The effect of forced and free convection in the discharge of a pressurized gas

It is noted that when gas issues from a pressurized container, the remaining gas expands and cools and the resulting heat flux from the walls acts on the mass transfer. Numerical solutions of the two-dimensional time-dependent governing equations for the resulting convective motion and mass transfer rate interaction are presented. Use is made of an explicit finite difference method. It is found that the region of numerical stability, as governed by the Courant-Friedrich-Lewy condition, is greatly enlarged by analytically 'filtering' the acoustic waves from the equations. The resulting equations make it possible to solve laminar flows in the non-Boussinesq regeme. The effect of discharging the gas at different angles from the gravity vector is demonstrated by several examples. In all cases, the resulting motion is characterized by three physically important flow regimes. Whereas initially forced convection dominates the flow, as thermal equilibrium is approached free convection prevails. Between these two regimes, as well as in the region of pressure equilibrium, the flow is controlled by the combined forced and free convection mechanisms; here the direction of discharge relative to the gravity vector is important.