Heat transfer and nucleation in pool-boiling

Abstract

The creation of bubbles is a fundamental process in boiling heat transfer. There must be a correlation between heat transfer and bubble action. In order to find out more about it a number of tubes of different materials, diameters and surface finish were investigated and their heat transfer coefficients were determined. Besides the overall heat transfer coefficient, local heat transfer coefficients on the circumference of a stainless steel tube were obtained. A comparison of measurements made in Stuttgart with those made in Paderborn (one of our partners in a joint program financed by the Deutsche Forschungsgemeinschaft) showed very good agreement. In order to count nucleation sites and determine nucleation site densities, observations were made over periods of 15 to 30 minutes (long term observations) and for periods of 1 / 60 second (short term observations). The results were called accordingly long term- or short term nucleation site densities. Results for either observation time are given in diagrams. Clear differences appear for different tube materials, different surface finish and long—or short term observations. Emery ground Cu-tubes show distinctly a different nucleation behaviour from gold-plated, sandblasted Cu-tubes. The heat flow from a single active site was determined and it was tried to correlate this with the average bubble spacing. So far, such a correlation is best acceptable for spacings larger than 1 mm, i.e. for only a very small nucleation site density.