Experimental Study of Horizontal Flow Boiling Heat Transfer of R134a at a Saturation Temperature of 18.6 °C

In spite of the extensive work in flow boiling in small-diameter tubes, the general characteristics and dominant mechanisms remain elusive. In this study, flow boiling heat transfer of R134a inside a 5mm I. D., smooth horizontal stainless steel pipe is experimentally studied. Local heat transfer coefficients (HTCs) were measured for heat fluxes from 3.9 to 47 kW/m(2) and mass fluxes from 200 to 400 kg/m(2) s at a saturation temperature of 18.6 degrees C. The studied cases have shown different behaviors at low and high heat fluxes. At low heat fluxes, the convective contribution looks to control the HTC, while at high heat fluxes the nucleation of vapor looks to be the dominant mechanism. Reducing the heat flux, the HTC approaches asymptotically a limit equivalent to the single-phase HTC defined in terms of the sum of the superficial liquid and vapor Reynolds numbers. A new correlation for dominant convective flow boiling is proposed and evaluated against experimental data from the literature.