HEAT TRANSFER TO R134a AND R123 DURING REFLUX CONDENSATION IN RECTANGULAR CHANNELS

摘要

Compact plate heat exchangers are often used for reflux condensation where the vapor enters the condenser at the bottom and flows upward while the condensate flows downward due to gravity. In such condensers the hydraulic diameter of the flow channels formed between two plates is 5-10 mm, and the flow channels are inclined. Fiedler and Auracher (2004) investigated reflux condensation in an inclined 7 mm ID tube as a representation of a single flow passage in a compact condenser. Real flow passages are, however, often better represented by rectangular cross sectional areas. As shown by Fiedler and Auracher (2004) the heat transfer in inclined tubes is significantly increased compared to vertical ones. The asymmetric distribution of the liquid film forms areas of thin films with high heat transfer rates and areas of thick films that have mainly the purpose to transport the liquid to the bottom. Inside rectangular channels, where the wider walls are vertically arranged in the inclined case, the fraction of these two areas is larger than in inclined tubes. This should have a positive effect on the heat transfer compared with tubes. Therefore experiments with a rectangular channel with the same hydraulic diameter (7 mm) and the same length (500 mm) as in Fiedler and Auracher (2004) are carried out to enable a direct comparison with the former results. R134a and R123 are used as test fluids. Experiments are carried out for Reynolds numbers between 20 and 130 for inclination angles of 10° to 90° to the horizontal. It is found that an optimum inclination angle exists where the heat transfer is about 30% better than in the vertically arranged rectangular channel. The heat transfer in the rectangular channel is by about 50% better than in the tube for each inclination angle.