Search

Classical gas turbine thermodynamic cycle has undergone no change over the last decades. The most important efficiency improvements have been obtained by reducing thermal losses and raising the overall pressure ratio and peak temperature. Pressure gain combustion (PGC) represents an increasingly interesting solution to break out current technological limits. Indeed cycle models show that a pressure raise across the combustion process would reduce fuel consumption, increasing efficiency. Providing an efficiency close to the corresponding detonative technological concepts, constant volume combustion (CVC) represents a viable solution that still needs to be studied. In this work, the CV2 (constant-volume combustion vessel) installed at the Pprime laboratory (France) is numerically investigated using the high-fidelity compressible large eddy simulation (LES) solver AVBP. All the successive phases of the CVC cycle, i.e., air intake, fuel injection, spark-ignited combustion, and exhaust, are considered in the LES. Intake and exhaust valves are properly represented by novel boundary conditions able to mimic the valves impact on the flow without the need to directly consider their presence and dynamics during the simulation, reducing the computational costs. The spark ignition is modeled as an energy deposition term added to the energy equation. The combustion phase is treated by the dynamic version of the thickened flame model (DTFLES) extended to deal with nonconstant pressure combustion. Time-resolved particle imaging velocimetry (PIV) and pressure measurement inside the chamber reveal that cold and reactive turbulent flow are well captured in all the phases, showing the reliability of the approach and the models used. [ABSTRACT FROM AUTHOR]

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process. [ABSTRACT FROM AUTHOR]

The forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1 large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements. [ABSTRACT FROM AUTHOR]

The present paper reports experimental and numerical analyses of the macrostructures featured by a stratified swirling flame for varying stratification ratio (SR). The studies are performed with the Beihang Axial Swirler Independently Stratified (BASIS) burner, a novel double-swirled full-scale burner developed at Beihang University. Experimentally, it is found that depending on the ratio between the equivalence ratios of the methane-air mixtures from the two swirlers, the flame stabilizes with three different shapes: attached V-flame, attached stratified flame, and lifted flame. In order to better understand the mechanisms leading to the three macrostructures, large eddy simulations (LES) are performed via the open-source computational fluid dynamics (CFD) software OpenFOAM using the incompressible solver ReactingFoam. Changing SR, simulation results show good agreement with experimentally observed time-averaged flame shapes, demonstrating that the incompressible LES are able to fully characterize the different flame behaviors observed in stratified burners. When the LES account for heat loss from walls, they better capture the experimentally observed flame quenching in the outer shear layer (OSL). Finally, insights into the flame dynamics are provided by analyzing probes located near the two separate streams. [ABSTRACT FROM AUTHOR]

Modern combustion chambers of gas turbines for power generation and aero-engines suffer of thermo-acoustic combustion instabilities generated by the coupling of heat release rate fluctuations with pressure oscillations. The present article reports a numerical analysis of limit cycles arising in a longitudinal combustor. This corresponds to experiments carried out on the longitudinal rig for instability analysis (LRIA) test facility equipped with a full-scale lean-premixed burner. Heat release rate fluctuations are modeled considering a distributed flame describing function (DFDF), since the flame under analysis is not compact with respect to the wavelengths of the unstable modes recorded experimentally. For each point of the flame, a saturation model is assumed for the gain and the phase of the DFDF with increasing amplitude of velocity fluctuations. A weakly nonlinear stability analysis is performed by combining the DFDF with a Helmholtz solver to determine the limit cycle condition. The numerical approach is used to study two configurations of the rig characterized by different lengths of the combustion chamber. In each configuration, a good match has been found between numerical predictions and experiments in terms of frequency and wave shape of the unstable mode. Time-resolved pressure fluctuations in the system plenum and chamber are reconstructed and compared with measurements. A suitable estimate of the limit cycle oscillation is found. [ABSTRACT FROM AUTHOR]

