Date of Award

Spring 1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical Engineering

First Advisor

Mohamed E. Labib

Second Advisor

Rong-Yaw Chen

Third Advisor

Pasquale J. Florio

Fourth Advisor

Ralph Hensler

Fifth Advisor

Robert P. Kirchner

Abstract

A novel air bag inflator based on the evaporation ofliquefied carbon dioxide was developed. A detailed qualitative model wasestablished on the basis of an extensive experimental study. An integratedquantitative model of this inflator was constructed.

The system was studied by discharging the inflatorinto a tank and measuring pressure and temperature evolution (0-50 ms). Thedispersion of the two-phase spray duringinflation was investigated by high-speed cinematography.

The optimal storage pressure of the liquid CO2was found to be 2000 psig (at 22 °C). Two distinct inflator behaviors wereidentified. First, at conditions corresponding to an initial entropy below the critical point, atwo-phase evaporating spray was ejected from the inflator into the tank. Second, at an initial entropy above thecritical point, the inflationsequence constituted the expansion of a real gas without a significant phase transformation. The minimal flow section in thenozzle was found to control the dynamicsof this new inflator.

To prevent the formation of solid CO2during inflation, small amounts of organic liquids were added to the inflator. A significant increase in tanktemperature was observed, resultingin a profound improvement in performance. An explanation for the influence of organic liquids was developed based ona 'layered evaporation model'.

The qualitative model was based on the interactionof the flashing process with thetwo-phase outflow from the inflator. This interaction was manifested in twodifferent waves, namely a forerunnerand an evaporation wave which controlled the evacuation of the two-phase mixture from the inflator. The latterwas predominantly dispersed accordingto classical atomization mechanisms. The generated droplets evaporated partially by consuming their own internal energyand by interacting with tank gases. The characteristics of the condensate were evaluated by a detailedthermodynamic analysis.

The quantitative description of the inflatorinvolved the development of a transientone-dimensional, two-fluid model. Preliminary simulations show excellent agreement with the expected results. The tank modelwas formulated on the basis of an empiricalcorrelation for the atomization process, coupled with a simple droplet evaporation model, followed by a model for themixing of real gases.

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