Observing investigation of boiling characteristics around CHF for the downward facing heating surface

This paper investigates the boiling heat transfer characteristics near the critical heat flux (CHF) for the downward facing heating via the experimental observation and measurements. These boiling processes can be examined thoroughly by combining the heat flux histories, boiling curves, and the snapshots obtained from the high-speed video record. The fluctuation behaviors in the heat flux/temperature histories and boiling curves for the downward facing heating are mainly resulted from the iterative processes of bubble attachment/detachment on the heating surface for smaller oscillation in the nucleate boiling as well as the small bubble coalescence to an oblong bubble/this flat bubble film break-up into small bubbles for higher oscillation in the transition boiling. In addition, there are cyclic processes from the nucleate boiling regime to the transition boiling regime and back to the nucleate boiling regime. Several high peaks are revealed in the boiling curve and the averaged value of these peaks is defined as the CHF in this paper. The average CHF obtained for the pool boiling case in this study is close to those from the previous works. The injection flow effect on the enhancement of boiling heat transfer is also revealed in the measured data, especially for the smaller gap distance.