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At least 1,207 records · Page 67

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NASA NTRS1995

Design and development of an all-solid-state laser unit for microgravity combustion applications

The laser-induced fluorescence (LIF) technique is a sensitive and noninvasive method for imaging of flames and for monitoring of temperature and the presence of transient molecular species in flames. Our research effort has two major objectives: (1) to use LIF to study the fundamentals of microgravity combustion via spectroscopic characterization of free radicals, and (2) to design and develop an all-solid-state portable laser unit for combustion studies in a microgravity environment. Well-characterized free radicals, namely hydroxyl (OH) and methoxy (CH3O), have analytically convenient bands that can be laser-excited in the 280-298 nm spectral region with the newly discovered solid-state tunable laser, which is based on the LiCaAIF6:Ce3+(LiCAF:Ce) single crystal pumped by the quadrupled (266 nm) output of a Q-switched YAG:Nd laser. The 266 nm YAG radiation (of about 10 mJ pulse energy) was split into two beams of about equal intensity, one used for photolysis of the free radical precursor and the other used for pumping the laser crystal. Tunability of the LiCAF:Ce laser was provided by a step-motor-driven Littrow-mounted diffraction grating. Laser excitation spectra of the CH3O radical was recorded in the 291.5-296.5 nm region with 0.15 cm(exp -1) resolution. Laboratory experiments have thus successfully shown that the idea of using the LiCAF:Ce laser as a dual-purpose photolysis and excitation source can pay rich dividends and that such a laser can be the center piece of an all-solid-state portable device that can be used for routine analytical investigations of microgravity combustion phenomena.

Kamal, Mohammed M.↗

NASA NTRS1995

Supersonic combustion ramjet propulsion experiments in a shock tunnel

Measurements have been made of the propulsive effect of supersonic combustion ramjets incorporated into a simple axisymmetric model in a free piston shock tunnel. The nominal Mach number was 6, and the stagnation enthalpy varied from 2.8 MJ kg(exp -1) to 8.5 MJ kg(exp -1). A mixture of 13 percent silane and 87 percent hydrogen was used as fuel, and experiments were conducted at equivalence ratios up to approximately 0.8. The measurements involved the axial force on the model, and were made using a stress wave force balance, which is a recently developed technique for measuring forces in shock tunnels. A net thrust was experienced up to a stagnation enthalpy of 3.7 MJ kg(exp -1), but as the stagnation enthalpy increased, an increasing net drag was recorded. pitot and static pressure measurements showed that the combustion was supersonic. The results were found to compare satisfactorily with predictions based on established theoretical models, used with some simplifying approximations. The rapid reduction of net thrust with increasing stagnation enthalpy was seen to arise from increasing precombustion temperature, showing the need to control this variable if thrust performance was to be maintained over a range of stagnation enthalpies. Both the inviscid and viscous drag were seen to be relatively insensitive to stagnation enthalpy, with the combustion chambers making a particularly significant contribution to drag. The maximum fuel specific impulse achieved in the experiments was only 175 sec., but the theory indicates that there is considerable scope for improvement on this through aerodynamic design.

Paull, A.↗

NASA NTRS1995

The 3rd International Microgravity Combustion Workshop

This Conference Publication contains 71 papers presented at the Third International Microgravity Combustion Workshop held in Cleveland, Ohio, from April 11 to 13, 1995. The purpose of the workshop was twofold: to exchange information about the progress and promise of combustion science in microgravity and to provide a forum to discuss which areas in microgravity combustion science need to be expanded profitably and which should be included in upcoming NASA Research Announcements (NRA).

Howard D Ross↗

NASA NTRS1995

Microgravity smoldering combustion on the USML-1 Space Shuttle mission

Preliminary results from an experimental study of the smolder characteristics of a porous combustible material (flexible polyurethane foam) in normal and microgravity are presented. The experiments, limited in fuel sample size and power available for ignition, show that the smolder process was primarily controlled by heat losses from the reaction to the surrounding environment In microgravity, the reduced heat losses due to the absence of natural convection result in only slightly higher temperatures in the quiescent microgravity test than in normal gravity, but a dramatically larger production of combustion products in all microgravity tests. Particularly significant is the proportionately larger amount of carbon monoxide and light organic compounds produced in microgravity, despite comparable temperatures and similar char patterns. This excessive production of fuel-rich combustion products may be a generic characteristic of smoldering polyurethane in microgravity, with an associated increase in the toxic hazard of smolder in spacecraft.

