Detonation Diffraction in Mixtures with Various Degrees of Instability

Citation

Pintgen, Florian Peter (2005) Detonation Diffraction in Mixtures with Various Degrees of Instability. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YSG0-TH85. https://resolver.caltech.edu/CaltechETD:etd-02072005-173741

Abstract

Planar laser induced fluorescence (PLIF) is widely used in combustion diagnostics but has only recently been successfully applied to detonation. The strong spatial variations in temperature, pressure, and background composition under these conditions influence the quantitative link between OH-number density and fluorescence intensity seen on images. Up to now, this has lead to uncertainties in interpreting the features seen on PLIF images obtained in detonations. A one-dimensional fluorescence model has been developed, which takes into account light sheet attenuation by absorption, collisional quenching, and changing absorption line shape. The model predicts the fluorescence profile based on a one-dimensional distribution in pressure, temperature, and mixture composition. The fluorescence profiles based on a calculated ZND detonation profile were found to be in good agreement with experiments.

The PLIF technique is used to study the diffraction process of a self-sustained detonation wave into an unconfined space through an abrupt area change. Simultaneous schlieren images enable direct comparison of shock and reaction fronts. Two mixture types of different effective activation energy [theta] are studied in detail, these represent extreme cases in the classification of detonation front instability and cellular regularity. Striking differences are seen in the failure mechanisms for the very regular H2-O2-Ar mixture ([theta] ~ 4.5) and the highly irregular H2-N2O mixture ([theta] ~ 9.4). Detailed image analysis quantifies the observed differences. Stereoscopic imaging reveals the complex three-dimensional structure of the transverse detonation and its location with respect to the shock front. The study is concluded by using the experimentally-obtained shock and reaction front profiles in a simplified model to examine the decoupling of the shock from the chemical reaction. The rapid increase in activation energy for the H2-O2-Ar mixtures with decreasing shock velocity is proposed as an important new element in the analysis of diffraction for these mixture.

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