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Combustion of partially premixed spray jets
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 30 (2005) 2021–2028
www.elsevier.com/locate/proci
Combustion of partially premixed spray jets Masato Mikami*, Kazuhiro Yamamoto, Osamu Moriue, Naoya Kojima Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
Abstract Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called ‘‘partially premixed spray.’’ Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Spray combustion; Partially premixed spray; Group combustion; Burning behavior
1. Introduction In a Bunsen type burner, a premixed flame cone is established for a lean or stoichiometric mixture within an appropriate velocity range. By adding gaseous fuel to the mixture, the flame transits from a premixed flame to double flames with an internal premixed flame and an external diffusion flame [1] as illustrated in Fig. 1. A further addition of the fuel to the mixture eventually *
Corresponding author. Fax: +81 836 85 9101. E-mail address: [email protected] (M. Mikami).
causes the mixture equivalence ratio to exceed the rich flammability limit. Then, the internal premixed flame disappears, and only the external diffusion flame remains as shown in Fig. 1. Such a flame is called a ‘‘partially premixed flame.’’ A partially premixed flame is practically used to enhance the combustion intensity of diffusion flame [2]. Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from outside of the spray to complete diffusion combustion. Such a rich premixed spray is called a ‘‘partially
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.034
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M. Mikami et al. / Proceedings of the Combustion Institute 30 (2005) 2021–2028
mentally [13]. Multiple flame structures were found by these researches. However, combustion of a partially premixed spray jet has not been studied. The combustion characteristics of a spray depend largely on the droplet diameter [3]. However, few systematic studies of droplet diameter effects on the spray combustion have been conducted. Recently, Mikami et al. [14] reported droplet diameter effects on the spray combustion in a strained flow field. Droplet diameter effects were discussed through the droplet inertia effect and the vaporization effect. Such effects have not been reported on partially premixed spray combustion. In the present research, burning behaviors of partially premixed spray jets were experimentally studied with a newly developed spray burner. A fuel spray and a carrier gas flow consisting of oxygen and nitrogen were injected into quiescent air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the fuel flow rate and the total carrier gas flow rate. Burning behaviors were investigated for different atomization conditions and overall equivalence ratios of the spray jet.
2. Experimental apparatus and procedure Figure 2 shows a schematic of a partially premixed spray burner that was newly developed Fig. 1. Schematics of combustion of premixed gas and premixed spray jets with different overall equivalence ratios of the jet. / is the equivalence ratio of premixed gas, /L and /R are, respectively, the equivalence ratio at the lean and rich flammability limits, and /SJ is the overall equivalence ratio of spray jet (i.e., fuel spray/ carrier gas mixture).
premixed spray.’’ Partially premixed sprays have not been studied systematically although they are of practical importance. On the other hand, many studies have been conducted on the combustion of non-premixed sprays [3–6] and lean premixed sprays [7,8]. For example, Onuma and Ogasawara [4] experimentally investigated the structure of non-premixed spray flames. They showed group burning behaviors of non-premixed sprays. In lean-premixed spray combustion, the spray was divided into several droplet clusters during burning [7,8]. The whole spray combustion was found to be the collection of several cluster combustion. Akamatsu et al. [8] experimentally investigated group combustion characteristics of clusters. The partially premixed spray exists between the lean premixed spray and the non-premixed spray as illustrated in Fig. 1. Partially premixed spray combustion in counter flow fields was investigated numerically [9,10], analytically [11,12], and experi-
Fig. 2. Schematic of the partially premixed spray jet burner.
M. Mikami et al. / Proceedings of the Combustion Institute 30 (2005) 2021–2028
for the present research. A fuel spray with an oxidizer stream diluted by nitrogen was issued into a quiescent atmosphere to establish the flame. The inner diameter of the exit hole of the burner was 16.4 mm. A twin-fluid atomizer (1/8J type, Spraying Systems) was placed 15.0 mm below the exit of the burner, from which the liquid fuel and atomizing gas were injected. The outer diameter of the atomizer was 12.7 mm. The atomization condition was controlled by the atomizing gas flow rate Qatom [14]. As shown in Fig. 2, a supplementary gas was supplied to the spray so that the total carrier gas flow rate (i.e., atomizing gas flow rate Qatom and supplementary gas flow rate Qsup) could be kept constant, and therefore, the overall equivalence ratio /SJ of the spray jet (i.e., fuel spray/carrier gas mixture) could be kept constant, even if the atomization condition was varied. The total carrier gas flow rate was 5.0 L/min. The Reynolds number was about 430 based on the exit hole diameter of the burner, and the mean flow velocity estimated was based on the measured total carrier gas flow rate and exit hole area. n-Decane was used as a liquid fuel. The flow rate QF of the liquid fuel was 7.2 mL/min for all the conditions. The definition of the overall equivalence ratio /SJ of the spray jet was /SJ ¼
ðQF ql Þ=fðQatom þ Qsup Þqg g ; mF W F =ðmO2 W O2 =Y O2 Þ
ð1Þ
where ql, qg, mF, mO2 , WF, W O2 , and Y O2 are the liquid fuel density, carrier gas density, stoichiometric coefficient for fuel, stoichiometric coefficient for oxygen, molecular weight of fuel, molecular weight of oxygen, and oxygen mass fraction in the carrier gas, respectively. When air (oxygen mole fraction X O2 ¼ 0:21) was used as a carrier gas, the overall equivalence ratio /SJ of the spray jet was 12. In the present study, /SJ was varied with the oxygen concentration of the carrier gas. For oxygen mole fraction X O2 of 0.16, 0.30, and 0.40, /SJ were 16, 8.6, and 6.4, respectively. The atomization condition was varied with the flow rate Qatom of atomizing gas. The droplet size distribution was measured by a phase Doppler particle analyzer (PDPA) (2D-PDPA/RSA, TSI). The droplet sampling completed if the number of sample droplets attained 10,000 or the sampling duration attained 1 min. The transmitter and the receiver of PDPA were traversed two-dimensionally both in horizontal and vertical directions by stepping-motor driven stages. Figure 3 shows the arithmetic mean diameter d10 and Sauter mean diameter d32 of the fuel spray measured near the burner exit without combustion for different atomizing gas flow rates Qatom. The mean droplet diameters decreased with increasing Qatom. This shows that, in the present burner, the atomization condition of the fuel was controlled by Qatom for a constant fuel flow rate QF and a constant carrier
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Fig. 3. Variations of mean droplet diameters, d10 and d32, for different atomizing gas flow rates Qatom. The droplet diameter was measured near the exit of the burner without combustion. The inset is the droplet diameter distribution for Qatom = 3.0 L/min.
