Double converging jets stabilize propane-air flame at the end of a glass tube
I
DONALD
P.
DUCLOS, ALLAN SCHAFFER’, and ALI BULENT CAMBEL
Gas Dynamics Laboratory, Northwestern University, Evanston, 111.
Flame-Stabilizing Effects of Inclined Air Jets
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Schematic of combustion tunnel
and Cambel (3, 4 ) found that under certain circumstances a n opposing jet performed better than some types of physical flameholders. They also observed that blowout curves could be shifted markedly by utilizing fuel-air jets. Schaffer and Cambel introduced the concept of the critical zone, which they defined as a small pilot reactor from which the flame spreads, and they suggested that the conditions in this zone determine whether or not the flame will be stabilized. (The following terminology Present address, Los Angeles, Calif. VOL. 49, NO. 12
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Figure 1. Blowout curves for a single air iet at various incidence angles Inside diameter of tube, 0.055 inch Jet supply pressure, 70 pounds/square inch g a g e Jet mass now, 0.1 05 to 0.1 45 pound/minute
is suggested for jet flame holders: opposing jet = incidence angle of substantially 180'; traverse jet = incidence angle of substantially 90"; inclined jet = intermediate incidence angle.) Pohlmann ( 2 ) investigated the effect of jet size and jet pressure on the flame stabilization of a n opposing jet using a propane-air mixture as the jet fluid. He found that the blowout performance is a function of jet pressure, jet size, and jet equivalence ratio. I n related work, Golitzine (7) found that kerosine, injected upstream by a n air blast, would produce flame stability without the use of baffles in the main air stream. He found that the flame stability limits are narrowed by a n increase in main stream velocity, as well as by a decrease in main stream static pressure, and are widened by an increase in air blast pressure and flow. Thorpe and Browning (5) performed tests with a secondary air duct blowing across a Bunsen burner flame. They concluded that when the combustible mixture issuing from the flame tube is being ejected upstream, a greater degree of turbulence is present before the gases reach the combustion zone. Therefore, a richer mixture can be burned when the flame tube is in the upstream tilted position.
Apparatus Combustion Tunnel No. 3 . in the Gas Dynamics Laboratory a t Northwestern University was used for all tests (shown in schematic drawing).
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e Figure 2. Ratio of maximum blowout velocity, Ve at any incidence angle, 8, to maximum blowout velocity VO at an incidence angle of 0' vs. 8.
A premixed but not preheated flame of commercial propane (95 to 98y0 propane, 2 to 5% other saturated hydrocarbons) and air was stabilized by a n opposing air jet, or jets, entering the end of a 96% silica glass tube with inside diameter approximately 1.75 inches and 15 inches long. The arrangement is pictured. During the tests, the pressure in the combustion chamber was approximately atmospheric. The jet mass rate of flow varied somewhat even though the supply pressure to the jet tube was maintained at 70 pounds per square inch gage. I n addition to the size and length of the jet tube, heat transfer from the flame was found to influence the flow rate strongly. The total rate of flow in any test for single or double jets varied from 0.105 to 0.145 pound per minute. The jet pressure, as well as the gas temperature, a t the exit from the tube was unknown. The jet tube was made of stainless steel of '/s-inch outside diameter, and was about 6 inches long bent a t 90' in the middle. The inside diameter of the tube was 0.055 inch for the singlejet tests, and 0.036 inch for the doublejet tests. The different sizes of jet tubes were used in order to make the total mass rate of flow for the two jets approximately equal to that for the single jet. The tips of the jet tubes protruded into the end of the 96% silica glass tube only about ' / 8 inch so that incidence angles could be made large. The flame initiated in the stagnation region several inches upstream from the tips of the tubes, and thus all
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of the flame region important to the stabilization effect was confined in the tube. Jet mass flows were approximately 1% of the primary flow.
