FORCED IGNITION AND FLAME SPREAD TEST (FIST)
Schematic of FIST Apparatus
All
experiments are conducted in the FIST (Forced Ignition and flame Spread Test)
apparatus, developed at U.C. Berkeley under NASA sponsorship to test the
flammability of materials in environments expected in spacecraft. The
experimental apparatus consists of a small-scale combustion wind tunnel and
supporting instrumentation. The tunnel inlet section is a settling chamber and
converging nozzle used to supply the oxidizer flow into the test section. The
test section is 560 mm long, and has a rectangular cross section 127 mm by 100
mm. The floor of the duct and sample holder is phlogopite mica 6.35 mm thick.
Samples are flush mounted in the bottom surface of the duct with their
downstream edge adjacent to the wire igniter. The sidewalls of the test section
are made of 6 mm quartz windows for optical access. The wall opposite the fuel
specimen is fitted with two Research Inc. parabolic strip heaters that irradiate
the fuel sample with an approximately uniform heat flux. The oxidizer gas is
provided by mixing compressed air or oxygen with nitrogen. Ignition is induced
with an electrically heated Kanthal wire coil that acts as a pilot and is
located 10 mm downstream from the trailing edge of the fuel sample. The coil entirely covers the vapor fuel concentration
boundary layer thus eliminating the pilot location, in the direction normal to
the flow, as a parameter of the problem.
Sample front and back face temperatures are measured with type K thermocouples placed at the center of the test specimen. The data is used primarily to determine the instant of ignition by the sharp peak that appears in the thermocouple trace. Although care is taken to ensure that the thermocouple bead is embedded flush with the sample surface, the measured temperature should only be viewed as an approximation to the actual surface temperature due to movement of the thermocouple resulting from surface density changes. Surface thermocouple measurements are corroborated with comparison to IR images of the surface.
A Denver Instruments SI 224 data logging scale with 0.1 mg
resolution is positioned below the fuel sample and fitted with the sample holder
to record continuous mass loss during sample heating and ignition. The mass loss
data is used to calculate the mass flux using a finite difference method for a
period between 10 and 14 seconds bracketed around the point of interest.
Two solid thermoplastic fuels are used: Black Polymethylmethacrylate (PMMA) and a blended Polypropylene/fiber glass (PP/GL). PMMA is a solid polymer of homogeneous composition having well-known properties. The PP/GL is a laboratory-made composite material with a polypropylene matrix and chopped fiberglass filler. Uniform distribution of glass fibers within the polypropylene matrix results in globally isotropic properties that simplify theoretical modeling and analysis of experimental results. PP/GL samples in this work were 30% fiberglass by mass and 70% blackened polypropylene, 10 mm thick by 30 mm on edge.
The FIST tunnel intended for the International Space Station contains seven sample holders and a radiometer plate for calibration of heat flux reaching the surface of the samples. The sample holders are placed on a carrousel that rotates the samples into place below the heater and in the wind tunnel test section at the start of each test. Flow velocity, oxygen concentration and external heat flux are set at the start of each test. After a sample has ignited and measurements are complete, the sample is extinguished and rotated out of the test section.
