Aerospace Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2737
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Item FUNDAMENTAL STUDY OF INJECTION, MIXING AND STABILITY IN MODEL ROTATING DETONATION ENGINES(2022) Redhal , Shikha Chaudhry; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In light of the growing demand for more efficient aviation engines, detonation-based engines are being investigated as possible replacements to traditional rockets and jet engines. Rotating Detonation Engine (RDE) is one of the novel engine concepts, gaining much interest from the aero propulsion community, including both industry and academia. RDE is a continuous-detonation engine, which consists of an annular chamber where the reactants are injected axially while the detonation wave propagates along the chamber in an orthogonal direction to the flow axis. Potential advantages of RDEs include greater thermal efficiency, improved fuel economy, simpler design, reduced weight, better scalability, and possible exempt from combustion instability concerns. One of the purposes of this research is to better understand the nature of RDE propulsion concepts and pertinent interaction between various physical processes. The main focus is on the effect of injection and mixing on the detonation wave propagation and heat release inside RDE combustors. Complex interaction between detonation waves and injector flow-fields is investigated for various injector geometries and flow compositions. The effects of injection and mixing are investigated for two different types of injector geometry, including unlike impinging doublet injectors and recessed partially-premixed jet injectors. Counter propagating waves are observed in the detonation tunnel as well as in other RDE tests. Based on the present results, the onset of the counter-propagating waves can be attributed to the reignition of the unburned reactants trapped in the wake of the wave. By visualizing the injector internal flowfield, it was also shown that detonation wave propagates into the interior of the recessed injectors. This finding is important for properly predicting the refresh injection timing. Also, the RDE stability and mode selection phenomenon are investigated using Rayleigh criterion to provide physical explanations based on acoustic energy for sustenance of periodic motion. The experimental data for this analysis is acquired using simultaneous sampling of transient chemiluminescence and local pressure measurements in the extended linear model detonation engine. The results show that, in the case of decoupled detonation wave, there is a distinct time delay between the pressure peak and the peak in the chemiluminescence signal. The ensuing Rayleigh index analysis can be used to explain and predict RDE mode selection processes. The results provide both qualitative and quantitative insights on the effect of RDE geometry as well as on the detonation wave stability. They provide better understanding of the complex interaction between RDE combustor processes.Item NUMERICAL SIMULATION OF THE BLUE WHIRL: A REACTING VORTEX BREAKDOWN PHENOMENON(2019) Chung, Joseph Dong il; Oran, Elaine S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a new potential way for low-emission combustion. This thesis uses numerical simulations to present, for the first time, the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames - a diffusion flame and a premixed rich and lean flame - all of which meet in a fourth structure, a triple flame which appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. This thesis also presents the development and testing of the numerical algorithms used in the simulation of the blue whirl. This work is a critical step forward in understanding how to use this new form of clean combustion.Item Combustion Instability and Active Control: Alternative Fuels, Augmentors, and Modeling Heat Release(2016) Park, Sammy; 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 explore thermo-acoustic coupling during the onset of combustion instability in various air-breathing combustor configurations. These include a laboratory-scale 200-kW dump combustor and a 100-kW augmentor featuring a v-gutter flame holder. They were used to simulate main combustion chambers and afterburners in aero engines, respectively. The three primary themes of this work includes: 1) modeling heat release fluctuations for stability analysis, 2) conducting active combustion control with alternative fuels, and 3) demonstrating practical active control for augmentor instability suppression. The phenomenon of combustion instabilities remains an unsolved problem in propulsion engines, mainly because of the difficulty in predicting the fluctuating component of heat release without extensive testing. A hybrid model was developed to describe both the temporal and spatial variations in dynamic heat release, using a separation of variables approach that requires only a limited amount of experimental data. The use of sinusoidal basis functions further reduced the amount of data required. When the mean heat release behavior is known, the only experimental data needed for detailed stability analysis is one instantaneous