Strong flame interaction-induced collective dynamics of multi-element lean-premixed hydrogen flames

Multi-element injector configurations, typically comprising numerous small-scale jet nozzles, will be required to enable reliable lean-premixed gas turbine combustion operating with pure hydrogen or high hydrogen content fuels. The integration of large numbers of millimeter-scale injectors in tightly clustered arrays is highly likely to produce peculiar topological states determined by the collective dynamics of strongly interacting premixed hydrogen flames. To understand the influence of inter-element flame interactions in a multi-element nozzle environment, here we experimentally investigate the combustion dynamics of two different pure hydrogen flame ensembles, one relatively coarse array of 293 round jet nozzles, and the other a dense array of 421 nozzles. Measurements of self-induced instability were conducted for both configurations under 60–130 thermal power conditions, in conjunction with phase-synchronized OH PLIF and high-speed OH1 chemiluminescence emission measurements. We demonstrate that for both cases the characteristic dimension of a single injector element is the main determinant of the fundamental frequency of self-induced pressure oscillations, and that the oscillations are primarily driven by the combination of coherent structure-related flame surface rollup and local extinction/pinch-off. Whereas the coarse injector arrangement exhibits well-organized parallel propagation and deformation of isolated vortical structures, the dense array case manifests strong repulsive interactions between adjacent coherent structures, naturally creating lateral flame surface modulations as well as limited streamwise oscillations. Importantly, collision-induced lateral flame dynamics play a mechanistic role in the substantial growth of higher harmonics in the frequency range of 1–2 kHz, leading to the creation of multiple frequency excitation states of commensurate amplitude, which uniquely define the thermoacoustic state of clustered lean-premixed hydrogen flames.     

Effects of different preheated CO2/O2 jet in cross-flow on combustion instability and emissions in a lean-premixed combustor

To reveal the suppression mechanism of thermoacoustic instability flames under CO₂/O₂ jet in cross flow. Experiments on the effects of different preheated CO₂/O₂ jet in cross-flow (JICF) on combustion instability and NO_x emissions in a lean-premixed combustor were conducted in a model gas turbine combustor. Two variables of the JICF were investigated—the flow rate and the temperature. Results indicate that combustion instability and NOx emissions could be suppressed when the JICF flow rate increases from 1 to 5 L/min. The average pressure amplitude decreases from 18.6 Pa to 1.6 Pa, and the average NOx emission decreases from 26.4 ppm to 12.1 ppm. But the average pressures amplitude and NOx emissions increase as the JICF temperature grows up. The sound pressure and the flame heat release rate exhibits different mode-shifting characteristics. The oscillation frequency of the sound pressure almost unchanged under JICF injection. However, the oscillation frequency of the heat release rate jumps from 95 Hz to 275 Hz under different JICF temperatures. As the CO₂/O₂ JICF flow rate arrived 3 L/min, the oscillation frequency of flame heat release rate jumps from 85 Hz to 265 Hz. The color of the flame fronts and roots were changed by the JICF injection. The average length of flame under CO₂/O₂ JICF cases is shorter than the N₂/O₂ JICF cases. There are three different modes of flames when the CO₂/O₂ JICF flow rate varies, and two different modes of flames when the CO₂/O₂ JICF temperature varies. This article explored the joint effects of different CO₂/O₂ or N₂/O₂ JICF on combustion instability and NOx emissions, which could be instructive to the designing of safely and clean combustors in industrial gas turbines.     

Compressible large eddy simulation of turbulent combustion in complex geometry on unstructured meshes

Large-eddy simulations (LESs) of an industrial gas turbine burner are carried out for both nonreacting and reacting flow using a compressible unstructured solver. Results are compared with experimental data in terms of axial and azimuthal velocities (mean and RMS), averaged temperature, and existence of natural instabilities such as precessing vortex core (PVC). The LES is performed with a reduced two-step mechanism for methane-air combustion and a thickened flame model. The regime of combustion is partially premixed and the computation includes part of the swirler vanes. For this very complex geometry, results demonstrate the capacity of the LES to predict the mean flow, with and without combustion, as well as its main unstable modes: it is shown, for example, that the PVC mode is very strong for the cold flow but disappears with combustion.     

