Flow Regime And Pressure Drop Of Refrigerant Condensation In Microchannel Arrays

Advanced technology development demands high heat dissipation in microstructure instruments,which promotes the research of phase-change heat transfer in microchannel heat exchangers.However,studies on condensation flow in microchannels mostly focused on steam condensing in silicon microchannels with hydraulic diameters smaller than 1mm cooled from only one side,which is insufficient for practical applications when the microchannels are cooled symmetrically with refrigerant flowing inside.Therefore,the investigation of refrigerant condensation flow in microchannels with hydraulic diameters smaller than 1 mm is necessary and important.An experimental investigation was conducted for two-phase flow regimes,flow pattern transition and pressure drop during condensation of refrigerant R134 a and R1234ze(E)in oval microchannel arrays with a hydraulic diameter of 301.9 ?m.The flow pattern transition processes and mechanisms and their effects on pressure drop were analyzed.Spcifically,the effects of cooling methods,including symmetric cooling and asymmetric cooling,on the flow pattern transition and pressure drop were studied.Three annular-to-intermittent flow pattern transition modes were observed.A flow regime map is given and compared with flow regime criteria in the literature.Waves were observed in the thin liquid film along the flat zone and the semicircle zone of the channels,whose wave lengths and velocities were measured.The frictional pressure drop increases with increasing refrigerant mass flux and vapor quality.Lower refrigerant saturation temperature or higher cooling water inlet temperature cause higher frictional pressure drop.Compared with microchannels cooled asymmetrically,condensation flow in microchannels cooled symmetrically has a lower frictional pressure drop.The frictional pressure drop of R134 a is slightly lower than that of R1234ze(E).The frictional pressure drop data was compared with available correlations in the literature but most correlations could not accurately predict the the present data.A theoretical model for R134 a and R1234ze(E)condensation stable annular flow in the oval microchannels used in the present study was proposed.The liquid film thickness profile variation with condensation was given,based on which the local circumferential mean heat transfer coefficients were obtained.The local circumferential mean heat transfer coefficient decreases suddenly as the refrigerant vapor enters the microchannel then keeps constant along flow direction.The constant value varies little with different refrigerant mass fluxes.The liquid-vapor interface linear instability model was proposed for condensation flow in circular microchannels and the oval microchannels in the present study using normal mode method.The interfacial wave length and velocity were obtained by finding the greatest instability point.The wave length for condensation flow in circular microchannels increased with decreasing R134 a mass flux and quality,with jumps between two instability states.This instability state transition can be used to predict condensation flow regime transitions for intermittent flow.R134 a condensation flow regime data in the literature was predicted by this intermittent flow criterion.The criterion predicted the data well for higher mass fluxes.The wave length and velocity obtained with this model for the oval microchannels were compared with the experimental data in the present study.The predicted wave length agreed well the experimental data,however the experimental wave velocity was overestimated by the model.