| With the rapid development of electronic chip technology,the integration and miniaturization of microelectronic devices have been improved,and the power consumption of the whole electronic device has increased significantly.The problem of heat dissipation has become the bottleneck of its development.The traditional chip cooling methods,such as forced air cooling,forced water circulation cooling and thermoelectric refrigeration,have been unable to meet the thermal management needs of the chip.Microchannel heat transfer technology can better solve the problems of low heat dissipation,high energy consumption and space occupying space in the traditional heat exchanger,which has been widely used in the fields of aerospace,modern medical treatment,chemical biological engineering and so on.Based on the new idea of the passive thermal boundary layer interruption and active bubble turbulence,a series of novel microchannel heat exchangers are designed to verify the mechanism of heat transfer enhancement,including the periodically combining / dividing micro channel along the flow path direction and a sinusoidal microchannel with seed bubble disturbing flow.The physical essence of their enhanced heat transfer is systematically studied by numerical simulation.The paper discusses and analyzes the influence of geometric characteristics and main physical parameters on the flow and heat transfer characteristics in microchannels is discussed and analyzed.The specific progress includes the following: 1.Numerical simulation of the periodically combining / dividing micro channel along the flow path directionBased on the idea of thermal boundary layer interruption,the transverse partition is set along the flow direction in a rectangular microchannel,and the effect of no transverse partition on the flow and heat transfer in the microchannel is studied.The influence of the position distribution of the transverse partition on the convection heat transfer in the microchannel is analyzed and discussed.(1)The pressure field,temperature field and velocity field were analyzed and compared in straight pipe rectangular microchannel with the same hydraulic diameter and combining / dividing microchannel.It is found that the combining / dividing microchannel can significantly reduce the heating surface temperature and flow resistance,the distribution uniformity of temperature field is enhanced,and the heat transfer efficiency is 1.3 times as high as that of the straight tube.(2)By analyzing the thermal boundary layer in microchannel,it is found that the thermal boundary layer of the combining / dividing microchannel is thin and the temperature gradient is large.The thermal boundary layer is interrupted in every lateral partition area,and is formed and developed again when it enters the next microchannel.Most heat transfer in the channel is in the heat transfer mode at the entrance section of the pipeline,and the thermal boundary layer fails to develop,which significantly reduces the thermal resistance of the thermal boundary layer,so as to achieve the purpose of enhancing heat transfer.(3)In order to study the influence of the distribution of the transverse partition on the enhanced heat transfer of the combining / dividing microchannel,three different combining / dividing microchannels are designed,including the uniform combining / dividing microchannel,the risen order combining / dividing microchannel and the descending order combining / dividing microchannel.It is found that the flow stability and heat transfer efficiency of the microchannel with the descending order are the best,the heat exchange efficiency is 1.2 times,and the system pressure drop is reduced by 9.4%.2.Numerical simulation of heat transfer enhancement of bubbles in fluctuating micro channelBy using the theoretical model of laminar convection heat transfer and the geometric model of the sinusoidal microchannel,the mechanism of enhanced heat transfer by bubble turbulence is studied.The effects of the amplitude and wavelength of the sinusoidal microchannel,the size of the bubble and the liquid velocity on the enhanced heat transfer in the microchannel are analyzed and discussed.(1)According to the characteristics of bubble bouncing motion,the structure of sinusoidal microchannel is established,and the traditional straight tube microchannel is taken as the comparison object.It is found that the surface structure of sinusoidal ripple breaks off the thermal boundary near the wall.By studying the influence of sinusoidal structure with different amplitudes and wavelengths,a sinusoidal ripple microchannel #1 with strong heat transfer ability and low pressure drop was selected as the best physical model for bubble bouncing.(2)The flow and heat transfer process of a single bubble in the microchannel is studied.It is found that in one cycle,the bubbles undergo the process of bouncing and collision,which correspond to the jumping segments and the collision segments respectively.In the whole process of the movement of the bubble,the absorption heat is carried to the downstream area by the inertia of liquid fluid;on the other hand,the high temperature fluid is pushed to the downstream region by the disturbance of the thermal boundary layer near the wall of the collision section,which is the main way to strengthen the heat transfer.(3)The effects of bubble size and liquid flow velocity on the flow heat transfer in the optimized sinusoidal microchannel #1 were studied.It is found that the bubble flow with large bubbles(100?m)and high velocity(0.3m/s)can significantly increase the efficiency of microchannel heat transfer.In this paper,based on the new idea of hot edge base interruption and bubble turbulence intensification.A series of new microchannel heat exchangers are designed,which reveal the law of intensification of heat transfer on heat transfer heat boundary layer interruption and bubble disturbing,and optimize the structure of the microchannel heat exchanger.It has important guiding significance for the development of microchannel heat transfer technology and has wide value of industrial application. |
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