Preparation, measurement and analysis of the microstructure of evaporator surfaces

The processes of the phase change in boiling occur at the solid-liquid interface by heat transfer from a solid heating surface to the boiling liquid. The characteristic features of the heating surfaces are therefore of great interest to optimize the design of evaporators. The microstructure with all its peaks and cavities influences directly the wetting and rewetting conditions of the heated surface by the boiling liquid and hence bubble formation and heat transfer. The heating elements of various subprojects within the research group working on fundamentals in boiling heat transfer are different in form and size (e.g. plates and tubes). Their micro- and macrostructure are characterized quantitatively with regard to the cavities offered to nucleation. The surfaces of the heating elements of varying materials (copper, copper alloys, steel and different metal layers) are emery ground or sandblasted resulting in a deterministic microstructure or a stochastic one. The surfaces are investigated by a three-dimensional contactless roughness measurement technique combining the stylus technique with the near field acoustic microscopy. The method opens the possibility to obtain results according to standard for practical applications and delivers detailed informations about the three-dimensional shape of each cavity within the surface investigated on the other side. The analysis of the microstructure implies the total number of cavities, their local and size distribution calculated by the method of the envelope area.