Critical heat flux (CHF) under flow oscillation in boiling channel is drastically reduced to about 40 to 50 percent of the steady state value, whereas most of the safety design criteria for boiling systems are based on steady-state and well-controlled laboratory experiments, being not realistic as far as industrial systems often encountered flow transients are concerned. Several investigations on CHF under flow transients have proposed prediction methods, which do, however, not give relevant information on the mechanisms leading to CHF. So far conducted our investigations suggest that two-phase flow dynamics is fairly well described in terms of void wave propagation. This paper describes an extended version of the void wave propagation model to boiling heat transfer transients. The transient behavior in the boiling channel, fed with subcooled liquid, suffers from an influence of a propagation delay of enthalpy perturbation as well in the subcooled region, which brings about complexity in discussing boiling heat transfer and CHF. This enthalpy perturbation causes boiling boundary fluctuation, being of importance in considering void wave propagation. The boiling boundary as well as thermal flow behavior was detected through wall temperature measurement. Then, with the aid of void propagation and energy equations, thermal flow field under oscillatory flow is numerically simulated. A comparison with the measured data demonstrates a prime importance of the present approach to boiling heat transfer and CHF under transients.
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