Flame Front Expansion Velocity Calculator

Flame Front Expansion Velocity Calculator

A flame front observed from a fixed point in space moves faster than the underlying laminar burning velocity, because combustion products expand as they heat up and push the flame forward. This calculator converts burning velocity into that observed front velocity using the gas expansion ratio.

The Formula

V = S_L × (ρ_u / ρ_b)

Worked Example

Unburned air-fuel mixture density: 1.13 kg/m³. Burned gas density: 0.17 kg/m³ (a typical hot combustion product density). Expansion ratio = 1.13 / 0.17 = 6.6.

V = 0.4 m/s × 6.6 = 2.66 m/s

Notice the front moves nearly seven times faster than the laminar burning velocity itself - this expansion effect is exactly why open flames appear to move so much faster than the chemistry alone would suggest.

How to Use This Calculator

1. Enter the laminar burning velocity in m/s.
2. Enter the unburned gas density in kg/m³.
3. Enter the burned (combustion product) gas density in kg/m³.

Related: Laminar Burning Velocity Calculator.