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Flame data from a large residual fuel oil-fired burner
F L A M E D A T A FROM A LARGE R E S I D U A L F U E L O I L - F I R E D B U R N E R J. A. ARSCOTT AND A. M. GODRIDGE
Central Electricity Generating Board, Research Division, Marchwood Engineering Laboratories, Matchwood, Southampton, S04 4ZB
Data have been collected from the flame of a full size steam atomised burner operating with a residual fuel oil throughput of 8,100 kg h ~ (13,500 lb h-l). The results reported here are the first to be obtained for a steam atomised oil burner of this throughput. The flame measurements were carried out in the Babcock & Wilcox large burner test facility, which is essentially a water cooled box approximately 22 m long and 3.6 m square internally. Flame temperatures were measured with optical and suction pyrometers and part of the gas sample was analysed for 02, CO and CO 2. Flame radiation measurements were made with a wide angled (2~" steradians) ellipsoidal radiometer and a narrow angled total radiation pyrometer, located at the chamber ports. The latter instrument was also built into a water cooled probe and traversed through the flame. Gas velocities were measured with a fully water cooled double-headed pitot tube. For total heat fluxes a conductivity type heat flow meter was used. Three flame conditions, 'a', 'b' and 'c', were examined, having respectively 0.5% excess 02, 1.5% excess 02 and again 0.5% excess 02 but with vitiated air at the burner inlet. The directly measured data have been compared for the three flames and the results show, for example, that a maximum flame temperature of 1725~ + 20~ occurs in flames 'a' and 'b' but that the vitiated flame has a maximum temperature of only 1680~ + 20~ The maximum emissivity of 0.95 + 0.05 occurs in flame 'b' although the largest local absorption coefficient of 2.4 m - t +_ 0.70 m -~ was observed in flame 'a'. Some comparisons have also been made with previous results from other burners. A computerised mathematical model with a diffusion approximation followed by a Monte Carlo simulation of the radiant heat transfer has been used to determine the heat release distribution in the flame. It is concluded that the volumetric heat release (Q) decays exponentially with downstream distance (x) from the burner and can be represented by an equation of the form: Q = - - exp AD
-
where P and D are the thermal power and diameter respectively of the burner and A is 1.3.
Introduction Furnace heat transfer programs (Ref. 1) have been used to calculate the distribution of absorbed heat flux in future CEGB boilers. The accuracy of such predictions is limited by the quality of the input data. Sensitivity studies have shown that the biggest contribution to the uncertainty in local absorbed heat flux is the heat release distribution of the flames.
737
In principle heat release can be evaluated from velocity and chemical analysis of the gases at successive positions along the flame. However in large, very turbulent combusting systems these measurements are difficult to make and correlation of temporal fluctuations of velocity and composition may make simple measurements inadequate. In the
738
FURNACE COMBUSTION
early part of the flame, where heat release is greatest, it is almost impossible to obtain representative measurements of the amount of unburnt liquids and solids present. This direct method has therefore been abandoned as a means of obtaining heat release data in favour of an inferential method using mathematical models. Rig tests are carried out on a single burner of the type used in the furnace being studied, velocity distributions, gas absorptivity, wall emissivity, gas temperatures and wall heat fluxes being measured directly. The furnace heat transfer program computes the heat release distribution, the only remaining degree of freedom, required to produce the measured gas temperatures. The wall heat flux data is used as a check on the accuracy of the program. Gas sample data also serve to check that the computed heat release data are consistent with the observed progress of the flame. The heat release of the single test burner is assumed identical to that of a burner in a multiburner furnace. The justification for this is that in these large flames the heat release is primarily mixing controlled and occurs close to the burner where the velocity and turbulence are almost wholly determined by burner exit conditions. This paper describes the techniques used and presents the measurements obtained on a rig test of a single 6100 kg h -~ steam atomised oil burner of the type to be used in some of the CEGB's latest oil fired boilers.
The Test Facility & Measuring Instruments The tests were done in the Babcock & Wilcox Large Burner Test Facility at Renfrew, Scotland. This facility is essentially a water cooled box approximately 22 m long and 3.6 m square internally, with a refractory wall at the burner end and refractory on the last 5 m of wall and floor at the exit end of the rig. Air preheat is usually obtained by burning a light oil in the air supply duct, thus vitiating the air to about 19% Oz. However for some of these tests (see the Section on Test Work) oxygen injection was provided so that preheat could be obtained without vitiation. The burner (see Figure 1) was a B & W venturi type of 710 mm throat diameter and 990 mm exit diameter fitted with a thirty bladed stabiliser 406 mm in diameter mounted 216 mm upstream from the exit plane. A 15 hole Y-jet atomiser was used and the air preheat was 260~ The residual fuel oil (RFO) used had an average viscosity of 925 cS at 37.7~ (100~ a specific gravity of 0.97 and a calorific value of 42,600 kJ kg -~. The oil was preheated to an atomising temperature of around 130"C to reduce its atomising viscosity to the normal 15 cS at 100~
The present tests made use of the experience gained in burner trials on the Board's rigs and plant (Ref. 2.) and at the International Flame Research Foundation (Ref. 3.) A full description of the instruments used and the errors have generally been given in these references, so that the instruments need only be listed here with a brief description. (a) Flame Temperatures. A water-cooled 4m long suction pyrometer was used to measure local temperatures in the flame. In this instrument flue gas is sucked at a high velocity over a suitably refractory sheathed and shielded thermocouple. The amount of gas is very small compared with the burner throughput (i.e. 1:1000 by weight). Corrections to the measured temperatures to allow for heat losses from the thermocouple to the cooler furnace chamber walls can readily be obtained by 'calibrating' the instrument in situ. A hand held optical pyrometer was used to measure the line of sight temperatures. The single reading obtained represents the 'brightness temperature' of this flame at 0.65 I~m. The flame is not a black body however, so in order to evaluate the true radiant temperature, the reading obtained must be combined with a second reading obtained at another wavelength, (Ref 4.) In these tests the second reading was obtained using the total radiation pyrometer described under item (d). (b) Gas Analyses. Part of the gas sampled by the suction pyrometer was passed through filters to a paramagnetic O~ analyser and an infra-red CO2 and CO analyser. (c) Total Heat Fluxes. These were measured with a conductivity type heat flow meter having a blackened plug mounted at the end of a 0.7 m long water cooled probe. This instrument has been described in Reference 3. The difference between the measured total heat flux and radiant flux was used to estimate the convective heat transfer. (d) Radiation Measurements. Three instruments (all manufactured by Land Pyrometers Ltd) were used to make measurements of radiation from the flame. The first two both used an arsenic trisulphide lens to focus a very narrow angled beam of radiation on to a thermopile. The use of the special lens material ensures reception of radiation up to 13 t~m wavelength. These two instruments have angular fields of view of 1/50 and 1/20 of a steradian respectively. The first instrument was used at the ports in conjunction with the optical pyrometer. The second instrument was mounted in a water-cooled probe and focused on to a water-cooled target fixed some 0.254 m from the front end of the main body of the probe, This probe was used for making 'local' radiation measurements within the flame. The third radiation pyrometer used was an ellipsoidal radiometer in which all the incident radiation over an angle of 2xr steradians is detected (Ref. 2).
