Flow boiling of R1234ze(Z) and R1234ze(E) in horizontal micro-fin tubes for high temperature application

Low GWP refrigerants R1234ze(Z) and R1234ze(E) are anticipated to being potential refrigerants in high-temperature heat pump systems. This study presents flow boiling heat transfer coefficient and pressure gradient in a horizontal micro-fin tube of 5.35 mm ID at higher temperatures for these new refrigerants and conventional refrigerant R134a. The experiments were conducted at a saturation temperature of 30 °C, mass velocities from 100 to 400 kg m^-2s^-1, and heat fluxes from 5 to 15 kW m^-2. The measured heat transfer coefficient (HTC) and pressure gradient of R1234ze(Z) are higher than those of R134a, while the HTC and pressure gradient of R1234ze(E) are as high as those of R134a. The predicted HTC and pressure gradient by selected correlations reasonably agree with the experimental data. The measured circumferential temperature distribution of the tube wall changes the behavior at vapor qualities where the correlations indicate the transition from separated flow to annular flow and the inception of dryout.