Predicting Stability in Domestic Gas Appliances
Five of the burners used in this study are pictured above. Burners A, B, D, and E are commonly found on gas ranges. You will recognize the ring-types, A, D, and E as successors to the old fashioned star-type, B. C i s used frequently on water heaters CHANNING W. WILSON and GEORGE E. McGOWAN, Research Department, Baltimore Gas and Electric Co., Baltimore, Md.
Fo,
economic as well as technical reasons gas distribution companies must occasionally vary the composition of the fuel gas distributed to consumers. During cold weather. natural gas may be augmented with propane-air mixtures or manufactured oil gas. In distributing supplementary or substitute gas. the utility must assure itself that the mixture will perform satisfactorily in consumers’ appliances, especially that stable flames will be produced regardless of variation in chemical composition of the gas. Experience, the incidence of service complaints, and empirical tests have heretofore been the guide for judging the limitations of introducing the supplementary fuel. Current flame stability theory ( 4 ) describes conditions necessary for the occurrence of stable flames, and the variation in stability limits for different fuel gases and for variations in air-fuel mixture composition. Flame stability
limits can now be calculated for nearly any fuel gas from its analysis (2). However, this information must be combined with parameters of the burner to be used. Therefore practical appliance burner performance was studied to determine the nature of the parameter required to correlate burner performance with different fuel gases. The most significant limits of satisfactory performance of an appliance burner are the limits of the stable flame region and the occurrence of flash back and bloLvoff. In gas industry practice the occurrence of luminous or yellow flames under certain conditions, and of minute traces of carbon monoxide in the combustion products, is also regarded as unsatisfactory. However, this discussion is limited to a consideration of flame stability.
Port Dimensions,
556
(1)
(2)
Burner
Ports
Cm.
A B C D
40 56 68 40 4 22 4
0.28 (dia.) 0.30 (dia.) 0.27 (dia.) 0.28 (square) 0.23 0.64 X 0.24 irregular
E (6,7)
INDUSTRIAL AND ENGINEERING CHEMISTRY
G = W / 8 v
(1)
where X is a friction or resistance coefficient relating the boundary velocity to the average flow velocity through the port, and is characteristic of the port geometry and the nature of the fluid flow. Parameter A? representing the influence of the burner port geometry, has been found to have the form
Geometric Characteristics of Ports on Appliance Burners
Background Literature Performance of appliance burners, limits of satisfactory performance, performance curves of burners Flame stability characteristics of fuel gas mixtures; calculation of diagrams from fuel gas analysis Effect of fluid flow through burner ports and port geometry on production of stable flames; resistance coefficient
The theory of flame stability limits proposed by Lewis and von Elbe ( d ) may be applied with modification to appliance burner performance. By this theory limits of stable flame regions may be expressed as critical values of the velocity gradient a t the boundary of the stream of premixed fuel-air mixture issuing from the burner port. The boundary velocity gradient may be expressed as (2, 3, 6)
D and E are interchangeable heads.
Total Area, Sq. Cm.
Depth, Cm.
2.45 3.54 3.91 2.87
0.53 0.45 1.17 0.80
3.77
0.80
Distance between Centers, Cm. 0.73 0.69 0.62 0.50 2.00 0.91
X = a/Reb
(2)
for a variety of port geometries over a broad range of flow velocities expressed by the Reynolds number, R e (6). X may be estimated theoretically for the simpler port forms, or determined experimentally from pressure drop measurements ( 6 , 7). An indirect method of determination has also been suggested (2, 3, 6), in which critical flow rates are measured with a fuel gas for which the flame stability diagram is known. Substituting the measured critical flow rate and the critical value of G, appropriate to the fuel and the fuelair mixture composition used, in Equation l the corresponding value of X may be calculated. Data of Grumer and associates (2, 3 ) obtained in this manner with ports having circular and noncircular cross sections compare favorably with values obtained by pressure drop measurements. The performance of multiport burners such as those found in many contemporary gas appliances differs from that of an individual port (6): Unequal flow through ports may occur. influenced by the location of the Venturi. “Piloting” from one flame to another and formation of a coalesced flame occur with some port arrangements. Burner configuration may affect disposal of combustion products and access of secondary air to flames. Lifting or incipient detachment of flame from ports usually precedes blowOff.
