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The effect of scale on the performance of swirl stabilised pulverised coal burners
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, 1992/pp. 1365-1372
THE EFFECT OF SCALE ON THE SWIRL STABILISED PULVERISED
PERFORMANCE OF COAL BURNERS
J. P. SMART, D. J. MORGAN AND P. A. ROBERTS International Flame Research Foundation P 0 Box 10 000 1970 CA IJmuiden The Netherlands
This paper describes the results of an experimental programme to evaluate the effect of burner scale and burner scaling criteria on the performance of swirl stabilised pulverised coal burners. The main text contains an analysis of the dominant considerations to be borne in mind with respect to the scaling of these systems. The experiments relate to establishing the effect of employing two different scaling criteria on the performance of an experimental burner capable of operating under both low and high NOx operation. The scaling criteria employed were constant velocity and constant residence-time scaling. The baseline burner was a conceptual 50 MW burner. Results of parametric studies on both experimental burners showed that baseline NOx emissions are higher for constant velocity scaled flames for both swirl numbers studied whereas for low NOx operation, the NOx emissions are similar. Results are explained in terms of differences in flame ignition and propagation pattern. Detailed in-flame measurements are presented for baseline flames produced from both burners and are used as a platform to interpret the experimental results.
Introduction One outstanding question still remaining in combustion research, is how representative is a scaled down burner's performance compared to the full industrial scale situation. Ideally, when scaling down a burner, the objective is to achieve similarity in all fluid dynamic and thermo-chemical processes in the scaled flame. If achieved, then the performance of the scaled burner will be identical to the full scale situation. In reality, this is practically impossible as all the physical and chemical processes do not scale in the same way. 1-4
velocities and momentum ratios are maintained constant with scale reduction. For constant residence-time scaling the co-axial velocity and momentum ratios are still maintained constant with scale reduction, however, velocities scale with tbe burner diameter. The principles behind constant velocity and constant residence-time scaling are outlined below.
Constant Velocity Scaling:
This criterion allows the following relationships to be established as: Qo = po Uo Do2
Background It follows that:
Burner Scaling Criteria:
Traditionally, there are two practical scaling criteria for the scaling down of burners. These are constant velocity and constant residence, or mixingtime, scaling.2'OBoth criteria rely on the scaling of the large macro-scale turbulent mixing process. Geometric similarity of the burner is maintained in both cases, and if swirl is used, the swirl number, So, is held constant. Constant velocity scaling is the most commonly used by burner manufacturers. In this case, co-axial combustion air and fuel injection
Do ~ Qo~
Here Qo, Uo and Do are the burner throughput (scale), characteristic burner velocity (combustion air velocity) and characteristic burner diameter respectively (burner throat diameter). The variable p,, is a characteristic inlet fluid density, normally the burner combustion air density. When a burner is scaled down from a baseline
1365
FURNACES AND BURNERS
1366
throughput, Qo.b.... to a reduced scale Q ..... t~d, using this criterion: Oo, base/Q ..... led -~ (Do,rase/D ..... led)2
This allows the value of the characteristic burner dimension to be derived for a scaled down burner when values for a large scale burner are known. Constant Residence-Time Scaling:
When using the constant residence-time scaling criterion, the ratio (Do/Uo) remains constant as scale is reduced. The ratio (Do/Uo) is proportional to the large macro-mixing time-scale, rm. Thus as: rm oc Do/Uo = constant
It follows that: Qo = poUoDo2 ~ poDo a
Therefore: Do OCUo OCQo~
When a burner is sealed down from a baseline throughput, Qo,ba,e to a reduced scale Qo,*cated using constant residence-time scaling: Qo, base/Q . . . . . led = (Do,l~,e/D . . . . . led) 3
And: Qo, base/Q ..... tea ~- (Uo,base/U . . . . . led) 3
Thus, if values of Uo and Do are known for the large scale case, then values for the reduced scale burner can be derived.
Where the parameters in parentheses are proportional to the area over which the mixing is taking place, the concentration gradient and the turbulent diffusivity, respectively. Furthermore, burner scale dictates the amount to be mixed at the above rate. The amount to be mixed is the burner scale, Qo which is proportional to UoDo2. The corollary of the above analysis is that the fractional degree of mixing, Xf, at any point downstream of the burner exit, x, is proportional to the ratio of the mixing rate and the amount to be mixed i.e.:
xs~ (xDoUo)/(UoDo2) ~ x/Do Thus, independent of any scaling criteria or burner scale, the fractional degree of mixing is constant at constant values of x/Do. If all the combustion and pollutant formation reactions are everywhere fast compared to this mixing rate, the rate of reaction is dictated by the rate at which the reactants are mixed. If this is the case, then flames of any scale, or flames of identical scale but scaled down using different scaling criteria should show identical degrees of reaction at identical values of x/Do. Effect of Scale on Two Phase Interactions:
In a swirling pulverised coal flame, ignition and stability are governed by the degree of interaction between the incoming coal stream with the internal zone (IRZ) present in the near burner field of these flows, see Fig. 1. Coal particles penetrate this region of reverse flow if the coal particle momentum is sufficient to overcome local particle drag forces. The penetration depth into the IRZ, 6, is given by:6 6 = U, rp,
Considerations on the Effect of Scale and Scaling Criteria on Physical Processes Occurring in a Pulverised Coal Flame Effect of Scale on Turbulent Mixing:
In swirling pulverised coal flames the coal and air are initially segregated by a distance proportional to the characteristic burner diameter, Do. The turbulent mixing process must overcome this segregation before the dominant combustion and pollutant formation reactions can occur. This mixing process is scale dependent. It can be readily derived from standard mixing layer theory, ~ that the rate of mixing between initially segregated reactants, R,~ is given by: Rm :c (xDo)(1/Oo)(UoOo) oc xDoUo
where:
U, = KIUo - Ur and:
rp = dpZpp/181xg
Where Us is the slip velocity between the injected coal particles (velocity KzUo) and the reverse flow velocity in the IRZ (Ur). The parameter rp is the relaxation time of a coal particle of diameter dp, where pp and /xg are coal particle density and gas viscosity, respectively. The ratio of the inlet flow velocity to the reverse flow velocity in the IRZ is a constant (K2) for constant swirl number, thus: q~ = Uo(KI - K2)zp
and: 6/oo
= (tJo/Oo)(~:l - K~).~p
Thus, the normalised degree of particle penetration
SCALING OF PULVERISED COAL BURNERS
1367
par|icle trajectories
D o throat \
\x~
t
',
I
boundary
9 . ~,-~'~ "/i po'slllon 0
/
~.
layer secondary air
Zi
!~
near burner field
FIG. 1. Coal particle dispersion in the near burner field of experimental burner
of the IRZ is dependent on how the ratio Uo/Do varies as scale is changed. This parameter increases as burner scale is reduced when constant velocity scaling is used, but remains constant for constant residence-time scaling. This fact highlights a potential problem with constant velocity scaling of pulverised coal burners, in that if coal particle size remains constant when reducing scale, to preserve the chemical reaction rates, the normalized degree of particle penetration of the IRZ changes.
