Enhancement of R1234ze(Z) Pool Boiling Heat Transfer on Horizontal Titanium Tubes for High Temperature Heat Pumps

摘要

Low GWP refrigerant R1234ze(Z) is promising candidate of refrigerant used in industrial high temperature heat pumps. The heat exchangers of such heat pumps designed for waste heat recovery systems is exposed to the exhaust containing acid substances. Using titanium as the material can be one solution to prevent oxidation. In this study, pool boiling heat transfer characteristics outside of horizontal titanium tubes were experimentally investigated for R1234ze(Z) at relatively higher temperatures. A plain tube and three enhanced tubes having different fin geometry were tested in a pressure vessel and the bubble behavior was observed. The experiment covers saturation temperatures from 10 to 60 °C and heat fluxes from 0.55 to 79.8 kWm-2. With the plain tube, it was confirmed that the measured heat transfer coefficients on the plain tube agree with the calculated heat transfer coefficients by Jung et al. correlation proposed for other HFC refrigerants within ±15%. The notable difference to the typical material copper is thermal resistance over the tube wall. Because the thermal conductivity of titanium is only 19 W m-1K-1 (that of copper is approximately 360 W m-1K-1), the thermal resistance is onsiderable. Comparing to the plain tube, the tested enhanced tube exhibited 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range. This could compensate the disadvantage in thermal conductivity for titanium. The enhancement ratio predominantly depends on the saturation temperature and wall heat flux. At conditions of the higher saturation temperatures and lower heat fluxes, where smaller bubbles were observed, the test tube having smaller fin spaces exhibits higher heat transfer coefficient. The experimental results remark the importance of fin geometry optimization to the operation conditions.