Study On Single/Two Phase Flow And Heat Transfer Enhancement Characteristics Using Integrated Gradient Distribution Pin Fins For Intra-Chip Cooling

Recently,high power micro-electrical 2D and 3D chips with denser integration of power processor are in urgent need of micro-chip level cooling technologies to demand the heat removal requirement of background(100 W/cm~2)and hotspot(1000 W/cm~2).Micro-fluidic cooling provides feasible strategy by directly integrating cooling channels on the semiconductors and has been regarded as a promising cooling technique for high power micro-chip level thermal management.To further address multiple and non-uniform power map,the heat transfer performance needs to be enhanced by optimizing and designing micro-structures with elucidating single phase and flow boiling mechanism.At present work,the single-phase and flow boiling heat transfer characteristics in sillicon micro-pin fin chips are furtherly investigated by theoretical,numerical and experimental ways.On the one hand,the effect of micro-structure shape,pin fins distribution and non-uniform heat power map on pressure drop,heat transfer coefficient and chip surface temperature uniformity are studied.On the other hand,novel micro-pin fin chip with gradient distribution pin fins and enlargement bypass are prosossed and fabricated.The relevant heat transfer enhancement mechanism and flow patters are revealed.The main works are summerised as follows:Firstly,the effect of pin fin diameter and pin fin array porosity on flow and thermal resistance are studied based on a simplified theoretical model.It is found total thermal resistance decrease with the reduction of pin fin diameter ans porosity.However,a lower pin fin diameter and porosity leads to a higher pumping power.Thus,a moderate pin fin diameter range(100-200 um)and porosity(>0.65)is suggested from perspective of reducing thermal resistance at a reasonalble pumping power.Besides,a novel gradient micro-pin fins chip is proposed to decrease temperature difference across micro-fluidic cooling chip.Secondly,the single-phase flow and heat transfer characteristics in uniform and gradient distribution micro-pin fins are experimently compared using dielectric coolant(HFE7100).The Re covers a range of 100-1000 to investigate the effect of potential flow transition on heat transfer.On the one hand,the effect of flow rate,heat flux and coolant inlet temperature on heat transfer coefficient and temperature uniformity are elucidated.On the other hand,the enhanced heat transfer mechanism is elucidated with increasing Re and a new unified Nu correlation is proposed considering the influence of flow fluctuations.Strong flow fluctuations lead to the rapid rise of pressure drop and lower dependence of friction factor after a critical Re of 449.Compared to uniform micro-pin fins,the flow fluctuations are more strong in gradient distribution micro-pin fins.Such flow fluctuations and increased heat transfer area not only improve the averaged heat transfer coefficient but ony lead to an enhanced heat transfer zone downstream,effectively reducing chip surface temperature difference.For a constant flow rate(120 ml/min)and heat flux density(40 W/cm~2),the averaged heat transfer coefficient is increased by 36.8%with reduction of maximum surface temperature difference by 6.3 K.Thirdly,targeting at 3D chip cooling,the effect of non-uniform heat flux distribution on uniform and gradient distribution micro-pin fins are numerically investigated by dielectric coolant of HFE7100.For a constant heat flux of each chip layer,double-side heating power condition shows a significant influence on thermal performance,leading to an almost half reduction of addressed heat flux limit.Moreover,an abrupt rise of local temperature is caused by high heat flux density of hotspot.However,such effect is alleviated in gradient distribution pin fins.Compared to conventional solid pin fins,gradient distribution annular-cavity pin fins present an obvious strength in increasing heat transfer area and eliminating flow dead zone,resulting in on obvious decrease of local temperature.For double-side non-uniform heating power(background:40 W/cm~2;hotspot:300 W/cm~2),the temperature of hotspot and chip surface averaged temperature reduce by about 10 K.Finally,the flow boling characteristics of uniform and gradient distribution micro pin fins with enlargement bypass are experimentaly investigated using HFE7000 as the coolant.Through flow visualization observations,taper two-phase flow is observed and gradually move upstream with increasing heat flux density.Athough a higher heat flux density and wall superheat is required for the onset of nucleate boiling(ONB)for gradient distribution micro pin fins with enlargement bypass.It is note that the taper two-phase flow is inhibited by the gradient distribution micro-pin fins and enlargement bypass with fluid local acceleration,flushment and wetting effect with respect to uniform micro-pin fins.With increasing heat flux density,the bubbles and slug flow is broken and flushed into downstream,which avoid the blocking in micro-channels and improve flow boiling stability.As a result,the addressed maximum heat flux is increased by 10 W/cm~2 at a mass velocity of 737 kg/m~2s with maintaining surface temperature lower than 348 K.Moreover,surface temperature uniformity is obviously improved during flow boiling with respect to single-phase heat transfer.The surface temperature increases to the maximum value and keeps a platenu during flow boiling downstream,leading to a reduction of maximum surface temperature difference.