Hukam Chand Mongia

Large-Eddy Simulation of a Gas Turbine Combustor Flow

Large-eddy simulation (LES) of turbulent premixed reacting flows in a gas turbine combustor (General Electrics lean premixed dry low-NOx LM6000) has been carried out to evaluate the potential of LES for design studies of realistic hardware. A flamelet model for the premixed flame is combined with a dynamic model for the subgrid kinetic energy to simulate the propagation of the turbulent flame in this high swirl and high Reynolds number flow. Comparison of the computed results with experimental data indicate good agreement in spite of relatively coarse grid resolution employed in the LES. These results provide significant confidence that LES capability for design studies of practical interest is feasible in the near future.

Challenges and Progress in Controlling Dynamics in Gas Turbine Combustors

Combustion dynamics impact the design of both conventional diffusion e ame gas turbine combustors and lean premixed combustion systems. The occurrence of dynamics in diffusion e ame combustors can generally be eliminated through changes to the fueling system or operating characteristics driven primarily through empirical design know-how. Lean premixed combustors, such as industrial aeroderivative dry low-emissions combustors, have more persistent dynamics problems that are only partially ameliorated with application of empirical tools and controls. With continuing emphasis on reducing emissions and cost and increasing performance of turbopropulsion engine combustors, the design direction of e ight engines is approaching the lean premixed limit; thus, there is a need for improved tools and control strategies for combustion dynamics in these applications as well. The components of a combined analytical, experimental, and computational approach to understanding and controlling combustion dynamics are being developed. With successful implementation of such an analytical and control mechanism, further improvements in the emissions, stability, and durability characteristics of both e ight and industrial combustors will become possible.

Flow dynamics in a swirl combustor

A hybrid simulation approach is used to investigate the flow patterns in an axisymmetric swirl combustor configuration. Effective inlet boundary conditions are based on velocity data from Reynolds-averaged Navier-Stokes or actual laboratory measurements at the outlet of a fuel-injector nozzle, and large eddy simulations are used to study the unsteady non-reactive swirl flow dynamics downstream. Case studies ranging from single-swirler to more complex triple-swirler nozzles are presented to emphasize the importance of initial inlet conditions on the behaviour of the swirling flow entering a sudden expansion area, including swirl and radial numbers, inlet length and characteristic velocity profiles. Swirl of sufficient strength produces an adverse pressure gradient which can promote flow reversal or vortex breakdown, and the coupling between swirl and sudden expansion instabilities depends on the relative length of the inlet. The flow is found to be very sensitive to the detailed nature of the velocity radi...

AN ADVANCED SPRAY MODEL FOR APPLICATION TO THE PREDICTION OF GAS TURBINE COMBUSTOR FLOW FIELDS

It is well known that fuel preparation, its method of injection into a combustor, and its atomization characteristics have a significant impact on emissions. A simple dilute spray model, which assumes that droplet heating and vaporization occur in sequence, has been implemented in the past within computational fluid dynamics (CFD) codes at General Electric (GE) and has been used extensively for combustion applications. This spray model coupled with an appropriate combustion model makes reasonable predictions of the combustor pattern factor and emissions. To improve upon this predictive ability, a more advanced quasi-steady droplet vaporization model has been considered. This article describes the evaluation of this advanced model. In this new approach, droplet heating and vaporization take place simultaneously (which is more realistic). In addition, the transport properties of both the liquid and vapor phases are allowed to vary as a function of pressure, gas phase temperature, and droplet temperature. Th...It is well known that fuel preparation, its method of injection into a combustor, and its atomization characteristics have a significant impact on emissions. A simple dilute spray model, which assumes that droplet heating and vaporization occur in sequence, has been implemented in the past within computational fluid dynamics (CFD) codes at General Electric (GE) and has been used extensively for combustion applications. This spray model coupled with an appropriate combustion model makes reasonable predictions of the combustor pattern factor and emissions. To improve upon this predictive ability, a more advanced quasi-steady droplet vaporization model has been considered. This article describes the evaluation of this advanced model. In this new approach, droplet heating and vaporization take place simultaneously (which is more realistic). In addition, the transport properties of both the liquid and vapor phases are allowed to vary as a function of pressure, gas phase temperature, and droplet temperature. These transport properties, which are most up to date, have been compiled from various sources and appropriately curve-fit in the form of polynomials. Validation of this new approach for a single droplet was initially performed. Subsequently calculations of the flow and temperature field were conducted and emissions (NOx, CO, and UHC) were predicted for a modern single annular turbofan engine combustor using both the standard spray model and the advanced spray model. The effect of the number of droplet size ranges as well as the effect of stochastic treatment of the droplets were both investigated.

