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Effect of injector geometry on the performance of an internally mixed liquid atomizer
Fuel Processing Technology 91 (2010) 1650–1654
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Effect of injector geometry on the performance of an internally mixed liquid atomizer A. Kushari Department of Aerospace Engineering, Indian Institute of Technology, Kanpur, India
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 28 May 2010 Accepted 29 June 2010 Keywords: Atomization Twin-fluid Air-assisted Internally mixed
a b s t r a c t This paper presents the results of an experimental study of the effect of injector's geometry on the performance of an internally mixed, air-assisted, liquid injector. In this type of injector a small amount of air is injected into a liquid stream within the injector. The interaction of the liquid with the atomizing air inside the injector induces atomization. The results presented in this paper show that the size of the droplets produced by the investigated injector decreases with a decrease in the air injection area. This is due to the increase in atomizing air injection velocity that accompanies the decrease in the air injection area, which improves atomization. This study also shows that the droplet sizes decrease with an increase in the injector's length, which is attributed to the increase in total interactive force. © 2010 Elsevier B.V. All rights reserved.
1. Introduction This paper describes the effect of injector geometry on the performance of an internally mixed injector. The objective of this effort is to develop a fuel injector whose spray properties could be varied in a controlled fashion to accommodate various operating conditions [1,2]. For example, it may be desirable to keep the mean droplet size constant while varying the fuel flow rate or keep the fuel flow rate constant while varying the mean droplet size. It is believed that the availability and application of such “smart” injectors would improve the performance of gas turbines and other combustors by providing capabilities for, e.g., reducing emissions and damping combustion instabilities [3–5]. In contrast to single fluid atomizers requiring high pressures to produce fine sprays, twin fluid atomizers can provide a fine spray at relative lower supply pressures [5,6]. Internally mixed atomizers are gaining popularity because of the controllability over the atomization process and the improved quality of atomization provided by them [1,2,7–14]. In such atomizers, low flow rate of marginally pressurized air is introduced into the liquid inside the atomizer. In an internally mixed, air assisted atomizer the atomizing air interacts with the liquid inside the injector and assists in the atomization process. It is believed that two effects induce atomization in such an injector [2]. First, as both the liquid and the air share the same flow passage in the injector, the liquid is restricted to a smaller available flow area. The reduction in flow area accelerates the liquid, thus, increasing its kinetic energy, which induces fine atomization. Second, the relative motion between the air and the liquid phases produces shear forces at their interface leading to the onset of flow instability at the interface resulting in
E-mail address: [email protected]. 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.06.014
ligament formation. Furthermore, the shear force strips liquid droplets from the liquid filaments inducing atomization. The positive aspect of the internally mixed air assisted atomizer is that its atomization characteristics can be controlled [2,13]. Furthermore, internally mixed atomizers need a significantly lower flow rate of atomizing air compared to airblast atomizers [5,6]. Conceptually, these atomizers are similar to effervescent atomizers, which require a small amount of air to produce a very fine spray [6,15–21]. Good operation of an effervescent injector requires the formation of a mixture of air and liquid in a mixing chamber whose characteristics correspond to a bubbly two-phase flow [6,15,16,22–25]. Lately, there has been many studies reported in the literature involving effervescent atomizers over a range of air liquid mass ratio and liquid flow rates [17–21], which have been successfully used for atomizing heavy fuel oils for power generation [18] and water–oil emulsions [19]. Two parameters that control the flow rate of the liquid emerging from the injector and the size of the droplets produced by the injector are the flow rate of the atomizing air and the supply pressure of the liquid. It has been shown experimentally [1,2,13] that the liquid flow rate and the size of the droplets can be controlled independently by changing the air flow rate and the liquid supply pressure simultaneously. In order to predict the performance of the investigated injector under various operating conditions and to develop a design algorithm to guide the design of internally mixed atomizers, a theoretical model [1] was developed that depicted the two-phase air–liquid flow through the injector. The predictions of the developed model [1] suggested an influence of the injector's geometry on the performance of the investigated injector. This influence was expected because the atomization of the liquid in an internally mixed atomizer depends on the interaction between the liquid and the atomizing air inside the injector. The model predicted that the droplet diameters decrease with a decrease in the air injection area and also with an increase in the
A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
injector's length. This predicted dependence of the injector's performance on the injector's geometry encouraged an experimental study to validate the model predictions. This paper describes the experimental effort to study the effect of injector geometry on the performance of the investigated internally mixed injector. Specifically, this paper describes the effect of the air injection area and the injector's length on the performance of the investigated injector.
