Theses and Dissertations from UMD
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Item Investigation of Swirl Distributed Combustion with Experimental Diagnostics and Artificial Intelligence Approach(2022) Roy, Rishi; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Swirl Distributed Combustion was fundamentally investigated with experimental diagnostics and predictive analysis using machine learning and computer vision techniques. Ultra-low pollutants emission, stable operation, improved pattern factor, and fuel flexibility make distributed combustion an attractive technology for potential applications in high-intensity stationary gas turbines. Proper mixing of inlet fresh air and hot products for creating a hot and low-oxygen environment is critical to foster distributed combustion, followed by rapid mixing with the fuel. Such conditions result in a distributed thick reaction zone without hotspots found in (thin reaction front) of conventional diffusion flames leading to reduced NOx and CO emissions. The focus of this dissertation is to develop a detailed fundamental understanding of distributed combustion in a lab-based swirl combustor (to mimic gas turbine can combustor) at moderate heat release intensities in the range 5.72- 9.53 MW/m3-atm using various low-carbon gaseous fuels such as methane, propane, hydrogen-enriched fuels. The study of distributed combustion at moderate thermal intensity helped to understand the fundamental aspects such as reduction of flame fluctuation, mitigation of thermo-acoustic instability, flame shape evolution, flow field behavior, turbulence characteristics, variation of Damkӧhler number, vortex propagation, flame blowoff, and pollutant and CO2 emission reduction with gradual mixture preparation. Initial efforts were made to obtain the volumetric distribution ratio, evolution of flame shape in terms of OH* radical imaging, variation of flame standoff, thermal field uniformity, and NO and CO emissions when the flame transitions to distributed reaction zone. Further investigation was performed to study the mitigation of flame thermo-acoustics and precession vortex core (PVC) instabilities in swirl distributed combustion compared to swirl air combustion using the acoustic pressure and qualitative heat release fluctuation data at different dilution CO2 dilution levels with and without air preheats. Proper orthogonal decomposition (POD) technique was utilized to visualize the appearance of dynamic coherent structures in reactive flow fields and reduction of fluctuation energy. Vortex shedding was found responsible for the fluctuation in swirl air combustion while no significant flame fluctuation was observed in distributed combustion. Distributed combustion showed significantly reduced acoustic noise and much higher stability quantified by local and global Rayleigh index. This study was extended with hydrogen-enriched methane (vol. = 0, 10, 20, 40% H2) to compare the stability of the flow field in conventional air combustion and distributed combustion. Results were consistent and distributed reaction zones showed higher flame stability compared to conventional swirl air combustion. The study of lean blowoff in distributed combustion showed a higher lean blowoff equivalence ratio with gradual increase in heat release intensity, which was attributed to higher flow field instability due to enhanced inlet turbulence. Extension of lean blowoff (ϕLBO) was observed with gradual %H2 which showed decrease of lean blowoff equivalence ratio in distributed reaction zones. Additionally, the reduction in ϕLBO was achieved by adding preheats to the inlet airstream for different H2 enrichment cases due to enhanced flame stability gained from preheating. Examination of non-reactive flow field with particle image velocimetry (PIV) was performed to understand the fundamental differences between swirl flow and distributed reaction flow at constant heat release intensities. Higher rms fluctuation leading to healthy turbulence and higher Reynolds stress were found in distributed reaction flow cases signifying enhanced mixing characteristics in distributed combustion. Reduction of pollutant emission was an important focus of this research. Measurement of NO and CO emission at different mixture preparation levels exhibited significant reduction in NO emission (single digit) compared to swirl air combustion due to mitigation of spatial hotspots and temperature peaks. Additionally, better mixing and uniform stoichiometry supported reduced CO emissions in distributed combustion for every fuel. With increased H2 in the fuel, NO gradually increased for air combustion while reduction of NO was found in distributed combustion due to decrease in thermal and prompt NO generation. Finally, the use of machine learning and computer vision techniques was investigated for software-based prediction of combustion parameters (pollutants and flame temperature) and feature-based recognition of distributed combustion regimes. The primary goal of using artificial intelligence is to reduce the time of experimentation and frequent manual interference during experiments in order to enhance the overall accuracy by reducing human errors. Such predictions will help in developing data-driven smart-sensing of combustion parameters and reduce the dependence on experimental trials.Item Slot Film Cooling: A Comprehensive Experimental Characterization(2016) Raffan Montoya, Fernando; Marshall, Andre W; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)When components of a propulsion system are exposed to elevated flow temperatures there is a risk for catastrophic failure if the components are not properly protected from the thermal loads. Among several strategies, slot film cooling is one of the most commonly used, yet poorly understood active cooling techniques. Tangential injection of a relatively cool fluid layer protects the surface(s) in question, but the turbulent mixing between the hot mainstream and cooler film along with the presence of the wall presents an inherently complex problem where kinematics, thermal transport and multimodal heat transfer are coupled. Furthermore, new propulsion designs rely heavily on CFD analysis to verify their viability. These CFD models require validation of their results, and the current literature does not provide a comprehensive data set for film cooling that meets all the demands for proper validation, namely a comprehensive (kinematic, thermal and boundary condition data) data set obtained over a wide range of conditions. This body of work aims at solving the fundamental issue of validation by providing high quality comprehensive film cooling data (kinematics, thermal mixing, heat transfer). 