Stretched flames

Wu, C.K. and Law, C.K., On the determination of laminar flame speeds from stretched flames, Proc. Combust. Inst.,... 

Rotman, D.A. and Oppenheim, A.K., Aerothermo-dynamic properties of stretched flames in enclosures, Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp. 1303-1312,1986. 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

Experimentally, two modes of extinction, based on the separation between the twin flames are observed. Specifically, the extinction of lean counterflow flames of n-decane/02/N2 mixtures occurs with a finite separation distance, while that of rich flames exhibits a merging of two luminous flamelets. The two distinct extinction modes can be clearly seen in Figure 6.3.2. As discussed earlier, the reactivity of a positively stretched flame with Le smaller (greater) than unity increases (decreases) with the increasing stretch rate. Therefore, the experimental observation is in agreement with the... 

Buckmaster, J., The effects of radiation on stretched flames. Combust. Theory Model., 1, 1,1997. 

C.K. Wu and C.K. Law. On the Determination of Laminar Flame Speeds from Stretched Flames. Proc. Combust. Inst., 20 1941-1949,1984. 

The nature of the solution to equation (52) depends on the sign of k. Positive stretch corresponds to /c > 0 and negative stretch, flame-sheet compression, corresponds to k < 0. For positive stretch, according to the sign of in equation (52), material flows toward the flame from... 

For spray combustion in diesel engines and gas turbines, the flame types for premixed and non-premixed combustion fall mainly into the wrinkled and stretched flame regime in region (3), and into the distributed or perturbed flames in region (4). 

C.J. Sun, C.J. Sung, L. He, C.K. Law, Dynamics of weakly stretched flames quantitative description and extraction of global flame parameters. Combust. Flame 118, 108-128 (1999)... 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

After the skimmer, the ions must be prepared for mass analysis, and electronic lenses in front of the analyzer are used to adjust ion velocities and flight paths. The skimmer can be considered to be the end of the interface region stretching from the end of the plasma flame. Some sort of light stop must be used to prevent emitted light from the plasma reaching the ion collector in the mass analyzer (Figure 14.2). 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

The flow structures of lean limit methane and propane flames are compared in Figures 3.1.2 and 3.1.3. The structure depends on the Lewis number for the deficient reactant. A stretched lean limit methane flame (Lepreferential diffusion, giving it a higher burning intensity. Hence, the flame extinction limit is extended. On the other hand, for a stretched lean limit propane flame (Le>l), the same effect reduces the burning intensity, which can... 

The flame stretch rate distribution was calculated based on experimental PIV data, using the semitheoretical... 

The shape of the stretch rate distribution curve for the lean limit propane flame, shown in Figure 3.1.9b, is very similar. However, for this flame, the estimated contribution of the curvature is more important than with the methane flame, reaching about 40% of the maximum stretch rate. 

Stretch rate as a function of distance, from the top leading point along the flame front, (a) for a lean limit methane flame and (b) lean limit propane flame propagating upward in a standard cylindrical tube. 

Direct images of the three counterflow twin flames with decreasing stretch rate through the reduction of the mixture flow rate (from top photo to bottom photo), while keeping the flow mixture composition constant. 

It can also be noted that the slope of the S , .,f-Ka plot reflects the combined effect of stretch rate and non-equidiffusion on the flame speed. Figure 4.1.6 clearly shows that the flame response with stretch rate variation differs for lean and rich mixtures. In particular, as Ka increases, the S for stoichiometric and rich mixtures increases, but decreases for the mixture of equivalence ratio = 0.7. This is because the effective Lewis numbers of lean w-heptane/ air and lean /so-octane/air flames are... 

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