S Ruan

Modelling of turbulent lifted jet flames using flamelets: a priori assessment and a posteriori validation

This study focuses on the modelling of turbulent lifted jet flames using flamelets and a presumed Probability Density Function (PDF) approach with interest in both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes of the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for a turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction Z and progress variable c, including their statistical correlation, are obtained using a copula method, which is also validated using the DNS data. The statistically independent PDFs are found to be generally inadequate to represent the joint PDFs from the DNS data. The effects of Z–c correlation and the contribution from the non-premixed combustion mode on the flame lift-off height are studied systematically by including one effect at a time in the simulations used for a posteriori validation. A simple model including the effects of chemical kinetics and scalar dissipation rate is suggested and used for non-premixed combustion contributions. The results clearly show that both Z–c correlation and non-premixed combustion effects are required in the premixed flamelets approach to get good agreement with the measured flame lift-off heights as a function of jet velocity. The flame brush structure reported in earlier experimental studies is also captured reasonably well for various axial positions. It seems that flame stabilisation is influenced by both premixed and non-premixed combustion modes, and their mutual influences.

Simulation of Premixed Combustion with Varying Equivalence Ratio in Gas Turbine Combustor

Reynolds-averaged Navier–Stokes simulation of a gas turbine combustor with complex geometry is performed. The turbulence is modeled using a two-equation approach. The scalar mixing and combustion is modeled using a flamelet-based approach for partially premixed combustion. In this approach, transport equations to describe scalar mixing and reacting fields and their statistical interactions are solved along with standard conservation equations. The boundary conditions are specified using measured values. The computed spatial variations of averaged temperature and dry mole fractions of various scalars, including CO and NOx, compared quite well with measured values. The results of this combustion model are compared with the eddy-dissipation-concept model commonly used for gas turbine combustion calculation and this comparison showed that the results of partially premixed combustion model is improved considerably.

INVESTIGATION OF FLAME STRETCH IN TURBULENT LIFTED JET FLAME

DNS data of a laboratory-scale turbulent lifted hydrogen jet flame has been analyzed to show that this flame has mixed mode combustion not only at the flame base but also in downstream locations. The mixed mode combustion is observed in instantaneous structures as in earlier studies and in averaged structure, in which the predominant mode is found to be premixed combustion with varying equivalence ratio. The non-premixed combustion in the averaged structure is observed only in a narrow region at the edge of the jet shear layer. The analyzes of flame stretch show large probability for negative flame stretch leading to negative surface averaged flame stretch. The displacement speed-curvature correlation is observed to be negative contributing to the negative flame stretch and partial premixing resulting from jet entrainment acts to reduce the negative correlation. The contribution of turbulent straining to the flame stretch is observed to be negative when the scalar gradient aligns with the most extensive principal strain rate. The physics behind the negative flame stretch resulting from turbulent straining is discussed and elucidated through a simple analysis of the flame surface density transport equation.

Simulation of MILD combustion using Perfectly Stirred Reactor model

Abstract A simple model based on a Perfectly Stirred Reactor (PSR) is proposed for moderate or intense low-oxygen dilution (MILD) combustion. The PSR calculation is performed covering the entire flammability range and the tabulated chemistry approach is used with a presumed joint probability density function (PDF). The jet, in hot and diluted coflow experimental set-up under MILD conditions, is simulated using this reactor model for two oxygen dilution levels. The computed results for mean temperature, major and minor species mass fractions are compared with the experimental data and simulation results obtained recently using a multi-environment transported PDF approach. Overall, a good agreement is observed at three different axial locations for these comparisons despite the over-predicted peak value of CO formation. This suggests that MILD combustion can be effectively modelled by the proposed PSR model with lower computational cost.

Statistics of Reaction Progress Variable and Mixture Fraction Gradients from DNS of Turbulent Partially Premixed Flames

Statistically planar turbulent partially premixed flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios = 0.7 and = 1.0 using three-dimensional, simplified chemistry–based direct numerical simulations (DNS). The simulation parameters are chosen such that the flames represent the thin reaction zones regime combustion. A random bimodal distribution of equivalence ratio φ is introduced in the unburned gas ahead of the flame to account for the mixture inhomogeneity. The results suggest that the probability density functions (PDFs) of the mixture fraction gradient magnitude |∇ξ| (i.e., P(|∇ξ|)) can be reasonably approximated using a log-normal distribution. However, this presumed PDF distribution captures only the qualitative nature of the PDF of the reaction progress variable gradient magnitude |∇c| (i.e., P(|∇c|)). It has been found that a bivariate log-normal distribution does not sufficiently capture the quantitative behavior of the joint PDF of |∇ξ| and |∇c| (i.e., P(|∇ξ|, |∇c|)), and the agreement with the DNS data has been found to be poor in certain regions of the flame brush, particularly toward the burned gas side of the flame brush. Moreover, the variables |∇ξ| and |∇c| show appreciable correlation toward the burned gas side of the flame brush. These findings are corroborated further using a DNS data of a lifted jet flame to study the flame geometry dependence of these statistics.

