(376bq) Numerical Investigation of the Effect of Bend on the Gas Absorption Rate in Microchannels

Authors: 
Nishimoto, T. - Presenter, Tokushima University
Sotowa, K. I., Tokushima University
Horikawa, T., Tokushima University
Alcantara-Avila, J. R., Kyoto University
The internal volume of microreactor is much smaller than the conventional reactors, and thus microreactors are capable of achieving high mass transfer and energy transfer rates. Thus, microreaction technology is expected to help us realize highly efficient chemical processes and bring about dramatic reduction in the consumption of resources and energies.
When microreactor is applied to a multiphase system, the fluid shows various flow patterns. We focused on the gas-liquid slug flow which is one of the typical flow patterns, and studied the relationship between the fluid behavior and mass transfer in this flow using computational fluid dynamics. In this flow, gas and liquid slugs flow alternately and the gas-liquid interfacial area increases. It is known that the fluid in each liquid slug exhibits strong circulating motion. The circulating motion of the liquid enhances the mass transfer from the gas-liquid interface to the liquid bulk. This study focused on the gas absorption behavior in a gas-liquid slug flow in a bent microchannel. The relationship between fluid behavior and the volumetric mass transfer coefficient was investigated by means of computational fluid dynamics.
Before conducting a simulation study, a numerical model to express gas absorption rate was developed. The rate equation used in theoretical studies of gas absorption usually expresses the rate per unit interfacial area. This rate was converted to show the value per unit volume so that it can be used in the discretized equation in the finite volume framework. This enabled us to numerically simulate gas absorption behavior through a moving interface.
A series of numerical simulation was carried out to investigate the effects of fluid behavior on the volumetric mass transfer coefficient. A commercial CFD software Fluent was used in numerical simulation. The linear velocity of the slug flow was changed from 0.4 to 0.8. It was assumed that the slug length was 2.0 mm and a single liquid slug flowed through the channel. The distribution of the mass transfer coefficient along the channel was examined.
It was confirmed that the mass transfer rate increased at a bend in a microchannel. This is because the gas-liquid interfacial area became larger due to deformation of the slug. When the flow velocity was 0.4 m/s, the shape of the slug recovered quickly after bend, but did not recover when the velocity was 0.8 m/s. It was also observed that the volumetric mass transfer coefficient in the latter case was greater than in the former case. The presentation covers the details of the results including the relationship between the fluid motion and the gas absorption rate.