Effect o f Initial Temperature o n
...
Flash Back of laminar and Turbulent Burner Flames BURTON FINE, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio
SEVERAL
STUDIES have recently reported the effect of elevated temperature on flash back of burner flames. Bollinger and Edse (2) and Miller and Setzer (72) studied flash back when only the burner lip was heated, the gas stream being approximately at room temperature. Grumer and Harris (9) and Dugger (4) maintained nearly the same temperature for both the gas stream and burner lip; flash-back data could be treated by a simple critical boundary velocity gradient which did not involve a transverse temperature gradient. T h e present study extends the measurements of Grumer and Harris and of Dugger on laminar hydrocarbon-air flames to chemically related systems of higher reactivity and to the turbulent region.
Experimental Materials. Tank hydrogen (9976) and propane and ethylene were used as fuels. Tank compressed air and a prepared mixture. 50y0 oxygen and 50% nitrogen by volume, were used as oxidants. 411 materials were used without further purification. Apparatus. The loiv pressure combustion chamber and gas metering system: based on critical flow orifices, have been described in detail (6). A single burner ivas used: a straight brass tube about 125 cm. long, having an inside diameter of 1.016 cm. and a wall thickness of about 0.16 cm. I t was fitted to the low pressure combustion chamber, and the part of the burner outside the combustion chamber was wrapped with asbestos-covered h-ichrome heating wire and thermally insulated. The part inside the chamber was heated by a separate wrapping of Xichrome. A Chromel-Alumel thermocouple was held under its own tension in a well about 0.08 cm. deep drilled into the tube wall about 0.15 cm. below, the lip. The reading given by the thermocouple was taken to be the temperature of the burner lip. T h e gas temperature, determined only in the absence of a flame, was taken to be the reading given by a bare-wire thermocouple held a t the center of the tube mouth. This gas temperature was constant for several inches down the tube. Procedure. The absolute gas temperwas measured ature, constant to f2YGc, before and after each short series of C.P.
564
flash-back measurements, with the small inner heater energized to simulate partially the effect of heating of the burner lip by the flame. The difference between gas temperatures measured with and without energizing of the inner heating coil was never more than 10 O K. The gas temperature varied someivhat \\ith flow rate for a given energy input, decreasing slightly as flow changed from laminar to turbulent. This decrease was overcome by adding a predetermined amount of energy during a series of flash-back measurements. I n measuring flash back, a stable flame was established a t some pressure. The energy input to the small inner heater was then adjusted so that the lip temperature approximated the gas temperature to within 5' to 10' K. The pressure was then slowly increased, a t constant mass flow, until the flame flashed back (6). For the hotter flame systems, heating of the burner lip by the flame was so great that gas and lip temperatures could be equalized only for very high gas temperatures. To obtain results for hotter flames at lower initial temperatures (specifically, propane-oxygen-nitrogen a t 394' K.) the burner was joined to the combustion chamber with a brass flange, so that much of the heat fed to the burner lip by the flame was removed by conduction. In this case, the difference between gas and lip temperatures was 30" K. A value of the average stream velocity at flash back, was computed by
c,,
For laminar flames, critical boundary velocity gradients for flash back were computed by the expression used for fully developed laminar flow ( 7 7 )
For turbulent (77)
flames
g,(t) = 0.023
the expression
f' Refo.a D
(3)
was used, where Re, is Reynolds number at flash back defined as
where p is the mixture density and 1 is its viscosity. I n the region of laminar-
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turbulent transition, critical flash-back gradients were computed as for laminar flames. Justification for this procedure has been offered (5).
Results a n d Discussion Description of Results. Experimental data for the four flame systems studied are shown in the right-hand parts of Figures 1 to 4, plotted as log g, against log P. The dashed lines represent results previously obtained for initial mixtures a t room temperature (5-7); except for ethylene-air, these results were obtained in water-cooled burners. Results for ethylene-air, nominally at room temperature, were cross-plotted from the data of Garside, Forsyth, and Townend (a), who used an uncooled burner 1 cm. in diameter. For each flame, data are presented at a single equivalence ratio which is close to the equivalence ratio giving the maximum value of g,. Results for turbulent flames are shown for two systems, hydrogenair and propane-oxygen-nitrogen. Flash-back curves a t constant initial temperature show three main regions for the range of conditions shown in Figures 1 to 4: a laminar region for Reynolds numbers less than about 1300, a transition region between 1500 and about 2500, and a turbulent region for Reynolds numbers greater than 2500. In the laminar and turbulent regions critical boundary velocity gradients for flash back are dependent on both pressure and initial temperature. T h e exponential pressure dependence of g,, however, is nearly independent of initial temperature. Some curvature is found in the plots of log g, against log P at low Reynolds number, which is attributed to partial quenching of the flame by the wall (7). A small change in slope is sometimes found for turbulent flames a t high initial temperatures. This may be because the amount of additional energy added a t high flows was not sufficient to maintain the temperature at its nominal value. At the beginning of the transition region (Figures 1 and 2), the flash-back pressure a t constant initial temperature goes through a slight maximum, levels off, and then remains constant with increasing Reynolds number until flames are fully turbulent. The flash-back pressure a t the maximum increases slightly with increasing initial temperature, so
Figure 2
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Figure 4
The critical boundary velocity gradient for flash back increases with initial gas temperature
Flash-back curves at constant initial temperature show three main regions: 2500, and turbulent > 2500
that the Reynolds number of the maximum remains about constant.
