The thermoacoustic behavior of a non-premixed, industrial RQL-type combustor operated with hydrogen is investigated experimentally. Flame transfer functions (FTF) are obtained utilizing the multimicrophone method, aiming to quantify the acoustic response of the swirl-stabilized hydrogen diffusion flame to pressure perturbations from either the primary combustion air passing through the burner itself, or the dilution air pathway. Flame images are captured using OH*-chemiluminescence and analyzed in terms of shape and intensity distribution. The basis of the experimental investigation is formed by a sweep in fuel-air ratio (FAR) at two different preheating temperatures with constant primary and secondary air mass flows, accompanied by the assessment of additional parameters.
Thermoacoustic analysis remains a key component during the development process of new combustion chambers. Whereas a vast amount of literature exists on the interaction of planar acoustic and propagation-stabilized flames, research on reheat flames, especially, on how they respond to transverse modes is scarce. With the study presented here, we show the isolated effect of transversal velocity perturbations on a reheat flame. This is done for two distinct flame stabilization cases occurring in a lab-scale reheat combustor. For the first, the flame is partly autoignition-stabilized but also has propagation-stabilized regions in the shear layer because of recirculation zones induced by a backward-facing step. The second features only a minimal step height and therefore only minor recirculation zones, leading to an almost purely autoignition-stabilized flame. The two different flame stabilization cases are investigated using Reynolds–averaged Navier–Stokes simulations integrating an in-house reheat combustion model. The analysis shows that transverse velocity perturbations have no effect on flames that are purely stabilized by autoignition. In the presence of propagation-stabilized flame regions within the shear layer, transverse velocity perturbations do induce heat release rate fluctuations, as expected.
This study investigates the inference of heat release fields from time-averaged and time-resolved particle image velocimetry data using physics-informed neural networks. The method assimilates density fields using a continuity equation, from which heat release fields are calculated a posteriori using an enthalpy equation. The methodology is applied to data of a laminar, premixed methane V-flame of 1.53 kW thermal power and an equivalence ratio of 0.73 that is forced acoustically in the unsteady cases. Validation for the steady case is provided by comparing the assimilated density fields to fields obtained from the particle image velocimetry seeding concentration and by comparing the inferred heat release distribution to Abel de-convoluted OH* chemiluminescence images. The identified unsteady heat release rate fields are validated against global heat release readings from an OH*-filtered photomultiplier tube. This validation data is also transferred into a flame transfer function. The results indicate that dynamics of laminar, premixed flames can be identified solely from velocity data and detailed insights can be gained without any measurement of the heat release rate.
This work investigates the response of a turbulent, technically premixed methane air flame to flow perturbations, with a focus on how fuel injector impedance affects system dynamics. The flame transfer function (FTF) is determined via system identification (SI) applied to time-series data from forced, high-fidelity large-eddy simulations (LES) and is validated against experimental mean flame shape and position. A Multiple Input, Single Output SI approach is used to separately identify the flame’s response to fluctuations in premixture velocity and equivalence ratio. The simulated premixture velocity FTF agrees well with the premixed experimental FTF, confirming that the simulations have a reasonable capture of flame dynamics. However, the airline fluctuation results show poorer agreement with the technically premixed measurements, indicating that injector impedance plays a significant role in those experiments. A low-order acoustic network model (LOM) is employed to evaluate the system transfer matrix, incorporating injector impedance along with the extracted transfer functions. This allows the injector impedance and the effects of fuel-line aperture and fuel-flow perturbations to be inferred. The results show that fuel injector impedance is significant even in the cold flow measurements, and that its combined effect with the flame’s response to fuel flow perturbations substantially influences the dynamics of the reacting case. These findings highlight the importance of ensuring injector acoustic stiffness across the full operating range in technically premixed systems, or alternatively, of characterizing the response to fuel flow perturbations and accounting for their influence on the measured flame dynamics.
