Effect of System Pressure on Film-Boiling Heat Transfer, Minimum Heat Flux, and Minimum Temperature

Film-boiling heat transfer on a horizontal test heater in a pool of saturated and subcooled water was investigated at pressures ranging from 20 kPa to 2 MPa. Platinum rods of 0.7, 1.2, 2, 3, and 5 mm in diameter were used as the test heater. A semiempirical equation and a modified Bromley equation were given, both of which could express the saturated film-boiling heat transfer coefficients within ±5% error. The heat transfer coefficients for a certain range of heater diameters under saturated and subcooled conditions were expressed within ±10% error by the two-phase boundary-layer film-boiling model with the boundary condition of equal liquid and vapor interfacial velocities. Pressure dependence of the minimum film-boiling temperature for pressure <1.1 MPa was clearly different from that for pressure >1.1 MPa. Minimum temperature in the lower pressure region seems to be determined by the hydrodynamic Taylor instability and that in the higher pressure region by the heterogeneous spontaneous nucleation limit. However, minimum temperature and heat flux of saturated film boiling in the former region did not agree with those of conventional equations based on the Taylor instability. Empirical equations of interfacial wave length, departing bubble diameter, and frequency near the minimum film-boiling temperature for the lower pressure region were given. Minimum temperature and heat flux equations were presented based on these empirical equations.