| With the development of electronic devices in the direction of smaller size and higher power,the problem of high heat flux heat dissipation has become a major challenge for the thermal management of electronic devices.Due to its high performance and compactness,two phase microchannel heat dissipation system has been identified as a promising heat dissipation technology.The microchannel evaporator is the core heat dissipation element,and the study of its working characteristics and internal heat transfer mechanism has important academic significance and application value.At present,many studies have been carried out on the flow boiling heat transfer characteristics of the microchannel evaporator.However,due to the inherent complexity and diversity of boiling heat transfer phenomenon and limitations of current testing methods,current understanding of the internal characteristics of microchannel flow boiling such as flow characteristics and heat transfer mechanism is not deep enough.Moreover,through literature research,it is found that most of the existing researches are based on the low L_t/D_h ratio microchannel evaporators with L_t/D_h<60.For a microchannel evaporator,higher L_t/D_hratio means that the heat transfer along the channel is more complicated,and it is more likely to appear heat transfer deterioration at the area close to the outlet,and the critical heat flux(CHF)will decrease.Based on this,the use of high L_t/D_h ratio microchannel evaporator means that it has to face more complicated working characteristics and the heat dissipation is more likely to deteriorate.Therefore,in order to cope with more complex heat dissipation conditions,it is necessary to study the heat transfer characteristics and flow boiling instability of microchannel evaporators.In this paper,a microchannel evaporator with the aspect ratio of L_t/D_h=100(higher than most of the microchannel evaporators in previous studies)is designed for a heat dissipation surface with an area of 10 mm×50 mm.The cross section size of each microchannel is 0.5 mm×0.5 mm,and the length is 50 mm.The interval between adjacent microchannels is 0.5 mm.Based on the microchannel evaporator,a visualization experimental device for microchannel flow boiling experiment is built.The boiling heat transfer characteristics and boiling instability of the microchannel evaporator are studied with deionized water as working fluid.The experimental research on heat transfer characteristics takes heat flux and flow rate as variables respectively.In the experiment of constant flow rate and variable heat flux,the change of wall temperature distribution along the channel during the heat flux gradual increase is focused on,and the heat transfer mechanism is studied by analyzing the flow pattern.A phase diagram of the wall temperature distribution state is made for the integration of the experimental conditions,and the changes of two phase heat transfer coefficient with heat flux and vapor quality are studied.In the experiment of constant heat flux and variable flow rate,the wall temperature variation regularity along the channel with the increase of flow rate is studied.The flow pattern,pressure and temperature data are analyzed,and the heat transfer mechanism leading to different variation regularity is studied.In the research on boiling instability,the mechanism of boiling instability and the adverse effects caused by boiling instability are first studied,and then the influence of flow rate on boiling instability is studied experimentally.The experimental study on microchannel flow boiling heat transfer characteristics shows that:at a constant flow rate,when the heat flux gradually increases from initial heat flux of two phase flow,because the dryout of film first appears at the local area close to the outlet of the microchannel evaporator,the wall temperature in that area is locally higher.As the heat flux continues to increases,when the steam convective heat transfer effect in that local area cannot stabilize the wall surface temperature,the CHF is reached.It can be seen from the two-phase heat transfer coefficient that in the initial stage of boiling,convective boiling dominates at the local area close to the outlet,and the heat transfer coefficient reaches the maximum(up to 175 k W/(m~2·K)).With the increase of heat flux and vapor quality,thin liquid film evaporation gradually dominates,and the heat transfer coefficient drops rapidly and gradually stabilizes(the range of heat transfer coefficient is around 75-100 k W/(m~2·K)).When the heat flux increases to a certain level,the heat transfer deteriorates,and the heat transfer coefficient decreases again(reduce to less than 50 k W/(m~2·K)).When the flow rate is gradually increased at a constant heat flux,with the increase of the flow rate,there will be two kinds of wall temperature variation regularity.When the flow rate is relatively small,with the increase of the flow rate,the rapid weakening of the reverse flow effect at the entrance of the channel causes the wall temperature near the entrance of the channel has a greater temperature drop,and the wall temperature uniformity along the channel gradually get worse.When the flow rate increases to the point where the reverse flow effect disappears,as the flow rate increases,the flow regime in the channel develops steadily,the temperature drop at the area close to the outlet becomes slightly larger than the area close to the inlet,and the wall temperature uniformity along the channel gradually get better.Research on the flow boiling instability of microchannels shows that:the reason for the flow boiling instability is that the steam produced by boiling in the confined space of the microchannel diffuses upstream,resulting in the reverse flow.The alternate appearance of reverse flow and incoming liquid causes the pressure oscillation at channel inlet and outlet and the continuous change of the flow regime in the channel,which in turn leads to the continuous change of the heat transfer on the wall and causes the wall temperature oscillation.With the increase of heat flux,the flow boiling instability of the microchannel evaporator becomes more and more severe.In the quasi-steady state,CHF may be advanced due to the instability of the flow rate.Therefore,the quasi-steady state can be regarded as a signal that the flow boiling instability of the microchannel evaporator is too high and the heat transfer deterioration is about to occur.In order to stably control the temperature of the electronic device,when the two phase microchannel heat dissipation system is working,it should try to keep the wall temperature distribution state in the stable state. |