Experimental An experiment is described that attempts to evaluate the effect of burner scale and scaling criteria on the performance of an experimental, swirling, pulverised coal burner in terms of the above scale dependent concepts. The experiments were executed on IFRF Furnace Number 1. A detailed description of this furnace may be found elsewhere. 2"7 The experimental burner used in this work can function under high and low NOx operation, dependent on the mean coal particle trajectories in the near burner field. A range of coal particle trajectories are possible in this burner and are shown in Fig. 1. If the coal particle trajectories are predominantly of type 1 and 2, the devolatilised volatile species, including volatile nitrogen are oxidised on the internal recirculation zone boundary in a region of high oxygen availability in the shear layer that surrounds it. This results in efficient conversion of fuel nitrogen to NOx. By contrast, if type 3 and 4 particle trajectories predominate, devola-
tilisation occurs primarily within the boundary of the internal recirculation zone in a region of low oxygen concentration. This promotes preferential reaction pathwaYTsfrom volatile fuel nitrogen to molecular nitrogen. Low NOx operation is most simply achieved by inserting the coal injector downstream of the burner throat, see Fig. 1. For the experiments, two burners were used. The first burner was a standard 2.5 MW version. 7 The velocities employed in this burner are typical of those regularly used in large scale utility boiler burners, i.e. 40 m/s for the combustion air and 20 m/s for the transport air. For the second burner, the 2.5 MW datum burner was theoretically scaled up to 50 MW using the constant velocity scaling criterion outlined above. This burner was subsequently scaled down from 50 to 2.5 MW using the constant residence-time scaling criterion. Both burners were geometrically similar, see Fig. 2. The combustion air and primary transport air velocities were 14 m/s and 7 m/s, respectively, for the constant residence-time burner. Swirl on the combustion air was generated using IFRF moveable block swirlers. The burner throat diameter, A, is selected as the characteristic burner diameter, Do. The coal used in the experiments was G6ttleborn hvBb (Composition (dry basis): C = 71.65, H = 4.61, S = 0.90, N = 1.51, O = 11.67, VM = 36.33; Ash = 9.50, LCV = 29.18 MJ/kg). The coal was pulverised to 75% < 75 ixm before being transported to the burner at a transport air to fuel ratio of 2:1 on a mass basis. The excess air factor was maintained at 1.15 for each burner. Combustion and
FURNACES AND BURNERS
1368
coal injector
secondary air
0
position =
L
q coal injector
~
L'
J
i
I
/_ burner quorl
movable block swirler
FIc. 2. Experimental burner designs (Reference burner: 50 MW, B = 2093 mm, A = 1046 mm, a = 628 ram, L = 1046 ram) (Constant velocity burner: 2.5 MW, B = 468 ram, A = 234 ram, a = 140 mm, L = 234 mm) (Constant residence-time burner: 2.5 MW, B = 771 ram, A = 385 mm, a --- 231 mm, L = 385 mm)
transport air temperatures were maintained at 300 and 70~ respectively. For each burner, the effect of coal injector position and swirl level was evaluated in terms of flue gas output conditions. For selected flames, in-flame maps of gas composition and temperature were performed using standard IFRF measurement probes.7 In this paper, selected in-flame measurements from two Type 27 baseline flames are compared.
Results and Discussion
Baseline--Type 2 Flames: These flames were generated with the coal injector positioned at the burner throat, x/Do = O. Under these conditions, the NOx emissions are higher for the constant velocity burner than the constant residence-time burner for both swirl numbers studied (So = 1.0 and 0.6), see Fig. 3. This is due to the mixing rate between fuel and air in the shear layer on the IRZ boundary being proportional to the product UoDo, see Section 3. This indicates that the combustion air mixes more rapidly into the shear mixing layer for the constant velocity burner and as a consequence markedly effects the flame ignition and propagation pattern.
Effect of Ignition Patter--Type 2 Flames: For the baseline, Type 2 flames studied, ignition takes place on the centreline side of the shear mix-
ing layer surrounding the IRZ. The majority of the coal particle trajectories are located in this mixing layer and thus the majority of devolatalisation takes place in this region, see Figs. 1 and 4(a). Ignition is initiated due to mixing between the hot recircuated combustion products in the IRZ and the devolatalisation products from the coal particles. Ignition occurs at a lean flammability limit. After ignition, the flame front propagates into the fuel stream into an increasingly richer mixture due to devolatilisation occurring ahead of the flame front as a result of radiation from the flame front and local refractory surfaces. The continuous entrainment of oxidant into the shear mixing layer will subsequently cause the flame front to propagate into a progressively leaner mixture. The flame front will then ultimately be convected downstream propagating at the lean limit. In a constant velocity scaled flame, the mixing of oxidant with the fuel stream is more rapid than for the constant residence-time case due to the value of UoDo being larger. This would indicate that flame propagation in the shear mixing layer of a constant velocity scaled flame is globally more lean than the corresponding residence-time situation. The corollary of the aforementioned discussion is that as the constant velocity scaled flame is propagating globally leaner than a constant residencetime flame of similar scale, a higher global conversion of volatile nitrogen species to NOx occurs. Moreover, the potential for more coal particles to penetrate the IRZ in the constant velocity case would augment this effect, as less coal will devolatalise on
SCAIANG OF PULVER1SED COAL BURNERS
looo/,~L~,..,.
1369
the IRZ boundary and the local gaseous environment on the IRZ boundary will be even leaner.
