| In this paper,the design of micro-channel evaporator in automotive air conditioning is taken as the research background.The environmental protection,safety and thermal performance of the refrigerant are considered comprehensively,and HFO1234 yf is used as substitute for R134 a.As the channel size decreases,the boiling heat transfer disciplinarian in the conventional channel is no longer suitable for micro-channel.Although the flow pattern,heat transfer and pressure drop characteristics in microchannels have been studied,the study of large-scale micro-channels is still absent.Therefore,the boiling heat transfer mechanism of HFO1234 yf in large-scale micro-channels is deeply explored in this paper,the flow pattern,heat transfer coefficient and pressure drop in the flow boiling process are taken as the starting point,and the theoretical analysis,visualization experiments,and numerical simulation studies were conducted.A rectangular micro-channel with a hydraulic diameter of 0.5 mm and length of 400 mm was designed,and the micro-channel flow boiling visualization experimental bench was set up.The experiments were conducted at mass flux range of 251-579 kg/m2 s,heat flux range of 8.21-32.68 kW/m2 and saturation temperature range of 0-10?.The effects of saturation temperature,mass flux and heat flux on the heat transfer coefficient and pressure drop of the flow boiling process in the micro-channel were studied.The high-speed camera was used to visualize the micro-channel two-phase flow pattern.The classification and definition of the flow pattern was carried out,and the relationship between flow pattern and heat transfer coefficient and pressure drop was analyzed.The correlation of heat transfer coefficient and pressure drop based on flow pattern were proposed.For the analysis of the experiment,different flow patterns were observed in the experiment: bubbly flow,slug flow,elongated slug flow,churn flow,wavy annular flow,throat-annular flow,annular flow and dry out.In terms of heat transfer coefficient,the mass flux has little effect on the heat transfer coefficient in low vapor quality,but has a significant effect in high vapor quality.However,the heat flux has a significant effect in low vapor quality,but has no significant effect in high vapor quality.The heat transfer coefficient is positively correlated with mass flux and heat flux.In terms of pressure drop,the effect of mass flux on pressure drop is significant,and the pressure drop is positively correlated withmass flux.However,the heat flux has no significant effect on the change of pressure drop,and the pressure drop is negatively correlated with saturation temperature.For the analysis of heat transfer coefficient correlation,based on the Chen correlation,the effects of Reynolds number Re,Weber number We,boiling number Bo and Martinell number X are considered to fit the new Chen-type correlation.The new Chen-type correlation predicts 86% of the data points within the error band of ±25%.However,the trend of different flow patterns cannot be accurately predicted,because the influence of micro-channel flow pattern transition is not considered.Therefore,the slug flow and the annular flow are distinguished to refit the heat transfer correlation based on the flow pattern.The new correlation predicts 95.5% of the data points within the error band of ±25%.The average relative deviation and the average absolute deviation were respectively 2% and 18%.For the analysis of pressure drop correlation,based on the Lockhart-Martinelli correlation,the effects of capillary number Ca,pure liquid Reynolds number Relo and Suratman number Su are considered to fit the new Lockhart-Martinell type correlation.The new correlation has 66% and 90% of the data points falling within the error band of ±25%and ±40%,respectively.Since the influence of the micro-channel flow pattern transition is not considered,the prediction of the pressure drop trend of the annular flow is not accurate.Therefore,according to the structure of slug flow and annular flow,the heat transfer correlation based on the flow pattern is refitted.The new correlation predicts 98% of the data points within the error band of ±25%,the average relative deviation and the average absolute deviation were 1% and 15%,respectively.For the analysis of numerical simulation,the Fluent software is used to construct the vapor-liquid two-phase flow heat transfer model,and the VOF model is selected to track the phase interface.The user-defined function is programmed to realize the control of the vapor-liquid two-phase heat and mass exchange.The growth and departure of a single bubble in the micro-channel are captured,the effect of wall contact angle on bubble nucleation is investigated,and the vapor-liquid volume fraction field,velocity vector field,temperature field and pressure field are obtained.It is found that the vapor-liquid two-phase prevalence in the numerical simulation is highly consistent with the experimental results.The average absolute deviation of the numerical simulation and experimental results are respectively16.3% and 19.6% for heat transfer coefficient and pressure drop. |
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