| The booming development of microelectronics technology,especially the military electronics and microwave devices,has pushed the high-power electronic chips to develop towards miniaturization and multi-cores level.The decreasing feature size and increasing transistors integration trends not only increase the whole heat density generated,but also contribute to the high non uniform heat distribution problems on the electronic chip surfaces.The great temperature gradient and high junction temperature caused by the heterogeneous power distribution could definitely reduce the performance,the reliability and life time of electronic chips.Therefore,it is an urgent job to improve the thermal management of electronic chips with hotspots problems and design some novel adaptive cooling technologies for the random distributed hotspots to avoid the local thermal failure problem.Therefore,a new smart fractal microchannel heat sink(SFMHS)with smart thermo-responsive hydrogels is proposed for the thermal management of the microchips with randomly distributed hotspots.The hydrogels are integrated aiming to reallocate the coolant flow rate for hotspots-targeted cooling by changing the coolant flow area with its thermal-responsive adaptive deformation under unpredictable hotspots condition.3D fluid-solid heat transfer models are employed to optimize the structure for better cooling performance firstly and then investigate the cooling performance and flow characteristic of this new smart fractal microchannel heat sink under different heat flux condition,including the hotspot temperature rise,hotspot temperature non-uniformity,the temperature rise of the heat sink,the effective heat dissipation of each branches,the branches flow allocation and the total pump power needed for the heat sink.Meanwhile,solutions to improve the heat flux that the SFMHS could process are proposed based on current situation of chip heat generation and cooling technology.The integration location and size of hydrogels are also been compared and optimized for different heat cases.The main conclusions of this paper are as follows:It is an effective local optimization method to change the shunt of the fractal microchannel heat sink from sharp-shape into the platform-shape or curve-shape.The temperature of optimized heat sink is 1.25K lower with the inlet flow at 12.5 ml/min and the background heat flux at 25W/cm~2.The integration of the thermal-responsive hydrogels in fractal microchannel heat sink could sense the hotspots achieve the adaptive cooling for the hotspots.But the adaptive cooling ability cannot be achieved under all heat conditions,the relative position between the hydrogels and the hotspots has great influence on that ability.With the decrease of the initial size of the hydrogel,the flow reallocation effects resulted from the deformation weakens but the total pumping power is less.Therefore,when designing the size of the hydrogel,the heat situation and the pumping power supplied should be considered.Under even heat flux,all the integrated hydrogels are at the same state,so there is no flow reallocation in the heat sinks.The heat source is cooled uniformly and there is no local over cooling problem.The pumping power of SFMHS is 48.47 Mw with the inlet flow at 200 ml/min.While,under hotspots condition,the hydrogels near the hotspots shrink because the heating temperature reaches the LCST,as a result,the flow rate in the branches where the hydrogels shrink increase for the hotspots-targeted cooling.The hotspot temperature rise of SFMHS is obviously lower than FMHS and the temperature is more uniform.When the hotspot heat flux at HSB is 400 W/cm~2,the maximum temperature rise,temperature difference and the temperature standard deviation of the heat source of SFMHS are 4.47K,4.28K and 0.97K lower than FMHS,respectively.The more hydrogels shrink,the more flow gets reallocated by the deformation of hydrogels and the less the total pumping power is.The superiority of the SFMHS for the random hotspots cooling gets enhanced with the increase of the heat area and heat flux.However,not all heat cases could be handled by the SFMHS because only under the condition that the integrated hydrogels are not at the same state could the flow be reallocated.Too small and too high heat flux would result in the same state of the hydrogels.For hotspots located in different locations,SFMHS are more likely to sense hotspots that are closer.The further the hotspot is,more difficult it is for the hydrogels to sense the hotspots.Improving the heat dissipation capacity of the heat sink itself and increasing the transition temperature of the hydrogel itself are two effective ways to improve the processible background heat flux of the SFMHS.Under the optimal conditions proposed in this paper,the maximum background heat flux that can be processed by the SFMHS with hydrogels LCST at 316K is 93.3 W/cm~2,which meets the cooling requirements of existing working chips.Compared with the heat sink SFMHS 2 with hydrogels embedded in 2nd level channel,the heat sink SFMHS1 with hydrogels embedded in 1st level presents a stronger heat dissipation ability,and the heat sink SFMHS 1+2 with hydrogels both embedded in 1st and 2nd level presents the strongest heat dissipation ability and the highest pumping power under even heat flux.The pumping power of SFMHS 1 and SFMHS 1+2 are 0.33 times and 1.16 times of that of SFMHS 2,respectively.Under higher heat flux condition(higher background heat flux and higher hotspot heat flux),the temperature of 2nd level is too higher that all the hydrogels temperature is over LCST and there is no flow reallocation in SFMHS 2,so SFMHS 2 presents the highest temperature rise.When the background heat flux at 25 W/cm~2 and hotspot heat flux at 400 W/cm~2,the maximum temperature rise of SFMHS 1+2 is 5.23K and 3.24K lower than SFMHS 2 and SFMHS 1,respectively.The temperature difference of heat source of SFMHS 1+2 is 6.50K and 4.37K lower than SFMHS 2 and SFMHS 1,respectively.The temperature standard deviation of heat source of SFMHS 1+2 is 1.47K and 0.98K lower than SFMHS 2 and SFMHS 1,respectively.While,under lower heat flux condition(lower background heat flux and lower hotspot heat flux),the temperature of 1st level is too lower to reach the LCST,so the hydrogels in SFMHS 1are difficult to sense the temperature rise caused by the hotspots and remove the hotspots,as a result SFMHS 1 presents the highest temperature rise.Only when all the3 hydrogels in one branche shrink would the SFMHS 1+2 shows better hotspots cooling performance than SFMHS 2.For the SFMHS 1+2 with hydrogels integrated in both 1st and 2nd level channel,the hydrogels in 2nd level could sense the hotspots and reallocate the flow when SFMHS 1 is ineffective and the hydrogels in 1st level could sense the hotspots and reallocate the flow when SFMHS 2 is ineffective with all hydrogels in 2nd level shrink.Therefore,SFMHS 1+2 can be applied to a wider range of heat flux and sense and process hotspots at more locations,but the overall pumping power is higher.In this study,a new smart fractal microchannel heat sink with smart thermo-responsive hydrogels is proposed for the thermal management of the microchips with randomly distributed hotspots.The adaptive cooling performance under different random hotspots conditions of this new smart fractal microchannel heat sink is obtained and the integrated location of the hydrogels has been optimized by CFD numerical simulation.This numerical study is aimed to provide valuable theoretical reference for the further relative randomly distributed hotpots-targeted cooling researches and significant guidance for the applications. |
