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Experimental studies on air-assisted impinging jet atomization
International Journal of Multiphase Flow 57 (2013) 88–101
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International Journal of Multiphase Flow j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j m u l fl o w
Experimental studies on air-assisted impinging jet atomization Madan Mohan Avulapati ⇑, Ravikrishna Rayavarapu Venkata Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 11 March 2013 Received in revised form 24 June 2013 Accepted 25 July 2013 Available online 7 August 2013 Keywords: Air-assisted impinging jet atomization Spray characterization PDIA Twin-fluid atomization
a b s t r a c t In the present study, a novel air-assisted impinging jet atomization is demonstrated. A configuration in which a gas jet is directed on to the impinging point of two liquid jets is used to improve the atomization. The effect of liquid properties such as viscosity and surface tension, angle between liquid jets and gas injection orifice diameter on spray characteristics has been experimentally studied. Backlit imaging and particle/droplet imaging and analysis techniques are utilized to characterize the sprays. The experimental results indicate that the effect of liquid viscosity is significant on the liquid sheet break up formed by the impinging jets. However, surface tension does not affect the spray structure significantly in this mode of atomization. At low liquid jet velocity, the prompt mode of atomization is observed where as atomization occurs in classical mode at higher liquid jet velocity. Results showed that variation in the angle between liquid jets do not affect the breakup phenomenon significantly. The spray angle is computed by finding the angle between the lines joining the impinging point and spray edge at an axial distance of 15 mm downstream of the impinging point from the ensemble-averaged data over 100 spray images. It was observed that effect of liquid jets impinging angle on the spray angle is higher at higher liquid velocity. Higher viscosity liquids exhibit lower spray angles. Droplet size measurements indicate a radial variation in the spray. An overall Sauter Mean Diameter (SMD) value is obtained by combining the droplet statistics at all radial locations at a fixed axial location. A very interesting trend is that the SMD is constant beyond a critical Gas to Liquid Ratio (GLR) and momentum ratio for a large variation in liquid viscosity and surface tension. This observation has important ramifications for fuel flexible systems. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Liquid atomization is an important process in many applications such as IC engines, gas turbines, spray-painting, and chemical reactions. Various liquid atomization strategies are employed as reported in the literature to obtain desired performance. Twinfluid atomization is one such method where the gas energy is used to assist liquid atomization. Air-assisted atomization and airblast atomization are examples of twin-fluid atomization methods, in which kinetic energy of air is used to aid liquid breakup. The fundamental difference between air-assisted and airblast atomization relates to the quantity of air used (Lefebvre, 1989). Air-assisted atomization uses a relatively less amount of air at high pressure while relatively large quantity of air at low pressure is used in airblast atomization. These methods have been used in the literature extensively to atomize various kinds of liquids (Lefebvre, 1989; Ashgriz, 2011). Various designs of air-assist and airblast atomizers used in the literature are discussed in a review article by Lefebvre ⇑ Corresponding author. Tel.: +91 8022932352. E-mail addresses: [email protected] (M.M. Avulapati), [email protected] (R. Rayavarapu Venkata). 0301-9322/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2013.07.007
(2000). In most of these studies, either a liquid sheet is blasted with air jets, or gas is allowed to mix with the liquid inside the atomizer to form a spray. These methods require a large amount of gas to achieve droplet sizes of the order of 50 lm. Beck et al. (1991) achieved a Sauter Mean Diameter (SMD) of 75 lm with water at a gas-to-liquid ratio (GLR) of the order of 1.0. Experimental studies on swirl airblast atomizers by Feras et al. (2010) showed that an SMD of the order 50 lm can be achieved at a GLR of 8. On the other hand, impinging jet atomization has been studied in applications to rocket engines and chemical processes due to its good atomization and mixing characteristics (Dombrowski and Hooper, 1964; Ibrahim and Przekwas, 1991; Bailardi et al., 2010; Yang et al., 2012). Bremond and Villermaux (2006) studied the dynamics of impinging jet atomization mechanism and effect of impinging angle on the liquid sheet formation and resulting droplet size using water and ethanol as liquids. Choo and Kang (2003) studied the effect of impinging angle on droplet velocities. Bailardi et al. (2010) conducted experiments with various liquids and identified seven different breakup patterns and corresponding Reynolds and Weber numbers. Bush and Hasha (2004) also studied various patterns produced by impingement of laminar liquid jets. Ren and Marshall (2003) studied the sheet breakup at high liquid
