A numerical model is developed to simulate the wrinkled laminar flame sheet flapping in weakly turbulent premixed combustion. The wrinkled laminar flame sheet is represented by a discrete distribution of volume sources called source disks. These source disks are utilized to produce the acceleration of combustion products behind the flame sheet. The laminar flame speed is allowed to vary according to flame stretch. A modified source model is proposed against the background of the existing source model's physically unrealistic symmetric expansion in both the upstream and the downstream directions. This flame model also includes flame-generated vorticity which is associated with the increasing entropy intrinsic to any system going through an irreversible process. The flame-generated vorticity is treated as discrete vortex disks. Vorticity created on the surface of the flame holder is computed with the vortex sheet method and diffuses into the surrounding flow in the form of vortex disks. The freestream turbulence is simulated by injecting vortex disks into an initially uniform freestream. Flame-flow interactions are studied when a thin circular cylinder, a large circular cylinder, and a flat plate normal to freestream are used as flame holders. Results sho\v that the modified source model gives more accurate prediction of flame angle than the existing source model does, the relative errors can be reduced by as much as four times. The modified source model also produces velocity profiles closer to those found in experiment, the deviations are cut by half at most sampling points in the flow. The vorticity shed from a thin circular cylinder flame stabilizer is found to only influence downstream regions very close to the cylinder. The eddy shedding behind a bluff body flame holder is suppressed in reacting flow simulations and the computed recirculating zone in a reacting flow is nearly half as long as that in a cold flow. When the relative size of the flame holder is one order of magnitude larger than the thickness of flame sheet, the vorticity shed by the flame holder can no longer be neglected. Flame wrinkling and flame extinction caused by vortical fluid motion behind the flame holder are found through numerical simulation. / Ph. D.

Investigations of the three-dimensional flow around a 6:1 prolate spheroid model 1.37 m long were conducted in the separation and near wake regions along the leeward side. Mean velocity flow field measurements, at α = 10∘ and 15∘ , and at Re = 1.3 x 10⁶ (U_re= 15.2 mls) and 4.0x 10⁶ (Ure=45.7 m/s), were obtained at four axial locations along the afterbody. Boundary layer profiles and Reynolds shear stress measurements were obtained at two axial locations, with a = 100 and Re=4.0 X 10⁶. Results of the flow field measurements indicate vortical flow along the surface of the body, growing in strength with increasing Reynolds number and increasing angle of attack. Skewing of the three-dimensional boundary layer is seen in the boundary layer profiles, with the surface shear stress direction lagging the local free-stream velocity direction. Growth of the boundary layer is evident circumferentially and axially along the body. Results of the turbulence measurements show that the distribution of Reynolds stress quantities is different from that of a two-dimensional flow over a flat plate, due to the three-dimensional flow and separation that is present. Estimates of x and z eddy viscosities show that the eddy viscosity is not isotropic. Estimates of the mixing length compared to values for a two-dimensional flow model indicate that the model predicts high values for the mixing length. Comparisons made with results obtained at DFVLR in West Germany show good agreement for the mean velocity and Reynolds normal stress values; however, the agreement of the Reynolds shear stresses is not as good. / Master of Science

Indiana University-Purdue University Indianapolis (IUPUI) / A constant-volume combustor is used to investigate the ignition initiated by a traversing jet of reactive hot gas, in support of combustion engine applications that include novel wave-rotor constant-volume combustion gas turbines and pre-chamber IC engines. The hot-jet ignition constant-volume combustor rig at the Combustion and Propulsion Research Laboratory at the Purdue School of Engineering and Technology at Indiana University-Purdue University Indianapolis (IUPUI) was used for this study. Lean premixed combustible mixture in a rectangular cuboid constant-volume combustor is ignited by a hot-jet traversing at different fixed speeds. The hot jet is issued via a converging nozzle from a cylindrical pre-chamber where partially combusted products of combustion are produced by spark- igniting a rich ethylene-air mixture. The main constant-volume combustor (CVC) chamber uses methane-air, hydrogen-methane-air and ethylene-air mixtures in the lean equivalence ratio range of 0.8 to 0.4. Ignition delay times and ignitability of these combustible mixtures as affected by jet traverse speed, equivalence ratio, and fuel type are investigated in this study.

Indiana University-Purdue University Indianapolis (IUPUI) / Ignition of a combustible mixture by a transient jet of hot reactive gas is important for safety of mines, pre-chamber ignition in IC engines, detonation initiation, and in novel constant-volume combustors. The present work is a numerical study of the hot-jet ignition process in a long constant-volume combustor (CVC) that represents a wave-rotor channel. The mixing of hot jet with cold mixture in the main chamber is first studied using non-reacting simulations. The stationary and traversing hot jets of combustion products from a pre-chamber is injected through a converging nozzle into the main CVC chamber containing a premixed fuel-air mixture. Combustion in a two-dimensional analogue of the CVC chamber is modeled using global reaction mechanisms, skeletal mechanisms, and detailed reaction mechanisms for four hydrocarbon fuels: methane, propane, ethylene, and hydrogen. The jet and ignition behavior are compared with high-speed video images from a prior experiment. Hybrid turbulent-kinetic schemes using some skeletal reaction mechanisms and detailed mechanisms are good predictors of the experimental data. Shock-flame interaction is seen to significantly increase the overall reaction rate due to baroclinic vorticity generation, flame area increase, stirring of non-uniform density regions, the resulting mixing, and shock compression. The less easily ignitable methane mixture is found to show higher ignition delay time compared to slower initial reaction and greater dependence on shock interaction than propane and ethylene. The confined jet is observed to behave initially as a wall jet and later as a wall-impinging jet. The jet evolution, vortex structure and mixing behavior are significantly different for traversing jets, stationary centered jets, and near-wall jets. Production of unstable intermediate species like C2H4 and CH3 appears to depend significantly on the initial jet location while relatively stable species like OH are less sensitive. Inclusion of minor radical species in the hot-jet is observed to reduce the ignition delay by 0.2 ms for methane mixture in the main chamber. Reaction pathways analysis shows that ignition delay and combustion progress process are entirely different for hybrid turbulent-kinetic scheme and kinetics-only scheme.