Microgravity Combustion: Droplet Flame Lab

Combustion Rack

Gravity 0.00 g

Ambient O₂ 21%

Droplet diameter 1.20 mm

Readouts

Standing by — press Ignite.

Droplet Ø

1.20 mm

Flame stand-off

— mm

Burn-rate K

— mm²/s

Burn time

0.0 s

How it works

Fiber-supported droplet combustion is a real ISS experiment family (FLEX, ACME) flown in the Combustion Integrated Rack. A liquid fuel droplet burns inside a spherical diffusion flame whose size and shape follow the classical d²-law:

D(t)² = D₀² − K·t
K = K₀ · ln(1 + B) · (1 + C·√g)

D is droplet diameter, D₀ the initial diameter, K the burning-rate constant, B the Spalding transfer number (rises with ambient O₂), and the √g term is a simplified stand-in for the natural-convection (Grashof-number) enhancement that only exists when buoyancy exists.

Gravity = 0 — no buoyancy, so fuel vapor and oxidizer meet purely by diffusion. The flame sits as a near-perfect blue sphere around the droplet, burns slower and cleaner (little to no soot), and can undergo two-stage "cool flame" burning at low O₂.
Gravity → 1g — a buoyant plume rises off the flame, dragging in extra oxidizer and stretching the envelope into the familiar yellow, sooty teardrop seen on Earth. Convection raises K, so the same droplet burns faster.
Ambient O₂ — more oxidizer raises the transfer number B, driving both flame temperature and K up at any gravity level.
The flame extinguishes when the droplet is fully consumed, or — in the low-g / low-O₂ corner — when radiative heat loss from the flame outpaces the (slow, diffusion-only) heat release, exactly the radiative-extinction limit ISS crews study to understand spacecraft fire safety.

Real-world relevance: microgravity droplet-flame data feeds directly into spacecraft fire-safety standards and cleaner terrestrial diesel/spray-combustion models, since both are diffusion flames around a shrinking droplet.

Diffusion flame Soot / convection plume

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