extended surface, Nusselt number, heat transfer characteristics, heat sink, maldistribution, temperature non-uniformities, heat flux, thermal resistance, optimization techniques