Real gas effects in heated gas inflators

Currently, almost all new vehicles are equipped with airbags. A common type of airbag inflator is the Heated Gas Inflator (HGI). These inflators are cylindrical shaped canisters that are filled to very high pressures with a gaseous mixture of fuel and air. The mixture is ignited from one or both ends of the cylinder. The resulting high temperatures from combustion heats the excess air, which is then used to inflate the airbag. Once the mixture is ignited, large pressure waves form, traveling along the length of the tube. These waves, inherent to the design of the inflator, do not allow the use of a volume averaged assumption for the combustion chamber. Therefore, it is necessary to use a Computational Fluid Dynamics (CFD) code to model the dynamic nature of the inflator. Commercial CFD codes are readily available that could be used to model the HGI. These codes use the Ideal Gas Law to calculate the properties of the mixture. The high pressures in an HGI do allow for the use of an ideal gas assumption. Instead, a Real Gas equation of state must be used.

An existing Airbag Inflator Model that was capable of Real Gas equation of state calculations had been previously created to simulate solid propellant inflators. In order to properly model the wave dynamics in an HGI and include Real Gas calculations, a CFD model has been added to the Airbag Inflator Model. The CFD model must be capable of handling multiple species of gases and be able to properly model the sharp gradients associated with large pressure waves and changes in chemical species. Therefore, a high-resolution shock capturing technique is used to handle the homogeneous part of the governing equations. The non-homogeneous terms of the governing equations are solved using an ordinary differential equations solver. In order to combine the solutions, a time splitting technique is used to combine the solutions from the homogeneous and non-homogeneous parts of the governing equations.

The addition of the CFD model to an Airbag Inflator Model with Real Gas equation of state capabilities provides a very useful tool in the design of HGIs. The model can be used to ensure that a design does not produce unexpected large magnitude pressure waves that could possibly cause dangerous mechanical failures. Later models of HGIs have ignitors at each end of the cylinder. The secondary ignitor can be delayed to vary the production rate of the exhaust gasses, depending on the severity of the crash. This time delay is an additional parameter that can have an effect on the wave dynamics in the HGI. The addition of the CFD model to the Airbag Inflator Model provides a fast and economical way to predict the outcome of any change in the design parameters of an HGI.