gas flow rate, i.e., for a constant overall equivalence ratio /SJ of the spray jet. The inset in Fig. 3 shows the droplet diameter distribution for Qatom of 3.0 L/min. The present spray was a polydisperse spray that had relatively large droplets with diameter of larger than 100 lm. Burning behaviors were basically photographed using a digital video camera (DCRPC100, SONY). The framing rate was 30 fps, and the exposure time was 1/30 s. 3. Results and discussion 3.1. Burning behaviors for different atomization conditions Direct photographs of burning sprays are shown for different atomization conditions in Fig. 4. Since spray flames showed flickering, a
Fig. 4. Direct photographs of burning sprays for different atomization conditions (/SJ = 12).
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photograph of a flame whose instantaneous height was close to the time-averaged value was picked up for each condition. The burning behaviors were affected by the atomization condition. When the atomizing gas flow rate Qatom was 1.0 L/min, yellow luminosity was observed in most parts of the group flame. As Qatom was increased, the yellow luminous area decreased, and blue flame area increased near the base of the flame as shown in Fig. 4. For Qatom higher than 4.0 L/min, especially for Qatom = 5.0 L/min, the flame attachment was unstable. Part of the flame base was sometimes away from the burner exit. If the oxygen concentration of the carrier gas was increased, the flame attachment became stable. The flame height decreased with increasing Qatom for larger Qatom than 2.0 L/min. The flame height for Qatom = 1.0 L/min was smaller than that for Qatom = 2.0 L/min. For Qatom = 1.0 L/min, many droplets were observed to fly off the yellow luminous group flame without burning. The existence of a large number of such droplets caused a smaller group flame. Since the present spray was a polydisperse spray, the spray had relatively large droplets that passed through the group flame due to large inertia and longer vaporization time [14]. If the droplet is too large to heat up sufficiently to vaporize vigorously even in a hot region of the flame, the droplet will fly off the group flame without burning. For Qatom P 2.0 L/min, some droplets were observed to burn in the form of the single droplet combustion outside the group flame although unburnt droplets were still observed. More droplets were found to burn in the form of single droplet combustion for larger Qatom. Next, the flame shape of the present spray is discussed in view of the group combustion of droplet cloud. Chiu and Liu [15] showed that the burning mode of a spherical droplet cloud could be classified through the group combustion number G. By neglecting the convective effect on the droplet vaporization, the group combustion number is reduced to G ¼ 2pndR2 ;
ð2Þ
where n, d, and R are the droplet number density, the droplet diameter, and the droplet cloud radius, respectively [5]. Chen and Gomez [5] discussed group combustion characteristics of nonpremixed spray jets using the group combustion number of Eq. (2) in which the flame radius was used as R instead of the droplet cloud radius. In the present spray, the group combustion number of Eq. (2), based on the Sauter mean diameter as a representative droplet diameter and the flame radius instead of the droplet cloud radius, was of the order of 10. Such a spray is categorized in the external group combustion [15]. As shown in Fig. 4, the present sprays were surrounded by an exter-
nal group flame although single droplet combustion was observed in part due to polydispersity of the spray. This observation seems to correspond to the prediction of the group combustion theory. In group combustion theory, oxygen diffuses into spray from the ambient gas to form an external group flame for high G sprays and an internal group flame for low G sprays [16]. In the present case, however, oxygen was added to the spray jet. Combustion of such a partially premixed spray should be different from the group combustion of non-premixed sprays. The classification of the spray combustion through the group combustion number G cannot be directly applied to the present partially premixed spray combustion. Effects of oxygen addition to the spray jet on group burning behaviors were found in the lower part of the burning spray. As shown in Fig. 5, an internal flame was observed inside the external group flame for Qatom = 1.0 and 2.0 L/min. For Qatom P 3.0 L/min, the internal flame did not appear. Spray combustion with the internal flame seems similar to combustion with an internal flame of gaseous rich mixtures as illustrated in Fig. 1. The internal flames observed for Qatom = 1.0 and 2.0 L/ min are expected to be premixed flames. The appearance of the combustion mode with the internal flame is examined in detail in the next subsection. 3.2. Combustion modes with and without internal flame As can be seen in Fig. 5, an internal flame was observed for Qatom = 1.0 and 2.0 L/min but did not appear for Qatom = 3.0 L/min. Under these conditions, the fuel spray and air were premixed prior to combustion. The overall equivalence ratio /SJ of the spray jet issued from the burner exit was 12. Although the overall equivalence ratio /SJ was quite large, a local equivalence ratio in the gas phase depended on the vaporization of droplets. Figure 5 suggests that for Qatom = 1.0 and 2.0 L/min, a local equivalence ratio before the internal flame attained unity and that a premixed flame was established inside the external group diffusion flame. For Qatom = 3.0 L/min, an internal flame was not observed. Since a mean droplet diameter for Qatom = 3.0 L/min was smaller than that for Qatom = 2.0 L/min as shown in Fig. 3, local equivalence ratio was larger than that for Qatom = 2.0 L/ min and was conceivably out of the flammability range. If a local equivalence ratio comes within the flammability range by addition of oxygen to the spray jet for Qatom = 3.0 L/min, an internal flame is expected to appear. Figure 6 shows direct photographs showing lower parts of spray flames for different oxygen mole fractions X O2 of the carrier gas. The overall equivalence ratio of the spray jet was 8.6 for
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Fig. 5. Direct photographs of the lower part of the burning sprays for different atomization conditions (/SJ = 12).
