Paul V. Ferkul

Prevention of Over-Pressurization During Combustion in a Sealed Chamber

The combustion of flammable material in a sealed chamber invariably leads to an initial pressure rise in the volume. The pressure rise is due to the increase in the total number of gaseous moles (condensed fuel plus chamber oxygen combining to form gaseous carbon dioxide and water vapor) and, most importantly, the temperature rise of the gas in the chamber. Though the rise in temperature and pressure would reduce with time after flame extinguishment due to the absorption of heat by the walls and contents of the sealed spacecraft, the initial pressure rise from a fire, if large enough, could lead to a vehicle overpressure and the release of gas through the pressure relief valve. This paper presents a simple lumped-parameter model of the pressure rise in a sealed chamber resulting from the heat release during combustion. The transient model considers the increase in gaseous moles due to combustion, and heat transfer to the chamber walls by convection and radiation and to the fuel-sample holder by conduction, as a function of the burning rate of the material. The results of the model are compared to the pressure rise in an experimental chamber during flame spread tests as well as to the pressure fall-off after flame extinguishment. The experiments involve flame spread over thin solid fuel samples. Estimates of the heat release rate profiles for input to the model come from the assumed stoichiometric burning of the fuel along with the observed flame spread behavior. The sensitivity of the model to predict maximum chamber pressure is determined with respect to the uncertainties in input parameters. Model predictions are also presented for the pressure profile anticipated in the Fire Safety-1 experiment, a material flammability and fire safety experiment proposed for the European Space Agency (ESA) Automated Transfer Vehicle (ATV). Computations are done for a range of scenarios including various initial pressures and sample sizes. Based on these results, various mitigation approaches are suggested to prevent vehicle over-pressurization and help guide the definition of the space experiment. Nomenclature Af = area of the flame over the fuel-sample surface, m 2 Aw = area of the total available surfaces heat is convected to, m 2

Solid fuel combustion experiments in microgravity using a continuous fuel dispenser and related numerical simulations

The conventional way of determining the flammability characteristics of a material involves a number of tedious single-sample tests to distinguish flammable from non-flammable conditions. A novel test device and fuel configuration has been developed that permits multiple successive tests for indefinite lengths of thin solid materials. In this device, a spreading flame can be established and held at a fixed location in front of optimized diagnostics while continuous variations of test parameters are made. This device is especially well-suited to conducting experiments in space (e.g. aboard the International Space Station) where the limited resources of stowage, volume, and crew time pose major constraints. A prototype version of this device was tested successfully in both a normal gravity laboratory and during low-gravity aircraft trials. As part of this ongoing study of material flammability behavior, a numerical model of concurrent-flow flame spread is used to simulate the flame. Two and three-dimensional steady-state forms of the compressible Navier-Stokes equations with chemical reactions and gas and solid radiation are solved. The model is used to assist in the design of the test apparatus and to interpret the results of microgravity experiments. This paper describes details of the fuel testing device and planned experiment diagnostics. A special fuel, developed to optimize use of the special testing device, is described. Some results of the numerical flame spread model are presented to explain the three-dimensional nature of flames spreading in concurrent flow and to show how the model is used as an experiment design tool.

Solid Inflammability Boundary at Low Speed (SIBAL)

This research program is concerned with the effect of low-speed, concurrent flow on the spreading and extinction processes of flames over solid fuels. The primary objective is to verify the theoretically predicted extinction boundary, using oxygen percentage and flow velocity as coordinates. Of particular interest are the low-speed quenching limits and the existence of the critical oxygen flammability limit. Detailed flame spread characteristics, including flame spread rate, flame size, and flame structure are sought. Since the predicted flame behavior depends on the inclusion of flame and surface radiation, the measured results will also be used to assess the importance of radiative heat transfer by direct comparison to a comprehensive numerical model. The solid fuel used in this experiment is a custom-made fabric consisting of a 1:1 blend of cotton and fiberglass. This choice was made following an extensive search to yield a material with favorable properties, namely, rollability, non-cracking behavior during combustion, strength after combustion, and flammability in a range of oxygen limits permissible within the Combustion Integrated Rack (CIR) on the International Space Station. At the present time, an effort is being made to characterize both the radiative properties of the fuel and the flame spreading behavior in normal gravity at reduced pressure. These will provide a basis for comparison with the microgravity results as well as aid in bracketing the anticipated flammability boundary for the flight experiment. An overview of recent work, with emphasis on theoretical results, is presented.