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER
739
PLENUM CHAMBER PRESSURE
CARRIER TUBE FOR OIL AND STEAM PIPES
STABILISER WITH CENTRAL ATOMISER AIR SLEEVE
BOILER FRONT
AIR SLEEVE CYLINDER
MAIN COVER VE NTURI THROAT RING
J, CENTURI THROAT 2750
F1c. 1. Sketch of the Babcock & Wilcox Burner of the type used in the present tests (dimensions,
ram). (e) Gas Velocities. These were measured with a fully water-cooled double headed pitot tube. The ends of the probe were shaped to give a cosine response to yawed flow so that the probe measured the axial component of the velocity head. A propeller anemometer was also used in the isothermal flow tests. The optical pyrometer, total heat flux meter and radiation measuring instruments were all calibrated before and after the tests.
Test Work and Results Isothermal tests were first conducted with and without a gauze situated 1 m downstream of the burner mouth, the gauze being theoretically required to make the isothermal jet behave like the burning jet (Ref. 5). It was expected that the isothermal data obtained from tests with a gauze could be used to
supplement the limited amount of velocity data obtainable under firing conditions. When firing the rig the excess O~ and CO content of the flue gases were measured by two probes in the exit duct and conditions adjusted to give the desired values (i.e. 0.5% and 1.5% excess O~ and 0.01% CO but see Table 1). A sheathed thermocouple nearby indicated gas exit temperatures and conditions were considered to be at equilibrium when this temperature remained constant. The normal operating conditions were logged and they show that the rig exit temperature throughout the tests only varied between 965 and 995~ The variations in other conditions are also summarised in Table 1. Although the spread of the excess 02 figures is large (0.2% to 0.9%) the bulk of the measurements were between 0.4% and 0.65%, for the nominal 0.5% condition. The oil flow was very steady, varying by less than +1%. Three flame conditions were studied, these were as follows: (a) maximum throughput or maximum
740
FURNACE COMBUSTION TABLE I Summary of variations in operating conditions Test conditions
Design Excess. 02 % Excess CO % Gas exit temperature ~ Oil flow kg h - l Steam flow kg h -~ Oil pressure bar Steam pressure bar Steam flow %
condition
(a) 0.5% excess Oz
(b) 1.5% excess 02
(c) 0.5% excess O2(V )
0.50 0.01 -6,070 240 15.2 8.65 4
0,2-0.9 (Av. 0.5) 0.04-0.005 965-995 5,985-6,095 205-311 10.6-13.8 8.20-9.00 3.4-5.1
1.35-1.9 (Av. 1.5) 0.02-0.005 975-995 6,030-6,090 238-246 11.4-12.6 7.20-8.80 3.9-4.1
0,45-0.75 0.02 970-990 6,030 235-239 12.5-12.6 8.20~8.40 3.9-4.0
continuous rating (MCR) with 0.5% excess 02 at the furnace outlet and O 2 injection into the preheated air; (b) the same conditions but 1.5% excess 0 2 at the furnace outlet and (c) MCR with 0.5% excess 02 at the furnace outlet but no oxygen injection into the preheated air (i.e. vitiated air was used). The first condition was investigated very extensively with all the instruments available. The second and third conditions (flames b and c) were studied to a lesser extent. Probing was usually along a horizontal line from
SCALE
the ports to the b u r n e r centre line. Temperature contours from these traverses for the three flames are shown in Figure 2 and centre line temperatures are compared in Figure 3A. The maximum corrected suction pyrometer temperature was 1725~ + 20~ The radiant (optical) temperatures are shown in Figure 3B with a maximum line of sight value of 1640~ _+ 35~ The observed temperature differences between the three flames are considered in the Discussion. The total emissivity, e T, of a flame is related to
i
Im
I
I I 5
r~
4
\
\Ill
Z3
II
/
I
A\ I~II /l l' , . X ' , ~\11 I I,
z "2
u. 1 Z
:[ l | BUR~NERt FLAME 'a' (0.5% EXCESS 0 2 AT FURNACE EXIT)
FLAME 'b'
(I .5% EXCESS 0 2 AT FURNACE EXIT)
FLAME %' (VITIATED AIR AT BURNER AND 0.5% EXCESS 0 2 AT FURNACE EXIT)
FIG. 2. Suction pyrometer temperature contours on the horizontal plane through the burner axis (~
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER the total absorption coefficient, Kr, by the equation eT = 1
exp(- KTL)
-
(1)
where L is the flame path length (Ref. 4). The present emissivity values for flames (a) and (b) peak at 0.90 • 0.05 and 0.95 • 0.05 and have minimum values of 0.43 • 0.02 and 0.63 • 0.3 respectively, the low values being observed in the flame region up to one 'burner diameter' downstream from the burner. For the shorter path lengths within the body of the flame the water cooled local radiation probe described earlier, was used. If the local 'gas temperature' (TT) is obtained from the suction pyrometer then the two readings can be combined to give the local flame emissivity or more usefully the local total flame absorption coefficient, as follows: ere~ T ~ = o'T ~ 4
thus
(2)
4
e~ = T s / T T
(3)
and from equation (1) PRESENT WORK x
FLAN~ 'a'
:
t700
g I I I r l I I ~ I I 2 4 6 8 10 DISTANCE FROM THE BURNER I N 'BURNER DI.'/U~TERS'
3A. FLAME CENTRE LINE TEMI~ERATURES
1700
x II ~M~
~.ESEN'TWORK - FLAME 'a'
9
PRESEN'TWORK - F~dC/E 'bT
9
SMALLERSTEAM ATOMISED
T
BURNER
,OOOR.OOE, .OLT
,,oo
1200
i
i
2
i
l
4
t
I
6
I
t
I
8
MATT.E .
T~/T~
741
= 1 - e x p ( - K ~ L).
The value of T, for the measured radiometer output was obtained from a black body calibration curve. An examination of the KT values for all the flames (see Figure 4) shows, as would be expected, the values peaking in areas where the oil spray was observed to be most dense. The emissivity and absorption coefficients for all three flames are considered in the Discussion. The gas analyses for flame 'a' showed CO levels of about 0.6% at a distance of four burner diameters downstream from the burner. At all distances from the burner the core of the flame had more CO than was found in the rest of the flame. This is in line with lower temperatures (see Figure 2) in the early part (say, two burner diameters from the burner) of the flame core. The propeller anemometer measurements (see Figures 5A and 5B in which the flow data has been non-dimensionalised with respect to downstream conditions) clearly indicate the lack of axial symmetry in the flow. The pitot measurements for the hot condition (flame 'a'; see Figure 5C) also show the flame to be directed towards the side of the rig with the probing ports. The spread of the jet in the horizontal plane through the burner centre appears to be wider in the firing case than in either isothermal case, although traverses completely across the rig would be required to confirm this. Because of the presence of unburnt oil, causing probe blockage, readings were not obtained in the body of the flame at ports 1 and 2 (0.2 m and 1.1 m downstream from the burner). The first complete traverse to the centre line was at port 3 (2.1 m), where the maximum velocities were 65 • 15 ms -~ The total incident heat fluxes (convective plus radiant) and the radiant heat fluxes above are plotted in Figure 6. Here it can be seen that the heat fluxes increase very rapidly from about 180 • 40 kW m -2 at port 1--this value agrees reasonably well with that of 160 • 30 kW m -~ measured on the burner wall--to a maximum value of 370 + 60 kW m -~ four and a half burner diameters downstream of the burner. Tests to determine the error from not holding the probes in exactly the same positions at the ports (i.e. varying tilt and insertion depth) indicated variations in both cases of about +20kW m -~. The averaged radiant fluxes were always less than the total heat fluxes, as expected.
