Doctor of Philosophy with a Major in Aerospace Engineering

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Doctor of Philosophy with a Major in Aerospace Engineering

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https://hdl.handle.net/1853/72952

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Degree Series

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Daniel Guggenheim School of Aerospace Engineering

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Now showing 1 - 10 of 36

The role of droplets in the autoignition of a polydisperse Jet-A spray in vitiated co-flow

(Georgia Institute of Technology, 2019-06-20) Williams, Aimee

The objective of this study is to understand the underlying mechanisms of autoignition of a polydisperse fuel spray. Understanding and predicting autoignition of fuel sprays is important to the design of modern gas turbine engines, especially in the interest of developing a flame-holder-less afterburner concept. In this system, liquid fuel is injected into a high temperature, flowing, vitiated air flow. Previous studies of fuel spray autoignition have suggested multiple mechanisms for a fuel spray to autoignite, including single droplet and droplet cloud ignition behavior. The majority of liquid-fueled autoignition studies have been parametric in nature and describe the overall effect of droplet size, equivalence ratio, turbulence intensity, etc. on ignition delay time but do not investigate the phenomena controlling the local behavior of autoignition kernel formation and growth. Autoignition studies of cold gaseous fuel jets in hot oxidizer cross flows have shown the importance of local mixture fraction. A test facility was developed that is capable of reproducing flow conditions in an aero-engine reheat combustor. Fuel is injected using a reproduction of a commercially available spray nozzle installed on an aerodynamically shaped body centered in the flow by three aerodynamic pylons. High speed chemiluminescence and UV PLIF were used to determine the dependence of the locations where autoignition kernels form, upon the flow temperature and velocity. Analysis of the scatter in the time-resolved ignition locations revealed the importance of temperature fluctuations in the vitiated flow. Specifically, the most upstream ignition locations likely correspond to the hottest and, therefore, most reactive fluid packets. The distribution of the fuel spray was found to affect the appearance of most upstream autoignition kernels. A near stationary (on average) flame was found to exist at high co-flow temperatures, being stabilized by autoignition as distinct kernels were formed upstream of the main flame region.
Simulation of full-scale combustion instabilities in small-scale rigs using actively controlled boundary conditions

