High-pressure Turbulent Flames Research Articles

Large eddy simulations of turbulent combustion of kerosene-air in a dual swirl gas turbine model combustor at high pressures

In advanced gas turbine engines, high-pressure combustion generally occurs with combustion chamber pressure exceeding the critical pressure of the engine fuel. In this paper, large eddy simulations are conducted in a dual swirl gas turbine model combustor to analyze the effects of different fuel-air equivalence ratios and chamber pressures on swirling injection and turbulent combustion. Numerical results reveal that intricate interactions exist between swirling flows and high-pressure turbulent flame. Strong swirling injection of air could cause reverse flows in the inner injector, which lead to a V shaped partially premixed turbulent flame at a chamber pressure of 3 MPa. As the operating pressure increases to 5 MPa, decreased injection velocity would prevent reverse flows in the inner injector, leading to the turbulent diffusive flame. On the other hand, combustion causes flow acceleration, gaseous expansion, and strong thermophysical property variations, which would influence the reverse flows, increase the expansion angle in swirling injection, and dictate the size of central recirculation zone and the oscillation frequency of precessing vortex core. Results obtained herein would help gain fundamental understanding on high-pressure swirling flows and flame dynamics in advanced gas turbine engines.

Numerical modelling of unburned hydrocarbon emissions in gas engines with varied fuels

A numerical model is developed for the prediction of unburned hydrocarbons (UHC) emissions in gas engines. Local UHC sources in the engine combustion chamber are simulated and the unburned fuel leaving the engine is calculated and compared with experimental data from a single cylinder research engine for different operating conditions. The calculation procedure is applicable for complex three dimensional geometries and regards the local flow and turbulence conditions. A hybrid model is proposed to integrate the detailed reaction kinetics via separate sub models coupled with Computational Fluid Dynamics (CFD). A high pressure turbulent flame propagation model is applied, being based on detailed reaction kinetic simulations of the laminar flame speeds for the engine conditions. Flame wall quenching, crevices and post-oxidation of UHC are treated as separate submodels. The UHC emissions are solved as scalars to account for the convection and diffusion transport in the engine, incorporating a one-step post-oxidation model with detailed chemistry. The 3D-CFD results are validated with experiments, showing on one hand side the fidelity of the proposed numerical approach and on the other hand side the possibility to differentiate the different sources of UHC. Additionally a numerical study is conducted to investigate the influence of stoichiometric gas engines with exhaust gas recirculation. The model is applicable also for biogas engines with reduced reactivity.

Soot and spectral radiation modeling for high-pressure turbulent spray flames

A transported probability density function (PDF) method and a photon Monte Carlo/line-by-line (PMC/LBL) spectral model are exercised to generate physical insight into soot processes and spectral radiation characteristics in transient high-pressure turbulent n-dodecane spray flames, under conditions that are relevant for compression-ignition piston engines. PDF model results are compared with experimental measurements and with results from a locally well-stirred reactor (WSR) model that neglects unresolved turbulent fluctuations in composition and temperature. Computed total soot mass and soot spatial distributions are highly sensitive to the modeling of unresolved turbulent fluctuations. To achieve reasonable agreement between model and experiment and to capture the highly intermittent nature of soot in the turbulent flame, it is necessary to accurately represent mixing and the low diffusivity of soot particles. This is accomplished in the PDF framework using a mixing model that enforces locality in the gas-phase composition space, while not mixing the transported soot variables. The results suggest that mixing is at least as important as kinetics in controlling soot formation and evolution in high-pressure turbulent flames. Regarding radiation, radiant fractions and global influences of radiation in these flames are relatively small. Nevertheless, an examination of spectral radiative heat transfer provides valuable insight into the nature and modeling of radiation in high-pressure turbulent combustion systems. There are complex spectral interactions that are revealed using PMC/LBL. CO2 dominates the total radiative emission and reabsorption, but most of the emitted CO2 radiation is reabsorbed before reaching the walls. On the other hand, most of the emitted soot radiation reaches the walls, but soot radiation is a small contribution overall; H2O dominates the radiation that reaches the walls. Global turbulence–radiation interactions (TRI) effects are small, but radiative emission from soot increases by approximately a factor two when TRI are considered. Radiative transfer contributes both to energy redistribution in the vessel and to wall heat losses. The results suggest that a simple model that considers soot radiation and the principal CO2 and H2O spectral bands might be sufficient to capture the key influences of radiation in engine CFD. It is expected that these findings will contribute to the development of truly predictive CFD models for engines and other high-pressure turbulent combustion systems.

Validation of a Turbulent Flame Speed Model across Combustion Regimes

A flame speed expression proposed recently is studied in detail and extensively validated in this study. This expression has no adjustable parameters, and its constants are closely tied to the physics of scalar mixing at small scales. In the weak turbulence limit, the flame speed expression recovers a linear dependence of the turbulent to laminar flame speed ratio, , on the normalized turbulence rms velocity, , in accordance with Damköhler's classical result. However, in the limit of intense turbulence, Damköhler's result gives a square-root dependence, while the new expression gives a combination of linear and square-root terms. Predictions of the new expression are compared to a wide range of experimental data of S T from various flame configurations and conditions, with the same values of the model constants. The quantitative comparisons are found to be very good with experimental data beyond the usually restricted range of of existing models. Predictions of S T for high-pressure turbulent flames, up to 3 MPa, also compare acceptably well with experimental measurements. The flame speed is found to increase when the mean curvature of the flame brush is positive, and this dependence seems to be linear.

Characterization of the effects of hydrogen addition in premixed methane/air flames

The use of hydrogenated fuels shows considerable promise for applications in gas turbines and internal combustion engines. In the present work, the effects of hydrogen addition in methane/air flames are investigated using both a laminar flame propagation facility and a high-pressure turbulent flame facility. The aim of this research is to contribute to the characterization of lean methane/hydrogen/air premixed turbulent flames at high pressures, by studying the flame front geometry, the flame surface density and the instantaneous flame front thermal thickness distributions. The experiments and analyses show that a small amount of hydrogen addition in turbulent premixed methane–air flames introduces changes in both instantaneous and average flame characteristics.