An Overview of Surface Roughness Effects on Nucleate Boiling Heat Transfer

ABSTRACT

The paper aims at an overview of heating surface microstructure effects on nucleate boiling heat transfer. A comprehensive chronological literature survey is presented followed by an analysis of the results of an experimental investigation. Boiling data of refrigerants R-134a and R-123 on 19 mm diameter copper and brass tubes of average surface roughness varying from 0.07 µm to 10.5 µm have been gathered under the present investigation. Though most of the data confirm previous literature trends, according to which the heat transfer coefficient increases with surface roughness, very rough surfaces present a peculiar behavior with respect to that of the smoother surfaces (Ra<3.0 µm). Heat transfer performance of very rough surfaces is superior to the smoother ones at low heat fluxes (q<20 kW/m2). However, decay in their thermal performance is observed in the high heat fluxes range. An in depth analysis of the effect of roughness on the slope, “m” of the h vs q curves has been performed with a general conclusion being that “m” is affected not only by de pressure but also by the surface roughness. Finally, an analysis of the active cavities density is performed using some models from the literature in order to evaluate the effect of surface roughness in cavity activation. The obtained results are generally inconclusive. However, it has been determined that surface microstructure effects must not be neglected in future models.

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