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Characteristics of burning liquid sprays with additional diluents in the primary air stream
COMBUSTION AND FLAME 34:141-151 (1979)
141
Characteristics of Burning Liquid Sprays with Additional Diluents in the Primary Air Stream SUBRAMANYAM R. GOLLAHALLI School of Aerospace, Mechanical and Nuclear Engineering, The University of Oklahoma, Norman, Oklahoma 73019
An investigation has been conducted into the effects of introducing additional diluents into the primary air stream of pressure-jet and air-blast atomizers on their flame characteristics. The effects of primary air/fuel ratio and concentration of diluent (N 2 or CO2) on flame appearance, flame length, fraction of heat release radiated, temperature profiles, and oxygen concentration profiles in the spray flames of ASTM number 2 fuel oil and kerosene are presented. The results show that the structure of flames from both types of atomizers is significantly affected by the additional diluents. Flame length, emission of soot, and fraction of heat release radiated all decrease with an increase in the amount of additional diluent in the primary air.
INTRODUCTION Exhaust gas recirculation is a simple and effective method of reducing the emission of smoke and gaseous pollutants like NOx from combustion devices [1]. Several publications are available concerning the effects of exhaust gas recirculation on overall combustion characteristics, such as flame length, stability, and radiation of gas jet flames [2, 3, 4]. Recently the effects of diluents on the structure of a gas jet flame have been studied in detail [5]. Similar studies on the effects of diluents on the characteristics of burning liquid sprays are very limited. The experimental studies of Dunn [6] and Cooper et al. [7] have shown that in pressure-jet oil burners, flames become nonluminous and soot concentration suddenly drops when a critical amount of exhaust gases or additional diluents is inducted into the combustion chamber. Both of these investigations indicated that similar effects could be achieved if the individual constituents of exhaust gases such as nitrogen, carbon dioxide, and water vapor at ambient temperature were introduced, instead of the hot exhaust products. Also, the study of Cooper et al [7] showed that such a reduction in Copyright © 1979 by The Combustion Institute Published by Elsevier North Holland, Inc.
the soot concentration could not be obtained if the inlet air was preheated i n the absence of recirculation of diluents, which suggested that the thermal effect of exhaust gases was not a strong factor in reducing the soot formation. The experiments of Sjt~gren [8] have confirmed the findings of Dunn [6] and Cooper et al [7]. Although these experiments have clearly demonstrated the effect of recirculation in reducing smoke formation, the mechanism by which this effect occurs is not at all clear. Dunn [6] indicated that some complex reactions occurring during combustion are responsible for the reduction in soot when exhaust gases are recirculated, whereas Cooper et al. [7], suggested that the balance between the reactions producing condensed soot and the reactions generating gaseous products is altered by the thermal diffusion characteristics of the diluents. The mechanism proposed by Cooper et al [7] contradicts the observation that preheating of air does not produce any significant effect. Also, it does not substantiate the observation that a larger proportion of nitrogen is required than carbon dioxide, in spite of the fact that nitrogen has a higher thermal diffusivity than carbon dioxide. Sj6gren [8] has argued that the
0010-2180/79/020141+ 11 $01.75
142 reduction of extinction velocity of drops in flames, caused by the addition of diluents, is the primary reason for the decrease of soot formation in burning sprays. The mechanism suggested by Sj6gren is based on the premise that droplets burn individually and not in an envelope of gases, which concept itself is currently disputed by various investigators [9, 10]. A recent theoretical study by Chiu and Liu [11 ] has shown that the occurrence of individual drop combustion is not likely in practical sprays. Even if individual droplet burning is possible in sprays, Sj6gren's argument, to be valid, further requires that wake flames of drops behave as premixed flames. The studies of Refs. [12-14], however, have shown that wake flames of drops do not behave always as premixed flames. There is no agreement among various investigators on the effects of exhaust gas recirculation on the flame temperature. Brown et al. [15] have shown that considerable reductions in the amounts of oxides of nitrogen occur when exhaust gases are recirculated, which they attribute to the decrease of flame temperature. However, Dunn's study [6] showed that the refractory walls of the combustion chamber fused as a consequence of exhaust gas recirculation, which he attributed to increased flame temperatures. Since no measurements of spectral radiant emittance of flames are reported by Dunn, it is not possible to determine whether the changes in temperature or concentrations of emitting species caused the increased wall temperatures in his combustor. Thus, the mechanism by which exhaust gas recirculation affects the characteristics of burning sprays is uncertain. Although several studies on the structure and pollutant formation in burning sprays from both pressure-jet atomizers [16-20] and air-blast atomizers [20-23] have recently appeared in the literature, the effects of introducing additional diluents have not yet been diagnostically studied. This paper presents the results of a detailed experimental study directed to provide information on the effects of diluents on the flame length, radiation, and temperature fields of open flames of burning sprays. The experimental results of studies on both pressure-jet and twin-fluid atomizer burners are presented.