2. Experimental efforts A cross sectional view of the internally mixed injector investigated in this study is shown in Fig. 1. Liquid entered through the wide opening of a tube that narrowed down into a 1.9 mm diameter tube. Air was radially injected into the liquid flow through several holes upstream of the injector's exit. The air injection holes were provided circumferentially along two rows, 3 mm apart, as shown in Fig. 1. Several injectors, with different geometry, were fabricated and tested to study the effect of injector geometry on the performance of the injector. The geometric details of the injectors investigated in this study are presented in Table 1. The first three injectors (i.e., injectors 1 (4 holes per row at 90° to each other), 2 (3 holes per row at 120° to each other) and 3 (2 holes per row at 180° to each other), see Table 1) were of the same length but had different air injection areas. These three injectors were characterized to study the effect of the air injection area on the performance of the injector. The injectors 4, 5, 6 and 7 had the same geometry of the air injection ports. However, they were of different length and, hence, they were characterized to study the effect of injector's length on the performance of the injector. The effect of the injector's length on the performance of the investigated injector was studied by characterizing seven injectors of different length. However, the results of the characterization of only four injectors (i.e., 4, 5, 6 and 7) are presented in this paper so that the results can the presented distinctly without obscuring each other. Fig. 2 shows a schematic of the experimental setup that had been developed to investigate the performance of this injector. The injector was installed at the top of a 0.91 m high, 0.2 m diameter cylindrical chamber. The chamber was pressurized by an inflow of compressed air and its pressure was varied by throttling a valve in its exhaust line. The chamber was designed to withstand pressures up to 1.37 MPa and had a relief valve that opened at 1.03 MPa to ensure adequate safety. The chamber has three windows at the same elevation that provide optical access to the spray. The angle between two of the windows equals 30° to allow optical access for the transmitter and receiver beams of the Phase Doppler Particle Analyzer (PDPA) system that was used to characterize the spray. The third window provides visual access to the spray. All the windows are circular and 0.127 m in diameter. The chamber is mounted on a traversing mechanism and can be moved along three mutually perpendicular directions with 0.025 mm resolution. During testing, a low velocity air, i.e., v = 1–2 m/s, is introduced into the chamber through a honeycomb structure above the test section to prevent misting of the windows by re-circulating liquid droplets. This air leaves the test chamber through another honeycomb structure at the bottom, just downstream of the test section.
Fig. 1. A schematic of the investigated internally mixed injector.
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Table 1 Descriptions of the tested injectors. Injector number 1 2 3 4 5 6 7
Injector's length (Xle) mm
No. of air holes
Diameter of air holes mm
Total air injection area 10−6 m2
15 15 15 5 20 25 35
8 6 4 6 6 6 6
0.61 0.51 0.36 0.51 0.51 0.51 0.51
2.34 1.225 0.41 1.225 1.225 1.225 1.225
Droplet size and axial droplet velocity were measured downstream of the injector's exit using a TSI® PDPA system. An argon-ion laser that provided a 5 W power green light (i.e., 514.5 μm) was used by the PDPA. A transmitting lens with a focal length of 300 mm and a receiving lens with a focal length of 750 mm were used in the optical path of the PDPA. The receiver was oriented at a 30° forward scatter position with respect to the transmitter to gather light scattered by the droplets. Both the transmitter and the receiver were mounted on stationary, rigid, platforms while the chamber was moved by the traversing mechanism relative to the PDPA. That ensured that any point in the spray could be brought into the intersection of the laser beams. In principle, the PDPA is able to measure droplet sizes in the 0.7 to 220 μm range. However, in practice, when the maximum size to be detected by the PDPA is set to, say, 220 μm, the minimum size that can be detected is 1/35th of the maximum, which is 6.3 μm. The PDPA system used in this study cannot characterize the dense sprays produced near the exit of the injector. Therefore, the droplet size measurements were taken at a distance of 0.0625 m downstream of the injector's exit, where the spray was dilute enough for the PDPA to measure droplet diameters with reasonable accuracy. On the average, the percentage of valid data ranged from 70% to 98% depending on the measurement location and the droplet size. Each data point was obtained using at least 5000 valid measurements. The mass flow rates and supply pressures of both the liquid and the atomizing air were monitored using standard, calibrated, flow meters and pressure gages as shown in Fig. 2. The liquid and the air flow rates were controlled with a pressure regulating valve and a needle valve, respectively. Check valves in the supply lines of both the liquid and the air prevented these flows from entering the supply line of one another. Distilled water from a pressurized tank, pressurized by a nitrogen bottle, was used in these tests. The atomizing air, which was at room
Fig. 2. A schematic of the experimental setup.
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A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
temperature, was obtained from an accumulator that was filled by compressed air from a laboratory compressor. 3. Results and discussions In order to study the effects of injector geometry on the performance of an internally mixed atomizer, several injectors were designed and fabricated (see Table 1) and their sprays were characterized inside the closed tank shown in Fig. 2. Two geometric parameters that affect the performance of these injectors [1] are the total air injection area and the length of the injector (Xle). The total air injection area is calculated by summing the areas of all the air injection holes. The effect of air injection port's area on the performance of an internally mixed atomizer was studied by experimentally determining the characteristics of the sprays produced by injectors 1, 2 and 3, which differ in the number of air injection ports and their areas. Distilled water was supplied from a tank, which was pressurized to 137 kPa, and the test chamber was maintained at atmospheric pressure. The droplet sizes were measured at the centerline of the spray at a distance of 6.25 cm downstream of the injector's exit. For each injector the air to liquid mass ratio (ALR) was varied over a range by changing the air flow rate and the corresponding changes in the liquid flow rate and the Sauter mean diameter (SMD) of the droplets were measured and the results are presented in Figs. 3 and 4. The data presented in Fig. 3 show that the liquid flow rate does not change with the change in the air injection area