3 distinct velocity ratios (VR=uc/u∞) are examined corresponding to wall-wake (VR~0.5), min-shear (VR ~ 1.0), and wall-jet (VR~2.0) type flows at injection, while the temperature ratio TR= T∞/Tc is approximately 1.5 for all cases. Turbulence intensities at injection are 2-4% for the mainstream (urms/u∞, vrms/u∞,), and on the order of 8-10% for the coolant (urms/uc, vrms/uc,). A special emphasis is placed on inlet characterization, since inlet data in the literature is often incomplete or is of relatively low quality for CFD development. The data reveals that min-shear injection provides the best performance, followed by the wall-jet. The wall-wake case is comparably poor in performance. The comprehensive data suggests that this relative performance is due to the mixing strength of each case, as well as the location of regions of strong mixing with respect to the wall. Kinematic and thermal data show that strong mixing occurs in the wall-jet away from the wall (y/s>1), while strong mixing in the wall-wake occurs much closer to the wall (y/s<1). Min-shear cases exhibit noticeably weaker mixing confined to about y/s=1. Additionally to these general observations, the experimental data obtained in this work is analyzed to reveal scaling laws for the inlets, near-wall scaling, detecting and characterizing coherent structures in the flow as well as to provide data reduction strategies for comparison to CFD models (RANS and LES).Item AN EXPERIMENTAL INVESTIGATION ON SOLUTE NATURAL CONVECTION IN A VERTICAL HELE-SHAW CELL(2014) Ehyaei, Dana; Kiger, Ken T; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental analogue was developed to investigate instability propagation of a multicomponent fluid system in porous media. This type of flow pattern has been observed in a broad range of applications from oil enhanced recovery to geological storage of byproduct materials such as CO2. The main focus of this study is on the engineering instrumentation and implementation of experimental measurement techniques in microfluidic systems, more specifically in a thin-gap device that is used as a model for a saturated porous medium. Initially, quantitative in-plane velocity measurement by means of particle image velocimetry (PIV) within thin gap devices subject to a large depth-of-focus and Poiseuille flow conditions is studied extensively. The temporal velocity measurement is then coupled with a simultaneous concentration measurement by means of LED induced fluorescence (LIF). The primary obstacles to a reliable quantitative PIV measurement are due to the effects of the inherent wall-normal velocity gradient and the inertial migration of particles in the wall-normal direction. After quantification of both effects, a novel measurement technique is proposed to make quantitative velocity measurement in microfluidic systems and narrow devices by manipulating the particles to their equilibrium position through inertial induced migration. This single camera technique is significantly simpler and cheaper to apply comparing to the existing multi-camera systems as well as micro-PIV implementations, which are restricted to a small field-of-view. A demonstration of a reliable PIV measurement under appropriate parameter design is then discussed for diffusive Rayleigh-Bénard convection in a Hele Shaw cell. For concentration measurements, the main difficulty of making LIF quantitative is its highly sensitive response to the experimental settings due to extreme sensitivity of the fluorescence to the environment factors and illumination conditions. A calibration procedure is required prior to performing any meaningful quantitative measurements. Additionally, the effect of photobleaching can be significant, which impairs the measurement as will be discussed later in further detail. Eventually after calibration and correction methods for velocity and concentration measurement techniques, a simultaneous PIV/LIF is performed to quantify the behavior of instability fingers in the developed experimental system.Item EFFECTS OF BLADE TIP SHAPE ON ROTOR IN-GROUND-EFFECT AERODYNAMICS(2011) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-speed videographic flow visualization and detailed two-component particle image velocimetry (PIV) measurements were conducted to examine the wake produced by a hovering single-bladed rotor as it interacted with a horizontal ground plane. The mo- tivation of the work was to better understand the nature of the flow field at the ground and the possible aerodynamic mechanisms that create brownout dust clouds when rotorcraft take off and land over surfaces covered with loose sediment. Rotors with four different blade tips were tested: 1. A baseline rectangular tip, 2. A simple 20◦ swept tip, 3. A BERP-like tip, and 4. A slotted tip. Flow visualization was performed using a high- repetition rate Nd:YLF laser that illuminated appropriately seeded flows in radial planes, with imaging performed using a high-speed CMOS camera. PIV measurements were performed in regions near the blades and at the ground plane by using a Nd:YAG laser with a CCD camera. Measurements as functions of wake age were obtained to examine the morphology of the vortical rotor wake during its interaction with the ground. The results showed that the wake was subjected to powerful curvature and straining effects as it interacted with the ground plane and was deflected into a radially outward direction along the plane. Reintensification of the tip vortices during the interaction caused them to remain very distinct features in the