Air fountains in the erosion of gaseous stratifications

Large eddy simulation of a light gas stratification break-up by an entraining turbulent fountain

“…In order to compare numerically obtained results for the temporal evolution of the stratification -which is of major interest for most applied studies [12,13] -one needs to find a definition of the stratification front to allow tracking of such. Finding a proper definition of the stratification is desirable since -in contrast to mixing of immiscible fluids or free surface flows -there is no sharp interface between the layers which the stratification is built upon.…”

“…Here, ∂ρ m /∂z is obtained from the assumption of a three-layer environment (see e.g. [12]). The three-layer model assumes the stratification to be composed of a homogeneous layer of dense source fluid, a linearly Downloaded by [University of York] at 07:14 10 June 2016 stratified layer above, and another homogeneous layer of less dense fluid on top.…”

“…For the validation simulation, the global flow properties of the subsequent case of an entraining turbulent fountain are employed. Thus, the bulk Reynolds number of the jet Re D = U b D/ν = 6800 -with the pipe diameter D, the bulk velocity U b and the kinematic viscosity ν -are chosen to be identical to the one encountered in Deri et al [12]. Since the validation case is performed for single component flow, the fluid properties of air at ambient conditions are applied.…”

“…Apart from the previously mentioned disciplines of geophysics and environmental engineering, the phenomenon of fountain-stratification interaction has recently brought attention to groups working in nuclear engineering [12][13][14] who investigate the erosion of stably stratified layers of gases in confined geometries. The motivation in that regard is mostly linked to reactor thermohydraulics and the potential discharge and accumulation of hydrogen inside nuclear safety containments if severe accidents are caused.…”

“…However, since experimental endeavours are made within related campaigns, non-explosion-prone helium is used as a substitute for hydrogen in the corresponding experiments and -for consistency -also within the computations. The particular containment configuration considered within the following is proposed by Deri et al, [12] whose experimentally obtained data for the helium concentration over time at different vertical positions inside the containment are in turn exploited as reference fields. In detail, the investigated problem outlines a stably stratified layer of helium underneath the top of a confined geometry which is gradually eroded by means of a turbulent fountain being directed upwards towards the stratification.…”