August 1953
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
(16) Hain, G. M., Jones, D. T., Merker, R. L., and Zisman, W. A,, IND. ENG.CHEM.,39,500 (1947). (17) Hardiman, E. W., and Nissan, A H., J. Inst. Petroleum. Technot., 31,255 (1945). (18) Horne, W. A,, IND. ENC. CHEM.,42, 2428 (1950). (19) Lamb, C., and Murphy, C. M., “Low Temperature Crankcase
Lubricants,” Naval Research Laboratory, N R L Rept.
P-3273 (1948). (20) Military Specification (Air Force-Navy Specification AN0 - l l ) , “Oil, Lubricating, Aircraft Instrument (Low Volatility),” April 12, 1948. (21) Military Specification MIL-L-7808, “Lubricating Oil, Gas Turbine, Aircraft,” December 5,1951. (22) Military Specification MIL-L-17353 (BuOrd), “Lubricating Oil, Low Temperature, Special,” October 2, 1952. (23) Military Specification MIL-0-6085, “Oil, Lubricating, Aircraft Instrument (Low VolatiIity),” March 30, 1950. (24) Military Specification [Bureau of Ordnance (Navy) 14-0-20
(Ord)], “Oil, Lubricating, Instrument (Synthetic),” December
pl
10,1946. (25) Miller, R. W., Craig, P. N., and Wolfe, J. K., “Synthesis of Esters of Dibasic Acids for Use in the Development of Lubricants,” Naval Research Laboratory, N R L Rept. P-2573 (1945). (26) Miller, R. W., and Wolfe, J. K., U. S. Patent 2,522,529 (Sept. 19,1950).
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(27) Murphy, C . M.,, et al., IND.ENG.C H E M . , 119 ~ ~ ,(1953). (28) Murphy, C. M., and Ravner, H., Ibid., 44, 1607 (1952). (29) Murphy, C. M., and Ravner, H., “Oxidation Inhibitors for Tur-
bojet Engines and Other High Temperature Applications,” Naval Research Laboratory, N R L Letter Rept. 3270206/49 hl (June 29,1949). (30) Murphy, C. M., Ravner, H., and Smith, N. L., IND. ENG. CHEM.,42,2479 (1950). (31) Murphy, C . M., Romans, J. B., and Zisman, W. A., Trans. Am. SOC.Mech. Engrs., 71, 561 (1949). (32) Murphy, C. M., and Zisman, W. A., IND.ENG. CHEM.,42, 2415 (1950). (33) O’Rear, J. G., “Synthesis and Characterization of Esters and
Ethers for Nonspreading Lubricants,” Naval Research Laboratory, N R L Rept. 3891 (1951). (34) Schiessler, R. W., Clark, D. G., Rowland, C. S., Sloatman, W. S.,and Herr, C. H,, Proc. Am. Petroleum I n s t , 111, 24, 49 (1943). (35) Tingle, E. D., J . Inst. Petroleum, 34,743 (1948). (36) Zisman, W. A., “Engineering Possibilities of Synthetic Lubri-
cants,” S.A.E. National Fuels and Lubricants Meeting, Reprint 855 (Nov. 6,1952). (37) Zisman, W. A., IND. ENG. CHEM.,45, 1406 (1953). (38) Zorn, H., “Esters as Lubricants,” U. S. Air Force Translation Report F-T-S-957RE (1946-47). RECEIVED for review February 17, 1953.
ACCEPTEDMarch 24, 1953.
Nature of Gas Burner Flash Back on Turndown JOSEPH GRUMER Flame Research Branch, Explosives and Physical Sciences Division, Bureau of Mines, Pittsburgh, Pa.