Zero-Gravity Centrifuge Used for the Evaluation of Material Flammability in Lunar Gravity

A new research apparatus has been developed to study the effect of gravity levels in a drop tower environment. A centrifuge test device was installed inside a drop rig used in the Zero-Gravity Facility at NASA Glenn Research Center. The centrifuge consists of a large rotating chamber with rotation rates up to 1 RPS with no measurable effect on the overall Zero-Gravity drop bus. Gravity levels from zero to 1g (at a radius of rotation = 30 cm) can be produced. Experiments were conducted to examine the maximum oxygen concentration at which three different materials ignited in lunar gravity would self-extinguish. All three materials burned to lower oxygen concentrations in lunar gravity than in normal gravity, although the low-g extinction limit criteria are not the same as 1g due to time constraints in drop testing. The margin of safety of the 1g test method relative to actual low gravity material performance is presented. In broader application, a modified Test 1 protocol is suggested to provide the option of selecting better materials based on the best margin of safety relative to the use environment, as opposed to what would be considered just “passing” from a flammability point of view. This paper discusses these results and demonstrates that the Zero-Gravity Centrifuge is a useful tool for researchers interested in the effects of partial gravity on experiments, especially as NASA embarks on future missions which may be conducted in non-Earth gravity.

Combustion experiments on the Mir Space Station

Combustion tests were carried out on the Mir Space Station. Flat sheets of paper, polyethyleneinsulated wires, cylindrical cellulosic samples, and candles were burned in microgravity. The test parameters included sample size, fuel preheating levels, and lowspeed air velocity. Data were collected mainly through video cameras, audio recordings of crew observations, and 35 mm still pictures. For many of these tests, thermocouples permitted the recording of temperatures of the gas phase flame and/or solid fuel. After the flight, the flame images and temperature data were compared to numerical simulations. Several unique phenomena were observed and the results have implications for spacecraft fire safety. These include the influence of airflow, fuel melting and bubbling, and fuel-vapor generation, and condensation after the flame extinguished.

Design and Analysis of a Handheld Fire Extinguisher for the Crew Exploration Vehicle

Prior work has revealed the unique aspects of fire suppression in reduced gravity and presented the results of trade studies for both the Crew Exploration Vehicle (CEV) and Lunar landers. The trade studies for both spacecraft showed that water mist systems are the best solution; extremely effective, non-toxic, easy clean-up and minimal interference with spacecraft life support systems. The only downside to water mist is the relatively low Technology Readiness Level (TRL). In the event that the TRL for water mist does not accelerate to a suitable level, the trade analyses showed that water-based foam agents represent an acceptable alternative. There exists no standardized design or testing protocol for spacecraft fire suppression systems (either handheld or total flooding designs). This paper discusses the design of handheld extinguishers, the design of standardized acceptance tests for candidate extinguishers and the results of tests for both water-based foam and water-mist extinguishers.

The Effect of Gravity on Flame Spread over PMMA Cylinders

Fire safety is a concern in space travel, particularly with the current plans of increasing the length of the manned space missions, and of using spacecraft atmospheres different than in Earth, such as microgravity, low-velocity gas flow, low pressure and elevated oxygen concentration. In this work, the spread of flame over a thermoplastic polymer, polymethyl methacrylate (PMMA), was conducted in the International Space Station and on Earth. The tests consisted of determining the opposed flame spread rate over PMMA cylinders under low-flow velocities ranging from 0.4 to 8 cm/s and oxygen concentrations from 15% to 21%. The data show that as the opposed flow velocity is increased, the flame spread rate first increases, and then decreases, different from that on Earth. The unique data are significant because they have only been predicted theoretically but not been observed experimentally before. Results also show that flame spread in microgravity could be faster and sustained at lower oxygen concentration (17%) than in normal gravity (18%). These findings suggest that under certain environmental conditions there could be a higher fire risk and a more difficult fire suppression in microgravity than on Earth, which would have significant implications for spacecraft fire safety.