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jet Weber numbers. Impinging jet experiments with water by Shen et al. (1997) showed that a jet velocity of the order of 12.5 m/s was required to obtain an SMD in the range of 300 lm. Studies by Lai et al. (1999) showed that an SMD of 50 lm can be achieved with impingement liquid jets with a velocity of the order of 40 m/s. At such a high velocity, the radial spread of the spray is higher which results in droplet impingement on the walls. In the present study, a novel strategy of combining the effect of impinging jets and air assisted atomization is used to achieve smaller droplets at low liquid injection velocities. In this method of atomization, a liquid sheet is formed using two impinging liquid jets, and an air jet is directed on to this sheet to aid the atomization process. The present study focuses on the effect of liquid properties, specifically viscosity and surface tension, angle between the liquid jets, and gas injection orifice diameter on spray characteristics such as spray structure, spray angle and the droplet sizes. According to Lefebvre (1989), there exist two fundamental atomization modes in case of air blast/air-assist atomizers, viz. classical mode and prompt mode of atomization. The classic mode of atomization is a mechanism where small disturbances, either within or on the surface of a liquid jet or sheet promote the formation of waves that eventually lead to disintegration into ligaments and then droplets. The mechanism is called prompt atomization when the breakup takes place very rapidly. Under these conditions, the jet or sheet has no time to develop a wavy structure, but is immediately torn into fragments by its vigorous interaction with the surrounding air. The spray structure images and droplet sizing data are analyzed to find the conditions at which these processes occur.
2. Experimental setup The experimental setup consists of two parts, (i) liquid and gas supply system to the atomizer and (ii) optical setup to measure spray characteristics. In the supply system, high pressure nitrogen from a gas bottle is regulated to the gas reservoir tank and a part of it is used to compress the liquid in the reservoir. The liquid and gas flow rates are controlled using needle valves in their respective lines. The gas flow rate is measured using a thermal mass flow meter while the liquid flow is measured using a gear flow meter. Pressure gages are placed closer to the atomizer for injection pressure measurements. The spray is injected in a quiescent ambient condition. The optical setup for spray characteristic measurements is based on backlit imaging method. In this setup, a pulsed Nd:YAG laser along with a fluorescent diffuser is used as a light source to provide back-illumination for the spray along with a CCD camera with a resolution of 2048 2048 pixels to record spray images. A laser pulse duration of 10 ns is used for the experiments. A schematic of the experimental setup is shown in Fig. 1. In the present study, two types of image based measurements are performed, one to study the spray structure and the other for droplet size measurements. For the spray structure measurements, images span a distance from the exit of the orifice to 80-mm downstream. The relevant portion of the spray is then imaged and analyzed using the using Particle/Droplet Imaging Analysis (PDIA) technique for droplet size measurements. Specifically, this method involves the use of a long distance microscope (Questar: QM1). The atomizer configuration used in the current study has two identical liquid injection orifices and a gas injection orifice as shown in Fig. 2. Two different gas orifices of diameter 0.76-mm and 1.1-mm are used while the liquid injection orifices of the diameter 0.76-mm each. The length of liquid injection orifice is about 50 mm which gives an L/D ratio of 65 and ensures fully developed flow profile at the exit. The angle between the liquid jets is varied between 60° and 120°. The gas jet is always placed above the impinging point of liquid jets making equal angles with the
