January 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
(19) Othmer, D. F., Anal. Chem., 20, 763 (1948). (20) Othmer, D. F., and Benenati, R. F., IND. ENG.CHEM.,37, 299 (1945). (21) Othmer, D. F., Friedland, D., and Schiebel, E. C., in “Distillation Equilibrium Data” by Ju Chin Chu, p. 146, New York, Reinhold Publishing Corp., 1960. (22) Piret, E. L., and Hall, M. W., IND. ENQ.CHEM.,40, 661 (1948). (23) Pyle, C., and Lane, J. -4.U. S. Patent 2,527,654 (Oct. 30, 1950). (24) Reynolds, B. M., Ibid., 2,256,497 (Sept. 23, 1941). (25) Uchida, S., and Kato, H , J . Soe. Chem. Ind. (Japan), 37, 525 (1934). ( 2 6 ) Walker, J. F., “Formaldehyde,” ACS LMonographSeries, New York, Reinhold Publishing Corp., 1944 and 1953.
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(27) (28) (29) (30)
Walker, J. F., IND.ENG.CHEM.,32, 1016 (1940). Walker, J. F., J. Ana. Chem. Soc., 55, 2821, 2826 (1933). Walker, J. F.. J. Phys. Chem., 35, 1104 (1931). Wallace, R. D., and McKinney, R. W., IND. ENU.CHEM.,44, 1508 (1952). (31) Zawidski, H., Z. p h y s i k . Chem., 35, 129 (1900). (32) Zimmerli, A., IND.ENG.CHEM.,19, 524 (1927). (33) Zimmerli, A., U. S. Patent 1,662,179 (March 13, 1928). RECEIVED for review March 9, 1954. ACCEPTBDSeptember 29, 1954.
Based on 8 dissertation in ohemical engineering presented by Stanley J. Green to the Drexel Institute of Teohnology in partial fulfillment of the requirements for the degree of master of science.
Flame Stability of Preheated PropaneAir Mixtures GORDON L. DUGGER’ National Advisory Committee f o r Aeronautics, Lewis Flight Propulsion Laboratory, Cleveland, Ohio
T
HE present investigation was concerned with the deter-
mination and interpretation of the stability limits of open Bunsen-type flames of preheated homogeneous mixtures of propane and air. I n this paper the preheat temperature is referred to as the initial temperature. It represents both the temperature of the unburned gas leaving the burner port and the temperature of the burner wall or port, because the burner was operated isothermally. For an open flame, blowoff occurs when the flow rate of the unburned gas is increased until the stream velocity is everywhere greater than the flame velocity, so that a stationary flame can no longer be held a t the burner port. Division of this critical volumetric flow rate by the port area gives the critical average stream velocity. (With a rich mixture, a stable lifted flame may be obtained some distance above the port as a result of mixture with and dilution by secondary air; only blowoff from the port is considered in the present investigation.) Conversely, when the mixture velocity is smaller than the flame velocity over a portion of the burner cross section, the flame flashes back into the burner [see the discussion by Lewis and Since the flame velocity increases with the von Elbe (@I. absolute temperature of the gas mixture raised to a power greater than 1 (4), both blowoff and flash back would be expected to occur a t higher gas velocities when the mixture has been preheated. Wohl, Kapp, and GazXey (16) have pointed out that the conditions for stability may be described in terms of laminar flow, regardless of whether the flow in the tube is laminar or turbulent, because in either case there is a laminar sublayer a t the stream boundary. Any point of equality between flow velocity and flame velocity must lie within the laminar sublayer, because the gas velocity a t the boundary between the sublayer and the turbulent core is greater than the flame velocity. The velocity gradient in this region near the stream boundary where stabilization must occur may be assumed constant if the width of the region is small in comparison to the tube diameter, [Lewis and von Elbe’s curves for flash back of natural gas flames (11) indicate that this assumption is satisfactory for correlating flashback data for various tube diameters, except for tube sizes comparable to the flame-quenching diameter.] The critical boundary velocity gradients for flash back and blowoff (see Apparatus and Present address, 818 Michigan Ave., Lakeland, Fla.