suggesting that the liquid flow rate is independent of the air injection area. However, for a particular injector, the liquid flow rate decreases with an increase in the ALR. This occurs because the pressure inside the atomizer increases as the air flow rate increases, thus decreasing the liquid flow rate through the injector. Furthermore, an increase in the ALR, due to an increase in the atomizing air flow rate, results in an increase in the area occupied by the air flow. This reduces the cross sectional area through which the liquid can flow and, thus, the liquid flow rate. The data in Fig. 4 show that for a particular injector the spray's SMD decreases with an increase in ALR, which is believed to be caused by two effects [1,2]. First, the increase in the air flow rate is accompanied by an increase in the area occupied by the air, resulting in a reduction in the available flow area for the liquid, which accelerates the liquid flow. This, in turn, increases the kinetic energy of the liquid, resulting in improved atomization. Second, the increase in air flow rate is accompanied by an increase of the air velocity and, thus, the shear force that it exerts upon the liquid, resulting in finer atomization. On the other hand, the liquid flow rate decreases with an increase in ALR (see Fig. 3), which should lead to a decrease in liquid velocity. It seems that the effect of reduction in flow area is predominant at lower ALR's resulting in a fast reduction in liquid
flow rate (as seen in Fig. 3). Lal et al. [26] have shown that the rate of change of discharge coefficient (i.e., the ratio of actual liquid flow area to the total flow area) with ALR for a twin fluid atomizer is inversely proportional to the ALR and hence, at higher values of ALR, the rate of change in liquid flow rate is less sensitive to the changes in ALR. At higher values of ALR, the variation in both the area occupied by liquid (decreasing [26], resulting in an increase in velocity) and the liquid flow rate (decreasing as seen in Fig. 3, resulting in a decrease in velocity) are quite small. Therefore, the velocity of the liquid was either constant or decreased as has been shown earlier [1,2]. The data in Fig. 4 also show that for a particular ALR, a reduction in the air injection area reduces the SMD of the droplets produced. Fig. 5 presents the radial distribution of the droplet SMD produced by injectors 1, 2 and 3 when the ALR was kept constant at 0.087 (i.e., 8.7 percent). It shows that the droplet sizes decrease everywhere in the spray when the air injection area is reduced. The behavior exhibited by the data plotted in Figs. 4 and 5 can be attributed to the fact that the air enters the injector at a higher velocity and, thus, higher kinetic energy, through a smaller area because its mass flow rate is conserved. Since the atomization in internally mixed atomizers is due to the transfer of kinetic energy from the air to the liquid, an increase in the kinetic energy of the incoming atomizing air results in the formation of smaller droplets. The minimum SMD of the droplets produced by injector 1 was 54.4 μm whereas the minimum droplet's SMD from injector 3 was 34.8 μm. Thus, the minimum achievable droplet diameter was reduced by 19.6 μm (i.e., 36%) by a reduction in the air injection area. It should be further noted that in Fig. 5, the SMD increases with the radial distance. Similar behavior has been reported earlier by Kushari et al. [2] for this range of ALR, which can be attributed to the formation
Fig. 3. Dependence of liquid flow rate on ALR for different air injection areas.
Fig. 5. Radial distribution of the droplets SMD for different injectors for an ALR of 0.087.
Fig. 4. Dependence of the droplets SMD on ALR for different air injection areas.
A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
of annular or dispersed two-phase flow inside the atomizer due to the penetration of air to the centre of the tube owing to its high momentum with respect to the liquid momentum. Furthermore, towards the edge of the spray, the SMD of the injector 3 seems to be equal or bigger than that of injector 2. The air injection port diameter in injector 3 is smaller than that of injector 2 and hence, the velocity of air inside the injector 3 at a given ALR is higher. However, the flow rates are same for both the injectors as seen in Fig. 3. Therefore, in order to maintain the conservation of mass, the liquid in the vicinity of the tube wall (i.e., towards the edge of the spray) has to move slower in injector 3 than in injector 2, resulting in larger SMD at those locations for injector 3. Figs. 6, 7 and 8 describe the dependence of the injector's performance upon the injector's length. This was determined by comparing the characteristics of the sprays produced by injectors 4, 5, 6 and 7. Fig. 6 shows that the liquid flow rate decreases slightly with an increase in injector's length. This can be attributed to the fact that the pressure gradient (dP/dx) inside the injector decreases when the injector's length is increased, which reduces the liquid momentum inside the injector resulting in a reduction in liquid flow area and, thus, the liquid flow rate. Furthermore, increased injector length causes a thicker boundary layer at the exit of the injector causing a decrease in discharge coefficient and hence the liquid flow rate. The droplet sizes also decrease as the injector's length increases, as shown in Fig. 7. The minimum droplets SMD produced by injector 4 (i.e., Xle = 5 mm) was 64.1 μm and the minimum droplets SMD from injector 7 (i.e., Xle = 35 mm) was 51.1 μm. Arguably this is due to the fact that the total force exerted by the air on the liquid increases as the injector's length increases because this force acts over a longer distance, resulting in a higher droplet velocity (see Fig. 8) due to better transfer of momentum and energy between the phases. This increase in interactive force results in the formation of smaller droplets. Therefore, for a particular ALR, the droplet diameter decreases with an increase in the injector's length. One interesting feature of the present study is that although in general there is a decrease in SMD with an increase in ALR, when the ALR is bigger than about 0.2, the SMD increases slightly. Similar trends can be seen in Figs. 4 and 7. As has been discussed earlier, the liquid velocity is expected to decrease due to the decrease in flow rate with an increase in ALR and is expected to increase due to a reduction in flow area. Therefore, there are two opposing effects that govern the liquid velocity and hence its kinetic energy. Furthermore, as has been discussed by Lal et al. [26] and Henry and Fauske [27], at these values of ALR, the flow area is quite insensitive to the variations in ALR and therefore, the spray characteristics are expected to be governed by the reduction in flow rate. Looking at the variation in droplet velocity with ALR, presented in Fig. 8, one can see that at ALR's beyond 0.2, the droplet velocity starts to decrease with an increase in ALR, suggesting
Fig. 6. Dependence of the liquid flow rate on ALR for different injector lengths.
