Investigation of the Effects of Heater Characteristics on CHF and Post-CHF Performance of a Long Vertical Annulus in High Pressure Water

Abstract

<p> CHF and Post-CHF tests were performed in water at 9.7 MPa using two vertical test assemblies having identical, internally heated annular flow channels, one heated directly, the other indirectly. Experiments were conducted to determine the effect of these methods of heating on CHF and Post-CHF heat transfer. </p> <p> For the range of the test conditions investigated, the results show that the direct and indirect heaters have similar CHF performance. At heat fluxes above CHF and mass fluxes of 2.0 and 3.5 Mg.s^-1 .m^-2 , the indicated maximum wall temperatures of the heaters were similar, but at the highest mass flux for the tests, 5.0 Mg.s^-1 .m^-2 , the indirect heater had lower indicated maximum wall temperatures than the direct heater for a given heat flux above CHF. </p> <p> A multi-fluid model, of the type used previously in the prediction of CHF, was derived and tested against the experimental data. The model, which considers droplet entrainment, deposition and evaporation in the annular flow regime, assumes dryout to occur when the liquid film flow on the inner rod approaches zero. The CHF predictions were in fairly good agreement with the experimental results. In general, the model under-predicted CHF at low inlet subcoolings and over-predicted CHF at high inlet subcooling. The error trend is consistent with that of the CHF prediction models of other researchers. </p> <p> In addition to the CHF prediction model, a Post-CHF model to predict the vapour temperatures, and hence, the heated wall temperature is also presented in the report. The theory is based on a physical model of heat transfer in the liquid deficient regime. In the model, heat in the dry region is assumed to transfer from the heated wall to superheat the steam and some of this heat, in turn, is used to evaporate the droplets which are entrained in the vapour core. Droplet entrainment and deposition at the shroud (outer tube) film-vapour interface are modelled. Heat transfer enhancement due to increased turbulence downstream of the rod centering spacers is incorporated through an empirical correlation. The predicted results were compared to the direct heater experiments. In general, the predicted wall temperatures were in agreement with those in the experiments. </p>