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Browsing by Author "Meadows, Joseph"

Now showing 1 - 20 of 25
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    Computational Methods for Optimizing Rotating Detonation Combustor (RDC) to Integrate with Gas Turbine
    Raj, Piyush (Virginia Tech, 2024-07-05)
    Pressure Gain Combustion (PGC) systems have gained significant focus in recent years due to its potential for increased thermodynamic efficiency over a constant pressure cycle (or Brayton cycle). A rotating detonation combustor (RDC) is a type of PGC system, which is thermodynamically more efficient than the conventional gas turbine combustor. One of the main aspects of the detonation process is the rapid burning of the fuel-oxidizer mixture, due to which there is not enough time for the pressure to equilibrate. Therefore, the process is thermodynamically closer to a constant volume process, which is thermodynamically more efficient than a constant pressure cycle. RDC, if integrated successfully with a turbine, can increase thermal efficiency and reduce carbon emissions, especially when hydrogen is introduced into the fuel stream. However, due to highly unsteady flow generated from RDC, a systematic approach to transition the flow exiting the RDC to supply steady, subsonic flow at the turbine inlet has not been developed so far. Numerical simulations serve as a valuable tool to provide insight into the flow physics and to optimize the RDE design. Numerical studies have explored RDC by utilizing high-fidelity 3D simulations. However, these CFD studies require significant computational resources, due to the large differences in length and time scales between the flow field and the chemical reactions involved. The motivation of this dissertation is to investigate these research gaps and to develop computationally efficient methods for RDC designs to be integrated with downstream turbine section. First, this research work develops a methodology to predict the unsteady flow field exiting an RDC using 2D reacting simulations and to validate the approach using experimental measurements. Next, computational techniques are applied to condition the flow within the annulus by strategically constricting the flow area. A design of experiment (DoE) study is used to optimize the area profiling of the combustor. Additionally, the performance of the profiled design is compared against the baseline and the conventional nozzle design used in the literature. However, these numerical works use a perfectly premixed condition, whereas, the actual setup consists of discrete fuel/oxidizer injectors providing a non-uniform mixture in the combustor. To eliminate the assumption of perfectly premixed conditions, a method is developed to model the dynamic injector response of fuel/oxidizer plenums. The goal of this approach is to provide an inhomogeneous mixture composition without having to resolve/mesh the individual injectors. This research work provides a robust and computationally efficient methods for minimizing unsteadiness, maximizing pressure gain, and modeling dynamic injector response of an RDC.
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    Design and Characterization of a Coaxial Plasma Railgun for Jet Collision Experiments
    Coleman, Mathew Riley (Virginia Tech, 2021-03-17)
    Plasma railguns are electromagnetic accelerators used to produce controlled high velocity plasma jets. This thesis discusses the design and characterization of a small coaxial plasma railgun intended to accelerate argon-helium plasma jets. The railgun will be used for the study of plasma shocks in jet collisions. The railgun is mounted on a KF-40 vacuum port and operated using a 90 kA, 11 kV LC pulse forming network. Existing knowledge of coaxial railgun plasma instabilities and material interactions at vacuum and plasma interfaces are applied to the design. The design of individual gun components is detailed. Jet velocity and density are characterized by analyzing diagnostic data collected from a Rogowski coil, interferometer, and photodiode. Peak line-integrated electron number densities of approximately 8 × 1015 cm-2 and jet velocities of tens of km/s are inferred from the data recorded from ten experimental pulses.
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    Development of Color Ratio Thin Filament Pyrometry Approach for Applications in High Speed Flames
    Hagmann, Kai Alexander (Virginia Tech, 2023-07-07)
    Thin filament pyrometry is a proven technique used to measure flame temperature by capturing the spectral radiance produced by the immersion of silicon carbide filaments in a hot gas environment. In this study a commercially available CMOS color camera was used, and the spectral response of each color channel was integrated with respect to the assumed graybody radiation spectrum to form a look up table between color ratio and temperature. Interpolated filament temperatures are then corrected for radiation losses via an energy balance to determine the flame temperature. Verification of the technique was performed on the Holthuis and Associates Flat Flame Burner, formerly known as the Mckenna Burner, and the results are directly compared to literature values measured on a similar burner. The results are also supported by radiation corrected measurements taken using a type B thermocouple on the same burner setup. An error propagation analysis was performed to determine which factors contribute the most to the final measurement uncertainty and confidence intervals are calculated for the results. Uncertainty values for a single point measurement were determined to be between ±15 and ±50 K depending on the color ratio and the total uncertainty associated with day-to-day changes in the measurement setup was found to be ±55 K.