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Fig. 7. Dependence of the droplets SMD on ALR for different injector lengths.
the dominance of the effect of the reduction in mass flow rate in these regimes. And the reduction of flow velocity results in an increase in the droplet diameter due to the decrease in liquid kinetic energy. The results reported in this paper are in accordance with the results reported by Jedelsky et al. [21] who have shown a decrease in droplet SMD with an increase in relative area of aeration holes and relative mixing distance. However, the smallest possible diameter reported by Jedelsky et al. [21] was about 30 μm at a relatively higher ALR of 1.0 at 200 kPa pressure. Ramamurthi et al. [17] have reported a minimum droplet SMD of 30 μm at a liquid supply pressure of 300 kPa and Broniarz-Press et al. [20] have reported the minimum SMD of the order of 1 mm. Ferreira et al. [18] have reported the smallest SMD of 20 μm but at much higher supply pressure (600 kPa). Therefore, the minimum SMD of 34.8 μm at a supply pressure of only 137 kPa makes this atomizer quite attractive for practical applications as it is at par with other reported studies.
4. Conclusions The results presented in this paper suggest that the performance of an internally mixed, air-assisted, injector depends on the injector's geometry. Although the liquid flow rate is independent of the air injection area, the size of the droplets produced by the injector decreases with a decrease in the air injection area. The liquid flow rate and the droplet diameter decrease with an increase in the injector's length. It is believed that in all the investigated cases the reductions in the droplets SMD are due to the increase in the forces exerted by the
Fig. 8. Variation of mean droplet velocity with ALR for different injector lengths.
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A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
air flow upon the liquid, which increased the liquid kinetic energy and/or the stripping of droplets off the liquid ligaments. The results presented in this paper suggest that reducing the air injection area and/or increasing the injector's length can modify the performance of the internally mixed injectors. References [1] A. Kushari, Study of an internally mixed liquid injector for active control of atomization process, Ph.D. Thesis, Georgia Institute of Technology, Atlanta, GA, 2000. [2] A. Kushari, Y. Neumeier, O. Israeli, E. Lubarsky, B.T. Zinn, Internally mixed liquid injector for active control of atomization process, J. Propul. Power 17 (4) (2001) 878–882. [3] A.H. Lefebvre, Fuel effects on gas turbine combustion – ignition, stability and combustion efficiency, J. Engg. Gas Turbines Power 107 (1985) 24–37. [4] K.K. Rink, A.H. Lefebvre, Pollutant formation in heterogeneous mixtures of fuel drops and air, J. Propul. 3 (1) (1987) 5–10. [5] A.H. Lefebvre, Gas turbine combustion, Hemisphere, New York, 1983. [6] A.H. Lefebvre, Atomization and Spray, Taylor and Francis, 1989. [7] P.J. Mullinger, N.A. Chigier, The design and performance of internally-mixing multijet twin-fluid atomizers, J. Inst. Fuel 47 (1974) 251–261. [8] T.C. Roesler, A.H. Lefebvre, Studies on aerated liquid atomization, Int. J. Turbo Jet Engines 6 (3–4) (1989) 221–230. [9] M.N. Biswas, Atomization in two-phase critical flow, Proc. 2nd Int. Conf. Liquid Atomization Spray Systems (ICLASS-82), 1982, pp. 145–151. [10] J.M. Chawla, Atomization of liquid employing the low sonic velocity of liquid/gas mixture, Proc. 3rd Int. Conf. Liquid Atomization Spray Systems (ICLASS-85), 1985, LP/1A/5/1-LP/1A/5/7. [11] J.S. Chin, Effervescent atomization and internal mixing air-assisted atomization, Int. J. Turbo Jet Engines 12 (1995) 119–127. [12] J. Karnawat, A. Kushari, Spray evolution in a twin-fluid swirl atomizer, Atomization Sprays 18 (5) (2008) 449–470.
