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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
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 NTRS1991

The Implications of Experimentally Controlled Gravitational Accelerations for Combustion Science

An overview of basic combustion problems which have been investigated under the condition of reduced gravity is presented to identify promising research directions. Attention is given to the broad categories of gas-jet diffusion flames, droplet combustions, particle clouds, flame spreading over liquid pools, smoldering, and flame spreading over solid fuels. Fire safety in spacecraft is the primary application that is addressed by the studies of combustion under microgravity. The need for more complete testing of the issues discussed in orbiting spacecraft is identified in the light of limited earth-based testing. Attention is also directed toward the need for advanced diagnostic methods for in-flight and other combustion investigations.

Sacksteder, Kurt R.↗
NASA NTRS1991

An investigation of flame spread over shallow liquid pools in microgravity and nonair environments

Experiments of interest to combustion fundamentals and spacecraft fire safety investigated flame spread of alcohol fuels over shallow, 15 cm diameter pools in a 5.2 sec free-fall, microgravity facility. Results showed that, independent O2 concentrations, alcohol fuel, and diluent types, microgravity flame spread rates were nearly identical to those corresponding normal-gravity flames for conditions where the normal gravity flames spread uniformly. This similarity indicated buoyancy-related convection in either phase does not affect flame spread, at least for the physical scale of the experiments. However, microgravity extinction coincided with the onset conditions for pulsating spread in normal gravity, implicating gas phase, buoyant flow as a requirement for pulsating spread. When the atmospheric nitrogen was replaced with argon, the conditions for the onset of normal-gravity pulsating flame spread and microgravity flame extinction were changed, in agreement with the expected lowering of the flash point through the thermal properties of the diluent. Helium-diluted flames, however, showed unexpected results with a shift to apparently higher flash-point temperatures and high normal gravity pulsation amplitudes.

Ross, Howard D.↗
NASA NTRS1992

Determination of the natural convection coefficient in low-gravity

Fire safety is an important issue in the current space program; ignition in low-g needs to be studied. The reduction in the gravitational acceleration causes changes in the ignition process. This paper examines the effect of gravity on natural convection, which is one of the important parameters in the ignition process. The NASA-Lewis 2.2 Second Drop Tower provided the low-gravity environment for the experiments. A series of experiments was conducted to measure the temperature of a small copper plate which was heated by a high intensity lamp. These experiments verified that in low-gravity the plate temperature increased faster than in the corresponding 1-g cases, and that the natural convection coefficient rapidly decreased in the low-gravity environment.

Goldmeer, J.↗
NASA NTRS1993

An Experimental and Theoretical Study of Radiative Extinction of Diffusion Flames

In a recent paper on 'Observations of candle flames under various atmospheres in microgravity' by Ross et al., it was found that for the same atmosphere, the burning rate per unit wick surface area and the flame temperature were considerably reduced in microgravity as compared with normal gravity. Also, the flame (spherical in microgravity) was much thicker and further removed from the wick. It thus appears that the flame becomes 'weaker' in microgravity due to the absence of buoyancy generated flow which serves to transport the oxidizer to the combustion zone and remove the hot combustion products from it. The buoyant flow, which may be characterized by the strain rate, assists the diffusion process to execute these essential functions for the survival of the flame. Thus, the diffusion flame is 'weak' at very low strain rates and as the strain rate increases the flame is initially 'strengthened' and eventually it may be 'blown out'. The computed flammability boundaries of T'ien show that such a reversal in material flammability occurs at strain rates around 5 sec. At very low or zero strain rates, flame radiation is expected to considerably affect this 'weak' diffusion flame because: (1) the concentration of combustion products which participate in gas radiation is high in the flame zone; and (2) low strain rates provide sufficient residence time for substantial amounts of soot to form which is usually responsible for a major portion of the radiative heat loss. We anticipate that flame radiation will eventually extinguish this flame. Thus, the objective of this project is to perform an experimental and theoretical investigation of radiation-induced extinction of diffusion flames under microgravity conditions. This is important for spacecraft fire safety.

Atreya, Arvind↗
NASA NTRS1993

Soot formation and radiation in turbulent jet diffusion flames under normal and reduced gravity conditions

Most practical combustion processes, as well as fires and explosions, exhibit some characteristics of turbulent diffusion flames. For hydrocarbon fuels, the presence of soot particles significantly increases the level of radiative heat transfer from flames. In some cases, flame radiation can reach up to 75 percent of the heat release by combustion. Laminar diffusion flame results show that radiation becomes stronger under reduced gravity conditions. Therefore, detailed soot formation and radiation must be included in the flame structure analysis. A study of sooting turbulent diffusion flames under reduced-gravity conditions will not only provide necessary information for such practical issues as spacecraft fire safety, but also develop better understanding of fundamentals for diffusion combustion. In this paper, a summary of the work to date and of future plans is reported.

