Richard Pettegrew

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.

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.

Effect of Sample Orientation on the Flammability of Thin Paper

Experiments were carried out to examine the combustion of thin, cellulosic tissue paper samples burning downward in normal gravity. The ambient oxygen mole fraction, total pressure, and sample orientation were varied For the two sample orientations studied, the flammability limit and the spread rates were different, attributed to differences in solid properties for the two directions. While the samples appear uniform, the fact that there is a consistent difference in the two directions implies that there is some structural anisotropy accounting for the behavior. Most likely, solid conductivity through the individual fibers of the paper promote pyrolysis in the general direction of fiber alignment, compared to the cross-fiber direction. Accepting that sample orientation is important even for these very thin specimens near the blowoff limit, it is suggested that the same effect is significant near the microgravity quenching extinction limit.

Emissivity measurement of a thin, pyrolyzing solid fuel

Numerical models of burning solids often rely on empirical relations to predict the solid phase temperature and burning rate, owing to the complexity of pyrolysis, char formation, and volatile production. An accurate experimental determination of the radiative properties of a burning surface allows direct comparison between theoretical and experimental results, and thus is a test of the solid phase model. lnfiared radiometry allows two-dimensional surface temperature maps to be made as a function of time, without the intrusive nature or spatial limitations of thermocouples. However, an accurate measurement requires a detailed understanding of fuel surface emissivity (which, for a given fuel, is a function of temperature, wavelength and viewing angle). Infrared surface temperature measurements are usually made using a constant value of emissivity, which is measured at a single defmed temperature (usually ambient or 90°C), and at a single wavelength (typically 650 run). This approach can lead to large and unquantified errors in the measured surface temperature. This paper presents a technique for determining emissivity as a function of temperature of a thin solid-surface, partially transmitting to infrared radiation, which undergoes a phase change due to pyrolysis. This technique involved heating the *National Center for Microgravity Research + NASA Lewis Research Center Copyright

FLAME SPREADING OVER THIN FUEL SAMPLES IN PARTIAL GRAVITY ENVIRONMENTS

This papers summarizes aspects of conducting flame spread experiments in a partial gravity environment aboard a NASA aircraft flying parabolic trajectories. The acceleration environment is increasingly disturbed by variations in the acceleration environment called gjitter as the nominal acceleration approaches zero. Results of flame spreading tests for thin solid fuels spreading downward in partial gravity are reviewed. Both material flammability and flame spread rates are shown to be enhanced at Lunar and Martian gravity levels compared to results at normal Earth gravity. An improved method for correlating these data using dimensionless spread rate, Damkohler number and a radiative loss parameter is presented.