SUBRAMANYAM R. GOLLAHALLI EXPERIMENTAL PROCEDURE In experiments with a pressure-jet atomizer, a commercial domestic furnace oil bumer rated at 0.063 l/min and 0.689 MPa with the provision for swirling the primary air was used. This burner was provided with an electric spark igniter located close to the nozzle. The ignition circuit was modified such that the burner could be operated with either continuous ignition or ignition only at the beginning of the tests. The flow rate of primary air drawn by the burner was measured by means of a Calibrated orifice plate mounted on the suction side of the blower. The primary air/fuel ratio was varied by means of a damper provided on the inlet of the blower. In the air-blast atomizer studies, the fuel atomization was achieved by the shearing action between the high-velocity air and the fuel film in a venturi-shaped nozzle. Fuel was forced to the throat of the venturi from a fuel tank under the pressure of bottled nitrogen. Atomization characteristics were controlled by varying the air velocity. A diverging conical section attached to the nozzle allowed the stable operation of the burner over a wide range of air and fuel flow rates. Flow rates of diluents and air were measured by means of calibrated rotameters. Fuel-supply rate was measured by timing a known quantity of fuel flow. Flames were photographed in color to record their lift-off distance, flame length, and appearance. Temperature profiles were measured by means of Pt-Pt 13% Rh thermocouples with 0.25mm dia. bead. Oxygen concentration was determined by sampling gases through a heated microprobe and analyzing them with a gas chromatograph provided with a thermal conductivity detector, using Mol-sieve 5A column. Radiant emittance of the flames was measured with a thermopile placed at such a distance in the direction normal to the axis of the flames that the inverse squarelaw was followed. The fraction of heat release radiated was computed according to the method used earlier for hydrocarbon flames [24, 25]. The size distribution of droplets in the regions near the axis of the flames was measured by the impaction method with magnesium oxide coated slides. Experiments were performed with kerosene
BURNING LIQUID SPRAYS
~i¸
143
~!i~
% (a)
(b)
:
(c)
~}
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(d)
Fig. 1. P h o t o g r a p h s o f b u r n i n g sprays o f n u m b e r 2 fuel oil (tex p = 1 s) over a pressure-jet atomizer; (a) and (c) side- and end-views without any additional diluent, (b)
and (d) side- and end-views with CO2 added to primary air (Y = 0.2).
and ASTM number 2 fuel oil. Carbon dioxide and nigrogen (99.9% purity) were used as diluents. The best estimates of uncertainties in flame length, radiation emittance, and temperatures are +-1%, -+2%, and -+1%, respectively.
OBSERVATIONS Pressure-Jet Atomizer Tests Flame Appearance and Length Figure 1 shows the side- and end-view photographs of typical flames over the pressure-jet atomizer, with and without additional diluents in the primary air. Based on color photographs and visual
observations the structure of these flames can be diagrammatically represented as shown in Fig. 2a. In the absence of additional diluents the flames exhibit a distinct four-zone structure. In the near nozzle region a faint blue flame extends from the igniter to about three burner diameters in the axial region. This flame, stabilized in the wake of the injector itself (region 1), appears like a premixed gas flame. This central core is surrounded by a relatively dark annular zone (region 2), which in turn is enveloped by an outer bright yellow flame (region 3). The flame in the region away from the nozzle (region 4) appears orange-red in color. As the amount of primary air is increased, flame in the region 1 becomes more bluish and the
144
SUBRAMANYAM R. GOLLAHALLI
-~ . . . . f
/
FUEL
FUEL " PUMP
NOZZLE
(a)
(b) Fig. 2. Structure of the burning sprays over (a) a pressure-let atomizer (b) an air-blast atomizer.
total visible flame length (axial distance from the burner mouth to the tip of the flame) decreases. When the primary air/fuel ratio becomes more than stoichiometric, the effects of further increasing primary air follow the law of diminishing returns. Even at the condition of flame blow-out a large part of the flame remains yellow. When additional diluents (C02 or N2) are introduced into the primary air, flame in the region 1 initially becomes elongated, but with larger mass fractions of diluent (Ydil > 0.2) it becomes extinguished. This effect is clearly seen in Fig. 1. The hole in the end-view photograph d of the flame with diluent in the primary air is caused by the extinguishment of the flame in the central core, which can also be seen by comparing the side-view photographs a and b in Fig. 1. In addition to this effect on the flame in region 1, the flame in region 3 also turns bluish, with the induction of diluents into the primary stream. Figure 3 shows the effects of adding C02 on flame length over pressure-jet atomizers burning number 2 fuel oil, whereas Fig. 4 compares the effects of the two fuels and two diluents in this study. These figures reveal the following: (i) Flame length decreases with the increase in primary air/fuel ratio. In the absence of diluents, when this ratio is increased by a factor of three,
flame length decreases by a factor of about two. (ii) The effect of diluents is to decrease the flame length and this effect decreases progressivley with the increase in the primary air/fuel ratio. (iii) No significant differences in the trends of the variation of flame length with the quantity of diluent added are noticed between the two fuels (kerosene and number 2 fuel oil) and the diluents (C02 and N2).
Flame Radiation Figures 3 and 4 also show the variation of the fraction (F) of heat release radiated from the flames with the mass fraction of diluents in the primary air stream. These figures indicate that (i) a considerable decrease in the heat radiated from the flame occurs when diluents are added to the primary air stream, (ii) the extent of reduction in F depends upon the primary air/fuel ratio, (iii) number 2 fuel oil flames radiates more than kerosene flame, and (iv) in the absence of diluents, F decreases when the primary air/fuel ratio is increased. Temperature and Oxygen Concentration Profiles Axial and radial profiles of the temperature and oxygen concentration in the flames of number 2
BURNING LIQUID SPRAYS
145
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Effects o f diluent ( C 0 2 ) mass fraction and p r i m a r y fuel/air ratio
(Mf/Ma) on
flame length and fraction of heat release radiated in number 2 fuel oil flames over a pressure-jet atomizer. fuel oil over the pressure-jet atomizer, with and without the diluent addition to the primary air, are shown in Fig. 5. It is seen that (i) both axial temperature and oxygen concentration decrease considerably in the near nozzle region when the diluent is added, (ii) there are valleys in the radial temperature and concentration profiles in the near nozzle region (x = 0.1 m) and these valleys flatten when the diluent (Y = 0.35) is added, and (iii) the effects of diluent addition on local temperature and concentration are smaller in the far nozzle region than in the near nozzle region.