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both liquid jets. The exit of the gas jet is placed about 10 mm above the impinging point of the liquid jet. The free length of liquid jet, which is defined as the distance from the exit of the liquid orifice to the point of impingement is maintained at about 8 mm. An image-based, non intrusive droplet size measurement technique, Particle/Droplet Imaging Analysis (PDIA) is used for droplet size measurements. In this method, back-lit spray images obtained in a region typically 5 mm 5 mm and analyzed using image processing algorithms. Significant improvements to this technique have been reported by Kim and Kim (1994) and Koh et al. (2001). This technique has been used extensively for particle/droplet size measurements in recent times due to developments in computational algorithms and technological improvements in digital imaging equipment (Blaisot and Yon 2005; Esmail et al. 2010). Use of short duration laser illumination with pulse-width of the order 10 ns has extended the applicability of this method to high velocity diesel sprays (Blaisot and Yon 2005) and gasoline sprays (Esmail et al., 2010). Studies by Kashdan et al. (2004) and Esmail et al. (2010) showed that PDIA compares well with Phase Doppler Anemometry (PDA). Also, PDIA has additional advantages of providing particle shape information and characterizing non-spherical droplets accurately (Kashdan et al., 2004; Lee and Kim, 2003). Since PDIA is a planar imaging technique, the depth-of-focus (DOF) for larger droplets is higher compared to that for smaller droplets. According to Kim and Kim (1994), the depth of focus varies linearly with droplet size. A DOF correction can be applied to droplet statistics to minimize the error due to variation in depth of focus (Kashdan et al., 2004). To study the effect of liquid properties on this mode of atomization, a range of liquids with variation in viscosity and surface tension have been used. Water and glycerol are mixed in different proportions to get the variation in viscosity while the variation in surface tension is achieved by mixing water and ethanol. The properties of the various liquids used in the study at room temperature are shown in Table 1. A rotational viscometer is used to measure the viscosity of the liquids while the surface tension is measured using Du-Nouy ring type tensiometer. The accuracy of the instruments is verified by measuring the properties of standard fluids viz. water and ethanol. Densities of the liquids are calculated using the linear mixing rule. The uncertainty in the measurement of spray cone angle is estimated by varying the threshold value used in the image processing algorithm. The uncertainty in the spray cone with threshold is of the order ±5% of the mean value. The accuracy of the PDIA technique and the processing was evaluated by performing droplet sizing measurements on a stream of monodisperse droplets created by a mono-disperse droplet generator (Artium Technologies Inc.: MDG 100). The pre-set value of the Sauter Mean Diameter (SMD) in the droplet generator was 135 lm. Using the PDIA technique, the value of SMD obtained was 136.8 lm with an arithmetic mean diameter of 136.7 lm. Thus, the accuracy of the technique in measuring the SMD can be observed to be within 1% for the mono-disperse droplets studied. For the spray, the uncertainty is estimated to be <±5% (which is the cumulative error considering precision, accuracy and repeatability).
3. Results and discussion To access the merit of the proposed atomization strategy, initially, a comparison of the conventional external mixing atomization with the proposed method is conducted. The spray images at similar gas and liquid flow rates obtained with both the configurations are shown in Fig. 3. With the conventional method, an unbroken liquid jet with surface instabilities is observed. In the
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Fig. 1. Schematic of the experimental setup.
Fig. 2. Schematic of the atomizer configuration.
proposed configuration, the liquid jets breakup completely and result in a well atomized spray. The following sub-sections present details of the spray structure and droplet sizes. 3.1. Spray structure 3.1.1. Effect of liquid properties The spray structure images are taken using the back-lit direct imaging method. When the two liquid jets impinge on each other in the absence of air injection, they form a liquid sheet in the plane making a right angle to both the liquid jets. The plane in which the liquid jets are present is called the ‘plane of jets’ while the plane in which a sheet is formed is termed the ‘plane of sheet’. In the present study, images are taken mainly in the plane of sheet. The effect of liquid properties, angle between the liquid jets, gas flow rate and the gas orifice diameter on the spray structure are studied.
