Analysis of the topography of microchannels with different sizes

Abstract

The longterm aim in the design of modern evaporators is to find a heat transfer calculation method based on the phenomena of growing and departing vapour bubbles on the heated surface. New developments in computer calculation and in measurement technique support the local analysis of the heat transfer, bubble formation and the microstructure of the heated surface. All three kinds of data originate from the same equipment and from experiments performed with equal detail and precision. The influence of different rough and structured surfaces on the heat transfer coefficient will be investigated by a joint research project of Brunel and Hannover Universities with refrigerants boiling in single microchannels of different inner diameter. The microchannels are capillary tubes and they are manufactured by industrial processes. The samples of the microchannels have to be prepared by special methods, because the surfaces inside the microchannels cannot be measured in the same way as used for the heat transfer measurements. The microstructure of the surfaces of three microchannels (D = 0.4, 0.5 and 0.8mm) is measured by a contact stylus instrument. The paper will focus on the detailed description of the profiles and the topographies of the samples. The roughness parameter in accordance with standard practice are calculated for ca 1000 runs (profiles). The roughness parameters of the smaller microchannels are twofold higher than those of the largest microchannel investigated. The scattering of the roughness of the largest microchannel (D = 0.8 mm) is less and the cavities are smaller than those of the other ones (D = 0.4 and 0.5 mm). All surfaces investigated demonstrate a deterministic microstructure as for emery ground surfaces on smooth tubes in the literature.