Mean Droplet Size The Sauter mean diameter of the droplets in the axial region at x = 100 mm determined by the
impaction method using magnesium oxide coated slides in number 2 fuel oil flames with diluents (C02, Y = 0.35) was 48/a, compared to a value of 30 /~ in the flames without additional diluents in the primary air. Air-Blast Atomizer Tests Appearance and Flame Length Figure 2(b) shows a diagrammatic representation of the structure of the flames from the air-blast atomizer as observed in several color photographs. The regions 1 and 2 in this sketch correspond to bluish and luminous yellow zones which appear similar to premixed and diffusion-controlled gas flames. With the increase in the flow rate of pri-
146
SUBRAMANYAM R. GOLLAHALLI
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Fig. 4. Comparison of the effects o f fuel and diluent types on flame length and fraction of heat release radiated in flames over a pressure-jet atomizer (,~I##I a = 0.06).
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Fig. 5. Axial and radial profdes of temperature and oxygen concentration in number 2 fuel oil flames over a pressure-jet atomizer with and without diluent addition.
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BURNING LIQUID SPRAYS
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Fig. 6. Effectsof diluent mass fraction, atomizing gas flow rate (Vg), and nature of diluent on flame length and fraction of heat release radiated in flames of number 2 fuel oil over an air-blast atomizer. mary air, region 2 also turns bluish, and the entire flame eventually becomes blue before it blows out. Figure 6 shows the variation of luminous flame length, measured as the distance from the burner mouth to the tip of the flame, with primary air/fuel ratio, the concentration and type of diluent in atomizing air of number 2 oil spray. In the tests with diluents, the total flow rate of atomizing gas (air + diluent) was kept constant by reducing air flow corresponding to the diluent added, in order to keep the atomization characteristics the same. In the absence of diluents, when the air flow was increased, an increase in the length of bluish zone and a reduction in the total flame length occurred. The addition of diluents to the primary stream, without a significant change in the flow rate of atomizing gas, shows a further increase in the length of the blue zone and a decrease of the total flame length.
F l a m e Radiation
Figure 6 also shows the variation of the fraction of heat release radiated with the mass fraction of diluents (CO 2, N2) in the atomizing gas stream and the flow rate of atomizing gas. As in the flames over the pressure-jet atomizer, increases in the flow rates of primary air and diluents cause considerable reductions in the fraction of heat release that is radiated. The decrease of the radiant emission depends upon the primary air flow rate (Vs). At large Vg the effect of diluent becomes minimal. Temperature and O x y g e n C o n c e n t r a t i o n
Profiles The axial and radial profiles of temperature and oxygen concentration in the number 2 oil flame~ over the air-blast atomizer, with and withoul additional diluent in the primary air stream, are
148
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SUBRAMANYAM R. GOLLAHALLI
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Fig. 7. Axial and radial profiles of temperature and oxygen concentration in number 2 fuel oil flames over an air-blast atomizer with and without diluent addition.
shown in Fig. 7. The axial temperature in the near nozzle region decreases considerably when the diluent is added, whereas the peak temperature remains essentially the same. The radial temperature profile in the near nozzle region develops a large off-axis peak when the diluent is introduced. In contrast, the radial temperature profdes in the far nozzle region are not significantly affected in shape, although the magnitude of temperature drops slightly. The addition of diluent also causes a drop in the oxygen concentration near the nozzle and the flattening of radial profdes.
Mean Doplet Size In this case also, mean droplet size (SMD)in the lxial region of fuel oil flames at x = 20 mm was :'ound to increase to 65 /~ when diluent (CO2, Y = 0.2) was added, from 48 p without any tiluent. Since the total flow of atomizing gas air + diluent) was kept constant, the atomization :haracteristics are expected to remain invariant. -Ience, this change in droplet size is expected to be )fimafily caused by the reduction in evaporation ate in the core.
DISCUSSION The commerical pressure-jet atomizer used in this study yields a hollow cone spray similar to that reported by McCreath and Chigier [16] in their flame structure studies. The air-blast nozzle causes the atomization of fuel by the shearing action of a high-velocity air stream (approximately sonic) and the annular liquid film, and is similar to that used in earlier investigations [21-23]. This is evidenced by the significant resemblance of oxygen concentration and temperature profiles for undiluted flames shown in Figs. 5 and 7 with the data reported for pressure-jet flames [16, 19, 20] and air-blast atomized spray flames [20-23] in the literatuare. Hence, the burners used in this study can be considered satisfactory to investigate the effects of the flow rates of primary air and diluents on the behavior of burning sprays.