Numerical investigation into the structural characteristics of a hydrogen dual-swirl combustor with slight temperature rise combustion

For decades, hydrogen has been identified as the most promising potential fuel to replace fossil fuels. In order to fully implement it and to promote the rationality of the design of hydrogen combustion chamber structure, it is very essential to understand the hydrogen/air combustion mechanism based on structural variations. The structural characteristics of a novel dual-swirl burner for hydrogen-air non-premixed combustion was studied numerically in this study. The influences of air distributions, swirling directions and nozzle configurations of the dual-swirl burner were studied, and the combustion performance was evaluated from various aspects. The numerical results showed that there was a trade-off between lower total pressure loss and the risk of fusing when considering air distribution strategies. The co-rotating swirl burner exhibited better uniformity of temperature distribution at the downstream of the combustor. The multi jet orifices showed superior penetration depth than the circular seam. Efficient and stable combustion could be achieved, which was beneficial to improve gas turbine efficiency and stable operation.     

On the observation of high-order,multi-mode,thermo-acoustic combustion instability in a model gas turbine combustor firing hydrogen containing syngases

This paper explicates the characteristics of high-order multi-mode combustion instability generated in a model GE7EA combustor that has a partially premixed flame, which can prohibit flashback when firing fast-burning hydrogen-containing fuels. Five test cases varying both the fuel composition and heat input were selected, which generates a high multi-mode without a fundamental mode. Multi-mode instability phenomena were observed by 11 dynamic pressure sensors in both the time and frequency domains and by OH planar laser induced fluorescence (PLIF) images, which were time-synchronized with a dynamic pressure signal. A distortion in pressure waves, which resulted from the superposition of several harmonic modes of combustion instability, was observed, whereas distortion in heat release was not clearly observed. PLIF images and their statistical analysis showed scientific clues that proved the dependency of the flame location and movement on the combustion instability. The driving source of the combustion instability of the longitudinal mode was not only the longitudinal variations but also the radial variations in the center of intensity (COI) of the flame images so the Pythagorean addition of the interquartile ranges of both longitudinal and radial directions was suggested as a good indicator of the size of the longitudinal mode's combustion instability. The time series trajectory of the COI also provided meaningful information on the combustion instability mechanism. A linear and longer COI trajectory was likely to generate more intense instability because the one-way movement of the flame can be more effective in driving instability by transferring consistent force in the same direction.     

The nonlinear heat release response of stratified lean-premixed flames to acoustic velocity oscillations

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to high-amplitude acoustic forcing in a laboratory-scale stratified burner operated with CH₄ and air at atmospheric pressure. The double-swirler, double-channel annular burner was specially designed to generate high-amplitude acoustic velocity oscillations and a radial equivalence ratio gradient at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the response are not considered in the present investigation. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH^∗/CH^∗ interference filters. Time-averaged and phase-synchronized CH^∗ chemiluminescence intensities were measured using an intensified CCD camera. The measurements show that flame stabilization mechanisms vary depending on equivalence ratio gradients for a constant global equivalence ratio (?_g = 0.60). Under uniformly premixed conditions, an enveloped M-shaped flame is observed. In contrast, under stratified conditions, a dihedral V-flame and a toroidal detached flame develop in the outer stream and inner stream fuel enrichment cases, respectively. The modification of the stabilization mechanism has a significant impact on the nonlinear response of stratified flames to high-amplitude acoustic forcing (u′/U ∼ 0.45 and f = 60, 160 Hz). Outer stream enrichment tends to improve the flame’s stiffness with respect to incident acoustic/vortical disturbances, whereas inner stream stratification tends to enhance the nonlinear flame dynamics, as manifested by the complex interaction between the swirl flame and large-scale coherent vortices with different length scales and shedding points. It was found that the behavior of the measured flame describing functions (FDF), which depend on radial fuel stratification, are well correlated with previous measurements of the intensity of self-excited combustion instabilities in the stratified swirl burner. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, its effect on flame location and the interaction with unsteady flow structures.     

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO₂, owing to the reduced amount of carbonaceous fuel.     

