Ian Campbell

Performance of discrete ordinates method in a gas turbine combustor simulator

Abstract Predictive accuracy of discrete ordinates method (DOM) was assessed by applying it to the prediction of incident radiative fluxes on the walls of a gas turbine combustor simulator (GTCS), and comparing its predictions with measurements. Input data utilized for the DOM were measured gas concentration and temperature profiles and inner wall temperatures of the GTCS which is a cylindrical enclosure containing a turbulent diffusion flame of propane and air. Effects of order of approximation (S4 and S6) and using uniform and non-uniform gas absorption coefficients for the non-homogeneous medium on the accuracy of the predicted heat fluxes were also investigated. Comparisons show that S4 approximation is adequate for the prediction of incident wall heat fluxes and that the use of an absorption coefficient profile based on measured gas concentrations and temperatures improves the accuracy significantly.

An Attempt at Large Eddy Simulation for Combustor Modeling

Large eddy simulation (LES) is recognized as a promising method for numerical simulation in combustion systems. A LES attempt in a model combustor has been made, and a few important issues including grid size, inflow condition, wall boundary conditions, physical sub-models and data sampling, have been carefully considered. It is found that the turbulence forcing with the vortex method at the air/fuel inlets does not affect the LES results for the present configuration and the turbulence can develop naturally in the inlet section. Moreover, significant computing power is required for LES to capture both the high and low frequencies of interest in a turbulent reacting flow. In the paper, some of the numerical results are presented and compared with a comprehensive experimental database, which indicates that LES can provide reasonable predictions for the mean axial velocity and temperature distributions inside the combustion chamber. However, in order to make LES a valuable and cost-effective tool in the development of advanced combustion systems, some fundamental questions remain to be addressed and more validation efforts are required.Copyright

Turbulence Modeling in a Model Combustor

The flow field of a propane-air diffusion flame combustor with interior and exterior conjugate heat transfers was numerically investigated. Solutions obtained from four turbulence models together with a laminar flamelet combustion model, discrete ordinates radiation model and enhanced wall treatment are presented and discussed. The numerical results are compared, in detail, with a comprehensive database obtained from a series of experimental measurements. It is found that the Reynolds stress model (RSM), a second moment closure, illustrates superior performance over three popular two-equation eddy-viscosity models. Although the main flow features are captured by all four turbulence models, only the RSM is able to successfully predict the lengths of both recirculation zones and the turbulence kinetic energy distribution in the combustor chamber. In addition, it provides fairly good predictions for all Reynolds stress components, except for the circumferential normal stress at downstream sections. However, the superiority of the RSM is not so obvious for the temperature and species predictions in comparison with eddy-viscosity models, except for the standard k-e model. This suggests that coupling between the RSM and combustion models needs to be further improved in order to enhance its applications in practical combustion systems.Copyright

Radiation Bench-Marking in a Model Combustor

Radiation heat transfer in a model combustor with interior and exterior conjugate heat transfers has been numerically studied. The previous investigations on turbulence, combustion and scalar transfer modeling (Reynolds analogy), and comparisons with a comprehensive experimental database provide a reliable base to evaluate the effect of radiation heat transfer on the flow field and NO emission in the combustor. Some of the numerical results with and without radiation are presented and compared with the experimental measurements. It is found that the total radiation heat flux through the combustor wall is about 4.2% of the total energy released from the input fuel. The effect of radiation on the flow field is minor, particularly to the velocity field. In contrast, it has significant effects on the NO field, where the predicted values without radiation are two times higher than those with radiation or the experimental data. A considerable effect of radiation on the combustor wall temperature is also observed. In summary, to provide valuable predictions of NO emission and combustor liner temperature, the radiation heat transfer should be properly taken into account in numerical simulations.Copyright

Reynolds Analog in Combustor Modeling

Accurate temperature prediction is vital for the development of advanced combustion systems. The Reynolds analogy concept has been almost exclusively used in current turbulent reacting flow RANS simulations. In this paper, this hypothesis applied to a diffusion flame model combustor is discussed and assessed. Some of the numerical results obtained from a flamelet combustion model with the turbulence Prandtl/Schmidt number from 0.25 to 0.85 are presented, and compared with a benchmark experimental database. It is found that the turbulence Prandtl/Schmidt number has significant effect on the predicted temperature and species fields inside the combustor, as well as the temperature profile at the combustor wall. In contrast, its effect on the velocity field is insignificant in the range assessed. With the optimized turbulence Prandtl/Schmidt number, both velocity and scalar fields can be reasonably and quantitatively predicted. For the present configuration and operating conditions, the optimal Prandtl/Schmidt number is 0.5, lower than the commonly accepted values, ∼0.70. This study suggests that for accurate prediction of scalar transfers in turbulent reacting flows, the Reynolds analogy concept should be improved and new approaches should be developed.© 2007 ASME