Effects of Primary Air-Flow Rate The flow rate of primary air plays a significant role in forming a small, well-mixed reaction zone in the initial region, where the fuel vapors from the evaporating droplets can mix quickly with the pri-
BURNING LIQUID SPRAYS mary air stream crossing the spray sheath. The blue region near the axis, noticed in the color photographs of pressure-jet flames corresponds to this zone. The dark region corresponds to the location of the spray sheath, and the outer yellow region corresponds to the diffusion-controlled reaction zones of evaporated fuel and its pyrolysis products. The dip in the radial temperature profile in the near nozzle region seen in Fig. 5, supported by the data of temperature contours presented in Refs. [16, 19] conforms to the above description of the structure of the pressurejet spray flames. The increase of the extent of blue region and the decreases of yellow region and the total flame length, with the increase of primary air flow rate in the pressure-jet atomizer, indicate that the relative proportions of fuel burning in the form of soot decrease when the primary air flow is increased. Further evidence for this can be seen in the variation of the fraction of heat release radiated with the primary air/fuel ratio (Fig. 3). The decrease of the amount of soot buming, and the consequent reduction in the continuum radiation probably causes the decrease of the heat radiated. In air-blast atomizers, mixing between primary air and the fuel droplets is much stronger than in the case of pressure-jet atomizers because of the difference in the atomization mechanism. Hence, combustion in a large part of the core occurs similar to that in a premixed gas flame, which is manifested by a continuous decrease in the radial temperature profde close to the nozzle (at x = 20 mm in Fig. 7). The decrease of the total flame length and the fraction of heat release radiated with the increase of primary air flow rate are again caused, mostly, by the reduction in the amount of soot burning, as discussed earlier. Effects of Diluents Addition of diluents is seen to result in reductions of flame length and radiation which are accompanied by the lowered oxygen concentration and temperatures in the near nozzle region in both pressure-jet and air-blast atomized sprays. Further-
149 more, in pressure-jet atomized sprays, when diluents are added the radial temperature profde shows a continuous increase from the axial to the peak value, instead of the dip between them seen in the absence of additional diluents. This change in the temperature profdes and the hole in the end-view photograph [Fig. l(d)] indicate that combustion in the core is suppressed when diluents are added. Reduction in the oxygen available in the core, as shown by the concentration proFdes, is the most likely cause for it. Similar changes leading to an increase in the extent of interface combustion are noticed in the temperature and concentration profiles of the air-blast atomized spray. The increases of both primary air and diluent concentration are seen to decrease the flame length, amount of soot burning, and radiation emitted, although they have opposite effects on the oxygen available in the core. This suggests that the mechanisms of their effects are different. The increase of primary air flow rate was found to elongate the blue region near the nozzle, which indicates that the gas phase oxidation reactions increase as a consequence. That would lead to a reduction of the proportion of the fuel that pyrolyzes and forms soot. On the other hand, the addition of diluents reduces the oxygen available in the core and still results in the decrease of soot burning, which suggests that the reduction of pyrolysis and soot-forming reactions is likely to be the more dominant mechanism of their action. Similar effects of diluents on propane gas flames were noticed earlier [5]. SjiSgren [8] suggested that the reduction in the amount of soot produced in burning sprays is caused by the transformation of envelope flames around the individual droplets into wake flames, which burn as premixed flames with oxygen entrained into the wake [11]. The formation ot premixed flames requires that sufficient oxygen be available in the core. Since concentration profiles show that the core becomes oxygen-deficienl when diluents are introduced, the fuel vapors burn at the edge of the core in the form of a diffusio~ flame, rather than as a premixed flame. This i,, substantiated by the peaking of the radial tempera
150 ture profde at the edge of the flame. Since a significant reduction in the heat radiated from the flames occurs when diluents are introduced, it suggests that the heat transferred in the radiant sections could decrease when exhaust gases are recirculated i n boilers. However, the decrease in soot concentration may cause an enhancement in the contributions of wall radiation and convective heat transfer. This effect is a probable factor in the changes in heat transfer pattern observed in industrial boilers with exhaust gas recirculation [26]. Although the combustion conditions in spark-ignition engines have no direct similarity to those of the present study, it is interesting to note that unburnt hydrocarbon emissions and chances of misfiring increase significantly with an increase in exhaust gas recirculation. Hence, introduction of diluents into either spray systems or homogeneous mixture systems necessitates some operational and design modifications to prevent undesirable effects. One such modification is the extended spark duration method developed by Aiman [27] for engines with high exhaust gas recirculation. CONCLUSIONS The present investigation has dealt with the effects of primary air/fuel ratio and additional diluents in primary air on the characteristics of hydrocarbon liquid spray flames from pressure-jet and air-blast atomizers. Variations of appearance of the flames, flame length, fraction of heat release radiated, temperature profdes, and oxygen concentration profiles with the concentration of additional diluents in the primary air have been documented. The results indicate that (i) flame length, soot burning, and radiation emitted from liquid spray flames decrease significantly with an increase in primary air/fuel ratio and the concentration of dfluents in the primary air, (ii) the effects of diluents on flame characteristics are similar in pressure-jet and air-blast atomizers, and (iii) introduction of additional diluents into combustion systems for the purpose of reduction of soot may require operational and design modifications.
SUBRAMANYAM R. GOLLAHALLI
REFERENCES 1. Hedley, A. B., and Jackson, E. W., J. lnst. Fuel 38: 290 (1965). 2. Arthur, J. R., and Napier, D., Fifth Symposium [Int.] on Combustion, Reinhold: New York, 1955, p. 303. 3. Mclintock, I.S., Combust. Flame 12:127 (1968). 4. Wright, F. J., Fuel53:232 (1974). 5. GoUahalli, S. R., Combust. Sci. Technol. 15:147 (1977). 6. Dunn, Jr., F. R., Improved oil burner performance with recirculafion of combustion gases, Amer. Petrol. Inst., conference paper 61-8 (1961). 7. Cooper, P. W., Kamo, R., Marek, C. J., and Solbrig, C. W., Recirculation and fuel-air mixing as related to burner design, Amer. Petrol. Inst. Publ. No. 1723 (1964). 8. Sjtigren, A., Fourteenth Symposium (lnt.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 919. 9. Chigier, N. A., Fifteenth Symposium (Int.} on Combustion, The Combustion Institute, Pittsburgh, Pa., 1975, p. 398. 10. Abdel-Khalik, S. I., Tamuru, T., and El-Wakil, M. M., Fifteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1975, p. 389. 11. Chiu, H. H., and Liu. T. M., Combust. ScL Techol. 17:127 (1977). 12. GollahaUi, S. R., and Brzustowski, T. A., Fourteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 1333. 13. GollahaUi, S. R., and Brzustowski, T. A., Combust. Flame 24:273 (1975). 14. Gollahalli, S. R., Combust. Flame 29:21 (1977). 15. Brown, T. D., Mitchell, E. R., and Lee, G. K., First European Symposium on Combustion (F. J. Weinberg, Ed.) Academic Press, New York, 1973, p. 487. 16. McCreath, C. G., and Chigier, N. A., Fourteenth Symposium (Int.} on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 1355. 17. Dombrowski, N., Home, W., and Williams, A., Combust. Sci. Technol. 9:247 (1974). 18. Khalil, E. E., and Whitelaw, J. H., Sixteenth Symposium (lnt.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 569. 19. Tuttle, J. H., Shisler, R. A., and Mellor, A. M., Combust. Sci. Technol. 14:229 (1976). 20. Komiyama, K., Flagan, R. C., and Heywood, J. B., Sixteenth Symposium (lnt.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 549. 21. Onuma, Y., and Ogasawara, M., Fifteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1975, p. 453. 22. Onuma, Y., Ogasawara, M., and Inoue, T., Sixteenth
BURNING LIQUID SPRAYS Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 561. 23. Styles, A. C., and Chigier, N. A., Sixteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 619. 24. Brzustowski, T. A., GoUahaUi, S. R., Kaptein, M., Gupta, M., and Sullivan, H. F., Radiant heating from flares, ASME paper 75-HT-4, 1975. 25. Gollahalli, S. R., and Sullivan, H. F., Effect of pool shape on burning rate, radiation, and flame height of liquid pool fires. Paper presented at the Technical
151 meeting of the Eastern Section of the Combustion Institute. John Hopkins University, Oct. 1974. 26. Breen, B. P., in, Emissions from Continuous Combustion Systems (W. Cornelius and W. G. Agnew, Eds.) Plenum Press, New York, 1972, p. 325. 27. Aiman, W. R., Combust. Sci. Technol. 15:129 (1977).
