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Daniel Guggenheim School of Aerospace Engineering

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

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UNCERTAINTY QUANTIFICATION OF DIMP PYROLYSIS KINETICS

(Georgia Institute of Technology, 2022-05-13) Patel, Pavan

To develop effective explosives and strategies for the rapid destruction of sarin stockpiles, a reliable understanding of sarin’s chemical kinetics is needed. Kinetic mechanisms of sarin simulants such as di-isopropyl methyl phosphonate (DIMP) are developed instead because they have a similar chemical structure as sarin and are less toxic. A detailed DIMP kinetics mechanism has been developed in the past; however, there is a considerable amount of uncertainty surrounding it. This uncertainty manifests through the choice of pathways, and their respective reaction rates, leading to large variations in outcomes predicted through simulations. Out of the many reaction pathways involved in the decomposition of DIMP, the initiating steps are the most crucial. Out of the two possible initiating pathways in the destruction of DIMP, the lower activation energy pathway is dominant for all temperatures. The purpose of this study is to investigate the uncertainties associated with the dominant initiating pathways of the DIMP kinetics mechanism. Propagating rate parameter uncertainties of the dominant pathway through computational models yields large uncertainties in predicting DIMP survivability at different temperatures. The prediction uncertainties are larger at lower temperatures than at high temperatures. This can significantly impact the ability to precisely predict collateral damage caused by partially destroyed DIMP in the far-field of an explosion. After reducing these rate parameter uncertainties, using Bayesian inference, the prediction uncertainties were within reasonable limits. The results here provide a reduced subspace for uncertainties associated with the first and most important step in the breakdown of DIMP, which shall enable more reliable predictions.
Numerical simulations of real-gas flows with phase-equilibrium thermodynamics

(Georgia Institute of Technology, 2020-07-20) Tudisco, Principio

Motivated by the complex physics of multi-component mixtures in strongly non-ideal, real-gas (RG) conditions reported in the field of chemical engineering, this work aims to address the behavior of multi-phase thermodynamics from a broader point of view. The focus is to evaluate the differences, as well as the possible sources of errors that would arise in a computational fluid-dynamics (CFD) simulation when conventional single-phase and multi-phase equilibrium RG thermodynamics are employed: an area of research that despite the active interest in many communities (especially CFD), has not been completely understood. Knowledge of the effects that multi-phase RG thermodynamics with the assumption of vapor-liquid equilibrium (VLE) can have on a flow dynamics is important because it establishes the relevance of the fully coupled CFD-VLE solver. In fact, this relevance may go beyond the stand-alone calculation of a multi-phase state, providing important insights about the physics that may not be captured if the single-phase assumption is invoked. This work provides an extensive study of RG mixtures from a physical and numerical point of view. The difficulties associated with their modeling are discussed in detail and solutions are provided accordingly. Emphasis is given to the occurrence and suppression of numerical noise in form of pressure oscillations that can pollute the simulation to the point that it cannot be performed. Extension of existing models to eliminate such problem is achieved by incorporating the effects of VLE thermodynamics in a consistent manner, ultimately forming a new and robust tool to investigate the physics further. The resulting model is applied to non-reacting and reacting flows in canonical setups where emphasis is devoted to the discussion of the differences and sources of errors that would occur if this multi-phase behavior is not taken into account. Results show that the different thermodynamic states reached by this advanced model can have an impact on the flow physics, especially in a non-reacting (or more in general cold) regime. In particular, the strong non-linear coupling between the VLE thermodynamics and the transport properties is identified as a key element of difference with respect to the single-phase model counterpart. These differences manifest into the occurrence of localized changes in the fluid properties (such as density) that affect the flow-field in their vicinity, causing visible discrepancies even when time-averaging is performed. Concurrently, results obtained on the reacting side and carried out (for the first time) with finite-rate kinetics suggest that any VLE formation between the products and the reactants may be considered of minor importance. The latter conclusion is supported by the analysis conducted on the multi-phase field which appears to be largely composed of the vapor solution, as expected, hence limiting the analogous effect observed the non-reacting system where a broader range of phase-separation appears instead.
Reduced-order model for prediction of staged-combustor NOx emissions with detailed chemistry and finite-rate mixing

