Aerodynamics of buoyant gases within a longitudinally ventilated tunnel: experiments in two different reduced scale models

摘要

We have investigated the dynamics of buoyant releases in a longitudinally ventilated tunnel in order to draw the dependence of the ventilation critical velocity on the source condition. To that purpose we have combined the experimental results collected in two reduced scale models simulating the fire heat release rate by the injection of two different kind of buoyant fluid: hot air and helium. The results show four main features: 1. The flow dynamics produced within the tunnel, and hence the critical velocity, do not depend on the ratio of the density of the buoyant and of the ambient fluid. The flow dynamics for non Boussinesq releases is therefore dynamically similar to induced by Boussinesq releases; 2. The non-dimensional critical velocity is ∝(Γ_0)~(1/3) for lazy releases and seems to be ∝Γ_0 for highly forced releases; 3. The increase of the source size reduces the influence of the source buoyancy flux on the critical velocity; 4. The role of ambient turbulence in the longitudinal tunnel flow does not influence the mixing of the buoyant fluid with the ambient fluid and therefore does not produce any modification in the value of the critical velocity.