~so =~.o
Effect of Coal Injector Position: f:~O-ccr~t~t " ~ . ~
E
velocity
N Z
z~o-b~rr~ I
"~'~=
~z 200-
o
I
0 6.~ ~2 ~
64 65 06 d.7 68 6.9
~
norrnalised cool injector I:x~tion (X/Do)
1200
[
I
Lso=o.6
-- l(II)~
const(~ntresi~7.e time burner
burn~l
z
2000 0
0'.~ 0.2 o3 & ~
o.6 0.7 d.s 6.9 ~o
The effect of coal injector position on NOx emissions is also shown in Fig. 3. It can be seen that coal injector insertion results in significant NOx reduction. An interesting observation is that tbr both swirl numbers, the normalised insertion distance at which point NOx begins to decrease is less for the constant velocity scaled flame. This can be explained with reference to Section 3, where it is shown that the normalised degree of penetration of the internal recirculation zone tbr any coal injector position is dependent on the scaling criterion used. and the coal particle size. Maintaining coal particle size constant, the coal jet from a constant vclocity scaled flame will penetrate relatively fluther into the IRZ than a constant residence-time tlan,c due to the larger value of the slip velocity U,.. As previous work on this bunaer ha.s shown, to reduce NOx a specific degree of IRZ penetration is required. 7 This degree of penetration will be achieved at a lesser normalised degree of coal injector insertion for the constant, veltx:ity scaled flames than tbr the constant residence-tilne cast'.
nocmfllist~lcoal injector position (X/Do)
l'.ffect of Ignition Pattern--Penetration t,'hmw.s. FIG. 3. Effect of normalised coal injector position on NOx emissions
~1
c ~ t jet
tmject~y\
,,1
primary oir , I *c~l
-
,'~V'/.,,~
~
/
~
"~
L~'=o===J'~'-f I ome ignition
-~-..
IRZ boundary
For penetration flames, the ignition ixdten~ is significantly different from baseline, Tylx' 2; flames, l)ue to the coal jet penetrating the IRZ, the centreline region of the IBZ does m~t contain recirculated condmstion products, see Figs. 1 and -I (b). In this case, the coal jet will ignite at the lean limit but on the outside of the coal jet. The flame front will then propagate into the coal stream into a progressively richer mixture limited hv devulatilisation, until the rich limit is reached. At this point, the flame front will be convected do~llstream at the rich limit, limited by oxygen availability. Due to the flame propagation pr~x'ess in penetratiun flames being predominantly raider rich conditions the result is reduced NOx emissions.
\'
Detailed In-Flame Measurenwnts--Type 2 Flames:
trajectory
FIG. 4. Coal jet trajectory and ignition pattern for baseline and penetration flames: (a) baseline Type 2 flame, (b) penetration flame
Here, tile conslant velocity and constant residence-time baseline flames are compared, lrluc gas output conditions and in-flame measurements between x/D,, = 1.0 and 8.0 are given in Fig. 5 (a and b). At the quarl exit (x/D, = 1.0) the shear mixing laver is identified by the peaks in CO concentrations for both flames. Correspmading temperature profiles confirm ignition of these flame on the IRZ side of the shear mixing laver. N()x is also observed to be forming in this region. The CO concentrations in this region are higher in the constant
FURNACES AND BURNERS
1370
I species
(o)
temp.
CO
~
OZ COz
910
NN(ppm) 211 T{~) / IB] 1600t
21. 18, 161 ~15 12~
98
quo,rl
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9
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6~
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,8
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'=4
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~
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61
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i
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;
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FIG. 5. In-flame data. (a) baseline constant velocity burner.--So = 1.0, Coal Injector position = 0 (Flue gas NOx = 1185 ppm (0%Oz), Tx = 99,6%) (b) baseline constant residence-time burner--So = 1.0, Coal Injector position = 0 (Flue gas NOx = 1100 ppm (0%O2), Tx = 99,6%) residence-time flame than the constant velocity case. This indicates a richer flame propagation in the former case. Observing the in-flame profiles between the quarl exit, x/Do = 1.0, and x/Do = 8.0, the following comments can be made. Between the quarl exit and x/Do = 3.0 the peak CO concentrations are higher for the constant residence-time flame due to the ef-
fect of the richer global flame propagation pattern. In this region it may also be observed that the NOx levels for the constant residence-time flame decrease to values below the corresponding values for the constant velocity case. At x/Do = 8.0, all radial variations in concentration and temperature are negligible, and values are representative of those found in the furnace exhaust. This indicates the
SCALING OF PULVERISED COAL BURNERS completion of turbulent macro-mixing. The observation that NOx does not change significantly suggests that NOx is effectively "frozen" in concentration after the completion of macro-mixing at the termination of volatile combustion.
1371
Acknowledgements The permission of the IEA CCS A2 Executive Committee and the Joint Committee of the I F R F to publish this paper is gratefully acknowledged. REFERENCES
Conclusions Two burners were studied in this work firing Gfttleborn hvBb coal. The two burners were 2.5 MW constant velocity and constant residence-time scaled versions of a conceptual 50 MW burner. For the baseline, Type 2, flames, the NOx emissions in the furnace exhaust were higher for the constant velocity burner than the constant residence-time case. In-flame measurements indicate that ignition of these flames takes place on the IRZ side of the coal jet. The difference in NOx is explained in terms of the differences in flame propagation pattern. With coal injector insertion into the burner quarl, NOx decreased for both burners. The NOx decreased at a lesser normalised coal injector insertion distance for the constant velocity flame than the constant residence-time flame. This is explained in terms of the different relative degree of IRZ penetration by the coal particles in each case.
l. LAWN, C. J., CUNNINGHAM,A. T. S., STREET, P. J., MATrHEWS, K. J., SARGEANT, M. AND GODRIDGE, A. M.: Principles of Combustion Engineering for Boilers (C. J. Lawn Ed.), Chap. 2, p. 61, Academic Press, 1987. 2. SMART,J. P., VAN DE KAMP, W. L. AND MORGAN, M. E.: Fuel, 70, 1350, (1990). 3. SALVl, G. AND PA'~'NE, R.: Combustion System Scaling, International Flame Research Foundation Report D07/a/69, 1Jmuiden, (1978). 4. SPALDING, D. B.: Ninth Symposium (International) on Combustion, p. 833, The Combustion Institute, 1963. 5. HAWTHORNE, W. R., WEDELL, D. S. AND HOTTEL, H. C.: Third Symposium (International) on Combustion, p. 266, The Combustion Institute (1949). 6. FIELD, M. A., GILL, D. W. MORGAN, B. B. AND HAWKSLEY, P. G. W.: Combustion of Pulverised Coal, Chap. 2, p. 25, BCURA, Leatherhead, (1967). 7. SMART,J. P. AND WEBER, R.: J. Inst. E, 62, ,937, (1990).