This paper describes a methodology to thermoacoustically account for different flame types in a single Finite-Element-computation. To do so, the flame segmentation mechanism presented in previous studies is used to characterize the different flame zones. Differentiation is done between propagation-stabilized shear-layer flames and autoignition flames that respond to acoustic perturbations very differently. While autoignition flames mainly respond to acoustic pressure and temperature fluctuations, propagation-stabilized flames respond to acoustic velocity perturbations. Thus, flame transfer functions specific to each flame type are analytically implemented within the frequency domain Finite-Element-computation. Using the novel framework, it is shown that the global flame transfer function obtained from Computational Fluid Dynamics (CFD) simulations of a backward facing step reheat combustor can be reproduced accurately in the low-frequency regime for an operating point where the flame is forced in a planar manner. The investigated operating point operates on hydrogen fully premixed at lean and autoignitive conditions. The autoignition framework is validated by comparison to one-dimensional direct numerical simulation. The time-averaged CFD heat release rate result is validated with large eddy simulation data.
The computation of flame transfer functions (FTFs), which characterize the flame response to acoustic fluctuations, is of central importance to the assessment of thermoacoustic stability. Hydrogen is an important carbon-free fuel to ensure sustainable power production in gas turbines. Since hydrogen possesses different reactivity and thermo-diffusive characteristics in comparison to natural gas, the nominal flame structure is different from natural gas, and therefore it also exhibits different dynamics. In the context of computing hydrogen flames, the non-unity Lewis numbers call for transport models that consider the enhanced molecular diffusion of hydrogen. This article answers an open question of how present-day computational fluid dynamics (CFD) solvers and transport models perform in the calculation of flame dynamics. Computations of FTFs of laminar premixed hydrogen flames are performed using OpenFOAM, ANSYS Fluent, S3D DNS code, and the AVBP solver. These solvers encompass the spectrum of numerical schemes and transport models used in computational combustion. Transport models taking into account the enhanced molecular diffusion of hydrogen are used in each solver. The results show that despite using similar transport models and identical chemical mechanisms, quantitative differences in the laminar flame speed, mean flame shapes, and the FTFs are seen between the solvers. However, the qualitative features of the FTF remain the same and the quantitative differences in terms of the root-mean-square error metric are within 15 % in gain and 6 % in phase. However, with transport models, larger quantitative differences in the phase of the FTF are observed.
In the pursuit of decarbonizing the aviation sector, hydrogen poses an alternative to kerosene. To facilitate this transition with minimal hardware modifications to existing engines, the air-staged, rich-quench-lean combustion principle is considered for retrofitting. With the importance of the primary zone in this setup in mind, the study examines the thermoacoustic behavior of non-premixed hydrogen flames within the primary zone of an aero-engine prototype combustor under atmospheric, rich and lean conditions. Pairs of rich and lean operating points with similar adiabatic flame temperature are selected for investigation, enhancing the comparability of the pairs. Additional lean operating points are defined to cover a broader range of interest. Stationary flame images are recorded based on OH * chemiluminescence. A non-monotonous dependency between the recorded stationary flame shape and the equivalence ratio is found. The shape of near-stoichiometric and rich flames does not significantly change compared to the observed lean flames. Furthermore, flame-transfer-functions are measured with the multi-microphone-method. Overall, the flame-transfer-function ( FTF ) trends show little sensitivity to the broad range of investigated equivalence ratios, except for the leanest point. Lean, non-premixed hydrogen flames display both an equivalence ratio and frequency dependent behavior. Under near-stoichiometric and rich operating conditions, the equivalence ratio dependency appears dominant.
Although hydrogen combustion has been extensively investigated in recent years, several challenges remain in accurately assessing jet flame transfer functions (FTFs) under intermediate thermal load conditions with a thermal power over ( P th ≥ 100 kW ). The well-established multi-microphone technique, commonly used for swirl-stabilized methane flames, faces several difficulties when applied to jet hydrogen combustion. These challenges are connected to the non-acoustically compact extension of the mixing tube in hydrogen jet burners and to significant changes in the gas properties under non-reactive (air) and reactive (air-hydrogen) conditions. In previous studies, the impact of changes in fuel properties on the multi-microphone method and burner transfer matrix (BTM) reconstruction was demonstrated. However, this approach was limited to well-mixed configurations. In the present work, we extend the methodology to technically premixed single-jet burner systems by introducing an array of four microphones within the mixing tube. This configuration significantly reduces the complexity of the BTM. We demonstrate this experimentally on a technically premixed hydrogen jet burner. Using low-order acoustic modeling of the BTM, we show through sensitivity analysis that the relocated microphone array enables a more robust assessment of the flame transfer matrix for the considered jet–burner configuration, and thus the FTF. Finally, a distributed time-delay model is fitted to the measured FTF to analyze flame dynamics across the relevant frequency range.
