Effect of surface roughness on the mass flow rate predictions for adiabatic capillary tubes

Abstract

Abstract In this paper, the effect of surface roughness on the mass flow rate through adiabatic capillary tubes was analyzed. Initially, relevant information about the surface roughness of capillary tubes and available correlations to estimate the sand-grain roughness were summarized. Subsequently, experimental measurements by stylus profilometry were performed for two different capillary tubes, presenting the Ra, Rq, Rp, and Rz as the roughness parameters. Algebraic solutions for straight and coiled adiabatic capillary tubes were considered for the refrigerants CO2 and R600a. A simulation-based factorial design was performed and the results showed that the effect of altering the surface roughness was about 2.5 times more prominent for CO2 than for R600a. The roughness change was found to be more significative than the shape alteration, for CO2, while for R600a the effect was the inverse. Besides, the mass flow rate was slightly more affected for the straight capillary tube than for the coiled one, for both refrigerants. Concerning the correlations to estimate the sand-grain roughness, it was shown that differences greater than 3 µm could be found by altering the estimator used. For the conditions investigated, two correlations demonstrated to be suitable for characterizing the surface roughness of capillary tubes, presenting 95% and 91.7% of the predicted mass flow rates within a ± 10% error band.