Active and Potential Bubble Nucleation Sites on Different Structured Heated Surfaces

Abstract

Characteristic features of the heating surface and bubble formation should be introduced into calculation methods for heat transfer in boiling. Different types of microstructure are investigated by new three-dimensional roughness measurement techniques without contact of the stylus with a variety of surfaces, such as deterministic emery ground surfaces, stochastic sandblasted surfaces and also milled surfaces resulting from commercial manufacturing processes. The method opens the possibility of getting more detailed information about the three-dimensional shape of the cavities, as shown by selected examples. The microscopic structure with all its peaks and cavities influences directly the wetting condition of the heated surface by the fluid and hence the bubble formation and the heat transfer. Therefore the analysis of the surface roughness profiles with an enveloped area to calculate the size distributions of parameters describing the cavities possibly acting as nucleation sites is transferred to the third dimension. The local distribution of the potential nucleation sites and the size distribution of the cavities are discussed and linked to the size results of studies of the active nucleation sites by high speed video technique. The interaction between the sites is demonstrated by the size distribution of the distances between nearest neighbouring sites.