Stocker, Dennis P.↗

NASA NTRS1995

Combustion synthesis of ceramic-metal composite materials in microgravity

Combustion synthesis, self-propagating high temperature synthesis (SHS) or reactive synthesis provides an attractive alternative to conventional methods of producing advanced materials since this technology is based on the ability of highly exothermic reactions to be self sustaining and, therefore, energetically efficient. The exothermic SHS reaction is initiated at the ignition temperature, T(sub ig), and generates heat which is manifested in a maximum or combustion temperature, T(sub c), which can exceed 3000 K . Such high combustion temperatures are capable of melting and/or volatilizing reactant and product species and, therefore, present an opportunity for producing structure and property modification and control through liquid-solid, vapor-liquid-solid, and vapor-solid transformations.

Moore, John↗

NASA NTRS1995

Quantitative measurement of oxygen in microgravity combustion

This research combines two innovations in an experimental system which should result in a new capability for quantitative, nonintrusive measurement of major combustion species. Using a newly available vertical cavity surface-emitting diode laser (VCSEL) and an improved spatial scanning method, we plan to measure the temporal and spatial profiles of the concentrations and temperatures of molecular oxygen in a candle flame and in a solid fuel (cellulose sheet) system. The required sensitivity for detecting oxygen is achieved by the use of high frequency wavelength modulation spectroscopy (WMS). Measurements will be performed in the NASA Lewis 2.2-second Drop Tower Facility. The objective of this research is twofold. First, we want to develop a better understanding of the relative roles of diffusion and reaction of oxygen in microgravity combustion. As the primary oxidizer species, oxygen plays a major role in controlling the observed properties of flames, including flame front speed (in solid or liquid flames), extinguishment characteristics, flame size, and flame temperature. The second objective is to develop better diagnostics based on diode laser absorption which can be of real value in microgravity combustion research. We will also demonstrate diode lasers' potential usefulness for compact, intrinsically-safe monitoring sensors aboard spacecraft. Such sensors could be used to monitor any of the major cabin gases as well as important pollutants.

Silver, Joel A.↗

NASA NTRS1990

Combustion and flow modelling applied to the OMV VTE

A predictive tool for hypergolic bipropellant spray combustion and flow evolution in the OMV VTE (orbital maneuvering vehicle variable thrust engine) is described. It encompasses a computational technique for the gas phase governing equations, a discrete particle method for liquid bipropellant sprays, and constitutive models for combustion chemistry, interphase exchanges, and unlike impinging liquid hypergolic stream interactions. Emphasis is placed on the phenomenological modelling of the hypergolic liquid bipropellant gasification processes. An application to the OMV VTE combustion chamber is given in order to show some of the capabilities and inadequacies of this tool.

Larosiliere, Louis M.↗

NASA NTRS1995

Spray combustion stability

The central purpose of this project is the improvement of liquid-fueled rocket motor design technology in order to assist the establishment of economical commercial access to space through the development of engines with enhanced performance and reliability. Specific research effort in the project is focused on spray physics and associated combustion instability phenomena. Results garnered from this work will contribute to the development of new computational tools for design of stable liquid propellant rocket engines. The specific objectives of the research effort include identifying and evaluating physical submodels which pertain to spray combustion stability with the idea of enhancing or refining existing submodels with a more comprehensive approach. In particular, any refinements to the spray combustion physical submodels which are achieved during the project will be channeled back to Rocketdyne for incorporation in their ARICC liquid rocket combustor code as second generation improvements. Also, as the ARICC code forms the basis or future CFD development, some effort is devoted to an evaluation of the code's capability for modeling oscillating pressure waves within the combustor.

