Study of secondary pool boiling effects with functionalized surfaces created via Femtosecond Laser Surface Processing

A pool boiling phenomenon referred to as secondary boiling effects is discussed. Based on experimental trends and numerical simulations, a mechanism is proposed that identifies the parameters that lead to this phenomenon. Secondary boiling effects refer to a distinct decrease in the wall superheat temperature near the critical heat flux due to a significant increase in the heat transfer coefficient. Recent pool boiling heat transfer experiments on femtosecond laser processed Inconel and stainless steel multiscale surfaces consistently displayed secondary boiling effects. Through experimental and numerical studies, it was determined that secondary boiling effects are a result of both temperature drop along the microstructures and nucleation characteristic length scales. The temperature drop is a function of microstructure height and thermal conductivity. An increased microstructure height and a decreased thermal conductivity result in a significant temperature drop along the microstructures. This temperature drop becomes more pronounced at higher heat fluxes and along with the right nucleation characteristic length scales results in a change of the boiling dynamics. Nucleation spreads from the bottom of the microstructure valleys to the top resulting in a decreased surface superheat with increasing heat flux. This decrease in wall superheat at higher heat fluxes is reflected by a hook back of the traditional boiling curve and is thus referred to as secondary boiling effects. In addition, a boiling hysteresis during increasing and decreasing heat flux develops due to the secondary boiling effects. This hysteresis further validates the existence of secondary boiling effects.