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sult the heat transfer is greater, the cone relatively shorter. For higher velocities the cone height is about constant. It was observed that a definite region of measurable thickness exists in the Bunsen flame, bounded by two superimposed cones. It is probable that the thickness of this section measures the rate a t which chemical combination occurs. This conclusion is borne out by the fact that the thickness increases with increased velocity. Table I1 illustrates the magnitude of this effect. Table 11-Effect of Port Velocity on the Thickness of the Bunsen Cone Port Diameter Height of cone Thickness Air-gas velocity of port in inches of cone ratio Ft./sec. Inches Upper Lower Inches 2.7 2.6 0.1 6.8 5.6 1.2 o.610 1.6 1.6 0.0 4.0 3.5 0 .. 5 . 5 0.469 2.3 2.1 0.2 3.4 2.9 0.5 3.7 1.1 4.8
Vol. 17, No. 12
A higher temperature difference results in a higher rate of heat transfer. Also, the ignition temperature of the mixture probably decrease^.^ 9
Falk, J. A m . Chem. Soc., as, 1517 (1906); 29, 1536 (1907); Dixon, Soc. ( L o n d o n ) , 93, 661 (1938); Taffanel, Compt. rend., lST, 714
J. Chem.
(1913); 118,42 (1914).
Factors Influencing Length of a Gas Flame Burning in Secondary Air By E. W. Rembert and R. T. Haslam
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MASSACHUSETTS INSTITUTE OF TRCHNOLOOY, CAMBRIDGE, MASS.
Experiments were carried out to determine the effects of port velocity, port diameter, and the primary air-gas ratio upon the over-all length of a gas flame burning in E F F E C TO F P O R T free space. It was found that the length of the flame is DIAMETER-w i t h a n determined by an equation of the form: increase in the diamL = K log u B log D E eter of the port, the l e n g t h of t h e p a t h where u is the port velocity, D is the port diameter, and t h r o u g h which heat K,B, and E depend on the primary air-gas ratio. A practical application of the results in connection must be t r a n s f e r r e d from the zone of sec- with the rational design of furnaces is included in the ondary combustion in- paper. creases for any given K a previous paper' the authors have discussed the height of cone, resultmechanism of combustion of a stream of gas premixed ing in a decrease in the with air. It was shown that port velocity, port diameter, over-all rate of heat t r a n s f e r . T h i s d e - and the primary air-gas ratio affect the dimensions of the crease in heat transfer Bunsen cone in such a way as to indicate that the rate of must manifest itself in combustion under these conditions is controlled by the rate an increased rate of a t which he$ is transferred from' the flame to the cold air c h a n g e of cone area mixture. In industrial furnace practice, combustion of a gas with prewith the amount of gas WRT MAMCTCR - INWCS mixed air offers no serious problems except in the design of burned. Figure 5 A variation in the burners. RIost of the combustion problems encountered diameter of the port, for any given velocity and air-gas ratio, in the industries arise in the process of burning in secondary also affects the shape of the flame surface. The ratio of the air; yet our knowledge of the phenomena involved is meager. mass of gas in turbulent motion to the total gas flowing in- The designer is forced to base his work upon a qualitative creases with increase in port diameter, resulting in a higher knowledge of the factors involved and upon empirical data over-all rate of heat obtained in field tests. Practically the only work that involves a thorough study of the factors influencing the rate of flow. 14 The data (Figure 5) combustion of a gas in secondary air is that contained in the indicate that the com- U. S. Bureau of Mines 12 b i n e d e f f e c t s of in- Bulletin 135. By samcreased heat transfer pling the reacting gases and a decrease in the a t various points along 10 curvature of the cone the flame, the length of s u r f a c e actually de- combustion space rei s crease the height of quired for various cona cone for larger values ditions of furnace operof diameter, the ve- ation was determined. 1 6 locity and air-gas ratio The data included in $ this work also show the Y remaining constant. EFFECTOF AIR-GAS marked tendency of hot RATIO-A typical set gases to stratify ac2 of curves showing the cording to their density, variation of cone height making sampling of with the air-gas ratio is stack gases difficult. 0 The present work 2 3 4 shown in Figure 6. As a & - RATIO the air-gas ratio in- includes a study of Figure 6 c r e a s e s the tempera- the effects of port velocity, port diameter, ture of the flame approaches a maximum corresponding to the =RAT0 GAS 1 Page 1233,this issue. theoretical air requirement of the gas under consideration. Figure 1
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INDUSTRIAL A N D EiVGIiVEERIiZ‘G CHEXISTRY
December, 1925
and the primary air-gas ratio upon the length of a gas flame burning in secondary air. Experimental
The apparatus and method of procedure were the same as described in the paper on the Bunsen cone;I the flame lengths measured directly for any given set of burner conditions.