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    Dynamics of Lean Direct Injection Combustors
    Aradhey, Yogesh Sachin (Virginia Tech, 2023-11-10)
    Improvements to heritage gas turbine engines will be needed in the coming years as the demand made on these systems increase. While industry continually presses for higher performance of both military and civilian aero engines, the government simultaneously raises the bar for emissions standards in the commercial sector to support public health. The next generation of aerospace gas turbine engines will be defined by their ability to operate at high power conditions while maintaining efficiency. This challenge is compounded by airlines' proposition of a return to supersonic flight- an operating regime characterized by higher total temperatures, and thus more NOx production. Lean Direct Injection (LDI) is a combustion scheme that was proposed by NASA, and inherently addresses the needs of both the private sector and the military. LDI is a liquid fueled combustor that promotes rapid mixing of fuel and air at the entrance of the combustor. Despite the benefits of LDI, it has never been implemented, nor has any other lean burning scheme been implemented in an aircraft due to the system level complications of such technology. This dissertation focuses on the dynamics of thermoacoustic instability and lean blowout (LBO), two of the major complications that industry will face when they attempt to incorporate LDI in a production engine. The present dissertation investigates these dynamics from a fundamental and applications standpoint. Fundamental insights on thermoacoustic instabilities are developed by investigating droplet dynamics in a self-excited flow field, and significant oscillations in droplet diameters are discerned. PDPA measurement will be taken to identify coupling of the fuel spray with the instability, and a phase locking algorithm will be used to develop a new spray parameter than is more indicative of combustion heat release that the standard Sauter mean diameter. Next, while varying the swirl number and the venturi geometry of the combustor, the evolution of the flow field will be characterized. An in-house innovation called the Direct Rotation Swirler (DRS) is built for this purpose. The DRS uses an active geometry to provide continuously variable swirl number modulation. The effects of these changes on lean blow out, pressure drop and NOx emissions will then be experimentally determined. Venturis were rapidly manufactured using a ii casting procedure that was developed to make venturi geometries from a commercially available ceramic at very low cost.
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    Dynamics of Thermoacoustic Oscillations in Swirl Stabilized Combustor without and with Porous Inert Media
    Dowd, Cody; Meadows, Joseph (Hindawi, 2022-02-21)
    Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. These processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. To better study these instabilities, quantitative experimental resolution of heat release is necessary, but it presents a significant challenge. Most combustion systems are partially premixed and therefore will have spatially varying equivalence ratios, resulting in spatially variant heat release rates. For laminar premixed flames, optical diagnostics, such as OH chemiluminescence, are proportionally related to heat release. This is not true for turbulent and partially premixed flames, which are common in commercial combustors. Turbulent eddies effect the strain on flame sheets which alter light emission, such that there is no longer a proportional relationship. In this study, phased, averaged, and spatially varying heat release measurements are performed during a self-excited thermoacoustic instability without and with porous inert media (PIM). Previous studies have shown that PIM can passively mitigate thermoacoustic instabilities, and to the best of the authors’ knowledge, this is the first-time that heat release rates have been quantified for investigating the mechanisms responsible for mitigating instabilities using PIM. Heat release is determined from high-speed PIV and Abel inverted chemiluminescence emission. OH chemiluminescence is used with a correction factor, computed from a chemical kinetics solver, to calculate heat release. The results and discussion show that along with significant acoustic damping, PIM eliminates the direct path in which heat release regions can be influenced by incoming perturbations, through disruption of the higher energy containing flow structures and improved mixing.
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    The Effects of Porous Inert Media in a Self-Excited Thermoacoustic Instability: A Study of Heat Release and Reduced Order Modelling
    Dowd, Cody Stewart (Virginia Tech, 2021-03-23)
    In the effort to reduce emission and fuel consumption in industrial gas turbines, lean premixed combustion is utilized but is susceptible to thermoacoustic instabilities. These instabilities occur due to an in-phase relationship between acoustic pressure and unsteady heat release in a combustor. Thermoacoustic instabilities have been shown to cause structural damage and limit operability of combustors. To mitigate these instabilities, a variety of active and passive methods can be employed. The addition of porous inert media (PIM) is a passive mitigation technique that has been shown to be effective at mitigating an instability. Practical industrial application of a mitigation strategy requires early-stage design considerations such as reduced order modeling, which is often used to study a systems' stability response to geometric changes and mitigation approaches. These reduced order models rely on flame transfer functions (FTF) which numerically represent the relationship between heat release and acoustic perturbations. The accurate quantification of heat release is critical in the study of these instabilities and is a necessary component to improve the reduced order model's predictive capability. Heat release quantification presents numerous challenges. Previous work resolving heat release has used optical diagnostics. For perfectly premixed, laminar flames, it has been shown there are proportional relationships between OH* or CH* chemiluminescence to heat release. This is an ideal case; in reality, practical burners produce turbulent and partially premixed flames. Due to the additional straining of the flame caused by turbulence, the heat release is no longer proportional to chemiluminescence quantities. Also, partially premixed systems have spatially varying equivalence ratios and heat release rates, meaning analysis reliant on perfectly premixed assumptions cannot be used and techniques that can handle spatial variations is needed. The objective of this thesis is to incorporate PIM effects into a reduced order model and resolve quantities vital to understand how PIM is mitigating thermoacoustic instabilities in a partially premixed, turbulent combustion environment. The initial work presented in this thesis is the development of a reduced order model for predicting mode shapes and system stability with and without PIM. This was the first time that a reduced order model was developed to study PIM effects on the thermoacoustic response. Model development used a linear FTF and can predict the system frequency and stability response. Through the frequency response, mode shapes can be constructed which show the axial variation in acoustic values, along with node and anti-node locations. Stability trends can be predicted, such as the independent effects of system parameter variation, to determine its stability response. The model was compared to canonical case studies