Automobile air bag inflation system using pressurized carbon dioxide
Department of Mechanical Engineering
Doctor of Philosophy
Labib, Mohamed E.
Florio, Pasquale J.
Kirchner, Robert P.
Air bag restraint systems.
A novel air bag inflator based on the evaporation of liquefied carbon dioxide was developed. A detailed qualitative model was established on the basis of an extensive experimental study. An integrated quantitative model of this inflator was constructed.
The system was studied by discharging the inflator into a tank and measuring pressure and temperature evolution (0-50 ms). The dispersion of the two-phase spray during inflation was investigated by high-speed cinematography.
The optimal storage pressure of the liquid CO2 was found to be 2000 psig (at 22 °C). Two distinct inflator behaviors were identified. First, at conditions corresponding to an initial entropy below the critical point, a two-phase evaporating spray was ejected from the inflator into the tank. Second, at an initial entropy above the critical point, the inflation sequence constituted the expansion of a real gas without a significant phase transformation. The minimal flow section in the nozzle was found to control the dynamics of this new inflator.
To prevent the formation of solid CO2 during inflation, small amounts of organic liquids were added to the inflator. A significant increase in tank temperature was observed, resulting in a profound improvement in performance. An explanation for the influence of organic liquids was developed based on a 'layered evaporation model'.
The qualitative model was based on the interaction of the flashing process with the two-phase outflow from the inflator. This interaction was manifested in two different waves, namely a forerunner and an evaporation wave which controlled the evacuation of the two-phase mixture from the inflator. The latter was predominantly dispersed according to classical atomization mechanisms. The generated droplets evaporated partially by consuming their own internal energy and by interacting with tank gases. The characteristics of the condensate were evaluated by a detailed thermodynamic analysis.
The quantitative description of the inflator involved the development of a transient one-dimensional, two-fluid model. Preliminary simulations show excellent agreement with the expected results. The tank model was formulated on the basis of an empirical correlation for the atomization process, coupled with a simple droplet evaporation model, followed by a model for the mixing of real gases.
njit-etd1998-034 (255 pages ~ 10,747 KB pdf)
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Created February 9, 2010