The Effect of Hydrogen Enrichment on the Thermoacoustic Behaviour of Lean Premixed Flames
Creators: name: Æsøy, Eirik
Publisher: name: Zenodo
Year: 2022

description: Gas turbines (GT) burning hydrogen can help deliver large-scale zero carbon power generation and facilitate rapid decarbonisation over the short to medium term. They can also play a crucial role in increasing the penetration of renewable energy sources via power to hydrogen to power cycles where excess electricity is used to produce hydrogen which is then burned in GTs during periods of high demand for electricity. However, operating with hydrogen in lean premixed combustion regimes can lead to flashback, auto-ignition, and thermoacoustic instabilities (TIs). This thesis puts the focus on the latter with the aim of investigating the effect of hydrogen enrichment on the thermoacoustic response of flames operated in the lean premixed regime. The work primarily consists of experimental measurements performed on single and multiple interacting flames conducted in laboratory scale can combustors. To assess thermoacoustic stability through linear stability analysis, the acoustic energy source posed by the unsteady flame was characterised through experimental measurements of flame transfer functions (FTFs). The FTF relates the coherent fluctuations in the heat release rate (HRR) to the acoustic velocity and was measured for a wide range of operating conditions and different fuels, ranging from pure methane to pure hydrogen flames. The work can be divided into three main topics considering different aspects of TIs. In the first part, the effect of hydrogen enrichment on the linear and non linear response were investigated in single flames and multiple interacting flames. Measurements of the linear response were modeled using a distributed time lag (DTL) model where the response in HRR to acoustic velocity fluctuations was described as a superposition of delayed responses. The model was shown to successfully capture all the features of the linear response, e.g. excess gain, cut-off frequency behaviour, time delay, and gain and phase modulations caused by acoustic/convective interference. From experimental measurements a scaling procedure was developed where these features were linked to flame and flow parameters through the DTL model constants. For sufficiently high hydrogen contents, the parameters scaled linearly with these and the procedure was shown to capture changes over a wide range of operating conditions. The predictability of hydrogen enriched flames through this scaling procedure showed a promising method to describe the stability of the combustion system when operated in a flexible manner where the thermal load and fuel composition are varied. In general the increased flame speed imposed by hydrogen leads to more compact flames which exhibit shorter time delays and larger cut-off frequencies, thereby potentially increasing the susceptibility to TIs. When studying the effect of hydrogen enrichment, the data revealed that the increase in the cut-off frequency gave rise to significant acoustic/convective interference appearing as large modulations of the gain and phase of the FTF below the cut-off frequency. Previously this phenomena was observed in swirling flames and was attributed to swirl number fluctuations. In the second part of the work the origin of this behaviour was investigated in detail through a series of experiments designed to capture different aspects of acoustic/convective interference. It was found that the modulations arise when the vortex shedding from the upstream geometry locks onto the acoustic field. A method was proposed, where the flame response could be tailored by utilizing targeted acoustic/convective interference by tuning the time delay through careful placement of the upstream geometry. The method was shown to have potential to damp TI in GT combustors when operating in a flexible manner where the thermal load and fuel mixture are varied over wider ranges than GIs are usually operated over today. In the third part the effect of simultaneous longitudinal and transverse acoustic oscillations on the hydrodynamic response of a turbulent jet was investigated. The aim was to simulate a simplified but similar flow condition to the one flames placed in annular combustors experience when exhibiting azimuthal TIs. It was found that the response in between the pressure and velocity nodes, where the flame exhibits both types of fluctuations simultaneously, the hydrodynamic mode of the jet resembled a superposition of the symmetric and anti-symmetric modes observed with only longitudinal or transverse forcing. The results support the method of superposition, where the combined flame response is reconstructed from individual measurements of the two modes.