Investigation of mass transfer in milli-channels using high-resolution microfocus X-ray imaging

In this work, the absorption rate of a single Taylor bubble of carbon dioxide in water was investigated within vertical capillaries using high resolution X-ray imaging. The liquid side mass transfer coefficient was calculated from the changes in the size of the bubble at constant pressure obtained from the high-resolution X-ray images by image processing. The bubbles were continuously monitored by holding the bubble stationary in a downward flow of liquid. The processed images, which give the volume of the bubble with high accuracy as a function of time, were processed to evaluate the liquid side mass transfer coefficient between bubble and liquid using the mass conservation equation. The experiments covered a large range of initial Taylor bubble length varying from 5 to 22 mm. The results show that the measured Sherwood numbers depend strongly on the bubble length and also equivalent diameter, which is the same trend as in previously reported results for larger pipe sizes. However the values of measured Sherwood numbers could not be predicted by available correlations. As a result a new mass transfer coefficient in the form of Sherwood number and as a function of Peclet number as well as the ratio of bubble equivalent diameter to capillary diameter (d(eq)/D) is presented. The proposed correlation is applicable for a large range of the ratio d(ed)/D that varies from 0.8 to 1.6. The average relative error between measured Sherwood number and the one calculated with the new correlation is less than 9.6%. (C) 2015 Elsevier Ltd. All rights reserved.