Research On The Structure And Heat Dissipation Performance Of Fractal Microchannel For Cooling Of Electronic Devices

Microchannel heat sink has a good development prospect in the heat dissipation of electronic equipment due to its outstanding heat dissipation performance and smaller volume.As compared with the traditional straight microchannel heat sink,the fractal microchannel heat sink has lower overall thermal resistance,and the surface temperature distribution of the base is more uniform.In this work,the structures of fractal microchannel were modeled on the conditions that the constant inlet cross-sectional area,the constant inlet hydraulic diameter,variable hydraulic diameter based on Murray's law(P.NATL.ACAD.SCI.,1926,12:207-214)and Bejan et al.'s(INT.J.THERM.SCI.,2000,39(9-11):949-960)research on fractal microchannel,the heat transfer and hydraulic characteristics were numerically studied,and the flow of nanofluids in fractal microchannels was also studied.Main work is summarized as follows:Ten different fractal microchannel heat sink structures were constructed according to the different aspect ratio(0.333,0.5,1,2,3)of entrance cross-sectional and bifurcation level(two or three bifurcations)on the condition that the cross-sectional area of the entrance is constant.The results showed that the higher the bifurcation level and the Reynolds number were,the better the heat dissipation effect of the fractal microchannel heat sink would be,the pressure drop was also larger at the same time.The influence of aspect ratio on pressure drop and heat transfer characteristics was no specific rules.The heat sink with aspect ratio of 0.333 had the highest pressure drop,thermal resistance and the mean base temperature under the same branching level and Reynolds number.Ten different fractal microchannel heat sink structures were constructed according to the different aspect ratio(0.333,0.5,1,2,3)of entrance cross-sectional and bifurcation level(two or three bifurcations)on the condition that the hydraulic diameter of the entrance is constant.The numerical simulation results showed that the pressure drop of the fractal microchannel heat sink increased with the increased of bifurcation level and Reynolds number,increasing the aspect ratio would reduce the pressure drop(Except for the heat sink with a aspect ratio of 3 at three level bifurcations).The influence of aspect ratio on heat transfer characteristics was no specific rules.The mean base temperature and thermal resistance of the fractal microchannel heat sink with three bifurcation levels and aspect ratio of 3 were the lowest,it also had the second highest Nusselt number and the fifth lowest pressure drop.Eight different fractal microchannel heat sink physical models were constructed according to the difference of bifurcation level(two bifurcations,three bifurcations),variable hydraulic diameter and hydraulic diameter ratio.The results showed that the variable hydraulic diameter conditions,higher hydraulic diameter ratio and bifurcation level could improve the mean heat transfer coefficient of microchannel heat sink,but it was at the cost of pressure drop.Hydraulic diameter ratio influenced the hydraulic and heat transfer characteristics of the microchannel heat sink to a great extent.The evaluation of COP showed that the heat sinks with better heat transfer performance did not have larger COP,because the increment of pressure drop was much larger than that of heat transfer performance.The cooling medium in the fractal microchannel heat sink with aspect ratio of 1 was replaced by Al2O3-water and SiO2-water nanofluids.The effects of different volume fractions(1%,2%,3%,4%)of nanofluids on the performance of fractal microchannel heat sinks were studied.The results showed that nanofluids had better heat transfer performance and higher pressure drop than pure base fluids.The heat transfer performance and pressure drop were increasing with the increase of the volume fraction of nanoparticles in the nanofluids.As compared with pure base fluids,the thermal resistance and pressure drop of 4%Al2O3-water nanofluids decreased by 3.52%and 28.58%respectively.The effect of Al2O3 nanoparticles on heat transfer performance and pressure drop was higher than that of SiO2 nanoparticles at the same volume fraction.The COP/COP0 results showed that the COP of the nanofluids was lower than that of the pure water coolant,and with the increased of volume fraction,the COP/COP0 value was decreased,the COP for 4%nanofluids was about 76.5%of the pure base fluids.