Discussion
I
10
OfSTANCE FROM THE BURNER IN 'BURNER DIAMETERS~ 3B. RADIAN~T{CORRECTED 'OPTICAL LINE OF
SIGHT') TEMPERATURES
FIG. 3. Flame temperatures for a number of flames.
Flame Comparisons Figures 2 and 3 show that in the present work flame 'b' with 1.5% excess 02, develops its highest temperature more quickly than the other two flames
742
F U R N A C E COMBUSTION
(i.e. flames 'a' and "c') and has a larger high temperature region. However, the maximum temperatures achieved by flames 'a' and'b' are identical, at 1725~ The vitiated flame ('c') never achieves this temperature, the maximum value recorded being 1680~ Nevertheless, all three flames show very similar temperature conditions at six 'burner diameters' downstream from the burner, for example where the centre line values are all within 1505~ + 25~ A steam atomised flame from a similar burner, (Ref, 6) but with a smaller oil throughput of 3060 kg h - t has been examined in the Marchwood Engineering Laboratory rig, which has a 50% refractory wall area. The maximum flame temperature measured with the same suction pyrometer as used in the present work was 1740~ + 20~ The excess O 2 was 1,0% and the maximum radiant temperature was 1625~ which is between the flame 'a' (0.5% excess O2) and 'b' (1.5% excess O2) values of 1640~ and 1600~ respectively. The local absorption coefficients for the present three flames (see Figure 4) show flame 'b' with the highest absorption coefficient (1.7 m -~) at one 'burner diameter' from the burner but flame 'a' having the highest overall value (2.4 m -~) at two 'burner diameters' downstream. The vitiated flame 'c' has the lowest absorption coefficients. The different values observed in the three flames are related to the different rates of soot formation and burnout at the different oxygen and resultant temperature levels. The local flame absorption coefficients mea-
-!
sured earlier on the smaller flame at 1% excess 02 varied between 0.45 m -~ + 0.18 m -~ at the edge of the flame and 1.85 m -~ + 0.7 m -~ at the centre of the flame, These values are similar to the present results which are in the range 0.41 m -1 + 0.15 m -j to 2.4 m -1 + 0.70 m -~ at excess Oz values of 0.5% to 1,5%. Leukel (Ref. 7) has reported values in the range 0.5 m -~ to 8.0 m -~ for a smaller oil burner operating with higher excess O~ values (i.e. oil throughput 144 kg h -~ and excess O~ up to 3%).
Heat Release Distribution In studies of power station boilers it has proved convenient to regard the axial distribution of heat release as obeying the equation:
Q:
where Q P D x A
= = = = =
9
I
" II'A'J)
I
heat release/unit axial distance thermal input power burner diameter axial distance a coefficient
1~.511 k2l
;,,;,.-,.,
I
I
Y/'l.-\r.,
I BUR~IER I
-
The axial disposition of the heat release is important in determining the relative heat absorption of front and rear walls of a boiler and the magnitude
o 51
II
exp
(f'(0o:; ,,
k. l ) J ) ,
, I
,
k -J.
/ I BURJlNERI
I BUR~IER I
r FLAME 'a' (0.5% EXCESS O2AT FURNACE EXIT)
FLAME 'b' (I .5% EXCESS O2AT FURNACE EXIT)
FLAME 'c ~ (VITIATED AIR AT BURNER AND 0.5% EXCESS O2AT FURNACE EXIT)
FIG. 4. Contours for measured local total absorption coefficients m -~
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER
743
9
<
8
5 ~ z7 6
Z
a~
~4 I.---
:E
03 z< 2
1
- GAUZE
8
i 2 ,
o (A) COLD A I R - N O GAUZE
(B) COLD AIR WITH GAUZE
7.2
,
I 2J
i
3
METRES ACROSS RIG
(C) FLAME O N
rh = 19.22 kg $-1
rh = 19.22 kgs -1
rh TOTAL = 2 5 . 6 7 kg s - I
g = 13.5 ~
O = 13.5~
O
EXIT
= 987 ~
F1c. 5. Axial velocities on the horizontal plane through the burner axis (non-dimensionalised). of peak heat fluxes on the burners and adjacent walls where waterside dry-out may be a problem. The axial distribution of heat release, characterised by the coefficient ;~, has been evaluated from measurements taken on a single burner firing in a test rig by the use of two related computer programs. In the first program, called CEDRUF, the furnace chamber was represented by 900 zones and the main input data to the program consisted of wall emissivity, estimated convective heat transfer coefficient, gas absorption coefficient and gas temperature in every zone, and gas mass flow through each face of every zone. The program used a diffusion approximation to radiation transport followed by a MonteCarlo simulation to evaluate the heat release that must have occurred in each zone for the measured temperature field to exist. The absorbed heat flux at the furnace wall zones was also available as output. The second program, called FURDEC, used the same furnace zones but the heat release in each zone was used as input data instead of gas temperatures. This program also employed a diffusion approximation followed by a Monte Carlo simulation of the radiant heat transfer to compute gas temperatures in each zone and the absorbed heat flux at the walls. In composing the gas flow data for these programs the limited number of flow measurements made in
the firing rig had to be supplemented by isothermal cold flow measurements. A gauze was fitted across the jet downstream of the burner to increase the isothermal jet diameter to match the flame diameter, (Ref 5). Initial analysis of the results using the program CEDRUF yielded on uneven heat release distribution. This was partly due to small gas temperature errors but mainly due to the mismatch of the isothermal flow field with the actual one that existed under firing conditions (see Figure 5). However, when the irregularities in the heat release distribution predicted by CEDRUF were removed and the results entered into the FURDEC program a few manually iterative computer runs produced a reasonable match to the measured gas temperatures, and absorbed heat fluxes and also yielded a smooth spatial distribution of heat release. For the 6100 kg h -~ steam atomised oil flame "a" the best fit to the measured gas temperatures was obtained with the coefficient in the exponential heat release equation ~ --- 1.3. The corresponding computed and measured absorbed heat fluxes are shown on Figure 6. The value A -= 1.3 may be compared with a value of A = 1.5 used by the author (Ref 1.) to match measured plant data on a large boiler fired by 3600 kg h -l pressure jet oil burners. Thus, expressed in terms of burner diameters, the present burner, which is steam atomised, releases
744
FURNACE COMBUSTION COMPUTED TOTAL HEAT FLUXES x
MEASUREDTOTAL HEAT FLUXES - FLAME ' a' AV1ERAGEDOVER T W O DAYS READINGS
9
MEASURED RADIANT HtEAT FLUXES - FLAME ' a ' AVERAGED OVER TWO DAYS READINGS
observed in the non-vitiated flames. (iii) The axial distribution of heat release in such flames can be represented by the equation Q = P / AD exp ( - x / A D ) where P = input power and D = b u r n e r diameter. The factor )t, found to match the measured flame data, was 1.3 for the steam atomised b u r n e r tested at 0.5% excess 02.
