High-frequency transverse combustion instabilities of lean-premixed multislit hydrogen-air flames

Abstract

The present experimental investigation is concerned with combustion dynamics of lean-premixed multislit hydrogen-air flames, with the aim of addressing some of the central technical problems associated with the development of low-emission carbon-free gas turbine combustion technology. To mitigate flashback risk in relatively fast premixed hydrogen flames, we use a multislit injector assembly with a slit width of 1.5 mm, of the same order of magnitude as the characteristic thickness of lean-premixed hydrogen flames. This new perspective on the characteristic nozzle dimension makes it possible to scrutinize the dynamics of multisheet hydrogen flames under extreme conditions. We carry out extensive measurements of self-excited pressure oscillations over a broad range of operating conditions between 30 and 100 kW thermal power. The experimental datasets are analyzed using the acoustic wave decomposition method, low-order thermoacoustic network modeling, and FEM-based three-dimensional Helmholtz simulations. We show that lean-premixed multislit hydrogen flames – unlike the multi-element hydrogen-air flame ensembles considered in our earlier investigations – undergo strong high-frequency pressure oscillations between 3.1 and 3.5 kHz in excess of 25 kPa, originating from the triggering of the first tangential mode of the combustion chamber. Whereas the frequency of the first tangential mode is well defined by the square root dependence of adiabatic flame temperature, the magnitude of pressure fluctuations is observed to be intensified exponentially with increasing flame temperature up to 2200 K. We demonstrate that the transverse mode is characterized by high-amplitude spinning modes in the clockwise direction under relatively short combustor length conditions, discontinuously switching to counter-clockwise spinning modes at relatively long combustor length, and eventually transitioning to a moderate standing wave mode at the same resonant frequency. These observations demonstrate the previously unidentified complex modal dynamics induced by lean-premixed multislit hydrogen-air flames of nozzle dimension comparable to the characteristic length scale of the flame.