Multi-Modal Turbulent Combustion under Autoignitive Conditions: Separability of Combustion Modes and Implications for Modeling
Abstract
In diesel engines and gas turbine engines, the primary propulsion systems used in Army operations, flames are not attached to fuel and oxidizer injection nozzles but are stabilized at a distance downstream of the nozzle, that is, the lift-off height. The lift-off height and its dynamics in turbulent flows play a crucial role in the stability of the system as well as in its emissions characteristics. At low oxidizer temperatures, lifted flame stabilization occurs through the interaction of premixed flame propagation and nonpremixed combustion. At the stabilization point, a fuel-rich premixed flame, a fuel-lean premixed flame, a nonpremixed flame form a ÒtripleÓ flame structure, with the propagation of this structure against the incoming flow responsible for stabilization of the flame. At high oxidizer temperatures, lifted flame stabilization occurs through autoignition as unburned fuel/air mixtures are carried downstream of the nozzle. However, in recent computational studies, regimes have been discovered where lifted laminar flame stabilization occurs by both premixed flame propagation and autoignition simultaneously. Furthermore, when subjected to oscillations in velocity, these laminar flames were found to exhibit a transition between different stabilization mechanisms and a corresponding change in lift-off height. These studies directly motivate the need to better understand the role of turbulent fluctuations in turbulent lifted flames in these new regimes and understand how turbulence affects the dynamics of the transition between the different stabilization mechanisms. To address the role of turbulence in turbulent lifted flame stabilization, the proposed research first seeks to develop a new general yet computationally efficient combustion model for Large Eddy Simulation (LES) that can account for all three modes of combustion (premixed flame propagation, nonpremixed combustion, and homogeneous autoignition) as well as all intermediate regimes, such as the strong multi-modal interactions involved in turbulent lifted flame stabilization. The foundation of the LES model is a proposed set of two-dimensional flamelet equations that account for all three combustion modes with only two dimensions, a key realization that makes the model computationally viable. The two coordinates describe changes in local fuel-air ratio and changes in the overall progress of reaction from unburned to equilibrium. These new flamelet equations will first be validated through comparisons with detailed laminar flame simulations in partially premixed coflow and counterflow configurations under autoignitive conditions when all three modes of combustion are simultaneously active. The two-dimensional flamelet model will then be integrated into a LES framework that includes a novel adaptive tabulation strategy to accommodate the high-dimensional model. To validate the LES model, comparisons will be made to experimental measurements and Direct Numerical Simulations (LES) of turbulent lifted flames with stabilization dominated by either premixed flame propagation or autoignition. The validated LES model will then be applied to engine-relevant conditions when both stabilization mechanisms are active to investigate and understand the role of turbulence in dictating the stabilization mechanism, its transition, and its dynamics.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 06, 2018
- Source ID
- W911NF1710391
Entities
People
- Michael Mueller
Organizations
- Army Contracting Command
- Princeton University
- United States Army