Observation of local heat transfer during pool boiling of a highly wetting fluid

Abstract

This thesis addressed fundamental insight into the mechanism by which CHF is triggered. Proper physics-based understanding of the mechanisms by which critical heat flux (CHF) occurs is crucial in order to guide the development of high heat removal devices. Although many mechanisms by which critical heat flux occurs during pool boiling have been proposed, these models are not consistent with the majority of the data. One of reasons is the lack of reliable local information that can enable models to be tested. In this study, the experimental work was undertaken to investigate the process by which pool-boiling critical heat flux (CHF) occurs using an IR camera to measure the local temperature and heat transfer coefficients on a heated silicon surface. The wetted area fraction (WF), the contact line length density (CLD), the frequency between dryout events, the lifetime of the dry patches, the speed of the advancing and receding contact lines, the dry patch size distribution, and the regional heat transfer on the surface were measured throughout the boiling curve. It was found that the heat flux through the liquid covered area was relatively small compared to the heat flux on the contact line, however, contributes the most to the overall boiling heat transfer. The heat transfer on the liquid covered area accounted for at least 85% of the overall heat transfer at CHF. Furthermore, quantitative analysis of this data at high heat flux and transition through CHF revealed that the boiling curve can simply be obtained by weighting the heat flux from the liquid-covered areas by WF. It was observed that CHF occurs when the heat transfer through the liquid covered area is not enough to remove the heat since the dry patch size increases faster than the increase in heat transfer through the liquid area. Finally, CHF mechanisms proposed in the literature were evaluated against the observations.