(Georgia Institute of Technology, 2020-04-24) Goh, Edwin

The ground power industry is targeting combined cycle plant efficiencies of 65% and above, which can be achieved primarily through higher combustor firing temperatures. Because conventional combustors fail to meet NOx regulations at such temperatures, there is a pressing need for high-temperature, low-emissions combustors. In this regard, the staged combustion architecture is one such concept that shows promise due to its enhanced emissions performance and operational flexibility. The prohibitive cost of building prototypes relegates full-scale testing to the final stages of the product design cycle, while accurate models with turbulence and detailed chemistry cannot be used to efficiently explore the design space. Therefore, an efficient computational model is necessary to study a broad range of architectures. Despite extensive research on staged combustion and the related jet-in-crossflow (JICF) problem, there is little published research regarding the minimum NOx levels achievable by staged combustion architectures. The first contribution of this thesis presents a set of fundamental minimum NOx levels that are obtained by wrapping a constrained optimization routine around a reduced-order staged combustor model. For a firing temperature of 1975 K which corresponds to 65% efficiency, the minimum NO levels are determined to be roughly 1 ppm. Sensitivities of these minimum NOx levels to operational, geometric and computational parameters are identified and discussed. Recognizing that a turbulent flow field affects NOx chemistry primarily through mixing, the second contribution presents a reduced-order Limited Mixing and Entrainment (LiME) reactor model to predict emissions based on mixing and entrainment time scales. Molecular mixing is simulated based on the Interaction by Exchange with the Mean (IEM) model through interacting Lagrangian particles. The consensus is that better JICF mixing leads to lower NOx emissions, but little work has been done to characterize the effects of large-scale entrainment and small-scale mixing on NOx in isolation. The third contribution elucidates the sensitivity of NOx to mixing and entrainment time scales using reduced-order models and demonstrates a potential use case of this model in a constrained design optimization problem to identify minimum NOx levels under fixed entrainment rates. The impact of fuel and air staging on NOx under mixing and entrainment-limited scenarios is elucidated.
Effects of preheat temperature and vitiation on reaction kinetics of higher hydrocarbon fuels

(Georgia Institute of Technology, 2019-07-26) Adusumilli, Sampath

Conducting full-scale experiments as part of the design process of jet engine combustors is a costly and time-consuming process. Therefore engine developers have been increasingly using numerical modeling approaches to assess new designs or design changes. The reaction chemistry, which is dependent on the flow conditions, the fuel composition, and the oxidizer composition, plays an important role in the accuracy of these simulations. The kinetic mechanisms that describe this chemistry need to be validated. Various global combustion characteristics are used to validate mechanisms against experimental data; one of these is laminar flame speed (SL). In this work, laminar flame speeds of various fuels relevant to jet engine combustion are measured using a previously developed, modified Bunsen Flame Technique (BFT). The accuracy of the BFT is examined here, both through a comparison to experimental results from other standard approaches for a range of fuels and through a detailed analysis of the impact of flame stretch. The measured flame speeds are also used to test leading chemical kinetic mechanisms, primarily the NUI and USC models. Laminar flame speeds of n-decane, ethylene and propylene are measured at conditions relevant to jet engine main combustors and afterburners. The experimental conditions include high preheat temperatures (up to 650 K) and reduced O2 levels (down to 15% mole fraction in the oxidizer); the latter is relevant to vitiation, where there is partial pre-burning of the oxidizing flow. Furthermore, vitiation introduces combustion products such as CO2 into the reactant stream that can participate in the combustion chemistry. Therefore, flame speeds are measured using dilution with both CO2 and N2 (considered non-reactive) to study these effects. SL measurements for alkenes using BFT are within 10% of measurements from literature and chemical kinetic mechanism predictions at 300 K and atmospheric pressure. At high preheat temperatures, the mechanisms accurately predict SL for ethylene mixtures, while they over predict SL of propylene mixtures at 650 K. Vitiation studies at 650 K preheat show that for N2 dilution and ethylene, the reduction in flame speed is mostly due to thermal effects. Some chemical effects were observed when the O2 level in the oxidizer was reduced to 15% (vol.). For propylene, reducing O2 had a larger impact on flame speed than that predicted by the mechanisms. With CO2 as a diluent, the mechanisms over predicted the flame speed, and the prediction error increased with higher levels of CO2. Reactions involving the allyl (C3H5-A) radical were identified as a likely source of the propylene flame speed errors, increase in the pre-exponential rate factor of the allyl-H recombination reaction improved the predictive capability of the mechanism at high preheat temperatures. Similarly, analysis of different sources of errors with CO2 dilution suggest the third-body efficiency of CO2 is underestimated in a three-body association (such as H + O2 (+M) <=> HO2 (+M)) type of reactions.
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.
Effect of jet fuel composition on forced ignition in gas turbine combustors

