Anil K. Tolpadi

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.

Heat Transfer Measurements and Predictions for a Modern, High-Pressure, Transonic Turbine, Including Endwalls

This paper presents both measurements and predictions of the hot-gas-side heat transfer to a modern, 1 1 / 2 stage high-pressure, transonic turbine. Comparisons of the predicted and measured heat transfer are presented for each airfoil at three locations, as well as on the various endwalls and rotor tip. The measurements were performed using the Ohio State University Gas Turbine Laboratory Test Facility (TTF). The research program utilized an uncooled turbine stage at a range of operating conditions representative of the engine: in terms of corrected speed, flow function, stage pressure ratio, and gas-to-metal temperature ratio. All three airfoils were heavily instrumented for both pressure and heat transfer measurements at multiple locations. A 3D, compressible, Reynolds-averaged Navier-Stokes computational fluid dynamics (CFD) solver with k-ω turbulence modeling was used for the CFD predictions. The entire 11 2 stage turbine was solved using a single computation, at two different Reynolds numbers. The CFD solutions were steady, with tangentially mass-averaged inlet/exit boundary condition profiles exchanged between adjacent airfoil-rows. Overall, the CFD heat transfer predictions compared very favorably with both the global operation of the turbine and with the local measurements of heat transfer. A discussion of the features of the turbine heat transfer distributions, and their association with the corresponding flow-physics, has been included.

Turbulence Model Assessment for Conjugate Heat Transfer in a High Pressure Turbine Vane Model

An assessment of steady state Reynolds Averaged Navier-Stokes (RANS) models has been undertaken for conjugate heat transfer of an internally cooled high-pressure turbine vane with and without film cooling. The assessment includes near wall treatment and different 2-equation Eddy Viscosity Models (EVM) and 6-equation Reynolds Stress Models (RSM) models. The present study was conducted using CFX v11.0 with unstructured tetrahedral meshes with near wall prism layers. The validation cases are the 1983 NASA C3X internally cooled vane and the 1988 NASA C3X internally and film cooled vane. Internal cooling for both cases is achieved with ten radial cooling channels of constant cross-sectional area. Film cooling is achieved for the same airfoil geometry but with three separately fed upstream plenums feeding various rows of film cooling holes. Predictions obtained with the different modeling strategies are compared to documented metal surface pressures and temperatures and the differences are discussed. A conjugate heat transfer assessment is made using the vane Biot number. In general good agreement with experimental data is obtained for wall integration meshes with the k-ω and SST turbulence models.Copyright

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.

Coupled Lagrangian Monte Carlo PDF-CFD computation of gas turbine combustor flowfields with finite-rate chemistry

A coupled Lagrangian Monte Carlo Probability Density Function (PDF) -Eulerian Computational Fluid Dynamics (CFD) technique is presented for calculating steady three-dimensional turbulent reacting flow 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 transport equation was solved using a Lagrangian particle tracking Monte Carlo (MC) method. The PDF modeled was over composition only. This MC module has been coupled with CONCERT, which is a fully elliptic three-dimensional body-fitted CFD code based on pressure correction techniques. In an earlier paper (Tolpadi et al., 1995), this computational approach was described, but only fast chemistry calculations were presented in a typical aircraft engine combustor. In the present paper, reduced chemistry schemes were incorporated into the MC module that enabled the modeling of finite rate effects in gas turbine flames and therefore the prediction of CO and NO x emissions. With the inclusion of these finite rate effects, the gas temperatures obtained were also more realistic. Initially, a two scalar scheme was implemented that allowed validation against Raman data taken in a recirculating bluff body stabilized CO/H 2 /N 2 -air flame. Good agreement of the temperature and major species were obtained. Next, finite rate computations were performed in a single annular aircraft engine combustor by incorporating a simple three scalar reduced chemistry scheme for Jet A fuel. This three scalar scheme was an extension of the two scalar scheme for CO/H 2 /N 2 fuel. The solutions obtained using the present approach were compared with those obtained using the fast chemistry PDF transport approach (Tolpadi et al., 1995) as well as the presumed shape PDF method. The calculated exhaust gas temperature using the finite rate model showed the best agreement with measurements made by a thermocouple rake. In addition, the CO and NO x emission indices were also computed and compared with corresponding data.

Combustion technology for low-emissions gas-turbines: Some recent modeling results

Computer modeling of low-emissions gas-turbine combustors requires inclusion of finite-rate chemistry and its interactions with turbulence. The purpose of this review is to outline some recent developments in and applications of the physical models of combusting flows. The models reviewed included the sophisticated and computationally intensive velocity-composition pdf transport method, with applications shown for both a laboratory flame and for a practical gas-turbine combustor, as well as a new and computationally fast PSR-microstructure-based method, with applications shown for both premixed and nonpremixed flames. Calculations are compared with laser-based spectroscopic data where available. The review concentrates on natural-gas-fueled machines, and liquid-fueled machines operating at high power, such that spray vaporization effects can be neglected. Radiation and heat transfer is also outside the scope of this review.

