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NASA NTRS1991

Homogeneous Premixed and Nonpremixed Flames in Microgravity - A Review

Combustion studies under microgravitational conditions are discussed in which fire-safety issues, near-limit phenomena, and flammability limits are addressed. The studies use ground-based drop towers to produce up to 5 s of about 10 exp -6 g and aircraft facilities to generate up to 20 s of about 10 exp -2 g. The limits of flammability and radiative-heat loss are important research objectives, and experimentally derived values are discussed. Other phenomena are reported including stationary spherical flames and self-extinguishing flames, and the effects of the Taylor instability are documented in the experiments. The flame-surface geometry is examined concurrently by means of numerical simulations, and flame processes are concluded to be unique under microgravity except when the flow velocities are large.

Faeth, G. M.↗

NASA NTRS1995

Combustion of interacting droplet arrays in a microgravity environment

This research program involves the study of one and two dimensional arrays of droplets in a buoyant-free environment. The purpose of the work is to extend the database and theories that exist for single droplets into the regime where droplet interactions are important. The eventual goal being to use the results of this work as inputs to models on spray combustion where droplets seldom burn individually; instead the combustion history of a droplet is strongly influenced by the presence of the neighboring droplets. Throughout the course of the work, a number of related aspects of isolated droplet combustion have also been investigated. This paper will review our progress in microgravity droplet array combustion, advanced diagnostics (specifically L2) applied to isolated droplet combustion, and radiative extinction large droplet flames. A small-scale droplet combustion experiment being developed for the Space Shuttle will also be described.

Dietrich, Daniel L.↗

NASA NTRS2013

Methanol Droplet Combustion in Oxygen-Inert Environments in Microgravity

The Flame Extinguishment (FLEX) experiment that is currently underway in the Combustion Integrated Rack facility onboard the International Space Station is aimed at understanding the effects of inert diluents on the flammability of condensed phase fuels. To this end, droplets of various fuels, including alkanes and alcohols, are burned in a quiescent microgravity environment with varying amounts of oxygen and inert diluents to determine the limiting oxygen index (LOI) for these fuels. In this study we report experimental observations of methanol droplets burning in oxygen-nitrogen-carbon dioxide and oxygen-nitrogen-helium gas mixtures at 0.7 and 1 atmospheric pressures. The initial droplet size varied between approximately 1.5 mm and 4 mm to capture both diffusive extinction brought about by insufficient residence time at the flame and radiative extinction caused by excessive heat loss from the flame zone. The ambient oxygen concentration varied from a high value of 30% by volume to as low as 12%, approaching the limiting oxygen index for the fuel. The inert dilution by carbon dioxide and helium varied over a range of 0% to 70% by volume. In these experiments, both freely floated and tethered droplets were ignited using symmetrically opposed hot-wire igniters and the burning histories were recorded onboard using digital cameras, downlinked later to the ground for analysis. The digital images yielded droplet and flame diameters as functions of time and subsequently droplet burning rate, flame standoff ratio, and initial and extinction droplet diameters. Simplified theoretical models correlate the measured burning rate constant and the flame standoff ratio reasonably well. An activation energy asymptotic theory accounting for time-dependent water dissolution or evaporation from the droplet is shown to predict the measured diffusive extinction conditions well. The experiments also show that the limiting oxygen index for methanol in these diluent gases is around 12% to 13% oxygen by volume.

Nayagam, Vedha↗

NASA NTRS2003

Modular Avionics Concept Developed for Microgravity Space Experiments: FEANICS

The Flow Enclosure Accommodating Novel Investigations in Combustion of Solids (FEANICS) is a facility being developed at the NASA Glenn Research Center that will be integrated into the Combustion Integrated Rack onboard the International Space Station. FEANICS will support various experimental studies of solid combustion in a microgravity environment. The experiments will vary, and the FEANICS facility hardware combined with the Combustion Integrated Rack hardware is designed to meet the science requirements of the individual experiments.