(Georgia Institute of Technology, 2019-01-22) Wei, Sheng

The rapid growth in the aviation industry means increasing consumption of jet fuels, which is leading to greater interest in alternate and sustainable fuel sources. The overall properties of these alternative fuels can be designed to meet existing standards. Nevertheless, the compositional differences between alternative and conventional fuels can lead to important variations in chemical and physical properties that impact engine performance. For example, ignition is of paramount importance to ensure reliable operation, especially in extreme conditions like cold starts and high altitude relights. For aircraft engines, ignition is the process of creating self-sustaining flames starting with a high-temperature source located near a combustor liner. This thesis is devoted to studying the differences in ignition behavior due to the variations in fuel composition. Fuel variations in ignition are studied in a well-characterized test facility that is readily amenable to modeling and simulation. The experiments employ a sunken-fire ignitor, like those typically employed in aircraft engines, operating at 15 Hz with ~1.25J spark energy. Performance differences among fuels are characterized through their ignition probabilities. To understand both the chemical and physical fuel effects on ignition, both prevaporized fuels and liquid fuel sprays are examined. The purpose of prevaporizing the fuel is to remove the process of liquid to gas transition and to focus on combustion chemistry alone. In the forced ignition of liquid fuel sprays, which mimics the situation encountered in aviation gas turbine engines, both physical and chemical properties of the fuel are relevant. Statistically significant differences between fuel ignition probabilities are observed. The droplet heating time is shown to correlate well with ignition probability. A particle Doppler phase analyzer (PDPA) is used to study droplet size distribution near the ignitor. These droplet distribution measurements can be useful for future CFD modeling. In addition to differentiating fuel performances through ignition probability, advanced diagnostic techniques are employed to understand the evolution of a spark kernels as it interacts with combustible mixtures. These techniques include high speed OH planar laser induced fluorescence, OH* chemiluminescence, and schlieren imaging. The results reveal the entrainment of ambient fluid into the convecting spark kernel, the decomposition of vaporized jet fuel in the high temperature kernel, and the transition from local “hot spots” within the spark kernel to a self-sustaining flame. In addition to the experiments, reduced order modeling is used to better understand the physics and chemistry of ignition for both prevaporized and liquid fuels. Chemical differences are found to depend on the relative distribution between intermediate breakdown products (e.g., ethylene, propene and isobutene) from the parent fuels, as these intermediates have drastically different chemical rates as a function of temperature. The energy transfer mechanisms important in the ignition of liquid fuel sprays are also identified. The chemical heat release and the dilution cooling rates are orders of magnitudes larger than the heat required for the droplets’ heating and vaporization. However, the droplet heating time is shown to have the largest impact on ignition performance
Combustion and flow characteristics of staged combustors involving confined jets in crossflow

(Georgia Institute of Technology, 2019-01-14) Jain, Nishant

Staged combustion offers many advantages in high performance aero-propulsion and power generation applications of gas turbine engines. For example, staged combustors can operate at low overall equivalence-ratio and temperature, thereby, pollutant emissions, while maintaining robustness, e.g., ignitability and flame stability over a greater operational range. To be effective, axial staging approaches require rapid mixing and burning of the staged reactants with the vitiated products from the pilot zone. In practice, this is achieved by utilizing a multiple jets-in-cross-flow (JICF) configuration in a highly reactive and confined combustor environment. While most previous work has focused on studying the properties of single, unconfined JICF, there is a paucity of work employing detailed diagnostics to study multiple and confined JICF (CJICF). This thesis examines the mixing, velocity and combustion characteristics of CJICF in air-staged (Rich-Quench-Lean, RQL), and fuel-air-staged (Lean-Quench-Lean, LQL) configurations using natural gas and air at atmospheric pressure and high temperature conditions. The well-characterized facility developed for this study allows for injection from five round jets, each produced by sudden contraction; two jets from the top wall and three interlaced jets from the bottom wall, with independent control of each set. Results are presented for parallel (one-sided injection), and staggered-opposed (two-sided injection) jets in a vitiated crossflow. High speed (10 kHz) stereo particle image velocimetry results are used to elucidate the mixing and flow characteristics, while OH* chemiluminescence imaging is used to study the combustion zone. Chemical reactor modeling is also used to help interpret the combustion results. For multiple confined, high momentum jets, the jet-wall and, to a lesser extent, the jet-jet interactions are found to have a major influence on the flowfield and the mixing characteristics of the jets with the crossflow. For example, the stagnation region where a jet interact with the opposite wall creates an upstream recirculation zone that redirects the crossflow away from the wall. Downstream of the jets, the crossflow can rapidly mix with jet fluid, which is even more noticeable in the regions between the jets due to lateral movement of jet fluid as it is redirected near the wall stagnation zone. The jet-wall impact appears to be more influenced by the total mass injection (or air split ratio in this study) rather than the momentum flux ratio, which is the parameter considered most influential for single, unconfined JICF configurations. In RQL conditions, with the high temperature crossflow containing H2 and CO, chemical times (autoignition delays) are sufficiently fast (~1-100μs), such that flames are stabilized near the jet exits and combustion is found to be mixing limited rather than chemistry limited. For LQL conditions, most of the burning likely occurs due to flame propagation, though a sufficiently high temperature crossflow can lead to enhanced flame stabilization, and burning of the premixed jets before significant mixing with the crossflow. Thus when stabilized in a high temperature crossflow, the LQL jets can burn in the opposed wall stagnation region, while the RQL burning is delayed until mixing with the crossflow occurs.
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.
Dynamics of Harmonically Forced Nonpremixed Flames