Ku, Jerry C.↗
NASA NTRS1993

Combustion of solid fuel in very low speed oxygen streams

In reduced gravity, the combustion of solid fuel in low-speed flow can be studied. The flame behavior in this low-speed regime will fill a void in our understanding of the flow effect on combustion. In addition, it is important for spacecraft fire safety considerations. In this work, modeling and experimental work on low-speed forced-concurrent-flow flame spread are carried out. In addition, experiments on reduced-gravity buoyant-flow flame spread are performed.

Tien, James S.↗
NASA NTRS1991

SSTAC/ARTS review of the draft Integrated Technology Plan (ITP). Volume 5: Human Support

Viewgraphs of briefings from the Space Systems and Technology Advisory Committee (SSTAC)/ARTS review of the draft integrated technology plan (ITP) on human support are included. Topics covered include: human support program; human factors; life support technology; fire safety; medical support technology; advanced refrigeration technology; EVA suit system; advanced PLSS technology; and ARC-EVA systems research program.

Source record↗
NASA NTRS1993

Downward Diffusion Flame Spread and Extinction in Variable Gravitational Fields - Lunar and Martian Simulations

This paper describes experimental observations of downward, opposed-flow flame spreading made under partial-gravity conditions aboard NASA research aircraft. Flame spreading and flammability limit behavior of a thin cellulosic fuel tested at normal pressure in oxygen/nitrogen mixtures of 21 percent oxygen, by volume, and below are described over effective acceleration levels ranging from 0.05 to 0.6 times normal earth gravity. Downward burning flammability and flame spread rates are shown to be enhanced by reductions in gravitational acceleration. These data have fire safety implications for the planning of lunar and Martian outposts.

Sacksteder, Kurt R.↗
NASA NTRS1993

Heat Transfer from Radiatively Heated Material in a Low Reynolds Number Microgravity Environment

A mathematical model of the transient three-dimensional heat transfer between a slowly moving ambient gas stream and a thermally thick or thin flat surface heated by external radiation in a microgravity environment is presented. The problem is motivated in part by fire safety issues in spacecraft. The gas phase is represented by variable property convection-diffusion energy and mass conservation equations valid at low Reynolds numbers. The absence of gravity and low Reynolds number together permit the flow to be represented by a self-consistent velocity potential determined by the ambient velocity and the thermal expansion in the gas. The solid exchanges energy with the gas by conduction/convection and with the surroundings by surface absorption and re-emission of radiation. Heat conduction in the solid is assumed to be one dimensional at each point on the surface as a consequence of the limited times (of order of 10 seconds) of interest in these simulations. Despite the apparent simplicity of the model, the results show a complex thermally induced flow near the heated surface. The thermal exchange between the gas and solid produces an outward sourcelike flow upstream of the center of the irradiated area and a sinklike flow downstream. The responses of the temperature fields and the associated flows to changes in the intensity of the external radiation and the ambient velocity are discussed.

Yamashita, H.↗
NASA NTRS1992

Flame resistant fibrous structures development

The purpose of the current program was (1) to investigate potentially useful new polymers, both for fire safety and mechanical properties, (2) to produce fibers from these polymers if necessary, and (3) to produce sufficient quantities of qualified fibrous structures, composites, or laminates for use in various areas of the Space Shuttle and Space Station Programs. During the past six years, development efforts have been expended in several major areas in support of Space Shuttle missions and Space Station Freedom projects. The summarized results of several of these major efforts are included in this report.

Coskren, Robert J.↗
NASA NTRS1995

Microgravity Combustion Science: 1995 Program Update

Microgravity greatly benefits the study of fundamental combustion processes. In this environment, buoyancy-induced flow is nearly eliminated, weak or normally obscured forces and flows can be isolated, gravitational settling or sedimentation is nearly eliminated, and temporal and spatial scales can be expanded. This document reviews the state of knowledge in microgravity combustion science with the emphasis on NASA-sponsored developments in the current period of 1992 to early 1995. The subjects cover basic research in gaseous premixed and diffusion-flame systems, flame structure and sooting, liquid droplets and pools, and solid-surface ignition and flame spread. They also cover applied research in combustion synthesis of ceramic-metal composites, advanced diagnostic instrumentation, and on-orbit fire safety. The review promotes continuing research by describing the opportunities for Principal Investigator participation through the NASA Research Announcement program and the available NASA Lewis Research Center ground-based facilities and spaceflight accommodations. This review is compiled by the members and associates of the NASA Lewis Microgravity Combustion Branch, and it serves as an update of two previous overview reports.