[13] J. Karnawat, A. Kushari, Controlled atomization using a twin-fluid swirl atomizer, Expt. Fluids 41 (2006) 649–663. [14] A. Kufferath, B. Wende, W. Leuckel, Influence of liquid flow conditions on spray characteristics of internal-mixing twin-fluid atomizers, Int. J. Heat Fluid Flow 20 (1999) 513–519. [15] S.D. Sovani, P.E. Sojka, A.H. Lefebvre, Effervescent atomization, Prog. Energy Combust. Sci. 27 (4) (2001) 483–521. [16] J. Li, A.H. Lefebvre, J.R. Rollbuhler, Effervescent Atomizers for Small Gas Turbines, ASME (paper), 1994, 94-GT-495. [17] K. Ramamurthi, U.K. Sarkar, B.N. Raghunandan, Performance characteristics of effervescent atomizers is different flow regimes, Atomization Sprays 19 (1) (2009) 41–56. [18] G. Ferreira, F. Barreras, A. Lozano, J.A. Garcia, E. Lincheta, Effect of inner two- phase flow on the performance of an industrial twin-fluid nozzle with an internal mixing chamber, Atomization Sprays 19 (9) (2009) 873–884. [19] G. Ferreira, J.A. Garcia, F. Barreras, A. Lozano, E. Lincheta, Design optimization of twin-fluid atomizers with an internal mixing chamber for heavy fuel oils, Fuel Processing Technol. 90 (2009) 270–278. [20] L. Broniarz-Press, M. Ochowiak, J. Rozanski, S. Woziwodzki, The atomization of water–oil emulsions, Expt. Therm. Fluid Sci. 33 (2009) 955–962. [21] J. Jedelsky, M. Jicha, J. Slama, J. Otahal, Development of an effervescent atomizer for industrial burners, Energy Fuels 23 (12) (2009) 6121-6120. [22] G.B. Wallis, One Dimensional Two-Phase Flows, McGrew-Hill Publications, 1969. [23] M.E.G. Ferguson, P.T. Spedding, Measurement and prediction of pressure drop in two-phase flow, J. Chem. Tech. Biotechnol. 62 (1995) 262–278. [24] S. Rastonin, Z. Schmidt, D.R. Doty, A review of multiphase flow through chokes, Multiphase Flow in Wells and Pipelines, ASME FED-144, 1992. [25] P.L. Spedding, V.T. Nguyen, Regime maps for air water two-phase flow, Chem. Eng. Sci. (35) (1980) 779–793. [26] S. Lal, A. Kushari, M. Gupta, J.C. Kapoor, S. Maji, Experimental study of an air assisted mist generator, Exp. Therm. Fluid Sci. (2010), doi:10.1016/j. expthermflusci.2010.03.006. [27] R.E. Henry, H.K. Fauske, The two-phase critical flow of one component mixture in nozzles, orifices and short tubes, J. Heat Transfer 520 (2) (1971) 179–187 (Trans. ASME) 93.
Contents lists available at ScienceDirect
Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Effect of injector geometry on the performance of an internally mixed liquid atomizer A. Kushari Department of Aerospace Engineering, Indian Institute of Technology, Kanpur, India
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 28 May 2010 Accepted 29 June 2010 Keywords: Atomization Twin-fluid Air-assisted Internally mixed
a b s t r a c t This paper presents the results of an experimental study of the effect of injector's geometry on the performance of an internally mixed, air-assisted, liquid injector. In this type of injector a small amount of air is injected into a liquid stream within the injector. The interaction of the liquid with the atomizing air inside the injector induces atomization. The results presented in this paper show that the size of the droplets produced by the investigated injector decreases with a decrease in the air injection area. This is due to the increase in atomizing air injection velocity that accompanies the decrease in the air injection area, which improves atomization. This study also shows that the droplet sizes decrease with an increase in the injector's length, which is attributed to the increase in total interactive force. © 2010 Elsevier B.V. All rights reserved.
1. Introduction This paper describes the effect of injector geometry on the performance of an internally mixed injector. The objective of this effort is to develop a fuel injector whose spray properties could be varied in a controlled fashion to accommodate various operating conditions [1,2]. For example, it may be desirable to keep the mean droplet size constant while varying the fuel flow rate or keep the fuel flow rate constant while varying the mean droplet size. It is believed that the availability and application of such “smart” injectors would improve the performance of gas turbines and other combustors by providing capabilities for, e.g., reducing emissions and damping combustion instabilities [3–5]. In contrast to single fluid atomizers requiring high pressures to produce fine sprays, twin fluid atomizers can provide a fine spray at relative lower supply pressures [5,6]. Internally mixed atomizers are gaining popularity because of the controllability over the atomization process and the improved quality of atomization provided by them [1,2,7–14]. In such atomizers, low flow rate of marginally pressurized air is introduced into the liquid inside the atomizer. In an internally mixed, air assisted atomizer the atomizing air interacts with the liquid inside the injector and assists in the atomization process. It is believed that two effects induce atomization in such an injector [2]. First, as both the liquid and the air share the same flow passage in the injector, the liquid is restricted to a smaller available flow area. The reduction in flow area accelerates the liquid, thus, increasing its kinetic energy, which induces fine atomization. Second, the relative motion between the air and the liquid phases produces shear forces at their interface leading to the onset of flow instability at the interface resulting in
E-mail address: [email protected]. 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.06.014
ligament formation. Furthermore, the shear force strips liquid droplets from the liquid filaments inducing atomization. The positive aspect of the internally mixed air assisted atomizer is that its atomization characteristics can be controlled [2,13]. Furthermore, internally mixed atomizers need a significantly lower flow rate of atomizing air compared to airblast atomizers [5,6]. Conceptually, these atomizers are similar to effervescent atomizers, which require a small amount of air to produce a very fine spray [6,15–21]. Good operation of an effervescent injector requires the formation of a mixture of air and liquid in a mixing chamber whose characteristics correspond to a bubbly two-phase flow [6,15,16,22–25]. Lately, there has been many studies reported in the literature involving effervescent atomizers over a range of air liquid mass ratio and liquid flow rates [17–21], which have been successfully used for atomizing heavy fuel oils for power generation [18] and water–oil emulsions [19]. Two parameters that control the flow rate of the liquid emerging from the injector and the size of the droplets produced by the injector are the flow rate of the atomizing air and the supply pressure of the liquid. It has been shown experimentally [1,2,13] that the liquid flow rate and the size of the droplets can be controlled independently by changing the air flow rate and the liquid supply pressure simultaneously. In order to predict the performance of the investigated injector under various operating conditions and to develop a design algorithm to guide the design of internally mixed atomizers, a theoretical model [1] was developed that depicted the two-phase air–liquid flow through the injector. The predictions of the developed model [1] suggested an influence of the injector's geometry on the performance of the investigated injector. This influence was expected because the atomization of the liquid in an internally mixed atomizer depends on the interaction between the liquid and the atomizing air inside the injector. The model predicted that the droplet diameters decrease with a decrease in the air injection area and also with an increase in the
A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
injector's length. This predicted dependence of the injector's performance on the injector's geometry encouraged an experimental study to validate the model predictions. This paper describes the experimental effort to study the effect of injector geometry on the performance of the investigated internally mixed injector. Specifically, this paper describes the effect of the air injection area and the injector's length on the performance of the investigated injector.
