Iris Z. Hu

Calculations of turbulent flames using a PSR microstructural library

An alternative to the laminar flamelet model is developed for intensely turbulent flame calculations. The turbulent structure of the flame is assumed to be in the distributed-reaction-zone regime. A library or lookup table of thermochemical quantities (species, temperature, and density) is constructed a priori by performing PSR calculations over a range of mixture fractions and a range of residence times corresponding to the expected range in the mixing scales. The PDF of residence times is related to the PDF of the scalar dissipation rate. The model is implemented in a two-dimensional axisymmetric k-e-g code, assuming a beta function PDF for the mixture fraction and a log-normal PDF for the scalar dissipation rate. Calculations are compared with Raman data from three turbulent bluff-body-stabilized flames (1) a non-premixed CO/H2/N2-air flame, (2) a non-premixed CH4/H2-air flame, and (3) a premixed CH4-air flame, with very encouraging results. The shapes of the profiles are similar, and the quantitative agreement is generally within 200 K or better on mean temperature and within a factor of 2 or better even on highly nonequilibrium quantities such as CO in the core of a CH4-air flame.

Aerodynamics of a Fuel Spoke in a Gas Turbine Combustor

The three-dimensional unsteady flow in a gas turbine combustor was studied using CFD means. The flow structure around a fuel spoke is of interest not only because of pollutant issues, but also because of combustor operating issues such as combustion acoustics and potential flame-holding in the premixer. The CFD model was tested extensively in terms of grid density and lime-marching step size before the final calculation was made. It was shown that when a swirling flow crosses over a cylindrical fuel spoke, wake vortices are formed and a strong secondary flow is generated along the spanwise direction. A secondary vortex existed near the tip of the spoke. This complex flow structure affects the quality of fuel and air mixing and can be addressed by CFD-based design methods.Copyright

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 (3–D) 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 3–D 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 NOx 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/H2/N2–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/H2/N2 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 temperatures using the finite rate model showed the best agreement with measurements made by a thermocouple rake. In addition, the CO and NOx emission indices were also computed and compared with corresponding data.Copyright