Study On Boiling Heat Transfer Enhancement By Vapor-Liquid Phase Distribution Regulation

With the development of micro-fabrication,the integration degree of electronic chips is getting higher and higher,the heat flux of equipment increases sharply,and the high heat flux generated has extremely adverse effect on the stability of performance and service life of electronic equipment.Conventional heat dissipation methods such as air cooling,wind cooling,etc.,can no longer continue to meet such requirements of large heat flux removal.Microchannel structures allow for more efficient removal of high heat flux due to their higher specific surface area,on the other hand,The microchannel boiling heat exchange process occurs when the working fluid changes from liquid to vapor and absorbs a large amount of latent heat of vaporization.It is an efficient heat dissipation method.For flow boiling heat transfer in a common microchannel,the liquid working medium is in a high superheated state under high wall superheat.Once bubble nucleation occurs at the nucleation site,the small bubbles rapidly grow to fill the entire microchannel,and the energy stored in the superheated liquid is concentrated and rapidly released through a limited vapor-liquid interface,The vapor-liquid interfaces expand rapidly,causing large fluctuations in flow,pressure drop,temperature,etc.and the vapor-liquid interface may even flow back to the reservoir of the microchannel inlet,and at the same time generate thermal stress,which may cause thermal fatigue.It has an adverse effect on the operation of the microchannel heat exchanger.In order to improve this situation,this paper proposes two new structures for microchannel flow boiling to control the vapor-liquid phase distribution and enhance flow boiling heat transfer.Two types of micro-channel structures enhanced boiling heat transfer which are based on the staggered split of vapor-liquid multiphase fluids and vapor-liquid phase separation were constructed.The boiling heat transfer characteristics in split microchannel structure,separate phase microchannel structure and ordinary parallel microchannel have been studied.With high-speed visualization,the flow patterns in separate microchannel structure and split microchannel structure were studied.The mechanisms of heat transfer enhancement for the two types of structure regulated phase distribution were revealed.It is found that for the phase-splitted microchannel,the working fluid enters the microchannel to generate vapor-liquid phase change,and the microchannel array is intercrossed along the flow process,which can realize five types of enhanced heat transfer modes at the same time.(1)vapor phase separation makes the bubble constant in its total volume but the surface area increases,thereby increasing the vapor-liquid latent heat exchange area and enhancing latent heat transfer;(2)the vapor phase division increases the number of bubbles,thereby increasing the bubble tail area and enhancing the microconvection heat transfer around the bubble;(3)thermal boundary layer detachment and redevelopment of the liquid phase enhances convective sensible heat transfer;(4)the division of the vapor phase delays the deterioration of heat transfer caused by vapor film covering heating surface,and increases the critical heat flux;(5)the periodic partitioning of the liquid phase can cause the thermal boundary layer in the near-wall region to be periodically detached and redeveloped.Due to the large thermal resistance of the thermal boundary layer,periodically destroying the boundary layer can significantly reduce the thermal resistance in the near-wall region.and improve convection sensible heat exchange.For phase-separated microchannel,when there is no phase change in the microchannel,due to the laterally expanding structure and the channels on both sides are wider,the two lateral channels have less flow resistance and their flow is greater than that in the middle area.When phase change occurs,a small part of vapors start to enter the narrow channel in the middle area,most of vapors flow into the wide channels on both sides of the channel,and most of the bubbles flow to the area where resistance is small through the gradually expanding channel.And the downstream microchannel acts as a bubble filter to a certain extent,realizing that the bubbles flow on sides,and mostly liquid phase flow in the middle channel area.The instability caused by the rapid expansion of the interface when the vapor-liquid mixed flow in the channel is suppressed.As the average vapor increases,more steam accumulates on both sides,creating a vapor blocking effect.The vapor-liquid interface in the channels on both sides expands toward the central area channel in the laterally expanding structure.As the vapor pressure overcomes the interfacial tension,the steam begins to enter the downstream channel and forms the slug flow(annulus-like flow)in the channel,the thin film evaporation mode of this annulus-like flow greatly enhances heat transfer.It was found that the phase-separated microchannel has the best heat transfer performance.The parallel-type straight microchannel has the worst heat transfer performance.The phase-splitted microchannel has the largest press drop,and the phase-seperated microchannel has the smallest press drop.Compared with ordinary parallel microchannels,under the condition of G=1.5g/s and q=320kW/m2,it was found that the maximum comprehensive evaluation factors for phase-separated microchannel and the phase-splitted microchannel reache a maximum of 1.17 and 1.93,respectively.