Burn rate (chemistry)
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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. Burn rate is measured in length over time, such as "mm/second" or "inches/second". Burn rate is a property of combustible substance and it quantifies the combustion rates. Burn rates can be measured and are different for a given substance at different pressures (equal to, above or below ambient pressure). Burning rate typically increases with pressure and temperature. There are few exceptions in which the burn rates are either neutral (e.g. black powder) or varies inversely with pressure. A substance is characterized through burn rate vs pressure chart and burn rate vs temperature chart.
[edit] Measurement
One apparatus 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 & temperature. For solid fuel propellant, the most common method of measuring burn rate is the Crawford Type Strand Burning Rate Bomb System, as described in MIL-STD-286C. Also known as the Crawford Burner or Strand Burner.
[edit] Characterization
If a sample burns (or otherwise reacts such as by shock wave) at a rate in excess of Mach 1 or 1,138 feet per second (347 m/s) it is called "detonation". If sample burn rate is in few meters per second it is called "deflagration". If sample burn rate is few centimeters per second it is generally understood that the sample neither detonates nor deflagrates, but rather burns or smolders. Between the range of 0.01 mm/s and 100 mm/s most scientists agree the sample is burning not deflagrating on the basis that deflagrating uses the term "decomposes rapidly" to characterize it. However, there is difference in opinion in differentiating the three in absence of firm numbers at given pressure or temperature.
