Critical velocity of active air jet required to enhance free opening rectangular exhaust hood

Abstract

Using a radial active air jet ejected from the slot surrounding a circular exhaust hood opening can increase the air speed generated by the hood as well as achieve a larger capacity to exhaust pollutants. Critical parameters of the Aaberg hood have been identified to ensure the positive effect of the active jet. This paper proposed a novel type of enhanced exhaust hood using the rectangular opening in place of the circular one in Aaberg hood to meet the needs of different shape and size of exhaust hood. To understand the performance of this kind of exhaust hood and to determine the critical velocity of the active air jet, the flow characteristics and the centerline velocity are investigated by changing the exhaust rate, hood size, jet angle, jet velocity and jet slot width via experimental and numerical methods. Results show that the critical jet velocity exists between two working modes of the novel exhaust hood, one is the ineffective mode with the short-circuit jet, and the other is jet-enhanced mode with the developed radial jet. The near-wall circulation along the hood opening perimeter is not attributed to the existence of the jet. The suggested equation which error is proven to be within 12.5% for the critical velocity presented in this paper is simple in structure, and will help to facilitate the choice of the energy-efficient exhaust hood design in residential and industrial buildings to save energy that would be wasted on increasing the exhaust rate.