Flammability Limit Dependence on Pressure

Pressure has little effect on the LFL except at very low pressures ( 50 mm Hg absolute), where flames do not propagate. 

The UFL increases significantly as the pressure is increased, broadening the flammability range. An empirical expression for the UFL for vapors as a function of pressure is available 6 

UFL is the upper flammable limit (volume % of fuel plus air at 1 atm). 

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The flammability limits depend on the temperature and pressure. At elevated temperatures and pressures, a given mixture may be flammable, even if this is not the case at ambient conditions. It is a key objective of the design of the inlet zone of a CPO reactor to prevent ignition and thermal combustion. 

Flammability limits of vapor Dependence on temperature T, °C Dependence on pressure... 

An alternate method for flash point prediction is the method of Gmehling and Rasmussen and depends on the lower flammabihty limit (discussed later). Vapor pressure as a function of temperature is also required. The method is generally not as accurate as the preceding method as flammability limit errors are propagated. The authors have also extended the method to defined mixtures of organics. 

A confined explosion occurs in a confined space, such as a vessel or a building. The two most common confined explosion scenarios involve explosive vapors and explosive dusts. Empirical studies have shown that the nature of the explosion is a function of several experimentally determined characteristics. These characteristics depend on the explosive material used and include flammability or explosive limits, the rate of pressure rise after the flammable mixture is ignited, and the maximum pressure after ignition. These characteristics are determined using two similar laboratory devices, shown in Figures 6-14 and 6-17. 

Flammability limits are not absolute, but are dependant on temperature, pressure, and other variables. Care must be exercised in using flammability limit data when conditions are different from ambient. For example, in reactors and thermal oxidizers. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

Pressure oscillations in the first arrangement depended on the equivalence ratio of the flow in the annulus and decreased with velocities in the pilot stream greater than that in the main flow due to decrease in size of the recirculation zone behind the annular ring and its deflection towards the wall. Increase in swirl number of the second arrangement caused the lean flammability limit to decrease, and the pressure oscillations to increase at smaller values of equivalence ratio. Unpremixedness associated with large fuel concentrations at the centre of the duct increased the pressure oscillations. Pressure oscillations caused the position of flame attachment to move downstream in both flows with a decrease in amplitude of oscillations. 

Burgoyne and Thomas (17G) have passed mixture air through an iron-electrode arc prior to passing into a Bunsen tube. The effect of the minute iron oxide particles contained in the stream is to lower the lean limit of flammability of hydrogen. Sanger (33G) deals with such topics as the dependency of combustion on pressure, temperature, and the moisture content of powders, and gives a series of equations in support of his hypothesis. 

Flammability limits. The range of flammable vapor-air or gas-air mixtures between the upper and lower flammable limits. Flammability limits are usually expressed in volume percent. Flammability limits are affected by pressure, temperature, direction of flame propagation, oxygen content, type of inerts, and other factors. The precise values depend on the test method. 

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