Liang, Pak-Yan↗

NASA NTRS1996

Investigation of shock-induced combustion past blunt projectiles

A numerical study is conducted to simulate shock-induced combustion in premixed hydrogen-air mixtures at various free-stream conditions and parameters. Two-dimensional axisymmetric, reacting viscous flow over blunt projectiles is computed to study shock-induced combustion at Mach 5.11 and Mach 6.46 in hydrogen-air mixture. A seven-species, seven reactions finite rate hydrogen-air chemical reaction mechanism is used combined with a finite-difference, shock-fitting method to solve the complete set of Navier-Stokes and species conservation equations. The study has allowed an improved understanding of the physics of shock-induced combustion over blunt projectiles and the numerical results can now be explained more readily with one-dimensional wave-interaction model.

Ahuja, J. K.↗

NASA NTRS1996

A numerical investigation of premixed combustion in wave rotors

Wave rotor cycles which utilize premixed combustion processes within the passages are examined numerically using a one-dimensional CFD-based simulation. Internal-combustion wave rotors are envisioned for use as pressure-gain combustors in gas turbine engines. The simulation methodology is described, including a presentation of the assumed governing equations for the flow and reaction in the channels, the numerical integration method used, and the modeling of external components such as recirculation ducts. A number of cycle simulations are then presented which illustrate both turbulent-deflagration and detonation modes of combustion. Estimates of performance and rotor wall temperatures for the various cycles are made, and the advantages and disadvantages of each are discussed.

Nalim, M. Razi↗

NASA NTRS1996

Spray Combustion Modeling with VOF and Finite-Rate Chemistry

A spray atomization and combustion model is developed based on the volume-of-fluid (VOF) transport equation with finite-rate chemistry model. The gas-liquid interface mass, momentum and energy conservation laws are modeled by continuum surface force mechanisms. A new solution method is developed such that the present VOF model can be applied for all-speed range flows. The objectives of the present study are: (1) to develop and verify the fractional volume-of-fluid (VOF) cell partitioning approach into a predictor-corrector algorithm to deal with multiphase (gas-liquid) free surface flow problems; (2) to implement the developed unified algorithm in a general purpose computational fluid dynamics (CFD) code, Finite Difference Navier-Stokes (FDNS), with droplet dynamics and finite-rate chemistry models; and (3) to demonstrate the effectiveness of the present approach by simulating benchmark problems of jet breakup/spray atomization and combustion. Modeling multiphase fluid flows poses a significant challenge because a required boundary must be applied to a transient, irregular surface that is discontinuous, and the flow regimes considered can range from incompressible to highspeed compressible flows. The flow-process modeling is further complicated by surface tension, interfacial heat and mass transfer, spray formation and turbulence, and their interactions. The major contribution of the present method is to combine the novel feature of the Volume of Fluid (VOF) method and the Eulerian/Lagrangian method into a unified algorithm for efficient noniterative, time-accurate calculations of multiphase free surface flows valid at all speeds. The proposed method reformulated the VOF equation to strongly couple two distinct phases (liquid and gas), and tracks droplets on a Lagrangian frame when spray model is required, using a unified predictor-corrector technique to account for the non-linear linkages through the convective contributions of VOF. The discontinuities within the sharp interface will be modeled as a volume force to avoid stiffness. Formations of droplets, tracking of droplet dynamics and modeling of the droplet breakup/evaporation, are handled through the same unified predictor-corrector procedure. Thus the new algorithm is non-iterative and is flexible for general geometries with arbitrarily complex topology in free surfaces. The FDNS finite-difference Navier-Stokes code is employed as the baseline of the current development. Benchmark test cases of shear coaxial LOX/H2 liquid jet with atomization/combustion and impinging jet test cases are investigated in the present work. Preliminary data comparisons show good qualitative agreement between data and the present analysis. It is indicative from these results that the present method has great potential to become a general engineering design analysis and diagnostics tool for problems involving spray combustion.