E
-~ 300
Acknowledgments
u_
100
1
I
2
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i
4
t
t
6
t
I
8
I
t
[
10
DISTANCE FROM THE BURNER I N 'BURNER DIAMETERS'
FIG. 6. Total and radiant heat fluxes for flame 'a'. heat earlier than does the smaller pressure atomised type in current use. The lateral spread of heat release appears roughly Gaussian in form. There is some evidence to suggest that the heat release is greatest along the cone of the oil spray and rather lower on the axis itself. However even the 900 zones used in this work are not sufficient to study such detail. In predicting the wall heat fluxes in a large multi burner furnace an accurate knowledge of the lateral spread of heat release is not as important as the longitudinal distribution and the characterisation of the heat release by the single coefficient A has been found sufficient.
We are indebted to Babcock and Wilcox (Research) Ltd, who modified and operated the rig, Under contract. Mr. W. B. Bryce and Mr. W. I. Joyce of B & W organised this part of the work and Marchwood Engineering Laboratories staff probed the flame (Mr. R. O. Denham, Mr. P. L. ]oyner, Mr. C. J. Norman and Mr. J. S. Holt). This paper is p u b l i s h e d with the permission of the Central Electricity Generating Board.
REFERENCES 1. ARSCOTT, ]. A., GraB, J. AND JENNER, R., First European Symposium on Combustion, p. 674, Academic Press, 1973. 2, GODmDGE, A. M. ANO READ, A. W., Prog. Energy and Combust. Sci. 2, 83, (1976). 3, C~EOA~LLE, J. AND BnAuo, Y., Industrial Flames, I, Measurements in Flames, E d w a r d Arnold, 1972. 4. Daws, L. F. AND T~R[NG, M. W., ]. Inst. Fuel,
25, 528, (1958). Conclusions
(i) Temperatures up to 1725~ + 20~ and local absorption coefficients in the range 0.41 m -~ + 0.15 m -~ to 2.4 m -~ + 0.70 m -~ have been measured in the flame from a large b u r n e r run under normal oxygen conditions. (ii) When supplied with air vitiated to 19% 0 2, the flame developed lower temperatures and local absorption coefficients close to the burner than those
5. DAVlSON, F. J., Nozzle Scaling in Isothermal Furnace Models, C E G B Laboratory Note R D / M / M 2 5 , 1967. 6. GODRIDGE, A. M., HOLT, J. S. AND MATTHEWS, K. J., The Collection of Flame Data From a Large Steam Atomised Flame, C E G B Laboratory Note R / M / N 9 3 8 , 1977. 7. LEUKEL, W., International Flame Research Foundation, 1st Members Conf. IJmuiden, 22-23 May, 1969.
COMMENTS A. N~metli, Computer Centre for Universities, Hungary. Recently some measurements have been
to soot and if so, what technique has been used?
carried out in Hungary concerning soot radiation in an oil fueled furnace using " w i n d o w s " in spectra where presumably only soot was emitting. I would be interested to know whether you have measured how much of the total radiation was due
Authors" Reply. Provided the gas composition is known, the non-luminous radiation can be calculated from published tables. This contribution can then be removed from the total radiation, w h i c h we have measured and reported, by using the equations for
F L A M E DATA F R O M A L A R G E R E S I D U A L F U E L O I L - F I R E D B U R N E R total and soot radiation given in References 2 and 3 of our paper.
1. 1, Macfarlane, Imperial
College,
England.
Some light w a s t h r o w n on the effect of vitiation on distillate fuel c o m b u s t i o n by my colleague Dr. B. P. Mullins at the National Gas T u r b i n e Establishm e n t at Pyestock d u r i n g the early 1950's. This work is still available in the form of N G T E reports and explains the effect of vitiation on the flame behavior of distillate fuels via reduction of the m a x i m u m flame temperature. T h e blue-flame effect referred to in the p r e s e n t d i s c u s s i o n w o u l d surely not apply to residual fuel oils w h e r e solid particle residues w o u l d still be p r o d u c e d in the flame.
Authors" Reply. It will be useful to examine the work referred to by Dr. Macfarlane. We think that the influence of the large residual fuel oil coke particles will be small in c o m p a r i s o n w i t h the soot. T h i s is b e c a u s e there are very many soot particles and only a few coke particles.
T. F. Wall, University of Newcastle, Australia. I am interested to k n o w h o w your data might be transferred into y o u r furnace heat transfer p r o g r a m s of larger p o w e r station furnaces. Presumably, the zones y o u use in these latter models are quite large so that you will require average values, for example, for a b s o r p t i o n coefficient. H o w w o u l d y o u translate your exhaustive data into these mean values? Secondly, in order to use the exponentially decaying heat release e q u a t i o n proposed, this m u s t use a zone s h a p e in the p o w e r station model similar to the flame shape. Do y o u r m o d e l s have this capacity, and if so, h o w sensitive are the predicitions of the m o d e l s to the accuracy of the heat release equation?
Authors'Reply. It w o u l d have been nice to analyse our rig data u s i n g the same size zones as w e use for complete furnace studies. However, s u c h zones can be as large as 3m x 3m x 3m d e p e n d i n g on the b u r n e r p i t c h i n g and furnace size. T h e bulk of the heat release in a furnace flame occurs in about 3 zones in front of each burner, most occurring in the first zone. O u r main interest was in the spatial distribution of heat release as the heat flux at the
745
b u r n e r wall is quite sensitive to it (10% change in the coefficient k changes the local heat flux by a b o u t 3%) b u t the total furnace heat a b s o r p t i o n is not very sensitive to it. T h e classical m e t h o d of deriving the heat release distribution from m e a s u r e m e n t of the b u r n o u t of chemical species w a s not attempted due to the difficulty of obtaining m e a s u r e m e n t s in the early part of the flame where b o t h solids and l i q u i d exist. I n o u r m e t h o d the heat release distribution w a s f o u n d by the use of a zone m o d e l of furnace heat transfer. U s i n g m e a s u r e d optical properties and gas m a s s flow data the heat release in each zone w a s adjusted until the program r e p r o d u c e d the m e a s u r e d gas t e m p e r a t u r e s and heat fluxes. It w a s to be able to assess the a d e q u a c y of the model representation o f the m e a s u r e d data that we u s e d so m a n y small zones. You are quite right in a s s u m i n g we c a n n o t use s u c h small zones for a w h o l e furnace s t u d y a n d m u s t therefore translate this small zone data to larger zone values. The heat release, our main interest, p r e s e n t s no p r o b l e m s b e i n g s i m p l y integrated over appropriate volumes. T h e optical properties may be more complex but there is less sensitivity to a b s o r p t i o n coefficient then heat release. T h e very h i g h values of a b s o r p t i o n coefficient exist w i t h i n s u c h small v o l u m e s that the average value over the large zone v o l u m e may be satisfactory. T h i s is the stage of this study w e are about to tackle.
B. L. Vollerin, Battelle, Geneva Research Centre, Switzerland. You reported results with vitiated air. C o u l d y o u give us some details on the a m o u n t of flue-gas is u s e d for this vitiation and on the techn i q u e w h i c h is u s e d on your testing? D u e to the s h a p e of the experimental c o m b u t i o n c h a m b e r u s e d in your tests, the c o n f i n e m e n t of the flame is certainly of major importance in the results p r e s e n t e d here. C o u l d you c o m m e n t on that?
Authors'Reply. The air was preheated by b u r n i n g a light fuel oil in the air duct leading to the burner. T h i s r e d u c e d the oxygen content of the air to 19%. T h e flame is c o n f i n e d by the walls of the c h a m b e r b u t the v o l u m e available for the flame to e x p a n d into is r o u g h l y equivalent to that available in a full scale boiler, each flame b e i n g confined by the adjacent flame.
