Numerical Simulation On Flow Boiling Heat Transfer Characteristics In Wavy Microchannel

Flow boiling heat transfer in microchannel can take away a large amount of heat in smaller space,which will be applied for electric chip cooling.Wavy microchannel is expected to improve the flow boiling heat transfer by changing the channel structure.In this paper,nine sets of sinusoidal wavy microchannels and one straight microchannel are designed.Based on the VOF model of CFD software,the theoretical model of flow boiling heat transfer in the microchannel is established properly by supplying user defined function.Then the flow boiling heat transfer process is simulated numerically.The influences of wave structure parameters including wave amplitude and wave pitch on bubble morphology,two-phase flow pattern,flow boiling stability and flow boiling heat transfer effect are studied.Meanwhile,the results are compared with straight microchannels.The main work and conclusions of this paper are as follows:?1?Compared the velocity field and the streamlines between the wavy microchannels and straight microchannel,secondary flow is found in the wavy microchannels.And then compared the bubble morphology in wavy microchannels and straight microchannel,it is found that due to the presence of secondary flow,the bubbles in wavy microchannels detach more easily.What's more,the bubbles in the microchannels are smaller and more in quantity,maintaining nucleate boiling in wavy microchannel.?2?The main flow patterns in straight microchannels are:confined bubbly flow,slug flow,elongated bubble flow,and annular flow.While the main flow patterns in wavy microchannel are:isolated bubble flow,slug flow,enlongated bubble flow,annular flow.However,the vapor blocks in wavy microchannels are short,making it difficult to block the microchannels.And when it becomes the annular flow,there is still a liquid film between the bubble and the heating wall,which is beneficial to keeping a reliable flow boiling.?3?By comparing the differences of flow boiling stability between the wavy microchannels and the straight microchannel,on the condition of T_in=368 K,v_in=0.4m·s^-1,q_w=250 kW·m^-2,the temperature of heating wall and pressure drop fluctuations are smaller in wavy microchannel during t=20 ms³0 ms and the flow boiling is relatively stable.While in straight microchannel,under the same conditions,the corresponding temperature and pressure drop fluctuation are both high in amplitude and frequency.In addition,the temperature of heating wall continues to rise,which makes the heat transfer deterioration and instability of the flow boiling.Further study has found that increasing the amplitude or decreasing the wave pitch are both beneficial to the stability of the flow boiling in wavy microchannel.?4?Wave structure is beneficial to improve the reliability of flow boiling in microchannels,but the amplitude and wave pitch should be controlled within a certain range.When the wave pitch reduces to a certain extent,continuously increasing the wave amplitude may result in heat transfer deterioration.?5?Compared with straight microchannel,the flow resistance and flow boiling heat transfer coefficient in wavy microchannels are both high,and they both increase with Reynolds number.Moreover,the wave amplitude and wave pitch have effects on the flow boiling heat transfer and flow resistance,and increasing the amplitude or decreasing the wave pitch can make the flow boiling heat transfer coefficient and flow resistance both increase.However,the wave amplitude has greater influence on the flow boiling coefficient,and the influence of the wave pitch on the flow resistance is more prominent.Besides,at the same wave pitch,the wave amplitude increased by 4times,the flow resistance and the flow boiling heat transfer coefficient increased by24%and 26.5%respectively.Under the same wave amplitude,the wave pitch decreased by 4 times,and the flow resistance and boiling heat transfer coefficient increased by 40%and 16%respectively.?6?Using comprehensive evaluation factor to judge the overall heat transfer performance of ten groups of microchannels.It can be concluded that the overall performance of#4 wavy microchannel?A=40?m,?=0.5 mm?is best.