A method for aircraft afterburner combustion without flameholders

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.Ph.D.Committee Chair: Zinn, Ben; Committee Member: Fuller, Thomas; Committee Member: Gaeta, Rick; Committee Member: Jagoda, Jeff; Committee Member: Neumeier, Yedidi

Identification and detection of anomalies through SSME data analysis

The goal of the ongoing research described in this paper is to analyze real-time ground test data in order to identify patterns associated with the anomalous engine behavior, and on the basis of this analysis to develop an expert system which detects anomalous engine behavior in the early stages of fault development. A prototype of the expert system has been developed and tested on the high frequency data of two SSME tests, namely Test #901-0516 and Test #904-044. The comparison of our results with the post-test analyses indicates that the expert system detected the presence of the anomalies in a significantly early stage of fault development

A Comparative Analysis of Design Techniques for the Construction of an Expert System for Aircraft Engine Diagnostics

The lack of knowledge and understanding of diagnostic aircraft propulsion systems causes inappropriate problem diagnosis. Because of increasing complexity, technicians are incapable of performing the necessary tasks in accordance with standard regulations. More sophisticated systems are needed today to assist the user technician in decision-making. This work provided a study of rule-based and frame-based expert system techniques to determine the most appropriate solution in the domain of complex diagnosis using large amounts of deterministic data. The study produced a framework that facilitates the diagnosing of faults on aircraft engines, thus reducing the burden on the aircraft mechanic regardless of experience level. An intelligent system, the Virtually Automated Maintenance Analysis System (V AMAS), was created as a test model. It was used to compare the relative efficiency of the different expert systems techniques and the effectiveness of expert systems. One aviation malfunction problem was identified. Information collected for the Main Ignition Malfunction was developed into question sets and coded. Six specific subsets of problems were addressed. This research compared the rule-based and frame-based knowledge representation techniques using a set of evaluation factors: computational efficiency, correctness, expressiveness, and consistency. From the analysis it was concluded that the frame based knowledge representation technique ranked higher than the rule-based representation, and is suitable for use with an expert system to represent an aircraft propulsion system \u27s deterministic data

Aeronautical engineering: A continuing bibliography with indexes (supplement 257)

This bibliography lists 560 reports, articles, and other documents introduced into the NASA scientific and technical information system in September 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Effects of Fuel Unsaturation on Pollutant Emissions from the Laminar Flames of Prevaporized Petroleum and Biodiesel Blends in Air

Biodiesels, considered as alternative fuels to petroleum diesel, are defined as fatty acid methyl or ethyl esters derived from triglycerides of vegetable oils or animal fats. The utilization of biodiesels reduces greenhouse gas emissions, assists in sustainable energy development, and enhances energy independence due to the renewable and biodegradable nature of these fuels. Besides being close to environmentally carbon-neutral, biodiesels have properties similar to those of petroleum fuels with comparable energy content and can be blended with petroleum fuels and used in existing engines without major modifications. Furthermore, they contain fuel-bound oxygen while being free of aromatic content; therefore, blends of biodiesels and petroleum fuels present the capability of reducing soot emissions from engines. Blending of biodiesels with petroleum fuels is considered feasible in the near term due to limited current availability of the commercial biodiesels and the lack of experience on the long term effects of storage, handling, transportation and combustion of these biodiesels and blends on the engines and the environment. Several studies in engine literature have revealed that the use of biodiesels and their blends in a compression ignition engine resulted in an appreciable reduction in the emissions of particulate matter (PM), unburnt hydrocarbons (UHC) and CO, compared to the use of diesel fuel. However, in case of nitric oxides (NOx) emissions, the results are variable and case dependent. The average effect of biodiesel on NOx emission was seen to be small, but with a high variance, which resulted in difficulty in discerning a clear pattern. Nitric oxides are categorized as one of the key pollutants in engine emissions that can affect human respiratory system and vegetation. Therefore, it is crucial to understand the effect of various fuel and engine operating parameters on biodiesel NOx emissions to develop enhanced mitigation and abatement techniques for the widespread use of biodiesels in transportation. In engine literature, fuel unsaturation has been attributed to the observed change in NOx emissions with the use of biodiesels in compression ignition engines. Several results indicated the existence of a strong relationship between NOx emissions and iodine number, used as a measure of the fuel unsaturation of vegetable oils and fatty acid methyl esters. However, relevance of iodine number as a measure of total unsaturation of petroleum fuels like diesel, Jet A and their blends with biodiesels is debatable due to the significant differences in the reactivity of iodine with petroleum fuels. Bromine number, used as a measure of aliphatic unsaturation in petrofuel samples, does not account for the aromatic unsaturation from petroleum fuels. Hence, a common parameter that is relevant for both biodiesels and petroleum fuels needs to be identified to quantify the fuel unsaturation. A parameter, termed “Degree of Unsaturation (DOU),” which accounts for the total unsaturation of the fuel from all sources such as double and triple bonds, aromatics and other ring structures irrespective of the families of the fuels (alkanes, alkenes, alkynes, aromatics, ether or ester) that has been used in organic chemistry literature is proposed in this work and identified as a potential indicator of NOx emissions from biodiesel blends. In this dissertation work, experimental correlations between DOU and the NO emission index on a mass basis (EINO) in laminar flames of neat prevaporized fuels such as methyl oleate (MO), soy methyl ester (SME), canola methyl ester (CME), rapeseed methyl ester (RME), palm methyl ester (PME), heptane, toluene, diesel, JetA and petroleum/biodiesel blends at various equivalence ratios (Ф = 0.9, 1.0, 1.2 and 1.5) are developed. The NO emission index of flames of biodiesel/petroleum blends was found to increase with DOU, but with varying trends depending on their families of origin. The effects of DOU on EINO were significantly influenced by the equivalence ratio, with the maximum influence at an equivalence ratio of 1.2. At the equivalence ratio (Ф) of 1.2, EINO increased from 2.4 g/kg at a DOU value of 1.7 to 4.4 g/kg at a DOU of 3.0 among biodiesels and their blends with petroleum fuel; toluene flame (100% aromatic content with a DOU of 4) produced an EINO of 6.94 g/kg. It is found that both NO and CO emission indices from the tested flames are influenced by two major parameters - equivalence ratio and total fuel unsaturation. Further, the presence of fuel aromatic content and the family of fuel were observed to significantly influence NOx formation particularly near stoichiometric equivalence ratios. Based on both global and inflame emission results along with the numerical analysis of tested flames, it is concluded that fuel unsaturation, fuel aromatic content, equivalence ratio and family of the respective fuel, together influence the NOx emissions in flames. The net effects of these parameters at a given condition establish the amount of EINO produced from the corresponding flames due to the fuel chemistry effect alone. Hence, DOU provides a common platform to compare and quantify the effects of fuel unsaturation across different fuel families and can be employed as an indicator of NOx emissions. DOU can be evaluated based on the average molecular formula of the fuel alone without involving complex and expensive experimental procedures such as those involved in the measurement of iodine number. The propensity of a biofuel blend for NOx emissions during combustion can be quickly ascertained with the successful development of Degree of Unsaturation (DOU) parameter, thus, providing a valuable tool for fuel developers