Swirling distributed combustion for clean energy conversion in gas turbine applications

Distributed combustion provides significant performance improvement of gas turbine combustors. Key features of distributed combustion includes uniform thermal field in the entire combustion chamber, thus avoiding hot-spot regions that promote NO_x emissions (from thermal NO_x) and significantly improved pattern factor. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of hot and active species present in the flame and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here for our quest to explore the beneficial aspects of such flows on clean combustion in simulated gas turbine combustion conditions. The goal is to develop high intensity combustor with ultra low emissions of NO and CO, and much improved pattern factor. Experimental results are reported from a cylindrical geometry combustor with different modes of fuel injection and gas exit stream location in the combustor. In all the configurations, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes for the geometries investigated here. Swirling flow configuration, wherein the product gas exits axially resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from traditional diffusion mode to distributed combustion regime. Results showed very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions even at rather high equivalence ratio of 0.7 at a high heat release intensity of 36 MW/m³-atm with non-premixed mode of combustion. Results are also reported on lean stability limit and OH^* chemiluminescence under both premixed and non-premixed conditions for determining the extent of distribution combustion conditions.     

Model-based control of combustion instabilities in annular combustors

Lean premixed combustors produce lower NO_x emissions, but are particularly prone to damaging combustion instabilities. Active control can be used to stabilize combustion instabilities. So far model-based control strategies have tended to focus on longitudinal rather than annular combustors, even though many gas turbines have annular geometries. In this work, a computational thermoacoustic model is used to simulate unstable annular combustors, providing a platform on which to develop and test control strategies. The model contains multiple fuel valves for actuation, which respond to multiple pressure sensors according to a controller matrix. Two strategies for designing the controller matrix are developed. The first involves stabilizing each of the unstable circumferential modes independently; the second involves controlling the transfer function matrix between sets of actuators and sensors. The resulting controllers are implemented in simulations using the thermoacoustic model. They are seen to stabilize instabilities in a variety of combustors, including one with nonaxisymmetry due to burner differences and one with both circumferential and longitudinal unstable modes.     

Plasma assisted combustion: Dynamics and chemistry

Plasma assisted combustion is a promising technology to improve engine performance, increase lean burn flame stability, reduce emissions, and enhance low temperature fuel oxidation and processing. Over the last decade, significant progress has been made towards the applications of plasma in engines and the understanding of the fundamental chemistry and dynamic processes in plasma assisted combustion via the synergetic efforts in advanced diagnostics, combustion chemistry, flame theory, and kinetic modeling. New observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported. Advances are made in the understanding of non-thermal and thermal enhancement effects, kinetic pathways of atomic O production, diagnostics of electronically and vibrationally excited species, plasma assisted combustion kinetics of sub-explosion limit ignition, plasma assisted low temperature combustion, flame regime transition of the classical ignition S-curve, dynamics of the minimum ignition energy, and the transport effect by non-equilibrium plasma discharge. These findings and advances have provided new opportunities in the development of efficient plasma discharges for practical applications and predictive, validated kinetic models and modeling tools for plasma assisted combustion at low temperature and high pressure conditions. This article is to provide a comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control.     

3D computation of hydrogen-fueled combustion around turbine blade-effect of arrangement of injector holes-

Recently,a number of environmental problems caused from fossil fuel combustion have been focused on.Inaddition,with the eventual depletion of fossil energy resources,hydrogen gas is expected to be an alternativeenergy resource in the near future.It is characterized by high energy per unit weight,high reaction rate,widerange of flammability and the low emission property.On the other hand,many researches have been underway inseveral countries to improve a propulsion system for an advanced aircraft.The system is required to have higherpower,lighter weight and lower emissions than existing ones.In such a future propulsion system,hydrogen gaswould be one of the promising fuels for realizing the requirements.Considering these backgrounds,our group hasproposed a new cycle concept for hydrogen-fueled aircraft propulsion system.In the present study,we perform 3dimensional computations of turbulent flow fields with hydrogen-fueled combustion around a turbine blade.Themain objective is to clarify the influence of arrangement of hydrogen injector holes.Changing the chordwise andspanwise spacings of the holes,the 3 dimensional nature of the flow and thermal fields is numerically studied.     