Received 8 February 1978; revised 6 May 1978
141
Characteristics of Burning Liquid Sprays with Additional Diluents in the Primary Air Stream SUBRAMANYAM R. GOLLAHALLI School of Aerospace, Mechanical and Nuclear Engineering, The University of Oklahoma, Norman, Oklahoma 73019
An investigation has been conducted into the effects of introducing additional diluents into the primary air stream of pressure-jet and air-blast atomizers on their flame characteristics. The effects of primary air/fuel ratio and concentration of diluent (N 2 or CO2) on flame appearance, flame length, fraction of heat release radiated, temperature profiles, and oxygen concentration profiles in the spray flames of ASTM number 2 fuel oil and kerosene are presented. The results show that the structure of flames from both types of atomizers is significantly affected by the additional diluents. Flame length, emission of soot, and fraction of heat release radiated all decrease with an increase in the amount of additional diluent in the primary air.
INTRODUCTION Exhaust gas recirculation is a simple and effective method of reducing the emission of smoke and gaseous pollutants like NOx from combustion devices [1]. Several publications are available concerning the effects of exhaust gas recirculation on overall combustion characteristics, such as flame length, stability, and radiation of gas jet flames [2, 3, 4]. Recently the effects of diluents on the structure of a gas jet flame have been studied in detail [5]. Similar studies on the effects of diluents on the characteristics of burning liquid sprays are very limited. The experimental studies of Dunn [6] and Cooper et al. [7] have shown that in pressure-jet oil burners, flames become nonluminous and soot concentration suddenly drops when a critical amount of exhaust gases or additional diluents is inducted into the combustion chamber. Both of these investigations indicated that similar effects could be achieved if the individual constituents of exhaust gases such as nitrogen, carbon dioxide, and water vapor at ambient temperature were introduced, instead of the hot exhaust products. Also, the study of Cooper et al [7] showed that such a reduction in Copyright © 1979 by The Combustion Institute Published by Elsevier North Holland, Inc.
the soot concentration could not be obtained if the inlet air was preheated i n the absence of recirculation of diluents, which suggested that the thermal effect of exhaust gases was not a strong factor in reducing the soot formation. The experiments of Sjt~gren [8] have confirmed the findings of Dunn [6] and Cooper et al [7]. Although these experiments have clearly demonstrated the effect of recirculation in reducing smoke formation, the mechanism by which this effect occurs is not at all clear. Dunn [6] indicated that some complex reactions occurring during combustion are responsible for the reduction in soot when exhaust gases are recirculated, whereas Cooper et al. [7], suggested that the balance between the reactions producing condensed soot and the reactions generating gaseous products is altered by the thermal diffusion characteristics of the diluents. The mechanism proposed by Cooper et al [7] contradicts the observation that preheating of air does not produce any significant effect. Also, it does not substantiate the observation that a larger proportion of nitrogen is required than carbon dioxide, in spite of the fact that nitrogen has a higher thermal diffusivity than carbon dioxide. Sj6gren [8] has argued that the
0010-2180/79/020141+ 11 $01.75
142 reduction of extinction velocity of drops in flames, caused by the addition of diluents, is the primary reason for the decrease of soot formation in burning sprays. The mechanism suggested by Sj6gren is based on the premise that droplets burn individually and not in an envelope of gases, which concept itself is currently disputed by various investigators [9, 10]. A recent theoretical study by Chiu and Liu [11 ] has shown that the occurrence of individual drop combustion is not likely in practical sprays. Even if individual droplet burning is possible in sprays, Sj6gren's argument, to be valid, further requires that wake flames of drops behave as premixed flames. The studies of Refs. [12-14], however, have shown that wake flames of drops do not behave always as premixed flames. There is no agreement among various investigators on the effects of exhaust gas recirculation on the flame temperature. Brown et al. [15] have shown that considerable reductions in the amounts of oxides of nitrogen occur when exhaust gases are recirculated, which they attribute to the decrease of flame temperature. However, Dunn's study [6] showed that the refractory walls of the combustion chamber fused as a consequence of exhaust gas recirculation, which he attributed to increased flame temperatures. Since no measurements of spectral radiant emittance of flames are reported by Dunn, it is not possible to determine whether the changes in temperature or concentrations of emitting species caused the increased wall temperatures in his combustor. Thus, the mechanism by which exhaust gas recirculation affects the characteristics of burning sprays is uncertain. Although several studies on the structure and pollutant formation in burning sprays from both pressure-jet atomizers [16-20] and air-blast atomizers [20-23] have recently appeared in the literature, the effects of introducing additional diluents have not yet been diagnostically studied. This paper presents the results of a detailed experimental study directed to provide information on the effects of diluents on the flame length, radiation, and temperature fields of open flames of burning sprays. The experimental results of studies on both pressure-jet and twin-fluid atomizer burners are presented.
