Experimental Investigation of Boiling Heat Transfer Under an Impinging Water Jet

Abstract

The current study is an experimental and analytical investigation of JIB within the nucleate and transition boiling regimes. This study focuses on studying JIB within the stagnation zone of a free water jet. An experimental setup has been designed and built at the Thermal Processing Laboratory (TPL) with the capability of carrying out boiling experiments at heat fluxes up to 12 MW/m2. The JIB curves have been obtained under steady-state conditions for a wide range of jet conditions, higher than those considered during previous JIB studies. The effect of jet velocity, up to 3.8 m/s, and degree of subcooling, up to 49 °C, on the JIB curve has been studied. The results showed that both jet velocity and degree of subcooling have a weak effect on the nucleate boiling regime and significantly affect the transition boiling regime. Bubble dynamics under the impinging jet within the nucleate boiling regime and the stability of the vapor layer within the transition boiling regime have been investigated. An analytical mechanistic model, based on force balance and thermal balance equations, has been developed to predict the bubble growth rate and the BDD. The developed model was validated using current experimental data. The model gave a relative deviation of 17.8 %. Results of the mechanistic model within the stagnation zone showed that, amongst the three heat transfer mechanisms that affect bubble growth (i.e., the microlayer evaporation, the heat from the superheated layer, the convection heat loss to subcooled liquid), the microlayer evaporation is the most significant contributor to the rate of bubble growth. The current work conducted within the transition boiling regime was focused on the determination of the total wall heat flux within the stagnation zone, both experimentally and analytically. Steady-state experiments have been carried out during which the vapor layer stability was examined. The vapor layer breakup frequency was measured using a fiber-optic probe. Experiments were conducted at a jet velocity of 1 m/s and degrees of subcooling between 11 and 49 ºC.