Jet quenching phenomena during emergency cooling of high temperature solid surface

Abstract

In the case of loss of coolant accident, LOCA in nuclear reactor, heat removal can be successful only when water rewets the surface. Jet impingement is a highly potential technique of heat removal from high temperature solid surface. After the impingement of liquid jet on hot solid surface it is not capable to wet the surface immediately, it takes time (wetting delay) to get a favorable condition by the liquid to wet and then propagate over the surface. Generation of vapor and its explosion creates the barrier for the liquid front to move. An experimental investigation has been conducted during sub-cooled water jet impingement quenching of three different cylindrical blocks of copper, brass and steel. The jet velocity varied from 3 to15 m/s, jet sub-cooling was 5-80 K and initial block temperature was 250 to 600 ºC. The study was conducted to determine the dominating parameters of maximum heat flux, wetting delay, quench front movement, boiling region size and finally developed a correlation of maximum heat flux for steel. After elapsing the wetting delay period, a visible smaller boiling region was observed to move radially from the impinged centre. The leading edge of this boiling region is designated as the quench front. The boiling region is a vital region as the maximum heat flux occurred at this region during cooling. The boiling region increases with the movement of the quench front and therefore the position of maximum heat flux moves accordingly. The boiling region width increases with material conductivity and decreases with liquid sub-cooling and liquid velocity. The maximum heat flux increases with jet velocity, liquid sub-cooling, solid material conductivity and it is almost independent of solid initial temperature. Wetting delay increases with block initial temperature and decreases with jet velocity and liquid sub-cooling.