This study investigates H 2 -air flames obtained with a laboratory scale coaxial dual-swirl injector in which fuel and oxidizer are injected separately. Two flame archetypes are observed experimentally for the same global equivalence ratio ϕ g ≈ 0.45 and two different thermal powers: a flame anchored to the injector (≈ 4 kW) and an aerodynamically stabilized flame exhibiting a characteristic V-shape (≈ 10 kW). Large Eddy Simulations (LES) allow to retrieve both regimes and are used to investigate these two stabilization modes. The numerical setup is first validated against isothermal and reactive Particle Image Velocimetry measurements and OH * chemiluminescence images. The mean velocity field of both operating conditions reveals the existence of a strong inner recirculation zone (IRZ) that, penetrating inside the injector nozzle, leads to a radial divergence of the central hydrogen jet, which ultimately favors one stabilization regime over the other. The first flame anchors on the hydrogen injector lip and it develops along the mixing layer between H 2 and air swirling jets. The lifted flame, instead, stabilizes in the inner shear layer between the IRZ and the exiting swirling jet of fresh gases, burning over a wide range of equivalence ratios. LES also unveil the flame structures typical of each flame: the anchored one is entirely controlled by diffusion, while the lifted flame is characterized by a first partially premixed branch and a second diffusion front. Finally, high-speed camera and LES are used to analyze the unsteady transition from lifted to anchored flames. [ABSTRACT FROM AUTHOR]

This work analyzes pressure and hydrogen enrichment effects on the stabilization and combustion dynamics of a partially-premixed swirled-stabilized methane flame operated at 1, 3 and 5 bar, with H 2 admixture up to 40% by volume. Large Eddy Simulations (LES) are performed to analyze flame stabilization and dynamics for all cases. When pressure is increased, both turbulence and chemical time scales are reduced leading to smaller turbulent scales and thinner flames which makes LES modeling challenging. To ensure that all flames are resolved similarly in the tested pressure range, the dynamic formulation of the thickened flame model (DTFLES) is used here with a Static Mesh Refinement (SMR) strategy. An Analytically Reduced Chemistry (ARC) scheme is employed to describe CH 4 -H 2 /Air chemistry. LES is validated against experimental multi-kHz repetition-rate OH* chemiluminescence, OH Planar Laser Induced Fluorescence (PLIF), stereoscopic Particle Image Velocimetry (sPIV) and pressure recordings. The dynamics of the different flames are then addressed. First, the impact of hydrogen at atmospheric pressure is investigated. While the reference natural gas flame (1 bar, 0% H 2) presents a lifted M-shape with a strong Precessing Vortex Core (PVC), 40% H 2 -enrichment modifies the flame which becomes an attached V-shape, with a weakened PVC and the triggering of a thermoacoustic oscillation at the combustion chamber first acoustic mode. Second, the impact of mean pressure is analyzed by fixing the H 2 -enrichment while increasing the mean pressure to 3 and then 5bar. As the pressure increases, the flame assumes a more compact M-shape. It is proven that the interaction with the turbulent smallest scales is not affected by pressure: Karlovitz number is constant. On the contrary, the flame response to the large turbulent structures is modified: Damkhler number reduces when mean pressure increases. If strong flame/PVC interactions are observed at atmospheric pressure leading to flame tip/root roll-up and local quenching, a more coherent flame is observed when pressure is raised. Furthermore, for these points, the thermoacoustic oscillation coupled with the first chamber acoustic mode rapidly disappears and stable conditions are recovered. [ABSTRACT FROM AUTHOR]

An Arrhenius-based one-step scheme is derived for hydrogen-air combustion simulations. A Pre-Exponential Adjustment approach, based on an explicit analytical dependence of reaction rate parameters on equivalence ratio and pressure is adopted, together with a correction to improve the prediction of thermal flame thickness. The reduced scheme is validated by computations of one-dimensional unstrained and strained laminar premixed flames for a wide range of pressures ([1; 30] atm), unburned gas temperatures ([300; 800] K), and equivalence ratios ([0.4; 6.0]), with a good agreement of predicted main flame parameters between reduced and reference kinetic schemes. Coupled to a high-fidelity Navier–Stokes compressible solver, the reduced scheme is successfully proved for the numerical simulation of canonical configurations such as one-dimensional and two-dimensional premixed flames under several mixture conditions, with a significant improvement of computational efficiency. [Display omitted] • Derivation of a one-step scheme for hydrogen-air mixtures based on Arrhenius law. • Pre-Exponential Adjustment to match typical combustion systems' operative ranges. • Introduction of a correction to predict flame thickness in all mixture conditions. • Good agreement of reduced and reference schemes found for canonical configurations. • Sensible reduction (> 50%) of computational costs in CFD applications. [ABSTRACT FROM AUTHOR]