as well as experimental data with reasonable agreement. With PIM addition, it was shown that a combustor would be under stable operation at more flow conditions than without PIM. The work also shows the stability sensitivity to different porous parameters and PIM locations within the combustor. The model has been used to aid in the design of other combustion systems developed at Virginia Tech's Advanced Propulsion and Power Laboratory. To better understand how PIM is affecting the system stability and demonstrate measurements for the improvement of a numerical FTF, experimental work to resolve the spatially varying equivalence ratio fluctuations was conducted in an atmospheric, swirl-stabilized combustor. The experimental studies worked to improve the fundamental understanding of PIM and its mitigation effects through spatially and temporally resolved equivalence ratios during a self-excited instability. The experimental combustor has an optically accessible flame region which allowed for high speed chemiluminescence to be captured during the instability. Equivalence ratio values were calculated from a relation involving OH*/CH* chemiluminescence ratio. The acoustic perturbations were studied to show how the equivalence ratio fluctuations were being generated and coupling with the acoustic waves. The fluctuation in equivalence ratio showed about 65% variation around its mean value during the period of an instability cycle. When porous media was added to the system, the fluctuation in equivalence ratio was mitigated and a reduction in peak frequency (sound pressure level) SPL of 38 dB was observed. Changes in the spatial distribution of equivalence ratio with PIM addition were shown to produce a more stable operation. Effects such as locally richer burning and changes to recirculation zones promoted more stable operation with PIM addition. Testing with forced acoustic input was also conducted to quantify the flame response. The results demonstrated that a flame in a system with PIM responds differently than without PIM, highlighting the need to develop FTF for systems with PIM. This experimental study was the first to study equivalence ratio in a turbulent, partially premixed combustor using PIM as a mitigation technique. A final experimental investigation was conducted to resolve the spatially defined heat release and its fluctuation during a thermoacoustic instability period. This was the first time that heat release had been investigated in a partially premixed, thermoacoustically unstable system, using PIM as a migration method. Heat release was quantified using equivalence ratio, strain rate, OH* intensity, and a correction factor determined from a chemical kinetic solver. The heat release analysis built upon the equivalence ratio study with additional flow field analysis using PIV. The velocity vectors showed prominent corner and central recirculation zones in the no PIM case which have been shown to be feedback mechanisms that support instability formation. With PIM addition, these flow features were reduced and decoupled from the combustor inlet reactants. The velocity results were decomposed using a spectral proper orthogonal decomposition (SPOD) method. The energy breakdown showed how PIM reduced and distributed the energy in the flow structures, creating a more stable flow field. Heat release results with velocity information demonstrated the significant coupling mechanisms in the flow field that were mitigated with the PIM addition. The no PIM case showed high heat release areas being directly influenced by the incoming flow fluctuations. The feedback mechanisms, both mean flow and acoustic, have a direct path to the incoming flow to the combustor. In the PIM case, there is significant mixing and burning taking place in locations where it is less likely that feedback can reach the incoming flow to couple to form an instability. The methodology to quantify heat release provides a framework for quantifying a non-linear FTF with PIM. The development and testing to determine a non-linear FTF with PIM are reserved for future work and discussed in the final chapter. The methodologies and modeling conducted here provided insight and understanding to answer why PIM is effective at mitigating a thermoacoustic instability and how it can be studied using a reduced order numerical tool.
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    Exploration and Development of Electrically Controllable Gel and Solid Propellants
    Gobin, Bradley Scott (Virginia Tech, 2023-05-26)
    Electrically controllable propellants (ECPs) provide a new method to increase the control and functionality of rocket motors in particular solid rockets. Traditional solid rockets do not have the capability to modify the burning rate on demand during operation, which greatly limits operational capabilities. The research outlined in this dissertation explores the fundamentals in the creation of ECPs to enable increased control in the burning rate of solid rockets. The research is organized into four studies which step through the fundamentals of ECPs, starting with a focus on the solid oxidizers, then moving into the creation of electrically controllable gel propellants (ECGPs). Next, electrically controllable solid propellants (ECSPs) were explored under atmospheric conditions, and then finally under elevated pressures. The first study explores the ability to electrically control the decomposition characteristics of various solid oxidizers. Typical composite solid propellants are composed of solid fuels and oxidizers and isolating the oxidizer in this study enables the ability to characterize critical components of ECSPs individually. This study discovered that certain solid oxidizers respond differently to applied voltages, but generally the decomposition rate of the solid oxidizers is greatly increased when voltage is applied using metal electrodes. The melt layer formed in the decomposition of the solid oxidizers was observed to be critical in the ability to manipulate the decomposition rate of the oxidizers. The second study built upon the knowledge that the melt layer was critical in the functionality of ECPs and explored the utilization of ECGPs which combined a viscous liquid polymer fuel in which solid oxidizers were dissolved. The ECGPs used in this study readily decomposed and ignited when a voltage potential was applied. The composition of the ECGPs along with the magnitude of the voltage being applied greatly impacted the ignition delay and overall burning characteristics of the propellants. This study illustrated the potential to create ECPs that enable increased control over the burning characteristics compared to conventional propellants. The third study utilized a solid polymer binder along with the solid oxidizers to create ECSPs that would readily decompose and ignite when a voltage potential was applied. Compositional changes in the propellant along with the magnitude of the applied voltage potential were observed to impact the regression rate of the ECSPs utilized in this study. The electrochemical decomposition characteristics of the ECSPs were explored to better characterize the contribution of the electrochemical reactions and how they differ from the more conventional thermochemical decomposition. The fourth and final study builds upon the prior ECSP study, but now experiments utilize compositions with electrically conductive additives to increase the responsiveness of the ECSPs to the applied voltage. This enabled the creation of ECSPs which ignite much more readily and with a higher degree of consistency. Experiments were also conducted at elevated pressures to analyze the combined impact that voltage and pressure play on the regression rate of the ECSPs.
