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 sur face 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 Schomann for the heat transfer near growing and depar ting bubbles, which were applied in the calculations, have been modified an d 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 tran sfer measurements show that d(A) and f are influenced by the azimuthal posi tion of the active nucleation sites on the tube and that the calculated res ults react sensitively to the input data for d(A) and f. A modification of the model is proposed. (C) 2000 Editions scientifiques et medicales Elsevie r SAS.