Burn rate (chemistry)

In chemistry, the burn rate (or burning rate) is a measure of the linear combustion rate of a compound or substance such as a candle or a solid propellant. It is measured in length over time, such as "mm/second" or "inches/second". Among the variables affecting burn rate are pressure and temperature.

Measurement

One device for measuring burning rate is a V shaped metal channel about 1–2 feet long wherein a sample is placed, with a cross-sectional dimension of approximately 6 mm or 1/4". The sample is ignited on one end and time is measured until the flame front gets to the other end. Burn rate (typically expressed in 'mm/s' or 'in/s') is the sample length over time at a given pressure and temperature. For solid fuel propellant, the most common method of measuring burn rate is the Crawford Type Strand Burning Rate Bomb System^[1] (also known as the Crawford Burner or Strand Burner), as described in MIL-STD-286C.^[2]

Characterization

A substance is characterized through burn rate vs pressure chart and burn rate vs temperature chart.

• Higher burn rate than the speed of sound in the material (usually several km/s): "detonation"
• A few meters per second: "deflagration"
• A few centimeters per second: "burn" or "smolder"
• 0.01 mm/s to 100 mm/s: "decomposing rapidly" to characterize it.

However, there is difference in opinion in differentiating the three in absence of firm numbers at given pressure or temperature.

References