Mass transfer measurements on periodic roughness in a circular pipe and downstream of orifice

摘要

This paper describes the mass transfer characteristics on periodic roughness in a circular pipe and that downstream of an orifice. The mass transfer enhancement ratio of the rough pipe with respect to the smooth pipe are measured by the plaster dissolution method for various combinations of roughness height and wavelength of the periodic roughness, and the influence of roughness on the mass transfer enhancement is examined experimentally. The mass transfer enhancement on periodic roughness is highly increased with increasing the roughness parameter, defined by the roughness height to the wavelength. It is found that the mass transfer enhancement of rough pipe is more than 2 times larger than that of the smooth pipe, which is due to the formation of the local recirculating region over the periodic roughness. On the other hand, minor mass transfer enhancement is observed on the flow downstream of an orifice. The examination of the flow and mass transfer over the periodic roughness and that downstream of orifice is carried out by the numerical simulation with low-Reynolds number k-ε model. The result indicates that the formation of local recirculating region on the periodic roughness is the major source of mass transfer enhancement on the rough pipe, while the minor mass transfer enhancement downstream of the orifice is attributed to the effect of separating shear layer originating from the orifice edge on the wall, which is dominant over the local effect of recirculating flow on the periodic roughness downstream of the orifice.