Reliable and computationally efficient prediction capabilities for combustion instabilities in liquid propellant rocket combustion chambers are still rare. This study uses a low-order tool based on the well-known acoustic network model principle to map the stability limits of two different research rocket combustors with different cryogenic propellant combinations. New elements were derived capable of describing any isentropic background flow field in a contoured chamber, resolving acoustic chamber modes in three dimensions, and supporting distributed flame response models. Furthermore, an improved boundary condition element for the sonic throat of a rocket nozzle was implemented. The new network elements were benchmarked against two different research rocket combustion chambers, one single injector experiment using liquid oxygen and natural gas, exhibiting longitudinal mode instabilities and one multi-element thrust chamber with transverse mode instabilities. The acoustic resonant frequencies are predicted with an average absolute error of less than 5% for both cases. The tool is capable of predicting stability based on classical time lag and gain parameters applied to the new distributed flame model. The resulting stability maps are consistent with the benchmark cases for flame response time lags which are close to those reported in literature from computational fluid dynamics simulations and experiments.
This work presents a computational study of atomization in an air-assisted coaxial atomizer utilizing a benchmark configuration, the Sydney Needle Spray, to examine both dense and dilute spray regimes under realistic gas-phase turbulence. A geometric volume-of-fluid method, coupled with large-eddy simulation and adaptive mesh refinement, resolves the interface and surrounding gas flow with high fidelity, and both phases are validated against experiment, providing a robust basis for simulation credibility. With a reasonably good agreement in the overall trend of droplet size distribution relative to the experiment, this study further evaluates the impact of numerical tolerances, showing that the strict settings commonly used in fundamental studies do not necessarily improve accuracy in terms of characteristic droplet size for large-scale spray simulations. Furthermore, the analysis identifies radial velocity fluctuations within the liquid column as an indicator of the onset of liquid-column instability. A thinner boundary layer at the air-pipe exit intensifies interfacial instabilities, leading to enhanced radial velocity fluctuations within the liquid column and improved atomization performance. Finally, the simulations elucidate the atomization sequence: the liquid column first regularizes the core gas flow, interfacial waves then develop and shed liquid sheets, which subsequently fragment into ligaments and droplets, producing intermittent mass-flow-rate signals downstream. Frequency-spectrum analysis of the liquid mass flow rate further reveals subharmonic frequency components corresponding to the merging of successive liquid sheets. The results establish a validated, high-fidelity framework for air-assisted sprays and offer guidance for large-scale spray simulations and an accurate description of the primary breakup processes.
Due to their multicomponent composition, fuels are challenging in thermoacoustic instability analysis, particularly sustainable aviation fuels (SAFs) sought as drop-in fuels. Understanding their dynamics is essential to prevent undesirable consequences in aircraft engines. This requires full comprehension of how each constituent of the liquid fuel affects combustion dynamics through atomization, evaporation, mixing with air, and ignition. This paper investigates a two-component SAF surrogate, composed of dodecane and isooctane, supplying three swirling spray flames subjected to a standing transverse acoustic mode. Results are compared with those of the pure constituents and n-heptane, used as a reference fuel. Using the downstream pressure-based flame describing function, the dynamical responses of isooctane and dodecane are found to be less efficient than n-heptane, and the SAF surrogate's response is weaker than its pure constituents. Phase Doppler Anemometry measurements highlighted dodecane's low evaporation rate and showed that the behaviors of isooctane and SAF surrogate cannot be explained solely by evaporation. High-speed OH* imaging showed longer flames for isooctane than for dodecane and n-heptane, while the SAF surrogate produced the longest. Compared to n-heptane, the region of maximum flame intensity oscillations () is weaker and shifted downstream for dodecane and isooctane, promoting more localized flame spreading outside the Rayleigh instability band. This attenuation effect is even more pronounced for the SAF surrogate. The different flame structures and responses arise from evaporation delay in dodecane, ignition delay in isooctane, and a combined effect of both in the SAF surrogate.
