Experimental study of onset of subcooled annular flow boiling

An experimental study on the onset of nucleate boiling (ONB) is performed for water annular flow to provide a systematic database for low pressure and velocity conditions. A parametric study has been conducted to investigate the effect of pressure, inlet subcooling, heat and mass flux on flow boiling. The test section includes a Pyrex tube with 21 mm inner diameter and a stainless steel (SS-304) rod with outer diameter of 6 mm. Pressure, heat and mass flux are in the range of 1.73 < P < 3.82 bar, 40 < q < 450 kW/m2 and 70 < G < 620 kg/m2 s, respectively. The results illustrate that inception heat flux is extremely dependent on pressure, inlet subcooling temperature and mass flux; for example in pressure, velocity and inlet subcooling as 3.27 bar, 230 kg/m2 s and 41.3 °C; consequently qw,ONB is 177.3 kW/m2. In other case with higher inlet temperature of 71.5 °C and with P, 3.13 bar and G, 232 kg/m2 s the inception heat flux reached to 101.6 kW/m2. The data of ONB heat flux are over estimated from the existing correlation, and maximum deviation of wall superheat (ΔTw,ONB) from correlations is 30%. Experimental data of inception heat flux are within 22% of that predicted from the correlation.