Chen, Yen-Sen↗

NASA NTRS1997

Fourth International Microgravity Combustion Workshop

This Conference Publication contains 84 papers presented at the Fourth International Microgravity Combustion Workshop held in Cleveland, Ohio, from May 19 to 21, 1997. The purpose of the workshop was twofold: to exchange information about the progress and promise of combustion science in microgravity and to provide a forum to discuss which areas in microgravity combustion science need to be expanded profitably and which should be included in upcoming NASA Research Announcements (NRA).

Sacksteder, Kurt R.↗

NASA NTRS1997

Selected Diagnostics for Microgravity Combustion Science

The evolution of our understanding of combustion phenomena occurring under reduced gravity conditions poses a continuing demand for diagnostic tools of increased sophistication. Existing methods have become insufficient to keep pace with emerging refinements in the underlying theories and predictive models. The coupling of physical mechanisms inherent in combustion systems mandates the simultaneous determination of numerous thermophysical quantities, principally temperature, velocity, and species concentrations. Observed differences attributable to variations in experimental configuration, fuels and diluents, and initial conditions enhance the difficulty of developing diagnostic methods suitable for the acquisition of the required data over the desired range of experimental parameters. Efforts to provide for ongoing diagnostic development supporting microgravity combustion science experiments are conducted within the Microgravity Science Division at the NASA-Lewis Research Center. The following describes recent results from several diagnostic development efforts conducted under this project.

Greenberg, Paul S.↗

NASA NTRS1997

A Novel Application of Ultrasonic Imaging to Study Smoldering Combustion

An ultrasonic imaging technique has been developed to examine the propagation of a smolder reaction within a porous combustible material. The technique provides information about the location of a propagating smolder front, as well as line-of-sight average permeability variations of the smoldering material. The method utilizes the observation that transmission of an ultrasonic signal through a porous material increases with permeability. Since a propagating smolder reaction leaves behind char with a higher permeability than the original material, ultrasound transmission can be employed to monitor smolder progress. The technique can also be used to track the char evolution as it continues to react. Experiments are presented where the technique is applied to smoldering combustion in a two-dimensional geometry. The results have furthered the understanding of two-dimensional smolder, especially in identifying the controlling mechanisms leading to the transition from smoldering to flaming. The applicability of ultrasonic tomography to smoldering combustion has also been investigated.

Tse, S. D.↗

NASA NTRS1997

Science Support for Space-Based Droplet Combustion: Drop Tower Experiments and Detailed Numerical Modeling

This program supports the engineering design, data analysis, and data interpretation requirements for the study of initially single component, spherically symmetric, isolated droplet combustion studies. Experimental emphasis is on the study of simple alcohols (methanol, ethanol) and alkanes (n-heptane, n-decane) as fuels with time dependent measurements of drop size, flame-stand-off, liquid-phase composition, and finally, extinction. Experiments have included bench-scale studies at Princeton, studies in the 2.2 and 5.18 drop towers at NASA-LeRC, and both the Fiber Supported Droplet Combustion (FSDC-1, FSDC-2) and the free Droplet Combustion Experiment (DCE) studies aboard the shuttle. Test matrix and data interpretation are performed through spherically-symmetric, time-dependent numerical computations which embody detailed sub-models for physical and chemical processes. The computed burning rate, flame stand-off, and extinction diameter are compared with the respective measurements for each individual experiment. In particular, the data from FSDC-1 and subsequent space-based experiments provide the opportunity to compare all three types of data simultaneously with the computed parameters. Recent numerical efforts are extending the computational tools to consider time dependent, axisymmetric 2-dimensional reactive flow situations.