A Critical Evaluation of NOx Modeling in a Model Combustor

Reliable NOx modeling depends on the accurate prediction of both velocity and temperature fields. The velocity and temperature fields of a propane diffusion flame combustor, with interior and exterior conjugate heat transfers, were first numerically studied. The results from three combustion models, together with the re-normalization group (RNG) k-e turbulence model and the discrete ordinates radiation model are discussed, and compared with comprehensive experimental measurements. The flow patterns and the recirculation zone length in the combustion chamber are excellently predicted, and the mean axial velocities are in fairly good agreement with the experimental data for all three combustion models. The mean temperature profiles are fairly well captured by the probability density function (PDF) and eddy dissipation (EDS) combustion models. However, the EDS-finite-rate combustion model fails to provide an acceptable temperature field. Based on the acceptable velocity and temperature fields, a number of NO modeling approaches were evaluated in a post-processing mode. The partial-equilibrium approach of O and OH radical concentrations shows a significant effect on the thermal NO formation rate. In contrast, the prompt NO, the NO re-burn mechanism and the third reaction of the extended Zeldovich mechanism have negligible effects on the overall NO formation in the present study. This study indicates that the semi-empirical, post-processing NO model can provide valuable NO simulations as long as the velocity and temperature fields are adequately predicted.Copyright

Study of Mixing Enhancement From a 12-Lobe Convoluted Mixer

Lobed mixers have been used in a variety of engineering applications, such as jet noise reduction, infrared suppression and improvement of propulsive efficiency for turbine engines. More recently, they have emerged as an attractive method to enhance mixing between fuel and air in advanced low-emission gas turbine combustors. The objectives of the present work were to assess the effectiveness of these devices for use inside the combustor and provide experimental data to validate CFD predictions. The mixing enhancements due to streamwise vortices generated from a 12-lobe convoluted mixer were characterized using Planar Laser Induced Fluorescence (PLIF) measurements, while 2-D PIV measurements established the underlying velocity field. The geometrical set-up of the mixing system is pertinent to many combustion systems using advanced lean premixed concepts with gaseous fuels. In addition to the benchmark case with no mixer, two different lobe geometries were considered, a semi-circle (or round) lobe and a square lobe. In this paper the experimental results are presented and discussed. Numerical predictions were performed for the semi-circle lobe geometry using a Reynolds-averaged Navier-Stokes (RANS) code and the results are compared with experimental measurements.© 2003 ASME

Application of Various Combustion Models to a Generic Combustor

The flow-field of a generic gas combustor with interior and exterior conjugate heat transfers was numerically studied. Results obtained from three combustion models, combined with the re-normalization group (RNG) k-e turbulence model, discrete ordinates radiation model, and partial equilibrium NOx model are presented and discussed. The numerical results are compared with a comprehensive database obtained from a series of experimental tests. The flow patterns and the recirculation zone length are excellently predicted, and the mean axial velocities are in fairly good agreement with the experimental measurements, particularly at downstream sections for all three combustion models. The mean temperature profiles are also fairly well captured by the probability density function (PDF) and eddy dissipation (EDS) combustion models. The EDS-finite-rate combustion model fails to provide acceptable temperature field. In general, the PDF shows some superiority over the EDS and EDS-finite-rate models. NOx levels predicted by the EDS model are in reasonable agreement with the experimental measurements.Copyright

Parametric Study to Optimize Air/Fuel Mixing for Lean, Premix Combustion Systems

New designs of gas turbine combustors for power generation applications have to meet ever-tightening emission standards (mainly NOx, CO and UHC) while operating at high combustor pressures. This requires a detailed understanding of the physical processes involved. The air-fuel mixture preparation is a critical step in most advanced gas turbine combustion strategies to achieve lower emissions. It has long been established that the level of unmixedness between the fuel and air is strongly tied with NOx levels. The present paper applies the statistical technique of Design Of Experiments (DOE) to a generic mixer set-up that includes an axial swirler, with fuel injected at discrete locations and transverse to the flow. The objective is to identify influential design and operating parameters that will provide rapid and enhanced mixing. The parameters tested include Swirl strength as measured by the Swirl number, Swirl type (Constant angle vs. Free vortex), number and momentum of fuel injection sites and gas temperature. Planar Laser Induced Fluorescence of acetone (PLIF) was used to quantify mixing at various planar locations in the mixing section. Commercial CFD software is used to model the flow field and predict the spatial mixing at selected conditions. Comparisons are made with experimental measurements with the aim to validate the CFD code and also on comparing the model results with the measurements.Copyright