Fig. 6. Direct photographs of the lower part of the burning sprays for different oxygen concentrations of the carrier gas. (Qatom = 3.0 L/min).
X O2 ¼ 0:30 and 6:4 for X O2 ¼ 0:40. Although the internal flame was not observed for X O2 ¼ 0:21, the internal flame appeared if the oxygen concentration of the carrier gas was increased. This corresponds to the above-mentioned expectation and suggests that the internal flame was a premixed flame. A discussion on the premixed-like character of the internal flame is made here. Continillo and Sirignano [9] numerically showed a double structure of counter flow premixed spray flames and reported that flames with both premixed-like and diffusionlike character were found. The first flame on the spray side had a predominant diffusion character, but some prevaporization occurred. A second, pure diffusion flame stabilized approximately at the stagnation plane. The first flame with a premixed-like character in counter flow premixed spray flame conceivably corresponds to the internal flame of the partially premixed spray jet. The overall equivalence ratio of the spray/air mixture was unity in [9], which is much smaller than that of the present rich premixed spray jet. The present internal flame should have a more premixed-like character than the first flame in [9]. Figure 7 shows intensified high-speed images of OH emission from a burning partially premixed spray. The image interval is 1/4500s. As can be seen in Fig. 7, the OH emission intensity of the internal flame was spatially non-uniform, and the internal flame oscillated. The non-uniformity
Fig. 9. Direct photographs of burning sprays, spatial distributions of droplet number measured per one minute by PDPA, and a mean droplet diameter d10 for different combustion modes with and without an internal flame (/SJ = 12).
in the OH emission is considered caused by nonuniform droplet spatial distribution. As shown in Fig. 3, the present spray was a polydisperse spray. The droplet spatial distribution was nonuniform in nature. The internal flame was mostly supported by vaporization of fine droplets. Relatively large droplets passed through the internal flame. Since the local equivalence ratio at the surface of an n-decane droplet is much larger than unity during vaporization, the passage of a droplet should cause a droplet-scaled non-uniformity in OH emission. As explained by Sornek et al. [17], the turbulent flow fluctuation leads to nonuniformity in the droplet special distribution. An extreme case of such spatial non-uniformity of droplets is a collection of droplet clusters. If a droplet cluster whose local equivalence ratio exceeds the rich flammability limit passes through the internal flame, the reaction zone deforms in
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Fig. 8. Combustion mode map of the partially premixed spray.
Fig. 7. Intensified high-speed images of OH emission from a burning partially premixed spray for Qatom = 3.0 L/min. The image interval is 1/4500 s.
a cluster scale. Figure 7 suggests that the present case locates between these two cases: the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame. As mentioned above, the combustion mode changed with the atomizing gas flow rate Qatom and the oxygen mole fraction X O2 of the carrier gas. In the preset study, atomization conditions, e.g., Sauter mean droplet diameter d32, were controlled by atomizing gas flow rate Qatom. The overall equivalence ratio /SJ of the spray jet was controlled by the oxygen mole fraction X O2 of the carrier gas. A combustion mode map of the present partially premixed spray jet is shown in d32–/SJ plane (Fig. 8). As the mean droplet diameter was increased for the same overall equivalence ratio /SJ of the spray jet, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. As the overall equivalence ratio /SJ of the spray jet was decreased under the same atomization condition, the same transition occurred. If the atomization is enhanced, the extreme limit is the combustion of the spatially uniform gaseous mixture shown in Fig. 1. As the mean droplet diameter increases, the degree of the spatial non-uniformity of fuel concentration increases. This suggests that such a spatial non-uniformity extends the range of the overall equivalence ratio of the spray jet within which the internal flame appears inside the external group flame.