COMMENTS John H. Pohl, Energy International, USA. 1. Was there any NO~ formation in the furnace outside the flame? 2. Did you make observations of the flames and were you able to relate the flame structure to the scaling laws and NO~ formation? 3. What is the status of the sealing law of Payne, et al. which concluded that the burner diameter should be scaled by the ratio of the diameter to the 2.5 power? Author's Reply. 1. From the in-flame measurements and visual observation of the volatile flame length, there was no significant NO~ formation/destruction after the termination of volatile combustion as the NOx values at this point are representative of those found in the furnace exhaust. 2. Visual observations of the flames suggested that the baseline (Type 2) flames for both scaling rules were structurally similar in terms of flame diameter and length with both scaling with the burner diameter. For the low-NO~ penetration flames (Type 1) the constant velocity scaled flame was longer,
relative to the burner diameter, in comparison to the constant residence time case. The lower NO~ level produced for the baseline constant residence time flame is due to the lower specific mixing rate between the fuel and air (proportional to voDo). This results in a globally more fuel rich flame propagation and reduced NO~ formation. 3. The evaluation of the sealing concept proposed by Salvi and Payne, 1 where the burner throughput (scale) is related to the burner diameter by: Q0 ~ Doz5 is currently being in progress. To this end two separate research programs have been initiated at the IFRF. The first program, from which this paper originates, is on scaling of pulverised coal burners. The second program financed by GR1 and IFRF is on sealing of natural gas burners, z
REFERENCES 1. SALVI, G. AND PAYNE, R.: Combustion System Sealing, IFRF Doe. No. D 07/a/69, 1978. 2. WEBER, R.: Overall Workplan for the Investi-
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FURNACES AND BURNERS
gation into Scaling of Aerodynamics and LowNO~ Characteristics of Natural Gas Burners in the Thermal Input Range 30 kW to 12 MW. IFRF Doc. No. D 95/y/1R, 1991.
L. D. Smoot, Brigham Young University, USA. 1. Simple scaling laws for such complex two-phase flows may not be generally applicable. Did the authors seek to evaluate the scaling laws and interpret the measurements through computations of the reacting b u r n e r flow? 2. Did the authors consider the method of dynamic scaling through dimensionless groups derived from dimensionless two-phase equations of change to establish scaling laws or explore limitations of scaling laws?
Author's Reply. 1. The authors agree that there is probably no generally applicable scaling rules that can produce identical flame aerodynamics, thermochemical structure in the scaled domain for burners at different scales. The objective of this work was to evaluate the effects of employing two simple scaling laws on the resultant flame aerodynamics and thermo-chemical structure. In the parent study, from which this work was drawn, ~'2 the authors evaluate the effects in more detail. More extensive in-flame measurements are presented in this work and are used to interpret the observed phenomena. Computations of the flames scaled according to the constant velocity criterion have been carried out, 3 while similar simulations for the constant residence time flames are in progress. 2. This was considered in detail in the parent study. ~ Many of the dimensionless groups thus derived cannot be maintained constant when changing scale or employing different scaling rules due to conflicting similarity requirements.
REFERENCES 1. SMART, J. P.: On the Effect of Burner Scale and Coal Quality on Low-NO~ Burner Performance. Ph.D. Thesis, Imperial College of Science Technology and Medicine, Thermofluids Section, Mechanical Engineering Department, June 1992. 2. SMART, J. P. AND MOBCAN, D. J.: The comparison between constant velocity and constant residence time scaling of the Aerodynamically Air Staged Burner, IFRF Doc. No. F 37/y/28, 1991. 3. VISSER, n. M. AND WEBER, R.: Mathematical modelling of swirl-stabilised coal flames of thermal input in the range 200 kW to 54 MW, VDIBerichte Nr 922, 1991.
T. F. Wall, University of Newcastle, Australia. Your results indicate an effect of flow type rather than the residence time of particles in the internal reverse flow zone. Is your conclusion, therefore, that the scaling law based on particle response is not important? If this is so the NO~ levels will not depend on the cx)al size distribution fed to the burner. Is this so? Author's Reply. The normalised degree of internal recirculation zone penetration by the coal jet, ~b/Do, is governed by the following relationship: --
Do
~
'/'p
Do
where zj, is the particle relaxation time, Do is the characteristic b u r n e r dimension and v~ is the slip velocity between the coal particles and the reverse flow in the internal recirculation zone; v~ is proportional to the characteristic burner velocity, Vo.~ Thus: --~
K--Tp
D0
Do
where K is a constant. This implies that a specific normalised degree of internal recirculation zone penetration, required for low-NO~ operation in the burner, is dependent on the two parameters (oo/Do) and %, the latter being dependent on coal particle size distribution. For constant velocity scaling, the value of (vo/Do) is larger than for constant residence-time scaling, and thus it is easier for the coal jet to penetrate the internal recirculation in the former case. In the work described, the coal injector was inserted downstream of the burner throat. In this case the above relationship may be rewritten as:
•176 Do Do
Do
where (Iv~Do) is the normalised coal injector position relative to the b u r n e r throat. A specific minimum level of internal recirculation zone penetration is required for low-NOT operation (q~/Do = (~/ Do),,,,). As coal particle size was maintained constant for both scaling rules, % is constant but (v0/ Do) is larger for constant velocity scaling. Thus, (~b/ D0),,~, is achievable at a lesser normalised coal injector position in this case, see Figure 3. REFERENCE 1.
SMART, J. P.: On the Effect of Burner Scale and Coal Quality on Low-NO, Burner Performance. Ph.D. Thesis, Imperial College of Science Technology and Medicine, Thermofluids Section, Mechanical Engineering Department, June 1992.
