Correlation of Normal Gravity Mixed Convection Blowoff Limits with Microgravity Forced Flow Blowoff Limits
Marcum, Jeremy W., Olson, Sandra L., Ferkul, Paul V.
Experiments: BASS, BASS-II, SAME
The axisymmetric rod geometry in upward axial stagnation flow provides a simple way to measure normal gravity blowoff limits to compare with microgravity Burning and Suppression of Solids - II (BASS-II) results recently obtained aboard the International Space Station. This testing utilized the same BASS-II concurrent rod geometry, but with the addition of normal gravity buoyant flow. Cast polymethylmethacrylate (PMMA) rods of diameters ranging from 0.635 cm to 3.81 cm were burned at oxygen concentrations ranging from 14 to 18% by volume. The forced flow velocity where blowoff occurred was determined for each rod size and oxygen concentration. These blowoff limits compare favorably with the BASS-II results when the buoyant stretch is included and the flow is corrected by considering the blockage factor of the fuel. From these results, the normal gravity blowoff boundary for this axisymmetric rod geometry is determined to be linear, with oxygen concentration directly proportional to flow speed. We describe a new normal gravity 'upward flame spread test' method which extrapolates the linear blowoff boundary to the zero stretch limit in order to resolve microgravity flammability limits-something current methods cannot do. This new test method can improve spacecraft fire safety for future exploration missions by providing a tractable way to obtain good estimates of material flammability in low gravity.
Test conditions.
Extracted from the paper by AI · “Verified” means a person checked it
| Fuel | O₂ | Pressure | Airflow | Sensors | Source | AI conf. | Status |
|---|---|---|---|---|---|---|---|
| Cast polymethylmethacrylate (PMMA) Axisymmetric rods (radii: 0.318, 0.476, 0.635, 1.270, 1.905 cm) |
14% to 18% by volume | — | Forced flow up to 165 SLPM (60.3 cm/s average tube speed) | Color video camera, Hot wire anemometer | p. 1 | 95% | Pending |
| Cast clear PMMA Rod (radius 0.318 cm) |
14% | — | 2 cm/s | Color video camera | p. 3 | 90% | Pending |
| Cast clear PMMA Rod (radius 0.318 cm) |
14% | — | 2 cm/s | Color video camera | p. 3 | 90% | Pending |
| Cast clear PMMA Rod |
15% | — | Higher flow speeds (specific value not stated for this specific test) | Color video camera | p. 4 | 90% | Pending |
Findings.
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01
The normal gravity blowoff boundary for the axisymmetric rod geometry is linear, with oxygen concentration directly proportional to flow speed.
Other · p. 1 · AI confidence 95% · Reported · Pending
“the normal gravity blowoff boundary for this axisymmetric rod geometry is determine d to be linear, with oxygen concentration directly proportional to flow speed.”
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02
Flame blowoff always occurs at the stagnation region, triggered by the development of a flame hole sufficient to destabilize the flame base.
Other · p. 3 · AI confidence 95% · Reported · Pending
“Flame blowoff always occurs at the stagnation region . T he triggering event appears to be the development of a flame hole [1 3 ] at the stagnation region of sufficient size to destabilize the flame base .”
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03
The forced stretch rate at blowoff varies linearly with oxygen concentration and as the square root of the rod radius.
Other · p. 4 · AI confidence 95% · Reported · Pending
“the forced stretch rate at blowoff varies linearly with oxygen in addition to varying as the square root of the rod radius .”
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04
Data from different rod radii collapse into a single blowoff boundary when corrected for flame curvature and flow acceleration (blockage factor).
Other · p. 7 · AI confidence 90% · Reported · Pending
“The data collapses into a single blowoff boundary for all the normal grav ity data.”
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05
At 14% oxygen in the quartz tube setup, ambient air entrainment increased local oxygen concentration, preventing blowoff at low flow rates.
Other · p. 3 · AI confidence 90% · Reported · Pending
“At a sufficiently low forced flow rate, entrainment of air into the quartz tube from the top might be mixing with the supplied gas, increasing the ambient oxygen concen tration so that the rod remained flammable.”