Monte Carlo Probability Density Function Method for Gas Turbine Combustor Flowfield Predictions

A coupled Lagrangian Monte Carlo (MC) probability density function (PDF), Eulerian computational e uid dynamics (CFD) technique is presented for calculating steady three-dimensional turbulent reacting e ow in a gas turbine combustor. PDF transport methods model turbulence ‐ combustion interactions more accurately than conventional turbulence models with an assumed-shape PDF. The PDF was over composition only. The PDF transport equation was solved using a Lagrangian particle-tracking MC method. This MC module has been coupled with CONCERT, which is a fully elliptic three-dimensional bodye tted CFD code based on pressure correction techniques. CONCERT calculates the mean velocity and mixing frequency e eld that are required by the composition PDF in the MC module, whereas the MC module computes the PDF from which the mean density e eld is extracted and supplied to CONCERT. This modeling approach was initially validated against Raman data taken in a recirculating bluff body stabilized e ame. The computed mixture fraction and its variance (as obtained from the calculated PDF ) compared very well against the corresponding measurements made along several radial lines at different axial downstream positions and along the axis. A typical single annular aircraft engine combustor was also analyzed. In this preliminary study, the e owe eld, fuel, and temperature distribution were obtained based on the assumption of fast chemistry. The solutions obtained using the present approach were compared with those obtained using a presumed-shape PDF method. The comparison of the calculated exhaust gas temperatures using these two approaches with measurements made by a thermocouple rake appeared to indicate better agreement with the PDF transport technique.

Validation of Near Wall Turbulence Models for Film-Cooling Applications in Combustors

Most of the cooling design of combustion liners generally involve several wall jet computations to simulate the machined ring film cooling arrangement and film cooling jets from multi-holes. The cooling system design requires that the metal temperatures and gradients be within acceptable limits for the most severe conditions imposed on the engine. To achieve this design, several arrangements of the cooling schemes, including machined ring film cooling, multihole effusion cooling and cooling flow splits to different sections of the combustor may have to be tried before arriving at an optimum design. For each cooling arrangement and cooling flow distribution, a CFD analysis is first performed and then a thermal analysis of the combustor walls is performed to obtain the distribution of wall temperatures, heat fluxes and temperature gradients. Thus accurate handling of these types of flows is a very important aspect of the analysis. The objective of the present work is to assess the performance of a few turbulence models for application to computation of film cooling effectiveness of modern combustors with multi-hole effusion cooled liners. INTRODUCTION In the modern combustors for aircraft engine gas turbines, the inlet temperatures and pressures are high. *Senior Engineer, Associate Fellow AIAA ++Manager Copyright

Prediction of NO emission from a lean dome gas turbine combustor

This paper describes the development and implementation of a model for predicting NO emissions from lean dome gas turbine combustors. Computational Combustion Dynamics models (CCD) have been used for providing qualitative guidance in the gas turbine combustor design process. Due to uncertainties in the turbulence and combustion models, boundary conditions and numerics, the CCD model has to be anchored to data before it can be used as a reliable design tool. The anchored CCD approach for rich dome combustors has been described previously by Danis, et al. (1996). Application of the rich dome combustor anchoring methods to lean dome combustors revealed several shortcomings with the models used for predicting gaseous emissions. The NO, CO and HC emissions were all under predicted. Subsequently, a NO formation model based on the steady state approximation for nitrogen atoms was implemented in the CCD code and gave much better agreement with the lean dome CFMDAC data.

Instantaneous flow structures in a reacting gas turbine combustor

While many researchers have studied swirl-stabilized and other model gas turbine combustors, very few have investigated the flow field and combustion within a gas turbine combustor operated with realistic Jet-A fuel at elevated pressures and inlet temperatures. In this paper we present results from an investigation into the flow field and flame locations within a unique combustor furnished with an injector designed by GE Aircraft Engines. The combustor provides high optical access which has allowed the use of Particle Imaging Velocimetry to probe both instantaneous vortex and recirculation zone structures in the flow and obtain ensemble averaged profiles of pertinent turbulence quantities. Both non-reacting and reacting conditions were studied to assess the eects of a flame on the flow within the combustor. All reacting conditions were run with liquid Jet A as fuel. This too provides a more realistic configuration for it accurately captures the eects of droplet/spray combustion along with the correct heat release from a heavy hydrocarbon fuel. The location of the flame front was imaged using Planar Laser Induced Fluorescence (PLIF) of formaldehyde. This study is one of the first to apply PLIF to a gas turbine combustor running liquid Jet A. Important flow features present in the combustor have been identified and key dierences between the reacting and non-reacting flow fields have been noted. With information about both the flow and flame front, structures that have not been highlighted in the past are shown to be important in the operation of the combustor. These results have indicated that the time-averaged flow field masks some very important details of the flow within a gas turbine combustor.

Automated cooling design methodology for combustor walls

An automated cooling design procedure for aircraft engine combustors has been developed which uses a converged CFD flow solution and produces a detailed temperature distribution in the liners, dome, etc. of a combustor. The output of the automated procedure is the following: a detailed wall temperature and heat flux distribution; arrangement of effusion cooling holes, starter slots, and impingement cooling holes; the cooling flow splits in various zones which produce acceptable wall temperatures for the most severe cycle conditions imposed on the engine.

Large Eddy Simulations of Reacting Flows in a Dump Combustor

Large-eddy simulations (LES) of turbulent premixed combustion in a dump combustor that is an accurate model for an actual gas turbine combustor (General Electrics lean premixed dry low NOx LM6000) has been carried out to evaluate the potential of LES for design studies of realistic hardware. A thin flame model for the premixed flame is combined with a dynamic model for the subgrid kinetics energy to simulate the propagation of the turbulent flame in this highly swirling and high Reynolds number flow field. Comparison of the computed results with experimental data indicate very good agreement in spite of relatively coarse grid resolution employed in the LES. These results provide significant confidence that advanced parallel LES capability for design studies of practical interest is feasible in the near future.