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    Influence of Fuel Inhomogeneity and Stratification Length Scales on Detonation Wave Propagation in a Rotating Detonation Combustor (RDC)
    Raj, Piyush (Virginia Tech, 2021-05-03)
    The detonation-based engine has the key advantage of increased thermodynamic efficiency over the traditional constant pressure combustor. These detonation-based engines are also known as Pressure Gain Combustion systems (PGC) and Rotating Detonation Combustor (RDC) is a form of PGC, in which the detonation wave propagates azimuthally around an annular combustor. Prior researchers have performed a high fidelity 3-D numerical simulation of a rotating detonation combustor (RDC) to understand the flow physics such as detonation wave velocity, pressure profile, wave structure; however, performing these 3-D simulations is computationally expensive. 2-D simulations are a potential alternative to reduce computational cost. In most RDCs, fuel and oxidizer are injected discretely from separate plenums, and this discrete fuel/air injection results in inhomogeneous mixing within the domain. Due to the discrete fuel injection locations, fuel/oxidizer will stratify to form localized pockets of rich and lean mixtures. The motivation of the present study is to investigate the impact of unmixedness and stratification length scales on the performance of an RDC using a 2-D numerical approach. Unmixedness, which is defined as the standard deviation of equivalence ratio normalized by the mean global equivalence ratio, is a measure of the degree of fuel-oxidizer inhomogeneity. To model the effect of unmixedness in a 2-D domain, a lognormal distribution of the fuel mass fraction is generated with a mean equivalence ratio of 1 and varying standard deviations at the inlet boundary as a numerical source term. Moreover, to model the effects of stratification length scales, fuel mass fraction at the inlet boundary cells is bundled for a given length scale, and the mass fractions for these bundles are updated based on the lognormal distribution after every three-time steps. Using this methodology, 2-D numerical analyses are carried out to investigate the performance of an RDC for an H2-air mixture with varying unmixedness and stratification length scales. Results show that mean detonation velocity decreases and wave speed variation increases with an increase in unmixedness. However, with an increase in stratification length scale mean velocity remain relatively unchanged but variation in local velocity increases. The detonation wave front corrugation also increases with an increase in mixture inhomogeneity. The mean detonation cell size increases with an increase in unmixedness. The cell shape becomes more distorted and irregular with an increase in stratification length scale and unmixedness. The combined effect of unmixedness and stratification length scale leads to a decrease in pressure gain. Overall, this concept is able to elucidate the effects of varying unmixedness and stratification length scales on the performance of an RDC.
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    Investigation of Fuel Geometry and Solid Fuel Combustion for Solid Fuel Ramjets
    Gallegos, Dominic Francisco (Virginia Tech, 2024-12-10)
    Solid fuel ramjets (SFRJs) are a simple means of sustaining supersonic flight. The utilization of solid fuels eliminates the need for moving parts or liquid delivery systems, and the solid fuels are typically inert, resulting in minimal handling requirements compared to solid propellants. Characteristic of SFRJ systems are the relatively high combustor velocities and the required gasification of the solid fuel prior to releasing heat through gas-phase reactions. The primary objectives of the current work were to investigate the decomposition behavior of model solid fuels typically used in SFRJ systems and to employ a novel fuel geometry to increase the flame-holding limits of an SFRJ. Two bench-scale solid fuel experiments were conducted to capture relevant performance metrics of five solid fuels. Performance parameters such as regression rates, surface temperature variations, molten layer thickness, and condensed-phase kinetic behavior were analyzed using a non-combusting laser pyrolysis experiment. Further investigations were performed for each fuel using a modified counterflow burner, which served as an analog for the boundary layer combustion in an SFRJ by introducing the effects of flame heat feedback to the fuel surface. General trends among the fuels were identified, and several mechanistic differences in the decomposition process were discussed with consideration of condensed-phase behavior. The results from the laser pyrolysis and counterflow burner studies were subsequently used as validation data for the development of a solid fuel decomposition model incorporating single-step decomposition, transient heat transfer, and surface heat losses. The developed model showed reasonable agreement with experimental pyrolysis results, particularly for regression rates and surface temperatures of polymethylmethacrylate (PMMA) and hydroxyl-terminated polybutadiene (HTPB). Investigations using two lab-scale SFRJs were conducted to determine the feasibility and performance impacts of implementing a cavity-style flame holder as a means of improving the flammability limits of a SFRJ. The results presented demonstrate the effectiveness of such a method showing that introducing a cavity flame holder enables significantly higher fuel loading in the present system. The effects of the alternate geometries on local regression rates are reported and a high local heat flux at the cavity corner is identified as a strong factor in the increased flame holding capability. The increased regression rates contribute to higher observed chamber pressures while the effects on combustion efficiency are observed to be minimal. Further investigation of the cavity geometries using an optically accessible SFRJ allowed the analysis of the reacting flow field. High-speed chemiluminescence, high-speed videography, and high-speed three-color camera pyrometry provided further insight into the reacting flow and identified key reaction regions relevant to flame holding. Observations of the spatial regression rate show similar trends to the initial experiments, revealing a large increase in regression rate associated with the cavity corner. The regression rates and observations regarding the size of the recirculation region were incorporated into a semi-empirical model describing the behavior of the recirculation region and point to the increased fuel flow rate resulting from the cavity corner as a contributing factor in the increased flammability of the cavity fuel grains.
