A Diffusive Gas Jet Model in PAM-SAFE for Airbag Inflation

Among the forces acting on an occupant during the deployment of an airbag, the impact force due to gas jets coming out of the inflator may be important. Consequently, the semi-analytical theory of high-velocity gas jets including velocity spreading due to turbulent diffusion has been incorporated into the explicit finite element computer program PAM-SAFE for occupant safety. The jet model, described in this paper, takes into account the geometry and thermodynamics of the inflator, the transfer of jet momentum to the moving airbag and the distribution of momentum due to diffusion. Since the intersection of every jet with the airbag is computed each timestep, the algorithm is consistent with numerical airbag unfolding.

Experimental validation of the jet model has been performed at Automotive Systems Laboratory Inc. by firing actual inflators on rigid flat plates at several distances. These results show that the total force exerted on the plate does not depend on the distance, which confirms the applicability of momentum conservation, and that the time and magnitude of the peak force is in good agreement with the numerical results.