Measurement and Prediction of the Onset of Intermittent Dryout During Blowdown Transients for Upward Annular Flow

Abstract

<p>The effect of pressure transients on the onset of intermittent dryout in upward annular flow was experimentally investigated in order to resolve the conflict between the observations drawn from two major data sets in the literature. A delay in time to the onset of dryout at the test section exit relative to the time predicted based on steady-state data was observed in the R-12 experiments of Celata et al (1988; 1991). Steady-state prediction methods were sufficient to predict the upstream progression of a pre-existing dryout front in the water experiments of Lyons and Swinnerton (1983). Steady state and pressure transient dryout experiments were performed using water with outlet pressures of 2 to 6 MPa and mass fluxes of 1000 to 2500 kg/m2/s in an electrically heated 1.32 m long 4.6 mm ID vertical Inconel 600 tube with depressurisation rates of up to 1.0 MPa/s. Transient experiments were performed with a small margin to dryout and with post-dryout initial conditions in order to test the hypothesis that these initial conditions influenced the onset of dryout during transients. The results of a comparison between the steady dryout data and two dryout prediction methods---the Biasi et al (1967) correlation and the 2006 CHF look-up table (Groeneveld et al, 2007)---were used to develop correction factor correlations to reduce systematic error when these methods were used to predict the transient time to dryout. These modified methods yielded mean predicted dryout delays of -0.1 and 1.5 s respectively with standard deviations of approximately 3 s. There was no statistically significant variation between the pre- and post-dryout initial conditions. Based on this result it was concluded that the initial conditions did not affect the observed time to dryout. The mean wall temperature exhibited a discontinuous decrease as the heat flux approached 92 to 95% of the dryout value. It was postulated that this was caused by a heat transfer regime change from liquid film evaporation to droplet evaporation based on the observations of Hewitt (1970), Doroschuk et al (1970) and Groeneveld (2011). For the range of conditions of the present work the onset of intermittent dryout (Groeneveld, 1986) was caused by deterioration of droplet evaporation heat transfer. Celata et al (1988) noted that in their pressure transient experiments the decrease in saturation temperature drove a rapid increase in the heat flux to the fluid. This was caused by the release of stored thermal energy as the test section wall cooled. Celata et al (1991) stated that the systematic dryout delay was observed for depressurisation rates greater than 0.2 MPa/s. Using Celata et al's (1988) pressure transient data it was concluded that the stored thermal energy transient did not influence the onset of intermittent dryout when rho_w c_pw L_w *(dT_sat/dt)<0.3*q''_a.</p>