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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 substances 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 vary inversely with pressure. A substance can be characterized by a burn rate vs pressure chart and burn rate vs temperature chart.
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 and temperature. Since burn rate depends on surface area, standard sample dimensions are used. 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. It is also known as the Crawford Burner or Strand Burner.
The burning rate measured from a strand burner is typically 4 to 12% less than actual burning rate observed in rockets. This is because the high temperature conditions in actual rocket are not simulated. The heat transfer characteristics are also different. Nevertheless, the strand burner experiment is easy to perform, can be repeated and a qualitative picture of the burning rate is obtained. The temperature sensitivity of burning rate is usually calculated from strand-burner test data.
Characterization
If a sample burns (or otherwise reacts such as by generating a shock wave) at a rate in excess of Mach 1 or the speed of sound it is called "detonation". If the sample burn rate is slower than Mach 1, but the reaction is several meters per second, it is called "deflagration". If sample burn rate is a 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 a difference of opinion in differentiating the three designations in absence of firm numbers at a given pressure or temperature.
In Pyrotechnic Compositions
Pyrotechnic compositions include fireworks, gunpowder, and rocket fuels.
The burn rate of a pyrotechnic composition is determined by seveal factors. All of these factors must be contolled in the manufacturing process for products to have reproducible peformance. Factors of burn rate include:
1. Selection of oxidizer, fuel, and other components, 2. Weight ratio of the components, 3. Particle size of the components, 4. Homogeneity of the blended composition, 5. Presence of moistue, 6. Consolidation pressure (of pressed materials), and 7. External temperatue and pressure when used.
Changing the burn rate
Based on these factors, there are several ways to intentionally change a burn rate. The starting chemicals can be changed: the type of fuel, oxidizer, or both. Metal fuels with highly exothermic rates of combustion and exdcellent thermal conductivity tend to increase the burn rate. Volatile fuels with a low melting point (such as sulfur) tend to slow the burn rate even though they enhance ignitability. The oxidizer-fuel ratio can also be changed. Specific chemicals known as catalysts and retardants can be included to increase or decrease the burn rate.
Burn rate increases as burning surface area increases. Therefore, small grains burn faster than large grains of fuel due to the greater surface area. Also, compositions loaded into a narrow tube will burn more slowly than when they are loaded into a wider tube.
An increase in burn rate usually occurs with an incease in external pessure. The pressure acts as a confinement to gas products and causes more heat to be retained by the burning materials, thereby accelerating the burn rate. External pressure will only affect the burn rate as it brings the substance closer or farther from ignition temperature. The presence of moisture can geatly slow the burn rate by absorbing substantial quantities of heat through vaporization.
Combustion rate will increase as loading density decreases. A lightly consolidated, porous powder will burn the fastest, while a highly consolidated mixture under considerable pressure will burn more slowly, because the effective surface area is decreased.
Examples
Ammonium Perchlorate Composite Propellant
Ammonium Perchlorate composite propellant (APCP) is a solid rocket propellant used in expeimental applications as well as in manned and unmanned space applications. Most formulations have a burn rate between 1-3 mm/s at STP and 6-12 mm/s at 68 atm. The burn characteistics (such as linear burn rate) are often determined prior to rocket motor firing using a strand burner test. This test allows the APCP manufacturer to characterize the burn rate as a function of pressure. Empirically, APCP adheres fairly well to the following power function model:[1]
.
It is worth noting that typically for APCP, 0.3 < n < 0.5, which indicates that APCP is sub-critically pressure sensitive. That is, if surface area were maintained constant during a burn the combustion reaction would not run away to theoretically infinite as the pressue would reach an internal equilibrium. This is not to say that APCP cannot cause an explosion, but rather that the explosion would be caused by the pressure surpassing the burst pressure of the container (rocket motor).
Other burn rates
Potassium Nitrate (KNO3) is a chemical compound used in rocket propellants and fireworks, and is also one of the constituents of gunpowder. It has been measured to have a burn rate of 3.8 mm/s,[2] which means that its burning reaction is classified as burning and not deflagrating.
Sugar and KNO3 based propellants generally have lower burn rates:
Sorbitol and KNO3: 2.7 mm/s,
Dextrose and KNO3: 1.8 mm/s,
Fructose and KNO3: 1.9 mm/s,
and Xylitol and KNO3: 2.6 mm/s.[3]
References