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    A Method for Measuring Spatially Varying Equivalence Ratios with Application to Thermoacoustics
    Hugger, Blaine Thomas (Virginia Tech, 2021-12-17)
    Computed tomography for flame chemiluminescence emissions allows for 3D spatially resolved flame measurements to be acquired using a series of discrete viewing angle camera images. To determine fuel/air ratios, the ratio of excited radical species (OH*/CH*) emissions using chemiluminescence can be employed. Following the process of high-resolution tomography reconstructions in this work allowed for flame tomography coupled with chemiluminescence emissions to be used for spatially resolved phase averaged equivalence ratio measurements. This is important as variations in local equivalence ratios can have a profound effect on flame behavior including but not limited to thermoacoustic instability, NOx and CO formation, and flame stabilization. Local equivalence ratios are determined from a OH*/CH* ratio of tomographically reconstructed intensity fields and relating them to equivalence ratio. The correlation of OH*/CH* to equivalence ratio is derived from an axisymmetric, commercially available flat flame burner (Holthuis and Associates Burner). To relate intensity field imaging (camera coordinate system) during the tomographic reconstruction to the real-world coordinate system of the burner a calibration procedure was performed and then validated. A calibration plate with 39 non-coplanar points was used in this procedure. It was then validated by comparing the Abel inverted flame images of the axisymmetric Holthuis and Associates burner with the tomographic reconstructed images. Results show a successful tomographic reconstruction of thermoacoustic self-excited cycle concluding equivalence ratio fluctuations coinciding with the 1st dominate frequency of the pressure fluctuations and influenced by a 2nd harmonic frequency.
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    Novel Approach for Computational Modeling of a Non-Premixed Rotating Detonation Engine
    Subramanian, Sathyanarayanan (Virginia Tech, 2019-07-17)
    Detonation cycles are identified as an efficient alternative to the Brayton cycles used in power and propulsion applications. Rotating Detonation Engine (RDE) operating on a detonation cycle works by compressing the working fluid across a detonation wave, thereby reducing the number of compressor stages required in the thermodynamic cycle. Numerical analyses of RDEs are flexible in understanding the flow field within the RDE, however, three-dimensional analyses are expensive due to the differences in time-scale required to resolve the combustion process and flow-field. The alternate two-dimensional analyses are generally modeled with perfectly premixed fuel injection and do not capture the effects of improper mixing arising due to discrete injection of fuel and oxidizer into the chamber. To model realistic injection in a 2-D analysis, the current work uses an approach in which, a Probability Density Function (PDF) of the fuel mass fraction at the chamber inlet is extracted from a 3-D, cold-flow simulation and is used as an inlet boundary condition for fuel mass fraction in the 2-D analysis. The 2-D simulation requires only 0.4% of the CPU hours for one revolution of the detonation compared to an equivalent 3-D simulation. Using this method, a perfectly premixed RDE is comparing with a non-premixed case. The performance is found to vary between the two cases. The mean detonation velocities, time-averaged static pressure profiles are found to be similar between the two cases, while the local detonation velocities and peak pressure values vary in the non-premixed case due to local pockets fuel rich/lean mixtures. The mean detonation cell sizes are similar, but the distribution in the non-premixed case is closer due to stronger shock structures. An analytical method is used to check the effects of fuel-product stratification and heat loss from the RDE and these effects adversely affect the local detonation velocity. Overall, this method of modeling captures the complex physics in an RDE with the advantage of reduced computational cost and therefore can be used for design and diagnostic purposes.
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    Numerical Investigation on Shape Impact of Deformable Droplets on Evaporation and Combustion: Method Development and Characterization
    Setiya, Meha (Virginia Tech, 2023-08-21)
    Inspired by the dilute spray regime in spray combustion, this dissertation explores the evaporation and combustion of an isolated droplet. Under a highly convective environment inside a gas combustor, due to imbalance of inertial and surface tension forces, the droplets of larger size in sprays exhibit notable deformations from spherical to non-spherical shapes. Such shape changes are generally observed but not quantified in experimental studies. Therefore, the effect of this deformation on droplet combustion dynamics is unknown yet. To bridge this gap, a comprehensive investigation of an isolated freely deforming droplet can be insightful as it can reveal more about the interaction of droplet shape with its evaporation and combustion. This work attempts to analyze and quantify the impact of such deformations on evaporation and combustion using interface-capturing Direct Numerical Simulation approach. With the focus on small-scale processes involved in evaporation as it is a pre-step for combustion, this dissertation first covers a thorough examination on evaporation of a deformable droplet under both natural and forced convection. A single component jet-fuel surrogate n-decane is chosen. To ensure that the droplet remains stationary throughout its lifetime, a novel numerical method called "gravity update method" is developed and implemented. The results obtained from these two separate studies are validated against experimental results and analytical correlations respectively. The findings from the investigation of droplet evaporation under forced convective flow at moderate Reynolds numbers are noteworthy. The droplet shape under such flow conditions is governed by Weber number (We) which is a ratio of inertial force to surface tension force. The results demonstrated upto 20% en- hancement in total evaporation rate for highly deformed droplets. This improvement is a net results of increased droplet surface area and alteration in the distribution of local evaporation flux ( m'' ). It is found that m'' is proportional to its curvature up to the point of flow separation which agrees with low Re theories on droplet evaporation by Tonini and Cossalli (International Journal of Heat and Mass Transfer 2013), Palmore (Journal of Heat Transfer 2022). Beyond the flow separation point, evaporation flux distribution depends on the boundary layer development and flow evolution downstream of the droplet. For highly deformed droplets, a larger wake region creates favorable fuel vapor gradients and promotes mixing in droplet wake, hence higher evaporation flux. Such positive impact of droplet deformation on total evaporation rate motivated further investigation on droplet combustion under a low Reynolds number convective flow. High pressure and temperature gas flow leads to Damköhler number is higher than 1. This fa- vors the generation of envelope type flame. The results show overall little sensitivity to combustion related parameters despite the droplet shape change and significant (upto 9%) enhancement in total evaporation rate. It is also noted that while burning, droplets do not reach critical deformation conditions and break-up even beyond the critical Weber number, suggesting the suppression of deformation due to faster evaporation rate. The findings presented in these studies provide substantial evidence for the interaction between droplet shape and flow dynamics. Therefore, it demonstrates the potential for enhancing the existing numerical models and analytical correlations by accounting the influence of droplet shape.