Temperature Dependence of Laminar and Turbulent Flash Back. The variation of g, is assumed to be described by a relation of the formg, a P a T b , so that a and b , taken to be constant, denote the "dependence" of g, on pressure and initial temperature. Because the dependence of g, on initial temperature is nearly independent of its dependence on pressure, the critical flashback gradient may be cross-plotted against T at constant P. Logarithmic plots of this kind are shown on the left-hand side of Figures 1 to 4 for both laminar and turbulent flames. The points shown in these cross plots are not experimental, but were taken from the smooth curves which describe g, as a
a laminar region for Reynolds numbers C:, a t elevated initial temperatures may be obtained by comparing values of Lrc for hydrogen-
Effect of Initial Temperature on Flash Back of Burner Flames
= burner diameter, cm. = activation energy, kcal. per
mole (7)
T h e fact that critical boundary velocity gradients for both laminar and turbulent flash back show the same dependence on initial temperature implies that the temperature dependence of the mean stream velocity at flash back will be different for laminar and turbulent flames. Similar differences exist between exponents on pressure and burner diameter a t constant inital temperature (5,7). The critical boundary velocity gradient for flash back a t constant pressure and initial temperature increases threefold with change from laminar to turbulent conditions. This increase may be explained on the assumption that turbulent flash back takes place in that portion of the boundary layer where conditions are essentially laminar. Thus, if g, is expressed as gf = LTh/6,where 6 is the so-called penetration distance, the critical burning velocity, Lrb, will remain constant with transition to turbulence, but the penetration distance will decrease by a factor of about 3. A possible justification has been offered for this decrease in penetration distance in terms of a heat transfer coefficient which varies across the turbulent boundary layer (5). Evidence has been previously obtained from measurements at room temperature that a turbulent flame a t flash back is stabilized in a near-laminar portion of the boundary layer. This has been done by showing that the maximum stream velocity in the boundary layer for which conditions are essentially laminar, Uc, is greater a t flash back than the normal laminar burning velocity at a given pressure (5, 7). Lrc,j is given, approximately by the expression
LTC,,= 0.75 DjRer-O.’
E
CHEM.48, 802-7 (1956). 13) Bover. M. H.. Friebertshauser. P. E. ’ Corndustz’on and Fiame 1, 264 1957). (4) Dugger, G., IND. EKG. CHEM.,47, 109-13 (1955). (5) Fine, B., Combustion and Fianie 2, 253-66 (1958). (6) Fine, B., Natl. Advisory Comm. .Aeronaut., Tech. Note 3977 (1957). (7) Ibid., 4031 (1957). (8) Garside, J. E., Forsyth, J. S.,Townend, D. T. h., J. Znst. Fuel 18, 175-85 (1945). (9) Grumer, J., Harris, M., IND. ENG. CHEM.46, 2424-31 (1954). (10) Heimel, S., Natl. Advisorv Comm. Aeronaut., Tech. Note 4156 If957). (11) Lewis, B., von Elbe, G., Combustion, Flames, and Explosions,” p. 279, Academic Press, New York, 1951. (12) Miller, E., Setzer, H. J., “Sixth Symposium (International) on Combustion,” p. 164, Reinhold, New York, 1957. (13) Schlichting, H., “Boundary Layer Theory,” p. 405, McGraw-Hill, New York, 1953. (14) Wohl, K., “Fourth Symposium (International) on Combustion,” pp. 68-89, Williams & Wilkins, Baltimore, 1953. RECEIVED for review July 25, 1958 ACCEPTED October 6, 1958 Division of Gas and Fuel Chemistry, ACS, Symposium on the Characteristics of Flames and of Gaseous and Liquid Fuels, Urbana, Ill., May 1958.
