Blowout velocity

Wells (Ref 2) recommends TBB for use as a fuel in high Mach number aircraft jet motors because of its v high flame speed and blowout velocity, with the unique safety feature that it doesn t bum when exposed to air, but must be sparyed before it exhibits its pyrophoric characteristics. .. ... 

I have vivid memories from the late 1940s of Stanley Wetterau s demonstration of blowout velocity in glass equipment at Hydrocarbon Research, Inc. s laboratory, near Trenton, New Jersey. At an air velocity a bit below blowout, he showed me a dense bed of powder, albeit carryover from the bed was evident to the eye. By adjusting the air flow just a minuscule amount upward, Wetterau caused the bed to vanish, almost in an instant. When someone presented Wetterau with a new powder, his first action was to determine blowout. He argued that this velocity was a better indicator of a powder s usefulness for a fluid-bed process than anything else he could measure. 

Experiments show that flame blowout occurs when vapor exit velocities are as high as 20 to 30% of the sonic velocity of the stack vapors. These results were obtained with small diameter pipes up to 0.152 inches. There is evidence that higher blowout velocities are attainable with pipes of larger diameter such as flare stacks but in the absence of data on blowout velocities for flare stacks, it is good practice to size flare stacks on a basis of 20% of the sonic velocity as the exit velocity. 

Figure 6.7.7 Fluidization regimes with fine particles (a minimum fluidization velocity, b beginning of bubbling, c minimum discharging velocity (terminal velocity in free fall), and d blowout velocity. Adapted from Froment and Bischoff (1990) Squires, Kwauk, and Avidan (1985) and Avidan and Shinnar (1990).

Fluidization regimes with fine particles. After Squires et al. [1985]. (a) Minimum buoyancy (b) Minimum bubbling (c) Terminal velocity (d) Blowout velocity. 

The best fit of velocity exponent n in Hp °c ug (Figure 4.3.11) for pure propane (n-butane) is n = 4.733 (3.638), corresponding to Sc = 1.37 (1.61) from n = (2Sc-l)/ (Sc -1), which agreed well with the suggested value of Sc = 1.376 (1.524). The experimental liftoff height data are shown in Figure 4.3.12 for various nozzle diameters and partial air dilutions to fuel [53]. It can be observed that the air dilution to fuel does not alter Ypst and S° sf The results substantiated the role of tri-brachial flames on flame stabilization in laminar jets. As mentioned previously. Equation 4.3.5 limits the maximum velocity Ug for Sc > 1, which corresponds to blowout condition. 

Flares are sometimes used after knockout drums. The objective of a flare is to burn the combustible or toxic gas to produce combustion products that are neither toxic nor combustible. The diameter of the flare must be suitable to maintain a stable flame and to prevent a blowout (when vapor velocities are greater than 20% of the sonic velocity). 

The actual construction details of blowout panels is beyond the scope of the text. A detached blowout panel moving at high velocity can cause considerable damage. Therefore a mechanism must be provided to retain the panel during the deflagration process. Furthermore, thermal insulation of panels is also required. Construction details are available in manufacturers literature. 

A patented water injection system has been devised for extinguishing oil and gas well fires in case of a blowout. The "Blowout Suppression System" (BOSS) consist of finely atomized water injected to the fluid stream of a gas and oil mixture before it exits a release point. The added water lowers the flame temperature and flame velocities thereby reducing the flame stability. In the case where the flame cannot be completely dissipated, the fire intensity is noticeably deceased, preserving structural integrity and allowing manual intervention activities. A precaution in the use of such a device is that, if a gas release fire is suppressed but the flow is not immediately isolated, a gas cloud may develop and exploded that would be more destructive that the pre-existing fire condition. 

In either case, bluff body or aerodynamic, blowout is the primary concern. In ramjets, the smallest frontal dimension for the highest flow velocity to be used is desirable in turbojets, it is the smallest volume of the primary recirculation zone that is of concern and in dump combustors, it is the least severe step. 

Several disadvantages are associated with the fluidized bed. The equipment tends to be large, gas velocities must be controlled to reduce particle blowout, deterioration of the equipment by abrasion occurs, and improper bed operation with large bubble sizes can drastically reduce conversion. 

Let us now consider continuous flows of premixed combustible gases and address the question of conditions necessary to retain a flame in the system [2]. This question is of practical significance for many power-production devices. To achieve high power densities, gas velocities in combustors exceed flame velocities, and so means must be found to stabilize flames against blowout, a condition at which the flames are transported through the exit of the burner so that combustion ceases. There are two main classes of stabilization techniques, stabilization by fluid streams and stabilization by solid elements. Although other stabilization methods may be envisioned, such as continuous or intermittent deposition of radiant or electrical energy, in the vast majority of practical continuous-flow systems, stabilization is obtained by techniques that fall within one of the two main classes. Stabilization by solid elements will be discussed first then stabilization by fluid streams will be considered. ... 

Correlations for normalized flame height as a function of the distance from the fuel orifice along the axis of the curved flame have also been developed. With an increase in crossflow velocity the flame height decreased up to a certain point, beyond which the flame height increased. However, when the blowout was approached, the flame height again decreased. 

The space velocity, often used in the technical literature, is the total volumetric feed rate under normal conditions, F o(Nm /hr) per unit catalyst volume (m X that is, PbF o/W. It is related to the inverse of the space time W/F g used in this text (with W in kg cat. and F q in kmol A/hr). It is seen that, for the nominal space velocity of 13,800 (m /m cat. hr) and inlet temperatures between 224 and 274 C, two top temperatures correspond to one inlet temperature. Below 224 C no autothermal operation is possible. This is the blowout temperature. By the same reasoning used in relation with Fig. 11.5.e-2 it can be seen that points on the left branch of the curve correspond to the unstable, those on the right branch to the upper stable steady state. The optimum top temperature (425°C), leading to a maximum conversion for the given amount of catalyst, is marked with a cross. The difference between the optimum operating top temperature and the blowout temperature is only 5°C, so that severe control of perturbations is required. Baddour et al. also studied the dynamic behavior, starting from the transient continuity and energy equations [26]. The dynamic behavior was shown to be linear for perturbations in the inlet temperature smaller than 5°C, around the conditions of maximum production. Use of approximate transfer functions was very successful in the description of the dynamic behavior. 

This example is a bit unrealistic in that the flame will most likely blow out due to the high exit velocity of the jet. As the flow velocity of the jet is increased, the flame moves downstream to a new location where the turbulent burning velocity equals the flame velocity. As the velocity is increased, a point is eventually reached where the burning location is so far downstream that the fuel concentration is below the lower flammability limit due to air entrainment. Mudan and Croce (1988) provide flame blowout criteria. 

Stability limits are provided in Fig. 6.6 for two inlet velocities, p = 5 bar and Tjj,j = 700 K, in terms of the critical heat transfer coefficient for extinction. For low thermal conductivities k < 2 W/mK), the reduced upstream heat transfer hinders catalytic ignition (light-off) and causes blowout. The stability limits at low fcs are narrower at higher inlet velocities (Fig. 6.6). In comparison to pure gas-phase combustion studies [18], there is a marked difference at the low behavior, which is discussed qualitatively (since the aforementioned work refers to different geometry and operating conditions). In gas-phase combustion, the blowout limits extend over a narrower range of ( 0.4-0.8 W/mK) and are nearly independent of h (the blowout limit line is almost parallel to the /i-axis). This is because low... 

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