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    On the Challenges of integrating a Rotating Detonation Combustor with an Industrial Gas Turbine and important design considerations for Row-1 Blades
    Rathod, Dharmik Sanjay (Virginia Tech, 2024-05-21)
    With the ever-growing demand for power generation to support the world economy and electric transportation needs, efficient gas turbine power cycles need to be investigated to match the anticipated high demands of the future. Decarbonization efforts around the world to achieve Net Carbon Zero by 2050 have also brought many new challenges for the development of these systems due to the unique constraints imposed by less carbon-intensive fuels. In this effort to increase the efficiency and performance of such gas turbine power cycles, pressure gain combustion (PGC) has gained significant interest. The potential for an increase in the thermodynamic efficiency over the constant-pressure Brayton Cycle has made detonation combustors, a type of PGC, an attractive alternative to traditional deflagration-type combustors. Since Rotating Detonation Combustors (RDC) can provide a quasi-steady mode of operation when compared to Pulse Detonation Combustors (PDC), research has been triggered to integrate RDC with power-generating gas turbines. However, the presence of subsonic and supersonic flow fields which are generated due to the shock waves that stem from the detonation wave front and the highly non-uniform temperature and velocity profiles may cause a depreciation in the turbine performance. The current study seeks to investigate the challenges of integrating the RDC with nozzle guide vanes (NGV) of an industrial, can-annular gas turbine and attempts to understand the major contributors that impact efficiency and identify the key areas of optimization that need to be considered for maximizing performance. In order to compare the results with an F class gas turbine engine condition, a geometric model of RDC developed by the Air Force Research Laboratory (AFRL) was scaled using a linear mass flow to area relationship, aiming to achieve a higher flow rate. The RDC was integrated with the NGVs through a non-optimized straight duct-type geometry with a diffuser cone. 3-Dimensional Numerical analyses were performed to investigate sources of total pressure loss and to understand the unsteady effects of RDC which contribute towards the deterioration of performance. The entropy generation at different regions of interest was calculated to identify the major irreversibility's in the system. Finally, total pressure and temperature distribution along the radial direction at the exit of the transitional duct is presented to understand the various constraints imposed by the RDC when integrating with an Industrial gas turbine engine NGV.
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    Phase Transform Time Delay Estimation to Counteract Spectral Haystacking Effects in Jet Exhaust Flow Measurements
    Silas, Kevin Alexander (Virginia Tech, 2021-09-01)
    This study determined a superior data processing technique for correlating an acoustic signal passing through a subsonic jet engine exhaust in order to estimate the traversal time of the signal. Thrust measurement is possible with enough time delay estimates across different portions of the exhaust. This preliminary study did not take the full array of data necessary to measure thrust, but did validate key aspects of the measurement process. The turbulent shear layers of the exhaust spectrally broaden the signal, creating the appearance of spectral "haystacks", making traditional correlation methods unworkable. An experiment was performed to evaluate the ability of a novel sound source to produce a signal from which a reliable and precise time delay estimate could be found. The test apparatus was installed on either side of a Honeywell TFE731-2 turbofan research engine exhaust cone, with the source and receivers placed near the jet exit plane. The signal was then directed across the jet exhaust. This flow environment is considered an extreme challenge for accurate acoustic signal propagation. A key contribution of this paper is the determination that the Phase Transform processor of the Generalized Cross-Correlation (GCC) method produces the most reliable time delay estimates, for the given signal and flow conditions. Several alternative time delay estimators and GCC processors were examined and evaluated on this data. A proposed explanation is provided for why this time delay estimation technique produces the most accurate results, as well as explanations for why the technique became less reliable as the flow environment became more challenging, with an observed 22% anomalous TDE selection rate for the N1Corr = 60% and N1Corr = 70% conditions combined, versus only 6% for the idle and N1Corr = 50% conditions combined. This paper also details the development and first use of a novel acoustic source that produces a two-tone narrowband signal emanating from a single point – the dual Hartmann generator.
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    Predicting Flow in Firebrand Pile using Pore Network Model
    Wu, Ditong (Virginia Tech, 2023-12-21)
    Firebrand pile ignition of adjacent materials requires an in-depth understanding of heat transfer and flow profile within the firebrand pile. Modeling the firebrand pile as a fibrous porous medium, this study identified a porosity-permeability correlation that accurately describes the transport properties of a firebrand pile. The conduction-based model and Kozeny-Carman model were identified and examined by experiment, where firebrand porosity and permeability were collected with a wind tunnel. The conduction-based model was more stable and more accurate in the porosity range of interest. Pore network models were developed for the simulation of flow profiles utilizing the permeability data collected. The non-uniform network, which better represents a randomly stack firebrand pile, resulted in a more complex multidimensional flow within the pile.
