Turbulent Combustion and Reacting Flows

Turbulence-Flame Interactions in Premixed Combustion

TESLa Members: Colin Towery, Nick Wimer, Clarissa Briner, and Peter Hamlington
Collaborators: Alexei Poludnenko, Elaine Oran, Matthias Ihme, and Javier Urzay

The interaction between a flame and the turbulence in which it propagates is a fundamental problem in combustion theory. The ability to understand and model these interactions -- both qualitatively and quantitatively -- is required for control of combustion in gas turbine, internal combustion, and supersonic combustion engines. Modeling these interactions is complicated, however, by the nonlinear coupling between turbulent and chemical processes, particularly in premixed reacting flows. The fundamental properties of turbulence in high-speed compressible reacting flows have been examined using three-dimensional, implicit LES of premixed flames. The interactions between turbulence and flames in premixed reacting flows have been studied for a broad range of turbulence intensities by analyzing scalar (reactant mass-fraction) gradient, vorticity, and strain rate fields. A decomposition of the total strain rate into components due to turbulence and the flame shows that vorticity suppression depends on the relative alignment between vorticity and the flame surface normal. Intermittency in premixed reacting flows is studied using numerical simulations of premixed flames at a range of turbulence intensities. The flames are modeled using a simplified reaction mechanism that represents a stoichiometric H2-air mixture. Intermittency is associated with high probabilities of large fluctuations in flow quantities, and these fluctuations can have substantial effects on the evolution and structure of premixed flames. Intermittency is characterized here using probability density functions (pdfs) and moments of the local enstrophy, pseudo-dissipation rate (strain rate magnitude), and scalar (reactant mass fraction) dissipation rate. The implications of these results for the internal structure of the flame are discussed, and we also propose a connection between reacting flow intermittency and anisotropic vorticity suppression by the flame. Particular emphasis in the future will be placed on understanding basic properties of the turbulence such as kinetic energy spectra, turbulent transport, and anisotropy. In order to study these properties in highly inhomogeneous flames, new diagnostics will be required; spatial structure functions will be used to gain spectral information about the turbulence, and standard turbulence metrics will be extended to account for the inhomogeneity introduced by flames and realistic system geometries.

Relevant Publications:
• Towery, C.A.Z., Poludnenko, A.Y., Urzay, J., Ihme, M., and Hamlington, P.E. (2014) Spectral energy dynamics in premixed flames. Center for Turbulence Research Proceedings of the Summer Program 2014, pp. 159-168. (pdf)
• O’Brien, J., Urzay, J., Poludnenko, A.Y., Hamlington, P.E., and Ihme, M. (2014) Counter-gradient subgrid-scale transport and energy backscatter in turbulent deflagrations. Center for Turbulence Research Proceedings of the Summer Program 2014, pp. 147-157. (pdf)
• Hamlington, P.E., Poludnenko, A.Y., and Oran, E.S. (2012) Intermittency in premixed turbulent reacting flows. Physics of Fluids, Vol. 24, 075111. (pdf)
• Hamlington, P.E., Poludnenko, A.Y., and Oran, E.S. (2011) Interactions between turbulence and flames in premixed reacting flows. Physics of Fluids, Vol. 23, 125111. (pdf)
• Hamlington, P.E., Poludnenko, A.Y., and Oran, E.S. (2011) Intermittency and premixed turbulent reacting flows. AIAA Paper No. AIAA-2011-0113, 49th AIAA Aerospace Sciences Meeting, 4-7 January 2011, Orlando, FL. (pdf)
• Hamlington, P.E., Poludnenko, A.Y., and Oran, E.S. (2010) Turbulence and scalar gradient dynamics in premixed reacting flows. AIAA Paper No. AIAA-2010-5027, 40th AIAA Fluid Dynamics Conference, 28 June - 1 July 2010, Chicago, IL. (pdf)

Turbulent Flow Effects in Detonation Engines

TESLa Members: Colin Towery, Kat Smith, and Peter Hamlington
Collaborators: Marthinus Van Schoor

Propulsion systems based on detonation waves, such as rotating and pulsed detonation engines, have the potential to substantially improve the efficiency and power density of gas turbine engines. Numerous technical challenges remain to be solved in such systems, however, including obtaining more efficient injection and mixing of air and fuels, more reliable detonation initiation, and better understanding of the flow in the ejection nozzle. These challenges can be addressed using numerical simulations. Such simulations are enormously challenging, however, since accurate descriptions of highly unsteady turbulent flow fields are required in the presence of combustion, shock waves, fluid-structure interactions, and other complex physical processes. In this study, we perform high-fidelity three dimensional simulations of rotating detonation engines and examine turbulent flow effects on the operation, performance, and efficiency of the engines. Along with experimental data, these simulations are being used to test the accuracy of commonly-used Reynolds averaged and subgrid-scale turbulence models when applied to detonation engines.

Relevant Publications:
• Towery, C.A.Z., Smith, K., Van Schoor, M., and Hamlington, P.E. (2014) Analysis and Modeling of Turbulent Flow Effects in Rotating Detonation Engines. AIAA Paper No. AIAA-2014-3031, 44th AIAA Fluid Dynamics Conference, 16-20 June 2014, Atlanta, GA. (pdf)