Sustainable Combustion Technologies Need Acoustics Research

This paper outlines the challenges of developing hydrogen-fueled combustion systems, which are safe from combustion instabilities. Combustion instabilities can occur in any configuration where a continuously burning flame is housed in an acoustic resonator, such as gas turbine engines, boilers and furnaces. They manifest themselves by large-amplitude oscillations of one or more resonator modes and can cause major damage to the combustion system. Their underlying cause is the following feedback loop: the flow carrying the combustible mixture is perturbed → the flame becomes unsteady and its heat release fluctuates → the fluctuating heat release generates sound → the sound perturbs the flow of the mixture, and the feedback cycle is closed. Various strategies exist to prevent instabilities in conventional combustion systems, which burn hydrocarbon fuels. However, if hydrogen or a hydrogen-blend is used as fuel, the flame's dynamic behaviour changes, and as a consequence, combustion instabilities occur under new and unexpected conditions. In order to give some explicit physical insight, a generic combustion system will be presented and described by an analytical model. Predictions will be made about the impact of hydrogen fuel and other parameters.