This study quantifies the behavior of a lean premixed prevaporized combustor operating at elevated temperatures and pressures, using both conventional and sustainable aviation fuel. Three different fuel compositions, 100% Jet-A, 100% hydrogenated esters and fatty acids (HEFA), and a 50/50 volumetric blend, are tested at various equivalence ratios and air preheat temperatures. Mie scattering is used to examine the liquid fuel spray pattern in the combustor; phase Doppler particle analysis is used for droplet sizing; 10-kHz OH planar laser induced fluorescence is used to determine the flame surface distribution and to examine the lean blowoff process. Experimental data show that the pure HEFA fuel had smaller droplets and lower fuel penetration depths because of favorable physical properties, but did not otherwise affect the combustor behavior. Despite the differences in fuel spray, the flame surface density fields are similar across fuels. Furthermore, the lean blowoff Damk & ouml;hler vs Reynolds-number behavior is the same for all fuels within experimental uncertainty. These results demonstrate promising fuel flexibility for low-emission lean premixed prevaporized combustor technologies.

This work explores the flame stabilization and blowoff characteristics of a novel lean premixed pre-vaporized (LPP) combustor design for potential commercial supersonic transport (CST) applications. CST operating conditions involve higher combustor inlet temperatures and lower inlet pressures than encountered in conventional subsonic aeroengines. Additionally, the LPP combustor investigated in this work is a multi-element subsonic flow design, with four bluff-body stabilized, annular "main" flames surrounding a central, swirl-stabilized pilot flame. Existing blowoff studies have not explored the combined effects of multi-element flames with varying temperature and pressure simultaneously. This work presents experimental analysis of the conditions that lead to blowoff in such a combustor. The experimental data consists of simultaneous pressure fluctuations, high-speed OH* chemiluminescence imaging and low-speed kerosene planar laser induced fluorescence imaging. The operating conditions for each data set were used to evaluate the Reynolds number (Re) and Damkohler number (Da). Plots of Da versus Re indicate that the Da at blowoff is lower than that reported in literature for flames stabilized on axisymmetric bluff bodies. It is hypothesized that the observed trend is due, in part, to the presence of the central pilot flame which breaks the axisymmetry of the individual main flames and acts as an ignition source for low Da conditions. An investigation of a case with intermittent blowoff reveals the presence and effects of unforeseen hysteresis in the combustor.

The effects of external forcing on a turbulent, liquid-fuelled, swirl-stabilized gas turbine combustor op-erating at a pressure of approximately 1 MPa are explored experimentally. In particular, the dynamics and coupling between the hydrodynamics, flame dynamics and acoustics are compared for various forcing am-plitudes at a fixed forcing frequency ff. The hydrodynamics were characterized via laser Mie scattering from droplets in the fuel spray, while the flame dynamics were qualitatively measured using chemiluminescence (CL) emissions in the 312 & PLUSMN; 12.5 nm wavelength range, both at 10 kHz. The dynamics at the frequencies of interest were extracted using spectral proper orthogonal decomposition (SPOD). In the unforced case, the spray and CL oscillations exhibited similar dynamics, dominated by oscillations at frequency f 0 , whereas the pressure fluctuations were predominantly at fP. As the forcing amplitude was increased from zero, the spray and CL exhibited changes in their power spectra characteristic of the suppression route to synchronization. The pressure fluctuations, however, were observed to follow the phase-locking route to synchronization. In contrast with expectations from synchronization theory, the amplitude of the pressure fluctuations increased significantly not only after lock-on, but also as the frequency detuning with ff decreased. It is shown that this increase in amplitude is not due to intermittency in the frequency of the pressure oscillations. The si-multaneous occurrence of phase-locking and suppression illustrates the rich variety of dynamics that can occur in practical combustor systems. In addition, the amplification of the pressure oscillations based on the frequency detuning with the forcing suggests that classical reasoning based on the Rayleigh Index may not be sufficient to understand the high amplitude behaviour of multimodal systems. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Several simultaneous optical and probe-based diagnostics were used to characterize the behavior of a novel lean premixed prevaporized (LPP) combustor with relevance to future commercial supersonic transport (CST) applications. The burner was tested at several fuel/air ratios while subjected to externally applied forcing. The use of fuel planar laser induced fluorescence (PLIF) revealed the stiffness of the fuel injection system to periodic oscillations within the combustor. High-speed OH* chemiluminescence and stereo particle image velocimetry were used to analyze the flow-flame coupling across different cases. Overall, the combustor exhibited no bifurcations as the fuel/air ratio and forcing frequency and amplitude were varied.