2. Experimental efforts A cross sectional view of the internally mixed injector investigated in this study is shown in Fig. 1. Liquid entered through the wide opening of a tube that narrowed down into a 1.9 mm diameter tube. Air was radially injected into the liquid flow through several holes upstream of the injector's exit. The air injection holes were provided circumferentially along two rows, 3 mm apart, as shown in Fig. 1. Several injectors, with different geometry, were fabricated and tested to study the effect of injector geometry on the performance of the injector. The geometric details of the injectors investigated in this study are presented in Table 1. The first three injectors (i.e., injectors 1 (4 holes per row at 90° to each other), 2 (3 holes per row at 120° to each other) and 3 (2 holes per row at 180° to each other), see Table 1) were of the same length but had different air injection areas. These three injectors were characterized to study the effect of the air injection area on the performance of the injector. The injectors 4, 5, 6 and 7 had the same geometry of the air injection ports. However, they were of different length and, hence, they were characterized to study the effect of injector's length on the performance of the injector. The effect of the injector's length on the performance of the investigated injector was studied by characterizing seven injectors of different length. However, the results of the characterization of only four injectors (i.e., 4, 5, 6 and 7) are presented in this paper so that the results can the presented distinctly without obscuring each other. Fig. 2 shows a schematic of the experimental setup that had been developed to investigate the performance of this injector. The injector was installed at the top of a 0.91 m high, 0.2 m diameter cylindrical chamber. The chamber was pressurized by an inflow of compressed air and its pressure was varied by throttling a valve in its exhaust line. The chamber was designed to withstand pressures up to 1.37 MPa and had a relief valve that opened at 1.03 MPa to ensure adequate safety. The chamber has three windows at the same elevation that provide optical access to the spray. The angle between two of the windows equals 30° to allow optical access for the transmitter and receiver beams of the Phase Doppler Particle Analyzer (PDPA) system that was used to characterize the spray. The third window provides visual access to the spray. All the windows are circular and 0.127 m in diameter. The chamber is mounted on a traversing mechanism and can be moved along three mutually perpendicular directions with 0.025 mm resolution. During testing, a low velocity air, i.e., v = 1–2 m/s, is introduced into the chamber through a honeycomb structure above the test section to prevent misting of the windows by re-circulating liquid droplets. This air leaves the test chamber through another honeycomb structure at the bottom, just downstream of the test section.
Fig. 1. A schematic of the investigated internally mixed injector.
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Table 1 Descriptions of the tested injectors. Injector number 1 2 3 4 5 6 7
Injector's length (Xle) mm
No. of air holes
Diameter of air holes mm
Total air injection area 10−6 m2
15 15 15 5 20 25 35
8 6 4 6 6 6 6
0.61 0.51 0.36 0.51 0.51 0.51 0.51
2.34 1.225 0.41 1.225 1.225 1.225 1.225
Droplet size and axial droplet velocity were measured downstream of the injector's exit using a TSI® PDPA system. An argon-ion laser that provided a 5 W power green light (i.e., 514.5 μm) was used by the PDPA. A transmitting lens with a focal length of 300 mm and a receiving lens with a focal length of 750 mm were used in the optical path of the PDPA. The receiver was oriented at a 30° forward scatter position with respect to the transmitter to gather light scattered by the droplets. Both the transmitter and the receiver were mounted on stationary, rigid, platforms while the chamber was moved by the traversing mechanism relative to the PDPA. That ensured that any point in the spray could be brought into the intersection of the laser beams. In principle, the PDPA is able to measure droplet sizes in the 0.7 to 220 μm range. However, in practice, when the maximum size to be detected by the PDPA is set to, say, 220 μm, the minimum size that can be detected is 1/35th of the maximum, which is 6.3 μm. The PDPA system used in this study cannot characterize the dense sprays produced near the exit of the injector. Therefore, the droplet size measurements were taken at a distance of 0.0625 m downstream of the injector's exit, where the spray was dilute enough for the PDPA to measure droplet diameters with reasonable accuracy. On the average, the percentage of valid data ranged from 70% to 98% depending on the measurement location and the droplet size. Each data point was obtained using at least 5000 valid measurements. The mass flow rates and supply pressures of both the liquid and the atomizing air were monitored using standard, calibrated, flow meters and pressure gages as shown in Fig. 2. The liquid and the air flow rates were controlled with a pressure regulating valve and a needle valve, respectively. Check valves in the supply lines of both the liquid and the air prevented these flows from entering the supply line of one another. Distilled water from a pressurized tank, pressurized by a nitrogen bottle, was used in these tests. The atomizing air, which was at room
Fig. 2. A schematic of the experimental setup.
