Study On Dryout Characteristics Of Parallel Mini-Channels In Flow Boiling Process

The mini-channel heat exchanger has the advantages of high heat exchange efficiency,easy sealing and small size,which makes it show great potential in the fields of aerospace,electronic equipment,chemical preparation,etc.The drying characteristics are studied.This article takes an experimental approach to explore the drying characteristics of refrigerant R141b in parallel small-channel heat exchangers(the heat exchanger is made up of 9 parallel 2 × 2 ×250mm mini-channels).This paper first studies the drying characteristics of parallel small channels during boiling heat transfer.Through visual observation,it was found that in the process of boiling heat transfer,the parallel small channels will sequentially have five different flow patterns:bubbly flow,slug flow,annular flow,dryout and mist flow.The temperature signals under the five flow patterns are calculated,and it is found that in the small channel boiling heat transfer process,the heat transfer coefficient of the annular flow is the highest and the heat transfer effect is the best.When the channel is dryout,the heat transfer effect of the channel is significantly reduced.In this paper,the dynamic characteristics of the channel are revealed by analyzing the differential pressure signals under different flow patterns.Due to the chaotic phenomenon in the process of boiling heat transfer,this paper uses three signal analysis methods(AOK-TFR analysis method,AR-power spectrum analysis method and recursive graph analysis method)to analyze the pressure difference signal in the frequency domain.And based on the dynamic analysis,by summarizing the recursive characteristic parameters and summarizing,the dryout warning prediction model based on the pressure drop signal is obtained as:RR<0.38,DET>0.65,and LAM>0.82.When the recursive characteristic parameters of a flow pattern pressure drop signal conform to the above rules,the flow pattern is an annular early warning flow pattern.At this time,it is necessary to adjust the flow rate or the inlet temperature of the working medium to prevent dryout and realize the dynamic characteristics through dry Predict the purpose of drying up.This paper continues to study the drying characteristics of parallel mini-channels under electric field interference.Through visual research,it is found that the small channel has a special flow pattern change phenomenon under the interference of the electric field force,that is,there will be bubbles in the channel when the small channel is not interfered by the electric field force.And with the development of bubbles,the flow pattern evolves,and under the action of electric field force,no bubbles are generated in the channel and the single-phase liquid flow has been maintained.After the dryout point appears at the bottom of the channel(in other words,the channel reaches the critical heat flux density),the flow pattern in the channel instantly changes to dryout and mist flow patterns.The temperature and pressure signals in the channels are analyzed under the interference of electric field force.The results show that the wall temperature when the channel reaches the critical point increases with increasing voltage.Before reaching the drying point,the differential pressure signal of the channel decreases as the voltage increases.The thermodynamic parameters including heat transfer coefficient,critical heat flux density and effective heat absorption rate in the channel all increase with increasing voltage.In order to better predict the critical heat flux of the channel under the interference of electric field force,a new dimensionless parameter-dimensionless electric field force is proposed in this paper,and on this basis,a prediction model of the critical heat flux density under the interference of electric field force.Through verification,the prediction accuracy of the new critical heat flux prediction formula proposed in this paper is within ±15%.