Central Electricity Generating Board, Research Division, Marchwood Engineering Laboratories, Matchwood, Southampton, S04 4ZB
Data have been collected from the flame of a full size steam atomised burner operating with a residual fuel oil throughput of 8,100 kg h ~ (13,500 lb h-l). The results reported here are the first to be obtained for a steam atomised oil burner of this throughput. The flame measurements were carried out in the Babcock & Wilcox large burner test facility, which is essentially a water cooled box approximately 22 m long and 3.6 m square internally. Flame temperatures were measured with optical and suction pyrometers and part of the gas sample was analysed for 02, CO and CO 2. Flame radiation measurements were made with a wide angled (2~" steradians) ellipsoidal radiometer and a narrow angled total radiation pyrometer, located at the chamber ports. The latter instrument was also built into a water cooled probe and traversed through the flame. Gas velocities were measured with a fully water cooled double-headed pitot tube. For total heat fluxes a conductivity type heat flow meter was used. Three flame conditions, 'a', 'b' and 'c', were examined, having respectively 0.5% excess 02, 1.5% excess 02 and again 0.5% excess 02 but with vitiated air at the burner inlet. The directly measured data have been compared for the three flames and the results show, for example, that a maximum flame temperature of 1725~ + 20~ occurs in flames 'a' and 'b' but that the vitiated flame has a maximum temperature of only 1680~ + 20~ The maximum emissivity of 0.95 + 0.05 occurs in flame 'b' although the largest local absorption coefficient of 2.4 m - t +_ 0.70 m -~ was observed in flame 'a'. Some comparisons have also been made with previous results from other burners. A computerised mathematical model with a diffusion approximation followed by a Monte Carlo simulation of the radiant heat transfer has been used to determine the heat release distribution in the flame. It is concluded that the volumetric heat release (Q) decays exponentially with downstream distance (x) from the burner and can be represented by an equation of the form: Q = - - exp AD
-
where P and D are the thermal power and diameter respectively of the burner and A is 1.3.
Introduction Furnace heat transfer programs (Ref. 1) have been used to calculate the distribution of absorbed heat flux in future CEGB boilers. The accuracy of such predictions is limited by the quality of the input data. Sensitivity studies have shown that the biggest contribution to the uncertainty in local absorbed heat flux is the heat release distribution of the flames.
737
In principle heat release can be evaluated from velocity and chemical analysis of the gases at successive positions along the flame. However in large, very turbulent combusting systems these measurements are difficult to make and correlation of temporal fluctuations of velocity and composition may make simple measurements inadequate. In the
738
FURNACE COMBUSTION
early part of the flame, where heat release is greatest, it is almost impossible to obtain representative measurements of the amount of unburnt liquids and solids present. This direct method has therefore been abandoned as a means of obtaining heat release data in favour of an inferential method using mathematical models. Rig tests are carried out on a single burner of the type used in the furnace being studied, velocity distributions, gas absorptivity, wall emissivity, gas temperatures and wall heat fluxes being measured directly. The furnace heat transfer program computes the heat release distribution, the only remaining degree of freedom, required to produce the measured gas temperatures. The wall heat flux data is used as a check on the accuracy of the program. Gas sample data also serve to check that the computed heat release data are consistent with the observed progress of the flame. The heat release of the single test burner is assumed identical to that of a burner in a multiburner furnace. The justification for this is that in these large flames the heat release is primarily mixing controlled and occurs close to the burner where the velocity and turbulence are almost wholly determined by burner exit conditions. This paper describes the techniques used and presents the measurements obtained on a rig test of a single 6100 kg h -~ steam atomised oil burner of the type to be used in some of the CEGB's latest oil fired boilers.
The Test Facility & Measuring Instruments The tests were done in the Babcock & Wilcox Large Burner Test Facility at Renfrew, Scotland. This facility is essentially a water cooled box approximately 22 m long and 3.6 m square internally, with a refractory wall at the burner end and refractory on the last 5 m of wall and floor at the exit end of the rig. Air preheat is usually obtained by burning a light oil in the air supply duct, thus vitiating the air to about 19% Oz. However for some of these tests (see the Section on Test Work) oxygen injection was provided so that preheat could be obtained without vitiation. The burner (see Figure 1) was a B & W venturi type of 710 mm throat diameter and 990 mm exit diameter fitted with a thirty bladed stabiliser 406 mm in diameter mounted 216 mm upstream from the exit plane. A 15 hole Y-jet atomiser was used and the air preheat was 260~ The residual fuel oil (RFO) used had an average viscosity of 925 cS at 37.7~ (100~ a specific gravity of 0.97 and a calorific value of 42,600 kJ kg -~. The oil was preheated to an atomising temperature of around 130"C to reduce its atomising viscosity to the normal 15 cS at 100~
The present tests made use of the experience gained in burner trials on the Board's rigs and plant (Ref. 2.) and at the International Flame Research Foundation (Ref. 3.) A full description of the instruments used and the errors have generally been given in these references, so that the instruments need only be listed here with a brief description. (a) Flame Temperatures. A water-cooled 4m long suction pyrometer was used to measure local temperatures in the flame. In this instrument flue gas is sucked at a high velocity over a suitably refractory sheathed and shielded thermocouple. The amount of gas is very small compared with the burner throughput (i.e. 1:1000 by weight). Corrections to the measured temperatures to allow for heat losses from the thermocouple to the cooler furnace chamber walls can readily be obtained by 'calibrating' the instrument in situ. A hand held optical pyrometer was used to measure the line of sight temperatures. The single reading obtained represents the 'brightness temperature' of this flame at 0.65 I~m. The flame is not a black body however, so in order to evaluate the true radiant temperature, the reading obtained must be combined with a second reading obtained at another wavelength, (Ref 4.) In these tests the second reading was obtained using the total radiation pyrometer described under item (d). (b) Gas Analyses. Part of the gas sampled by the suction pyrometer was passed through filters to a paramagnetic O~ analyser and an infra-red CO2 and CO analyser. (c) Total Heat Fluxes. These were measured with a conductivity type heat flow meter having a blackened plug mounted at the end of a 0.7 m long water cooled probe. This instrument has been described in Reference 3. The difference between the measured total heat flux and radiant flux was used to estimate the convective heat transfer. (d) Radiation Measurements. Three instruments (all manufactured by Land Pyrometers Ltd) were used to make measurements of radiation from the flame. The first two both used an arsenic trisulphide lens to focus a very narrow angled beam of radiation on to a thermopile. The use of the special lens material ensures reception of radiation up to 13 t~m wavelength. These two instruments have angular fields of view of 1/50 and 1/20 of a steradian respectively. The first instrument was used at the ports in conjunction with the optical pyrometer. The second instrument was mounted in a water-cooled probe and focused on to a water-cooled target fixed some 0.254 m from the front end of the main body of the probe, This probe was used for making 'local' radiation measurements within the flame. The third radiation pyrometer used was an ellipsoidal radiometer in which all the incident radiation over an angle of 2xr steradians is detected (Ref. 2).