to describe the motion of a jet of viscous fluid flowing into a body of the same fluid a t rest, is sufficient evidence of the difficulties. It is found that by p l o t t i n g the logarithm of port velocity against the flame length a straight line is obtained. A typical set of curves is shown in Figure 5 . That is :
F
PORT VELOUTY-FEET PER Y C
Figure 2
A typical set of experimental curves is shown in Figure 1 giving the variation of flame length with the primary airgas ratio at constant rates of flow of fuel gas. For purposes of discussion, these curves have been calculated over to the types shown in Figures 2, 3, and 4, showing the variation of flame length with port velocity, port diameter, and the primary air-gas ratio, respectively, other factors remaining constant. It was observed that the ratio of secondary air to gas had practically no effect upon the length of the free-burning flame, provided this ratio did not fall below that required for combustion of 75 per cent of the fuel gas. It is true that the flame would, in the case of the pure fuel gas, become quite smoky and “lazy,” as the secondary air supply was diminished, but the length was not affected.
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= K log u
+ C (1)
where F = l e n g t h of flame, in inches u = port veloci t y , in f e e t per second K, C = coefficients depending on port diameter and p r i m a r y air-gas ratio
Figure 4
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If one plots C against the logarithm of the port diameter,
D ,a series of lines such as shown in Figure 6 results. The constant C may be resolved into B log D
Discussion of Results
If a constant quantity of fuel gas is supplied to a burner per unit time and the amount of primary air mixed with the gas increased from zero, the length of the flame decreases (Figure 1). It follows that the length of a gas flame burning in secondary air is determined by the rate a t z z which the oxygen of T this air is mixed with EY the fuel gas. Figures 2, 3, and 4 show the 3 v a r i a t i o n in flame il length with port velocity, port diameter, and primary air-gas ratio, respectively. It is not proposed t o a t t e m p t a rigid o s treatment of theprobR)RT oimcrm-HwEs lem from] a matheFigure 3 matical point of view. The failure of such men as K e l ~ i nH, ~e l m h ~ l t zand , ~ Rayleigh4
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Nature, I O , 524, 549, 573, 597 (1863). “Wilssenschaftliche Abhandlungen,” Vol. I, p . 152. Phrl. M a g , 54, 59, 177 (1892).
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and Intercepts in Equation F K log u C Diameter of port, inches Air-gas 4 . 2 8 1 -0.360-0.469-0.610-0.820ratioK C K C K C K C K C 0.0 17.6 7 . 0 5 1 7 . 6 9.6 17.5 1 4 . 4 1 7 . 1 1 7 . 9 17.5 2 2 . 3 0 . 5 22.8 4.4 23.3 - 0 . 5 23.6 5 . 0 2 3 . 2 10.0 2 3 . 5 1 5 . 9 1.0 23.0 9 . 1 23.6 -5.1 23.4 23.6 5 . 1 23.6 10.5 0 2 . 0 21.2 -11.5 2 2 . 8 -9.1 2 1 . 0 - 3 . 6 2 1 . 2 1 . 2 2 1 . 4 5 . 1 Table I-Slopes
FLAME LENGTH
Figure 5
- INCHES
+ E , in which B
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and E are the slopes and intercepts in Figure 6. The complete equation is therefore, F = K log u B logD E (2) The Coefficients in the complete Equation 2 (functions of the air-gas ratio) are summarized in Table 11.
