Effects of pressure, subcooling and the spacer grid on the minimum film boiling temperature under high flooding rate conditions

Post-dryout heat transfer is a critical phenomena in the analysis of nuclear fuel rods during a hypothetical loss of coolant accident (LOCA). Heat transfer in transition boiling increases as intermittent wetting of the heated surface takes place. In film boiling, a vapor film covers the heated wall and prevents direct heat transfer from the liquid to the rod. The boundary between the transition and film boiling heat transfer regimes occurs at Tmin, the minimum film boiling temperature, sometimes called the Leidenfrost point. Understanding the sensitivity of Tmin to pressure, subcooling, surface conditions, and flow rates is critical because Tmin determines when the vapor film breaks, and the heat transfer significantly improves. In this work, the minimum film boiling temperature has been determined for different ranges of pressure, subcooling, and flow rate for high flooding rate tests. Pressure, liquid subcooling, and flow rate were varied from 0.14 MPa to 0.41 MPa (20 to 60 psia), 5 K to 83 K (9 to 150°F), and 8 cm/s to 21 cm/s (3 to 8 in/s) respectively with the peak power varying from 1.31 kW/m to 2.3 kW/m (0.4 to 0.7 kW/ft) and the initial peak rod temperature varying from 1033 K to 1144 K (1400 to 1600 °F). The experiments were performed using an electrically heated 7×7, 3.66 m (12 ft) Inconel rod bundle array with constant and variable flooding rate capabilities at the NRC-PSU Rod Bundle Heat Transfer (RBHT) facility. The RBHT is equipped with seven mixing vane spacer grids with which the effect of spacer grid on Tmin was also studied. Data reduction was performed by using an inverse heat conduction code to calculate the surface temperature and heat flux, based on the inner cladding temperatures. The results indicate that the system pressure, inlet subcooling, flooding rate and spacer grids have impact on the minimum film boiling temperature. The experimental results presented in this work can be used to determine the transition boiling regime in the limit of TCHF<T<Tmin.