Numerical Study Of Optimal Geometric Structure For Microchannel Heat Sink And Modified Condensation Structure For Modulating Flow Pattern

With elcetronic technology growing rapidly,miniaturization of equipments is the trend of the technological development.High density integrated circuit brings high power density and high heat density.Increased heat production can compromise the equipment performance and shorten equipment life.Thus,the microelectronic cooling became a hot issue in the industrial production and scientific research.Cooling demand is growing rapidly with chip density and heat production increasing.The electronic cooling technology is currently under continuous improvement.Under the limited space of microelectronics dense packaging,the main task of electronic cooling is how to improve heat dissipation performance of heat exchanger with the limited space.the cooling performance of the equipment depends on its geometric structure,properties of material and flow patterns of coolant fluids.According to the above factors,our research is mainly focused on structure improvement of cooling devices.The influence of properties of material and flow patterns are also studied.The research on electronic cooling is carried on from both direct device and indirect component to improve the cooling device performance.Microchannel heat sink as the electronic cooling device is studied firstly.Our work proposed an optimization approach combining the simplified conjugate-gradient-method and a fully three-dimensional nanofluid-cooled microchannel heat sink model to look for optimal geometric design with minimum total thermal resistance as the objective function.A three dimensional fluid-solid conjugated MCHS model combining the simplified-conjugate-gradient-method was used as the optimization tool.Nanofluids as a replacement working medium can effectively improve performance of microchannel heat sink.Geometric structure of MCHS is optimized based on the nanofluid cooling fluid to obtain the maximum heat transfer performance.The results showed that after optimization of geometric structure,the performance of nanofluid-cooled microchannel heat sink is significant improved.Compared to single variable optimization multi-variable optimization could obtain or at least approach the global optimal design.The optimal geometry for the nanofluidcooled MCHS is different from the water-cooled counterpart since heat transfer characteristics for nanofluids differ from those for water.Microchannel heat sink with different cooling fluid could not play their best heat dissipation performance at the same geometry.From the viewpoint of practical operation of MCHS three geometric parameters,including channel number,channel aspect ratio,and width ratio of channel to pitch,are simultaneously optimized at fixed inlet volume flow rate,fixed pumping power,and fixed pressure drop as constraint condition.The optimal geometric structure is different under various inlet volume flow rates,various pumping powers,and various pressure drops.Larger channel number and smaller width ratio of channel to pitch should be adopted when nanofluidcooled MCHS operates under fixed inlet volume flow rate,however,smaller channel number and larger width ratio of channel to pitch should be adopted when nanofluid-cooled MCHS operates under fixed pumping power or under fixed pressure drop.The improvement in cooling performance of nanofluid-cooled MCHS is attributed that optimal geometric structure increases inlet flow velocity and effective thermal conductivity of nanofluid,which enhances convective heat transfer between nanofluid and channel wall.When the demand of heat dissipation increases or lower working temperature is required,phase change heat transfer is another important method of the electronic cooling due to its larger heat absorption ability.Most of the phase-change electronic cooling research published has been focused on removing heat from the electronic device using micro-channel heat sinks,jet impingement or sprays.Far less emphasis has been placed on high-flux heat rejection from the two-phase cooling system.A key reason behind this trend is a common perception that a commercial condenser can always be found to reject the heat from virtually any phase-change cooling system.However,commercial condensers are often far too large to meet the size and packaging constraints of defense electronics.substantial enhancement in heat dissipation from the device can be achieved by improving the condenser heat transfer.The present study proposed a innovative phase separation mirco-condenser formed by populating an enclosed micro-membrane at the microchannel center.When gas phase interacts with the membrane,the gas liquid interface cannot break through the pin-fin holes due to the increased effect of surface tension in microscale.In contrast to the bare duct section,the fully phase separation section significantly decreased liquid film thicknesses.Consequently heat transfer is improved.The 2D numerical simulation using the Volume of Fluid(VOF)method is carried on the relationship among the different factors.Investigation shows that after phase separation the thickness of liquid film around bubble is several times less in the fully phase separation section in contrast to the bare section.The increase of gas flow rates shortened the separation length.The ultra-large gas flow rate yields large pressures inside the bubbleto exceed the capillary pressure limit that can be provided by the interface curvature within the membrane holes.Thus,the bubble breakup happens.Because the capillary pressure is inversely proportional to the hole size(d),the decrease of d increases the capillary pressure to extend the gas flow rate operation range.Further,a three-dimensional model is build for phase separation micro condenser.The heat transfer enhancement is related to Ar(the bubble project area relative to the bottom surface area)and an averaged liquid film thickness.In contrast to the bare duct section,the fully phase separation section significantly increased Ar and decreased liquid film thicknesses.The comprehensive heat transfer enhancement ratios could reach ten covering the present data range.The effect of phase separation structure is studied.The arrangement of the micro-pin-fin has significant influence on the phase separation.Under different arrangement structure all can achieve gas-liquid separation,separation effect is different.The smaller cross section area of the side region is,the shorter the separation distance is.It is suitable for high gas flow rate.The higher pressure difference between the micro-pin-fin results in phase separated quickly.The structure with large cross section area of the side region is more suitable for low gas flow rate.not only the phase separation is achieve,but additional pressure drop is small.The conclusions of this paper will extend micro-scale flow and heat transfer characteristics of theoretical knowledge,and provide a theoretical basis to improve the electronic cooling equipment performance.