THE EFFECT OF SCALE ON THE SWIRL STABILISED PULVERISED
PERFORMANCE OF COAL BURNERS
J. P. SMART, D. J. MORGAN AND P. A. ROBERTS International Flame Research Foundation P 0 Box 10 000 1970 CA IJmuiden The Netherlands
This paper describes the results of an experimental programme to evaluate the effect of burner scale and burner scaling criteria on the performance of swirl stabilised pulverised coal burners. The main text contains an analysis of the dominant considerations to be borne in mind with respect to the scaling of these systems. The experiments relate to establishing the effect of employing two different scaling criteria on the performance of an experimental burner capable of operating under both low and high NOx operation. The scaling criteria employed were constant velocity and constant residence-time scaling. The baseline burner was a conceptual 50 MW burner. Results of parametric studies on both experimental burners showed that baseline NOx emissions are higher for constant velocity scaled flames for both swirl numbers studied whereas for low NOx operation, the NOx emissions are similar. Results are explained in terms of differences in flame ignition and propagation pattern. Detailed in-flame measurements are presented for baseline flames produced from both burners and are used as a platform to interpret the experimental results.
Introduction One outstanding question still remaining in combustion research, is how representative is a scaled down burner's performance compared to the full industrial scale situation. Ideally, when scaling down a burner, the objective is to achieve similarity in all fluid dynamic and thermo-chemical processes in the scaled flame. If achieved, then the performance of the scaled burner will be identical to the full scale situation. In reality, this is practically impossible as all the physical and chemical processes do not scale in the same way. 1-4
velocities and momentum ratios are maintained constant with scale reduction. For constant residence-time scaling the co-axial velocity and momentum ratios are still maintained constant with scale reduction, however, velocities scale with tbe burner diameter. The principles behind constant velocity and constant residence-time scaling are outlined below.
Constant Velocity Scaling:
This criterion allows the following relationships to be established as: Qo = po Uo Do2
Background It follows that:
Burner Scaling Criteria:
Traditionally, there are two practical scaling criteria for the scaling down of burners. These are constant velocity and constant residence, or mixingtime, scaling.2'OBoth criteria rely on the scaling of the large macro-scale turbulent mixing process. Geometric similarity of the burner is maintained in both cases, and if swirl is used, the swirl number, So, is held constant. Constant velocity scaling is the most commonly used by burner manufacturers. In this case, co-axial combustion air and fuel injection
Do ~ Qo~
Here Qo, Uo and Do are the burner throughput (scale), characteristic burner velocity (combustion air velocity) and characteristic burner diameter respectively (burner throat diameter). The variable p,, is a characteristic inlet fluid density, normally the burner combustion air density. When a burner is scaled down from a baseline
1365
FURNACES AND BURNERS
1366
throughput, Qo.b.... to a reduced scale Q ..... t~d, using this criterion: Oo, base/Q ..... led -~ (Do,rase/D ..... led)2
This allows the value of the characteristic burner dimension to be derived for a scaled down burner when values for a large scale burner are known. Constant Residence-Time Scaling:
When using the constant residence-time scaling criterion, the ratio (Do/Uo) remains constant as scale is reduced. The ratio (Do/Uo) is proportional to the large macro-mixing time-scale, rm. Thus as: rm oc Do/Uo = constant
It follows that: Qo = poUoDo2 ~ poDo a
Therefore: Do OCUo OCQo~
When a burner is sealed down from a baseline throughput, Qo,ba,e to a reduced scale Qo,*cated using constant residence-time scaling: Qo, base/Q . . . . . led = (Do,l~,e/D . . . . . led) 3
And: Qo, base/Q ..... tea ~- (Uo,base/U . . . . . led) 3
Thus, if values of Uo and Do are known for the large scale case, then values for the reduced scale burner can be derived.
Where the parameters in parentheses are proportional to the area over which the mixing is taking place, the concentration gradient and the turbulent diffusivity, respectively. Furthermore, burner scale dictates the amount to be mixed at the above rate. The amount to be mixed is the burner scale, Qo which is proportional to UoDo2. The corollary of the above analysis is that the fractional degree of mixing, Xf, at any point downstream of the burner exit, x, is proportional to the ratio of the mixing rate and the amount to be mixed i.e.:
xs~ (xDoUo)/(UoDo2) ~ x/Do Thus, independent of any scaling criteria or burner scale, the fractional degree of mixing is constant at constant values of x/Do. If all the combustion and pollutant formation reactions are everywhere fast compared to this mixing rate, the rate of reaction is dictated by the rate at which the reactants are mixed. If this is the case, then flames of any scale, or flames of identical scale but scaled down using different scaling criteria should show identical degrees of reaction at identical values of x/Do. Effect of Scale on Two Phase Interactions:
In a swirling pulverised coal flame, ignition and stability are governed by the degree of interaction between the incoming coal stream with the internal zone (IRZ) present in the near burner field of these flows, see Fig. 1. Coal particles penetrate this region of reverse flow if the coal particle momentum is sufficient to overcome local particle drag forces. The penetration depth into the IRZ, 6, is given by:6 6 = U, rp,
Considerations on the Effect of Scale and Scaling Criteria on Physical Processes Occurring in a Pulverised Coal Flame Effect of Scale on Turbulent Mixing:
In swirling pulverised coal flames the coal and air are initially segregated by a distance proportional to the characteristic burner diameter, Do. The turbulent mixing process must overcome this segregation before the dominant combustion and pollutant formation reactions can occur. This mixing process is scale dependent. It can be readily derived from standard mixing layer theory, ~ that the rate of mixing between initially segregated reactants, R,~ is given by: Rm :c (xDo)(1/Oo)(UoOo) oc xDoUo
where:
U, = KIUo - Ur and:
rp = dpZpp/181xg
Where Us is the slip velocity between the injected coal particles (velocity KzUo) and the reverse flow velocity in the IRZ (Ur). The parameter rp is the relaxation time of a coal particle of diameter dp, where pp and /xg are coal particle density and gas viscosity, respectively. The ratio of the inlet flow velocity to the reverse flow velocity in the IRZ is a constant (K2) for constant swirl number, thus: q~ = Uo(KI - K2)zp
and: 6/oo
= (tJo/Oo)(~:l - K~).~p
Thus, the normalised degree of particle penetration
SCALING OF PULVERISED COAL BURNERS
1367
par|icle trajectories
D o throat \
\x~
t
',
I
boundary
9 . ~,-~'~ "/i po'slllon 0
/
~.
layer secondary air
Zi
!~
near burner field
FIG. 1. Coal particle dispersion in the near burner field of experimental burner
of the IRZ is dependent on how the ratio Uo/Do varies as scale is changed. This parameter increases as burner scale is reduced when constant velocity scaling is used, but remains constant for constant residence-time scaling. This fact highlights a potential problem with constant velocity scaling of pulverised coal burners, in that if coal particle size remains constant when reducing scale, to preserve the chemical reaction rates, the normalized degree of particle penetration of the IRZ changes.
