Film thickness and heat transfer characteristics of R1233zd(E) falling film with nucleate boiling on an inclined plate

Abstract

Falling film evaporation is an effective heat transfer method for achieving high heat flux and wide-area cooling with less refrigerant. As the film thickness and wall superheat increase, nucleate boiling occurs in the liquid film. In this case, the superficial liquid film thickness becomes thicker due to the existence of vapor bubbles. Although such superficial film thickness is an important parameter for expressing the film structure, the liquid film thickness of hydrofluorocarbon and hydrofluoroolefin refrigerants under nucleate boiling conditions has not yet been reported. This study experimentally investigated the liquid film thickness and heat transfer coefficients on an inclined plate with the width of 50 mm and the length of 75 mm. R1233zd(E) was used as the working fluid at a saturation temperature of 20°C. The film Reynolds number at the inlet was varied from 4.4 × 102 to 2.7 × 103, and the heat flux was varied from 5.6 to 138 kW/m2. As a result, the mean film thickness under adiabatic conditions increased when the flow rate increased or the inclination angle decreased from 30° to 15°, and ranged from 0.14 to 0.37 mm. The values were in good agreement with preexisting theories and correlations. Under heating conditions without boiling, the film thickness was almost equal to that under adiabatic conditions, and the heat transfer coefficients were slightly higher than the typical heat transfer correlation of the saturated falling film of water. With nucleate boiling in the high heat flux range, the superficial film thickness reached 1–3 mm at the location where vapor bubbles existed. The frequency of the thickness exceeding 1 mm increased with the heat flux as nucleate boiling became more active; thus, the average thickness also increased. The heat transfer coefficients under the developed nucleate boiling conditions were dominated by heat flux and were in good agreement with the existing pool boiling correlation.