Mixed convection heat transfer of supercritical pressure R1234yf in horizontal flow: Comparison study as alternative to R134a in organic Rankine cycles

The low GWP working fluid of R1234yf is a prospective alternative to the conventional R134a used in organic Rankine cycles. As studies regarding the heat transfer of supercritical pressure R1234yf are rare, supercritical heat transfer experiments of R1234yf were conducted for the first time to evaluate the feasibility of R1234yf in replacing R134a from the perspective of supercritical heat transfer performance. First, the effects of pressure, mass flux, and heat flux were analyzed. Subsequently, the supercritical heat transfer performances of the two fluids were compared experimentally and numerically. Results show that the heat transfer characteristics of R1234yf and R134a are similar at the same operating condition with the heat transfer coefficient of R1234yf being approximately 10% higher than that of R134a, while the pressure drop of R1234yf is higher. Numerical analyses on detailed flow and thermal field information show that the heat transfer difference between the two fluids is primarily caused by the difference in density, thereby causing a stronger buoyancy effect in R134a at the same operating condition. Finally, the applicability of the R134a-based buoyancy criterion and heat transfer correlation were evaluated for R1234yf, and results indicate that they can be used for R1234yf as well.