INVESTIGATION OF HEAT EXCHANGE IN FUEL ROD BUNDLES AT SUPERCRITICAL WATER PRESSURE

Abstract

Calculations to substantiate the characteristics of a reactor cooled by water with supercritical steam parameters and an adjustable neutron spectrum require clarification of data on heat transfer characteristics. The features and data on heat transfer in round pipes and rod bundles at supercritical pressures (SCP) are considered. In analysis of hydrodynamics and heat transfer dates with the flow of water of supercritical parameters in round pipes, an area of both more intense and degraded convective heat transfer was detected compared to subcritical pressures. It is assumed that one of the reasons for the deteriorated heat exchange is the presence of a near-wall layer consisting of a "gas" phase with low thermal conductivity and a central flow region in the form of a liquid-like phase having a lower temperature. A brief review of data on heat transfer in round tubes and rod bundles with SCP is presented. Since experiments on water are complex and expensive, many results, including the SSC RF - IPPE, were obtained on model media (CO2 and freon-12). Shown that despite the diversity of recommendations on the boundary regime with the deterioration of heat transfer, the proposed ratios give a different dependence of the heat flux density on the mass velocity. They do not consider the influence of such parameters as pressure, channel geometry, temperature of heat exchange surface, etc., they are obtained for a narrow range of parameters. And only extensive analytical studies of Yu.A. Zeigarnik et al. (JIHT RAS, NRU "MEI") allowed to summarize the data on heat transfer in round pipes at SCP. Only limited data was obtained by Russian, Ukrainian and Chinese specialists in heat transfer in bundles with a small number of pipes. The results of Chinese experts have shown that heat transfer in rod bundles at supercritical water pressures is higher and more stable than when water moves in pipes and annular channels. In close bundles, the deterioration of heat transfer occurred at low mass velocities and high heat fluxes, and in widespread deterioration of heat transfer was not observed. The growth of heat transfer in the rod bundles is promoted by mixing with spacer and mixing grids, which destroy the wall barrier layer, which prevents the transfer of heat under supercritical pressure from the wall to the center of the flow. The accumulation of new experimental data and additional analysis of research are necessary. A description and technical characteristics of the thermo-hydraulic facilities of the SCP on water (SVD-1 and SVD-2) and freon-12 (STF) available at SSC RF - IPPE is provided. The results of experiments at SSC RF - IPPE and technical approach and methodology of future experiments in hydrodynamics and heat transfer at SCP is presented and discussed.