SUBRAMANYAM R. GOLLAHALLI EXPERIMENTAL PROCEDURE In experiments with a pressure-jet atomizer, a commercial domestic furnace oil bumer rated at 0.063 l/min and 0.689 MPa with the provision for swirling the primary air was used. This burner was provided with an electric spark igniter located close to the nozzle. The ignition circuit was modified such that the burner could be operated with either continuous ignition or ignition only at the beginning of the tests. The flow rate of primary air drawn by the burner was measured by means of a Calibrated orifice plate mounted on the suction side of the blower. The primary air/fuel ratio was varied by means of a damper provided on the inlet of the blower. In the air-blast atomizer studies, the fuel atomization was achieved by the shearing action between the high-velocity air and the fuel film in a venturi-shaped nozzle. Fuel was forced to the throat of the venturi from a fuel tank under the pressure of bottled nitrogen. Atomization characteristics were controlled by varying the air velocity. A diverging conical section attached to the nozzle allowed the stable operation of the burner over a wide range of air and fuel flow rates. Flow rates of diluents and air were measured by means of calibrated rotameters. Fuel-supply rate was measured by timing a known quantity of fuel flow. Flames were photographed in color to record their lift-off distance, flame length, and appearance. Temperature profiles were measured by means of Pt-Pt 13% Rh thermocouples with 0.25mm dia. bead. Oxygen concentration was determined by sampling gases through a heated microprobe and analyzing them with a gas chromatograph provided with a thermal conductivity detector, using Mol-sieve 5A column. Radiant emittance of the flames was measured with a thermopile placed at such a distance in the direction normal to the axis of the flames that the inverse squarelaw was followed. The fraction of heat release radiated was computed according to the method used earlier for hydrocarbon flames [24, 25]. The size distribution of droplets in the regions near the axis of the flames was measured by the impaction method with magnesium oxide coated slides. Experiments were performed with kerosene
BURNING LIQUID SPRAYS
~i¸
143
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% (a)
(b)
:
(c)
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(d)
Fig. 1. P h o t o g r a p h s o f b u r n i n g sprays o f n u m b e r 2 fuel oil (tex p = 1 s) over a pressure-jet atomizer; (a) and (c) side- and end-views without any additional diluent, (b)
and (d) side- and end-views with CO2 added to primary air (Y = 0.2).
and ASTM number 2 fuel oil. Carbon dioxide and nigrogen (99.9% purity) were used as diluents. The best estimates of uncertainties in flame length, radiation emittance, and temperatures are +-1%, -+2%, and -+1%, respectively.
OBSERVATIONS Pressure-Jet Atomizer Tests Flame Appearance and Length Figure 1 shows the side- and end-view photographs of typical flames over the pressure-jet atomizer, with and without additional diluents in the primary air. Based on color photographs and visual
observations the structure of these flames can be diagrammatically represented as shown in Fig. 2a. In the absence of additional diluents the flames exhibit a distinct four-zone structure. In the near nozzle region a faint blue flame extends from the igniter to about three burner diameters in the axial region. This flame, stabilized in the wake of the injector itself (region 1), appears like a premixed gas flame. This central core is surrounded by a relatively dark annular zone (region 2), which in turn is enveloped by an outer bright yellow flame (region 3). The flame in the region away from the nozzle (region 4) appears orange-red in color. As the amount of primary air is increased, flame in the region 1 becomes more bluish and the
144
SUBRAMANYAM R. GOLLAHALLI
-~ . . . . f
/
FUEL
FUEL " PUMP
NOZZLE
(a)
(b) Fig. 2. Structure of the burning sprays over (a) a pressure-let atomizer (b) an air-blast atomizer.
total visible flame length (axial distance from the burner mouth to the tip of the flame) decreases. When the primary air/fuel ratio becomes more than stoichiometric, the effects of further increasing primary air follow the law of diminishing returns. Even at the condition of flame blow-out a large part of the flame remains yellow. When additional diluents (C02 or N2) are introduced into the primary air, flame in the region 1 initially becomes elongated, but with larger mass fractions of diluent (Ydil > 0.2) it becomes extinguished. This effect is clearly seen in Fig. 1. The hole in the end-view photograph d of the flame with diluent in the primary air is caused by the extinguishment of the flame in the central core, which can also be seen by comparing the side-view photographs a and b in Fig. 1. In addition to this effect on the flame in region 1, the flame in region 3 also turns bluish, with the induction of diluents into the primary stream. Figure 3 shows the effects of adding C02 on flame length over pressure-jet atomizers burning number 2 fuel oil, whereas Fig. 4 compares the effects of the two fuels and two diluents in this study. These figures reveal the following: (i) Flame length decreases with the increase in primary air/fuel ratio. In the absence of diluents, when this ratio is increased by a factor of three,
flame length decreases by a factor of about two. (ii) The effect of diluents is to decrease the flame length and this effect decreases progressivley with the increase in the primary air/fuel ratio. (iii) No significant differences in the trends of the variation of flame length with the quantity of diluent added are noticed between the two fuels (kerosene and number 2 fuel oil) and the diluents (C02 and N2).