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER
739
PLENUM CHAMBER PRESSURE
CARRIER TUBE FOR OIL AND STEAM PIPES
STABILISER WITH CENTRAL ATOMISER AIR SLEEVE
BOILER FRONT
AIR SLEEVE CYLINDER
MAIN COVER VE NTURI THROAT RING
J, CENTURI THROAT 2750
F1c. 1. Sketch of the Babcock & Wilcox Burner of the type used in the present tests (dimensions,
ram). (e) Gas Velocities. These were measured with a fully water-cooled double headed pitot tube. The ends of the probe were shaped to give a cosine response to yawed flow so that the probe measured the axial component of the velocity head. A propeller anemometer was also used in the isothermal flow tests. The optical pyrometer, total heat flux meter and radiation measuring instruments were all calibrated before and after the tests.
Test Work and Results Isothermal tests were first conducted with and without a gauze situated 1 m downstream of the burner mouth, the gauze being theoretically required to make the isothermal jet behave like the burning jet (Ref. 5). It was expected that the isothermal data obtained from tests with a gauze could be used to
supplement the limited amount of velocity data obtainable under firing conditions. When firing the rig the excess O~ and CO content of the flue gases were measured by two probes in the exit duct and conditions adjusted to give the desired values (i.e. 0.5% and 1.5% excess O~ and 0.01% CO but see Table 1). A sheathed thermocouple nearby indicated gas exit temperatures and conditions were considered to be at equilibrium when this temperature remained constant. The normal operating conditions were logged and they show that the rig exit temperature throughout the tests only varied between 965 and 995~ The variations in other conditions are also summarised in Table 1. Although the spread of the excess 02 figures is large (0.2% to 0.9%) the bulk of the measurements were between 0.4% and 0.65%, for the nominal 0.5% condition. The oil flow was very steady, varying by less than +1%. Three flame conditions were studied, these were as follows: (a) maximum throughput or maximum
740
FURNACE COMBUSTION TABLE I Summary of variations in operating conditions Test conditions
Design Excess. 02 % Excess CO % Gas exit temperature ~ Oil flow kg h - l Steam flow kg h -~ Oil pressure bar Steam pressure bar Steam flow %
condition
(a) 0.5% excess Oz
(b) 1.5% excess 02
(c) 0.5% excess O2(V )
0.50 0.01 -6,070 240 15.2 8.65 4
0,2-0.9 (Av. 0.5) 0.04-0.005 965-995 5,985-6,095 205-311 10.6-13.8 8.20-9.00 3.4-5.1
1.35-1.9 (Av. 1.5) 0.02-0.005 975-995 6,030-6,090 238-246 11.4-12.6 7.20-8.80 3.9-4.1
0,45-0.75 0.02 970-990 6,030 235-239 12.5-12.6 8.20~8.40 3.9-4.0
continuous rating (MCR) with 0.5% excess 02 at the furnace outlet and O 2 injection into the preheated air; (b) the same conditions but 1.5% excess 0 2 at the furnace outlet and (c) MCR with 0.5% excess 02 at the furnace outlet but no oxygen injection into the preheated air (i.e. vitiated air was used). The first condition was investigated very extensively with all the instruments available. The second and third conditions (flames b and c) were studied to a lesser extent. Probing was usually along a horizontal line from
SCALE
the ports to the b u r n e r centre line. Temperature contours from these traverses for the three flames are shown in Figure 2 and centre line temperatures are compared in Figure 3A. The maximum corrected suction pyrometer temperature was 1725~ + 20~ The radiant (optical) temperatures are shown in Figure 3B with a maximum line of sight value of 1640~ _+ 35~ The observed temperature differences between the three flames are considered in the Discussion. The total emissivity, e T, of a flame is related to
i
Im
I
I I 5
r~
4
\
\Ill
Z3
II
/
I
A\ I~II /l l' , . X ' , ~\11 I I,
z "2
u. 1 Z
:[ l | BUR~NERt FLAME 'a' (0.5% EXCESS 0 2 AT FURNACE EXIT)
FLAME 'b'
(I .5% EXCESS 0 2 AT FURNACE EXIT)
FLAME %' (VITIATED AIR AT BURNER AND 0.5% EXCESS 0 2 AT FURNACE EXIT)
FIG. 2. Suction pyrometer temperature contours on the horizontal plane through the burner axis (~
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER the total absorption coefficient, Kr, by the equation eT = 1
exp(- KTL)
-
(1)
where L is the flame path length (Ref. 4). The present emissivity values for flames (a) and (b) peak at 0.90 • 0.05 and 0.95 • 0.05 and have minimum values of 0.43 • 0.02 and 0.63 • 0.3 respectively, the low values being observed in the flame region up to one 'burner diameter' downstream from the burner. For the shorter path lengths within the body of the flame the water cooled local radiation probe described earlier, was used. If the local 'gas temperature' (TT) is obtained from the suction pyrometer then the two readings can be combined to give the local flame emissivity or more usefully the local total flame absorption coefficient, as follows: ere~ T ~ = o'T ~ 4
thus
(2)
4
e~ = T s / T T
(3)
and from equation (1) PRESENT WORK x
FLAN~ 'a'
:
t700
g I I I r l I I ~ I I 2 4 6 8 10 DISTANCE FROM THE BURNER I N 'BURNER DI.'/U~TERS'
3A. FLAME CENTRE LINE TEMI~ERATURES
1700
x II ~M~
~.ESEN'TWORK - FLAME 'a'
9
PRESEN'TWORK - F~dC/E 'bT
9
SMALLERSTEAM ATOMISED
T
BURNER
,OOOR.OOE, .OLT
,,oo
1200
i
i
2
i
l
4
t
I
6
I
t
I
8
MATT.E .
T~/T~
741
= 1 - e x p ( - K ~ L).
The value of T, for the measured radiometer output was obtained from a black body calibration curve. An examination of the KT values for all the flames (see Figure 4) shows, as would be expected, the values peaking in areas where the oil spray was observed to be most dense. The emissivity and absorption coefficients for all three flames are considered in the Discussion. The gas analyses for flame 'a' showed CO levels of about 0.6% at a distance of four burner diameters downstream from the burner. At all distances from the burner the core of the flame had more CO than was found in the rest of the flame. This is in line with lower temperatures (see Figure 2) in the early part (say, two burner diameters from the burner) of the flame core. The propeller anemometer measurements (see Figures 5A and 5B in which the flow data has been non-dimensionalised with respect to downstream conditions) clearly indicate the lack of axial symmetry in the flow. The pitot measurements for the hot condition (flame 'a'; see Figure 5C) also show the flame to be directed towards the side of the rig with the probing ports. The spread of the jet in the horizontal plane through the burner centre appears to be wider in the firing case than in either isothermal case, although traverses completely across the rig would be required to confirm this. Because of the presence of unburnt oil, causing probe blockage, readings were not obtained in the body of the flame at ports 1 and 2 (0.2 m and 1.1 m downstream from the burner). The first complete traverse to the centre line was at port 3 (2.1 m), where the maximum velocities were 65 • 15 ms -~ The total incident heat fluxes (convective plus radiant) and the radiant heat fluxes above are plotted in Figure 6. Here it can be seen that the heat fluxes increase very rapidly from about 180 • 40 kW m -2 at port 1--this value agrees reasonably well with that of 160 • 30 kW m -~ measured on the burner wall--to a maximum value of 370 + 60 kW m -~ four and a half burner diameters downstream of the burner. Tests to determine the error from not holding the probes in exactly the same positions at the ports (i.e. varying tilt and insertion depth) indicated variations in both cases of about +20kW m -~. The averaged radiant fluxes were always less than the total heat fluxes, as expected.
