Experimental study on the effect of turbulence on hot-smoke dispersion ejecting in a cross flow

Abstract

This study aims to investigate the hot-smoke dispersion behavior released from a chimney in a cross flow experimentally using specially designed wind tunnel. An artificially obtained quasi-isotropic turbulence was generated using an active turbulence generator developed by Makita. A heated jet and an unheated jet with smoke are injected into the cross flow from the vertically-oriented chimney installed in the test section. Smoke motion was captured by high-speed camera to obtain instantaneous patterns of the smoke dispersion. Six-kinds of the featured patterns are clearly identified, such as; (I)(II) bifurcated vortex tubes with and without a strong mutual interaction, (III) connected hairpin-type vortices, (IV)(V) the mixture of the coherent and turbulent vortices, (VI) downwash-type structure. These smoke pattern in the downstream field from the chimney are found to be depended on buoyancy, turbulent motion, and inertia forces. Under the quasi-isotropic turbulence, the smoke dispersion prefers to exhibit a meandering pattern (Mode V) under wide range of adopted flow velocities, which is hardly observed using the grid turbulent test device. Interestingly, as compared to the unheated jet, the meandering smoke structure with heated jet ejection was also observed even at the lower jet velocities and the higher cross flow velocities, suggesting that the buoyancy force shall play an important role on appearance of meandering motion and control the smoke dispersion. Direct smoke exposure case (Mode VI) is preferred to be observed when the quasi-isotropic turbulence is imposed, although the trend of appearance depending on the jet temperature can be predicted even using grid turbulent device. It is concluded that using grid turbulence would not be suitable to predict on the smoke dispersion problem in the actual scale. The time averaged smoke concentrations profiles are analyzed at locations along the cross flow direction and it is revealed that the effective diffusion becomes stronger when the quasi-isotropic turbulence is imposed in the cross wind. Further, it is confirmed that the smoke dispersion behavior can be well-characterized by the existing prediction method based on the point source model.