Fig. 4 shows the structure of the spray using water as liquid at different gas mass flow rates and liquid jet velocities. The corresponding Gas to Liquid Ratio (GLR) values are specified on each image. Gas to Liquid Ratio (GLR) is defined as the ratio of gas mass injected to the liquid mass injected. At a liquid jet velocity of 1.8 m/s, in the absence of gas injection, the two liquid jets impinge and form a sheet which is bounded by a rim. A stream of ligaments and droplets are produced at the bottom end of the sheet due to impingement of the two arms of the rim. This mode of break up is termed as ‘closed rim’ mode by Bailardi et al. (2010). Increasing the liquid jet velocity to 3.7 m/s increases the sheet width. The mode of sheet breakup shifted to ‘open rim mode’ where the lower part of liquid sheet breaks up due to Kelvin–Helmholtz-type instabilities (Bailardi et al., 2010). The shedding of droplets and ligaments from the edge of the liquid sheet is observed at this condition. Increasing the liquid velocity further to 5.5 m/s makes the liquid sheets break up rapidly due to the impact waves created by the impinging of the liquid jets. At this condition, the mode of Table 1 Properties of liquids used in the study. Liquid
Density (kg/ m3)
Viscosity (mPa s)
Surface tension (mN/m)
Water Water–glycerol (50:50) Water–glycerol (30:70) Water–glycerol (20:80) Water–ethanol (90:10) Water–ethanol (75:25) Ethanol
998 1129
1 5
72 67.6
1182
16
66.7
1208
39
66.1
978
1
55.1
944
1
37.9
780
1
22.3
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Fig. 3. Comparison of the proposed impinging jet air-assisted atomization strategy with the conventional method at similar gas and liquid flow rates.
Fig. 4. Spray structure for water at various GLRs (listed on the images).
breakup is similar to the ‘Ruffled sheet ligaments mode ‘described by Bailardi et al. (2010). A similar observation is made at a higher liquid velocity. Introducing a small amount of gas flow (6 grams per minute (gpm)) directed towards the liquid jets impinging point is observed to have a positive effect on the breakup of the liquid sheet. It was observed that the presence of gas jet eliminates the sheet formation at a liquid jet velocity of 1.8 m/s, while the liquid sheet is reduced significantly in terms of width at a liquid jet velocity of 3.7 m/s. This is due to the fact that the impact of gas jet on liquid jets reduces the momentum of the liquid jets at the impinging point and hence reduces the radial expansion of the liquid sheet. However, at liquid jet velocities of 5.5 m/s and 7.3 m/s, there is a reduced effect on the breakup except that the spray is confined to a smaller width when compared to the situation in the absence of gas flow. Increasing the gas flow rate further to 12 gpm showed a complete breakup of the liquid jets into a well atomized spray. Liquid sheet formation is not observed up to 5.5 m/s liquid jet velocity. In terms of the GLR, for a fixed gas flow rate, increasing the GLR showed improvement in the atomization. However, for a
fixed GLR, improved atomization is observed at a higher gas flow rate. Instantaneous images of the spray using a water–glycerol (50:50) mixture of viscosity 5 mPa s are shown Fig. 5. In the absence of gas jet, the sheet formation is observed to be similar to that of water. The rim bounding the liquid sheet is distinctly visible in this case. However, as the liquid velocity increases, perforations start to appear and the growth of these perforations eventually lead to the breakup of the liquid sheet. Unlike with water, a clear liquid sheet is observed even at a liquid jet velocity of 7.3 m/s. Akin to a water spray, introducing the gas jet improves the overall atomization. This improvement in the atomization is clear even at higher liquid velocity. Increasing the liquid viscosity further to 16 mPa s shows a further delay in the liquid sheet break up. Fig. 6 shows the spray images for a water–glycerol (20:80) mixture of viscosity 39 mPa s. A complete breakup of the liquid is observed at a gas flow rate of 12 gpm and a liquid jet velocity upto 3.7 m/s. It is interesting to note that the proposed strategy leads to effective atomization
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Fig. 5. Spray structure for water–glycerol (50:50) mixture (viscosity: 5 mPa s) at various GLRs (listed on the images).
Fig. 6. Spray structure for water–glycerol (20:80) mixture (viscosity: 39 mPa s) at various GLRs (listed on the images).
of high viscosity liquid at very low injection pressures (of the order 3–4 bar). The detailed dropsizing measurements are discussed in the later section.
The spray images using ethanol as the liquid are shown in Fig. 7. The effect of surface tension on this mode of atomization can be observed by comparing Figs. 4 and 7. Due to lower surface tension,
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Fig. 7. Spray structure for ethanol at various GLRs (listed on the images).
the sheet formed due to the impinging of the liquid jets is larger than that of water. The sheet is broken into droplets and ligaments in the presence of the gas jet. It is observed that the effect of surface tension is not significant on the spray structure in the presence of the gas jet.
