Modeling and numerical simulation of the internal thermochemistry of automotive airbag inflators

Abstract

This paper provides a comprehensive description of the coupled thermochemical processes that occur during the firing of an automotive airbag inflator. A mathematical model is developed to simulate the transient, thermochemical events associated with ignition and combustion of a pyrotechnic automotive airbag gas-generator unit (inflator). The governing equations for the airbag inflator model are derived by expressing conservation conditions for mass and energy in the interior combustion chamber, filter/cooling screens, exterior plenum, and discharge tank. Following a brief description of the model development and physical assumptions made in the analysis, two series of test calculations are presented. The first series of calculations is for a baseline test case of a conventional pyrotechnic inflator system that is characteristic of a standard discharge tank validation experiment. Transient pressure and temperature profiles generated by the airbag inflator model are presented along with properties at the exit nozzles. A parametric study demonstrates the usefulness of airbag inflator simulations in assessing the sensitivity of airbag pressure curves to various design parameters such as propellant and hardware properties and hardware dimensions. The second series of calculations illustrates the influence of pre-pressurized inert gas on the performance of a pre-pressurized pyrotechnic inflator system. Performance of the inflators is measured in terms of pressure-time and temperature-time profiles in the inflator and discharge tank as well as pressure-time integrals at specified times after ignition. The pre-pressurized pyrotechnic inflator shows certain advantages over conventional pyrotechnic units, including significantly lower requirements for solid propellant mass, lower operating temperature, more uniform performance at hot and cold ambient conditions, and higher thermal efficiency. The chemical composition of the inert gas, pre-pressurized system is also shown to influence the working process of the inflator.