Bubble Behaviors And Flow Boiling Heat Transfer Characteristics In Microchannel With Cavities

Flow boiling heat transfer in microchannel has the advantages of high heat transfer coefficient and small thermal stress due to the use of latent heat of vaporization,and it is expected to dissipate high heat flux in a small space.The cavities made on the heating wall can increase nucleation sites,promote the nucleation and detachment of bubbles,and enhance the flow boiling heat transfer and its stability in microchannel.Study on bubble behaviors in the process of flow boiling heat transfer in microchannel with cavities is helpful to understand and explain the internal relationship between bubble behaviors and the evolution of vapor-liquid two-phase flow pattern and the mechanism of flow boiling heat transfer.In this paper,based on the VOF two-phase flow model and user-defined function,the flow boiling heat transfer in microchannel with V-shaped,trapezoidal,square,and dovetail-shaped cavities on the heating wall has been numerically simulated.The effects of cavity shape on the bubble behaviors such as bubble nucleation,growth,detachment,and coalescence are analyzed,and the influences of cavity shape and working condition（variation of working fluid inlet velocity,heat flux）on the bubble detachment diameter and frequency are also obtained.Then,based on the evolution of vapor-liquid two-phase flow pattern in microchannel,the effects of cavity shape or working condition on the characteristics of flow boiling heat transfer in microchannel,such as heat transfer stability,wall heat transfer coefficient,pressure drop,are discussed.The main research work and conclusions are as follows:（1）The bubble behaviors in the process of flow boiling in microchannel with cavities are studied firstly,focusing on the effects of cavity shape on the characteristics of bubble nucleation,growth and detachment.It is found that the onset of bubble nucleation time varies with the different cavity shape,and the dovetail cavity in microchannel activates bubble nucleation earliest.The change of cavity shape takes no obvious effect on the bubble detachment diameter,but takes significant effect on the bubble detachment frequency.The bubble detachment frequency（64.9Hz）in microchannel with dovetail cavities is the highest,and the lowest bubble detachment frequency（33.8Hz）occurs in microchannel with trapezoidal cavities.It shows that the dovetail cavities in microchannel can enhance the microchannel flow boiling heat transfer much more.（2）The effects of working condition variation on the bubble detachment characteristics are compared and analyzed.With the increase of the inlet velocity of working fluid,the bubble detachment diameter in microchannel decreases,but the bubble detachment frequency increases.When inlet velocity changes from 0.1m·s^-1 to0.8m·s^-1,the bubble detachment diameter in the microchannel decreases from 0.19mm to 0.09mm,and the bubble detachment frequency increases from 53.2Hz to 112.9Hz.With the increase of heat flux on the heating wall,the bubble detachment diameter and frequency increase.When heat flux changes from 100k W·m^-2 to 500k W·m^-2,the bubble detachment diameter varies from 0.1295mm to 0.1395mm,and the bubble detachment frequency increases from 22Hz to 137Hz.Compared with the variation of the inlet velocity,the effect of heat flux on bubble detachment frequency is more significant.（3）The difference in the evolution of two-phase flow pattern between the microchannel with cavities and the straight microchannel is compared and analyzed,and the correlation between bubble behavior and the transition of two-phase flow pattern is also concerned.In the stage of bubbly flow,the number of bubbles in microchannel with the cavities is more than that of the straight microchannel.When confined bubble flow pattern occurs,the number of confined bubbles in microchannel with the cavities is less,occupying a smaller zone in microchannel.The cavity can delay the transition of two-phase flow pattern from bubbly flow to confined bubble flow in the microchannel,which is beneficial to improve the stability and reliability of flow boiling heat transfer in microchannel.（4）The effects of cavity shape and working condition variation on the flow boiling heat transfer stability in microchannel are investigated.In microchannel with different cavity shapes,both the standard deviation of pressure drop and wall temperature in microchannel are different.Among them,the standard deviation of pressure drop and wall temperature in microchannel with dovetail cavities is the smallest,showing the best stability of flow boiling heat transfer.Furthermore,the standard deviation of pressure drop and wall temperature decreases with increasing the inlet velocity of working fluid or decreasing the heat flux on the heating wall,which could improve the stability of flow boiling heat transfer in microchannel.（5）The effects of different cavity shape on the wall heat transfer coefficient and pressure drop of microchannel are compared,in order to analyze the enhancement of flow boiling heat transfer in microchannel.Compared with the straight microchannel,the wall heat transfer coefficient and pressure drop of flow boiling heat transfer in microchannel with the cavities can be promoted,and both increase with Reynolds number of the working fluid and heat flux on the heating wall.Under the same condition,the enhancement of flow boiling heat transfer in microchannel with dovetail cavities is much more outstanding.