Large Eddy Simulation of High-Speed, Premixed Ethylene Combustion (Invited)

Abstract

A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) methodology is used to simulate premixed ethylene-air combustion in a model scramjet designed for dual mode operation and equipped with a cavity for flameholding. A 22-species reduced mechanism for ethylene-air combustion is employed, and the calculations are performed on a mesh containing 93 million cells.Fuel plumes injected at the isolator entrance are processed by the isolator shock train, yielding a premixed fuel-air mixture at an equivalence ratio of 0.42 at the cavity entrance plane. A premixed flame is anchored within the cavity and propagates toward the opposite wall. Near complete combustion of ethylene is obtained. The combustor is highly dynamic, exhibiting a large-scale oscillation in global heat release and mass flow rate with a period of about 2.8 ms. Maximum heat release occurs when the flame front reaches its most downstream extent, as the flame surface area is larger. Minimum heat release is associated with flame propagation toward the cavity and occurs through a reduction in core flow velocity that is correlated with an upstream movement of the shock train.Reasonable agreement between simulation results and available wall pressure, particle image velocimetry, and OH-PLIF data is obtained, but it is not yet clear whether the system-level oscillations seen in the calculations are actually present in the experiment.

OH PLIF Visualization of a Premixed Ethylene-fueled Dual-Mode Scramjet Combustor

“…Absorption of the laser sheet (as happened with the NO PLIF experiment) may perhaps have attenuated the laser intensity as it penetrated into the flow resulting in an asymmetric image; however, this optical phenomenon does not affect the CARS signal since information is determined from shape of spectrum not signal intensity, hence the asymmetry is real. This asymmetry is consistent with CFD calculation 33,34 of the nozzle, inlet isolator and combustor with non-uniform-temperature inflow conditions based on CARS measurements; these calculations showed similar non-uniformities in predicted OH distribution to those that were measured.…”

“…This representation suggests a repeatable flame oscillation. LES/RANS simulations 34 predicted thermo-acoustic oscillations that created similar flame motions, as shown in Figure 15. The frequency of the oscillation was calculated at about 350 Hz in the CFD.…”

“…These data illustrate the streamwise flame formations and enable the calculation of a flame angle. Figure 13 represents a selection of OH PLIF selected single shots (top) of the base case (Run 4) compared with the corresponding OH LES/RANS CDF simulation performed by Ramesh et al 34 The CFD image color scale was modified to match the PLIF images. As previously stated, it was not possible to record one single image to cover the entire combustor length, so the complete stream-wise view was obtained by combining 3 images which were recorded at different times.…”

High-Spatial-Resolution OH PLIF Visualization in a Cavity-Stabilized Ethylene-Air Turbulent Flame

“…11 Previous computational and particle image velocimetry investigations of this flowpath estimated the = 5 mm, root-mean square velocity = 50 m/s, and kinematic viscosity = 4 to 5 m 2 /s. 12 The Kolmogorov length scale can be estimated 13 using Eq. (1) at 7 to 8 µm.…”

Modeling and Analysis of Different Aspects of Finite-rate Chemical Kinetics in Hypersonic Combustors

“…For example recent LES/RANS supersonic reacting flow simulation by Edwards and coworkers [109,110] have used a reduced ethylene oxidation chemistry model recently developed by Tianfeng Lu and coworkers [112]. This reduced model is based on the University of Southern California detailed mechanism (USCMechII [70]).…”