Quantitative Surface Emissivity and Temperature Measurements of a Burning Solid Fuel Accompanied by Soot Formation

Surface radiometry is an established technique for noncontact temperature measurement of solids. We adapt this technique to the study of solid surface combustion where the solid fuel undergoes physical and chemical changes as pyrolysis proceeds, and additionally may produce soot. The physical and chemical changes alter the fuel surface emissivity, and soot contributes to the infrared signature in the same spectral band as the signal of interest. We have developed a measurement that isolates the fuels surface emissions in the presence of soot, and determine the surface emissivity as a function of temperature. A commercially available infrared camera images the two-dimensional surface of ashless filter paper burning in concurrent flow. The camera is sensitive in the 2 to 5 gm band, but spectrally filtered to reduce the interference from hot gas phase combustion products. Results show a strong functional dependence of emissivity on temperature, attributed to the combined effects of thermal and oxidative processes. Using the measured emissivity, radiance measurements from several burning samples were corrected for the presence of soot and for changes in emissivity, to yield quantitative surface temperature measurements. Ultimately the results will be used to develop a full-field, non-contact temperature measurement that will be used in spacebased combustion investigations.

Boundary Layer Effect on Opposed-Flow Flame Spread and Flame Length over Thin Polymethyl-Methacrylate in Microgravity

ABSTRACT Flame spread and flame length are two of the most important characteristics to determine flame growth and heat transfer to a solid fuel. Depending on the intensity of the opposed flow, and therefore the oxidizer residence time in the burning region, flame spread can be divided into three different regimes. In the thermal regime the residence time is much larger than the chemical time of the reactions, and the flame spread is independent on the opposing flow velocity. Reducing the residence time, the flame enters in the kinetic regime where the flame eventually experiences blow-off extinction. In a quiescent environment, possible only in microgravity, oxygen can reach the flame region only by diffusion, and it might not be fast enough to guarantee the reactions to occur. In this regime, called radiative regime, the flame eventually extinguishes, since the heat losses are larger than the heat released by the reactions. In this work, the role played by the boundary layer due to very low flow velocities in the radiative regime is studied, both experimentally and computationally. Experiments were carried out on the International Space Station, using thin sheets of polymethyl-methacrylate as fuel. Parameters such as flow velocity, oxygen concentration, sample width, and fuel thickness were varied in these experiments. The flame size changes significantly as the flame spread across a developing boundary layer, as predicted by the computational model. However, over the limited range of boundary layer development length, the experiments did not completely agree with the rise in spread rate in a thinning boundary layer as expected from the simulations.

Sooting and Radiation Effects in Microgravity Droplet Combustion

Today, despite efforts to develop and utilize natural gas and renewable energy sources, nearly 97% of the energy used for transportation is derived from combustion of liquid fuels, principally derived from petroleum. While society continues to rely on liquid petroleum-based fuels as a major energy source in spite of their finite supply, it is of paramount importance to maximize the efficiency and minimize the environmental impact of the devices that burn these fuels. The development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to meeting both local and regional air quality standards. This development requires improvements in computational design tools for applied energy conversion systems, which in turn requires more robust sub-model components for combustion chemistry, transport, energy transport (including radiation), and pollutant emissions (soot formation and burnout). The study of isolated droplet burning as a unidimensional, time dependent model diffusion flame system facilitates extensions of these mechanisms to include fuel molecular sizes and pollutants typical of conventional and alternative liquid fuels used in the transportation sector. Because of the simplified geometry, sub-model components from the most detailed to those reduced to sizes compatible for use in multi-dimensional, time dependent applied models can be developed, compared and validated against experimental diffusion flame processes, and tested against one another. Based on observations in microgravity experiments on droplet combustion, it appears that the formation and lingering presence of soot within the fuel-rich region of isolated droplets can modify the burning rate, flame structure and extinction, soot aerosol properties, and the effective thermophysical properties. These observations led to the belief that perhaps one of the most important outstanding contributions of microgravity droplet combustion is the observation that in the absence of asymmetrical forced and natural convection, a soot shell is formed between the droplet surface and the flame, exerting an influence on the droplet combustion response far greater than previously recognized. The effects of soot on droplet burning parameters, including burning rate, soot shell dynamics, flame structure, and extinction phenomena provide significant testing parameters for studying the structure and coupling of soot models with other sub-model components.