Procedure for equations) have been used successfully both to correlate flash-back or blowoff data obtained with burners of various sizes and shapes and to correlate turbulent blowoff data with laminar blowoff data (1, 6, 8, 11, 16, 16). The temperature dependence of these critical boundary velocity gradients should not be far different from the temperature dependence of the flame velocity, because it is reasonable to expect that the distance from the burner wall to the stabilization point would change only slightly as the temperature of the unburned gas and the burner wall was changed. The small changes in thiR distance that do occur are the result of changes in the wallquenching and secondary-air-dilution effects. The effects of wall quenching and secondary-air dilution may be illustrated by the ratio of fundamental flame velocity to critical boundary velocity gradient, which has been called the “penetration distance” (11,16,16). This ratio represents the distance from the burner wall a t which the local stream velocity is equal to the fundamental flame velocity (Figure 1). For the case of flash back it approximates the depth of penetration of the quenching effect of a single wall-that is, the distance from the wall a t which the local flame velocity, ur,becomes smaller than “thefundamental flame velocity, ufo,as a result of quenching. The penetration distance is to be distinguished from the smaller “dead space a t the wall” (Figure 1, A ) or “dead space above the rim” (Figure 1, B ) where quenching is complete, so that the luminous zone assumes a position parallel t o the gas flow lines. It is reasonable to expect that the depth of penetration of wall quenching would be a function of the temperature drop between the flame and the wall. Since a given change in initial temperature causes a smaller change in flame temperature, the temperature drop between the flame and the wall, and hence the penetration distance for flash back, would decrease slightly as the initial temperature is increased. In the case of blowoff of a fuel-lean flame, local flame velocities in the boundary region are reduced not only by wall quenching, but also by the dilution of the unburned gas which occurs as secondary air diffuses into the mixture from the surrounding atmosphere. The rate of diffusion of secondary air would increase with an increase in temperature, thus tending to increase the penetration distance and t o offset the decrease resulting from reduced wall quenching. For any fuel concentration greater than that giving the maximum u,’, secondary-air dilution might actually increase flame velocity and decrease the penetration distance.
INDUSTRIAL AND ENGINEERING CHEMISTRY
110
Actually the picture of stabilization points and penetration distances given by Figure 1 may be oversimplified, because, as shown in Figure 2, a flame velocity-radial distance curve of the same general shape as the one determined experinientally by
.L
Figure 1. Relation between Flame Velocity arid Gas Velocity a t Stability Limits
Vol. 47, No. 1
proximate theoretical treatment they related the critical boundary velocity gradient, the flame temperature, and an "ignition temperature" which is obtained by implicit argument. I n the present paper flash-back and blowoff data for propaneair flames burning from a l.5G-em. tube are reported for initial mixt,ure temperatures of 3OG", 422O, 50G3, and 617" K. Stream flow Reynolds numbers varied from 240 to 7400. The data are presented in terms of critical average stream velocity, critical st,ream flow Reynolds number, critical boundary velocit'y gradient, and penetration distance. Some typical empirical equatione for critical average stream velocities as functions of temperature are given. -4 comparison is made between calculated penet,ration distances and quenching dist,ances from t'he literature, and a reason for the observed discrepancies is postulated. The theoretical treatment for flash back given by Grumer and Harris ( 7 ) is discussed. 4 modified form of this treatment based on the Semenov equation for flame velocity is shown to give satisfact,ory predictions of critical boundary velocity gradients. APPARATUS AlriD PROCEDURE
A. B.
The burner arrangement %--asthe same as that reported for an investigation of t'he effect of temperature on flame velocity
&.
(4).
Flash back Blowoff Penetration distance for flash back p , ~ . Penetration distance for blowoff Dead space at wall dr. Dead space at rim For flash back the tube diameter is considered to be at least tw.ice the quenching dinmeter
P.F.