If the overall equivalence ratio /SJ is sufficiently small, like a lean premixed spray, what happens to the burning behavior? If the flammable mixture is not formed only through the vaporization of fine droplets, the continuous internal premixed flame is not established any more. Since flammable mixtures exist only around droplet clusters in such a case, the spray is expected to burn in a collection of the droplet cluster combustion. Such burning behaviors were observed in lean premixed sprays [7,8]. It is desirable to clarify the transition from the partially premixed spray combustion with the internal flame to the droplet cluster combustion of lean premixed spray in future studies. 3.3. Effects of internal flame The appearance of the internal flame should affect burning behaviors in a post-premixedflame zone. The spatial distributions of the droplet number measured per 1 min by PDPA and the arithmetic mean droplet diameter d10 are shown in Fig. 9 for the case with the internal flame (Qatom = 2.0 L/min) and that without it (Qatom = 3.0 L/min). The PDPA measurement areas are also shown in Fig. 9. For Qatom = 2.0 L/min, the droplet number in the post-internal flame zone was smaller than that in the same area for Qatom = 3.0 L/min. The spray for Qatom = 2.0 L/min originally included larger droplets, which vaporize more slowly, than that for Qatom = 3.0 L/min. The mean droplet diameter in the post-internal flame zone for Qatom = 2.0 L/min was larger than that in the same area for Qatom = 3.0 L/min. These results suggest that the heat released from the internal premixed flame enhanced droplet vaporization in the post-internal flame zone, even in the spray core region. Without the internal flame, the spray
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core region should be maintained at a relatively low temperature. Only droplets near the spray outer region vaporize actively by heating from the external group flame. Next, the effect of the internal flame on the flame height is discussed. Figure 10 shows dependences of the flame height on the mean droplet diameter d32 for different overall equivalence ratios /SJ of the spray jet. Since the flame height varied with time due to flickering, flame heights taken from 10 consecutive video images were plotted for each condition in Fig. 10. For the case with d32 = 50 lm, the atomizing gas flow rate Qatom was 5.0 L/min and for the case with d32 = 53 lm, the atomizing gas flow rate
Fig. 10. Dependences of flame height on a mean droplet diameter for different overall equivalence ratios of the spray jet.
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Qatom was 4.0 L/min. The average flame height for Qatom = 5.0 L/min was much smaller than that for Qatom = 4.0 L/min although the mean droplet diameter d32 was close to each other. This is because effects of turbulence generated near the twin-fluid atomizer by the atomizing gas flow on the flame height were remarkable for Qatom = 5.0 L/min. For the case with d32 = 104 lm, the existence of a large number of droplets flying off the group flame without burning caused a smaller group flame as explained above. Therefore, dependences of the flame height on the mean droplet diameter d32 are discussed except for these two atomization conditions. As shown in Figs. 10A and B, the average flame height increased with the mean droplet diameter without the internal flame. This is because the droplet vaporization time is roughly proportional to the square of the droplet diameter. However, the average flame height was less dependent on the mean droplet diameter with the internal flame as shown in Figs. 10C and D. These results support the assumption that the heat from the internal flame enhanced droplet vaporization in the post-internal flame zone. If the droplets vaporize completely in passing the internal flame, the flame height becomes independent of the initial droplet diameter because the external flame becomes a gaseous diffusion flame in such a case. The internal premixed flame also plays a role in supplying the combustion products to the postpremixed-flame zone. This results in that the oxygen diffusion might be prevented by the product supply. However, effects of the product supply were not observed in the present results.
Fig. 11. Schematics of partially premixed spray jet combustion with and without internal premixed flame.
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4. Conclusions Burning behaviors of partially premixed sprays were experimentally studied with a newly developed spray burner. The overall equivalence ratio of the spray jet was set quite larger than unity to establish partially premixed spray combustion. Two combustion modes with and without an internal flame were observed. A schematic of partially premixed sprays is illustrated in Fig. 11. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame. In future studies, it is desirable to clarify the transition from the partially premixed spray combustion with the internal flame to the droplet cluster combustion of a lean premixed spray if the overall equivalence ratio of the spray jet is decreased.
Acknowledgments We thank Mr. Yuta Kikuchi and Mr. Keita Nakamoto for their valuable help in experiments. This research was partly subsidized by Grant-inAid for Young Scientists (B), 15760133, 2003,
from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] A.G. Gaydon, H.G. Wolfhard, Flames, fourth ed. Chapman and Hall, London, 1979, pp. 10–11. [2] Y. Mizutani, Combustion Engineering (in Japanese). Morikita Press, Tokyo, 1989, p. 76. [3] G.M. Faeth, Prog. Energy Combust. Sci 9 (1/2) (1983) 1–76. [4] Y. Onuma, M. Ogasawara, Proc. Combust. Inst. 15 (1975) 453–465. [5] G. Chen, A. Gomez, Combust. Flame 110 (3) (1997) 392–404. [6] A.N. Karpetis, A. Gomez, Combust. Flame 121 (1) (2000) 1–23. [7] K. Nakabe, Y. Mizutani, T. Hirano, S. Tanimura, Combust. Flame 74 (1) (1988) 39–51. [8] F. Akamatsu, Y. Mizutani, M. Katsuki, S. Tsushima, Y.D. Cho, Proc. Combust. Inst. 26 (1996) 1723–1730. [9] G. Continillo, W.A. Sirignano, Combust. Flame 81 (1990) 325–340. [10] E. Gutheil, Combust. Theory Model. 5 (2001) 131–145. [11] J.B. Greenberg, N. Sarig, Combust. Flame 104 (4) (1996) 431–459. [12] J.B. Greenberg, N. Sarig, Proc. Combust. Inst. 26 (1996) 1705–1711. [13] S.C. Li, Prog. Energy Combust. Sci. 23 (1997) 303–347. [14] M. Mikami, S. Miyamoto, N. Kojima, Proc. Combust. Inst. 29 (2002) 593–599. [15] H.H. Chiu, T.M. Liu, Combust. Sci. Technol. 17 (1977) 127–142. [16] H.H. Chiu, H.Y. Kim, E.J. Croke, Proc. Combust. Inst. 19 (1982) 971–980. [17] R.J. Sornek, R. Dobashi, T. Hirano, Proc. Combust. Inst. 28 (2000) 1055–1062.
