Pool boiling heat transfer from horizontal tubes with different surface roughness

The influence of the roughness of a heating surface on the heat transfer coefficient has been investigated by experiments with propane boiling on single horizontal copper and steel tubes with different surface roughnesses (emery ground, fine or rough sandblasted). The results can be correlated in a preliminary manner by no longer separating the influence of surface roughness from the other parameters in an empirical prediction method. A detailed description of the three surfaces on the steel tube examined is given, with surface profiles, quasi three-dimensional topography presentation, SEM photographs, and with a statistical analysis of a selected roughness parameter. The size distributions of active nucleation sites have been determined using the model assumptions of Schomann of the heat transfer near growing and departing bubbles. 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 for 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 on the input data for d(A) and f. (C) 1998 Elsevier Science and IIR.