Sensitivity analysis of a time-delayed thermo-acoustic system via an adjoint-based approach

TL;DR

This paper employs adjoint-based sensitivity analysis to investigate and identify effective passive control mechanisms for a time-delayed thermo-acoustic system, specifically a Rijke tube with hot wires, to mitigate oscillations.

Contribution

It introduces an adjoint-based sensitivity analysis framework for thermo-acoustic systems, enabling the identification of passive control strategies and their optimal placement.

Findings

Force opposite to velocity stabilizes oscillations.

Downstream placement maximizes control effectiveness.

Sensitivity analysis guides passive control design.

Abstract

We apply adjoint-based sensitivity analysis to a time-delayed thermo-acoustic system: a Rijke tube containing a hot wire. We calculate how the growth rate and frequency of small oscillations about a base state are affected either by a generic passive control element in the system (the structural sensitivity analysis) or by a generic change to its base state (the base-state sensitivity analysis). We illustrate the structural sensitivity by calculating the effect of a second hot wire with a small heat release parameter. In a single calculation, this shows how the second hot wire changes the growth rate and frequency of the small oscillations, as a function of its position in the tube. We then examine the components of the structural sensitivity in order to determine the passive control mechanism that has the strongest influence on the growth rate. We find that a force applied to the acoustic momentum equation in the opposite direction to the instantaneous velocity is the most stabilizing feedback mechanism. We also find that its effect is maximized when it is placed at the downstream end of the tube. This feedback mechanism could be supplied, for example, by an adiabatic mesh. We illustrate the base-state sensitivity by calculating the effects of small variations in the damping factor, the heat-release time-delay coefficient, the heat-release parameter, and the hot wire location. The successful application of sensitivity analysis to thermo-acoustics opens up new possibilities for the passive control of thermo-acoustic oscillations by providing gradient information that can be combined with constrained optimization algorithms in order to reduce linear growth rates.