Comment Julian Tishkoff, Air Force Office of Scientific Research, USA. The richer flames that you studied appear pale blue, whereas the leaner, but still rich flames show high luminosity. Can you comment on the temperature and fuel concentration behavior that would explain this difference in appearance. Reply. Since the internal premixed flame supplies heat to the post internal flame zone, the temperature
in the post internal flame zone should be higher and the local mixture richer, due to enhancement of droplet vaporization, than those in the combustion mode without the internal premixed flame. This is the reason why the flames in the combustion mode with the internal premixed flame show high luminosity, whereas the flames in the combustion mode without the internal premixed flame appear pale blue.
Combustion Institute
Proceedings of the Combustion Institute 30 (2005) 2021–2028
www.elsevier.com/locate/proci
Combustion of partially premixed spray jets Masato Mikami*, Kazuhiro Yamamoto, Osamu Moriue, Naoya Kojima Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan
Abstract Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called ‘‘partially premixed spray.’’ Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Spray combustion; Partially premixed spray; Group combustion; Burning behavior
1. Introduction In a Bunsen type burner, a premixed flame cone is established for a lean or stoichiometric mixture within an appropriate velocity range. By adding gaseous fuel to the mixture, the flame transits from a premixed flame to double flames with an internal premixed flame and an external diffusion flame [1] as illustrated in Fig. 1. A further addition of the fuel to the mixture eventually *
Corresponding author. Fax: +81 836 85 9101. E-mail address: [email protected] (M. Mikami).
causes the mixture equivalence ratio to exceed the rich flammability limit. Then, the internal premixed flame disappears, and only the external diffusion flame remains as shown in Fig. 1. Such a flame is called a ‘‘partially premixed flame.’’ A partially premixed flame is practically used to enhance the combustion intensity of diffusion flame [2]. Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from outside of the spray to complete diffusion combustion. Such a rich premixed spray is called a ‘‘partially
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.034
2022
M. Mikami et al. / Proceedings of the Combustion Institute 30 (2005) 2021–2028
mentally [13]. Multiple flame structures were found by these researches. However, combustion of a partially premixed spray jet has not been studied. The combustion characteristics of a spray depend largely on the droplet diameter [3]. However, few systematic studies of droplet diameter effects on the spray combustion have been conducted. Recently, Mikami et al. [14] reported droplet diameter effects on the spray combustion in a strained flow field. Droplet diameter effects were discussed through the droplet inertia effect and the vaporization effect. Such effects have not been reported on partially premixed spray combustion. In the present research, burning behaviors of partially premixed spray jets were experimentally studied with a newly developed spray burner. A fuel spray and a carrier gas flow consisting of oxygen and nitrogen were injected into quiescent air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the fuel flow rate and the total carrier gas flow rate. Burning behaviors were investigated for different atomization conditions and overall equivalence ratios of the spray jet.
2. Experimental apparatus and procedure Figure 2 shows a schematic of a partially premixed spray burner that was newly developed Fig. 1. Schematics of combustion of premixed gas and premixed spray jets with different overall equivalence ratios of the jet. / is the equivalence ratio of premixed gas, /L and /R are, respectively, the equivalence ratio at the lean and rich flammability limits, and /SJ is the overall equivalence ratio of spray jet (i.e., fuel spray/ carrier gas mixture).
premixed spray.’’ Partially premixed sprays have not been studied systematically although they are of practical importance. On the other hand, many studies have been conducted on the combustion of non-premixed sprays [3–6] and lean premixed sprays [7,8]. For example, Onuma and Ogasawara [4] experimentally investigated the structure of non-premixed spray flames. They showed group burning behaviors of non-premixed sprays. In lean-premixed spray combustion, the spray was divided into several droplet clusters during burning [7,8]. The whole spray combustion was found to be the collection of several cluster combustion. Akamatsu et al. [8] experimentally investigated group combustion characteristics of clusters. The partially premixed spray exists between the lean premixed spray and the non-premixed spray as illustrated in Fig. 1. Partially premixed spray combustion in counter flow fields was investigated numerically [9,10], analytically [11,12], and experi-
Fig. 2. Schematic of the partially premixed spray jet burner.
