WILLIAM F. KENNEYl and LYLE F. ALBRIGHT Purdue University, Lafayette, Ind.
Energy Release in
Flames Stabilized by Cyclonic Flow
IT
HAS BEEN reported (7: 2, 4, 6 ) that stable flames result from unique flow patterns of gases flowing cyclonically (rotational motion around their flow axis). This cyclonic motion results in low static pressure along the flow axis which in turn causes a reverse flow (2, 3-5) that pilots fresh feed gases and stabilizes the flame. Energy release rates have been reported for only a few cyclonic flames ( 2 , 4 ) . With one apparatus, the rate was as high as 1,500,000 B.t.u. per hour per square foot, and with another it was as high as 10,000,000. Thus, these rates may vary significantly with flow rate, air-fuel ratio, duct length, and degree of cyclonic flow. The present investigation was made to determine quantitatively the effect of these variables and to characterize further cyclonic flames.
Equipment and Procedure Compressed air and propane gas from bottles were metered separately and mixed before entering the nozzle (Figure 1) through the annular space benveen the 2- and ','*-inch pipes. The gases entered the s ~ i r chamber l of the nozzle through eight curved vanes. T h e position of the vane tips, and hence the degree of cyclonic motion imparted to the gas, was controlled with a '/d-inch thick disk, either 5 or 5.188 inches in diameter, which was attached to the end of the 1/8-inch pipe. This pipe of the cvclonic nozzle permitted the introduction of propane gas into the s\virl chamber where it was mixed with air introduced through the vanes. Gases in the swirl chamber \Yere exhausted through the converging noz7le having a throat diameter of 2 inches. Glass or metal ducts of \,ariable length and about 2 inches in diameter were attached a t the nozzle exhaust by holders screwed to the face plate of the burner. Water sprayed from a quenching device extinguished the flame a t the duct outlet. Part of the gases in the quenching device was withdrawn and analyzed with a standard Burrell apparatus; the remainder was evacuated into a ventilating hood. Hydrocarbons \yere analyzed with a catalytic oxidation unit. Present address, Brookhaven National Laboratory, Upton, N. Y.
Discussion of Results Cyclonic flames were stabilized in 2inch ducts which were 9.5, 19, and 28.5 inches long. With the vane tips positioned with a 5-inch disk, stabilitv limits and combustion efficiencies were determined a t propane flows from 0.16 to 0.39 mole per hour. Measurements were also made in the 19-inch duct when the vanes \vere positioned with a 5.188-inch disk. The flames could be viewed when glass ducts were used, and appeared similar to those previously observed ( 7 , 2, 4 ) . T h e appearance. stability limits, and othcr characteristics of the flames in the ducts did not change when the quencher was positioned a t the end of the duct. T h e quencher extinguished the flames a t the exit of the duct and cooled the exhaust gases to temperatures significantly beloiv those of combustion Only a t the highest flow rates Lvere noticeable quantities of steam produced. Combustion in all flames was complete ( 10070 combustion efficiencv) when the flame front reached the duct wall a t air-to-fuel flow ratios equal to or exceeding 24. the stoichiometric ratio. The flame front did reach the duct wall, ho\\ever, in several cases when less than the stoichiometric amount of air was present. Incomplete combustion indicated that no air was drawn back into the duct by the reverse flow in flow pattern. If the quenching apparatus had not been present. however. exhaust gases from the duct would probably have mixed with air in the room to allow complete combustion. T h e combustion efficiency was calculated from the gas analyses and thermo-
dynamic data. The superficial velocity is defined here as that in the nozzle throat, assuming flow was linear and gases were unburned and a t average room conditions (84' F. and 750 mm. of mercury). The combustion efficiency curves a t air-to-fuel ratios greater than the stoichiometric ratio are relatively parallel to the top stability curve and the curves a t lower ratios are parallel to the lower stability curve (Figure 2 ) . Thus: combustion efficiency tended to decrease with increased velocities and hence tvith the corresponding decrease of the residence time of the g a s w in the duct. For a given flow condition, the combustion efficiency tended to be higher with longer ducts, and the range of flow conditions for complete combustion \vas larger as the duct length increased. In fact, the end of the complete combustion region for the flame in the 28.5-inch duct could not be determined because insufficient air was available. Also, the area of that portion of the region which was determined was more than twice that of the entire region for the 19-inch duct. T h e maximum combustion efficiency for a given superficial velocity generally occurred at approximately the stoichiometric ratio of air to propane. This agrees with previous observations for cyclonic flames with natural gas and air ( 7 , 2). However, maximum velocities a t which 80 and 90yc efficiencies w.ere obtained occurred a t air-to-fuel ratios slightly less than the stoichiometric ratio (Figure 2, B). The curves for still lower efficiencies probably would exhibit similar behayior: but could not be measured because of the limited air supply available. The combustion efficiency for a given superficial velocity decreased rapidly as the air-to-fuel ratio changed from the stoichiometric ratio, especially at ratios higher than the stoichiometric ratio. At ratios just before blowout conditions, the combustion efficiency was low, often less than 10%. These low efficiencies can be explained in part a t least by the excess of air in one case or of the fuel in the other. The flame temperatures were as a result lowered causing a slower rate of flame propagation. The flame front. tvhich n .