This paper reports flame transfer function (FTF) measurements obtained in a high-pressure lean-burn aero-engine combustor using acoustic and optical methods. A liquid fueled lean premixed pre-vaporized combustor was operated at various combustor fuel air ratios (FAR) relevant for supersonic transport. A high-pressure siren was employed to modulate the combustor inlet air flow at frequencies ranging from 30Hz up to 1050Hz. Acoustic FTFs were obtained via multi-microphone method using signals from multiple dynamic pressure sensors. Optical FTFs were obtained simultaneously with acoustic measurements by imaging OH*, CH* and C2* chemiluminescence from the combustor., Specifically, line-of-sight chemiluminescence signal for each species was collected with a fiber optic probe and imaged onto photomultiplier tubes with 10nm bandpass filters centered at 310nm, 430nm and 515nm, respectively. Complementary 2D high speed OH* chemiluminescence images were also acquired using a high-speed camera to gain insight into the flame dynamics and support the interpretation of the FTF findings. A comparison of FTFs using the acoustic and optical methods show good agreement in phases and magnitude trends. The measured FTFs show significant variations with varying fuel-air ratios at various frequency ranges confirming a strong dependency of FTFs with flame shapes.

Emissions and optical diagnostics were used to characterize a novel lean premixed pre-vaporized (LPP) combustor of relevance to future commercial supersonic transport (CST) applications. The burner consists of four annular flames with cylindrical bluff bodies, stabilized by a swirling pilot flame. Experiments were performed at pressures at and above nominal cruise, with various air inlet temperatures and fuel/air ratios. NOx, CO and unburnt hydrocarbons (UHC) emissions were measured using a traversable water-cooled probe. OH* chemiluminescence, fuel droplet Mie scattering, fuel vapor planar laser induced fluorescence and 2D laser induced incandescence were used to describe the heat release distribution, any residual fuel spray, fuel/air mixing and nonvolatile particulate matter, respectively. The optical diagnostics demonstrate flame structures corresponding to partially premixed bluff-body stabilized flames, albeit with significant spatial and temporal variations. The measured emissions demonstrate an encouraging potential of LPP combustion for CST.

Combustion dynamics is one of the most important factors to be understood and navigated in the design of modern gas turbine combustors. For liquid-fueled combustors this becomes especially challenging given the complexity of additional physics involved, which includes fuel atomization and transport, mixing, reactive kinetics, and acoustics. In this paper an analytical approach to model combustion dynamics is described for an industrially relevant liquid fuel nozzle. For determining the flame fluctuating heat release response to inflow perturbations, an analytical liquid-fuel model was leveraged, developed as an extension and augmentation of traditional diffusion flame models. The acoustic response of the combustor was calculated using 3D finite-element models, including acoustic damping effects of key geometric features. These individual responses were then utilized in a time-domain Green’s function based approach to calculate the response, including growth and saturation, of pressure oscillations. To gain modeling approach confidence and enhanced accuracy, some model parameters impacted by real effects were calibrated manually to achieve better general agreement with the breadth of experimental and computational data available. This included measured flame transfer functions and dynamics metrics, both frequencies and amplitudes, and computed mode shapes and flame shapes. The calibrated modeling approach was then applied to two different combustors, a single-cup and full-annular configurations. It was found that the results agreed well with test data, especially trend-wise, across a modest range of operating conditions. However, at conditions which extended too far beyond the bounds of the data used for model calibration, model inaccuracies became evident. Lastly, sources of model inaccuracies and areas for improvement were discussed.

This paper describes an experimental approach and study of thermo-acoustic flame transfer functions in a high-pressure liquid-fueled rich burn combustor. The presence of high background flame luminosity in high-pressure sooty flame combustors precludes the application of any direct optical flame transfer function method. Instead, an acoustic method based on multiple microphones was employed to characterize the combustor acoustic pressure and velocity responses to acoustic forcing. A high-pressure siren device was employed to acoustically excite the combustor air flow over a broad range of frequencies from 150–1000Hz and modulate the combustor inlet dynamic pressure amplitudes. The acoustic pressures measured from the microphones located upstream and downstream of the flame were processed to obtain swirler impedances and flame transfer functions. Nonlinear behavior of the liquid fuel flame transfer function was studied by systematically varying the siren excitation pressure amplitudes. A parametric study of varying inlet air pressure, inlet air temperature, and thermal power was performed to study the impact of operating conditions on the measured liquid flame transfer function.