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for the present research. A fuel spray with an oxidizer stream diluted by nitrogen was issued into a quiescent atmosphere to establish the flame. The inner diameter of the exit hole of the burner was 16.4 mm. A twin-fluid atomizer (1/8J type, Spraying Systems) was placed 15.0 mm below the exit of the burner, from which the liquid fuel and atomizing gas were injected. The outer diameter of the atomizer was 12.7 mm. The atomization condition was controlled by the atomizing gas flow rate Qatom [14]. As shown in Fig. 2, a supplementary gas was supplied to the spray so that the total carrier gas flow rate (i.e., atomizing gas flow rate Qatom and supplementary gas flow rate Qsup) could be kept constant, and therefore, the overall equivalence ratio /SJ of the spray jet (i.e., fuel spray/carrier gas mixture) could be kept constant, even if the atomization condition was varied. The total carrier gas flow rate was 5.0 L/min. The Reynolds number was about 430 based on the exit hole diameter of the burner, and the mean flow velocity estimated was based on the measured total carrier gas flow rate and exit hole area. n-Decane was used as a liquid fuel. The flow rate QF of the liquid fuel was 7.2 mL/min for all the conditions. The definition of the overall equivalence ratio /SJ of the spray jet was /SJ ¼
ðQF ql Þ=fðQatom þ Qsup Þqg g ; mF W F =ðmO2 W O2 =Y O2 Þ
ð1Þ
where ql, qg, mF, mO2 , WF, W O2 , and Y O2 are the liquid fuel density, carrier gas density, stoichiometric coefficient for fuel, stoichiometric coefficient for oxygen, molecular weight of fuel, molecular weight of oxygen, and oxygen mass fraction in the carrier gas, respectively. When air (oxygen mole fraction X O2 ¼ 0:21) was used as a carrier gas, the overall equivalence ratio /SJ of the spray jet was 12. In the present study, /SJ was varied with the oxygen concentration of the carrier gas. For oxygen mole fraction X O2 of 0.16, 0.30, and 0.40, /SJ were 16, 8.6, and 6.4, respectively. The atomization condition was varied with the flow rate Qatom of atomizing gas. The droplet size distribution was measured by a phase Doppler particle analyzer (PDPA) (2D-PDPA/RSA, TSI). The droplet sampling completed if the number of sample droplets attained 10,000 or the sampling duration attained 1 min. The transmitter and the receiver of PDPA were traversed two-dimensionally both in horizontal and vertical directions by stepping-motor driven stages. Figure 3 shows the arithmetic mean diameter d10 and Sauter mean diameter d32 of the fuel spray measured near the burner exit without combustion for different atomizing gas flow rates Qatom. The mean droplet diameters decreased with increasing Qatom. This shows that, in the present burner, the atomization condition of the fuel was controlled by Qatom for a constant fuel flow rate QF and a constant carrier
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Fig. 3. Variations of mean droplet diameters, d10 and d32, for different atomizing gas flow rates Qatom. The droplet diameter was measured near the exit of the burner without combustion. The inset is the droplet diameter distribution for Qatom = 3.0 L/min.
gas flow rate, i.e., for a constant overall equivalence ratio /SJ of the spray jet. The inset in Fig. 3 shows the droplet diameter distribution for Qatom of 3.0 L/min. The present spray was a polydisperse spray that had relatively large droplets with diameter of larger than 100 lm. Burning behaviors were basically photographed using a digital video camera (DCRPC100, SONY). The framing rate was 30 fps, and the exposure time was 1/30 s. 3. Results and discussion 3.1. Burning behaviors for different atomization conditions Direct photographs of burning sprays are shown for different atomization conditions in Fig. 4. Since spray flames showed flickering, a
Fig. 4. Direct photographs of burning sprays for different atomization conditions (/SJ = 12).
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photograph of a flame whose instantaneous height was close to the time-averaged value was picked up for each condition. The burning behaviors were affected by the atomization condition. When the atomizing gas flow rate Qatom was 1.0 L/min, yellow luminosity was observed in most parts of the group flame. As Qatom was increased, the yellow luminous area decreased, and blue flame area increased near the base of the flame as shown in Fig. 4. For Qatom higher than 4.0 L/min, especially for Qatom = 5.0 L/min, the flame attachment was unstable. Part of the flame base was sometimes away from the burner exit. If the oxygen concentration of the carrier gas was increased, the flame attachment became stable. The flame height decreased with increasing Qatom for larger Qatom than 2.0 L/min. The flame height for Qatom = 1.0 L/min was smaller than that for Qatom = 2.0 L/min. For Qatom = 1.0 L/min, many droplets were observed to fly off the yellow luminous group flame without burning. The existence of a large number of such droplets caused a smaller group flame. Since the present spray was a polydisperse spray, the spray had relatively large droplets that passed through the group flame due to large inertia and longer vaporization time [14]. If the droplet is too large to heat up sufficiently to vaporize vigorously even in a hot region of the flame, the droplet will fly off the group flame without burning. For Qatom P 2.0 L/min, some droplets were observed to burn in the form of the single droplet combustion outside the group flame although unburnt droplets were still observed. More droplets were found to burn in the form of single droplet combustion for larger Qatom. Next, the flame shape of the present spray is discussed in view of the group combustion of droplet cloud. Chiu and Liu [15] showed that the burning mode of a spherical droplet cloud could be classified through the group combustion number G. By neglecting the convective effect on the droplet vaporization, the group combustion number is reduced to G ¼ 2pndR2 ;
ð2Þ
where n, d, and R are the droplet number density, the droplet diameter, and the droplet cloud radius, respectively [5]. Chen and Gomez [5] discussed group combustion characteristics of nonpremixed spray jets using the group combustion number of Eq. (2) in which the flame radius was used as R instead of the droplet cloud radius. In the present spray, the group combustion number of Eq. (2), based on the Sauter mean diameter as a representative droplet diameter and the flame radius instead of the droplet cloud radius, was of the order of 10. Such a spray is categorized in the external group combustion [15]. As shown in Fig. 4, the present sprays were surrounded by an exter-
