Rotational symmetry-driven modal dynamics of high-frequency transverse instabilities in a lean-premixed multislit hydrogen combustor

Abstract

Decarbonization of the power grid targeting the reduction of greenhouse gas emissions has elicited the necessity of a pure hydrogen combustion gas turbine system. However, dissimilarity in combustion characteristics between hydrogen gas and traditional fossil fuels such as kerosene or natural gas requested industries to develop a new type of combustors designed for lean-premixed pure hydrogen combustion. Recent studies about the dynamics of hydrogen flames suggested multitube injector, comprised of multiple small-scale jets with high flow speed, therefore effectively removing the flashback risk of an ultra-fast hydrogen flame. The present study introduces a new concept of multislit combustor, which resembles the idea of high-speed flow of the multitube injector, but consists of multiple slits. Slit width was set to be 1.5 mm, which shows similarity in the order of magnitude with the flame thickness of hydrogen, successfully mitigating the front-merging mechanism which induces longitudinal combustion instability. This design, however, showed characteristic high-frequency oscillations with frequency of 3.0 ~ 3.5 kHz, while typical multitube hydrogen flame were coupled with longitudinal acoustic field that has a natural frequency of 400 ~ 600 Hz. Remarkedly, signals from five pressure transducers placed circumferentially around the dump plane revealed that these oscillations were transverse standing/spinning mode combustion instabilities, which rarely takes place in the laboratory-scale, atmospheric pressure combustors. Various tests on the four types of slit segment patterns were conducted to investigate the influence of nozzle geometry in the triggering of self-excited transverse oscillation, in the wide range of operations from 30 to 100 kW thermal power. In general, the excitation of transverse mode required sufficient thermal power but is also controlled by the arrangement of slits, mainly related to the breaking of rotational symmetry in the slit pattern. With probabilistic approaches, tendencies were found for nodal lines of standing mode to be positioned along the most asymmetrical lines of the combustor pattern. Unlike the intuition that the spinning mode will occur in the rotationally-symmetric nozzles, the rotational invariance hindered the locking of nodal line position, resulting in the unsteady mode transition between standing and spinning modes. Discoveries of the present study can be further extended for the suppression of strong transverse modes in the rocket combustors by intentional symmetry-breaking.