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Swirling jets are canonical flow fields used to stabilize flames in many gas-turbine combustion systems. These flows amplify background acoustic disturbances and perturb the flame, producing combustion noise and potentially combustion instabilities. Hydrodynamic flow response to acoustic excitations in reacting flows is an important modeling task for understanding combustion noise and combustion instabilities. This study utilizes a global hydrodynamic stability analysis in a biglobal and triglobal framework to model the vortical hydrodynamic modes of a reacting, swirling large eddy simulations (LES) mean-flow based on a commercial nozzle. After conducting an unforced, natural biglobal stability analysis to study the unstable eigenmodes of the flow, linearized equations of motion are used to study flow response of the flow to forcing. An empirical velocity transfer function is constructed by determining the flow response to a harmonically varying 150-1500 Hzu' velocity disturbance at the inflow boundary. These disturbances are strongly amplified, with amplification factors ranging from about 20 to 50 and peaking at about 1050Hz, or St=0.375 study with a Cartesian, three-dimensional (3D) reacting base-flow is also performed where the base-flow varies spatially in three-dimensions. The study utilizes a stable, high accuracy, but computationally effective centered finite difference scheme based on a three-dimensional structured mesh. Illustrative results present the qualitative modes and quantitative transfer functions for Biglobal and Triglobal stability analyses. [ABSTRACT FROM AUTHOR]

This study analyses the stability characteristics of the shear layer vortices (SLV) in a reacting jet in crossflow, analysing effects of flame position, momentum flux ratio ($J$) and density ratio ($S$). It utilizes 40 kHz particle image velocimetry to characterize the dominant SLV frequencies, streamwise evolution and convective/global stability characteristics for three different canonical configurations, one non-reacting and two reacting ('R1' and 'R2'). In the non-reacting case, both convective and global instability is observed, depending upon $S$ and $J$. Qualitatively similar $S$ dependencies occur for the R1 reacting case where the radial flame position lies outside the jet shear layer, albeit with slower SLV growth rates. When the flame lies inside the jet shear layer, the R2 reacting case, a qualitatively different behaviour is observed, as vorticity concentration in the shear layers is suppressed almost completely. Finally, we show that frequency and stability characteristics of the non-reacting and R1 cases can be scaled in a unified manner using a counter-current shear layer model. This model relates these SLV behaviours to a vorticity layer thickness, a velocity scale and an effective density ratio (noting that there are three distinct densities associated with the jet, the crossflow and the burned gases). These parameters were extracted from the data and used to collapse the frequency scaling, and to explain the transition to self-excited oscillatory behaviour. [ABSTRACT FROM AUTHOR]

Flow data are often decomposed using proper orthogonal decomposition (POD) of the space–time separated form, $\boldsymbol {q}'\left (\boldsymbol {x},t\right)=\sum _j a_j\left (t\right)\boldsymbol {\phi }_j\left (\boldsymbol {x}\right)$ , which targets spatially correlated flow structures in an optimal manner. This paper analyses permuted POD (PPOD), which decomposes data as $\boldsymbol {q}'\left (\boldsymbol {x},t\right)=\sum _j a_j\left (\boldsymbol {n}\right)\boldsymbol {\phi }_j\left (s,t\right)$ , where $\boldsymbol {x}=(s,\boldsymbol {n})$ is a general spatial coordinate system, $s$ is the coordinate along the bulk advection direction and $\boldsymbol {n}=(n_1,n_2)$ are along mutually orthogonal directions normal to the advection characteristic. This separation of variables is associated with a fundamentally different inner product space for which PPOD is optimal and targets correlations in $s,t$ space. This paper presents mathematical features of PPOD, followed by analysis of three experimental datasets from high-Reynolds-number, turbulent shear flows: a wake, a swirling annular jet and a jet in cross-flow. In the wake and swirling jet cases, the leading PPOD and space-only POD modes focus on similar features but differ in convergence rates and fidelity in capturing spatial and temporal information. In contrast, the leading PPOD and space-only POD modes for the jet in cross-flow capture completely different features – advecting shear layer structures and flapping of the jet column, respectively. This example demonstrates how the different inner product spaces, which order the PPOD and space-only POD modes according to different measures of variance, provide unique 'lenses' into features of advection-dominated flows, allowing complementary insights. [ABSTRACT FROM AUTHOR]

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH 2 O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – "stage 1" characterized by local extinction, where the overall flame and flow field remain largely unchanged, and "stage 2", characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH 2 O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH 2 O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame. [ABSTRACT FROM AUTHOR]

To limit climate change and promote energy security, there is widespread interest toward transitioning existing fossil fueled combustion systems to sustainable, alternative fuels such as hydrogen (H2) and ammonia (NH3) without negatively impacting air quality. However, quantifying the emission rate of air pollutants such as nitrogen oxides (NOx) is a nuanced process when comparing pollutant emissions across different fuels, as discussed in our paper GT2022-80971 presented last year. That study indicated that the standardized approach for measuring combustion emissions in terms of dry, oxygen-referenced volumetric concentrations (i.e., dry ppmv at the reference O2 concentration (ppmvdr)) inflates reported pollutant emissions by up to 40% for hydrogen combustion relative to natural gas. In this paper, we extend our prior analysis of these so-called "indirect effects" on emissions values to ammonia (NH3) and cracked ammonia (i.e., molecular hydrogen and nitrogen, 3H2 per N2) fuel blends. The results reveal that ppmvdr-based pollutant reporting approaches have a less prominent influence on emissions interpretations for molecular ammonia-methane blends than for hydrogen-methane blends. Nonetheless, we still find that ppmvdr reporting induces up to a 10% relative increase in apparent emissions when comparing 100% NH3 and 100% methane (CH4) fuels at an equal mass-per-work emission rate. Cracking the ammonia is shown to increase this relative bias up to 21% in comparison to a methane system. Further analysis shows how drying, dilution, thermodynamic, and performance effects each influence the relationship between ppmvdr and mass-per-work emissions across the spectrum of fuels and fuel blends. Following discussion of these findings, we conclude that quantifying combustion emissions using ppmvdr is generally inappropriate for emissions comparisons and advise the combustion community to shift toward robust mass-per-energy metrics when quantifying pollutant emissions. [ABSTRACT FROM AUTHOR]