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    Pyrolysis and Flamelet Model for Polymethyl Methacrylate in Solid Fuel Sc(ramjet) Combustors
    Pace, Henry Rogers (Virginia Tech, 2024-10-28)
    Scramjets have been identified as a potential long-term replacement for rocket and ramjet propulsion systems due to their enhanced performance at high Mach numbers. The introduction of solid fuels in these scramjet systems allows for shaping of the solid fuel cavity by additive manufacturing and introduces the possibility of enhancing combustion rates and stability. The present investigation aims to develop a coupled, high-order computational model to study the combustion of solid fuel scramjets. The primary objectives are to identify the effects of changing geometry on combustion and to better characterize the combustion process and flow patterns within a solid fuel scramjet engine. The high-Mach number of the air inflow over a scramjet cavity introduces a strong coupling between fluid dynamics, combustion, and regression time scales. Existing models often use simplified treatments of melt-layer conditions and combustion models that over-predict experimental rates, along with highly dissipative numerical schemes that inhibit the study of thermo-acoustic interactions between coherent pressure waves and the burning walls of the cavity. These limitations in current models suggest the need for a Navier-Stokes solver based on a high-order, discontinuous Galerkin method, incorporating melt layer equations and enhanced combustion manifolds. These manifolds should account for the effects of pressure and high oxidizer temperatures on flamelet dynamics. The focus is on modeling the flow field with accurate chemical heat release and residence time, to better study the effects of heat flux on the solid surface and the resulting coupling. An investigation of solid fuel scramjets was performed, and the numerical methodology with which the problem was tackled is described. A novel combustion mechanism was developed using a counterflow burner to study the combustion and regression of solid model fuel polymethyl methacrylate (PMMA). The diffusion flame between the fuel and oxidizer was studied numerically using a solid fuel decomposition and melt layer model to simulate convection and pyrolysis of the material. This model was validated using new experimental data as well as previously published works. The foam layer parameters are critical to the success of the validation. Results showed that the increased residence time of the gas in the bubbles facilitates the fuel breakdown. Fully coupled fuel injection and solid fuel surface monitoring was implemented based on this counterflow model and was a function of heat flux. Fuel regression was handled using adaptive control points for a B-Spline basis that updates based on surface movement. This methodology was used due to its resilience against the creation of surface discontinuities likely to result from large temperature gradients during combustion. Fourth-order computational simulations of ramjet combustion without regressing fuel walls using an in-house Discontinuous Galerkin approach were performed with a fully conjugate solution for the thermal wave in the solid. Results in ramjet geometries showed the turbulent combustion strongly affects the heat feedback to the walls and thus increases both the regression and fuel injection rates. Scramjet geometries were also simulated using the flamelet-progress variable approach in two different oxidizer conditions. All of these simulations showed strong agreement with experimental data and helped to uncover flame holding characteristics of the scramjet cavities and the strong coupling between the recirculation region and pyrolysis of fuel. The analysis has led to a better understanding of the effects of solid fuel scramjet geometries on mixing, enhanced modeling of acoustic instabilities in solid fuel air-breathing propulsion, and improved fuel chemistry modeling. It has been shown that cavity design significantly influences heat transfer to the solid fuel in both ramjet and scramjet conditions. The presence and thickness of the melt layer will guide designs that aim to reduce or enhance mechanical removal of fuel. Additionally, ramjet results indicate that longer cavities can couple with acoustics to induce self-excited conditions, leading to increased heat transfer to the solid. The importance of self-sustained instability and its coupling with melt layer fuel injection will contribute to improved acoustic stability. Developing pressure/temperature-dependent manifolds and melt layer models will advance our understanding of solid fuel supersonic combustion and its effects on phenomena such as blowout, fuel residence time, and solid fuel dual-mode transition.
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    Simulation of Electrical Characteristics in Oxyfuel Flame Subject to An Electric Field
    Xu, Kemu (Virginia Tech, 2021-06-11)
    The oxyfuel cutting method is still widely used nowadays, even though it is not a fully autonomous process. Thisthesis presents a computational model to study ion and electron transport and current-voltage characteristics inside a methane-oxygen flame. By finding the relationship between current-voltage characteristics and critical parameters,such as standoff, fuel oxygen ratio, and flow rate, a control algorithm could be implemented into the system and make it autonomous. Star CCM+ software is used to develop preheat phase computational models by splitting the simulations into the combustion and electrochemical transport parts. Both the laminar and turbulent flows are considered. Several laboratory experiments are used to compare test data with the numerical results generated using this model. The initial and boundary conditions used in the simulation were to the extent possible similar to the experimental conditions in the laboratory experiment. In the combustion part, the general GRI3.0 mechanism plus three additional ionization reactions are applied, and the combustion part results are then used as input into the electrochemical transport part. A particular inspection line inside the domain is created to analyze the results of the electrochemical transport part. Ions, electrons number density, and current density are studied in the interval from -40V to 40V electric potential. The ions are heavier and more challenging to move than electrons. The results show that at both the torch and work surfaces, charged sheaths are formed, which cause three different regions of current-voltage relations to form in a similar manner as observed in the tests.