Discussion
I
10
OfSTANCE FROM THE BURNER IN 'BURNER DIAMETERS~ 3B. RADIAN~T{CORRECTED 'OPTICAL LINE OF
SIGHT') TEMPERATURES
FIG. 3. Flame temperatures for a number of flames.
Flame Comparisons Figures 2 and 3 show that in the present work flame 'b' with 1.5% excess 02, develops its highest temperature more quickly than the other two flames
742
F U R N A C E COMBUSTION
(i.e. flames 'a' and "c') and has a larger high temperature region. However, the maximum temperatures achieved by flames 'a' and'b' are identical, at 1725~ The vitiated flame ('c') never achieves this temperature, the maximum value recorded being 1680~ Nevertheless, all three flames show very similar temperature conditions at six 'burner diameters' downstream from the burner, for example where the centre line values are all within 1505~ + 25~ A steam atomised flame from a similar burner, (Ref, 6) but with a smaller oil throughput of 3060 kg h - t has been examined in the Marchwood Engineering Laboratory rig, which has a 50% refractory wall area. The maximum flame temperature measured with the same suction pyrometer as used in the present work was 1740~ + 20~ The excess O 2 was 1,0% and the maximum radiant temperature was 1625~ which is between the flame 'a' (0.5% excess O2) and 'b' (1.5% excess O2) values of 1640~ and 1600~ respectively. The local absorption coefficients for the present three flames (see Figure 4) show flame 'b' with the highest absorption coefficient (1.7 m -~) at one 'burner diameter' from the burner but flame 'a' having the highest overall value (2.4 m -~) at two 'burner diameters' downstream. The vitiated flame 'c' has the lowest absorption coefficients. The different values observed in the three flames are related to the different rates of soot formation and burnout at the different oxygen and resultant temperature levels. The local flame absorption coefficients mea-
-!
sured earlier on the smaller flame at 1% excess 02 varied between 0.45 m -~ + 0.18 m -~ at the edge of the flame and 1.85 m -~ + 0.7 m -~ at the centre of the flame, These values are similar to the present results which are in the range 0.41 m -1 + 0.15 m -j to 2.4 m -1 + 0.70 m -~ at excess Oz values of 0.5% to 1,5%. Leukel (Ref. 7) has reported values in the range 0.5 m -~ to 8.0 m -~ for a smaller oil burner operating with higher excess O~ values (i.e. oil throughput 144 kg h -~ and excess O~ up to 3%).
Heat Release Distribution In studies of power station boilers it has proved convenient to regard the axial distribution of heat release as obeying the equation:
Q:
where Q P D x A
= = = = =
9
I
" II'A'J)
I
heat release/unit axial distance thermal input power burner diameter axial distance a coefficient
1~.511 k2l
;,,;,.-,.,
I
I
Y/'l.-\r.,
I BUR~IER I
-
The axial disposition of the heat release is important in determining the relative heat absorption of front and rear walls of a boiler and the magnitude
o 51
II
exp
(f'(0o:; ,,
k. l ) J ) ,
, I
,
k -J.
/ I BURJlNERI
I BUR~IER I
r FLAME 'a' (0.5% EXCESS O2AT FURNACE EXIT)
FLAME 'b' (I .5% EXCESS O2AT FURNACE EXIT)
FLAME 'c ~ (VITIATED AIR AT BURNER AND 0.5% EXCESS O2AT FURNACE EXIT)
FIG. 4. Contours for measured local total absorption coefficients m -~
FLAME DATA FROM A LARGE RESIDUAL FUEL OIL-FIRED BURNER
743
9
<
8
5 ~ z7 6
Z
a~
~4 I.---
:E
03 z< 2
1
- GAUZE
8
i 2 ,
o (A) COLD A I R - N O GAUZE
(B) COLD AIR WITH GAUZE
7.2
,
I 2J
i
3
METRES ACROSS RIG
(C) FLAME O N
rh = 19.22 kg $-1
rh = 19.22 kgs -1
rh TOTAL = 2 5 . 6 7 kg s - I
g = 13.5 ~
O = 13.5~
O
EXIT
= 987 ~
F1c. 5. Axial velocities on the horizontal plane through the burner axis (non-dimensionalised). of peak heat fluxes on the burners and adjacent walls where waterside dry-out may be a problem. The axial distribution of heat release, characterised by the coefficient ;~, has been evaluated from measurements taken on a single burner firing in a test rig by the use of two related computer programs. In the first program, called CEDRUF, the furnace chamber was represented by 900 zones and the main input data to the program consisted of wall emissivity, estimated convective heat transfer coefficient, gas absorption coefficient and gas temperature in every zone, and gas mass flow through each face of every zone. The program used a diffusion approximation to radiation transport followed by a MonteCarlo simulation to evaluate the heat release that must have occurred in each zone for the measured temperature field to exist. The absorbed heat flux at the furnace wall zones was also available as output. The second program, called FURDEC, used the same furnace zones but the heat release in each zone was used as input data instead of gas temperatures. This program also employed a diffusion approximation followed by a Monte Carlo simulation of the radiant heat transfer to compute gas temperatures in each zone and the absorbed heat flux at the walls. In composing the gas flow data for these programs the limited number of flow measurements made in
the firing rig had to be supplemented by isothermal cold flow measurements. A gauze was fitted across the jet downstream of the burner to increase the isothermal jet diameter to match the flame diameter, (Ref 5). Initial analysis of the results using the program CEDRUF yielded on uneven heat release distribution. This was partly due to small gas temperature errors but mainly due to the mismatch of the isothermal flow field with the actual one that existed under firing conditions (see Figure 5). However, when the irregularities in the heat release distribution predicted by CEDRUF were removed and the results entered into the FURDEC program a few manually iterative computer runs produced a reasonable match to the measured gas temperatures, and absorbed heat fluxes and also yielded a smooth spatial distribution of heat release. For the 6100 kg h -~ steam atomised oil flame "a" the best fit to the measured gas temperatures was obtained with the coefficient in the exponential heat release equation ~ --- 1.3. The corresponding computed and measured absorbed heat fluxes are shown on Figure 6. The value A -= 1.3 may be compared with a value of A = 1.5 used by the author (Ref 1.) to match measured plant data on a large boiler fired by 3600 kg h -l pressure jet oil burners. Thus, expressed in terms of burner diameters, the present burner, which is steam atomised, releases
744
FURNACE COMBUSTION COMPUTED TOTAL HEAT FLUXES x
MEASUREDTOTAL HEAT FLUXES - FLAME ' a' AV1ERAGEDOVER T W O DAYS READINGS
9
MEASURED RADIANT HtEAT FLUXES - FLAME ' a ' AVERAGED OVER TWO DAYS READINGS
observed in the non-vitiated flames. (iii) The axial distribution of heat release in such flames can be represented by the equation Q = P / AD exp ( - x / A D ) where P = input power and D = b u r n e r diameter. The factor )t, found to match the measured flame data, was 1.3 for the steam atomised b u r n e r tested at 0.5% excess 02.
E
-~ 300
Acknowledgments
u_
100
1
I
2
I
i
4
t
t
6
t
I
8
I
t
[
10
DISTANCE FROM THE BURNER I N 'BURNER DIAMETERS'
FIG. 6. Total and radiant heat fluxes for flame 'a'. heat earlier than does the smaller pressure atomised type in current use. The lateral spread of heat release appears roughly Gaussian in form. There is some evidence to suggest that the heat release is greatest along the cone of the oil spray and rather lower on the axis itself. However even the 900 zones used in this work are not sufficient to study such detail. In predicting the wall heat fluxes in a large multi burner furnace an accurate knowledge of the lateral spread of heat release is not as important as the longitudinal distribution and the characterisation of the heat release by the single coefficient A has been found sufficient.
