Flame propagation rate

Some of the tests and criterion used to define fire resistance may be found in the Hterature (9). Additionally, the compression—ignition and hot manifold tests as defined in MIL-H-19457 and MIL-H-5606, respectively the Wick test as defined by Federal Standards 791, Method 352 flash point and fire point as defined in ASTM D92 autoignition temperature as defined in ASTM D2155 and linear flame propagation rate are defined in ASTM D5306 are used. 

Sometimes conveying (qv) velocities, typically 20—25 m/s, exceed the flame-propagation rate as determined by tests. Moreover, velocities vary throughout the system. If the flame-propagation rate exceeds the conveying velocities, consideration must be given to the isolation of parts of the system by choking, ie, the use of rotary values, screen conveyors, bins, etc. 

The concept of turbulent flame stretch was introduced by Karlovitz long ago in [15]. The turbulent Karlovitz number (Ka) can be defined as the ratio of a turbulent strain rate (s) to a characteristic reaction rate (to), which has been commonly used as a key nondimensional parameter to describe the flame propagation rates and flame quenching by turbulence. For turbulence s >/ />, where the dissipation rate e and u, L and v... 

It is interesting to note that stratified combustible gas mixtures can exist in tunnel-like conditions. The condition in a coal mine tunnel is an excellent example. The marsh gas (methane) is lighter than air and accumulates at the ceiling. Thus a stratified air-methane mixture exists. Experiments have shown that under the conditions described the flame propagation rate is very much faster than the stoichiometric laminar flame speed. In laboratory experiments simulating the mine-like conditions the actual rates were found to be affected by the laboratory simulated tunnel length and depth. In effect, the expansion of the reaction products of these type laboratory experiments drives the flame front developed. The overall effect is similar in context to the soap bubble type flame experiments discussed in Section C5c. In the soap bubble flame experiment measurements, the ambient condition is about 300 K and the stoichiometric flame temperature of the flame products for most hydrocarbon fuels... 

The above flame retardants, HMPN and TMP, along with another commercially available alkyl phosphate, triethyl phosphate (TEP), were systematically characterized by Xu et al. To quantify the flammability of the electrolytes so that the effectiveness of these flame retardants could be compared on a more reliable basis, these authors modified a standard test UL 94 HB, intended for solid polymer samples, and measured the self-extinguishing time (SET) instead of the universally used flame propagation rate. Compared with the UL 94 HB, this new quantity is more appropriate for the evaluation of the electrolytes of low flammability, since the electrolytes that are determined to be retarded or nonflammable by this method all showed zero flame propa-... 

We express the chemical reaction time r in terms of the normal flame propagation rate since the latter is fairly well studied experimentally as a function of various parameters (mixture composition, etc.). 

To take into account the stretch effect on the flame propagation rate, the consecutive theory needs the treatment of the flame structure. The phenomenological Markstein (1951) theory is the first in this direction,... 

Burning velocity, or flame speed or flame propagation rate, is defined, in simple terms, as the speed of a flame relative to the gas-oxidant mixture. The burning velocity is of critical importance in flame unit design. 

The art of designing flame units for fuels with relatively low flame propagation rates lies in surrounding the main flame port with a suitable number of pilot flames of appropriate diameter, through which the fuel-oxidant mixture emerges at the required velocity. An arrangement of pilot flames is shown in Fig. 4.20. 

Hydraulic fluid flammability is a difficult property to assess and flash point demonstrates only one aspect of that flammability, namely the temperature at which, under the conditions of the test, sufficient vapour is produced to generate a flash when a flame is applied to the vapour space above the liquid. Another approach is to determine the flame propagation rate, the rate at which a fire spreads once the fluid has ignited. Typical flame propagation rates for these fluids are stated in Table 11.2 [26] ... 

Table 11.2 Flame propagation rates for hydrocarbon-based hydraulic fluids...

The higher the flame propagation rate, the faster the fire spreads and therefore the more flammable the fluid, as measured by the test. As can be seen this approach ranks the fluids in the same order as the flash point, with H-515 being the most flammable and H-537 the least. There are a variety of methods that can be used to demonstrate flammability and some show the PAO fluids in a better light than others however, the ultimate test of flammability has to be the performance of the fluid in service. US Air Force statistics show quite clearly a significant reduction in aircraft losses since the introduction of the PAO-based H-537 into service. 

Flammable mixtures are formed above a flammable liquid pool only if the flash point is exceeded (Figure 20). However, at this temperature a flame does not persist because the flame propagation rate exceeds the rate at which the flammable mixture zone is formed. Sustained burning occurs above a liquid pool at a slightly higher temperature, the fire point. At this temperature, a range of vapor-air mixtures may be found above an open pool in air. In a quiescent atmosphere this range includes mixtures near the liquid surface with a vapor content above that found in a lower-limit mixture, and others with essentially no vapor some distance from the surface mixtures with intermediate concentrations are found in the space between these extremes. 

Flammability, flame propagation rate cm min ASTM D635 2.5 (5)... 

Water absorption Flammability Flame propagation rate Intrinsic viscosity... 

---