| In recent years,technologies such as concentrating photovoltaic systems and electronic information have developed in the direction of miniaturization and compactness,and heat exchange in micro-systems has become a new research direction.Microchannel cooling has been widely used in systems with high heat flow and limited space for heat dissipation due to the good heat transfer characteristics and small volume of the microchannel cooling technology.Two-phase heat dissipation is an effective method to dissipate a large amount of heat by using latent heat of phase change.Two-phase heat dissipation is an effective method to dissipate a large amount of heat by using the latent heat of phase change.Compared with the single-phase heat dissipation of microchannels,the flow boiling heat dissipation in the microchannel has the advantages in both the microscale effect and the phase change effect and can be used in a limited space.However,the micro-channel boiling two-phase flow is more prone to instability than the single-phase flow.Therefore,this dissertation focused on the flow boiling in the microchannels for concentrating photovoltaic heat dissipation and the flow and heat transfer characteristics under unsteady heat flux was investigated.In this paper,the flow and boiling heat transfer process of deionized water in a rectangular microchannel with a hydraulic diameter of 0.4mm was studied by a combination of experiments and simulations.In the experimental research part,a set of visual experimental equipment for the flow boiling heat transfer characteristics in the microchannel was built to study the influence of the inlet mass velocity,inlet temperature,and heat flux on the pressure drop and heat transfer coefficient of the microchannel under steady-state heat flow.The influence of the inlet mass velocity,inlet temperature,and heat flux increase amplitude on the pressure drop and heat transfer coefficient of the microchannel and the change of the flow pattern under the unsteady heat flow was discussed.The research results indicate that:(1)In the steady state heat flow,the pressure drop and heat transfer coefficient increased with the inlet temperature.The maximum heat transfer coefficient was increased from 23.56k W/m~2kto 43.09 k W/m~2k when the inlet temperature was increased from 45°C to 55°C.The pressure drop and heat transfer coefficient first decreased and then increased with the mass velocity,and increased with the heat flux.The increase of the pressure drop and heat transfer coefficient was small at low heat flux and started to get bigger when the heat flux increased to a certain value.(2)With an unsteady state heat flow,the pressure drop rised sharply after the heat flow increases suddenly due to the enhancement of the phase change and then stabilized which was showing obvious fluctuation characteristics.The pressure drop fluctuation intensity was affected by the initial conditions of the microchannel inlet.The pressure drop fluctuation intensity after the sudden increase in heat flow was decreased from 0.56 to 0.39 and the flow boiling instability decreased when the inlet temperature decreased.The pressure drop fluctuation after the sudden increase in heat flow can be effectively reduced when the mass velocity increased.For example,when the heat flow density increased to 97.96k W/m~2,the pressure drop fluctuation intensity was reduced from 0.57 to 0.05,and the flow boiling instability was reduced.Secondly,different heat flow increases had different effects on the fluctuation of pressure drop.When the heat flow increase was large,the pressure drop fluctuated greatly.For example,when the mass velocity was411.66kg/m~2s,the increase of pressure drop was increased from 3.40 k Pa to 6.82 k Pa,the flow boiling instability was greater,and vice versa.(3)When the heat flow was in an unsteady state,the gas phase in the microchannel increased accompanied by the change of the flow pattern after the heat flow increases suddenly.When the inlet temperature increased,the boiling start point was advanced and more gas phase was generated,and the instability of flow boiling increases;As the mass velocity increased,the temperature in the microchannel decreased,the gas phase in the microchannel decreased,and the time for the same flow pattern to occur went backward,the two-phase friction pressure decreased,the pressure drop increase and the fluctuation intensity showed a downward trend,and the instability of flow boiling was reduced.(4)In the case of unsteady heat flow,the heat transfer coefficient increased due to the enhancement of phase transition when the heat flow surged.Thereafter,the microchannel was blocked when the gas phase continues to increase and the bubble size was the same as that of the microchannel which resulted in the instability of flow boiling.Meanwhile,the heat transfer coefficient decreased.The fluctuation intensity of the heat transfer coefficient after the increase was affected by the initial conditions of the inlet of the microchannel.When the inlet temperature was reduced from 55°C to 45°C,the fluctuation of the heat transfer coefficient after the sudden increase of heat flow can be effectively weakened and the heat transfer coefficient after stabilization.When the mass velocity increased,the fluctuation of the heat transfer coefficient after a sudden increase in heat flow can be effectively reduced.For example,when the heat flow increased to97.96 k W/m~2,the fluctuation of the heat transfer coefficient after stability was reduced from 3.29k W/m~2K to 1.72k W/m~2K,and the instability of flow boiling was reduced.Secondly,the heat flow increase has different effects on the fluctuation of the heat transfer coefficient.When the heat flow increase was large,the heat transfer coefficient fluctuated greatly.For example,when the mass velocity was 411.66kg/m~2s,the fluctuation of the heat transfer coefficient after stability increased from 3.29k W/m~2K to4.65k W/m~2K,the flow boiling instability is greater and vice versa.In the numerical simulation part,a three-dimensional numerical simulation of microchannel flow boiling was established,and the VOF multiphase flow model was used to capture the vapor-liquid interface to simulate the real bubble separation process and flow pattern evolution.The influence of the increase of heat flux on the pressure drop of the microchannel and the change of the flow pattern was studied.Numerical simulation calculations showed that as the increase in heat flow increases,the heat was absorbed by the water in the microchannel increases,the gas phase generated in the microchannel increases,and the instability of flow boiling increases.Compared the gas-phase volume fraction diagrams of different heat flux increases,it can be found that the gas-phase volume fraction with a relatively large heat flux increase was larger and faster and the instability was greater.For example,when the increase in heat flux was 284.54k W/m~2,the maximum gas phase volume fraction was 0.425.When the heat flux increase was 97.96 k W/m~2,the maximum gas phase volume fraction was 0.104.The phase transition intensity was greater at the exit of the microchannel,and the gas phase was more and the instability was more likely to deteriorate near the exit of the microchannel. |
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