(Georgia Institute of Technology, 2017-11-09) Kim, Yong Jea

The onset of combustion instabilities (CIs) has hindered the development and performance of combustion systems employed in industrial, power generation and propulsion systems for many decades. Investigating CIs in actual “full-scale” engine tests are not practical because of the exorbitant cost of such tests, the large space required to house the full-sized engine, and the inability to equip full-scale engines with diagnostic systems. Because of these difficulties, most studies of CIs to date were performed in “small-scale” setups that were geometrically similar to but smaller than the full-scale engines combustors. While testing with these small-scale setups reduced the cost of testing and produced important results, the acoustic modes excited in the small-scale setups had considerably higher frequencies that did not simulate the lower frequency oscillations that are excited in the unstable full-scale engines. The above discussion indicates that in order to study the driving of CIs in full-scale engines in small-scale rigs, the latter must simulate the acoustic environments, the combustion processes, and the interactions between these processes in the unstable full-scale engine. This study developed a real time active acoustic boundary control approach to simulate the acoustic environment of the full-scale engine in the small-scale rig. For the study of the driving mechanism of longitudinal CIs, the small-scale rig consists of a shorter section of the full-scale engine and the “missing part” of the full-scale engine (consists of what has been “left over” after the small-scale rig has been removed from the full-scale combustor); see the first figure in Summary in this thesis. The goal of the actively controlled small-scale rig is to experimentally study the processes taking place in the corresponding region of small-scale rig section in an unstable full-scale engine. To attain this goal, the active control system (ACS) needs to generate an acoustic impedance at the actively controlled boundary of the small-scale rig that equals to the acoustic impedance at the corresponding location in the full-scale unstable engine. If this is accomplished, the acoustic oscillations in the small-scale rig and the corresponding region of small-scale rig section in full-scale engine would be identical. This study has developed a real time ACS, which enables the small-scale tube rig to simulate the longitudinal acoustic oscillations in the full-scale tubes (or engine), with the one-dimensional cold flow setup. In this setup, the speaker at the left end of the small-scale tube rig generated acoustic oscillations that simulate the driving by the combustion process, and the speaker at the right end was actively controlled to simulate the acoustic field of the full-scale system. It was demonstrated that the developed, actively controlled, small-scale, rig can simulate travelling and standing waves oscillations that are encountered in longer full-scale tubes. Additionally, this rig was used to demonstrate that standing acoustic waves CI in full-scale tubes having different lengths could be simulated in the developed, actively controlled, small-scale rig. This study also developed a theoretical model that determines in real time the acoustic boundary condition (BC) that must be generated by the ACS at the boundaries of a small-scale rig that simulates transverse (tangential) CI in an annular combustor similar to those used in gas turbines and jet engines. In this case, the small-scale rig consists of a small section of the annular combustor and the “missing part” of the full-scale engine (consists of what has been “left over” after the small-scale sector-rig has been removed from the annular combustor); see the second figure in Summary in this thesis. To determine the BCs that needed to be established at the boundaries of the actively controlled, small-scale rig, the developed model accounts for the effects of the combustion processes and flows through the reactants supply injectors and exhaust nozzles in the “missing part” of the engine, and for the presence of a tangential mean flow in the annular combustor. The developed model was numerically validated and used to investigate the effects of the exhaust nozzle, combustion process, and tangential mean flow component upon the characteristics of tangential CIs in an annular combustor. For example, its numerical solutions demonstrated that the presence and direction of the mean tangential flow component critically affect the characteristics of tangential (spinning) instabilities and an initially standing wave disturbance gradually transforms itself into a spinning wave that rotates around the annular combustor in the direction of the tangential mean flow; this finding is in agreement with previous experimental observations that have not been explained to date.
Experimental study of spray-formation processes in twin-fluid jet-in-crossflow at jet-engine operating conditions

(Georgia Institute of Technology, 2017-01-05) Tan, Zu Puayen

The jet-in-crossflow (JICF) fuel-injection technique is widely applied in modern jet-engine fuel-air mixers to provide rapid fuel atomization and mixing. However, the “Classical” JICF places large amounts of fuel into the initial jet/spray’s recirculation zone and the wall boundary-layer, both of which can risk flashback and fuel-coking on the wall, particularly for next-generation jet-engines that will operate at increasingly higher pressures and temperatures. Twin-Fluid (TF) JICF, where streams of air are co-injected with the fuel jet into the crossflow, is being considered as a way to mitigate the Classical-JICF’s shortcomings. However, the TF-JICF is a nascent fuel-injection technique that is not well understood, especially at the high operating pressures of jet-engines. This dissertation reports an experimental investigation of TF-JICF where liquid Jet-A fuel was co-injected with pressurized nitrogen into a crossflow of air. The developed fuel sprays were characterized using shadowgraphy. The fuel-to-crossflow momentum-flux ratios were varied from J=5-40, the air-nozzles pressure-drops were varied from dP=0-150% of crossflow pressure, and the crossflow Weber numbers were varied from Wecf=175-1050. These operating conditions allowed us to obtain a dataset that is both comparable with near-atmospheric studies of TF-JICF in the literature and applicable to jet-engines. The results show that TF-JICF can be classified into four spray-formation regimes (i.e., Classical-JICF, Air-Assist JICF, Airblast JICF and Airblast Spray-in-Crossflow), each containing a unique set of spray characteristics and mechanisms. In the Air-Assist regime that spans dP≈3-13%, the injected air formed a protective air-sheath around the initial fuel jet, which inhibited the development of Rayleigh-Taylor waves and surface-shearing (i.e., disturbances created by the crossflow), thus reducing the near-wall fuel concentrations. Applying higher levels of dP transitioned the spray into the Airblast JICF regime, where the intensified fuel-air impingement and shearing generated new disturbances on the jet. These generally caused the near-wall regions to become repopulated with fuel droplets (i.e., counter-productive towards mitigating flashback and wall-coking). When dP was higher than 100%, the jet became completely atomized by air prior to encountering the crossflow, producing an “Airblast Spray-in-Crossflow”. The resulting spray-plume’s penetration became related to the combination of the fuel and air’s momentum-fluxes, where increasing dP caused increasing separation between the spray-plume and test-channel wall. This reduces the near-wall fuel concentrations and is beneficial towards fuel-air mixer design, although the required levels of dP for this regime is likely too high for practical jet-engine operation.
Investigation of driving mechanisms of combustion instabilities in liquid rocket engines via the dynamic mode decomposition