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Figure 1. Cyclonic combustion nozzle. Gases enter the swirl chamber through eight curved vanes
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19-inch duct; 5-inch disk.
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REFERS TO COMBUSTION EFFICIENCY
Figure 3. For complete combustion, rates as high as 9,500,000 B.t.u. per hour per cubic foot were obtained. With higher rates, combustion was incomplete
00,000 BTU/hr. cu.ft.
SUPERFICIAL ' VELOCITY ( ' f p s 1
was piloted near the flow axis ( 7 , 2 ) in the swirl chamber or a t the entrance of the duct. did not have time to reach the wall of the duct, and the gases flowing a t the \Val1 passed out of the duct unburned. Tor Figure 3, the volume charged to the flame was that of the duct plus the estimated volume of the flame in the swirl chamber. T h e energy release curves are probably quasi-eliptical and form a closed loop at higher superficial velocities than could be obtained here. Energv release rates for flames in longer ducts could not be measured over as large a range of combustion efficiencies because of the limited air and propane flows. Sufficient data were obtained though, for the 19-inch duct with a vane setting of 5 inches to calculate the maximum energy release rate for complete combustion a t about 10,000.000 B.t.u. per hour per cubic foot. Both this study and that of Albright and Alexander (2) indicate that maximum energ\ release rates for complete combustion did not change significantly as the duct length was increased over the range of duct lengths studied. They (2) used natural gas and the same cyclonic burner used here and reported energy release rates of about 10,000,000 B.t.u. for complete
228
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combustion. Type of paraffin fuel used apparently does not affect the rates. T h e setting with a 5-inch disk allows more space between the vanes and causes the vanes to point in a more radial direction than the setting with the 5.188-inch disk. Therefore, for the smaller disk gases enter the swirl chamber a t both lower total and tangential velocities, and the degree of cyclonic flow is lower. Both higher combustion efficiencies and energy release rates were obtained in this investigation with more cyclonic motion (Figure 2, B and C). With the 5.188inch disk, a maximum value of 11,800,000 B.t.u. per hour per cubic foot was obtained in the 19-inch duct for the region of complete Combustion.
Table I. Increasing Cyclonic Motion Increases Pressure Drop (Av. values; 5 in. disk) vs, Superficial Press. Drop., In. Hg Veloc., F.P.S. APs.800 APj is8
INDUSTRIAL A N D ENGINEERING CHEMISTRY
20
30 40 50 60 70
0.53 1.1 1.8 2.7 3.6 4.6
2.6 4.6 6.8 9.3 12.0 15.0
19-inch duct; 5.1 88 inch disk
Pressure drops were more than three times higher for the fames u i t h the increased degree of cyclonic motion. No significant effect of duct length on the pressure drop was observed. T h e following equation correlates the ratio of pressure drops in the burner: APo.le8/-IP. 000 = 14 1's-0
35
Sum ma ry A cyclonic flame was obtained that had an energy release rate of 13,400.000 B.t.u per hour per cubic foot with a pressure drop of less than 4 inches of mercury. T h e highest rate of energy release for a flame with complete combustion was 11,800,000 B.t.u. per hour per cubic foot. Stable cyclonic flames were, however, obtained a t combustion efficiencies of significantly less than 10% and with very low rates of energy release. T h e combustion efficiency and rates of energy release depend primarilv on the air-to-fuel ratio, flow rates, and degree of cyclonic flow. References (1) Albright, L. F., Alexander, L. G., Jet Propulsion 26, 867 (1956). (2) .4lbright, L. F., Alexander. L. G.. 6th Sympogum on Combustion; Reinhold, New York, 1957. (3) Eckert, E. R. G., Hartnett, J. P., Proc. 4th Midwestern Conf. on Fluid Mechanics, Research Series 128, Engineering Experiment Station, Purdue University, Lafayette, Ind., 1955. (4) Hottel, H. C., Person, R. A , 4th Symposium on Combustion, p. 781, Williams and Wilkins Co., Baltimore, Md., 1953. (5) Morrison, W. M., M.S. thesis, University of Oklahoma, 1956. (6) Plaster, W. E., Zbid., 1955.
RECEIVED for review November 2, 1957 ACCEPTED October 27, 1958 Work sponsored in part by the Purdue Research Foundation.
