Investigation into an unsteady heat release model applied to an unstable model rocket combustor
Abstract
The development of a velocity-based mechanistic combustion response model and its application to liquid rocket combustors with a rearward facing dump configuration is described. A pressure-sensitive time lag model is applied to capture the periodic process of vortex shedding that is hypothesized to be the primary mechanism of combustion instability in the studied combustor configuration. The linearized Euler equations (LEE) solver is used to compute resonant frequencies, pressure and velocity mode shapes, and linear growth rates. The LEE model is multi-dimensional along the longitudinal axis, and includes mean flow and entropy wave effects, changes in cross sectional area, and user-defined boundary conditions on resonance frequencies and mode shapes. The mechanistic formulation is applied to a single-element unstable liquid rocket combustor that exhibited varying levels of spontaneous longitudinal instability depending on the geometry and flow conditions. Predicted resonant frequencies and linear growth rates are compared with experimental values. Only the first resonant mode is considered. Similar trends in stability are observed between the experimental and analytical approaches. Applying the mechanistic unsteady heat release model does not dramatically affect predicted resonant frequencies but significantly affects growth rates. It is concluded that the model as defined in this study does not accurately reflect the mechanism; however, results are encouraging for a mechanistic response function capable of representing the underlying physics in the combustor. Parametric studies were also conducted to characterize the system response to the time lag model. Three key variables in the mechanistic time lag formulation were selected for study. Study variables were systematically varied and resonant frequencies and growth rates were compared with test data from the unstable combustor. The results confirmed the expected trends in stability using a time lag model and provided a guideline for development of the mechanistic model. Future work includes extending the analysis to include the second resonant mode and using large eddy simulations (LES) to devise more accurate formulations for the mechanistic response function.
Degree
M.S.A.A.
Advisors
Anderson, Purdue University.
Subject Area
Aerospace engineering
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