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temperature, was obtained from an accumulator that was filled by compressed air from a laboratory compressor. 3. Results and discussions In order to study the effects of injector geometry on the performance of an internally mixed atomizer, several injectors were designed and fabricated (see Table 1) and their sprays were characterized inside the closed tank shown in Fig. 2. Two geometric parameters that affect the performance of these injectors [1] are the total air injection area and the length of the injector (Xle). The total air injection area is calculated by summing the areas of all the air injection holes. The effect of air injection port's area on the performance of an internally mixed atomizer was studied by experimentally determining the characteristics of the sprays produced by injectors 1, 2 and 3, which differ in the number of air injection ports and their areas. Distilled water was supplied from a tank, which was pressurized to 137 kPa, and the test chamber was maintained at atmospheric pressure. The droplet sizes were measured at the centerline of the spray at a distance of 6.25 cm downstream of the injector's exit. For each injector the air to liquid mass ratio (ALR) was varied over a range by changing the air flow rate and the corresponding changes in the liquid flow rate and the Sauter mean diameter (SMD) of the droplets were measured and the results are presented in Figs. 3 and 4. The data presented in Fig. 3 show that the liquid flow rate does not change with the change in the air injection area suggesting that the liquid flow rate is independent of the air injection area. However, for a particular injector, the liquid flow rate decreases with an increase in the ALR. This occurs because the pressure inside the atomizer increases as the air flow rate increases, thus decreasing the liquid flow rate through the injector. Furthermore, an increase in the ALR, due to an increase in the atomizing air flow rate, results in an increase in the area occupied by the air flow. This reduces the cross sectional area through which the liquid can flow and, thus, the liquid flow rate. The data in Fig. 4 show that for a particular injector the spray's SMD decreases with an increase in ALR, which is believed to be caused by two effects [1,2]. First, the increase in the air flow rate is accompanied by an increase in the area occupied by the air, resulting in a reduction in the available flow area for the liquid, which accelerates the liquid flow. This, in turn, increases the kinetic energy of the liquid, resulting in improved atomization. Second, the increase in air flow rate is accompanied by an increase of the air velocity and, thus, the shear force that it exerts upon the liquid, resulting in finer atomization. On the other hand, the liquid flow rate decreases with an increase in ALR (see Fig. 3), which should lead to a decrease in liquid velocity. It seems that the effect of reduction in flow area is predominant at lower ALR's resulting in a fast reduction in liquid
flow rate (as seen in Fig. 3). Lal et al. [26] have shown that the rate of change of discharge coefficient (i.e., the ratio of actual liquid flow area to the total flow area) with ALR for a twin fluid atomizer is inversely proportional to the ALR and hence, at higher values of ALR, the rate of change in liquid flow rate is less sensitive to the changes in ALR. At higher values of ALR, the variation in both the area occupied by liquid (decreasing [26], resulting in an increase in velocity) and the liquid flow rate (decreasing as seen in Fig. 3, resulting in a decrease in velocity) are quite small. Therefore, the velocity of the liquid was either constant or decreased as has been shown earlier [1,2]. The data in Fig. 4 also show that for a particular ALR, a reduction in the air injection area reduces the SMD of the droplets produced. Fig. 5 presents the radial distribution of the droplet SMD produced by injectors 1, 2 and 3 when the ALR was kept constant at 0.087 (i.e., 8.7 percent). It shows that the droplet sizes decrease everywhere in the spray when the air injection area is reduced. The behavior exhibited by the data plotted in Figs. 4 and 5 can be attributed to the fact that the air enters the injector at a higher velocity and, thus, higher kinetic energy, through a smaller area because its mass flow rate is conserved. Since the atomization in internally mixed atomizers is due to the transfer of kinetic energy from the air to the liquid, an increase in the kinetic energy of the incoming atomizing air results in the formation of smaller droplets. The minimum SMD of the droplets produced by injector 1 was 54.4 μm whereas the minimum droplet's SMD from injector 3 was 34.8 μm. Thus, the minimum achievable droplet diameter was reduced by 19.6 μm (i.e., 36%) by a reduction in the air injection area. It should be further noted that in Fig. 5, the SMD increases with the radial distance. Similar behavior has been reported earlier by Kushari et al. [2] for this range of ALR, which can be attributed to the formation
Fig. 3. Dependence of liquid flow rate on ALR for different air injection areas.
Fig. 5. Radial distribution of the droplets SMD for different injectors for an ALR of 0.087.
Fig. 4. Dependence of the droplets SMD on ALR for different air injection areas.