Numerical study of transient evolution of lifted jet flames: partially premixed flame propagation and influence of physical dimensions

Three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes simulations of a spark-ignited turbulent methane/air jet flame evolving from ignition to stabilisation are conducted for different jet velocities. A partially premixed combustion model is used involving a correlated joint probability density function and both premixed and non-premixed combustion mode contributions. The 3D simulation results for the temporal evolution of the flames leading edge are compared with previous two-dimensional (2D) results and experimental data. The comparison shows that the final stabilised flame lift-off height is well predicted by both 2D and 3D computations. However, the transient evolution of the flames leading edge computed from 3D simulation agrees reasonably well with experiment, whereas evident discrepancies were found in the previous 2D study. This difference suggests that the third physical dimension plays an important role during the flame transient evolution process. The flame brushs leading edge displacement speed resulting from reaction, normal and tangential diffusion processes are studied at different typical stages after ignition in order to understand the effect of the third physical dimension further. Substantial differences are found for the reaction and normal diffusion components between 2D and 3D simulations especially in the initial propagation stage. The evolution of reaction progress variable scalar gradients and its interaction with the flow and mixing field in the 3D physical space have an important effect on the flames leading edge propagation.

Computation of Forced Premixed Flames Dynamics

ABSTRACT Bluff body stabilized turbulent premixed flames subject to inlet velocity oscillation over a wide range of forcing frequency and amplitude are simulated using a flamelet-based combustion model. Two sets of detailed chemical kinetic schemes are used to model combustion chemistry. It is observed that the computed dynamics of forced flames agree reasonably well with experimental measurements. The flame elongation and shortening at a frequency of 40 Hz and strong flame-vortex interaction at a higher frequency of 160 Hz are captured well in the computations. The global flame describing function extracted from the computational results shows a linear response at 40 Hz and a nonlinear behavior at 160 Hz as observed in the experiments. The nonlinear response is due to vortex roll-up and its subsequent shedding. The quantitative agreement of the computed flame describing function (FDF) with experimental measurement is uniformly good over a wide range of forcing frequency and amplitude. Some influence of chemical kinetics on the FDFs is observed, which mainly stems from the difference in laminar burning velocity and spatial heat release rate distribution.

Modelling of Turbulent Premixed and Partially Premixed Combustion

A significant increase in our energy consumption, from 495 quadrillion Btu in 2007 to 739 quadrillion Btu in 2035 with about 1.4% annual increase, is predicted (US Energy Information Administraion, 2010). This increase is to be met in environmentally friendly means in order to protect our planet. Despite the renewable energy sources are identified to be the fastest growing in the near future, they are expected to meet only one third or less of this energy demand. Also, the renewable generation methods face significant barriers such as economical risks, high capital costs, cost for infrastructure development, low energy conversion efficiency, and low acceptance level from public (US Energy Information Administraion, 2010) at this time. It is likely that improvements will be made on all of these factors in due course. The role of nuclear technology in the energy market will vary from time to time for many cultural and political reasons, and the perceptions of the general public. In the current climate, however, it is clear that the fossil fuels will remain as the dominant source to meet the demand in energy consumption. Hence, optimised design of combustion and power generating systems for improved efficiency and emissions performance are crucial. The emissions of oxides of nitrogen and sulphur, and poly-aromatic hydrocarbons are known sources of atmospheric pollution from combustion. Their detrimental effects on environment and human health is well known (Sawyer, 2009) and green house gases such as oxides of carbon and some hydrocarbons are also included as pollutants in recent years. The emission of carbon dioxide (CO2) from fossil, liquid and solid, fuel combustion accounts for nearly 76% of the total emissions from fossil fuel burning and cement production in 2007 (Carbon Dioxide Information Analysis Center, 2007). The global mean CO2 level in the atmosphere increases each year by about 0.5% suggesting a global mean level of about 420 ppm by 2025 (Anastasi et al., 1990; US Department of Commerce, 2011) Such a forecasted increase has led to stringent emission regulations for combustion systems compelling us to find avenues to improve the environmental friendliness of these systems. Lean premixed combustion is known (Heywood, 1976) to have potentials for effective reduction in emissions and to increase efficiency simultaneously. Significant technological advances are yet to be made for developing fuel lean combustion systems operating over wide range of conditions with

A priori investigation of subgrid correlation of mixture fraction and progress variable in partially premixed flames

Subgrid correlation of mixture fraction, Z, and progress variable, c, is investigated using direct numerical dimulation (DNS) data of a hydrogen lifted jet flame. Joint subgrid behaviour of these two scalars are obtained using a Gaussian-type filter for a broad range of filter sizes. A joint probability density function (JPDF) constructed using single-snapshot DNS data is compared qualitatively with that computed using two independent β-PDFs and a copula method. Strong negative correlation observed at different streamwise locations in the flame is captured well by the copula method. The subgrid contribution to the Z–c correlation becomes important if the filter is of the size of the laminar flame thickness or larger. A priori assessment for the filtered reaction rate using the flamelet approach with independent β-PDFs and correlated JPDF is then performed. Comparison with the DNS data shows that both models provide reasonably good results for a range of filter sizes. However, the reaction rate computed using copula JPDF is found to have a better agreement with the DNS data for large filter sizes because the subgrid Z–c correlation effect is included.