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Table 11-Coefficients for Equation (Length of flame in inches K log u f B log D f E) Air-gas ratio K B E 0 17.6 33.9 25.2 0.5 23.3 32.6 18.9 1.0 23.0 43.0 14.0 39.0 8.6 2.0 21.5
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Factors Affecting Utility of Secondary Air in Gaseous Combustion By E. W. Rembert a n d R. T. Haslam MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE.MAS%
The application of the general equation to actual furnace design depends on the ability to evaluate the coefficients under actual operating conditions, these coefficients changing with the type of furnace and the kind of fuel gas used. If 1.0,
Vol. 17, No. 12
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These experiments were carried o u t to obtain s o m e definite ideas regarding t h e mechanism by which oxygen in secondary air is consumed by a combustible gas. T h e results show that t h e total amount of air supplied to a free-burning flame materially affects the fraction thereof that is utilized for combustion. I n all cases investigated, t h e r a t i o of utilized t o supplied air passes through a m a x i m u m with respect to supplied air, t h e actual values depending upon port velocity a n d diameter. T h e fraction of the supplied a i r that is utilized for combustion increases with both port velocity a n d diameter, owing probably t o an increased rate of mixing. Several practical aspects of t h e results are discussed, including t h e fact that, by plotting t h e log of t h e air supplied t o that a m o u n t which is utilized, a flat curve results which can be closely approximated by a straight line, thereby permitting accurate extrapolation of “two-point’’ d a t a o n excess air requirements.
I R introduced into a fire box, above that theoretically required for complete combustion of the fuel, decreases the thermal efficiency of a furnace, because the stack gases can seldom be cooled to the initial temperature of the air used for combustion. The use of a certain amount of excess air in a furnace operating with long-flame combustion is necessary; if this amount is judiciously chosen, the sensible heat carried away in the stack gases is equal to that which would be lost as potential heat were the secondary air reduced. P r a c t i c a l l y , the only data relating directly to the factors affecting the excess AIR air requirement of a furnace are those included in the U. s. B u r e a u of M i n e s Bulletin 135, attention to which has been called in a previous paper. The results of this work are open to two serious objections from a point of view of general appIication. In the first place, stratification of the gases in the flue renders the average values of the gas analyses of doubtful accuracy. Furthermore, t h e experiFigure 1 mental conditions are important in the determination of excess air requirements, because the baffle arrangement, gas velocity, and distribution will-vary in different furnaces. It is possible to obviate the difficulties involved in sampling by thoroughly mixing the total volume of flue gas, when working on a small scale.
A
C Figure 6
it is desired, for example, to extend the fire box of a gasfired furnace, using the same port arrangement and size, so that the furnace will burn twice the quantity of gas that it will handle at present, we have an equation of the form of L = K log 11 c for which the coefficients can be determined in the existing furnace by two simultaneous measurements of flame length and port velocity.
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Bureau of Standards Completes over 173,000 Tests During the fiscal year ended June 30, 1925, the Bureau of Standards completed 173,261 tests. This represents a considerable increase over previous years; 115,729 tests having been completed in 1923, and 135,852 in 1924. The fee value of the work is $547,543.35. This is also an increase over past years; the 1923 and 1924 figures being $419,915.70 and $509,850.87, respectively. All income so received is turned back into the Treasury of the United States, the expenses incurred in the tests being absorbed by the annual congressional appropriations for the bureau’s maintenance. 2 1 great variety of materials and devices were tested, including analytical weights, chemical glassware, thermometers, sugar samples, radium, cement and concrete, leather, and paper. Almost every industry and branch of the Government has been served by the bureau.
