Premixed Flames in Intense Turbulence and/or under Compressible Conditions

Turbulent premixed flames exhibit different structure and propagation characteristics with increasing upstream turbulence intensity starting from thin wrinkled flames in the Corrugated Flamelet regimes to a thicker flame in the Thin Reaction Zone Regime (TRZ) and finally, becoming more disorganized or broken in the Broken or Distributed Reaction Zone (B/DRZ) regimes under intense turbulence. A single comprehensive predictive model that can span all the regimes does not currently exist, and in this study we explore the ability of the linear-eddy mixing (LEM) model to capture the physics in all these regimes. Past successful predictions in the flamelet and TRZ regimes are revisited with specific focus on new experiments in the TRZ regime, and new simulations in the B/DRZ regimes are performed to qualitatively compare results from recent DNS literature. Further studies that use LEM as a subgrid closure in LES is also being carried out to correlate the 1D predictions against full 3D LES studies. Extension of the modeling to study such flame structures in very intense compressible turbulence is also being investigated using canonical isotropic turbulent premixed flow.

Parallel Experimental studies are underway to simulate flame kernel propagation in channel flows. Experiments are focused on creating a well-defined channel flow with actively generated inflow turbulence in methane-air premixed mixtures. Detailed characterization of the turbulence is being carried out using both conventional (3-component LDV and CTA) and laser diagnostics (HTV, PIV). Flame kernels are ignited in homogeneous mixtures under various turbulence conditions in order to assess the growth and propagation characteristic. Simultaneous OH and CH2O PLIF techniques are being used to capture the structure and the propagation of the kernels. Intense turbulence in channels with mean Mach number ranging from Mach Number 0.1 to 1.25 are planned to cover a wide range of compressibility and flow conditions.

Personnel Involved:
Computational: Dr. R. Ranjan, A. Panchal, T. Gibis
Experimental: B. Ochs, D. Fries
Experimental (Vanderbilt University): Prof. R. Pitz

Sponsor: AFOSR

Images:

  1. brush_front and brush_rear
  2. Turbulent flame brush structure for a high Karlovitz number (Ka_L = 920) flame, identified using iso-surfaces of c = 0.01 and c = 0.99. The two figures correspond to view from reactants and products side. The purple lines identify the region of high reaction rate. Symbols A and B show a representative pocket of cold reactants penetrating towards reaction zone and protruding structures, respectively.

  3. co_frac
  4. Inner flame structure on the central plane identified using mass fraction of CO in low (top) and high (bottom) Karlovitz number flames.