Liftoff height

A stationary lifted flame will be stabilized at (xV )/ where Yp=yps, and u = S. The liftoff height Hl=x and the limitation on jet velocity can be derived from Equation 4.3.3 as follows ... 

The relevance of nonpremixed edge flames to turbulent nonpremixed flames can be described in two aspects. One is the mechanism of turbulent nonpremixed lifted flames and the other, the flame-hole dynamics. For turbulent lifted flames in nonpremixed jets, the liftoff height is linearly dependent on jet velocity. There have... 

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. 

Liftoff height with jet velocity in free jet [10] (H attached flame length, quenching distance, Hp liftoff height, L premixed flame length). 

Correlation between liftoff height and jet velocity with partial premixing of air to fuel stream. (From Lee, B.J., Cha, M.S., and Chung, S.H., Combust. Sci. Technoh, 127, 55,1997.)... 

A link between laminar and turbulent lifted flames has been demonstrated based on the observation of a continuous transition from laminar to turbulent lifted flames, as shown in Figure 4.3.13 [56]. The flame attached to the nozzle lifted off in the laminar regime, experienced the transition by the jet breakup characteristics, and became turbulent lifted flames as the nozzle flow became turbulent. Subsequently, the liftoff height increased linearly and finally blowout (BO) occurred. This continuous transition suggested that tribrachial flames observed in laminar lifted flames could play an important role in the stabilization of turbulent lifted flames. Recent measurements supported the existence of tribrachial structure at turbulent lifted edges [57], with the OH zone indicating that the diffusion reaction zone is surrounded by the rich and lean reaction zones. 

This discussion represents only a broad outline of the ideas. There are a number of questions of detail, for example, concerning the radial position in the jet at which a condition like Xc Xe should be applied. A position might best be selected at which the concentration of stoichiometric surfaces is greatest this lies off the axis of symmetry, at least for low liftoff heights. At sufficiently high jet velocities, very few stoichiometric surfaces will occur anywhere with Xc Xe blowoff must occur. Thus the viewpoint is consistent qualitatively with observations. 

The second category includes BLEVE simulation, in which a pressurized, heated flask containing liquid or liquefied fuel is broken after the desired vapor pressure has been reached, and the released vapor is then ignited. Measurement of fireball diameter, liftoff time, combustion duration, and final height is captured by filming with high-speed cameras. Radiometers are used to measure radiation and temperature is measured by thermocouples or by determination of fireball color temperature (Lihou and Maund 1982). 

FIGURE 6.10 Variation of the character (height) of a gaseous diffusion flame as a function of fuel jet velocity showing experimental flame liftoff (after Linan and Williams [13] and Hawthorne etal. [14]). 

The first application of hydrogen as a fuel was starting in the late 18th century when it was tried to utilize the physical property of low density for flying balloons. The Frenchman Charles realized in 1783 the first liftoff of an H2 balloon ( Charliere ) filled with 40 m of H2 which he produced by spilling sulfuric acid onto iron. The balloon traveled a distance of 25 km in a height of up to about 1 km. 

Figure 6 Scanning Force Microscopic (SFM) image of PFS-6-PDMS diblock copolymer cylinders deposited along a prepattemed groove on a resist film, followed by liftoff with acetone followed by hydrogen plasma treatment the aligned nanoscopic line, height 4nm, is composed of clusters of Fe, Si, O, and C (adapted from Ref. 43).

Figures 11.11 and 11.12 present Uft-off characteristics of a hydrogen diffusion flame. The diagram in Fig. 11.11 was plotted using data from [23]. It is seen that liftoff occurs at a flow rate exceeding 600 m/s. The flame lift-off height grows linearly with increasing flow rate and can exceed the sound speed. Close results have been obtained in [30-32]. To evaluate the data discrepancy level, the line approximating the measured data from [23] is plotted with the experimental points obtained in [30-32].

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