(Georgia Institute of Technology, 2015-08-20) Quinlan, John Mathew

Combustion instability due to feedback coupling between unsteady heat release and natural acoustic modes can cause catastrophic failure in liquid rocket engines and to predict and prevent these instabilities the mechanisms that drive them must be further elucidated. With this goal in mind, the objective of this thesis was to develop techniques that improve the understanding of the specific underlying physical processes involved in these driving mechanisms. In particular, this work sought to develop a small-scale, optically accessible liquid rocket engine simulator and to apply modern, high-speed diagnostic techniques to characterize the reacting flow and acoustic field within the simulator. Specifically, high-speed (10 kHz), simultaneous data were acquired while the simulator was experiencing a 170 Hz combustion instability using particle image velocimetry, OH planar laser induced fluorescence, CH* chemiluminescence, and dynamic pressure measurements. In addition, this work sought to develop approaches to reduce the large quantities of data acquired, extracting key physical phenomena involved in the driving mechanisms. The initial data reduction approach was chosen based on the fact that the combustion instability problem is often simplified to the point that it can be characterized by an approximately linear constant coefficient system of equations. Consistent with this simplification, the experimental data were analyzed by the dynamic mode decomposition method. The developed approach to apply the dynamic mode decomposition to simultaneously acquired data located a coupled hydrodynamic/combustion/acoustic mode at 1017 Hz. On the other hand, the dynamic mode decomposition's assumed constant operator approach failed to locate any modes of interest near 170 Hz. This led to the development of two new data analysis techniques based on the dynamic mode decomposition and Floquet theory that assume that the experiment is governed by a linear, periodic system of equations. The new periodic-operator data analysis techniques, the Floquet decomposition and the ensemble Floquet decomposition, approximate, from experimental data, the largest moduli Floquet multipliers, which determine the stability of the periodic solution trajectory of the system. The unstable experiment dataset was analyzed with these techniques and the ensemble Floquet decomposition analysis found a large modulus Floquet multiplier and associated mode with a frequency of 169.6 Hz. Furthermore, the approximate Rayleigh criterion indicated that this mode was unstable with respect to combustion instability. Overall, based on the positive finding that the ensemble Floquet decomposition was able to locate an unstable combustion mode at 170 Hz when the operator's time period was set to 1 ms, suggests that the dynamic mode decomposition based 1017 Hz mode parametrically forces the 170 Hz mode, resulting in what could be characterized as a parametric combustion instability.
Breakup characteristics of a liquid jet in subsonic crossflow