Discussion of Results The study of the flame-holding ability of opposing jets reported here can be divided into three parts: investigation of a single jet a t various incidence angles, investigation of two nonconverging jets a t various distances apart, and investigation of two converging jets a t various angles and distances apart. The values of velocities in the curves are subject to a n estimated maximum error of 10% and equivalence ratios to an estimated maximum error of 1570. Because of variations in heat transfer from the flame to the jet gas, the jet exit temperature is far from constant for any curve, and thus there is an uncontrolled and unmeasured variable in each curve. Furthermore, this heat transfer effect makes the value of each blowout point dependent upon the rate at which the blowout is accomplished. The results of the tests using a single 0.055-inch inside diameter jet at angles of from 0 to 15 are shown in Figure 1. A peak could not be obtained for the O o curve because of insufficient air supply. The flame-holding ability of the single jet is reduced drastically as the jet angle is increased. In Figure 2 the ratio of the maximum blowout velocity at a given angle to the maximum velocity at 0" is plotted us. the angle. For example, this curve shows that the maxi-
FLAME STABILIZING
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mum velocity obtainable a t 15' is only about 27y0 of the maximum velocity obtainable a t 0". Larger angles were not investigated because in this apparatus a flame can be maintained with no flame holder a t about 30 feet per second or 11% of the maximum velocity at 0". A jet a t an angle of 90" was tried, however, and it was found to stabilize a flame at about 40 feet per second or 14.5y0 of the maximum velocity a t 0". I t is for this reason that the curve in Figure 2 is drawn asymptotic to a velocity ratio of 0.145 for larger angles. The reason for the decrease in flame stability as incidence angle was increased from 0" is not fully understood a t this time. The decrease may be due to a breaking up of the critical zone, or to a lowering of the rate of recirculation and mixing. Whether such a decrease will exist for larger jet diameters has not been determined. The resultis of tests on two 180' nonconverging 0.036-inch inside diameter jets are shown in Figure 3. A comparision of Figures 3 and 1 shows that two jets which do not converge do not perform as well as a single jet at the corresponding angle with the same total flow rate. Apparently there was virtually no interaction between the two jets at the distances investigated. Consequently blowout performance was essentially the same as for a single jet of 0.036-inch inside diameter. However, it was observed that the doublenonconverging jets do have the advantage of better flame spreading. The results of the tests on the doubleconverging 0.036-inch inside diameter jets are shown in Figures 4 and 5. Comparing these results with Figure 1 shows that converging jets can perform comparably to a single jet of the same total flow at the same angle. The curves for the double converging jets show a tendency toward widening the range of equivalence ratios over which the flame can be stabilized. The effect of the convergence of jets can best be shown by Figure 5. Blowout curves were run with the angle of the jets held constant at 15', while the distance between the tips of the tubes was varied. The performance increased as the distance between the tips was decreased, with the exception of the positions a t 1.25 and l 5 / 8 inches apart. For these latter two distances the experimental points fall on approximately the same curve as the effect of convergence is very slight at such distances. With the tips of the tubes only 7/16 inch apart, the performance was comparable to that of a single jet at 0 " with the same flow. The probable reason for this comparable performance is that as the distance between the jets is decreased, the jets impinge so
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Figure 3. Blowout curves for several configurations of nonconverging double air jets Inside diameter of tube, 0.036 inch Jet supply pressure, 70 pounds/square inch gage Total iet mass flow, 0.105 to 0.1 45 pound/minute
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Figure 4. Blowout curves for converging double air jets at various tube angles and at various distances apart Inside diameter of tube, 0.036 inch Jet supply pressure, 70 paunds/square inch-gage Total jet mass flow, 0.1 05 to 0.1 45 pound/minute
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as to take on a profile similar to that of a single jet a t O0, as shown in Figure 6. T h e photograph in Figure 6 is of a flame near rich blowout. The free jet streams are clearly visible in the flame. Such outlines of air jets can be seen only for rich mixtures. I t is also because of a rich mixture that a dark space appears just downstream of the jets. Combustion takes place in and around the stagnation region of the
jets because of the dilution of part of the main stream mixture with the jet air. The rest of the main stream burns outside of the glass tube upon mixing kvith the surrounding air. (The dark region appears darker than it actually is, however; the print had to be overexposed in order to show the jets clearly. For comparison, a photograph of a flame with a leaner main stream mixture is shown in Figure 7.)
Conclusions
The flame-holding ability of the single opposing jet tested decreased rapidly as the incidence angle was increased from 0". Multiple nonconverging jets do not perform better than a single jet a t the same angle, although the flame-holding ability of multiple jets at a given angle can be greatly improved if they converge. T h e blowout performance of a single jet at 0' appears to be the maximum that can be obtained with a jet flame holder for constant jet mass flow rate and supply pressure. Flamespreading can be increased by the use of multiple jets. Acknowledgment
This study was sponsored by the Department of Mechanical Engineering and financed rhrough funds from Faculty Research Project, F R P 103-54, T h e Technological Institute, Northwestern University. The authors gratefully acknowledge the help extended them by the faculty and staff of the Technological Institute. The constructive criticisms of the members of the Gas Dynamics Laboratory, in particular, those of Edward Pohlmann, are greatly appreciated. '
Literature Cited
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Blowout curves for converging double air jets at various distances apart I
Angle of each tube, 15' Inside diameter of tube, 0.036 inch Jet supply pressure, 70 pounds/square inch gage Total jet mass flow, 0.1 0 5 t o 0.1 45 pound/rninute
Figure 6 . Direct photograph of rich flame stabilized on double converging air jets
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(1) Golitzine, X., Natl. Aeronaut. Establ. Canada, Lab. Rept. 102 (May 1954). (2) Pohlmann, E., M.S. thesis, Department of Mechanical Engineering, Northwestern University, September 1954. ( 3 ) Schaffer, A., Cambel, A. B., Jet Propulsion 25 (No. 6), 284 (1955). (4) Zbid., 26 (No. 7, Pt. l ) , 576 (1956). ( 5 ) Thorpe, M. L., Browning, J. A . , IND. ENG.CHEM.46,2203 (1954).
RECEIVED for review August 27, 1956 ACCEPTED April 22, 1957
Figure 7. Direct photograph of flame slightly leaner than stoichiometric stabilized on double converging air jets