Different port diameters in one burner, and irregularities in construction due to commercial production methods, may cause nonuniform flow through ports. T o describe quantitatively the influence of these characteristics of a multiple port-burner, and including the resistance coefficient, A, a “performance coefficient.” PA, is proposed. If these items in combination influence blowoff and flash back bv different degrees, numerical values for the performance coefficient may not be the same at the two limits of satisfactory performance. I n this study the occurrence of lifting or incipient blowoff is selected, in accordance with gas industry practice, as the upper limit of satisfactory performance. Experimental
PA for a selected burner may be determined by indirect procedure (2, 3, 6 ) and calculated by an expression similar to Equation 1 : G
=
PACa/8v
(3)
During cold weather, gas distribution companies may have to augment their base supply of natural gas with manufactured oil gas or a propane-air mixture. O r a substitute gas may have to be put into the system. For these reasons, i t is important to be able to predict burner performance with different fuels flow through all the ports at the observed limit of satisfactory performance calculated from the volumetric rate of flow to the burner and the total port area; U = V / A . Data were obtained with four typical appliance burners. However, the method of correlation is not restricted to the types of burners illustrated. Several different fuel gases were used in the experiments, selected so that the desired range of variation of Reynolds number could be covered. For the most part natural gas, cylinder methane, or propane was used for lifting determinations. Flash back could often be obtained only with mixtures of a hydrocarbon gas and some hydrogen. These mixtures were prepared by mixing the individual constituents under pressure in a compressed gas cylinder. With predetermined rates of fuel flow to the burner, measured by a calibrated orifice flowmeter, sufficient primary air was premixed with fuel (the air shutter of the burner being completely closed) to produce either initial lifting or flash back, as desired. The composition of the fuel-air mixture a t the critical limit was determined by drawing a sample from the burner head through a Pauling oxygen analyzer. From the observed oxygen content, the per cent air and the per cent fuel in the combustible mixture were calculated. T h e value of P Afor the limiting conditions in each test was calculated by Equation 3. Required values of G were found from flame stability diagrams for each fuel gas determined experimentally with tubular burners, or with short cylindrical ports with rounded entrances in the manner described by Wilson and Hawkins (7). Kinematic viscosities of the combustible mixtures were estimated by Wilke’s method (5). Experimental data obtained with each burner were plotted on log-log coordinates. The data could be represented satisfactorily by curves of the form
Table I.
a
G is the critical boundary velocity gra-
A
dient for flash back or blowoff, at the pertinent fuel-air ratio, obtained from the flame stability diagram for the fuel used. i is the average linear rate of
c
104 503
D E
(4)
using the “hydraulic radius,” R H , to calculate R e for burner ports other than circular cross section. Constants of Equation 4 pertaining to each burner were evaluated by the method of le,ist squares. Data obtained with the star-shaped burner are plotted in Figure 1 for illustration. Individual data points represent performance coefficients, calculated with Equation 3. and the corresponding Reynolds number at the critical flow rates for initial lifting and for flash back. When consistent criteria are used for each limit, data obtained with different fuel gases fall on the same curves. This supports the hypothesis that the
I
I
O 03 0 o 4 1
omL
~
1
1 40
80 100 200 REYNOLDS N U M B E R , Re
KO
600
400
Figure 1. Performance cofficients of the star-shaped burner, 6, are not influenced b y combustion characteristics of gas or proportion of air and fuel Natural
A 75.3% methane $- 24.7% hydrogen 65.8% methane 34.2% hydrogen
+
Constants for Equations for Performance Coefficients
Burner
B
PA = a/Reb
157 436 335
(PA= a/Reb) Flash Back b 1.316 1.669 1.363 1.680 1.590
Lift in g
a 88
477 150 102 19
VOL. 51,
NO. 4
b
0.865 1.240 0.980 0.989 0.766
APRIL 1959
557
30
s
formance curves for a burner may be plotted, the value for PA,in Equation 4 may be substituted in Equation 3, and since Re = D ~ / v
(5)
or, more convenient for calculation, Log Re
REYNOLDS NUMBER,
REYNOLDS NUMBER,
Re
R~
Figure 2. Performance coefficients for initial lifting on typical appliance burners are function of Reynolds number
Figure 3. Performance coefficients for flash back on appliance burners are smaller than lifting coefficients
performance coefficients are parameters which will correlate the performance of the burner with different fuel gases, and are not influenced by the combustion characteristics of the fuel gas supplied or the proportions of air and fuel in the combustible mixture. Similar experimental data were obtained with the other burners illustrated. including the two heads, D and E , of the interchangeable-head burner, and all are summarized in Figures 2 and 3 (data points have been omitted for clarity). Figure 2 represents initial lifting of flames, and Figure 3, flash back. Data for each burner are represented by curves of the form of Equation 4, and agreement between the data points and the calculated curves was a t least equal to that illustrated by Figure 1 in all cases. Values of constants a and b of Equation 4 pertaining to each burner are listed in Table I.