We are indebted to Babcock and Wilcox (Research) Ltd, who modified and operated the rig, Under contract. Mr. W. B. Bryce and Mr. W. I. Joyce of B & W organised this part of the work and Marchwood Engineering Laboratories staff probed the flame (Mr. R. O. Denham, Mr. P. L. ]oyner, Mr. C. J. Norman and Mr. J. S. Holt). This paper is p u b l i s h e d with the permission of the Central Electricity Generating Board.
REFERENCES 1. ARSCOTT, ]. A., GraB, J. AND JENNER, R., First European Symposium on Combustion, p. 674, Academic Press, 1973. 2, GODmDGE, A. M. ANO READ, A. W., Prog. Energy and Combust. Sci. 2, 83, (1976). 3, C~EOA~LLE, J. AND BnAuo, Y., Industrial Flames, I, Measurements in Flames, E d w a r d Arnold, 1972. 4. Daws, L. F. AND T~R[NG, M. W., ]. Inst. Fuel,
25, 528, (1958). Conclusions
(i) Temperatures up to 1725~ + 20~ and local absorption coefficients in the range 0.41 m -~ + 0.15 m -~ to 2.4 m -~ + 0.70 m -~ have been measured in the flame from a large b u r n e r run under normal oxygen conditions. (ii) When supplied with air vitiated to 19% 0 2, the flame developed lower temperatures and local absorption coefficients close to the burner than those
5. DAVlSON, F. J., Nozzle Scaling in Isothermal Furnace Models, C E G B Laboratory Note R D / M / M 2 5 , 1967. 6. GODRIDGE, A. M., HOLT, J. S. AND MATTHEWS, K. J., The Collection of Flame Data From a Large Steam Atomised Flame, C E G B Laboratory Note R / M / N 9 3 8 , 1977. 7. LEUKEL, W., International Flame Research Foundation, 1st Members Conf. IJmuiden, 22-23 May, 1969.
COMMENTS A. N~metli, Computer Centre for Universities, Hungary. Recently some measurements have been
to soot and if so, what technique has been used?
carried out in Hungary concerning soot radiation in an oil fueled furnace using " w i n d o w s " in spectra where presumably only soot was emitting. I would be interested to know whether you have measured how much of the total radiation was due
Authors" Reply. Provided the gas composition is known, the non-luminous radiation can be calculated from published tables. This contribution can then be removed from the total radiation, w h i c h we have measured and reported, by using the equations for
F L A M E DATA F R O M A L A R G E R E S I D U A L F U E L O I L - F I R E D B U R N E R total and soot radiation given in References 2 and 3 of our paper.
1. 1, Macfarlane, Imperial
College,
England.
Some light w a s t h r o w n on the effect of vitiation on distillate fuel c o m b u s t i o n by my colleague Dr. B. P. Mullins at the National Gas T u r b i n e Establishm e n t at Pyestock d u r i n g the early 1950's. This work is still available in the form of N G T E reports and explains the effect of vitiation on the flame behavior of distillate fuels via reduction of the m a x i m u m flame temperature. T h e blue-flame effect referred to in the p r e s e n t d i s c u s s i o n w o u l d surely not apply to residual fuel oils w h e r e solid particle residues w o u l d still be p r o d u c e d in the flame.
Authors" Reply. It will be useful to examine the work referred to by Dr. Macfarlane. We think that the influence of the large residual fuel oil coke particles will be small in c o m p a r i s o n w i t h the soot. T h i s is b e c a u s e there are very many soot particles and only a few coke particles.
T. F. Wall, University of Newcastle, Australia. I am interested to k n o w h o w your data might be transferred into y o u r furnace heat transfer p r o g r a m s of larger p o w e r station furnaces. Presumably, the zones y o u use in these latter models are quite large so that you will require average values, for example, for a b s o r p t i o n coefficient. H o w w o u l d y o u translate your exhaustive data into these mean values? Secondly, in order to use the exponentially decaying heat release e q u a t i o n proposed, this m u s t use a zone s h a p e in the p o w e r station model similar to the flame shape. Do y o u r m o d e l s have this capacity, and if so, h o w sensitive are the predicitions of the m o d e l s to the accuracy of the heat release equation?
Authors'Reply. It w o u l d have been nice to analyse our rig data u s i n g the same size zones as w e use for complete furnace studies. However, s u c h zones can be as large as 3m x 3m x 3m d e p e n d i n g on the b u r n e r p i t c h i n g and furnace size. T h e bulk of the heat release in a furnace flame occurs in about 3 zones in front of each burner, most occurring in the first zone. O u r main interest was in the spatial distribution of heat release as the heat flux at the
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b u r n e r wall is quite sensitive to it (10% change in the coefficient k changes the local heat flux by a b o u t 3%) b u t the total furnace heat a b s o r p t i o n is not very sensitive to it. T h e classical m e t h o d of deriving the heat release distribution from m e a s u r e m e n t of the b u r n o u t of chemical species w a s not attempted due to the difficulty of obtaining m e a s u r e m e n t s in the early part of the flame where b o t h solids and l i q u i d exist. I n o u r m e t h o d the heat release distribution w a s f o u n d by the use of a zone m o d e l of furnace heat transfer. U s i n g m e a s u r e d optical properties and gas m a s s flow data the heat release in each zone w a s adjusted until the program r e p r o d u c e d the m e a s u r e d gas t e m p e r a t u r e s and heat fluxes. It w a s to be able to assess the a d e q u a c y of the model representation o f the m e a s u r e d data that we u s e d so m a n y small zones. You are quite right in a s s u m i n g we c a n n o t use s u c h small zones for a w h o l e furnace s t u d y a n d m u s t therefore translate this small zone data to larger zone values. The heat release, our main interest, p r e s e n t s no p r o b l e m s b e i n g s i m p l y integrated over appropriate volumes. T h e optical properties may be more complex but there is less sensitivity to a b s o r p t i o n coefficient then heat release. T h e very h i g h values of a b s o r p t i o n coefficient exist w i t h i n s u c h small v o l u m e s that the average value over the large zone v o l u m e may be satisfactory. T h i s is the stage of this study w e are about to tackle.
B. L. Vollerin, Battelle, Geneva Research Centre, Switzerland. You reported results with vitiated air. C o u l d y o u give us some details on the a m o u n t of flue-gas is u s e d for this vitiation and on the techn i q u e w h i c h is u s e d on your testing? D u e to the s h a p e of the experimental c o m b u t i o n c h a m b e r u s e d in your tests, the c o n f i n e m e n t of the flame is certainly of major importance in the results p r e s e n t e d here. C o u l d you c o m m e n t on that?
Authors'Reply. The air was preheated by b u r n i n g a light fuel oil in the air duct leading to the burner. T h i s r e d u c e d the oxygen content of the air to 19%. T h e flame is c o n f i n e d by the walls of the c h a m b e r b u t the v o l u m e available for the flame to e x p a n d into is r o u g h l y equivalent to that available in a full scale boiler, each flame b e i n g confined by the adjacent flame.