Hausmann, Clifford R.↗

NASA NTRS1996

Ignition and Combustion of Bulk Metals in a Microgravity Environment

This annual report summarizes the latest results obtained in a NASA-supported project to investigate the effect of gravity on the ignition and combustion of bulk metals. The experimental arrangement used for this purpose consists of a 1000-W xenon lamp that irradiates the top surface of cylindrical titanium and magnesium specimens, 4 mm in diameter and 4 mm in height, in a quiescent, pure-oxygen environment at 1 atm. Reduced gravity is obtained from the NASA LeRC DC-9 aircraft flying parabolic trajectories. Values of critical and ignition temperatures are obtained from thermocouple records. Qualitative observations and propagation rates are extracted from high-speed cinematography. Emission spectra of gas-phase reactions are obtained with an imaging spectrograph/diode array system. It was found that high applied heating rates and large internal conduction losses generate critical and ignition temperatures that are several hundred degrees above the values obtained from isothermal experiments. Because of high conduction and radiation heat losses, no appreciable effect on ignition temperatures with reduced convection in low gravity is detected. Lower propagation rates of the molten interface on titanium and of ignition waves on magnesium are obtained at reduced gravity. These rates are compared to theoretical results from heat conduction analyses with a diffusion/convection controlled reaction. The close agreement found between experimental and theoretical values indicates the importance of the influence of natural convection-enhanced oxygen transport on combustion rates. Lower oxygen flux and lack of oxide product removal in the absence of convective currents appear to be responsible for longer burning times of magnesium diffusion flames at reduced gravity. The accumulation of condensed oxide particles in the flame front at low gravity produces a previously unreported unsteady explosion phenomenon in bulk magnesium flames. This spherically symmetric explosion phenomenon seems to be driven by increased radiation heat transfer from the flame front to an evaporating metal core covered by a porous, flexible oxide coating. These important results have revealed the significant role of gravity on the burning of metals, and are now being used as the database for future experiments to be conducted with different metals at various pressures, oxygen concentrations and gravity levels.

Branch, M. C.↗

NASA NTRS1993

Visualization and imaging methods for flames in microgravity

The visualization and imaging of flames has long been acknowledged as the starting point for learning about and understanding combustion phenomena. It provides an essential overall picture of the time and length scales of processes and guides the application of other diagnostics. It is perhaps even more important in microgravity combustion studies, where it is often the only non-intrusive diagnostic measurement easily implemented. Imaging also aids in the interpretation of single-point measurements, such as temperature, provided by thermocouples, and velocity, by hot-wire anemometers. This paper outlines the efforts of the Microgravity Combustion Diagnostics staff at NASA Lewis Research Center in the area of visualization and imaging of flames, concentrating on methods applicable for reduced-gravity experimentation. Several techniques are under development: intensified array camera imaging, and two-dimensional temperature and species concentrations measurements. A brief summary of results in these areas is presented and future plans mentioned.

Weiland, Karen J.↗

NASA NTRS2004

Carbon Dioxide Dispersion in the Combustion Integrated Rack Simulated Numerically

When discharged into an International Space Station (ISS) payload rack, a carbon dioxide (CO2) portable fire extinguisher (PFE) must extinguish a fire by decreasing the oxygen in the rack by 50 percent within 60 sec. The length of time needed for this oxygen reduction throughout the rack and the length of time that the CO2 concentration remains high enough to prevent the fire from reigniting is important when determining the effectiveness of the response and postfire procedures. Furthermore, in the absence of gravity, the local flow velocity can make the difference between a fire that spreads rapidly and one that self-extinguishes after ignition. A numerical simulation of the discharge of CO2 from PFE into the Combustion Integrated Rack (CIR) in microgravity was performed to obtain the local velocity and CO2 concentration. The complicated flow field around the PFE nozzle exits was modeled by sources of equivalent mass and momentum flux at a location downstream of the nozzle. The time for the concentration of CO2 to reach a level that would extinguish a fire anywhere in the rack was determined using the Fire Dynamics Simulator (FDS), a computational fluid dynamics code developed by the National Institute of Standards and Technology specifically to evaluate the development of a fire and smoke transport. The simulation shows that CO2, as well as any smoke and combustion gases produced by a fire, would be discharged into the ISS cabin through the resource utility panel at the bottom of the rack. These simulations will be validated by comparing the results with velocity and CO2 concentration measurements obtained during the fire suppression system verification tests conducted on the CIR in March 2003. Once these numerical simulations are validated, portions of the ISS labs and living areas will be modeled to determine the local flow conditions before, during, and after a fire event. These simulations can yield specific information about how long it takes for smoke and combustion gases produced by a fire to reach a detector location, how large the fire would be when the detector alarms, and the behavior of the fire until it has been extinguished. This new capability could then be used to optimize the location of fire detectors and fire-suppression ports as well as to evaluate the effectiveness of fire suppressants and response strategies. Numerical data collected from these simulations could also be used to develop a virtual reality fire event for crew training and fire safety awareness. This work is funded by NASA's Bioastronautics Initiative, which has the objective of ensuring and enhancing the health, safety, and performance of humans in space. As part of this initiative, the Microgravity Combustion Science Branch at the NASA Glenn Research Center is conducting spacecraft fire safety research to significantly improve fire safety on inhabited spacecraft.