This paper demonstrates cross-frequency coupling between pressure, heat release rate, fuel spray and velocity oscillations in a model aeronautical gas turbine combustor operating at a pressure of approximately 10 atm. Heat release rate was characterized by 10 kHz chemiluminescence (CL) imaging of several species. Stereoscopic particle image velocimetry and laser Mie scattering from the fuel droplets were used to measure the gas velocity and spray dynamics, respectively, at 5 kHz. The pressure fluctuations were dominated by oscillations at a frequency $f_0$, whereas the spray, CL and velocity oscillated at approximately $2f_0$. All of these oscillations were nonstationary, exhibiting changes in frequency and amplitude. Comparing the time evolution of the dominant frequencies and amplitudes indicates a behavior consistent with mutually coupled self-oscillators; the observed dynamics of the 1:2 super-harmonic coupling is consistent synchronization via oscillation death. Furthermore, increases in the frequency of the ca. $f_0$ velocity oscillations away from the harmonic ratio (increased frequency detuning) were correlated with decreases in the power of the $f_0$ pressure oscillations. The corresponding nonreacting flow had a natural hydrodynamic mode at a frequency slightly greater than $2f_0$. Hence, the data are consistent with the $f_0$ acoustic mode pulling the hydrodynamic frequency towards the super-harmonic ratio.

This paper experimentally demonstrates the effects of combustion and cross-frequency thermoacoustic coupling on flow structures in a model aeronautical combustor operating at elevated pressure. Measurements from high-speed optical diagnostics are presented for two reacting conditions – corresponding to loud and quiet thermoacoustic oscillations – and a nonreacting case at the same air flow rate. Through a comparative analysis of the pressure, gas velocity, and heat release rate (via OH* chemiluminescence), the influence of cross-frequency coupling on the frequency of natural hydrodynamic oscillations is explored. The nonreacting flow exhibited a coherent flow structure at approximately 2.2 times the frequency of the thermoacoustic pressure oscillations in the reacting flows. In the reacting cases, neither the flow nor OH* oscillated at the frequency of the pressure oscillations. The quiet case exhibited an increase in the flow and OH* oscillation frequencies, which is consistent with results at low power densities. Contrary to expectations, the velocity and OH* oscillations in the loud case were at a frequency lower than the natural frequency, corresponding to the first super-harmonic of the pressure oscillations. It is hypothesized that the cross-frequency interactions between the super-harmonic heat release oscillations and pressure oscillations result in the stronger thermoacoustic dynamics. This difference in behavior between the loud and quiet cases stems from a switch in the nonlinear coupling mechanism that occurs for higher power conditions.

In this study, an experimental facility with two combustion cans was built and successfully replicated the field boundary conditions for heavy duty gas turbine combustors. Each combustor consisted of multiple Dry Low NOx (DLN) fuel nozzles, representative of a real gas turbine combustor headend. The two combustor cans were connected at the combustor exits to simulate the cross-talk area in a can-annular combustor configuration of a gas turbine. Moreover, a choked boundary condition, at the exit section of the cross-talk area, simulated the first-stage nozzle of a turbine. The push-push and push-pull tones were excited by varying the fuel flow splits among the various fuel nozzles in each combustor can. The thermoacoustic behavior of the two-can combustor was modeled using both a reduced-order network approach and a high-fidelity CFD approach. The modeling was carried out to guide rig design and to predict the frequency and relative amplitudes of the various dynamics modes from the experiments. Various combustion dynamics mitigation strategies were demonstrated via the experiments in reducing both push-pull and push-push dynamics tones. Moreover, stable combustor operation was demonstrated with complete mitigation of all dynamics tones.

Dynamics of thermoacoustics oscillations occurring in a liquid fueled aeronautical gas turbine model combustor burning Jet A fuel were investigated experimentally at a pressure of approximately 10 bar. Data was acquired using 5 kHz repetition-rate stereoscopic particle image velocimetry (S-PIV) for both gas phase and fuel droplet velocities, 10 kHz repetition-rate OH* chemiluminescence (CL), and a variety of pressure transducers. Methods for addressing challenges in the application of PIV at these conditions are presented. Analysis of the pressure and CL data showed two coexisting thermoacoustic modes at Strouhal numbers of St approximate to 10.3 and 0.8, both of which exhibited intermittent changes in the oscillation amplitudes. The spatial distribution of the transient pressure-heat release rate coupling, i.e., Rayleigh index, demonstrated repeated dynamics during intermittent oscillations. Specifically, different combustor regions added and/or removed energy from the oscillations at different times. For the tested experimental conditions, the gas phase velocity did not feature any detectable coherent oscillations. However, the fuel droplet velocities in the immediate vicinity of the combustor dump plane exhibited oscillations with a similar intermittent spectral signature as the pressure and CL, indicating coupling through oscillations in the fuel. To further investigate the fuel coupling, the laser scattering signal from the fuel droplets was evaluated. Coherent oscillations in the fuel droplet scattering persisted over the entire length of the fuel spray, which is consistent with an oscillating fuel supply being convected by a non-oscillating air supply for the conditions studied here. The amplitude of the total fuel droplet scattering oscillations was linearly correlated with that of the pressure oscillations. As the pressure amplitude increased, the droplet and pressure oscillation cycles became more coherently out-of-phase, which is hypothesized to lead to the observed intermittent behavior. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.