Heat transfer and size distribution of active nucleation sites in boiling propane outside a tube

Size distributions of active nucleation sites in pool boiling of propane on the outside of single horizontal copper and steel tubes with different surface finishes (emery ground and fine sandblasted) have been calculated from heat transfer measurements over wide ranges of heat flux and pressure. The model assumptions of Schömann for the heat transfer near growing and departing bubbles, which were applied in the calculations, have been modified and the calculated results have been compared to experimental investigations by high-speed video techniques. New measurements of the departure diameter d_A and frequency f of the bubbles at the same conditions as the heat transfer measurements show that d_A and f are influenced by the azimuthal position of the active nucleation sites on the tube and that the calculated results react sensitively to the input data for d_A and f . A modification of the model is proposed.