Mechanistic Modeling of Jet Impingement Boiling

Abstract

The current work presents a steady-state experimental study of nucleate and transition boiling under a planar water jet impinging on a flat horizontal copper surface. The maximum attained surface temperature is 560 C and the maximum wall heat flux is 5 MW/m2. The boiling curves under the impinging jet have been obtained for 0.6 and 0.7m/s jet velocity and 15 C of subcooling. The boiling curves are similar to flow boiling curves in the nucleate boiling regime. However, the curves are characterized by a region of constant heat flux (shoulder) in the transition boiling regime. The current work studies nucleate and transition impinging jet boiling. Determining the bubble size is essential for the heat flux estimation in the nucleate boiling regime. There are limited number of studies targeting bubble dynamics under impinging jets. The current study presents a new force balance for bubbles growing in the stagnation region for the nucleate boiling regime. The jet dynamics cause two more force components compared to the case of flow boiling: (1) an asymmetric bubble growth force for a moving fluid and (2) a pressure force caused by the jet stagnation. As the force balance depends on bubble growth rate, Zuber’s model for non-uniform temperature field has been found to best represent the current experimental data with different b values. b = /7 has been found to best fit the data at the stagnation region and b = 5 /6 for the parallel flow region. In transition boiling, the existence of the shoulder phenomenon has been attributed to vapor pockets break-up and formation on the heated surface. Vapor break-up and formation has been observed using high speed imaging. A new wall heat flux partitioning model has been developed based on the current physical understanding. The model has two components of heat transfer. Each component has been assumed to have a weighted contribution to the total heat flux based on the degree of superheat. The components are based on two mechanisms observed during vapor break-up. The first mechanism is due to the interaction between the jet dynamics and the vapor pockets in the stagnation region. The second mechanism which is more frequent at higher temperatures is due to Rayleigh-Taylor instability. The instability of the liquid-vapor interface grows and liquid wets the surface in the form of liquid columns. The modeled wall heat flux follows the experimental boiling curve reasonably. The model has a normalized root mean square error (NRMSE) of 33%. The wall heat flux has been found to be dependent on the vapor break-up frequency. A micro-size fiber optic has been used to measure the vapor oscillation frequency on the boiling surface. The probe has been produced by a new etching technique to assure minimal interference with the vapor formation or break-up. Based on the collected data, the break-up frequency has been modeled.