Simulation of multicomponent spray combustion in gas turbine engines

Liquid fuels are the dominant source of energy from combustion and will continue to be so until the maturity of emerging technologies. During this transition phase the use of Sustainable Aviation Fuels (SAF) as blends or in totality inside existing infrastructure is an attractive option. The operational aspects of these new fuels inside the combustion chambers are not known in detail. Further, gas turbine engines operate under high pressure ratios and lean conditions to achieve emission targets making them susceptible to thermo-acoustic oscillations. Large Eddy Simulations (LES) have proven to be a successful tool in understanding fuel combustion processes. The focus of this thesis is on the modelling and simulation of complex multicomponent spray flame combustion in realistic systems. First step deals with the multi-component evaporation of the liquid fuel. Realistic fuels have hundreds of components each with their vapourisation characteristics. The Abramzon-Siringnano evaporation in the AVBP solver is extended to handle this complex compositional aspect of realistic fuels. Comparison of the implemented model with experimental and numerical studies show a good agreement and ability to capture the preferential evaporation characteristics of multicomponent fuels. Second, this multicomponent evaporation model is used in a canonical 1D laminar spray flame setup. A three-component jet fuel surrogate is coupled with Analytically Reduced Chemistry (ARC) to study the effects of droplet sizes, equivalence ratios and relative velocities on the spray flame structures. Correlations developed to estimate the spray flame speed agree with the numerical experiments indicating the correct physical parameters have been chosen to describe multicomponent spray flame propagation. Third part of the thesis deals with the simulations of swirled multicomponent spray flames in a large-scale LOTAR configuration. A three component description of conventional jet fuel and sustainable aviation fuel spray is coupled with turbulent combustion models and complex chemistry description to perform 3D-LES. The fuels composition effects on the overall vapour distribution and its effects on the spray flame structure indicate the role of preferential evaporation on flame stabilisation and combustion regimes. Finally, the forced response of the spray flame in the configuration is studied. The flame transfer function extracted using global chemistry agrees well with the experimental trends. Varying injection patterns to account for the effects of forcing on the droplet distribution shows a change in the flame response. The multicomponent spray flame response shows a strong role of composition and volatility of the fuel components