Marchese, Anthony J.↗

NASA NTRS1997

Emulsion Droplet Combustion in Microgravity: Water/Heptane Emulsions

This presentation reviews a series of experiments to further examine parametric effects on sooting processes of droplet flames in microgravity. The particular focus is on a fuel droplet emulsified with water, specifically emulsions of n-heptane as the fuel-phase and water as the dispersed phase. Water was selected as the additive because of its anticipated effect on soot formation, and the heptane fuel phase was chosen to theoretically reduce the likelihood of microexplosions because its boiling point is nearly the same as that of water: 100 C for water and 98 C for heptane. The water content was varied while the initial droplet diameter was kept within a small range. The experiments were carried out in microgravity to reduce the effects of buoyancy and to promote spherical symmetry in the burning process. Spherically symmetric droplet burning is a convenient starting point for analysis, but experimental data are difficult to obtain for this situation as evidenced by the fact that no quantitative data have been reported on unsupported emulsion droplet combustion in a convection-free environment. The present study improves upon past work carried out on emulsion droplet combustion in microgravity which employed emulsion droplets suspended from a fiber. The fiber can be instrusive to the emulsion droplet burning process as it can promote coalescence of the dispersed water phase and heterogeneous nucleation on the fiber. Prior work has shown that the presence of water in liquid hydrocarbons can have both beneficial and detrimental effects on the combustion process. Water is known to reduce soot formation and radiation heat transfer to combustor walls Gollahalli (1979) reduce flame temperatures and thereby NOx emissions, and encourage secondary droplet atomization or microexplosion. Water also tends to retard ignition and and promote early extinction. The former effect restricted the range of water volume fractions as discussed below.

Avedisian, C. Thomas↗

NASA NTRS1997

Formation and Combustion of Unconfined Drop Clusters in Microgravity

Single-drop and droplet array studies have become common methods to isolate and investigate the effects of any of the complexities that enter into the drop combustion process. Microgravity environments are required to allow larger drops to be studied while minimizing or eliminating the confounding effects of buoyancy. Based on the results from current isolated drop, drop array, and spray studies funded through the Microgravity Science and Applications Division, it has become clear that even with the effects of buoyancy removed, the extrapolation of results from droplet array studies to spray flames is difficult. The problem occurs because even the simplest spray systems introduce complexities of multi-disperse drop sizes and drop-drop interactions, coupled with more complicated fluid dynamics. Not only do these features make the interpretation of experimental data difficult, they also make the problem very difficult to analyze computationally. Group combustion models, in which the interaction between droplets is treated on a statistical manner, have become a popular method to investigate the behavior of large numbers of interacting droplets, particularly through the work of Ryan et al. and Bellan and co-workers. While these models idealize the actual spray systems to a point where they can be treated computationally, the experimental analogy to these models is difficult to achieve because it requires the formation and Combustion of drop clusters without the effects of buoyancy. Therefore, even though these models have provided useful and insightful information, the verification of the results by direct comparison with experimental data is still lacking.

Liu, S.↗

NASA NTRS1997

Comparison of High Aspect Ratio Cooling Channel Designs for a Rocket Combustion Chamber

An analytical investigation on the effect of high aspect ratio (height/width) cooling channels, considering different coolant channel designs, on hot-gas-side wall temperature and coolant pressure drop for a liquid hydrogen cooled rocket combustion chamber, was performed. Coolant channel design elements considered were: length of combustion chamber in which high aspect ratio cooling was applied, number of coolant channels, and coolant channel shape. Seven coolant channel designs were investigated using a coupling of the Rocket Thermal Evaluation code and the Two-Dimensional Kinetics code. Initially, each coolant channel design was developed, without consideration for fabrication, to reduce the hot-gas-side wall temperature from a given conventional cooling channel baseline. These designs produced hot-gas-side wall temperature reductions up to 22 percent, with coolant pressure drop increases as low as 7.5 percent from the baseline. Fabrication constraints for milled channels were applied to the seven designs. These produced hot-gas-side wall temperature reductions of up to 20 percent, with coolant pressure drop increases as low as 2 percent. Using high aspect ratio cooling channels for the entire length of the combustion chamber had no additional benefit on hot-gas-side wall temperature over using high aspect ratio cooling channels only in the throat region, but increased coolant pressure drop 33 percent. Independent of coolant channel shape, high aspect ratio cooling was able to reduce the hot-gas-side wall temperature by at least 8 percent, with as low as a 2 percent increase in coolant pressure drop. The design with the highest overall benefit to hot-gas-side wall temperature and minimal coolant pressure drop cooling can now be done in relatively short periods of time with multiple iterations.

Wadel, Mary F.↗