Development of Simulations/Diagnostics and Their Use as a Combustor Design Tool

Abstract

This presentation describes the importance of CFD-based simulations and diagnostics in the design of gas turbine combustor main burners and augmentors.  Most combustor design simulations are based on solving the Reynolds averaged Navier-Stokes equations.  Reviews of high-speed movies of the flame structure in a practical combustor illustrate the highly dynamic nature of the flame.  Very few if any of the instantaneous snapshots of the flame appear to represent the “time𕢝 averaged flow field estimated with the Reynolds averaged codes.  This casts doubt on how well the “real” physics is represented by a Reynolds averaged CFD simulation that tacitly uses physics that is based on a time independent flow field.  These simulations also contain sub-models for fuel breakup, turbulent mixing, chemistry, etc., that are required for closure and completeness of the physical and chemical process.  Many of these processes are not well understood, especially under practical combustion conditions.  Indeed, considering the non repetitive nature of the dynamic flow field and the uncertainties associated with the sub-models, it is amazing how well the simulations estimate, with some calibration, the performance and emissions parameters associated with gas turbine main burners and augmentors.  However, there are conditions, such as combustion instabilities in augmentors, where the time dependence of the flow cannot be ignored.  The Air Force has been evolving developing sub-models for sprays, chemistry, etc.  for improving the accuracy of Reynolds averaged simulations.  Large eddy simulations are being developed on basic research programs to capture the time dependent nature of flows.  We have also been involved in a long-term (10+ years) development of a time dependent Navier-Stokes simulation called UNICORN (UNsteady Ignition and COmbustion with ReactioNs).  UNICORN is a research tool for studying the dynamic characteristics of flames.  It has been used as a design tool for advanced combustors.  From its conception, the development of UNICORN has been strongly coupled with experiments that are designed to evaluate the chemistry and transport models used in the code, and to challenge its ability to predict complex dynamic characteristics of combusting flows.  High-speed computing is essential to fully utilize UNICORN and other simulations being developed as combustor and augmentor design tools.  However, it is also important to integrate the high-speed computing capabilities with an experimental diagnostic program that can aid in evaluating and developing the simulations.  The value of an integrated simulation/diagnostic approach will be illustrated in this presentation.