Experimental Study on Onset of Nucleate Boiling in Vertical Annular Channel with a Short Heated Rod  Under Low Pressure

The onset of nucleate boiling (ONB) in a vertical annular channel with single short heated rod was studied by using deionized water under the conditions of inlet pressure ranges from 0.16 to 0.3 MPa and mass flux ranges from 100 to 300 kg·m -2 ·s -1 . In this study, the influence of mass flux, inlet temperature and pressure on ONB were studied and the transition point from single phase to onset of nucleate boiling was obtained by increasing the heat flux step by step. A high-speed camera was used to capture bubbles generating position under different heat flux. The experimental results show that the ONB occurs firstly near the outlet position of the test section, and the ONB moves gradually from the outlet of the test section to the lower part of the heated wall with the increase of heat flux and that the heat flux and wall superheat needed for reaching ONB increase with the increase of the mass flux, inlet subcooling and pressure. Based on the experimental data, the effect of inlet temperature and mass flux on the wall superheat and heat flux at ONB was investigated. In addition, conventional correlations were evaluated for predicting wall superheat and heat flux at ONB. It is found that heat flux at ONB calculated by these correlations is in poor agreement with experimental data in this study. Hence, a new correlation has been developed for predicting the heat flux at ONB in annular channel with a shorted heat rod under low pressure. The values calculated by the new correlation has a good agreement with the experimental data in this study.