An experimental study has been carried out to investigate the onset of nucleate boiling (ONB) characteristics of R134a in vertical and horizontal helically-coiled tubes. The experiments are carried out in a range of pressure from 412 to 850 kPa

inlet subcooling from 4.7 to 15.0℃

heat flux from 0.11 to 10.90 kW·m-2 and mass flux from 147.5 to 443.7 kg·m-2·s-1. The subcooled flow boiling curves of the vertical and horizontal helically-coiled tubes are analyzed from single-phase forced convection to ONB. A parametric study is explored to investigate the effects of the experimental parameters on ONB. The results indicate that the wall superheat and temperature undershoot at ONB in vertical coil are smaller than that in horizontal coil. The heat flux and wall superheat of the ONB increase with the increase of the mass flux and inlet subcooling but decrease with the increase of the system pressure and helical diameter for both vertical and horizontal helically-coiled tubes. Furthermore

the dimensionless parameters are introduced to develop a new correlation for predicting the heat flux at ONB in helically-coiled tube. It is shown that 95.8% of the experimental data of the heat flux at ONB are within 20% of that calculated from the new correlation.