Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item SIMULATION AND MODELING OF AN ACOUSTICALLY FORCED MODEL ROCKET INJECTOR(2010) Gers, David; Yu, Ken; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A numerical and experimental study was performed to assess the capability of the Loci-CHEM CFD solver in simulating dynamic interaction between hydrogen-oxygen turbulent diffusion flames and periodic pressure waves. Previous experimental studies involving a single-element shear-coaxial model injector revealed an unusual flame-acoustic interaction mechanism affecting combustion instability characteristics. To directly compare the simulation and experiments, various models in the present solver were examined and additional experiments conducted. A customized mesh and corresponding boundary conditions were designed and developed, closely approximating the experimental setup. Full 3-D simulations were conducted using a hybrid RANS/LES framework with appropriate chemistry and turbulence models. The results were compared for both reacting and non-reacting flows that were excited at various forcing frequencies representing both resonant and non-resonant behaviors. Although a good qualitative agreement was obtained for the most part, there was a significant discrepancy in simulating the flame-acoustic interaction behavior observed under non-resonant forcing conditions.Item The Role of Density Gradient in Liquid Rocket Engine Combustion Instability(2008-12-01) Ghosh, Amardip; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experimental and analytical studies were conducted to investigate key physical mechanisms responsible for flame-acoustic coupling during the onset of acoustically driven combustion instabilities in liquid rocket engines (LREs). Controlled experiments were conducted in which a turbulent hydrogen-oxygen (GH2-GO2) diffusion flame, established downstream of a two-dimensional model shear coaxial injector was acoustically forced by a compression driver unit mounted in a transverse direction and excited through a broad range of frequencies (200Hz-2000Hz) and amplitudes. Characteristic interactions between flame and acoustics visualized through OH* and CH* chemiluminescence imaging and dynamic pressure measurements obtained using high frequency dynamic pressure transducers indicated that small acoustic disturbances could be amplified by flame-acoustic coupling under certain conditions leading to substantial modulation in spatial heat release fluctuations. Density gradient between fuel and oxidizer was found to significantly affect the way acoustic waves interacted with density stratified flame fronts. The particular case of an asymmetric flame front oscillation under transverse acoustic forcing indicated that baroclinic vorticity, generated by the interactions between misaligned pressure gradient (across the acoustic wave) and density gradient (across the fuel oxidizer interface) could further amplify flame front distortions. Asymmetric interaction between flame and acoustics is shown to occur preferentially on flame fronts where controlled waves from the compression driver travel from lighter fluid to denser fluid and the amount of interaction between flame and acoustics is shown to depend strongly on the density ratio between the fluids on either sides of the flame front. This observation is in agreement with the baroclinic vorticity mechanism and a variant of the classical Rayleigh-Taylor instability mechanism. The results provide the first known experimental evidence that baroclinic vorticity could play a role in triggering flame-acoustic interactions associated with LRE shear coaxial injectors. Parametric studies investigating the sensitivity of flame-acoustic interaction on key physical parameters that govern shear coaxial injector operations (including density ratio, velocity ratio, momentum ratio and chemical composition of the fuel) were conducted by varying the parameter of interest independently while holding the other parameters relatively constant. Density ratios ranging from 1 to 16, velocity ratios ranging from 3.02 to 5.27, momentum ratios ranging from 0.67 to 2.12 and fuel mixtures ranging from pure hydrogen to 10%-90% GH2-GCH4 combination were tested. It is shown that in the ranges considered, flame-acoustic interaction is most sensitively affected by density ratio changes. Spectral measurements of flame front oscillations using local chemiluminescence measurements further revealed the non-linear nature of the interaction process : a flame system forced at 1150 Hz gave rise not only to 1150 Hz oscillations but also triggered flame oscillations occurring at substantially lower frequencies. Analytical models were developed to interpret and predict acoustic modes of a combustion chamber containing a density stratified flowfield subjected to transverse acoustic disturbances. Incorporating both the known phenomenon of jet mixing length and the new experimental result of preferential excitation, the models allow different resonant behaviors to occur for separate regions of the combustor bounded by sudden changes in density. For isothermal experiments where the flow temperature was known, calculated Eigen frequencies were in good agreement with measured frequencies. Overall, the identification of fuel-oxidizer density ratio as a critical parameter and the identification of baroclinic vorticity as a potential mechanism in flame acoustic coupling are significant because a reduction in the density gradient between fuel and oxidizer can be used as a control mechanism to improve flame stability in liquid rocket engines.Item Active Suppression of Vortex-Driven Combustion Instability Using Controlled Liquid-Fuel Injection(2005-09-09) Pang, Bin; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combustion instabilities remain one of the most challenging problems encountered in developing propulsion and power systems. Large amplitude pressure oscillations, driven by unsteady heat release, can produce numerous detrimental effects. Most previous active control studies utilized gaseous fuels to suppress combustion instabilities. However, using liquid fuel to suppress combustion instabilities is more realistic for propulsion applications. Active instability suppression in vortex-driven combustors using a direct liquid fuel injection strategy was theoretically established and experimentally demonstrated in this dissertation work. Droplet size measurements revealed that with pulsed fuel injection management, fuel droplet size could be modulated periodically. Consequently, desired heat release fluctuation could be created. If this oscillatory heat release is coupled with the natural pressure oscillation in an out of phase manner, combustion instabilities can be suppressed. To identify proper locations of supplying additional liquid fuel for the purpose of achieving control, the natural heat release pattern in a vortex-driven combustor was characterized in this study. It was found that at high Damköhler number oscillatory heat release pattern closely followed the evolving vortex front. However, when Damköhler number became close to unity, heat release fluctuation wave no longer coincided with the coherent structures. A heat release deficit area was found near the dump plane when combustor was operated in lean premixed conditions. Active combustion instability suppression experiments were performed in a dump combustor using a controlled liquid fuel injection strategy. High-speed Schlieren results illustrated that vortex shedding plays an important role in maintaining self-sustained combustion instabilities. Complete combustion instability control requires total suppression of these large-scale coherent structures. The sound pressure level at the excited dominant frequency was reduced by more than 20 dB with controlled liquid fuel injection method. Scaling issues were also investigated in this dump combustor to test the effectiveness of using pulsed liquid fuel injection strategies to suppress instabilities at higher power output conditions. With the liquid fuel injection control method, it was possible to suppress strong instabilities with initial amplitude of 5 psi down to the background noise level. The stable combustor operating range was also expanded from equivalence ratio of 0.75 to beyond 0.9.