Experimental An experiment is described that attempts to evaluate the effect of burner scale and scaling criteria on the performance of an experimental, swirling, pulverised coal burner in terms of the above scale dependent concepts. The experiments were executed on IFRF Furnace Number 1. A detailed description of this furnace may be found elsewhere. 2"7 The experimental burner used in this work can function under high and low NOx operation, dependent on the mean coal particle trajectories in the near burner field. A range of coal particle trajectories are possible in this burner and are shown in Fig. 1. If the coal particle trajectories are predominantly of type 1 and 2, the devolatilised volatile species, including volatile nitrogen are oxidised on the internal recirculation zone boundary in a region of high oxygen availability in the shear layer that surrounds it. This results in efficient conversion of fuel nitrogen to NOx. By contrast, if type 3 and 4 particle trajectories predominate, devola-
tilisation occurs primarily within the boundary of the internal recirculation zone in a region of low oxygen concentration. This promotes preferential reaction pathwaYTsfrom volatile fuel nitrogen to molecular nitrogen. Low NOx operation is most simply achieved by inserting the coal injector downstream of the burner throat, see Fig. 1. For the experiments, two burners were used. The first burner was a standard 2.5 MW version. 7 The velocities employed in this burner are typical of those regularly used in large scale utility boiler burners, i.e. 40 m/s for the combustion air and 20 m/s for the transport air. For the second burner, the 2.5 MW datum burner was theoretically scaled up to 50 MW using the constant velocity scaling criterion outlined above. This burner was subsequently scaled down from 50 to 2.5 MW using the constant residence-time scaling criterion. Both burners were geometrically similar, see Fig. 2. The combustion air and primary transport air velocities were 14 m/s and 7 m/s, respectively, for the constant residence-time burner. Swirl on the combustion air was generated using IFRF moveable block swirlers. The burner throat diameter, A, is selected as the characteristic burner diameter, Do. The coal used in the experiments was G6ttleborn hvBb (Composition (dry basis): C = 71.65, H = 4.61, S = 0.90, N = 1.51, O = 11.67, VM = 36.33; Ash = 9.50, LCV = 29.18 MJ/kg). The coal was pulverised to 75% < 75 ixm before being transported to the burner at a transport air to fuel ratio of 2:1 on a mass basis. The excess air factor was maintained at 1.15 for each burner. Combustion and
FURNACES AND BURNERS
1368
coal injector
secondary air
0
position =
L
q coal injector
~
L'
J
i
I
/_ burner quorl
movable block swirler
FIc. 2. Experimental burner designs (Reference burner: 50 MW, B = 2093 mm, A = 1046 mm, a = 628 ram, L = 1046 ram) (Constant velocity burner: 2.5 MW, B = 468 ram, A = 234 ram, a = 140 mm, L = 234 mm) (Constant residence-time burner: 2.5 MW, B = 771 ram, A = 385 mm, a --- 231 mm, L = 385 mm)
transport air temperatures were maintained at 300 and 70~ respectively. For each burner, the effect of coal injector position and swirl level was evaluated in terms of flue gas output conditions. For selected flames, in-flame maps of gas composition and temperature were performed using standard IFRF measurement probes.7 In this paper, selected in-flame measurements from two Type 27 baseline flames are compared.
Results and Discussion
Baseline--Type 2 Flames: These flames were generated with the coal injector positioned at the burner throat, x/Do = O. Under these conditions, the NOx emissions are higher for the constant velocity burner than the constant residence-time burner for both swirl numbers studied (So = 1.0 and 0.6), see Fig. 3. This is due to the mixing rate between fuel and air in the shear layer on the IRZ boundary being proportional to the product UoDo, see Section 3. This indicates that the combustion air mixes more rapidly into the shear mixing layer for the constant velocity burner and as a consequence markedly effects the flame ignition and propagation pattern.
Effect of Ignition Patter--Type 2 Flames: For the baseline, Type 2 flames studied, ignition takes place on the centreline side of the shear mix-
ing layer surrounding the IRZ. The majority of the coal particle trajectories are located in this mixing layer and thus the majority of devolatalisation takes place in this region, see Figs. 1 and 4(a). Ignition is initiated due to mixing between the hot recircuated combustion products in the IRZ and the devolatalisation products from the coal particles. Ignition occurs at a lean flammability limit. After ignition, the flame front propagates into the fuel stream into an increasingly richer mixture due to devolatilisation occurring ahead of the flame front as a result of radiation from the flame front and local refractory surfaces. The continuous entrainment of oxidant into the shear mixing layer will subsequently cause the flame front to propagate into a progressively leaner mixture. The flame front will then ultimately be convected downstream propagating at the lean limit. In a constant velocity scaled flame, the mixing of oxidant with the fuel stream is more rapid than for the constant residence-time case due to the value of UoDo being larger. This would indicate that flame propagation in the shear mixing layer of a constant velocity scaled flame is globally more lean than the corresponding residence-time situation. The corollary of the aforementioned discussion is that as the constant velocity scaled flame is propagating globally leaner than a constant residencetime flame of similar scale, a higher global conversion of volatile nitrogen species to NOx occurs. Moreover, the potential for more coal particles to penetrate the IRZ in the constant velocity case would augment this effect, as less coal will devolatalise on
SCAIANG OF PULVER1SED COAL BURNERS
looo/,~L~,..,.
1369
the IRZ boundary and the local gaseous environment on the IRZ boundary will be even leaner.
~so =~.o
Effect of Coal Injector Position: f:~O-ccr~t~t " ~ . ~
E
velocity
N Z
z~o-b~rr~ I
"~'~=
~z 200-
o
I
0 6.~ ~2 ~
64 65 06 d.7 68 6.9
~
norrnalised cool injector I:x~tion (X/Do)
1200
[
I
Lso=o.6
-- l(II)~
const(~ntresi~7.e time burner
burn~l
z
2000 0
0'.~ 0.2 o3 & ~
o.6 0.7 d.s 6.9 ~o
The effect of coal injector position on NOx emissions is also shown in Fig. 3. It can be seen that coal injector insertion results in significant NOx reduction. An interesting observation is that tbr both swirl numbers, the normalised insertion distance at which point NOx begins to decrease is less for the constant velocity scaled flame. This can be explained with reference to Section 3, where it is shown that the normalised degree of penetration of the internal recirculation zone tbr any coal injector position is dependent on the scaling criterion used. and the coal particle size. Maintaining coal particle size constant, the coal jet from a constant vclocity scaled flame will penetrate relatively fluther into the IRZ than a constant residence-time tlan,c due to the larger value of the slip velocity U,.. As previous work on this bunaer ha.s shown, to reduce NOx a specific degree of IRZ penetration is required. 7 This degree of penetration will be achieved at a lesser normalised degree of coal injector insertion for the constant, veltx:ity scaled flames than tbr the constant residence-tilne cast'.
nocmfllist~lcoal injector position (X/Do)
l'.ffect of Ignition Pattern--Penetration t,'hmw.s. FIG. 3. Effect of normalised coal injector position on NOx emissions
~1
c ~ t jet
tmject~y\
,,1
primary oir , I *c~l
-
,'~V'/.,,~
~
/
~
"~
L~'=o===J'~'-f I ome ignition
-~-..
