Deflagration is subsonic and detonation is supersonic. What determines the category of an explosion scenario?

• A. Rate of release
• B. Fuel type
• C. The speed of the combustion wave relative to the speed of sound
• D. Confinement level

Answer: (C)

The category is determined by how fast the burning front moves compared with the speed of sound in the mixture. If the combustion front travels slower than sound, heat transfer and diffusion govern the process, and you have a deflagration. If the front outruns sound and a shock wave forms ahead of the reaction zone, the process becomes a detonation. In air, the sound speed is about 343 m/s, while deflagration flame speeds are typically only a few meters per second, and detonations run at hundreds to thousands of meters per second. So the essential criterion is the wave speed relative to sound, not the rate of energy release, the fuel type, or the level of confinement (which can influence whether a deflagration can transition to detonation).