A vertical buoyant jet with high momentum in a long ventilated tunnel

Abstract

In this paper the flow resulting from the release of buoyant material within a long tunnel is investigated. The source fluid is discharged through a nozzle of small radius with sufficiently high flow rate to ensure that the axial lengthscale of the buoyant jet (subsequently called the ‘jet-length’) is several times the depth of the tunnel, d. The ends of the tunnel may be either open or closed and a number of ventilation points may exist along it. Consideration of a source with high momentum is an important development in confined jet flow models, as most previous models have assumed that the source has little or no initial momentum. It is found that circulation cells are driven near to the source and that the concentration within them increases to a steady-state maximum. At a distance of about 2.5d from the source the buoyancy forces are then sufficiently strong to drive a two-layered stratified counterflow. The steady-state conservation equations are analysed in order to calculate the mean flow variables. The flow past a ventilation point and the characteristics of the secondary outflow are derived, enabling the calculation of the total number of vents needed to vent the buoyant layer. The time dependence of the mean concentration in the circulation cell near to the source is also deduced. This could be used to calculate time-dependent solutions for the other mean flow variables. All of the theoretical results are compared with experimental measurements.