Ross, Howard D.↗
NASA NTRS1995

Solid surface combustion experiment flame spread in a quiescent, microgravity environment implications of spread rate and flame structure

A unique environment in which flame spreading, a phenomenon of fundamental, scientific interest, has importance to fire safety is that of spacecraft in which the gravitational acceleration is low compared with that of the Earth, i.e., microgravity. Experiments aboard eight Space Shuttle missions between October 1990 and February 1995 were conducted using the Solid Surface Combustion Experiment (SSCE) payload apparatus in an effort to determine the mechanisms of gas-phase flame spread over solid fuel surfaces in the absence of any buoyancy induced or externally imposed oxidizer flow. The overall SSCE effort began in December of 1984. The SSCE apparatus consists of a sealed container, approximately 0.039 cu m, that is filled with a specified O2/N2 mixture at a prescribed pressure. Five of the experiments used a thin cellulosic fuel, ashless filter paper, 3 cm wide x 10 cm long, 0.00825 cm half-thickness, ignited in five different ambient conditions. Three of the experiments, the most recent, used thick polymethylmethacrylate (PMMA) samples 0.635 cm wide x 2 cm long, 0.32 cm half-thickness. Three experiments, STS 41, 40 and 43, were designed to evaluate the effect of ambient pressure on flame spread over the thin cellulosic fuel while flights STS 50 and 47 were at the same pressure as two of the earlier flights but at a lower oxygen concentration in order to evaluate the effect of ambient oxygen level on the flame spread process at microgravity. For the PMMA flights, two experiments, STS 54 and 63, were at the same pressure but different oxygen concentrations while STS 64 was at the same oxygen concentration as STS 63 but at a higher pressure. Two orthogonal views of the experiments were recorded on 16 mm cine-cameras operating at 24 frames/s. In addition to filmed images of the side view of the flames and surface view of the burning samples, solid- and gas-phase temperatures were recorded using thermocouples. The experiment is battery powered and follows an automated sequence upon activation by the Shuttle Crew. In this study we separate the SSCE data into two groups according to the fuel type: (1) thin cellulose; and (2) thick PMMA. The experimental spread rates are compared with prediction from a number of models in an effort to uncover the important physics that characterize microgravity flame spread. Both steady and unsteady solutions are employed to explore the flame evolution, especially for thick fuels. Finally, the flame structure in downward spread is compared with the microgravity flame structure and modeling results to delineate the difference between the two configurations and the influence of normal gravity.

Bundy, Matthew↗
NASA NTRS1995

Combustion of a polymer (PMMA) sphere in microgravity

Polymer combustion is a highly complicated process where chemical reactions may occur not only in the gas phase, but also in the condensed phase as well as at the solid-gas interphase. The chemistry depends strongly on the coupling between the condensed phase and gas phase phenomena. For some polymers, additional complications arise due to the formation of char layers. For others, the behavior of the condensed phase involves swelling, bubbling, melting, sputtering, and multi-stage combustion. Some of these features bear resemblance to the phenomena observed in coal particle combustion. In addition to its relevance to spacecraft fire safety, the combustion of polymeric materials is related to many applications including solid and hybrid rocket propulsion, and of recent interest, waste incineration . The burning rate is one of the most important parameters used to characterize the combustion of polymers. It has been used to rank the polymer flammability under the same experimental conditions and to evaluate various modes of inhibiting polymer flammability. The main objective of this work is to measure the burning rates of a polymeric material in low gravity. Because of inherent logistical difficulties involved in microgravity experiments, it is impossible to examine a wide spectrum of polymeric materials. It is desirable to investigate a polymer whose combustion is less complicated, and yet will lead to a better understanding of the burning characteristics of other more complicated materials. Therefore, a typical non-charring polymer is selected for use in this experimental study. PMMA (polymethylmethacrylate) has been chosen because its thermo-physical properties are well characterized. Although the combustion of PMMA has been extensively studied in 1G experiments, only a limited amount of work has been conducted in low gravity. A spherical sample geometry is chosen in this study because it is the simplest configuration in terms of the microgravity hardware design requirements. Furthermore, a burning PMMA sphere in microgravity represents a one-dimensional flame with overall combustion characteristics expected to be analogous to the combustion of a liquid fuel droplet, a field with many well-developed theories and models. However, differences can also be expected such as the flame-front standoff ratios and the condensed phase processes occurring during combustion.