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    Spatially Resolved Equivalence Ratio Measurements Using Tomographic Reconstruction of OH*/CH* Chemiluminescence
    Giroux, Thomas Joseph III (Virginia Tech, 2020-07-27)
    Thermoacoustic instabilities in gas turbine operation arise due to unsteady fluctuations in heat release coupled with acoustic oscillations, often caused by varying equivalence ratio perturbations within the flame field. These instabilities can cause irreparable damage to critical turbine components, requiring an understanding of the spatial/temporal variations in equivalence ratio values to predict flame response. The technique of computed tomography for flame chemiluminescence emissions allows for 3D spatially resolved flame measurements to be acquired using a series of integral projections (camera images). High resolution tomography reconstructions require a selection of projection angles around the flame, while captured chemiluminescence of radical species intensity fields can be used to determine local fuel-air ratios. In this work, a tomographic reconstruction algorithm program was developed and utilized to reconstruct the intensity fields of CH* and OH*, and these reconstructions were used to quantify local equivalence ratios in an acoustically forced flame. A known phantom function was used to verify and validate the tomography algorithm, while convergence was determined by subsequent monitoring of selected iterative criteria. A documented method of camera calibration was also reproduced and presented here, with suggestions provided for future calibration improvement. Results are shown to highlight fluctuating equivalence ratio trends while illustrating the effectiveness of the developed tomography technique, providing a firm foundation for future study regarding heat release phenomena.
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    Spatiotemporally-Resolved Velocimetry for the Study of Large-Scale Turbulence in Supersonic Jets
    Saltzman, Ashley Joelle (Virginia Tech, 2021-01-08)
    The noise emitted from tactical supersonic aircraft presents a dangerous risk of noise-induced hearing loss for personnel who work near these jets. Although jet noise has many interacting features, large-scale turbulent structures are believed to dominate the noise produced by heated supersonic jets. To characterize the unsteady behavior of these large-scale turbulent structures, which can be correlated over several jet diameters, a velocimetry technique resolving a large region of the flow spatially and temporally is desired. This work details the development of time-resolved Doppler global velocimetry (TRDGV) for the study of large-scale turbulence in high-speed flows. The technique has been used to demonstrate three-component velocity measurements acquired at 250 kHz, and an analysis is presented to explore the implications of scaling the technique for studying large-scale turbulent behavior. The work suggests that the observation of low-wavenumber structures will not be affected by the large-scale measurement. Finally, a spatiotemporally-resolved measurement of a heated supersonic jet is achieved using large-scale TRDGV. By measuring a region spanning several jet diameters, the lifetime of turbulent features can be observed. The work presented in this dissertation suggests that TRDGV can be an invaluable tool for the discussion of turbulence with respect to aeroacoustics, providing a path for linking the flow to far-field noise.
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    Stability, LES, and Resolvent Analysis of Thermally Non-uniform Supersonic Jet Noise
    Chauhan, Monika (Virginia Tech, 2021-11-16)
    For decades noise-induced hearing loss has been a concern of the Department of Defense (DoD). My research investigates noise generation and dispersion in supersonic jets and focuses on the fluid-dynamic regime typical of high-performance turbojet and turbofan engines. The goal of my research is to understand how dispersion and propagation of wavepackets can be modified by noise reduction strategies based on secondary injections of fluid with a different temperature from the main jet. The research is organized into three studies that focus on instability, large eddy simulations, and resolvent modes. The first study is a computational investigation of the role of thermal non-uniformity on the development of instability modes in the shear-layer of a supersonic $M= 1.5$, $Re=850,000$ jet. Cold fluid is injected at the axis of a heated jet to introduce radial non-uniformity and control the spatial development of the shear layer. The mean flow is analyzed with an efficient 2D and 3D Reynolds-averaged Navier-Stokes (RANS) approach using the SU2 code platform for 3 different cases -baseline, centered, and offset injection. Different turbulence models are tested and compared with the experiments. The coherent perturbation is analyzed using linear parallel and parabolized stability equations (PSEs). The second study investigates novel formulations of large eddy simulation models using an arbitrary high order discontinuous Galerkin scheme. The LES analysis focuses on both numerical issues (such as convergence against the polynomial order of the mesh), modeling issues (such as the choice of subgrid model), and underlying physics (such as vortex stretching and noise generation). Wall models are used to capture the viscous sublayer at the nozzle. The Ffowcs Williams-Hawkings (FW-H) method is used for far-field noise predictions for all cases. Three-dimensionality is studied to investigate how injection in the shear layer acts to create a rotational inviscid core and affects the mixing of the cold fluid and noise dispersion. The third study extends the (first) instability study by considering (global) resolvent modes. Such optimally forced modes of the turbulent mean flow field will identify the turbulent coherent structures (wavepackets) for different turbulence models at $M=1.5$. The LES simulations performed in the second study will be used to extract the mean flow and the dynamic modes for comparison. My research plan is to perform the resolvent analysis of the axisymmetric mean flow fields for the thermally activated case (i.e., the centered injection) and compare it to the baseline jet case. Different turbulence models will be investigated to determine the correct alignment of dynamic and resolvent modes. Finally, I will consider the three-dimensional, non-axisymmetric mean flow created by offset injection described in the second study, which requires evaluating the convolution products of resolvent modes and base flow. Such three-dimensional resolvent compressible modes have never been identified in the context of supersonic jets.