The use of electrostatic fields to control the location of charged fuel droplets in a spray jet flame is investigated as a means of developing fuel-flexible combustion systems to accelerate the transition to carbon-neutral transportation. The focus of this work is on the effects of an external electric field parallel to the jet direction on the spray penetration and fuel vapour distribution, and the subsequent effects on flame shape and location. This study is conducted using large-eddy simulations under the assumption that there is negligible production of charged species during combustion. Non-reacting simulations at atmospheric conditions show that with practical electrostatic fields and droplet charges, it is possible to significantly change the spray penetration. Electric forces in the opposite direction to the spray jet lead to a reduction in the spray penetration, an effect that increases with stronger electrostatic fields. This leads to an increase in drag between the droplets and the air flow, resulting in a smaller penetration of the air jet. The reduced droplet penetration confines the fuel vapour to a region closer to the jet inlet and leads to a more compact flame in the corresponding reacting cases. Similar trends are observed in simulations performed at a higher pressure and temperature. This study suggests that electrostatic fields can be used with charged fuel droplets to provide an element of control over spray jet flames, which may allow for the development of novel hybrid thermal-electric combustion systems.
The global linear stability of a swirl-stabilized laminar flame is analyzed with a monolithic approach based on linearized reactive flow equations. The computational set-up is axisymmetric with an embedded and spatially resolved swirler model to circumvent the use of ad-hoc swirl profile and fluctuation at the inlet. An input-output analysis reveals that the azimuthal and axial components of inertial waves dominate the contribution of inertial waves in gain modulation at low and high frequencies, respectively. A resolvent analysis then identifies the optimal amplification mechanism, which is found to correspond to the flame angle oscillation mechanism observed in experiments. The large gain separation explains why this optimal mechanism also appears in experiments or simulations with acoustic forcing at the inlet. Finally, flame displacement is correlated with radial velocity fluctuations at the base of the flame, which are quasi-normal to the flame sheet. This component of the fluctuating velocity is amplified along the flame sheet until it reaches its tip only if flame-flow feedback is active.
This study examines the dynamics of diesel/hydrogen dual-fuel combustion in a constant-volume vessel under compression ignition engine conditions, where a small pilot n-heptane (a surrogate of diesel) injection precedes a long-duration hydrogen main injection containing 93.6% of the energy share. We investigate how pilot n-heptane flame and hydrogen injection affect emissions and combustion by systematically varying the dwell time ( Delta t between the end of n-heptane injection and the start of hydrogen injection) while keeping the injection rate profile fixed. Large eddy simulations with detailed chemistry are performed at vessel conditions of 890 K, 52 bar, and 21 vol.% O 2, and compared against Schlieren and apparent heat-release rate derived from measurements. The simulations show a good agreement with experimental results regarding ignition onset and flame propagation. The pilot n-heptane ignites rapidly at the spray tip, similar to 0.5 ms after injection; thereafter, the flame propagates upstream toward the injector nozzle. The hydrogen jet ignites nearly immediately upon its intersection with the n-heptane flame. Varying the dwell time between the end of n-heptane injection and the start of hydrogen injection significantly affects hydrogen ignition timing, peak heat release rates, and NO x emissions. Notably, while longer dwell times lead to delayed hydrogen ignition and lower NO x formation, medium dwell times result in higher NO x emissions, showing a complex non-linear relationship between hydrogen-diesel combustion and NO x emissions.
Combustion of steam-diluted fuels offers low emissions and fuel flexibility, but thermoacoustic instabilities remain a challenge. Burner acoustics play a crucial role in the prediction and control of such instabilities. This study reports an experimental investigation of the acoustic characteristics of a dual-swirl burner integrated in a humid combustor. In contrast to conventional two-port burners, this burner features three ports of acoustic flux: Two non-stiff inlets corresponding to air and steam-fuel mixture, and one outlet. A full characterisation of such a three-port is demanding for practical burners. Based on the assumption of acoustic compactness, a Reduced-order Model is proposed for the three-port Burner Transfer Matrix (BTM). The model treats the three-port as a combination of two compact two-ports with no interaction between the two inlets. It allows obtaining the reduced three-port BTM with only two linearly independent acoustic states, as is done for a two-port, but with additional microphones for the third port. First, the two-port characteristics of the burner were studied by blocking one of the two inlets at a time. The results substantiate the model assumptions for the three-port. However, the simplifications are valid for frequencies up to 400 Hz, beyond which some deviations are observed. The three-port shows higher resistive and reactive characteristics between the fuel inlet and the outlet ports, compared to the two-port case with the air inlet blocked. The ratio of flow momentum between the two inlets has only a mild effect on the BTM, whereas the speed of sound ratio exhibits a stronger influence.
