Film boiling heat transfer of a hot sphere in a subcooled liquid pool considering heat loss through its support rod

Highlights: • Experiments of film boiling were carried out by immersing a steel sphere in a pool of water. • Different immersion depths and different support rod diameters were used. • The heat loss through the support rod increases and saturates as immersion depth increases. • The actual film boiling heat transfer of a sphere was obtained. • New coefficient as a power function is suggested for the Michiyoshi correlation. - Abstract: Experiments were performed to determine the film boiling heat transfer of a hot stainless steel sphere of 10 mm diameter, whose initial temperature is about 850 °C, immersed in a 43–81 °C water pool. From the experimental results, and direct visual aid, it was found that non-negligible amounts of heat transfer exist from the sphere to the support rod, which is installed frequently in many studies to hold the specimen in place. The experimental heat transfer results for the sphere without immersion of the support rod showed the maximum deviation of 5.47%, despite the different support rod diameters. With the increase of the immersion depth, the heat ratios of the total heat transfer (Q_total) to the actual heat transfer for the sphere (Q_sphere) were increased and saturated to 1.13 and 1.97, at a sphere temperature of 650 °C, and rod diameters of 1.6 mm and 5.0 mm, respectively. It was found out that the heat loss through the rod (Q_rod) is linearly dependent on rod conductivity multiplied by its cross-sectional area (kA_c). Based on the current database obtained for the sphere without the immersion of the support rod, it is suggested to use the coefficient in the Michiyoshi correlation as a power function, rather than current value of 0.696. With the new coefficient by the authors, the normalized root mean square deviation (NRMSD) turns out to be 4.56%, which improves the prediction capability rather than that of Michiyoshi (10.43%) or Liu and Theofanous (17.87%). Utilizing the relationship between Q_rod and kA_c obtained from the present study, we reestimated the databases of Q_sphere from other previous studies. The deviations between the modified Michiyoshi model and the corrected databases for those previous studies were within 23.72% over the ranges of 300–950 °C for a sphere temperature between 30 °C and 50 °C for the degree of subcooling.