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Preparation and analysis of different roughness structures for evaporator tubes

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Abstract

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 roughness structures of different evaporator copper tubes with 8 or 25 mm diameter are characterized quantitatively with regard to the cavities offered to nucleation. The surfaces of the heating elements are sandblasted by different means resulting in a stochastic microstructure. 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 additionally delivers detailed information about the three-dimensional shape of each cavity within the surface investigated. The analysis of the microstructure implies the total number of cavities, their local and size distribution calculated by the method of the envelope area. The results of the surface analysis are linked to those of heat transfer and bubble formation discussed in a contribution by Kotthoff and Gorenflo.

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Abbreviations

N :

number of potential nucleation sites (–)

P a, P q, P p, P pm, P t, P z :

standardized roughness parameter acc. to DIN EN ISO 4287 (µm)

R B :

roller radius for calculating the envelope curves or areas (µm)

x, y, z :

coordinates of the topographies (µm)

α:

heat transfer coefficient

λc :

cut-off (mm)

φ:

azimuthal angle (°)

σ:

standard deviation

References

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Acknowledgments

The author appreciates financial support of Deutsche Forschungsgemeinschaft.

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Authors and Affiliations

  1. Institut für Thermodynamik, Callinstr. 36, 30167, Hannover, Germany

    Andrea Luke

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Correspondence to Andrea Luke.

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Luke, A. Preparation and analysis of different roughness structures for evaporator tubes. Heat Mass Transfer 45, 909–917 (2009). https://doi.org/10.1007/s00231-009-0481-1

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  • DOI: https://doi.org/10.1007/s00231-009-0481-1

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