Experimental Investigation of Minimum Film Boiling Temperature for Vertical Cylinders at Elevated Pressure

The minimum film boiling temperature, T_min, is of particular interest to nuclear safety. Many thermal-hydraulics codes depend on an accurate estimation of T_min to determine the appropriate heat transfer regime. Heat transfer coefficients increase considerably when the surface temperature decreases below T_min. This paper presents the results of an experimental investigation of T_min for vertical cylinders at elevated pressure. While several previous studies have investigated T_min under various conditions, relatively few have generated experimental data at pressures above atmospheric. As a result, correlations for T_min often predict unrealistic values at high pressure. The present investigation has generated an experimental database examining the effects of pressure, material, subcooling, surface roughness, and oxidation on T_min. The experimental apparatus and test specimens are described in addition to the evaluation procedure. T_min results are reported in this paper for Type 316 stainless steel, Type 1018 carbon steel, and Zircaloy-4. Pressure was varied from 0.101 MPa to 3.0 MPa, and liquid subcooling was ranged up to 30 °C. Zircaloy-4 samples with a thick oxide layer were also tested. The results show that T_min increases with pressure, liquid subcooling, surface roughness, and surface oxidation. T_min is a strong function of pressure at low pressure, but asymptotically approaches a constant value as pressure increases. T_min is also a function of surface material properties, with Zircaloy-4 being found to have the highest minimum film boiling temperature compared to carbon steel and stainless steel. For Zircaloy-4, an oxide layer was found to significantly increase T_min.