Effect of channel size and shape on condensation heat transfer of refrigerants HFO-1234yf and HFC-134a in rectangular microchannels

摘要

Refrigerant HFO-1234yf has similar properties to HFC-134a but much lower GWP value. It is possible to replace HFC-134a by directly drop into the air-condition systems and expected to be an appropriate replacement for these systems. MicroChannel heat exchangers have been commonly used in the vehicle air-condition systems attributed to their less weight and low air side pressure drops. However, there is no experimental study on condensation heat transfer performance of HFO-1234yf in microchannel with hydraulic diameter lower than 1 mm. This study provides an experimental comparison on condensation heat transfer and pressure drops of refrigerants HFO-1234yf and HFC-134a in multiport microchannels. The experimental results show that the effect of surface tension drainage force is important on condensation heat transfer in square channels especially for very small channels. The surface tension drainage force pulled the condensate to the corner of the square microchannels. This caused a very thin liquid film left on the tube wall and liquid conductivity became the major controlling property on the condensation heat transfer. Since the liquid conductivity of HFO-1234yf is 27% less than that of HFC-134a, the condensation heat transfer coefficients of HFO-1234yf are 15% to 23% lower than those of HFC-134a at various mass velocities and vapor qualities. The surface tension drainage force is in the direction of normal to the liquid and vapor flow. The experimental two-phase pressure drops are not affected by surface tension force and can be predicted reasonably well by using conventional correlations.