Modeling of transition boiling under an impinging water jet

Abstract

Liquid jet impingement is known for its high capability of extracting heat from the surface it impinges on. Steady state subcooled boiling experiments were carried out in the stagnation region of a water planar jet of 0.6 and 0.75m/s jet velocity and 15°C of subcooling. The transition boiling regime is found to have a region of constant heat flux. After the critical heat flux (CHF) is reached, the heat flux decreases till a local minimum value is reached; called the first minimum. The heat flux then increases till it reaches a relatively constant value, even with the increase of the surface degree of superheat. The constant heat flux region is called the shoulder heat flux. With the aid of high speed imaging, Rayleigh–Taylor instability has been observed at the liquid–vapor interface which is accelerated by gravity and jet dynamic pressure. A modified expression for the interface acceleration is proposed. The vapor layer formed in the transition boiling is not stable; it follows periodic cycles of break up and formation. Two surface wetting mechanisms are proposed based on observations: (i) direct liquid touching the surface at moderate degrees of superheat and (ii) liquid columns intruding the vapor layer and touching the surface at discrete locations at high degrees of superheat. A new wall heat flux partitioning model is proposed. The proposed model divides the wall heat flux, based on the two observed wetting mechanisms, into two components: (i) quenching heat flux and (ii) intrusion heat flux. The current model estimates the wall heat flux during transition boiling covering the surface superheat range from the CHF till the Leidenfrost point with a normalized root mean square error (NRMSE) of 33%.