(Georgia Institute of Technology, 2016-04-19) Magina, Nicholas A

This thesis describes the dynamics, both spatio-temporal and heat release, of harmonically excited non-premixed flames. Analytical, numerical, computational, and, experimental analyses were performed, along with combined analyses methods, to study excitation and evolution of wrinkles on the flame front. Explicit expressions for the dynamics were developed. Wrinkle convection at the mean axial flow speed, and wrinkle dissipation and dispersion were analytically identified in the Pe-->∞ and Pe>>1 limits, respectively. Altered inlet mixture fraction profiles and attachment point dynamics were shown to accompany axial diffusion effects. Some physical effects such as axial diffusion, forcing configuration, and anisotropic diffusion altered the wrinkle interference pattern/waveform characteristics, while others, such as confinement, dimensionality, and differential diffusion, altered the dynamics through modifying the mean flame location. Comparisons to established premixed flame dynamics were made throughout. Despite having similar space-time dynamics, the heat release dynamics of the two differed greatly, having different dominant contributions, as well as different asymptotic trends. Experimental results obtained validated previous findings as well as enabled advanced model development, revealing the importance of accurate mixture fraction field capture, particularly in the near burner exit region. Findings shed light onto model and predictive improvements for future works.
High energy spark ignition in non-premixed flowing combustors

(Georgia Institute of Technology, 2014-08-22) Sforzo, Brandon Anthony

In many practical combustion devices, including those used in gas turbine engines for aircraft and power generation, a high energy spark kernel is necessary to reliably ignite the turbulently flowing flammable gases. Complicating matters, the spark kernel is sometimes generated in a region where a non-flammable mixture is present, or where there is no fuel at all. This requires the spark kernel to travel to a flammable region before rapid combustion can begin in non-premixed or stratified flows. This transit time allows for chemical reactions to take place within the kernel as well as mixing with surrounding gases. Despite these demanding conditions, the majority of research in ignition has been for low energy sparks and premixed conditions, not resembling those found in many combustion devices. Similarly, there is little work addressing this issue of spark kernel evolution in the non-premixed flowing environment, and none available that control the time allowed for transit. The goal of this thesis is to understand the development of a spark kernel issued into a non-premixed flow and the sensitivities of the ignition process. To this effect, a stratified flow facility for ignition experiments has been fabricated utilizing a high speed schlieren and emission imaging system for visualizing the kernel motion and ignition success. Additionally, OH chemiluminescence and CH PLIF were used to track chemical species during the ignition process. This facility is also used to control the important variables regarding the flow and spark kernel interaction to quantify the influence on ignition probability. A reduced order model employing a perfectly stirred reactor (PSR) has also been developed based on experimental observations of the entrainment of fluid into the evolving kernel. The simulations provide additional insight to the chemical development in the kernel under different input conditions. This model was enhanced by introducing random perturbations to the input variables, mimicking a practical situation. A computationally efficient support vector machine was trained to replicate the numerical model outputs and predict ignition probabilities for nominal input conditions, providing comparison to experimental results. Experimental and numerical results show that initial mixing with non-flammable fluid quickly reduces the ability for the kernel to ignite the flammable flow, resulting in a strong influence of the inlet temperature and the kernel transit time on the probability of ignition. Once the kernel reaches the flammable mixture, entrainment of this flow occurs, which requires on the order of a vortex turn-over time before chemistry can begin. Initial chemical reactions include endothermic fuel decomposition, further reducing the kernel temperature prior to heat release, creating a competition between the cooling effect of additional mass entrainment and the delayed heat release reactions. CH PLIF results show that flame chemistry is initially confined to a thin region that corresponds to the interface layer where the flammable gases mix with the hot kernel fluid from the vortex entrainment of ambient gas. The dependence of the ignition probability to variations in flow conditions is captured reasonably well by the reduced order model, validating the PSR approach and the probability prediction tool. The development of this reduced order model is a major contribution of this work with the ability to predict the effects of the important physical ignition processes, which can be used when considering an ignition system's feasibility. This work will provide knowledge to guide the use and design practices in industry, as well as a simple model to test ignition feasibility based on mixing, entrainment, and chemical reactions. Furthermore, the flow facility is well characterized, and a database has been developed that can provide validation points for future computational simulations. Future modeling will be important to further understand fluid dynamic effects that are difficult to measure experimentally, and study a broader range of conditions.