(Georgia Institute of Technology, 2012-05-18) Gopala, Yogish

This thesis describes an experimental investigation of the breakup processes involved in the formation of a spray created by a liquid jet injected into a gaseous crossflow. This work is motivated by the utilization of this method to inject fuel in combustors and afterburners of airplane engines. This study aims to develop better understanding of the spray breakup processes and provide better experimental inputs to improve the fidelity of numerical models. This work adresses two key research areas: determining the time required for a liquid column to break up in the crossflow (i.e., primary breakup time) and the effect of injector geometry on spray properties. A new diagnostic technique, the liquid jet light guiding technique that utilizes ability of the liquid jet to act as a waveguide for laser light was developed to determine the location where the liquid column breaks up, in order to obtain the primary breakup time. This study found that the liquid jet Reynolds number was an important factor that governed the primary breakup time and improved the existing correlation. Optical diagnostic techniques such as Phase Doppler Particle Analyzer, Liquid Jet Light Guiding Technique, Particle Image Velocimetry and Imaging techniques were employed to measure the spray properties that include spray penetration, droplet sizes and velocities, velocity field on the surface of the liquid jet and the location of the primary breakup time. These properties were measured for two injectors: one with a sharp transition and the other with a smooth transition. It was found that the spray created by the injector with a sharp transition forms large irregular structures while one with smooth transition produces a smooth liquid jet. The spray transition creates a spray that penetrates deeper into the crossflow, breakup up earlier and produces larger droplets. Additionally, this study reports the phenomenon of the liquid jet splitting into two or more jets in sprays created by the injector with a smooth transition.
A method for aircraft afterburner combustion without flameholders

(Georgia Institute of Technology, 2009-03-02) Birmaher, Shai

State of the art aircraft afterburners employ spray bars to inject fuel and flameholders to stabilize the combustion process. Such afterburner designs significantly increase the length (and thus weight), pressure losses, and observability of the engine. This thesis presents a feasibility study of a compact prime and trigger (PAT) afterburner concept that eliminates the fuel spray bars and flameholders and, thus, eliminates the above-mentioned problems. In this concept, afterburner fuel is injected just upstream or in between the turbine stages. Downstream of the turbine stages, a low power pilot, or trigger , can be used to control the combustion process. The envisioned trigger for the PAT concept is a jet of product gas from ultra-rich hydrocarbon/air combustion that is injected through the afterburner liner. This partial oxidation (POx) gas, which consists mostly of H2, CO, and diluents, rapidly produces radicals and heat that accelerate the autoignition of the primed mixture and, thus, provide an anchor point for the afterburner combustion process. The objective of this research was to demonstrate the feasibility of the PAT concept by showing that (1) combustion of fuel injected within or upstream of turbine stages can occur only downstream of the turbine stages, and (2) the combustion zone is compact, stable and efficient. This was accomplished using two experimental facilities, a developed theoretical model, and Chemkin simulations. The first facility, termed the Afterburner Facility (AF), simulated the bulk flow temperature, velocity and O2 content through a turbojet combustor, turbine stage and afterburner. The second facility, termed the Propane Autoignition Combustor (PAC), was essentially a scaled-down, simplified version of the AF. The developed model was used to predict and interpret the AF results and to study the feasibility of the PAT concept at pressures outside the AF operating range. Finally, the Chemkin simulations were used to study the effect of several POx gas compositions on the afterburner combustion process.
Stochastic dynamical system identification applied to combustor stability margin assessment

(Georgia Institute of Technology, 2008-12-16) Cordeiro, Helio de Miranda

A new approach was developed to determine the operational stability margin of a laboratory scale combustor. Applying modern and robust techniques and tools from Dynamical System Theory, the approach was based on three basic steps. In the first step, a gray-box thermoacoustical model for the combustor was derived. The second step consisted in applying System Identification techniques to experimental data in order to validate the model and estimate its parameters. The application of these techniques to experimental data under different operating conditions allowed us to determine the functional dependence of the model parameters upon changes in an experimental control parameter. Finally, the third step consisted in using that functional dependence to predict the response of the system at different operating conditions and, ultimately, estimate its operational stability margin. The results indicated that a low-order stochastic non-linear model, including two excited modes, has been identified and the combustor operational stability margin could be estimated by applying a continuation method.
A Numerical Investigation of a Thermodielectric Power Generation System

(Georgia Institute of Technology, 2005-11-17) Sklar, Akiva A.