Evaluation of srk equation of state in calculating the thermophysical properties of gas turbine combustion gases

The purpose of this work is to evaluate the thermophysical properties of combustion gases of the gas turbine engine using the Soave-Redlich-Kwong equation of state and to compare the results with those obtained from the virial equation of state and the experimental values obtained from experiment and the generalized charts. The properties which have been considered in this work were, density, specific heat at constant pressure, enthalpy, entropy, viscosity and thermal conductivity. The temperature range was (200–2600 K) theoretically, while the pressure range was (3–12 atmospheres). The Soave-Redlich-Kwong (SRK) equation of state generally predicted better values for thermophysical properties than those predicted by the virial equation of state. A computer program, to evaluate the departure of thermophysical properties using virial and SRK equations of state, was used.     

Co-firing hydrogen and dimethyl ether in a gas turbine model combustor: Influence of hydrogen content and comparison with methane

To promote the utilization of hydrogen (H₂) in existing gas turbines, dimethyl ether (DME) was used to co-fire with H₂ in a model combustor. The swirl combustion characteristics of DME/H₂ mixtures were measured under the varying H₂ content up to 0.7. The results show that the flow velocity elevates as the H₂ content increases, which is associated with the increased flame temperature. The OH level firstly increases and subsequently keeps nearly unchanged as the H₂ content increases. Meanwhile, the OH area nonlinearly increases with the increasing H₂ content. Moreover, the increasing H₂ content induces almost linearly decreased lean blowout limit (LBO), increased NO emission, and intensified combustion acoustics. Furthermore, the combustion characteristics of the 0.46DME/0.54H₂ mixture and CH₄ with the same volumetric heat value were compared. The 0.46DME/0.54H₂ flame displays lower LBO and higher NO emission than the CH₄ flame, which mainly results from the higher reactivity of 0.46DME/0.54H₂ mixture.     

MILD combustion for hydrogen and syngas at elevated pressures

As gas recirculation constitutes a fundamental condition for the realization of MILD combustion, it is necessary to determine gas recirculation ratio before designing MILD combustor. MILD combustion model with gas recir- culation was used in this simulation work to evaluate the effect of fuel type and pressure on threshold gas recir- culation ratio of MILD mode. Ignition delay time is also an important design parameter for gas turbine combustor, this parameter is kinetically studied to analyze the effect of pressure on MILD mixture ignition. Threshold gas re- circulation ratio of hydrogen MILD combustion changes slightly and is nearly equal to that of 10 MJ/Nm3 syngas in the pressure range of 1-19 atm, under the conditions of 298 K fresh reactant temperature and 1373 K exhaust gas temperature, indicating that MILD regime is fuel flexible. Ignition delay calculation results show that pres- sure has a negative effect on ignition delay time of 10 MJ/Nm3 syngas MILD mixture, because OH mole fraction in MILD mixture drops down as pressure increases, resulting in the delay of the oxidation process.     

A consistent level set formulation for large-eddy simulation of premixed turbulent combustion

A consistent formulation of the G-equation approach for LES is developed. The unfiltered G equation is valid only at the instantaneous flame front location. Hence, in a filtering procedure applied to derive the appropriate LES equation, only the instantaneous unfiltered flame surface can be considered. A new filter kernel is provided, which averages along the flame surface. The filter kernel is used to derive the G equation for the filtered flame front location. This equation has two unclosed terms, involving a flame front conditional averaged flow velocity, and a filtered propagation term. A model for the conditional velocity is derived, expressing this quantity in terms of the Favre-filtered flow velocity, which is typically known from a flow solver. This model leads to the appearance of a density ratio in the propagation term of the G equation. LES of combustion in the thin reaction zones regime is discussed in the LES regime diagram. A new line is identified separating the thin reaction zones regime into two parts, where the broadened flame thickness is larger and smaller than the filter size, respectively. A model for the propagation term is provided. This leads to a term including the subfilter turbulent burning velocity and an additional term proportional to the resolved flame front curvature. For the former, an algebraic model is provided from an equation for the subfilter flame front wrinkling. The latter term depends on the inverse of the subfilter Damköhler number and disappears in the corrugated flamelets regime.     

