Enhancement of R1234ze(Z) pool boiling heat transfer on horizontal titanium tubes for high-temperature heat pumps

R1234ze(Z), which has a global warming potential of less than 1, is a promising alternative refrigerant for high-temperature heat pumps designed for heat recovery in the industrial sector. The use of titanium as the material for heat exchangers exposed to acid exhaust is one solution to prevent oxidation. In this study, the pool boiling heat transfer characteristics outside of horizontal titanium tubes were experimentally investigated for R1234ze(Z). A plain tube and three enhanced titanium tubes were tested at saturation temperatures from 10°C to 60°C and heat fluxes from 0.55 to 79.8 kW m⁻². Compared to the plain tube, the tested enhanced tube exhibited a 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range, which could compensate for the disadvantage in the thermal conductivity of titanium. The enhancement ratio predominantly depends on the saturation temperature and the wall heat flux. At conditions of higher saturation temperatures and lower heat flux, where smaller bubbles are observed, test tubes with smaller fin spaces exhibit higher heat transfer coefficients. The experimental results indicate the importance of fin geometry optimization to the operation conditions.