nal group flame although single droplet combustion was observed in part due to polydispersity of the spray. This observation seems to correspond to the prediction of the group combustion theory. In group combustion theory, oxygen diffuses into spray from the ambient gas to form an external group flame for high G sprays and an internal group flame for low G sprays [16]. In the present case, however, oxygen was added to the spray jet. Combustion of such a partially premixed spray should be different from the group combustion of non-premixed sprays. The classification of the spray combustion through the group combustion number G cannot be directly applied to the present partially premixed spray combustion. Effects of oxygen addition to the spray jet on group burning behaviors were found in the lower part of the burning spray. As shown in Fig. 5, an internal flame was observed inside the external group flame for Qatom = 1.0 and 2.0 L/min. For Qatom P 3.0 L/min, the internal flame did not appear. Spray combustion with the internal flame seems similar to combustion with an internal flame of gaseous rich mixtures as illustrated in Fig. 1. The internal flames observed for Qatom = 1.0 and 2.0 L/ min are expected to be premixed flames. The appearance of the combustion mode with the internal flame is examined in detail in the next subsection. 3.2. Combustion modes with and without internal flame As can be seen in Fig. 5, an internal flame was observed for Qatom = 1.0 and 2.0 L/min but did not appear for Qatom = 3.0 L/min. Under these conditions, the fuel spray and air were premixed prior to combustion. The overall equivalence ratio /SJ of the spray jet issued from the burner exit was 12. Although the overall equivalence ratio /SJ was quite large, a local equivalence ratio in the gas phase depended on the vaporization of droplets. Figure 5 suggests that for Qatom = 1.0 and 2.0 L/min, a local equivalence ratio before the internal flame attained unity and that a premixed flame was established inside the external group diffusion flame. For Qatom = 3.0 L/min, an internal flame was not observed. Since a mean droplet diameter for Qatom = 3.0 L/min was smaller than that for Qatom = 2.0 L/min as shown in Fig. 3, local equivalence ratio was larger than that for Qatom = 2.0 L/ min and was conceivably out of the flammability range. If a local equivalence ratio comes within the flammability range by addition of oxygen to the spray jet for Qatom = 3.0 L/min, an internal flame is expected to appear. Figure 6 shows direct photographs showing lower parts of spray flames for different oxygen mole fractions X O2 of the carrier gas. The overall equivalence ratio of the spray jet was 8.6 for
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Fig. 5. Direct photographs of the lower part of the burning sprays for different atomization conditions (/SJ = 12).
Fig. 6. Direct photographs of the lower part of the burning sprays for different oxygen concentrations of the carrier gas. (Qatom = 3.0 L/min).
X O2 ¼ 0:30 and 6:4 for X O2 ¼ 0:40. Although the internal flame was not observed for X O2 ¼ 0:21, the internal flame appeared if the oxygen concentration of the carrier gas was increased. This corresponds to the above-mentioned expectation and suggests that the internal flame was a premixed flame. A discussion on the premixed-like character of the internal flame is made here. Continillo and Sirignano [9] numerically showed a double structure of counter flow premixed spray flames and reported that flames with both premixed-like and diffusionlike character were found. The first flame on the spray side had a predominant diffusion character, but some prevaporization occurred. A second, pure diffusion flame stabilized approximately at the stagnation plane. The first flame with a premixed-like character in counter flow premixed spray flame conceivably corresponds to the internal flame of the partially premixed spray jet. The overall equivalence ratio of the spray/air mixture was unity in [9], which is much smaller than that of the present rich premixed spray jet. The present internal flame should have a more premixed-like character than the first flame in [9]. Figure 7 shows intensified high-speed images of OH emission from a burning partially premixed spray. The image interval is 1/4500s. As can be seen in Fig. 7, the OH emission intensity of the internal flame was spatially non-uniform, and the internal flame oscillated. The non-uniformity
Fig. 9. Direct photographs of burning sprays, spatial distributions of droplet number measured per one minute by PDPA, and a mean droplet diameter d10 for different combustion modes with and without an internal flame (/SJ = 12).
in the OH emission is considered caused by nonuniform droplet spatial distribution. As shown in Fig. 3, the present spray was a polydisperse spray. The droplet spatial distribution was nonuniform in nature. The internal flame was mostly supported by vaporization of fine droplets. Relatively large droplets passed through the internal flame. Since the local equivalence ratio at the surface of an n-decane droplet is much larger than unity during vaporization, the passage of a droplet should cause a droplet-scaled non-uniformity in OH emission. As explained by Sornek et al. [17], the turbulent flow fluctuation leads to nonuniformity in the droplet special distribution. An extreme case of such spatial non-uniformity of droplets is a collection of droplet clusters. If a droplet cluster whose local equivalence ratio exceeds the rich flammability limit passes through the internal flame, the reaction zone deforms in
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Fig. 8. Combustion mode map of the partially premixed spray.
Fig. 7. Intensified high-speed images of OH emission from a burning partially premixed spray for Qatom = 3.0 L/min. The image interval is 1/4500 s.
a cluster scale. Figure 7 suggests that the present case locates between these two cases: the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame. As mentioned above, the combustion mode changed with the atomizing gas flow rate Qatom and the oxygen mole fraction X O2 of the carrier gas. In the preset study, atomization conditions, e.g., Sauter mean droplet diameter d32, were controlled by atomizing gas flow rate Qatom. The overall equivalence ratio /SJ of the spray jet was controlled by the oxygen mole fraction X O2 of the carrier gas. A combustion mode map of the present partially premixed spray jet is shown in d32–/SJ plane (Fig. 8). As the mean droplet diameter was increased for the same overall equivalence ratio /SJ of the spray jet, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. As the overall equivalence ratio /SJ of the spray jet was decreased under the same atomization condition, the same transition occurred. If the atomization is enhanced, the extreme limit is the combustion of the spatially uniform gaseous mixture shown in Fig. 1. As the mean droplet diameter increases, the degree of the spatial non-uniformity of fuel concentration increases. This suggests that such a spatial non-uniformity extends the range of the overall equivalence ratio of the spray jet within which the internal flame appears inside the external group flame.