Effect of convex platform structure on hydrogen and oxygen combustion characteristics in micro combustor

The combustion characteristics of the micro combustor with a convex platform were simulated and the effects of the height of the convex platform and the inlet velocity on the combustion process were analyzed. The results show that the setting of convex platform can significantly increase the maximum velocity and reduce the outlet velocity. When the height of the boss continues to increase, the maximum velocity is more significant, but has little effect on the outlet velocity. At the same time, the increasing height of the convex platform increases, the turbulent kinetic energy and reduces the intensity of combustion on the axis. However further increase in the height does not reduce the effect significantly. The fuel conversion rate increases significantly, but the velocity decreases. In the micro combustor with a convex platform, increasing the inlet velocity increases the axial temperature, the fuel conversion rate decreases.     

An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions

Chemical kinetics in hydrogen combustion for elevated pressures have recently become more relevant because of the implementation of hydrogen as a fuel in future gas turbine combustion applications, such as IGCC or IRCC systems. The aim of this study is to identify a reaction mechanism that accurately represents H₂/O₂ kinetics over a large range of conditions, particularly at elevated pressures as present in a gas turbine combustor. Based on a literature review, six mechanisms of different research groups have been selected for further comparisons within this study. Reactor calculations of ignition delay times show that the mechanisms of Li et al. and Ó Conaire et al. yield the best agreement with data from shock tube experiments at pressures up to 33 bar. The investigation of one-dimensional laminar hydrogen flames indicate that these two mechanisms also yield the best agreement with experimental data of laminar flame speed, particularly for elevated pressures. The present study suggests that the Li mechanism is best suited for the prediction of H₂/O₂ chemistry since it includes more up-to date data for the range of interest.     

Feasibility of pulse combustion in micro gas turbines

In gas turbines, a fast decrease of efficiency appears when the output decreases; the efficiency of a large gas turbine (20…30 MW) is in the order of 40 %, the efficiency of a 30 kW gas turbine with a recuperator is in the order of 25 %, but the efficiency of a very small gas turbine (2…6 kW) in the order of 4…6 % (or 8…12 % with an optimal recuperator). This is mainly a result of the efficiency decrease in kinetic compressors, due to the Reynolds number effect. Losses in decelerating flow in a flow passage are sensitive to the Reynolds number effects. In contrary to the compression, the efficiency of expansion in turbines is not so sensitive to the Reynolds number; very small turbines are made with rather good efficiency because the flow acceleration stabilizes the boundary layer. This study presents a system where the kinetic compressor of a gas turbine is replaced with a pulse combustor. The combustor is filled with a combustible gas mixture, ignited, and the generated high pressure gas is expanded in the turbine. The process is repeated frequently, thus producing a pulsating flow to the turbine; or almost a uniform flow, if several parallel combustors are used and triggered alternately in a proper way. Almost all the compression work is made by the temperature increase from the combustion. This gas turbine type is investigated theoretically and its combustor also experimentally with the conclusion that in a 2 kW power size, the pulse flow gas turbine is not as attractive as expected due to the big size and weight of parallel combustors and due to the efficiency being in the order of 8 % to 10 %. However, in special applications having a very low power demand, below 1000 W, this solution has better properties when compared to the conventional gas turbine and it could be worth of a more detailed investigation.     

Reduction of a detailed reaction mechanism for hydrogen combustion under gas turbine conditions

The aim of this study is to find a reduced mechanism that accurately represents chemical kinetics for lean hydrogen combustion at elevated pressures, as present in a typical gas turbine combustor. Calculations of autoignition, extinction, and laminar premixed flames are used to identify the most relevant species and reactions and to compare the results of several reduced mechanisms with those of a detailed reaction mechanism. The investigations show that the species OH and H are generally the radicals with the highest concentrations, followed by the O radical. However, the accumulation of the radical pool in autoignition is dominated by HO₂ for temperatures above, and by H₂O₂ below the crossover temperature. The influence of H₂O₂ reactions is negligible for laminar flames and extinction, but becomes significant for autoignition. At least 11 elementary reactions are necessary for a satisfactory prediction of the processes of ignition, extinction, and laminar flame propagation under gas turbine conditions. A 4-step reduced mechanism using steady-state approximations for HO₂ and H₂O₂ yields good results for laminar flame speed and extinction limits, but fails to predict ignition delay at low temperatures. A further reduction to three steps using a steady-state approximation for O leads to significant errors in the prediction of the laminar flame speed and extinction limit.