Flame Radiation Figures 3 and 4 also show the variation of the fraction (F) of heat release radiated from the flames with the mass fraction of diluents in the primary air stream. These figures indicate that (i) a considerable decrease in the heat radiated from the flame occurs when diluents are added to the primary air stream, (ii) the extent of reduction in F depends upon the primary air/fuel ratio, (iii) number 2 fuel oil flames radiates more than kerosene flame, and (iv) in the absence of diluents, F decreases when the primary air/fuel ratio is increased. Temperature and Oxygen Concentration Profiles Axial and radial profiles of the temperature and oxygen concentration in the flames of number 2
BURNING LIQUID SPRAYS
145
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(Mf/Ma) on
flame length and fraction of heat release radiated in number 2 fuel oil flames over a pressure-jet atomizer. fuel oil over the pressure-jet atomizer, with and without the diluent addition to the primary air, are shown in Fig. 5. It is seen that (i) both axial temperature and oxygen concentration decrease considerably in the near nozzle region when the diluent is added, (ii) there are valleys in the radial temperature and concentration profiles in the near nozzle region (x = 0.1 m) and these valleys flatten when the diluent (Y = 0.35) is added, and (iii) the effects of diluent addition on local temperature and concentration are smaller in the far nozzle region than in the near nozzle region.
Mean Droplet Size The Sauter mean diameter of the droplets in the axial region at x = 100 mm determined by the
impaction method using magnesium oxide coated slides in number 2 fuel oil flames with diluents (C02, Y = 0.35) was 48/a, compared to a value of 30 /~ in the flames without additional diluents in the primary air. Air-Blast Atomizer Tests Appearance and Flame Length Figure 2(b) shows a diagrammatic representation of the structure of the flames from the air-blast atomizer as observed in several color photographs. The regions 1 and 2 in this sketch correspond to bluish and luminous yellow zones which appear similar to premixed and diffusion-controlled gas flames. With the increase in the flow rate of pri-
146
SUBRAMANYAM R. GOLLAHALLI
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BURNING LIQUID SPRAYS
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Fig. 6. Effectsof diluent mass fraction, atomizing gas flow rate (Vg), and nature of diluent on flame length and fraction of heat release radiated in flames of number 2 fuel oil over an air-blast atomizer. mary air, region 2 also turns bluish, and the entire flame eventually becomes blue before it blows out. Figure 6 shows the variation of luminous flame length, measured as the distance from the burner mouth to the tip of the flame, with primary air/fuel ratio, the concentration and type of diluent in atomizing air of number 2 oil spray. In the tests with diluents, the total flow rate of atomizing gas (air + diluent) was kept constant by reducing air flow corresponding to the diluent added, in order to keep the atomization characteristics the same. In the absence of diluents, when the air flow was increased, an increase in the length of bluish zone and a reduction in the total flame length occurred. The addition of diluents to the primary stream, without a significant change in the flow rate of atomizing gas, shows a further increase in the length of the blue zone and a decrease of the total flame length.
F l a m e Radiation
Figure 6 also shows the variation of the fraction of heat release radiated with the mass fraction of diluents (CO 2, N2) in the atomizing gas stream and the flow rate of atomizing gas. As in the flames over the pressure-jet atomizer, increases in the flow rates of primary air and diluents cause considerable reductions in the fraction of heat release that is radiated. The decrease of the radiant emission depends upon the primary air flow rate (Vs). At large Vg the effect of diluent becomes minimal. Temperature and O x y g e n C o n c e n t r a t i o n
Profiles The axial and radial profiles of temperature and oxygen concentration in the number 2 oil flame~ over the air-blast atomizer, with and withoul additional diluent in the primary air stream, are
148
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SUBRAMANYAM R. GOLLAHALLI
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Fig. 7. Axial and radial profiles of temperature and oxygen concentration in number 2 fuel oil flames over an air-blast atomizer with and without diluent addition.
shown in Fig. 7. The axial temperature in the near nozzle region decreases considerably when the diluent is added, whereas the peak temperature remains essentially the same. The radial temperature profile in the near nozzle region develops a large off-axis peak when the diluent is introduced. In contrast, the radial temperature profdes in the far nozzle region are not significantly affected in shape, although the magnitude of temperature drops slightly. The addition of diluent also causes a drop in the oxygen concentration near the nozzle and the flattening of radial profdes.
Mean Doplet Size In this case also, mean droplet size (SMD)in the lxial region of fuel oil flames at x = 20 mm was :'ound to increase to 65 /~ when diluent (CO2, Y = 0.2) was added, from 48 p without any tiluent. Since the total flow of atomizing gas air + diluent) was kept constant, the atomization :haracteristics are expected to remain invariant. -Ience, this change in droplet size is expected to be )fimafily caused by the reduction in evaporation ate in the core.
DISCUSSION The commerical pressure-jet atomizer used in this study yields a hollow cone spray similar to that reported by McCreath and Chigier [16] in their flame structure studies. The air-blast nozzle causes the atomization of fuel by the shearing action of a high-velocity air stream (approximately sonic) and the annular liquid film, and is similar to that used in earlier investigations [21-23]. This is evidenced by the significant resemblance of oxygen concentration and temperature profiles for undiluted flames shown in Figs. 5 and 7 with the data reported for pressure-jet flames [16, 19, 20] and air-blast atomized spray flames [20-23] in the literatuare. Hence, the burners used in this study can be considered satisfactory to investigate the effects of the flow rates of primary air and diluents on the behavior of burning sprays.