A. Kushari / Fuel Processing Technology 91 (2010) 1650–1654
of annular or dispersed two-phase flow inside the atomizer due to the penetration of air to the centre of the tube owing to its high momentum with respect to the liquid momentum. Furthermore, towards the edge of the spray, the SMD of the injector 3 seems to be equal or bigger than that of injector 2. The air injection port diameter in injector 3 is smaller than that of injector 2 and hence, the velocity of air inside the injector 3 at a given ALR is higher. However, the flow rates are same for both the injectors as seen in Fig. 3. Therefore, in order to maintain the conservation of mass, the liquid in the vicinity of the tube wall (i.e., towards the edge of the spray) has to move slower in injector 3 than in injector 2, resulting in larger SMD at those locations for injector 3. Figs. 6, 7 and 8 describe the dependence of the injector's performance upon the injector's length. This was determined by comparing the characteristics of the sprays produced by injectors 4, 5, 6 and 7. Fig. 6 shows that the liquid flow rate decreases slightly with an increase in injector's length. This can be attributed to the fact that the pressure gradient (dP/dx) inside the injector decreases when the injector's length is increased, which reduces the liquid momentum inside the injector resulting in a reduction in liquid flow area and, thus, the liquid flow rate. Furthermore, increased injector length causes a thicker boundary layer at the exit of the injector causing a decrease in discharge coefficient and hence the liquid flow rate. The droplet sizes also decrease as the injector's length increases, as shown in Fig. 7. The minimum droplets SMD produced by injector 4 (i.e., Xle = 5 mm) was 64.1 μm and the minimum droplets SMD from injector 7 (i.e., Xle = 35 mm) was 51.1 μm. Arguably this is due to the fact that the total force exerted by the air on the liquid increases as the injector's length increases because this force acts over a longer distance, resulting in a higher droplet velocity (see Fig. 8) due to better transfer of momentum and energy between the phases. This increase in interactive force results in the formation of smaller droplets. Therefore, for a particular ALR, the droplet diameter decreases with an increase in the injector's length. One interesting feature of the present study is that although in general there is a decrease in SMD with an increase in ALR, when the ALR is bigger than about 0.2, the SMD increases slightly. Similar trends can be seen in Figs. 4 and 7. As has been discussed earlier, the liquid velocity is expected to decrease due to the decrease in flow rate with an increase in ALR and is expected to increase due to a reduction in flow area. Therefore, there are two opposing effects that govern the liquid velocity and hence its kinetic energy. Furthermore, as has been discussed by Lal et al. [26] and Henry and Fauske [27], at these values of ALR, the flow area is quite insensitive to the variations in ALR and therefore, the spray characteristics are expected to be governed by the reduction in flow rate. Looking at the variation in droplet velocity with ALR, presented in Fig. 8, one can see that at ALR's beyond 0.2, the droplet velocity starts to decrease with an increase in ALR, suggesting
Fig. 6. Dependence of the liquid flow rate on ALR for different injector lengths.
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Fig. 7. Dependence of the droplets SMD on ALR for different injector lengths.
the dominance of the effect of the reduction in mass flow rate in these regimes. And the reduction of flow velocity results in an increase in the droplet diameter due to the decrease in liquid kinetic energy. The results reported in this paper are in accordance with the results reported by Jedelsky et al. [21] who have shown a decrease in droplet SMD with an increase in relative area of aeration holes and relative mixing distance. However, the smallest possible diameter reported by Jedelsky et al. [21] was about 30 μm at a relatively higher ALR of 1.0 at 200 kPa pressure. Ramamurthi et al. [17] have reported a minimum droplet SMD of 30 μm at a liquid supply pressure of 300 kPa and Broniarz-Press et al. [20] have reported the minimum SMD of the order of 1 mm. Ferreira et al. [18] have reported the smallest SMD of 20 μm but at much higher supply pressure (600 kPa). Therefore, the minimum SMD of 34.8 μm at a supply pressure of only 137 kPa makes this atomizer quite attractive for practical applications as it is at par with other reported studies.
4. Conclusions The results presented in this paper suggest that the performance of an internally mixed, air-assisted, injector depends on the injector's geometry. Although the liquid flow rate is independent of the air injection area, the size of the droplets produced by the injector decreases with a decrease in the air injection area. The liquid flow rate and the droplet diameter decrease with an increase in the injector's length. It is believed that in all the investigated cases the reductions in the droplets SMD are due to the increase in the forces exerted by the
Fig. 8. Variation of mean droplet velocity with ALR for different injector lengths.
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air flow upon the liquid, which increased the liquid kinetic energy and/or the stripping of droplets off the liquid ligaments. The results presented in this paper suggest that reducing the air injection area and/or increasing the injector's length can modify the performance of the internally mixed injectors. References [1] A. Kushari, Study of an internally mixed liquid injector for active control of atomization process, Ph.D. Thesis, Georgia Institute of Technology, Atlanta, GA, 2000. [2] A. Kushari, Y. Neumeier, O. Israeli, E. Lubarsky, B.T. Zinn, Internally mixed liquid injector for active control of atomization process, J. Propul. Power 17 (4) (2001) 878–882. [3] A.H. Lefebvre, Fuel effects on gas turbine combustion – ignition, stability and combustion efficiency, J. Engg. Gas Turbines Power 107 (1985) 24–37. [4] K.K. Rink, A.H. Lefebvre, Pollutant formation in heterogeneous mixtures of fuel drops and air, J. Propul. 3 (1) (1987) 5–10. [5] A.H. Lefebvre, Gas turbine combustion, Hemisphere, New York, 1983. [6] A.H. Lefebvre, Atomization and Spray, Taylor and Francis, 1989. [7] P.J. Mullinger, N.A. Chigier, The design and performance of internally-mixing multijet twin-fluid atomizers, J. Inst. Fuel 47 (1974) 251–261. [8] T.C. Roesler, A.H. Lefebvre, Studies on aerated liquid atomization, Int. J. Turbo Jet Engines 6 (3–4) (1989) 221–230. [9] M.N. Biswas, Atomization in two-phase critical flow, Proc. 2nd Int. Conf. Liquid Atomization Spray Systems (ICLASS-82), 1982, pp. 145–151. [10] J.M. Chawla, Atomization of liquid employing the low sonic velocity of liquid/gas mixture, Proc. 3rd Int. Conf. Liquid Atomization Spray Systems (ICLASS-85), 1985, LP/1A/5/1-LP/1A/5/7. [11] J.S. Chin, Effervescent atomization and internal mixing air-assisted atomization, Int. J. Turbo Jet Engines 12 (1995) 119–127. [12] J. Karnawat, A. Kushari, Spray evolution in a twin-fluid swirl atomizer, Atomization Sprays 18 (5) (2008) 449–470.
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