The performance of a novel micro-thermodielectric power generation device (MTDPG) was investigated in order to determine if thermodielectric power generation can compete with current portable power generation technologies. Thermodielectric power generation is a direct energy conversion technology that converts heat directly into high voltage direct current. It requires dielectric (i.e., capacitive) materials whose charge storing capabilities are a function of temperature. This property is exploited by heating these materials after they are charged; as their temperature increases, their charge storage capability decreases, forcing them to eject a portion of their surface charge to an appropriate electronic storage device. Previously, predicting the performance of a thermodielectric power generator was hindered by a poor understanding of the materials thermodynamic properties and the affect unsteady heat transfer losses have on system performance. In order to improve predictive capabilities in this study, a thermodielectric equation of state was developed that describes the relationship between the applied electric field, the surface charge stored by the thermodielectric material, and its temperature. This state equation was then used to derive expressions for the material's thermodynamic states (internal energy, entropy), which were subsequently used to determine the optimum material properties for power generation. Next, a numerical simulation code was developed to determine the heat transfer capabilities of a micro-scale parallel plate heat recuperator (MPPHR), a device designed specifically to a) provide the unsteady heating and cooling necessary for thermodielectric power generation and b) minimize the unsteady heat transfer losses of the system. The previously derived thermodynamic equations were then incorporated into the numerical simulation code, creating a tool capable of determining the thermodynamic performance of an MTDPG, in terms of the thermal efficiency, percent Carnot efficiency, and energy/power density, when the material properties and the operating regime of the MPPHR were varied. The performance of the MTDPG was optimized for an operating temperature range of 300 500 K. The optimization predicted that the MTDPG could provide a thermal efficiency of 29.7 percent. This corresponds to 74.2 percent of the Carnot efficiency. The power density of this MTDPG depends on the operating frequency and can exceed 1,000,000 W/m3.
A Numerical Investigation on the Influence of Engine Shape and Mixing Processes on Wave Engine Performance

(Georgia Institute of Technology, 2005-01-12) Erickson, Robert R.

Wave engines are a class of unsteady, air-breathing propulsion devices that use an intermittent combustion process to generate thrust. The inherently simple mechanical design of the wave engine allows for a relatively low cost per unit propulsion system, yet unsatisfactory overall performance has severely limited the development of commercially successful wave engines. The primary objective of this investigation was to develop a more detailed physical understanding of the influence of gas dynamic nonlinearities, unsteady combustion processes, and engine shape on overall wave engine performance. Within this study, several numerical models were developed and applied to wave engines and related applications. The first portion of this investigation examined the influence of duct shape on driven oscillations in acoustic compression devices, which represent a simplified physical system closely related in several ways to the wave engine. A numerical model based on an application of the Galerkin method was developed to simulate large amplitude, one-dimensional acoustic waves driven in closed ducts. Results from this portion of the investigation showed that gas-dynamic nonlinearities significantly influence the properties of driven oscillations by transferring acoustic energy from the fundamental driven mode into higher harmonic modes. The second portion of this investigation presented and analyzed results from a numerical model of wave engine dynamics based on the quasi one-dimensional conservation equations in addition to separate sub-models for mixing and heat release. This model was then used to perform parametric studies of the characteristics of mixing and engine shape. The objectives of these studies were to determine the influence of mixing characteristics and engine shape on overall wave engine performance and to develop insight into the physical processes controlling overall performance trends. Results from this model showed that wave engine performance was strongly dependent on the coupling between the unsteady heat release that drives oscillations in the engine and the characteristics that determine the acoustic properties of the engine such as engine shape and mean property gradients. Simulation results showed that average thrust generation decreased dramatically when the natural acoustic mode frequencies of the engine and the frequency content of the unsteady heat release were not aligned.
Modeling of combustion instabilities and their active control in a gas fueled combustor

(Georgia Institute of Technology, 1998-08) Mohanraj, Rajendran