Lei-Yong Jiang

A Critical Evaluation of NOx Modeling in a Model Combustor

Reliable NO x 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 renormalization 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 postprocessing 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 reburn 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 semiempirical, postprocessing NO model can provide valuable NO simulations as long as the velocity and temperature fields are adequately predicted.

Radiation Benchmarking 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.

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

CFD Investigation of Temperature Rise in a Combustor Test Rig Exhaust System

This paper presents a study of temperature rise in the exhaust system of a combustor test rig, Test Cell #1, at the Gas Turbine Laboratory, Institute for Aerospace Research, the National Research Council of Canada. As the flow regime is supersonic with a mixture of hot air & water vapour, condensation of water vapour in the system is suggested to explain the temperature rise observed along the exhaust pipe. The method of Computational Flow Dynamics is used to carry out the first investigation on this hypothesis. The exhaust system is reproduced by CAD, meshed and modelled by the ANSYS-FLUENT CFD package. Simulations of a two-phase complex mixture are performed. The numerical results indicate that the pressure control devices in the exhaust flow towards the stack create phenomena similar to nozzles and yield condensed water into the system. The simulations of liquid phase content and temperature fields are qualitatively consistent with experimental observations and support the hypothesis that condensation is occurring and may therefore threaten the structural integrity of the system through thermal effects.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

Flow-Field Investigation of Two Gas-Sampling Probes

The flow fields in and around two versions of a water-cooled gas-sampling probe, situated downstream of a gas turbine combustor, were numerically studied in an elevated pressure and temperature environment. The probes are of triple-walled stainless steel assembly, where the gas sample is transported through a centre tube, while preheated and pressurized cooling water flows through two surrounding annuli. Complex conjugate heat transfers amongst the exhaust mixture, cooling water and probe walls were modelled at a selected operating condition. The numerical results indicate over-heating and possible vaporization of water or cavitation in the upstream tip region of the probe with the original design. This is consistent with the evidence of damage observed in these probes from prolonged testing under similar conditions. For the modified probe, the effectiveness of cooling water is much improved, which is confirmed by long-term combustor rig testing. From this investigation, some recommendations for probe design and operation are provided. Moreover, the present study has proved that the numerical simulation is a valuable tool for probe design and trouble-shooting, and to accurately predict conjugate heat transfers in such flows, the laminar sub-layer in the near-wall region should be adequately resolved.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