IRZ boundary
For penetration flames, the ignition ixdten~ is significantly different from baseline, Tylx' 2; flames, l)ue to the coal jet penetrating the IRZ, the centreline region of the IBZ does m~t contain recirculated condmstion products, see Figs. 1 and -I (b). In this case, the coal jet will ignite at the lean limit but on the outside of the coal jet. The flame front will then propagate into the coal stream into a progressively richer mixture limited hv devulatilisation, until the rich limit is reached. At this point, the flame front will be convected do~llstream at the rich limit, limited by oxygen availability. Due to the flame propagation pr~x'ess in penetratiun flames being predominantly raider rich conditions the result is reduced NOx emissions.
\'
Detailed In-Flame Measurenwnts--Type 2 Flames:
trajectory
FIG. 4. Coal jet trajectory and ignition pattern for baseline and penetration flames: (a) baseline Type 2 flame, (b) penetration flame
Here, tile conslant velocity and constant residence-time baseline flames are compared, lrluc gas output conditions and in-flame measurements between x/D,, = 1.0 and 8.0 are given in Fig. 5 (a and b). At the quarl exit (x/D, = 1.0) the shear mixing laver is identified by the peaks in CO concentrations for both flames. Correspmading temperature profiles confirm ignition of these flame on the IRZ side of the shear mixing laver. N()x is also observed to be forming in this region. The CO concentrations in this region are higher in the constant
FURNACES AND BURNERS
1370
I species
(o)
temp.
CO
~
OZ COz
910
NN(ppm) 211 T{~) / IB] 1600t
21. 18, 161 ~15 12~
98
quo,rl
//Outlet
.o
BI 3 0
~6
9
(b) X/Do= 1
- 10
outlet
ts
2.0
2.5
O
-B
1200-
-6 I--I~1__
I
9
800-
-4
400-
-2
3. 0 ~0
o'.5 ~io &
0
2.o
z5
0
3.0
10
12
,0 3.0
0'.5
:-
192t
-B 1200 I
I
I
,6
I
-4
9 3-
4004
O0
-2 0.5
1.0
1.5
2'.5
9
0 3.0
i ,10
.
"B
__-o
BO( --=
0.L-- 0
~ 0
o.5
1.o
1.5
2.0
2'.s
ao
XlDo=B 10
I
~.o
X/Do = 3
:],6oot 151 120(
~:s
1.'0
XlDo=8 21. 1B,
6~
.6~
800
-0 to
X/Do= 3
15' 12
,8
/~qu~l
'=4
.2
21, 18. 1600-
9' 6'
-'51
o
Ai 0.5
15-
I
~
X/Do= 1
N0x m~n)
w
NOx
2.0
2'3
0 3.0
10
1i
-B
~2ooI
-6
800~. . .9 . 9 . ". . .
61
-2
3.
oi
0,
i
0
0.5
I
tO
;
1.5
"
2.0
2.5
0 3.0
FIG. 5. In-flame data. (a) baseline constant velocity burner.--So = 1.0, Coal Injector position = 0 (Flue gas NOx = 1185 ppm (0%Oz), Tx = 99,6%) (b) baseline constant residence-time burner--So = 1.0, Coal Injector position = 0 (Flue gas NOx = 1100 ppm (0%O2), Tx = 99,6%) residence-time flame than the constant velocity case. This indicates a richer flame propagation in the former case. Observing the in-flame profiles between the quarl exit, x/Do = 1.0, and x/Do = 8.0, the following comments can be made. Between the quarl exit and x/Do = 3.0 the peak CO concentrations are higher for the constant residence-time flame due to the ef-
fect of the richer global flame propagation pattern. In this region it may also be observed that the NOx levels for the constant residence-time flame decrease to values below the corresponding values for the constant velocity case. At x/Do = 8.0, all radial variations in concentration and temperature are negligible, and values are representative of those found in the furnace exhaust. This indicates the
SCALING OF PULVERISED COAL BURNERS completion of turbulent macro-mixing. The observation that NOx does not change significantly suggests that NOx is effectively "frozen" in concentration after the completion of macro-mixing at the termination of volatile combustion.
1371
Acknowledgements The permission of the IEA CCS A2 Executive Committee and the Joint Committee of the I F R F to publish this paper is gratefully acknowledged. REFERENCES
Conclusions Two burners were studied in this work firing Gfttleborn hvBb coal. The two burners were 2.5 MW constant velocity and constant residence-time scaled versions of a conceptual 50 MW burner. For the baseline, Type 2, flames, the NOx emissions in the furnace exhaust were higher for the constant velocity burner than the constant residence-time case. In-flame measurements indicate that ignition of these flames takes place on the IRZ side of the coal jet. The difference in NOx is explained in terms of the differences in flame propagation pattern. With coal injector insertion into the burner quarl, NOx decreased for both burners. The NOx decreased at a lesser normalised coal injector insertion distance for the constant velocity flame than the constant residence-time flame. This is explained in terms of the different relative degree of IRZ penetration by the coal particles in each case.
l. LAWN, C. J., CUNNINGHAM,A. T. S., STREET, P. J., MATrHEWS, K. J., SARGEANT, M. AND GODRIDGE, A. M.: Principles of Combustion Engineering for Boilers (C. J. Lawn Ed.), Chap. 2, p. 61, Academic Press, 1987. 2. SMART,J. P., VAN DE KAMP, W. L. AND MORGAN, M. E.: Fuel, 70, 1350, (1990). 3. SALVl, G. AND PA'~'NE, R.: Combustion System Scaling, International Flame Research Foundation Report D07/a/69, 1Jmuiden, (1978). 4. SPALDING, D. B.: Ninth Symposium (International) on Combustion, p. 833, The Combustion Institute, 1963. 5. HAWTHORNE, W. R., WEDELL, D. S. AND HOTTEL, H. C.: Third Symposium (International) on Combustion, p. 266, The Combustion Institute (1949). 6. FIELD, M. A., GILL, D. W. MORGAN, B. B. AND HAWKSLEY, P. G. W.: Combustion of Pulverised Coal, Chap. 2, p. 25, BCURA, Leatherhead, (1967). 7. SMART,J. P. AND WEBER, R.: J. Inst. E, 62, ,937, (1990).