T
HE Bureau of Mines and the American Gas Associations are cooperatively supporting a study of the combustion charac-
teristics of fuel gases intended to supply, first, a basic understanding of the processes involved in flame stabilization and burner performance and, second, a body of data for practical applications. To date, satisfying progress has been made with respect to the flash back and blow-off characteristics of fuel gases ( I , 3-6,8) and the interchangeability of gases as limited by flash back and blowoff (2, 6, 7 ) . Yellow tipping will be investigated also. An investigation of the nature of flash back on turndown was undertaken as an incidental phase of this project. This paper reports the findings and suggests possible remedies. Normally, flash back takes place on a burner when the bound- ‘ ary velocity gradient at the flame port rim is less than the critical boundary velocity gradient for flash back of the mixture flowing through the ports. Secondary factors may contribute to produce flash back under conditions not included in the above description. One may observe flash back on ignition, flash back on turndown, and flash back on extinction, in addition to normal flash back. These instances need t o be considered separately, because conditions are not identical for all four. Flash back on ignition may be caused, for example, by the fact that more air is entrained by.a burner before ignition than after. Furthermore, the sudden appearance of the backward thrust of the flame may momentarily modify the flow profile through the port so that the boundary velocity gradient is less than the critical value for flash back. Flash back on extinction may occur if the performance points of the burner during turnoff pass into the flashback region of the fuel. I t can also occur after turnoff if the flame lingers and air entering the burner manifold changes the mixture composition there to an explosive one. Flash back on turndown occurs through still another mechanism. The following discussion is restricted to flash back on turndown.
Flash back on turndown can occur on a two-setting burner when it is rapidly throttled from the high to low flame position. American Gas Association requirements for gas valves on such burners specify that the low flow channel become operative before the high flow channel goes out of register. During turndown, therefore, the gas flow is lowered without ever dropping below the low flame rate. The flow of primary air must decrease proportionally with the gas flow, otherwise the stream leaving the ports would have to carry more momentum than the fuel jet leaving the gas spud, which is a physical impos;ibility. Arguments t o the contrary, proposing momentary overaeration, may be answered as follows:
It may be supposed that excess air flows into the burner air ports during turndown owing t o its inertia of motion. However, air outside a burner is nearly stationary, and because of its nearly zero velocity bears nearly zero momentum. It, therefore, has no appreciable inertia of motion of its own. It may be supposed that the stream within the burner does not slow up during turndown owing t o its inertia of motion, thereby sucking excess air into the air ports. When turndown starts, however, the stream in the burner must immediately slow down because of friction with the burner walls and the backward thrust of the flame. Stack action would not cause overaeration either. Whatever air flows through the burner owing t o buoyancy or temperature of the burner or pressure heads in the gas appliance is included in the primary air a t both the high and low settings. As flash back does not take place at either setting, the contribution of primary air by these factors is not enough to make the burner stream so lean that it would flash back.
It is more reasonable t o suppose that flash back during turndown results from momentary underaeration. The sudden throttling of the fuel-gas jet causes the jet t o decay momentarily into eddies consuming much of the momentum flux that would otherwise entrain air. The same effect can be had by passing a
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 45, No. 8
the transient flame formed on turndotvn. This flame clearly diminishes in size, as the total flow drops, until it becomes smaller than the stable flame formed a t the low setting, shown in the fourth strip. The transient flame formed immediately following turndown is shown in the third strip. This is the flame of the overrich slug resulting from the fleeting break down of air entrainment. It is yellow-tipped, proving that it is richer than either of the two stable flames. The flow characteristics of the gas valve used in this experiment are given in Figure 2. Flames from Figure 1 are superimposed on the curve in Figure 2 to show the correspondence between flame characteristics and state of turndown of the gas valve. It is felt that the very sharp drop in gas flow from the high to the low setting causes the short-lived decay of air entrainment. One remedy for flash back on turndown would be to adjust mixture composition and flame port diameter so that the flame would be quenched on entering the ports. Another approach to the problem would be t h a t of holding the boundary velocity gradients obtained during turndown above the critical boundary velocity gradient for flash back of the high-setting mixture. The minimum value of the boundary velocity gradient possible during turndown would equal that of a stream flowing a t the rate corresponding to zero air entrainment. However, the above analysis suggests that a basic remedy for flash back on turndown would be a gas valve which would drop the gas flow gradually from a high to a low setting. 16
Figure 1. Burner Flames during Turndown of Two-Position Gas Valve Stable flame at high flow Transient flame on turndown C. Transient flame following turndown D . Stable flame at low setting
