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Experimental Characterization of Instability in Gaseous Detonation
Examination of gaseous detonation flow-fields represents a unique experimental challenge. High-speed shock interaction within a reactive mixture manifests combustion modes across a range of spatial-temporal scales. While the kinetics along the leading front are often characterized by adiabatic compression, simultaneously strong shear induces turbulent mixing in downstream portions of the flow. This all occurs within a wave structure typically traveling near 2000 m/s. To advance fundamental understanding, high-resolution diagnostics are required to make quantitative, time-resolved measurements of the unsteady detonation propagation.
In this work, detonations are experimentally studied in a single-shot, narrow channel using non-intrusive optical diagnostics. The change in wave structure between mixtures fueled by methane and natural gas was characterized using 175 kHz schlieren and CH* chemiluminescence imaging. The effect of the higher order alkanes in natural gas is speed up the reaction kinetics and produce a wave structure with smaller spatial scales and in which reaction occurs closer to the leading shock front.
A schlieren system operating at a rate of 5 MHz is then implemented to resolve the spatial-temporal oscillation of the leading shock front. These images are used to compute the lead shock normal speed, which enables a statistical analysis of the oscillating shock velocity. The moments of distribution are compared with computed instability levels of sixteen mixtures and a positive correlation is found. Simultaneous chemiluminescence is used to create joint distribution of shock speed and chemical length scale, which are then compared with the quasi-steady reaction zone solution.
Experiments are performed with highly nitrogen diluted mixtures of methane and oxygen to examine specific flow features. Different regimes of transverse wave reactivity are observed, from nonreactive to detonative. The transverse detonation wave structure is modeled using oblique shock relations and good agreement is found. The chemical length scales within the configuration are compared to the relevant expansion scales to explain the observed near-steady propagation. Distinct reactive processes following transverse wave collision are also captured. In one instance an explosion immediately occurs, while in the other a reactive gas jet grows from the point of collision. An unsteady reaction zone model is applied to understand the reaction mode within the jet.
Lastly, 300 kHz OH PLIF is performed to study small scale and weak reaction structures within the flow. The evolution of deflagrative burning mechanisms becomes resolvable using this technique, which highlights the benefit of its use.
- Doctor of Philosophy
- Mechanical Engineering
- West Lafayette