Experimental study of steady state nucleate boiling heat transfer under planar jet impingement

Experimental study was conducted to investigate the characteristics of subcooled flow boiling during free water jet impingement on horizontal upward facing flat surface. Different sub regimes within nucleate regime has been identified and quantified in terms of the range of surface superheat and the values and slope of heat flux for each range. The study was conducted under steady state, controlled temperature boiling conditions for range of jet velocity Vj=0.75 to 1.68 m/s and water degree of subcooling ΔTsub=10 to 28°C. The nozzle is of sudden contraction configuration and has rectangular cross section (w=1mm, 1=8 mm). The nucleate boiling sub regimes; namely: partial nucleate boiling and fully developed; in addition to the vicinity of the critical heat flux (CHF) were known to have different capacity of heat removal due to the corresponding governing mechanism of heat transfer. The current study addresses the changes to those mechanisms with the aid of visual observations and the relative changes in the boiling curves obtained from the temperature measurements. Two analysis approaches were used: averaged ID to study the general effect of the velocity and subcooling at a given heat flux and distributed 2D approach where the relative variations in boiling curve were investigated for stagnation (x/w=0) and downstream locations (x/w=2 and 4). Increasing jet velocity and subcooling delayed the initiation of net vapor generation and reduced the average bubble size and frequency. As the wall superheat increased both velocity and subcooling increased the range of nucleate boiling regime such that heat flux values in the order of 6 MW/m ² were obtained at surface superheat (ΔTs ∼100°C) with no burn out all over the boiling surface. A flat flux shoulder was observed after CHF at the lower values of the two parameters where as the heat flux experienced narrow step after the CHF and was then followed by a secondary increase at the higher levels of each factor. In addition, at stagnation less heat flux was obtained relative to downstream locations over most of nucleate boiling regime and in early transition boiling flux shoulder. This is on the contrary to its superior performance in single phase heat transfer. Flux shoulder extended for superheat range of 30°C at the lower bound of jet velocity and subcooling whereas it dropped at higher velocity and subcooling to below 10°C.