Wu, Ming-Shin↗

NASA NTRS1995

GASCan 2 payload integration

This project, conducted in cooperation with the NASA Advanced Space Design Program, is part of an ongoing effort to place an experiment package into space. The goal of this project is to build and test flight-ready hardware that can be launched from the Space Shuttle. Get Away Special Canister 2 (GASCan 2) consists of three separate experiments. The Ionospheric Properties and Propagation Experiment (IPPE) determines effects of the ionosphere on radio wave propagation. The Microgravity Ignition experiment (MGI) tests the effects of combustion in a microgravity environment. The Rotational Fluid Flow experiment (RFF) examines fluid behavior under varying levels of gravity. This year the following tasks were completed: design of the IPPE antenna, X- and J-cell battery boxes, J-cell battery box enclosure, and structural bumpers; construction of the MGI canisters, MGI mounting brackets, IPPE antenna, and battery boxes; and the selection of the RFF's operating fluid and the analysis of the fluid behavior under microgravity test conditions.

Cody, Dennis J.↗

NASA NTRS1999

Combustion of a Polymer (PMMA) Sphere in Microgravity

A series of low gravity, aircraft-based, experiments was conducted to investigate the combustion of supported thermoplastic polymer spheres under varying ambient conditions. The three types of thermoplastic investigated were polymethylmethacrylate (PMMA), polypropylene (PP). and polystyrene (PS). Spheres with diameters ranging from 2 mm to 6.35 mm were tested. The total initial pressure varied from 0.05 MPa to 0. 15 MPa whereas the ambient oxygen concentration varied from 19 % to 30 % (by volume). The ignition system consisted of a pair of retractable energized coils. Two CCD cameras recorded the burning histories of the spheres. The video sequences revealed a number of dynamic events including bubbling and sputtering, as well as soot shell formation and break-up during combustion of the spheres at reduced gravity. The ejection of combusting material from the burning spheres represents a fire hazard that must be considered at reduced gravity. The ejection process was found to be sensitive to polymer type. All average burning rates were measured to increase with initial sphere diameter and oxygen concentration, whereas the initial pressure had little effect. The three thermoplastic types exhibited different burning characteristics. For the same initial conditions, the burning rate of PP was slower than PMMA, whereas the burning rate of PS was comparable to PMMA. The transient diameter of the burning thermoplastic exhibited two distinct periods: an initial period (enduring approximately half of the total burn duration) when the diameter remained approximately constant, and a final period when the square of the diameter linearly decreased with time. A simple homogeneous two-phase model was developed to understand the changing diameter of the burning sphere. Its value is based on a competition between diameter reduction due to mass loss from burning and sputtering, and diameter expansion due to the processes of swelling (density decrease with heating) and bubble growth. The model relies on empirical parameters for input, such as the burning rate and the duration of the initial and final burning periods.

Yang, Jiann C.↗

NASA NTRS1999

Combustion of Interacting Droplet Arrays in a Microgravity Environment

Investigations into droplet interactions date back to Rex et al. Recently, Annamalai and Ryan and Annamalai published extensive reviews of droplet array and cloud combustion studies. The authors studied the change in the burning rate constant, k, (relative to that of the single droplet) that results from interactions. Under certain conditions, there exists a separation distance where the droplet lifetime reaches a minimum, or average burning rate constant is a maximum . Additionally, since inter-droplet separation distance, L, increases relative to the droplet size, D, as the burning proceeds, the burning rate is not constant throughout the burn, but changes continuously with time. Only Law and co-workers and Mikami et al. studied interactions under conditions where buoyant forces were negligible. Comparing their results with existing theory, Law and co-workers found that theory over predicted the persistency and intensity of droplet interactions. The droplet interactions also depended on the initial array configuration as well as the instantaneous array configuration. They also concluded that droplet heating was retarded due to interactions and that the burning process did not follow the "D-squared" law. Mikami et al. studied the combustion of a two-droplet array of heptane burning in air at one atm pressure in microgravity. They showed that the instantaneous burning rate constant increases throughout the droplet lifetime, even for a single droplet. Also, the burn time of the array reached a minimum at a critical inter-droplet spacing. In this article, we examine droplet interactions in normal and microgravity environments. The microgravity experiments were in the NASA GRC 2.2 and 5.2 second drop towers, and the JAMIC (Japan Microgravity Center) 10 second drop tower. Special emphasis is directed to combustion under conditions that yield finite extinction diameters, and to determine how droplet interactions affect the extinction process.