flow near the ground to as old as six or more ro- tor revolutions. The unsteady outward flow over the ground plane was shown to have similarities to a classical turbulent wall jet, especially further away from the rotor. Flow measurements were obtained deep into the boundary layer region at the ground, and in some cases into the laminar sublayer. The results showed certain common flow features between the four blade tips, but also differences in the flows that may ultimately affect the problem of brownout. The slotted-tip was shown to be particularly effective in diffusing the tip vortices and reducing the overall intensity of the fluctuating aspects of the flow at the ground.Item Flow Field and Performance Measurements of a Flapping-Wing Device Using Particle Image Velocimetry(2011) Ramsey, Joseph Patrick; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A flexible flapping wing was tested using various flow interrogation techniques including particle image velocimetry (PIV) to further the understanding of its complex, unsteady, three-dimensional flow field. The flow field was characterized using high-speed flow visualization (FV) in chordwise and spanwise planes to observe and characterize the evolution of flow structures produced. The formation, growth, convection, and shedding of a leading-edge vortex (LEV) was observed on the upper surface of the wing, which was found to mimic the classical process of dynamic stall. A motion-tracking system was used to characterize the complex wing kinematics and aeroelastic deformations of the flexible wing. These measurements were then used to estimate the noncirculatory forces and moments acting on the wing. Two-dimensional velocity fields around the wing contour and in its wake were obtained using PIV. These velocity fields were used to calculate the circulatory lift as well as the drag produced on the wing. It was found that the process of LEV formation, growth, and convection significantly increased the lift production on the flapping wing. The noncirculatory and circulatory lift measurements were then combined in amplitude and phase to calculate the total lift on the wing. It was shown that the noncirculatory contributions to the airloads were small except near pronation and supination. The flow field results were also used to calculate the lift-to-drag ratio during the wing stroke, where surprisingly it was found that the lift-to-drag ratio increased during the process of LEV formation and shedding. This observation perhaps suggests a reason why flapping-wing flyers intentionally produce LEVs during their wing stroke.Item Experimental Characterization of Slot Film Cooling Flows With Minimally Intrusive Diagnostics(2008) Raffan, Fernando; Marshall, Andre; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The performance of a propulsion system is directly proportional to its operating temperature; therefore, an optimum operation regime will undoubtedly generate intense thermal loads on system components. If the system is designed for reusability and/or long range missions, it may be necessary to perform active cooling of critical components to prevent premature failure of the system. One such method is film cooling, in which a layer of relatively cool gas is injected near the surface to be protected. This work describes the use of minimally intrusive diagnostics to characterize the kinematics, thermal dynamics and heat transfer of slot film cooling flows over a wide range of blowing ratios, generating a comprehensive database for detailed analysis, as well as for further use by model developers.Item Spray Flame and Exhaust Jet Characteristics of a Pressurized Swirl Combustor(2006-05-17) Linck, Martin Brendan; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work describes an investigation of swirl-stabilized flames, created in a combustor featuring co-annular swirling airflows, under unenclosed, enclosed, and submerged conditions. A centrally-located fuel nozzle, which uses air-assist atomization, creates a methanol fuel spray. This approach provides great control over fuel spray properties in a compact geometry. Factors affecting the structure of the flames, including the effect of the central atomization air jet, are investigated using three-dimensional particle image velocimetry, direct imaging, and phase-Doppler particle analysis techniques. Exhaust jet temperatures are measured. The dynamic events affecting two-phase exhaust jets from the combustor under submerged conditions are examined using high-speed cinematography and sound spectrum analysis. It is found that the structures of the flames examined, which feature low overall equivalence ratios, are closely linked to the features of the air flowfield in the combustor. Swirl numbers of flows emerging from twisted-vane swirl assemblies are characterized. The structure of the flow is affected by the swirl configuration, but does not depend heavily on the Reynolds number. The central atomization air jet (with or without fuel) reshapes the recirculation region in the swirling flow and has a significant, controllable effect on the structure of the airflow and flame. The effect is the same for nonreacting and reacting flows. In one unique case, the central atomization air interacts with the swirling flow to create two recirculation regions and a lifted flame. The lifted flame is more compact than similar non-lifted flames. The twin-fluid atomization approach is shown to provide effective atomization over a wide range of operating conditions. The two-phase interaction of the exhaust jet is found to depend on the pressure drop over the exhaust nozzle. The dynamic behavior of the exhaust jet is buoyancy-driven at low pressure drops, and is affected by complex instability mechanisms at high pressure drops. Strouhal numbers of large-scale unstable events occurring in the two-phase flow are two orders of magnitude smaller than those associated with instabilities in single-phase flows. Evidence is presented, indicating that acoustic pressure waves in the exhaust jet may be involved in the generation of bubbles surrounding exhaust jets at high pressure drops.