Lewis and von Elbe ( I O ) would indicate that the point of tangency between flame velocity and gas velocity curves which represents stabilization occurs with a flame velocity smaller than the fundamental value. The distance from the wall to the stabilization point is labeled (Istab in Figure 2. The distance from the wall a t which the line representing the critical boundary velocity gradient reaches a velocity equal t o til0 is the calculated penetration distance d, I,. d,, caie is somewhat uncertain because the calculation does not take into consideration t,he changes in boundary velocity gradient which may be produced by the back pressure of the flame. The local flame velocity first begins to fall below u j o a t a n even greater distance from the wall; t,his distance a t which wall quenching first begins t o affect flame velocity represents the true depth of penetration, d,,tru,. The shape of the flame is determined by the fact that the nornial component of the velocity vector for the unburned gas entering the flame front a t any point must be equal t o the local flame velocity. The literature on experimental investigations of the effect of initial mixture temperature on blowoff and flash-back limits is rather limited. -4n earlier investigat,ionby the author (6)shor?-ed that the blowoff velocity increased with increasing mixture temperature for propane and air burning above tubes of 3 / ~ - , 5 / g - , or T/8-inch inside diameter. The critica,l velocity gradient a t the tube wall also increased with temperature, but less rapidly. Heiligenstaedt [see Culsham and Garside ( 3 ) ] mixed coke-oven gas with heated air and determined t,he flash-back limb of the resulting mixtures. He observed that t,he critical average stream velocity for flash back increased linearly with the air temperature. Delbourg [Culshaw and Garside ( S ) ] reported that t'he average stream velocity a t flash back for mixtures of city gas and heated air was proportional t o the 1.G3rd power of the absolute air temperature in the laminar flow region and t o the 1.3rd power in the turbulent flow region. Browning and Thorpe ( a ) recently studied the stability of heterogeneous mixtures of partially vaporized benzene with oxygen a t temperat,uresranging from 339' t o 534" K. Their results, which show a linear increase in blowoff velocity with temperat'ure, are not directly comparable to those from homogeneous gaseous systems. Grumer and Harris ( 7 ) have just, reported flash-back and blowoff data for methane-air flames burning above short cylindrical ports with initial temperatures of 300' to 523" K. By an ap-
...
The burner was a 150-em. straight length of brass tubing 1.56 cm. in inside diameter. The burner tube x a s wrapped with asbestos-covered resistance wire and insulated t o permit control of t,he tube-wall temperature. A collnr 7.6 cm. in diameter wa8 silver-soldered t,o the lip of the burner and was provided with a separate heater. The preheating section, which was a 15O-cm. length of stainless steel tubing 0.05cm. in inside diameter n-as also wrapped with resistance xire and insulated. The commercial grade propane used had a purity of approrimately 95%; the principal impurities n'ere ethane and isobutane. Laboratory service air containing approximately 0.4 mole yo of water was used. Propane arid air mere metered by criticalflow orifices Xrhich were calibrated in place x i t h wet-test meters.
GAS VELOCITY
PROFILE;
b
RADIAL D I S T A N C E F R O M WALL>
Figure 2. Illustration of Why Actual Depth of Penetration of Wall Quenching on Flame Velocity >$lag Be Greater than d,. = i.(fo/gF
Gas temperatures rvere meaqilred by iron-constantRn thermocouples a t the metering-orifice inlcts, a t the burner inlet, and a t the burner port. The port thermocouple mas of the aspirating type; it, was pla.ced directly over the center of t,he port between runs. Tube wall t,emperatures were measured a t the burner inlet and port. The gas temperature a t the port as kept within 1 5 ' IC. of the desired value and the burner inlet gas temperature and port lip temperature were kept within & I O K. of t,he desired value for all except the 306' K. curve, in which case the lip temperature ran as much as 10' K. high. The procedure in determining a blowoff point \\-as t o enrich the mixture enough t o obtain a seated flame and then either to reduce the fuel rate gradually or t o increase the air flow gradually
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1955
until blowoff occurred. In determining a flash-back point, the air rate was set high enough to obtain a stable flame and then the fuel rate was either increased or decreased until flash back occurred, depending upon which side of the flash-back envelope the beginning condition fell.
a F = 100
aB
cp =
(o
=
GB =
200 IO0
-
cIT
40 0
1
2
0
1
2
0
1
2
0
1
2
3
EQUIVALENCE R AT10
0
Figure 3. Effect of Initial Mixture Temperature on Region of Stable Flames for Propane-Air Mixtures at Atmospheric Pressure Critical average stream velocities at flasli back and blowoff veraus equivalent ratio
Critical boundary velocity gradients for flash back, gF (in the present investigation this applies to laminar flow only) and blowoff, QB, were computed as follows: For laminar flow, Re
