Wall heat flux partitioning during subcooled flow film boiling of water on a vertical surface.

摘要

Inverted annular film boiling is a post-dryout phenomenon, which is characterized by a continuous liquid core separated from the wall by a vapor film. This type of boiling has been observed in experiments simulating the reflooding phase of the loss of coolant accident (LOCA) in light water reactors. A mechanistic modeling of inverted annular flow is essential if one has to predict, accurately, the void fraction and development of flow regimes downstream of the quench front. Although a significant number of investigations of inverted annular film boiling exist in the literature, the phenomenon has not been completely understood and there still exist lots of deficiencies in the existing models. The reason for this is that the data collected so far does not provide adequate information on the phenomenon. For example, there is not much knowledge on the vapor film thickness, vapor superheat, vapor velocity and interface to liquid heat flux. One needs this kind of information to build accurate models of the phenomenon. In order to develop a mechanistic model for the subcooled flow film boiling process, the key issues that need to be addressed are wall heat flux partitioning and interfacial heat transfer.;In this work, subcooled flow film boiling experiments were conducted on a vertical flat plate, 30.5 cm in height, and 3.175 cm wide with forced convective upflow of subcooled water at atmospheric pressure. Data have been obtained for mass flux ranging from 0 to 700 kg/m2s, inlet subcooling ranging from 0 to 25 °C and wall superheat ranging from 200 to 400 °C. Correlations for wall heat transfer coefficient and wall heat flux partitioning were developed as part of this work. These models derive their support from simultaneous measurements of the wall heat flux, fluid temperature profiles, liquid side heat flux and interfacial wave behavior during steady state flow film boiling. A new correlation for the minimum film collapse temperature was also deduced by considering the limiting case of subcooled film boiling. The premise of this deduction is that film collapse under subcooled conditions occurs when the entire wall heat flux is used in sensible heating of the subcooled liquid. The developed correlations have also been compared with data and correlations available in the literature.