Pressure effects on critical heat flux under low pressure and low flow conditions

Abstract

To investigate pressure effects on CHF and to get CHF data, experimental study has been performed with vertical round tubes under the condition of low pressure and low mass flux. Test sections are made of Inconel-625 tube and have the geometry of 8 and 10 mm in diameter, and 0.5 and 1.0 m in heated length. All experiments have been conducted at the pressure of under 9 bar, the mass flux of under $250 kg/m^2s$ and the inlet subcooling of 350 and 450 kJ/kg, for stable upward flow with water as a working fluid. Flow regime analysis is conducted for obtained CHF data with Mishima``s flow regime map, which reveals that most of the CHF conditions occur in the annular-mist flow regime. General parametric trends of the collected CHF data are consistent with those of previous But for the pressure effect on CHF, two different trends are observed: At relatively high mass flux($\ge 150 kg/m^2s$), CHF increases with increasing pressure but at relatively low mass flux($\le 100 kg/m^2s$), CHF decreases or is constant with increasing pressure. The CHF data are compared with predictions of existing well-known correlations and AECL 1995 look-up table resulting that Shah(1987) correlation yields the best prediction accuracy among them. Using modern data regression tool, ACE algorithm, a new CHF correlation for low pressure and low mass flux condition is developed based on local condition with the available 315 CHF data. The developed CHF correlation is based on six dimensionless parameters and shows good prediction accuracy; average error of -0.5\% and RMS error of 10.2\% for HBM.