and multiple port appliance burners. Finally, difference between corresponding curves for the drilled port burners may be accounted for only in part by differences in port depth. However, the limited published data do not suggest a change in slope with a difference in port depth but only a parallel displacement of the curves. The data for individual ports (2, 3 ) indicate differences in slope of curves for X us. Re for channels having square, rectangular, and triangular cross sections. These experiments cannot discriminaLe between the relative importance of each feature of burner construction of the limits of satisfactory performance, but they provide a sound basis for further study of these elements individually, which will lead to a clearer understanding of their relative influence. Practical use is made of the performance coefficients determined for any burner by combining such data with flame stability data for any selected fuel gas. This may be illustrated with most generality by calculating performance curves, which show directly the behavior of a burner supplied with a selected fuel, in relation to the heat input and air shutter adjustment. These curves have been familiar to the gas industry for many years (7). although it has heretofore been necessary to obtain them by direct cxperiment. This has limited their utility to fuel gases actually available. Because flame stability diagrams may now be derived for fuel gases having nearly any assumed chemical composition ( Z ) , the ability to estimate and predict burner performance with different fuels is greatly extended. T o calculate data from which per-
Discussion Neither flame stability theory nor the published results of experiments with individual burner ports (3, 6) suggest immediately that separate curves should be obtained representing flash back and blowoff. A small difference in flow between initial lifting and complete blowoff with square-edged ports has been reported ( 3 ) , but it is not of sufficient magnitude to account for the separation of the curves in Figure 1, or between corresponding curves in Figures 2 and 3. T h e major portion of the observed spread between the lifting and flash-back curves may be attributed to the combined effect of other differences itemized above between individual ports
558
INDUSTRIAL AND ENGINEERING CHEMISTRY
=
1 __ 2 - b log
8D2G
(7)
T o calculate the lifting curve for a burner, the values of the constant, a , and the exponent, 6, of the equation for its performance coefficient curve for lifting are used. At a series of fuelair ratios, for which values of the critical gradient a t blowoff are obtained from the flame stability diagram for the fuel gas selected, the Reynolds numbers corresponding to the critical flow rates are calculated with Equation 6 or 6a. As the linear and volumetric flow rates are related, zi = V / A , the critical flow rate a t lifting may be obtained from the critical Reynolds number by V = Re X A v / D
(7)
Having the critical total flow at lifting and the per cent fuel in the combustible mixture, the corresponding fuel rate and heat input to the burner are readily found. The curve for flash back is calculated in the same manner by using the equation for the flash back performance coefficient, and critical gradients at flash back from the flame stability diagram of the fuel. Performance curves, calculated by this procedure for burner A , when supplied with a manufactured oil gas, are shown in Figure 4. The flame stability diagram for the oil gas was prepared by methods outlined by Grumer and others (2) using the analvsis of the oil gas. The stable flame region is represented by the space bet\reen the flash back and blowoff curves. It is clear from this illustration that a broad range of fuel input rates extends to an aeration as high as 80% theoretical air. Practical adjustment of aeration for this burner would not normally exceed this value. The broad operating range therefore means that this type of burner would give good performance when supplied bvith a supplementarv manufactured oil gas. Similar calculated curves (Figure 5) illustrate the behavior of burner E. supplied with propane or propane-air mixtures. The intersection of the flash back and lifting curves clarifies an originally puzzling observation that onlv flash back occurred between 4000 and 11,000 B.t.u. per hour input when the primary aeration was between 110 and 85% of stoichiometric air. The lifting curve falls within the flash-back region, and under these conditions flame will propa-
F L A M E CHARACTERISTICS
0
0
4 6 8 10 12 14 16 18 20 HEAT I N P U T - THOUSANDS O F B t u PER HOUR
2
Figure 4. Calculated performance curves for burner A with supplementary oil gas
I
Figure 5. propane
2 ‘ 3 4 HEAT INPUT
5 6 7 8 9 IO - THOUSANDS OF Blu PER HOUR
II
12
Calculated performance curves for burner E with
Performance of this burner m a y b e faulty with propane-air fuel
This t y p e of burner gives good performance wilh supplementary manufactuied oil gas
gate back through the ports. T h e experimental points and calculated flash back curve appear to be in very satisfactory agreement. T h e relatively large flash-back region shown in Figure 5 indicates possible faulty performance when this burner is supplied with propane-air fuel. Even if the airshutter is adjusted with natural gas, so that a normallv conservative proportion of primary air is entrained, the greater specific gravity of the propane-air ivill result in increased aeration and a tendency toward flash back. If propane-air is to be mixed with natural gas by a distribution company, the maximum acceptable proportion of the supplementary gas in the mixture may be estimated by extending the calculations to mixtures of different proportions. I t is someivhat simpler to determine the behavior of a given burner, a t a fixed heat input and primary aeration adjustment; !