Dietrich, D. L.↗

NASA NTRS1995

Combustion and structure formation in SHS processes under microgravity conditions: SHS plans for microgravity experiments

This paper outlines ISMAN suggestions for the joint NASA-RSA project 'Combustion and Structure formation in SHS Processes under Microgravity Conditions'. The basic ideas of this work naturally follow from our almost 30-year experience in the field of SHS. As a matter of fact, we have already obtained some results in the following two directions closely related to the microgravity problem. One is the studies on SHS processes in the field of centrifugal forces. These studies aimed at the intensification of gravity-sensitive SHS processes in multicomponent highly caloric systems forming melts at high overloads (up to 2000 g). In other words, these studies had the objectives that are inverse to those in the microgravity studies. The second group of results directly relates to the microgravity problem and the project under consideration. These experiments played the important role in establishing links between SHS and microgravity.

Merzhanov, A. G.↗

NASA NTRS1999

Combustion and Flammability Characteristics of Solids at Microgravity in very Small Velocity Flows

Fires still remain as one of the most important safety risks in manned spacecraft. This problem will become even more important in long endurance non orbital flights in which maintenance will be non existing or very difficult. The basic process of a fire is the combustion of a solid at microgravity conditions in O2/N2 mixtures. Although a large number of research programs have been carried out on this problem, especially on flame spreading, several aspects of these processes are not yet well understood. It may be mentioned, for example, the temperature and characteristic of low emissivity flames in the visual range that take place in some conditions at microgravity; and there exists a lack of knowledge on the influence of key parameters, such as convective flow velocities of the order of magnitude of typical oxygen diffusion velocities. The "Departamento de Motopropulsion y Termofluidodinamica" of the "Universidad Politecnica de Madrid, Escuela Tecnica Superior de Ingenieros Aeronauticos" is conducting a research program on the combustion of solids at reduced gravity conditions within O2/N2 mixtures. The material utilized has been polymethylmethacrylate (PMMA) in the form of rectangular slabs and hollow cylinders. The main parameters of the process have been small convective flow velocities (including velocity angle with the direction of the spreading flame) and oxygen concentration. Some results have also been obtained on the influence of material thickness and gas pressure.

Sanchez-Tarifa, C.↗

NASA NTRS2001

Some Recent Observations on the Burning of Isolated N-Heptane and Alcohol Droplets

In a joint program involving Prof. F.A. Williams of the University of California, San Diego and Dr. V. Nayagam of the National Center for Microgravity Research on Combustion and Fluid Dynamics, the combustion of liquid fuel droplets of n-heptane, n-decane, methanol, methanol-water, ethanol and ethanol-water having initial diameters between about 1 mm and 6 mm continues to be studied. The objectives of the work are to improve fundamental knowledge of droplet combustion dynamics for pure fuels and fuel-water mixtures through microgravity experiments and theoretical analyses. The Princeton contributions to the collaborative program supports the engineering design, data analysis, and data interpretation requirements for the study of initially single component, spherically symmetric, isolated droplet combustion studies through experiments and numerical modeling. The complementary UCSD contributions apply asymptotic theoretical analyses and are described in the published literature and in a companion communication in this conference. The combined program continues to focus on analyses of results obtained from Fiber Supported Droplet Combustion (FSDC) experiments (FSDC-2, STS- 94) conducted with the above fuels in shuttle cabin air and Droplet Combustion Experiment (DCE) data obtained for unsupported and fiber supported droplets of n-heptane in Helium-Oxygen mixtures and cabin air (STS-83, STS-94). The program is preparing for a second DCE experimental mission using methanol/methanol-water as fuels and helium-oxygen-nitrogen environments. DCE-2 is to be conducted aboard the International Space Station. Emphases of recent Princeton work are on the study of simple alcohols (methanol, ethanol) and alcohol/water mixtures as fuels, with time-dependent measurements of drop size, flame-standoff, liquid-phase composition, and finally, extinction. Ground based experiments have included bench-scale studies at Princeton and collaborative experimental studies in the 2.2 second drop tower at NASA-Glenn Research Center.

Dryer, F. L.↗

NASA NTRS2001

Combustion of Interacting Droplet Arrays in a Microgravity Environment

Investigations into droplet interactions date back to Rex et al. Annamalai and Ryan and Annamalai published extensive reviews of droplet array and cloud combustion studies. In the majority of the reviewed studies, the authors examined the change in the burning rate constant, k, (relative to that of the single droplet) that results from interactions. More recently, Niioka and co-workers have examined ignition and flame propagation along arrays of interacting droplets with the goal of relating these phenomena in this simplified geometry to the more practical spray configuration. Our work has focussed on droplet interactions under conditions where flame extinction occurs at a finite droplet diameter. In our previous work, we reported that in normal gravity, reduced pressure conditions, droplet interactions improved flame stability and extended flammability limits (by inference). In our recent work, we examine droplet interactions under conditions where the flame extinguishes at a finite droplet diameter in microgravity. The microgravity experiments were in the NASA GRC 2.2 and 5.2 second drop towers, and the JAMIC (Japan Microgravity Center) 10 second drop tower. We also present progress on a numerical model of single droplet combustion that is in the process of being extended to model a binary droplet array.

