Investigation of pre- and post-dryout heat transfer in upward steam-water two-phase flow at low flow rate.

Abstract

A heat transfer experiment was performed on steam-water two-phase flow in an annular flow path with a uniformly heated rod under the conditions of the mass flow rates from <J.2 x 10' to 1.0 x 10 6 kg/m 2 • h, inlet qualities from 0.5 to 1.0, heat fluxes below 4.7 x 10 5 W /m 2 and pressure of 31 bar. Dryout of the heater rod surface was observed resulting in the sharp rise of the heater rod surface temperature. Measured heat transfer coefficients were compared with the several empirical and semi-empirical correlations with the emphasis on the applicability of the correlations to the present test conditions being important in the analysis of the thermal hydraulic behavior during a LOCA of a nuclear reactor. The measured heat transfer coefficient in the pre-dryout region is lower than the existing correlations. The cooling of the heat transfer surface by the liquid phase in the post-dryout region is significant, which is neglected in the existing correlations. The heat transfer coefficients calculated for the post-dryout region by the Groeneveld correlation show good agreement with the presently measured results within the accuracy of 0-27.%. KEYWOR.DS l. INTRODUCTIONThe wall temperature of a heated channel in which high quality two-phase fluid is flowing can be kept at a low value when the wall is wetted. However, when the dryout of the wall takes place, the wall temperature rises sharply due to abrupt deterioration in the heat transfer at the wall surface. The resultant temperature rise is often sufficient to cause the metallurgical change of the wall.Quite a few papers have been published on the pre-and post-dryout heat transfer. In the pre-dryout region the flow mechanism is complicated, thus the papers about analytical solutions of the heat transfer are few but many experimental correlations have been presented. For example, Schrock & Grossman°l performed a heat transfer experiment on the steam-water mixture for mass flow rates between 4.5 X 10 6 and 10.6 X 10 6 kg/m 2 • h and qualities between 0.05 and 0.59 at pressures between 4 and 13 bar and presented the heat transfer correlation for the forced convection evaporation regime. They also performed the experiment for the steam-water mixture of mass flow rates between 0.86 x 10" and 16.0 X 10 6 kg/m 2 ·h and qualities between 0.05 and 0.57 at pressures between 7 and 26 bar and presented the heat transfer correlation for the forced convection evaporation and boiling regime< 2 l. Collier et al.<'l performed the heat transfer experiment for the steam-water * Tokai-mura, Ibaraki-ken 319-11.mixtures of mass flow rates between 0.48 X 10" and 3.9 x 10" kg/m 2 • h and qualities below 0.66 at pressure 1.75 bar and presented the heat transfer correlation for the forced convection evaporation regime.In the post-dryout region many experimental and analytical works on the heat transfer have been done and they are summarized in Table 1. In Refs. (4)"-'(7) the heat transfer correlations modified from the generally accepted heat transfer equations for the singlephase flow a...