Modeling and Simulation of Combustion Chamber and Propellant Dynamics and Issues in Active Control of Combustion Instabilities

Citation

Isella, Giorgio Carlo (2001) Modeling and Simulation of Combustion Chamber and Propellant Dynamics and Issues in Active Control of Combustion Instabilities. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/k1rf-a525. https://resolver.caltech.edu/CaltechETD:etd-03012006-093758

Abstract

A method for a comprehensive approach to analysis of the dynamics of an actively controlled combustion chamber, with detailed analysis of the combustion models for the case of a solid rocket propellant, is presented here. The objective is to model the system as interconnected blocks describing the dynamics of the chamber, combustion and control (including sensors and actuators).

The analytical framework for the analysis of the dynamics of a combustion chamber is based on spatial averaging, as introduced by Culick. This method results in the determination of a set of coupled oscillator equations that are then integrated with the appropriate forcing terms deriving from combustion and control.

Combustion dynamics are analyzed for the case of a solid propellant. Considerable data exists suggesting that the response functions for many solid propellants tend to have higher values, in some ranges of frequencies, than predicted by the conventional quasi-steady theory. Hence, quasi-steady theory is extended to include the dynamics of the gas-phase and also of a surface layer interposed between the gaseous flame zone and the heated solid phase of the propellant. The models are constructed so that they produce a combustion response function for the solid propellant that can be immediately introduced in the our analytical framework. The principal objective of this analysis is to determine which characteristics of the solid propellant are responsible for the large sensitivity, observed experimentally, of propellant burning response to small variations in the conditions. We show that velocity coupling, and not pressure coupling, has the potential to be the mechanism responsible for that high sensitivity. Some issues related to the modeling of solid propellant are also discussed, namely the importance of particulate modeling and its effect on the global dynamics of the chamber and a revisited interpretation of the intrinsic stability limit for burning of solid propellants.

Active control is also considered in the analysis. A critical discussion about the most commonly used control strategies used in combustion allows us to define which are the most promising algorithms to use on future experiments. Particular attention is devoted to the effect of time delay (between sensing and actuation) on the control strategy; several methods to compensate for it are presented and discussed, with numerical examples based on the approximate analysis produced by our framework.

Experimental results are presented for the case of a Dump Combustor. The combustor exhibits an unstable burning mode, defined through the measurement of the pressure trace and shadowgraph imaging. The transition between stable and unstable modes of operation is characterized by the presence of hysteresis, also observed in other experimental works, and hence not a special characteristic of this combustor. Control is introduced in the form of pulsed secondary fuel. We show the capability of forcing the transition from unstable to stable burning, hence extending the stable operating regime of the combustor. The transition, characterized by the use of a shadowgraph movie sequence, is attributed to a combined fluid-mechanic and combustion mechanism.

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