The experiment is conducted in a small scale, vertically oriented, combustion chamber along with supporting instrumentation. The test section is a 300mm long, 120mm diameter cylinder, and is made of 3.2mm (1/8") thick, stainless steel tubing with the inside walls insulated with FiberFrax QF-180 white, insulating cement. The outside walls are wrapped in one layer of 3.2mm (1/8") FiberFrax 970-J insulating paper held in place with strips of aluminum tape. A flow diffuser and igniter are placed at the bottom of the test section. The oxidizer flow is diffused through a 1.6mm (0.062") thick 2mm pore-size, porous stainless steel plate. The igniter consists of a 0.38mm (0.015") Nichrome wire sandwiched between two 5mm thick, 120mm diameter, flat, porous honeycomb, ceramic plates that provide rigidity to the igniter. The igniter wire has a cold resistance of 7.9W. Porous FiberFrax Durablanket-S insulation, 50mm thick, is placed between the flow diffuser and the bottom ceramic plate of the igniter to insulate the bottom of the igniter. All tests are run using open cell, unretarded, polyurethane foam samples, with 26.5kg/m3 density and 0.975 void fraction. Bottled air is used as the oxidizer, with the flow rate being controlled by Tylan Model FC280S and Brooks Model 5858A mass flow controllers.
A temperature history of each smolder propagation is obtained via K-type, (chromel-alumel), thermocouples with a 0.8mm diameter stainless steel sheath in a 1.6mm (1/16") stainless steel protective sleeve, embedded into the foam through holes drilled in the side of the combustion chamber. The thermocouples are spaced 20mm apart along the fuel centerline with the first at the igniter-fuel interface. They are connected to a multiplexer board which is connected to an analog to digital converter card in a personal computer. Exhaust gas composition is measured by a Horiba gas analyzer connected to a probe suspended above the top of the sample. The output from the analyzers is also fed into the personal computer for logging. The sample exhaust gas is bubbled through an ice water slurry before being taken to the analyzers to prevent heavy hydrocarbons from fouling the gas lines and the analyzers themselves.
The test samples are cylinders cut to dimensions of 300mm long and 132mm in diameter, (approx. 104g). The over-sizing provides a 10% diameter compression of the foam which insures good contact between the sides of the chamber and the sample, thereby preventing any oxidizer by-passing the sample via leakage along the wall. The sample is packed into the chamber with 5mm protruding from the igniter end of the chamber. This protrusion is compressed against the ceramic honeycomb of the igniter when the apparatus is assembled, insuring good igniter to foam contact.
Two air flow rates are used during each experiment. An initial "ignition" air flow velocity, Vign, is used while the igniter is on, and a "smolder" flow velocity, Vox, is used after the igniter is switched off. Ignition flows with a velocity of 0.1mm/s are chosen to compare with the aforementioned NASA experiments. The igniter is run at various powers and times, and is controlled by a Variac rheostat regulating AC voltage. The igniter power is monitored by two digital multimeters measuring voltage across and current through the igniter. When the prescribed time for the igniter to be powered has elapsed, the igniter is switched off and the air flow rate is switched from the ignition flow velocity, Vign, to the smolder flow velocity, Vox. The thermocouples’ and gas analyzer output are continuously monitored and recorded to file using the personal computer.
Post combustion analyses include weighing the sample and residual char, visually examining the sample and recording observations and events, as well as determining smolder propagation extent and velocity.