Thermoacoustic Oscillations in Supercritical Fluid Flows

Pressure oscillations in supercritical Jet-A fuel flowing through four parallel, heated tubes connected to common manifolds have been observed in this study. Tests were performed with fuel inlet temperatures ranging from 70F to 700F, and fuel pressures ranging from 360-700 psi. Total fuel flow rate ranged from 5-55 lb/hr. Tubes were heated by blowing 800-950F nitrogen over them. Acoustic-mode oscillations, typically ranging from 100-500 Hz, occurred only when a large temperature gradient was created inside the heated fuel tubes. Pressure oscillation amplitudes ranged from 0.1-1.0 psi. Oscillations at the inlet and outlet manifolds that were caused by a mode with the characteristic length of a single fuel tube were separated by a phase lag that was a function of the manifold cross-passage diameter. A lower-frequency mode was also observed, which had a characteristic length based of the summed lengths of a single fuel tube and a single manifold passage. An acoustic simulation using the COMSOL Acoustics Module was performed to predict frequencies based on geometry and flow conditions of the experiment

Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations

Most energy requirements of modern life can be fulfilled by renewable energy sources, but it is impossible in the near future to provide an alternative energy source to combustion for airplanes. That being said, combustion in aviation can be made more sustainable by using alternative jet fuels, which are made from renewable sources like agricultural wastes, solid wastes, oils, and sugars. These alternative fuels can be used in commercial flights only after a long certification process by the Federal Aviation Agency (FAA) and ASTM International. Unfortunately, in over 50 years of fuel research, only five fuels have been certified. This research project aims to speed up the certification process with quicker testing of alternative fuels. Engine testing and even laboratory testing require large amounts of time and fuel. Simulations can make the process much more efficient, but accurately simulating highly turbulent flames in such complex geometries would need large amounts of computational resources. The goal of this thesis is to create an efficient computational framework, that can replicate different engine-like turbulent flow conditions in simple geometries with numerical tractability. The central idea is to decompose the flow field into ensemble mean and fluctuating quantities. The simulations then resolve only the fluctuations using simple computational domains, while emulating the effect of the mean flow using "forcing" terms. These forcing terms are calculated first for incompressible turbulence, and this method is later extended to turbulent reacting flows. In incompressible turbulence, Direct Numerical Simulations (DNS) performed on simple triply periodic cubic domains reasonably capture the statistically stationary shear turbulence, that is observed in free shear flows. The simulations are also performed in cuboidal domains, that are longer in one direction and with an inflow/outflow along it. Both changes are observed to not have a significant impact on the turbulence statistics. Finally, shear convection is applied to the turbulence simulations with inflow/outflow, which has a significant impact on the turbulence. These simulations accurately capture the turbulence anisotropy in free-shear flows. The study is extended to DNS of highly turbulent n-heptane-air flames performed under different flow conditions. Turbulent flames involve two-way coupling between fluid mechanics and combustion. The effects of the flame on the turbulence and the impact of the turbulent flow conditions on the flame behavior are analyzed. The focus is placed on the effects of turbulence production, shear convection, and pressure gradients. The anisotropy produced in the turbulence due to the different flow conditions and the flame are also compared and contrasted. While the global behavior and flow anisotropy were affected by these conditions, the local chemistry effects were unaffected, and depend only on the laminar flame properties and turbulence intensity. These findings can help predict turbulent flame behavior, and can expedite the search and testing of sustainable alternatives to conventional jet fuels.</p

Large eddy simulation of supersonic combustion with application to scramjet engines

This work evaluates the capabilities of the RANS and LES techniques for the simulation of high speed reacting flows. These methods are used to gain further insight into the physics encountered and regimes present in supersonic combustion. The target application of this research is the scramjet engine, a propulsion system of great promise for efficient hypersonic flight. In order to conduct this work a new highly parallelised code, PULSAR, is developed. PULSAR is capable of simulating complex chemistry combustion in highly compressible flows, based on a second order upwind method to provide a monotonic solution in the presence of high gradient physics. Through the simulation of a non-reacting supersonic coaxial helium jet the RANS method is shown to be sensitive to constants involved in the modelling process. The LES technique is more computationally demanding but is shown to be much less sensitive to these model parameters. Nevertheless, LES results are shown to be sensitive to the nature of turbulence at the inflow; however this information can be experimentally obtained. The SCHOLAR test case is used to validate the reacting aspects of PULSAR. Comparing RANS results from laminar chemistry and assumed PDF combustion model simulations, the influence of turbulence-chemistry interactions in supersonic combustion is shown to be small. In the presence of reactions, the RANS results are sensitive to inflow turbulence, due to its influence on mixing. From complex chemistry simulations the combustion behaviour is evaluated to sit between the flamelet and distributed reaction regimes. LES results allow an evaluation of the physics involved, with a pair of coherent vortices identified as the dominant influence on mixing for the oblique wall fuel injection method. It is shown that inflow turbulence has a significant impact on the behaviour of these vortices and hence it is vital for turbulence intensities and length scales to be measured by experimentalists, in order for accurate simulations to be possible.This work was supported by an EPSRC DTA studentship [grant number RG50668

Center for Advanced Space Propulsion Second Annual Technical Symposium Proceedings

The proceedings for the Center for Advanced Space Propulsion Second Annual Technical Symposium are divided as follows: Chemical Propulsion, CFD; Space Propulsion; Electric Propulsion; Artificial Intelligence; Low-G Fluid Management; and Rocket Engine Materials