Combustion dynamics of lean fully-premixed hydrogen-air flames in multinozzle/multislit arrays

Abstract

We first examine combustion instabilities in lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array

little is currently known about how these flames respond to acoustic perturbations. Several measurement techniques, including phase-synchronized OH* chemiluminescence imaging, OH Planar Laser Induced Fluorescence, acoustic pressure, and the two-microphone method, are used, together with reduced-order acoustic modeling, to identify the key physical properties of lean-premixed hydrogen-air flames, in comparison with lean-premixed methane-air flames. We show that extremely compact lean-premixed hydrogen flames are preferentially coupled to higher eigenmodes of a given system, L3 – L6, including approximately 1 kHz high-frequency instabilities, while the instabilities of methane-air flames are predominantly limited to the first longitudinal mode under the same range of operating conditions. This is attributed to the fact that the dynamics of the methane flames are governed by the collective motion of constituent flames, involving a complex process of the emergence, convection, and interaction of large-scale structures. By contrast, the hydrogen flames in the multinozzle configuration oscillate in isolation within a very short distance and without strong flame-to-flame interactions. This is particularly suitable for accommodating high-frequency heat release modulations. The triggering of intense sound generation from lean-premixed mesoscale hydrogen flames is correlated strongly with a combination of flame surface destruction due to front merging and the flames’ close proximity to a pressure antinode. These results, for the first time, highlight the key features of self-excited combustion instabilities of mesoscale multinozzle hydrogen-air flames in a well-controlled laboratory-scale experiment, and could pave the way for future carbon-neutral gas turbine combustion technology. In addition, local structures of lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array were studied in a tunable combustion test facility, using instantaneous OH PLIF measurements and subsequent image processing techniques. We observe that under thermoacoustically stable conditions, pure hydrogen flame ensemble takes on a conical structure and they are stabilized in isolation without strong flame-to-flame interactions. Under unstable conditions, however, marked flame front deformation and cusp formation occur due to high amplitude velocity fluctuations. Our curvature calculation results revealed that the probability density function distribution is characterized by a negative average value, meaning that the formation of concave contours towards reactant mixture is more pronounced in lean-premixed pure hydrogen combustion environment. Then, we present 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. Lastly, we studied the influence of radial slit components on the development of high-frequency transverse instabilities and found that the growth of transverse thermoacoustic instabilities is closely related to the thermal power supplied to the system under the same adiabatic flame temperature rather than the mean nozzle velocity. A sufficient energy supply makes it possible to establish well-defined limit cycle oscillations, but the absence of radial flame sheets affects the transverse modal dynamics. Especially for the standing mode, the preferred orientation of pressure nodal lines is considerably changed due to the reconstructed heat release distribution. These observations demonstrate the previously unidentified complex modal dynamics induced by multislit channel dimensions comparable to the characteristic length scale of the flame.