If the overall equivalence ratio /SJ is sufficiently small, like a lean premixed spray, what happens to the burning behavior? If the flammable mixture is not formed only through the vaporization of fine droplets, the continuous internal premixed flame is not established any more. Since flammable mixtures exist only around droplet clusters in such a case, the spray is expected to burn in a collection of the droplet cluster combustion. Such burning behaviors were observed in lean premixed sprays [7,8]. It is desirable to clarify the transition from the partially premixed spray combustion with the internal flame to the droplet cluster combustion of lean premixed spray in future studies. 3.3. Effects of internal flame The appearance of the internal flame should affect burning behaviors in a post-premixedflame zone. The spatial distributions of the droplet number measured per 1 min by PDPA and the arithmetic mean droplet diameter d10 are shown in Fig. 9 for the case with the internal flame (Qatom = 2.0 L/min) and that without it (Qatom = 3.0 L/min). The PDPA measurement areas are also shown in Fig. 9. For Qatom = 2.0 L/min, the droplet number in the post-internal flame zone was smaller than that in the same area for Qatom = 3.0 L/min. The spray for Qatom = 2.0 L/min originally included larger droplets, which vaporize more slowly, than that for Qatom = 3.0 L/min. The mean droplet diameter in the post-internal flame zone for Qatom = 2.0 L/min was larger than that in the same area for Qatom = 3.0 L/min. These results suggest that the heat released from the internal premixed flame enhanced droplet vaporization in the post-internal flame zone, even in the spray core region. Without the internal flame, the spray
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core region should be maintained at a relatively low temperature. Only droplets near the spray outer region vaporize actively by heating from the external group flame. Next, the effect of the internal flame on the flame height is discussed. Figure 10 shows dependences of the flame height on the mean droplet diameter d32 for different overall equivalence ratios /SJ of the spray jet. Since the flame height varied with time due to flickering, flame heights taken from 10 consecutive video images were plotted for each condition in Fig. 10. For the case with d32 = 50 lm, the atomizing gas flow rate Qatom was 5.0 L/min and for the case with d32 = 53 lm, the atomizing gas flow rate
Fig. 10. Dependences of flame height on a mean droplet diameter for different overall equivalence ratios of the spray jet.
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Qatom was 4.0 L/min. The average flame height for Qatom = 5.0 L/min was much smaller than that for Qatom = 4.0 L/min although the mean droplet diameter d32 was close to each other. This is because effects of turbulence generated near the twin-fluid atomizer by the atomizing gas flow on the flame height were remarkable for Qatom = 5.0 L/min. For the case with d32 = 104 lm, the existence of a large number of droplets flying off the group flame without burning caused a smaller group flame as explained above. Therefore, dependences of the flame height on the mean droplet diameter d32 are discussed except for these two atomization conditions. As shown in Figs. 10A and B, the average flame height increased with the mean droplet diameter without the internal flame. This is because the droplet vaporization time is roughly proportional to the square of the droplet diameter. However, the average flame height was less dependent on the mean droplet diameter with the internal flame as shown in Figs. 10C and D. These results support the assumption that the heat from the internal flame enhanced droplet vaporization in the post-internal flame zone. If the droplets vaporize completely in passing the internal flame, the flame height becomes independent of the initial droplet diameter because the external flame becomes a gaseous diffusion flame in such a case. The internal premixed flame also plays a role in supplying the combustion products to the postpremixed-flame zone. This results in that the oxygen diffusion might be prevented by the product supply. However, effects of the product supply were not observed in the present results.
Fig. 11. Schematics of partially premixed spray jet combustion with and without internal premixed flame.
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4. Conclusions Burning behaviors of partially premixed sprays were experimentally studied with a newly developed spray burner. The overall equivalence ratio of the spray jet was set quite larger than unity to establish partially premixed spray combustion. Two combustion modes with and without an internal flame were observed. A schematic of partially premixed sprays is illustrated in Fig. 11. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame. In future studies, it is desirable to clarify the transition from the partially premixed spray combustion with the internal flame to the droplet cluster combustion of a lean premixed spray if the overall equivalence ratio of the spray jet is decreased.
Acknowledgments We thank Mr. Yuta Kikuchi and Mr. Keita Nakamoto for their valuable help in experiments. This research was partly subsidized by Grant-inAid for Young Scientists (B), 15760133, 2003,
from the Ministry of Education, Science, Sports and Culture, Japan.
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Comment Julian Tishkoff, Air Force Office of Scientific Research, USA. The richer flames that you studied appear pale blue, whereas the leaner, but still rich flames show high luminosity. Can you comment on the temperature and fuel concentration behavior that would explain this difference in appearance. Reply. Since the internal premixed flame supplies heat to the post internal flame zone, the temperature
in the post internal flame zone should be higher and the local mixture richer, due to enhancement of droplet vaporization, than those in the combustion mode without the internal premixed flame. This is the reason why the flames in the combustion mode with the internal premixed flame show high luminosity, whereas the flames in the combustion mode without the internal premixed flame appear pale blue.