Dietrich, D. L.↗

NASA NTRS2001

Combustion Stages of a Single Heavy Oil Droplet in Microgravity

Heavy oil is a common fuel for industrial furnaces, boilers, marines and diesel engines. Previous studies showed that the combustion of heavy oil involves not only the complete burning of volatile matters but also the burn-out of coke residues. Detailed knowledge about heavy oil combustion therefore requires an understanding of the different burning stages of heavy oil droplets in the burner. This in turn, demands knowledge about the single droplet evaporation and combustion characteristics. This study measured the temperature and size histories of heavy oil (C glass) droplets burning in microgravity to elucidate the various stages that occur during combustion. The elimination of the gravity-induced gas convection in microgravity allows the droplet combustion to be studied in greater detail. Noting that the compositions of heavy oil are various, we also tested the fuel blends of a diesel light oil (LO) and a heavy oil residue (HOR).

Ikegami, M.↗

NASA NTRS2003

Recent Advances In Science Support For Isolated Droplet Combustion Experiments

In a joint program involving Prof. F.A. Williams of the University of California, San Diego and Dr. V. Nayagam of the National Center for Microgravity Research, the combustion characteristics of isolated liquid fuel droplets of n-heptane, n-decane, methanol, methanol-water, ethanol and ethanol-water having initial diameters between about 1 mm and 6 mm continues to be investigated. The objectives of the work are to improve fundamental knowledge of droplet combustion dynamics for pure fuels and fuel-water mixtures through microgravity experiments and theoretical analyses. The Princeton contributions support the engineering design, data analysis, and data interpretation requirements for the study of initially single component, spherically symmetric, isolated droplet combustion studies through experiments and numerical modeling. UCSD contributions are described in a companion communication in this conference. The Princeton effort also addresses the analyses of Fiber Supported Droplet Combustion (FSDC) experiments conducted with the above fuels and collaborative work with others who are investigating droplet combustion in the presence of steady convection. A thorough interpretation of droplet burning behavior for n-heptane and n-decane over a relatively wide range of conditions also involves the influences of sooting on the combustion behavior, and this particular aspect on isolated burning of droplets is under consideration in a collaborative program underway with Drexel University. This collaboration is addressed in another communication at this conference. The one-dimensional, time-dependent, numerical modeling approach that we have continued to evolve for analyzing isolated, quiescent droplet combustion data has been further applied to investigate several facets of isolated droplet burning of simple alcohols, n-heptane, and n-decane. Some of the new results are described below.

Dryer, F. L.↗

NASA NTRS1990

New findings and instrumentation from the NASA Lewis Microgravity Facility

The study of fundamental combustion and fluid physics phenomena in microgravity enables a new range of experiments to be performed since buoyancy-type flows are nearly eliminated, normally obscured forces and flows may be isolated, gravitational settling or sedimentation is nearly eliminated, and larger time or length scales in experiments become permissible. Some new phenomena found through ground-based microgravity research at NASA facilities is reviewed along with the instrumentation and drop tower facilities used in the research. Experiments are described in the areas of premixed gas combustion, droplet combustion, particle cloud combustion, liquid reorientation, and two-phase flow patterns. Future plans for instrumentation development are also outlined.

Ross, Howard D.↗

NASA NTRS1986

Equipment concept design and development plans for microgravity science and applications research on space station: Combustion tunnel, laser diagnostic system, advanced modular furnace, integrated electronics laboratory

Taking advantage of the microgravity environment of space NASA has initiated the preliminary design of a permanently manned space station that will support technological advances in process science and stimulate the development of new and improved materials having applications across the commercial spectrum. Previous studies have been performed to define from the researcher's perspective, the requirements for laboratory equipment to accommodate microgravity experiments on the space station. Functional requirements for the identified experimental apparatus and support equipment were determined. From these hardware requirements, several items were selected for concept designs and subsequent formulation of development plans. This report documents the concept designs and development plans for two items of experiment apparatus - the Combustion Tunnel and the Advanced Modular Furnace, and two items of support equipment the Laser Diagnostic System and the Integrated Electronics Laboratory. For each concept design, key technology developments were identified that are required to enable or enhance the development of the respective hardware.

Uhran, M. L.↗