Effects of Primary Air-Flow Rate The flow rate of primary air plays a significant role in forming a small, well-mixed reaction zone in the initial region, where the fuel vapors from the evaporating droplets can mix quickly with the pri-
BURNING LIQUID SPRAYS mary air stream crossing the spray sheath. The blue region near the axis, noticed in the color photographs of pressure-jet flames corresponds to this zone. The dark region corresponds to the location of the spray sheath, and the outer yellow region corresponds to the diffusion-controlled reaction zones of evaporated fuel and its pyrolysis products. The dip in the radial temperature profile in the near nozzle region seen in Fig. 5, supported by the data of temperature contours presented in Refs. [16, 19] conforms to the above description of the structure of the pressurejet spray flames. The increase of the extent of blue region and the decreases of yellow region and the total flame length, with the increase of primary air flow rate in the pressure-jet atomizer, indicate that the relative proportions of fuel burning in the form of soot decrease when the primary air flow is increased. Further evidence for this can be seen in the variation of the fraction of heat release radiated with the primary air/fuel ratio (Fig. 3). The decrease of the amount of soot buming, and the consequent reduction in the continuum radiation probably causes the decrease of the heat radiated. In air-blast atomizers, mixing between primary air and the fuel droplets is much stronger than in the case of pressure-jet atomizers because of the difference in the atomization mechanism. Hence, combustion in a large part of the core occurs similar to that in a premixed gas flame, which is manifested by a continuous decrease in the radial temperature profde close to the nozzle (at x = 20 mm in Fig. 7). The decrease of the total flame length and the fraction of heat release radiated with the increase of primary air flow rate are again caused, mostly, by the reduction in the amount of soot burning, as discussed earlier. Effects of Diluents Addition of diluents is seen to result in reductions of flame length and radiation which are accompanied by the lowered oxygen concentration and temperatures in the near nozzle region in both pressure-jet and air-blast atomized sprays. Further-
149 more, in pressure-jet atomized sprays, when diluents are added the radial temperature profde shows a continuous increase from the axial to the peak value, instead of the dip between them seen in the absence of additional diluents. This change in the temperature profdes and the hole in the end-view photograph [Fig. l(d)] indicate that combustion in the core is suppressed when diluents are added. Reduction in the oxygen available in the core, as shown by the concentration proFdes, is the most likely cause for it. Similar changes leading to an increase in the extent of interface combustion are noticed in the temperature and concentration profiles of the air-blast atomized spray. The increases of both primary air and diluent concentration are seen to decrease the flame length, amount of soot burning, and radiation emitted, although they have opposite effects on the oxygen available in the core. This suggests that the mechanisms of their effects are different. The increase of primary air flow rate was found to elongate the blue region near the nozzle, which indicates that the gas phase oxidation reactions increase as a consequence. That would lead to a reduction of the proportion of the fuel that pyrolyzes and forms soot. On the other hand, the addition of diluents reduces the oxygen available in the core and still results in the decrease of soot burning, which suggests that the reduction of pyrolysis and soot-forming reactions is likely to be the more dominant mechanism of their action. Similar effects of diluents on propane gas flames were noticed earlier [5]. SjiSgren [8] suggested that the reduction in the amount of soot produced in burning sprays is caused by the transformation of envelope flames around the individual droplets into wake flames, which burn as premixed flames with oxygen entrained into the wake [11]. The formation ot premixed flames requires that sufficient oxygen be available in the core. Since concentration profiles show that the core becomes oxygen-deficienl when diluents are introduced, the fuel vapors burn at the edge of the core in the form of a diffusio~ flame, rather than as a premixed flame. This i,, substantiated by the peaking of the radial tempera
150 ture profde at the edge of the flame. Since a significant reduction in the heat radiated from the flames occurs when diluents are introduced, it suggests that the heat transferred in the radiant sections could decrease when exhaust gases are recirculated i n boilers. However, the decrease in soot concentration may cause an enhancement in the contributions of wall radiation and convective heat transfer. This effect is a probable factor in the changes in heat transfer pattern observed in industrial boilers with exhaust gas recirculation [26]. Although the combustion conditions in spark-ignition engines have no direct similarity to those of the present study, it is interesting to note that unburnt hydrocarbon emissions and chances of misfiring increase significantly with an increase in exhaust gas recirculation. Hence, introduction of diluents into either spray systems or homogeneous mixture systems necessitates some operational and design modifications to prevent undesirable effects. One such modification is the extended spark duration method developed by Aiman [27] for engines with high exhaust gas recirculation. CONCLUSIONS The present investigation has dealt with the effects of primary air/fuel ratio and additional diluents in primary air on the characteristics of hydrocarbon liquid spray flames from pressure-jet and air-blast atomizers. Variations of appearance of the flames, flame length, fraction of heat release radiated, temperature profdes, and oxygen concentration profiles with the concentration of additional diluents in the primary air have been documented. The results indicate that (i) flame length, soot burning, and radiation emitted from liquid spray flames decrease significantly with an increase in primary air/fuel ratio and the concentration of dfluents in the primary air, (ii) the effects of diluents on flame characteristics are similar in pressure-jet and air-blast atomizers, and (iii) introduction of additional diluents into combustion systems for the purpose of reduction of soot may require operational and design modifications.
SUBRAMANYAM R. GOLLAHALLI
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BURNING LIQUID SPRAYS Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 561. 23. Styles, A. C., and Chigier, N. A., Sixteenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1977, p. 619. 24. Brzustowski, T. A., GoUahaUi, S. R., Kaptein, M., Gupta, M., and Sullivan, H. F., Radiant heating from flares, ASME paper 75-HT-4, 1975. 25. Gollahalli, S. R., and Sullivan, H. F., Effect of pool shape on burning rate, radiation, and flame height of liquid pool fires. Paper presented at the Technical
151 meeting of the Eastern Section of the Combustion Institute. John Hopkins University, Oct. 1974. 26. Breen, B. P., in, Emissions from Continuous Combustion Systems (W. Cornelius and W. G. Agnew, Eds.) Plenum Press, New York, 1972, p. 325. 27. Aiman, W. R., Combust. Sci. Technol. 15:129 (1977).
Received 8 February 1978; revised 6 May 1978