In this work, a new fuel injection concept for non-premixed jet stabilized hydrogen combustion was analysed and compared to a conventional one in an atmospheric single-nozzle FLOX (R) combustor. In both cases, the fuel nozzle was installed concentrically inside the air nozzle with flush nozzle exits, so that no premixing takes place before both fluids, fuel and air, enter the combustion chamber. The first fuel nozzle was a metal tube with an inner diameter of ID = 1.5 mm. The second novel fuel injector consisted of a three-armed cross-section with three outlet orifices (ID = 0.5 mm) in each arm. The effective total outlet cross-section of the injectors was kept the same at 1.76 mm2. To compare their performances at typical conditions for jet-stabilized combustion systems, the global operating condition was fixed with a bulk air coflow at vexit = 115 m/s preheated to Tpre = 573 K, an equivalence ratio of phi = 0.66, corresponding to an adiabatic flame temperature of Tadiabat = 2100 K and a thermal power of Pth = 10.5 kW. The resulting flames were monitored using OH* chemiluminescence imaging, showing a significantly reduced flame lift-off height with the novel fuel injector. Major species concentrations and gas temperature downstream of the nozzle exit were measured using 1 dimensional laser Raman spectroscopy. The reconstructed 2 dimensional distribution of mixture fraction clearly demonstrated an enhanced fuel-air mixing and more distributed combustion with the novel injector. Additional evidence of such enhancement was found at the exhaust, with about 10% reduction in NOx emission using the novel injector.
The intensification of regulations on greenhouse gas emissions and pollutants has underscored the necessity for advanced injector concepts that ensure fuel flexibility and scalability, addressing critical demands within the thermochemical energy conversion sector. An additively manufactured & micro;-slit injector is proposed and evaluated across a wide range of operating conditions, including variations in jet velocity, fuel loading, air preheating temperature, and fuel type. Detached fuel film dynamics are analysed using machine learning-based object detection, revealing reduced film length with increasing jet velocity and decreasing mass flow rate, ensuring uniform radial distribution. Phase Doppler interferometry confirms the production of fine droplets, with an average diameter of 20 & micro;m, inherently generated by the & micro;-scale fuel outlet design while maintaining a low pressure drop. Combustion performance, assessed via OH *-chemiluminescence for H 2, CH 4, ethanol, and Jet A1, shows excellent stability for H 2 across both low- and high-momentum jet regimes, attributed to enhanced turbulent mixing driven by the high density ratio. In contrast, CH 4 and Jet A1 exhibit similar lift-off trends. Ethanol and Jet A1 display significant pressure drop increases at high preheating condition, highlighting pre-vapourisation effects. Additionally, a correlation for gaseous fuels between the pressure drop and fluid property ratios is established, consistent with turbulent flow theory, and further extended by incorporating the injector discharge coefficient and compressibility effects. These results demonstrate that the & micro;-slit injector seamlessly accommodates both gaseous and liquid fuels, delivering exceptional fuel flexibility and scalability, and positioning it as a promising candidate for industrial burners, micro-gas turbines, and hybrid aero-engine systems.
Ammonia as a carbon-free fuel is promising for the transportation sector. Its direct injection in the liquid phase results in flash-boiling, significantly affecting the spray characteristics. This article characterizes the influence of the degree of superheat ( R p ) on spray characteristics by injecting liquid ammonia using a single-hole gasoline direct injection injector. R p is varied by changing ambient pressure (1–6 bar) and fuel temperature (−10 to 50°C). These conditions lead to sprays that are in flash-boiling and nonflash-boiling, as well as in a transition region. Near-field and far-field spray fluctuations are studied systematically for the first time. In the nonflash boiling conditions, when R p < 1, the spray parameters are mainly controlled by ambient conditions: spray cone angle, spray area, and spray width are reduced for lower ambient pressure. However, in transition and strong flash boiling conditions, spray parameters increased with increasing R p . Increasing R p > 20 contributes slightly to the radial expansion and increase in the spray area. For superheated ammonia spray, a significant cooling effect of vaporization is observed due to the higher latent heat of liquid ammonia. This resulted in the presence of more liquid mass near the nozzle field for strong flash boiling spray. These findings provide a database for liquid ammonia injections and can be used to validate models.