Heat Transfer Predictions of Film Cooled Stationary Turbine Airfoils

Conventional heat transfer design methods for high temperature gas turbine airfoils decouple the internal and external flow. Thermal boundary conditions from these decoupled analyses are applied to the blade surfaces to predict turbine life. Typically, the domain for the external flow includes the hot gas path and the film cooling holes while the domain for the internal flow includes the internal flow passages and film cooling holes. The solid blade itself couples the external and internal flow and heat transfer. Since film cooling flow physics can play a significant role on the overall turbine blade heat transfer, there has been increased interest in capturing these effects by including the hole geometry in the solution procedure. Ideally, the complete turbine blade heat transfer analysis would be provided by efficient CFD simulations for the coupled problem including the internal passages, film cooling holes and hot gas path. By prescribing both the external flow and internal flow inflow/outflow boundary conditions, the hole physics can be included in the solution. The current paper presents results obtained for coupled simulations of the NASA C3X vane and VKI rotor which models the internal passages, hole geometries and hot gas path. In both cases, cooling is achieved by rows of pressure-side, leading-edge and suction-side film cooling holes. The rows are independently fed by span-wise, constant area plenums. The former has a total of 152 cylindrical cooling holes whereas the later has a total of 110 cylindrical/shaped holes. In addition, the C3X vane consists of 10 internal radial cooling passages of cylindrical cross-section. The simulations were conducted with the Shear Stress Transport (SST) model on a grid that extended into the viscous sub-layer along all surfaces. The computed surface pressure and external heat transfer coefficient distributions at mid-span are compared to experimental data for both cases. Internal heat transfer predictions are also presented and discussed.Copyright

Predictions of the Effect of Roughness on Heat Transfer From Turbine Airfoils

The heat transfer and aerodynamic performance of turbine airfoils are greatly influenced by the gas side surface finish. In order to operate at higher efficiencies and to have reduced cooling requirements, airfoil designs require better surface finishing processes to create smoother surfaces. In this paper, three different cast airfoils were analyzed: the first airfoil was grit blasted and codep coated, the second airfoil was tumbled and aluminide coated, and the third airfoil was polished further. Each of these airfoils had different levels of roughness. The TEXSTAN boundary layer code was used to make predictions of the heat transfer along both the pressure and suction sides of all three airfoils. These predictions have been compared to corresponding heat transfer data reported earlier by Abuaf et al. (1997). The data were obtained over a wide range of Reynolds numbers simulating typical aircraft engine conditions. A three-parameter full-cone based roughness model was implemented in TEXSTAN and used for the predictions. The three parameters were the centerline average roughness, the cone height and the cone-to-cone pitch. The heat transfer coefficient predictions indicated good agreement with the data over most Reynolds numbers and for all airfoils-both pressure and suction sides. The transition location on the pressure side was well predicted for all airfoils; on the suction side, transition was well predicted at the higher Reynolds numbers but was computed to be somewhat early at the lower Reynolds numbers. Also, at lower Reynolds numbers, the heat transfer coefficients were not in very good agreement with the data on the suction side.Copyright

Parametric Modeling Approach to Gas Turbine Combustor Design

Recent advances in CAD/CAE based design tools have not only enabled the automation of parametric sensitivity analyses involving aero, thermal and structural calculations necessary for gas turbine combustor life predictions, but have also minimized the analysis cycle time required to perform such parametric analyses. Concepts involved in development of the parametric models for geometry creation, grid generation and unstructured Computational Fluid Dynamics (CFD) simulations of a gas turbine combustor are discussed. In addition, the advantages of utilizing the parametric geometry and analysis models in parametric sensitivity studies of combustor aero analysis are demonstrated. A CAD-based parametric master model for 3-D solid feature creation and aero-analysis parametric models were developed for a modern GE single annular aircraft gas turbine combustor. Using these parametric models, a matrix of geometries was generated to define the computational domain for performing a parametric sensitivity study involving aero analysis, for combustor exit temperature predictions. The present combustor parametric aero-analysis considers the effects of the location of dilution holes and film slots in the combustor inner and outer liner components on the gas turbine combustor exit temperature profiles. Comparisons of the predicted combustor exit temperatures with available test data for an 18° annular sector are presented.Copyright

Soot Modeling in Gas Turbine Combustors

A method is presented for predicting soot in gas turbine combustors. A soot formation/oxidation model due to Fairweather et al [1992] has been employed. This model has been implemented in the CONCERT code which is a fully elliptic three-dimensional (3-D) body-fitted computational fluid dynamics (CFD) code based on pressure correction techniques. The combustion model used here is based on an assumed probability density function (PDF) parameterized by the mean and variance of the mixture fraction and a β-PDF shape. In the soot modeling, two additional transport equations corresponding to the soot mass fraction and the soot number density are solved. As an initial validation, calculations were performed in a simple propane jet diffusion flame for which experimental soot concentration measurements along the centerline and along the radius at various axial downstream stations were available from the literature. Soot predictions were compared with measured data which showed reasonable agreement. Next, soot predictions were made in a 3-D model of a CF6-80LEC engine single annular combustor over a range of operating pressures and temperatures. Although the fuel in the combustor is Jet-A, the soot computations assumed propane to be the surrogate fuel. To account for this fuel change, the soot production term was increased by a factor of 10X. In addition, the oxidation term was increased by a factor of 4X to account for uncertainties in the assumed collision frequencies. The soot model was also tested against two other combustors, a CF6-80C and a CFM56-5B. Comparison of the predicted scot concentrations with measured smoke numbers showed fairly good correlation within the range of the soot model parameters studied. More work has to be performed to address several modeling issues including sensitivity to oxidation rate coefficients and scalar diffusion.Copyright