picture of heat release at the peak pressure phase. This model was successfully tested in the dump combustor experiments, reproducing the correct sign of the overall Rayleigh index as well as the remarkably accurate spatial distribution pattern of fluctuating heat release. Active combustion control was explored for fuel-flexible combustor operation using twelve different jet fuels including bio-synthetic and Fischer-Tropsch types. Analysis done using an actuated spray combustion model revealed that the combustion response times of these fuels were similar. Combined with experimental spray characterizations, this suggested that controller performance should remain effective with various alternative fuels. Active control experiments validated this analysis while demonstrating 50-70\% reduction in the peak spectral amplitude. A new model augmentor was built and tested for combustion dynamics using schlieren and chemiluminescence techniques. Novel active control techniques including pulsed air injection were implemented and the results were compared with the pulsed fuel injection approach. The pulsed injection of secondary air worked just as effectively for suppressing the augmentor instability, setting up the possibility of more efficient actuation strategy.Item Effect of Fin-Guided Fuel Injection on Supersonic Mixing and Combustion(2014) Aguilera Munoz, Camilo; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rapid mixing and combustion is a key challenge in supersonic combustors due to the extremely short flow residence time and the effect of compressibility. Mixing enhancement is therefore desirable to ensure timely mixing, reaction, and heat release. Fin-guided fuel injection is one approach that can be optimized for propulsion performance consideration. The present investigation examined the mixing and combustion characteristics of using this alternative fuel injection method to evaluate its performance in comparison to conventional transverse wall injection. This study was conducted in two parts: (1) fuel-air mixing experiments in a non-reacting Mach 2.2 flow with a test section Reynolds number of 1.15×106, and (2) combustion experiments using a high-enthalpy, vitiated air flow with a Mach 2.0 condition at the isolator inlet and Reynolds number of 1.14× 105. The non-reacting mixing study used either helium or ethanol, while the combustion study used either hydrogen or ethylene as fuel for each experiment. The mixing behavior of the gaseous and liquid jets was studied using schlieren and a laser sheet technique while quantitative assessments were made from pressure measurements. Similarly, the physical mechanisms in the reacting flow experiments were analyzed using schlieren visualizations while pressure measurements and chemiluminescence emission data were used for performance evaluation. The fuel-air mixing study highlighted possible tradeoffs between mixing enhancement and the stagnation pressure loss stemming from fuel jet-induced shocks. Since the fin was designed to weaken the oblique shock strength while shielding the fuel jet penetrating into the core airflow, it not only resulted in better mixing but also improved the pressure recovery. For gaseous fuel, fin-guided injection improved jet penetration by 100 to 200% for a momentum ratio between 0.15 and 0.03. It also resulted in 64 to 85% additional pressure recovery of the injection shock loss. Combustion experiments revealed that the fin could be used to extend the upper limit of supersonic combustion mode in the present configuration, from an equivalence ratio of 0.04 to 0.12, by preventing thermal choking caused by concentrated heat release near the baseline flame holder. This could be advantageous for certain systems by reducing the thermal protection requirements. However, the fin also made the wall cavity flame holder less effective by increasing fuel penetration away from the bottom wall. The net effects on propulsion system performance will ultimately depend on whether ramjet or scramjet mode is preferred for a given operation.Item Modeling of a High Energy Density Propulsion System Based on the Combustion of Aluminum and Steam(2007-12-13) Eagle, W. Ethan; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a thermodynamic analysis of a novel Rankine cycle aluminum/steam combustion power system being developed for use in Unmanned Underwater Vehicles (UUVs). The analysis is performed using a system modeling tool developed by the NASA Glenn Research Center called Numerical Propulsion System Solver (NPSS). Thermodynamic models of the individual components are created and linked together in NPSS, which then solves the system by enforcing mass and energy conservation. Design and off-design conditions are simulated and predicted performance is compared with predictions made by two other research groups. The simulations predict that this power system could provide at least five-fold increases in range and endurance for the US Navy's 'Sea Horse' UUV. A rudimentary sensitivity analysis is used to identify the factors which most strongly influence the performance of the design. Lastly, recommendations for future work and possible model improvements are discussed.