A. B.
EXPERIMENTAL PROCEDURE
A burner was built to meet these experimental conditions for propane, Motion pictures were taken of the flames preceding. during, immediately following, and after turndown. A commercial two-position gas valve was used. Four strips from this movie are shown in Figure 1. The first strip shows the stable flame a t the high setting. The top frame shows a photograph of the burner (laboratory Meker burner). The second strip s h o w
E
I
l
l
I
-12
3,290 -10
n
a
0
I 0 E
Fuel = propane
.
4,606 -14
HIGH SETTING -3,948
card over the gas spud. The total flow of gas and entrained air therefore momentarily falls greatly a t the upstream end of the burner and correspondingly through the burner ports, dropping below the flow for the low setting. However, the gas-air ratio is not the same a t the two ends of the burner. As there has not been time to purge the burner, the mixture composition a t the port still corresponds to the high setting. The mixture composition a t the upstream end of the burner is overrich. The flame flashes back because the boundary velocity gradient a t the ports has dropped below the critical boundary velocity gradient for flash back (1, 3, 8) of the mixture corresponding to the high setting. This explanation of the phenomenon of flash back on turndown does not violate any physical laws. I t s validity can be demonstrated experimentallv. Two experimental criteria may be applied to test the above theory; the flame size during turndown should shrink because the total flow of the initial high-setting mixture drops and this slug of the initial mixture should be followed by flow through the ports of a slug that is richer than the final low-setting mixture. The flames appearing during turndown can be made to reveal the passage of an overrich slug through the ports if the mixture c o r position a t the high and low settings is just lean of the yellowtip limit. Then any overrich mixture will momentarily produce a yellow-tipped flame. These two events can be observed on a burner where flash back on turndown is prevented by smalldiameter ports.
-
Y
2,632 2 - 8 w
=!
2 1,974 -6d V
LOW SETTING -1,316
-4
Y 658 - 2
\ O
0
2
0
DEGREES ROTATION OF VALVE FROM FULL ON
Figure 2. Air Entrainment and Flow of Fuel in Burner during Turndown of Two-Position Gas Valve LITERATURE CITED
Grumer, J., “Combustion Characteristics of Fuel Gases,” Amer Gas Assoc. Research and Utilization Conference, Cleveland, Ohio, June 5-6, 1952. (2) Grumer, J., IND. ENG.CHEM.,41, 2756 (1949). ( 3 ) Grumer, J., “Study of Combustion Characteristics of Fuel Gases,” Interim Report No. 1, Amer. Gas Assoc. Project PDC-3, October 1951. (4) Grumer, J., and Harris, RI. E., IND. ENG.CHEV.,44,1547 (1952). ( 5 ) Grumer. J.. Harris, M. E., and Rchultr. H., “Flame Stabilization on Burners with Short Ports or Koncircular Ports,” Fourth Svmposium (International) on Combustion, Massachusetts Institute of Technology, Sept. 1-5, 1952 (in print). (6) Grumer, J., Harris, A I . E., and Schultr, H., IND.E m . CHEW, 44, 1554 (1952). (7) Lewis, B., and Grumer, J., Gas Age, 105, 25 (May 11, 1950). (8) Lewis, B., and Von Elbe, G., J . Chem. Phys., 11, 75 (1943). (1j
RECEIVED for review March 28, 1953. ACCEPTEDApril 27, 1953 This research is supported in part by the Bmerican Gas Association (Project PDC-3-GU).
