Opposing flow

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

I. Wichman, Theory of opposed-flow flame spread, Prog. Energy Combust. Sci. 18 553-593,1992. 

Kee, R.J., Miller, J.A., Evans, G.H., and Dixon-Lewis, G., A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames, Proc. Combust. Inst., 22, 1479, 1988. 

Barlow, R.S., Karpetis, A.N., and Frank, J.H., Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames, Combust. Flame, 127, 2102,2001. 

ShaJSc113 as indicated by the thin solid line. This 0.67 power of Re agrees with the result of a turbulent heat transfer measurement on a rotating sphere [40], Since the flow induced by a rotating sphere is also characterized by an outflowing radial jet at the equator caused by the collosion of two opposing flow boundary layers on the sphere, the 0.67 power dependence on Re is clearly related to the radial flow stream away from the equator. 

Harkelroad, M., Quintiere, J., Walton, W., "Radiative Ignition and Opposed Flow Flame Spread Measurements on Materials", Report No. DOT/FAA-CT-83/28, FAA Technical Center, Atlantic City Airport, N.J., 1983... 

Figure 8.2 Dynamics of opposed flow surface flame spread...

This follows by a steady state energy balance of the surface heated by qe, outside the flame-heated region S. It appears that a critical temperature exists for flame spread in both wind-aided and opposed flow modes for thin and thick materials. Tstmn has not been shown to be a unique material property, but it appears to be constant for a given spread mode at least. Transient and chemical effects appear to be the cause of this flame spread limit exhibited by 7 smln. For example, at a slow enough speed, vp, the time for the pyrolysis may be slower than the effective burning time ... 

Thus, for opposed flow spread, the steady state thermal flame spread model appears valid. In wind-aided flame spread, it seems appropriate to modify our governing equation for the thermally thin case as... 

For opposed flow spread under natural convection conditions, Ito and Kashiwagi [14] have measured the q( (x) profile for PMMA (d = 0.47 cm) at several angles. Their results, shown in Figure 8.10, suggest values for... 

A more practical way to yield results for opposed flow spread is to recognize that the parameter... 

Table 8.1 Opposed flow properties for lateral flame spread on a vertical surface [16]...

Figure 8.12 (a) Flame front movement for a wood particle board under opposed flow spread in... 

Thus, we can replace u00 in Equation (8.36) and apply it to both opposed and wind-aided cases. For upward or wind-aided spread the speed increases as cos (f> increases to the vertical orientation. For downward or opposed flow spread, the speed is not significantly affected by changes in until the horizontal inclination is approached for the bottom orientation (—90 < wind-aided as a stagnation plane flow results from the bottom. Figure 8.19 gives sketches of the... 

Opposed flow flame spread, representing spread on a horizontal surface, e.g. floor, large chair, mattress, etc. 

Partial premixing has been proposed as a means of NOj, reduction in gas turbine engines by Jayavant Gore at Purdue University. An experimental and computational study was conducted to observe NO behavior under the circumstances of moderate stretch rate, opposed-flow, partially premixed flames. The results show that the minimum NO emissions at an optimal level of partial premixing result as a consequence of decrease in CH radical concentrations. Partial premixing appears to be a possible practical immediate solution for NO remediation in gas turbines. 

The geometry of the present opposed-flow burner is identical to the one designed by Puri and coworkers (see [18] for example). The burner consists of two opposing ducts with 20-millimeter diameter separated by 15 mm. The exhaust is extracted by a vacuum pump though a water-cooled annulus mounted around the bottom duct and a guard co-flow of nitrogen is issued from an annulus around the top duct. Experiments were performed with methane (99% purity) and premixed air introduced from the bottom duct and air admitted from the top duct. The flow rates were monitored using choked orifice meters. 

Figure 27.1 Measurements and predictions of mole fractions of CH4, O2, and N2 as a function of distance from the fuel duct for diffusion (a) and partially premixed opposed flow flames with b = 2.2 (6) and 1.42 (c), Tair = 560 K and Ttuei = 321 K, Fair = 70, 60, and 50 cm/s, Ffuei = 70 cm/s, distance between ducts 1.5 cm...

Figure 27.5 Integrated reaction rate of prompt and thermal NO initiation reactions (a) and peak temperature (6) as a function of fuel duct equivalence ratio 4>b for opposed flow flames with Tair = 560 K and Ttuei = 300 K Fair = 70 cm/s, Ftuei = 70 cm/s, distance between ducts 1.5 cm. 1 — CH -h N2 N -h HCN (240) 2 N2- -0-S N0- -N (-178) and 3 — total...

Lutz, A.E., R. J. Kee, J.F. Grcar, and F.M. Rupley. 1997. OPPDIF A Fortran program for computing opposed-flow diffusion flames. Sandia Report SAND96-8243. 

When heat and mass are transferred simultaneously, the two processes interact through the Gr and Gq terms in Eq. (10-12) and the energy and diffusion equations. Although solutions to the governing equations are not available for spheres, results should be qualitatively similar to those for flat plates (T4), where for aiding flows (Gr /Gq > 0) the transfer rate and surface shear stress are increased, and for opposing flows (Gr Gq < 0) the surface shear stress is predicted to drop to zero yielding an unstable flow. 

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