COMMENTS John H. Pohl, Energy International, USA. 1. Was there any NO~ formation in the furnace outside the flame? 2. Did you make observations of the flames and were you able to relate the flame structure to the scaling laws and NO~ formation? 3. What is the status of the sealing law of Payne, et al. which concluded that the burner diameter should be scaled by the ratio of the diameter to the 2.5 power? Author's Reply. 1. From the in-flame measurements and visual observation of the volatile flame length, there was no significant NO~ formation/destruction after the termination of volatile combustion as the NOx values at this point are representative of those found in the furnace exhaust. 2. Visual observations of the flames suggested that the baseline (Type 2) flames for both scaling rules were structurally similar in terms of flame diameter and length with both scaling with the burner diameter. For the low-NO~ penetration flames (Type 1) the constant velocity scaled flame was longer,
relative to the burner diameter, in comparison to the constant residence time case. The lower NO~ level produced for the baseline constant residence time flame is due to the lower specific mixing rate between the fuel and air (proportional to voDo). This results in a globally more fuel rich flame propagation and reduced NO~ formation. 3. The evaluation of the sealing concept proposed by Salvi and Payne, 1 where the burner throughput (scale) is related to the burner diameter by: Q0 ~ Doz5 is currently being in progress. To this end two separate research programs have been initiated at the IFRF. The first program, from which this paper originates, is on scaling of pulverised coal burners. The second program financed by GR1 and IFRF is on sealing of natural gas burners, z
REFERENCES 1. SALVI, G. AND PAYNE, R.: Combustion System Sealing, IFRF Doe. No. D 07/a/69, 1978. 2. WEBER, R.: Overall Workplan for the Investi-
1372
FURNACES AND BURNERS
gation into Scaling of Aerodynamics and LowNO~ Characteristics of Natural Gas Burners in the Thermal Input Range 30 kW to 12 MW. IFRF Doc. No. D 95/y/1R, 1991.
L. D. Smoot, Brigham Young University, USA. 1. Simple scaling laws for such complex two-phase flows may not be generally applicable. Did the authors seek to evaluate the scaling laws and interpret the measurements through computations of the reacting b u r n e r flow? 2. Did the authors consider the method of dynamic scaling through dimensionless groups derived from dimensionless two-phase equations of change to establish scaling laws or explore limitations of scaling laws?
Author's Reply. 1. The authors agree that there is probably no generally applicable scaling rules that can produce identical flame aerodynamics, thermochemical structure in the scaled domain for burners at different scales. The objective of this work was to evaluate the effects of employing two simple scaling laws on the resultant flame aerodynamics and thermo-chemical structure. In the parent study, from which this work was drawn, ~'2 the authors evaluate the effects in more detail. More extensive in-flame measurements are presented in this work and are used to interpret the observed phenomena. Computations of the flames scaled according to the constant velocity criterion have been carried out, 3 while similar simulations for the constant residence time flames are in progress. 2. This was considered in detail in the parent study. ~ Many of the dimensionless groups thus derived cannot be maintained constant when changing scale or employing different scaling rules due to conflicting similarity requirements.
REFERENCES 1. SMART, J. P.: On the Effect of Burner Scale and Coal Quality on Low-NO~ Burner Performance. Ph.D. Thesis, Imperial College of Science Technology and Medicine, Thermofluids Section, Mechanical Engineering Department, June 1992. 2. SMART, J. P. AND MOBCAN, D. J.: The comparison between constant velocity and constant residence time scaling of the Aerodynamically Air Staged Burner, IFRF Doc. No. F 37/y/28, 1991. 3. VISSER, n. M. AND WEBER, R.: Mathematical modelling of swirl-stabilised coal flames of thermal input in the range 200 kW to 54 MW, VDIBerichte Nr 922, 1991.
T. F. Wall, University of Newcastle, Australia. Your results indicate an effect of flow type rather than the residence time of particles in the internal reverse flow zone. Is your conclusion, therefore, that the scaling law based on particle response is not important? If this is so the NO~ levels will not depend on the cx)al size distribution fed to the burner. Is this so? Author's Reply. The normalised degree of internal recirculation zone penetration by the coal jet, ~b/Do, is governed by the following relationship: --
Do
~
'/'p
Do
where zj, is the particle relaxation time, Do is the characteristic b u r n e r dimension and v~ is the slip velocity between the coal particles and the reverse flow in the internal recirculation zone; v~ is proportional to the characteristic burner velocity, Vo.~ Thus: --~
K--Tp
D0
Do
where K is a constant. This implies that a specific normalised degree of internal recirculation zone penetration, required for low-NO~ operation in the burner, is dependent on the two parameters (oo/Do) and %, the latter being dependent on coal particle size distribution. For constant velocity scaling, the value of (vo/Do) is larger than for constant residence-time scaling, and thus it is easier for the coal jet to penetrate the internal recirculation in the former case. In the work described, the coal injector was inserted downstream of the burner throat. In this case the above relationship may be rewritten as:
•176 Do Do
Do
where (Iv~Do) is the normalised coal injector position relative to the b u r n e r throat. A specific minimum level of internal recirculation zone penetration is required for low-NOT operation (q~/Do = (~/ Do),,,,). As coal particle size was maintained constant for both scaling rules, % is constant but (v0/ Do) is larger for constant velocity scaling. Thus, (~b/ D0),,~, is achievable at a lesser normalised coal injector position in this case, see Figure 3. REFERENCE 1.
SMART, J. P.: On the Effect of Burner Scale and Coal Quality on Low-NO, Burner Performance. Ph.D. Thesis, Imperial College of Science Technology and Medicine, Thermofluids Section, Mechanical Engineering Department, June 1992.