Visualization study of critical heat flux mechanism on a small and horizontal copper heater

We examined the influence of the macrolayer under a large vapor mushroom on critical heat flux (CHF) during pool boiling with a small plate heater. Evidence of a macrolayer was provided, and its measured thickness was compared and well agreed with values reported in the literature. The classical CHF models of pool boiling do not consider the effect of the heater size. For a small and horizontal heater, a hydrodynamic liquid inflow increases the CHF beyond the predictions of most models, which are based on an infinite and horizontal heater. Using high speed visualization of CHF, we proposed a new CHF triggering mechanism for a small heater. Because the hydrodynamic liquid inflow supplies liquid to the edge of the heater, nucleate boiling is maintained in this region, even when a large dry patch beneath the large mushroom is generated at the center of the heater. The CHF occurs when the macrolayer at the edge of the heater dries out (i.e., becomes coated with a vapor film) and meets the large dry patch at the center of the heater. Finally, we proposed the shape of macrolayer beneath the large mushroom in order to explain the CHF triggering mechanism on the small heater.