Effect of subsurface tunnel on the nucleate pool boiling heat transfer of R1234ze(E), R1233zd(E) and R134a

Nucleate pool boiling heat transfer of R1234ze(E), R1233zd(E) and R134a were investigated outside two reentrant cavity tubes. The two tubes have different fin density while the size of subsurface tunnel width and fin height are almost the same. In the experiment, the pool boiling heat transfer is tested at the saturation temperature of 6 °C and heat flux of 10–80 kW/m². A brief summary of pool boiling experimental data of Hydrofluoroolefins(HFO) refrigerants on different enhanced tubes from recent work is also provided. It is found that the combinations of tube and refrigerant: R134a and R13234ze(E) outside Tube-B1 shows almost equivalent boiling heat transfer performance. The two combinations also yield the highest averaged overall heat transfer coefficient. At the lower heat flux less than 60 kW/m², for the tube with larger fin density and thinner fin thickness, the number of nucleation cavities is found to be more than that with larger fin thickness. At the heat flux more than 40 kW/m², the boiling heat transfer coefficient of R134a and R1234ze(E) for the two enhanced tubes nearly merges into a single curve. At the higher heat flux, boiling heat transfer shows weak dependence on the surface structures. The boiling heat transfer coefficient of R1233zd(E) is more than 40 percentages lower than R134a for the two tubes at the nearly identical conditions. The research is helpful for the designer to summarize the boiling heat transfer performance of some typical HFO refrigerants.