3-DIMENSIONAL DESCRIPTION OF THE MICROSTRUCTURE OF HEATED SURFACES IN NUCLEATE POOL BOILING

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Abstract
The trend for better understanding for fundamental processes connected with nucleate boiling heat transfer and the introduction of characteristic features of the heating surface and bubble formation in calculation methods for the heat transfer in boiling are supported by new developments in computer calculations and in measurement techniques. Because less quantitative information exists on the microstructure especially of the topography of the heated surfaces and on bubble formation than on heat transfer, little progress has been made so far in the development of theoretically based predictive methods, especially for heat transfer conditions relevant in practice. To improve the situation the surface roughness and the topography of the heated surface is measured before and after the heat transfer measurements, which are studied simultaneously with the bubble formation. The paper focuses on the detailed description of the structure of the heated surface. New three-dimensional roughness measurement techniques without contact of the stylus with the surface open the possibility to get more detailed information for the cavities in their three-dimensional form, as it will be shown by selected examples. The analysis of the surface roughness profile with an enveloped curve to calculate the size distributions of parameters describing the cavities possibly acting as nucleation sites can be transferred to the third dimension. The local distributions of potential nucleation sites are calculated and compared with the local distributions of the active sites.

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