Swirl and buoyancy effects on mixing performance of baffle-plate-type miniature confined multijet

Abstract

A three-dimensional numerical simulation was carried out to examine the effects of swirl and buoyancy-driven flows on the mixing performance of a baffle-plate-type miniature confined multijet. Swirl flow was produced by inclining the jet nozzles surrounding the central jet in the circumferential direction. The results obtained were compared with those of the non-swirl case. The swirl flow interrupted the radial secondary flow generated in the region adjacent to the baffle plate. This interruption decreased the size of the reverse flow region, resulting in a deterioration of the mixing performance. This behavior was more noticeable in the case of a large swirl number.

During the study of the buoyancy effect, the operating direction of the gravitational force was changed with respect to the central axis of the mixing chamber, i.e. the attack angles were 0°, 90° and 180°. The buoyancy effect was negligible when the operating direction was parallel to the chamber axis. However, when the attack angle was 90°, asymmetric distributions of mixture fraction and flow velocity were obtained, and the mixing performance was found to be poor. In the swirl-accompanied cases, the effect of buoyancy force was apparent even when the operating direction was parallel to the central axis. The swirl flow, which reduced the secondary flow effects of the reverse and radial flows, tended to increase the effect of the buoyancy force on the multijet flow. On the other hand, the swirl flow prevented the mixture fraction distribution to be asymmetric in the case of buoyancy with 90° attack angle. These tendencies became more conspicuous for flows with stronger swirl.