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

Abstract

The minimum film boiling temperature, Tmin, is of particular interest to nuclear safety. Many thermal-hydraulics codes depend on an accurate estimation of Tmin to determine the appropriate heat transfer regime. Heat transfer coefficients increase considerably when the surface temperature decreases below Tmin. This paper presents the results of an experimental investigation of Tmin for vertical cylinders at elevated pressure. While several previous studies have investigated Tmin under various conditions, relatively few have generated experimental data at pressures above atmospheric. As a result, correlations for Tmin 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 Tmin. The experimental apparatus and test specimens are described in addition to the evaluation procedure. Tmin 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 Tmin increases with pressure, liquid subcooling, surface roughness, and surface oxidation. Tmin is a strong function of pressure at low pressure, but asymptotically approaches a constant value as pressure increases. Tmin 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 Tmin.