Yang, Jiann C.↗
NASA NTRS1995

Effect of pressure on a burning solid in low-gravity

Venting, or depressurization, has been discussed as a possible technique for extinguishing fires on aircraft and spacecraft. Fire suppression plans for the International Space Station Alpha (ISSA) discuss the use of depressurization as a method for extinguishing fires. In the case of an uncontrollable fire, the affected compartment would be vented from an initial pressure of 1.0 atm (14.7 psia) to a final pressure 0.33 atm (4.8 psia) within 10 minutes. However, the lack of low pressure flammability data for solid materials in a low-gravity environment presents an uncertainty for the use of the venting technique. There are also transient effects that need to be considered. It is possible that the flows induced by the venting could intensify the fire. This occurred during flammability tests conducted on board Skylab. In addition, the extinction pressure could be a function of the depressurization rate. Studies conducted with solid propellants have shown that if the pressure is reduced quickly enough, the pressure at extinction will be greater than the steady-state extinction limit. This project, which was started in 1992, is examining both the quasi steady-state and transient effects of pressure reduction on a burning solid in low-gravity. This research will provide low-g extinguishment data upon which policies and practices can be formulated for fire safety in orbiting spacecraft.

Goldmeer, Jeffrey S.↗
NASA NTRS1995

Flame spread over solid fuel in low-speed concurrent flow

This research program is concerned with the effect of low speed flow on the spreading and extinction processes of flames over solid fuels. Primary attention is given to flame propagation in concurrent flow - the more hazardous situation from the point of view of fire safety.

T'Ien, James S.↗
NASA NTRS1995

Radiant extinction of gaseous diffusion flames

The absence of buoyancy-induced flows in microgravity significantly alters the fundamentals of many combustion processes. Substantial differences between normal-gravity and microgravity flames have been reported during droplet combustion, flame spread over solids, candle flames, and others. These differences are more basic than just in the visible flame shape. Longer residence time and higher concentration of combustion products create a thermochemical environment which changes the flame chemistry. Processes such as flame radiation, that are often ignored under normal gravity, become very important and sometimes even controlling. This is particularly true for conditions at extinction of a microgravity diffusion flame. Under normal-gravity, the buoyant flow, which may be characterized by the strain rate, assists the diffusion process to transport the fuel and oxidizer to the combustion zone and remove the hot combustion products from it. These are essential functions for the survival of the flame which needs fuel and oxidizer. Thus, as the strain rate is increased, the diffusion flame which is 'weak' (reduced burning rate per unit flame area) at low strain rates is initially 'strengthened' and eventually it may be 'blown-out'. Most of the previous research on diffusion flame extinction has been conducted at the high strain rate 'blow-off' limit. The literature substantially lacks information on low strain rate, radiation-induced, extinction of diffusion flames. At the low strain rates encountered in microgravity, flame radiation is enhanced due to: (1) build-up of combustion products in the flame zone which increases the gas radiation, and (2) low strain rates provide sufficient residence time for substantial amounts of soot to form which further increases the flame radiation. It is expected that this radiative heat loss will extinguish the already 'weak' diffusion flame under certain conditions. Identifying these conditions (ambient atmosphere, fuel flow rate, fuel type, etc.) is important for spacecraft fire safety. Thus, the objective is to experimentally and theoretically investigate the radiation-induced extinction of diffusion flames in microgravity and determine the effect of flame radiation on the 'weak' microgravity diffusion flame.

Atreya, Arvind↗
NASA NTRS1996

Potential Commercial Applications from Combustion and Fire Research in Space

The near-zero (microgravity) environment of orbiting spacecraft minimizes buoyant flows, greatly simplifying combustion processes and isolating important phenomena ordinarily concealed by the overwhelming gravity-driven forces and flows. Fundamental combustion understanding - the focus to date of the NASA microgravity-combustion program - has greatly benefited from analyses and experiments conducted in the microgravity environment. Because of the economic and commercial importance of combustion in practice, there is strong motivation to seek wider applications for the microgravity-combustion findings. This paper reviews selected technology developments to illustrate some emerging applications. Topics cover improved fire-safety technology in spacecraft and terrestrial systems, innovative combustor designs for aerospace and ground propulsion, applied sensors and controls for combustion processes, and self-sustaining synthesis techniques for advanced materials.

Friedman, Robert↗