3.1.2. Effect of impinging angle The effect of the liquid jet impingement angle on spray structure is studied by using three different impinging angles viz., 60°, 90° and 120°. The spray structure images for a water spray at 60°, 90° and 120° impinging angles are shown in Figs. 8, 4 and 9, respectively. It is observed from the images that changing the impinging angle does not affect the breakup phenomenon significantly. However, the spray spread is the main parameter affected by the angle. An increase in the spray spread is observed at a higher impinging angle. This is due to the fact that at higher impinging angles, impact forces are high and push the liquid radially outwards from the impinging point resulting in a wider spread. At lower angles, the forces are predominant in the vertical direction and hence the length of the liquid sheet is larger. The spray structure images at different impinging angles for a water–glycerol (20:80) mixture of viscosity 39 mPa s at 60°, 90° and 120° impinging angles are shown in Figs. 10, 6 and 11 respectively. The observations are similar for all the other liquids. Since the effect of the impinging angle has not shown any significant effect, further studies are performed at a fixed impinging angle of 90°.
3.1.3. Effect of gas orifice diameter The gas injection orifice diameter is an important parameter in this mode of atomization since it directly affects the gas jet exit velocity for the same mass flow rate. The effect of the gas jet orifice diameter is studied by using 1.1 mm and 0.76 mm gas orifices. The spray structure images are obtained at the same gas and liquid flow rates and hence at the same GLRs for both the orifices.
Fig. 12 shows the spray structure for water with two different gas orifices at different liquid and gas flow rates. At a gas flow rate of 6 gpm, reducing the gas orifice diameter shows an improvement in the atomization for liquid jet velocity up to 3.7 m/s. This is attributed to the higher gas jet velocity which tends to disintegrate the liquid jets before impingement, as in prompt atomization mode. However, the improvement in the atomization is not clear at higher liquid jet velocity. The higher liquid jet momentum precludes the prompt atomization mode. The advantage of using a smaller gas orifice is significant at a gas flow rate of 12 gpm. A well atomized spray is observed at all the liquid flow rates. The results of similar experiments conducted for water–glycerol mixture (20:80) are presented in Fig. 13. In this case also, the observations are similar to that of water. A significant improvement in the atomization is observed at a 12 gpm gas flow rate. The spray structure at liquid jet velocity below 5.5 m/s is almost independent of the liquid viscosity. This might be due to the fact that the liquid jets are broken completely before they impinge and prompt atomization might occur at these conditions. A similar observation is made with two other water–glycerol mixtures. Overall, the spray structure study has shown that introducing gas jet on top of impinging point of the liquid jets improves the atomization. The effect of liquid viscosity is shown to have a significant effect on the sheet formation and breakup while the surface tension shows a much lesser effect. Changing the liquid jets impinging angle has not altered the breakup mechanism, however, it has caused spray spread in the radial direction. Using a smaller gas orifice has shown improved breakup for similar gas and liquid flow rates. 3.2. Spray angle The spray angle in the plane of sheet for different liquids is calculated from the spray structure images. Calculations are performed at a gas flow rate of 12 gpm and at different liquid jet
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Fig. 8. Spray structure for water at an impinging angle of 60° at various GLRs (listed on the images).
Fig. 9. Spray structure for water at an impinging angle of 120° at various GLRs (listed on the images).
velocities. At each condition, an ensemble average image is calculated from 100 instantaneous spray images taken at a rate of 10
frames/s. The ensemble average image is binarized using a threshold value. The spray angle is computed by finding the angle be-
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Fig. 10. Spray structure for water–glycerol (20:80) mixture at an impinging angle of 60° at various GLRs (listed on the images).
Fig. 11. Spray structure for water–glycerol (20:80) mixture at an impinging angle of 120° at various GLRs (listed on the images).
tween the lines joining the impinging point and spray edge at an axial distance of 15 mm downstream of the impinging point. An image processing software routine is used to find the spray edges and calculate the spray angle from the binarized ensemble average
image. The uncertainty in the calculation of a spray angle with the threshold value is estimated to be of the order of 5%. The results of the spray angle calculations are shown in the figures from Figs. 14– 16. The Figs. 14 and 15 give the spray angle values for different liq-
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Fig. 12. Effect of gas orifice diameter on spray structure for water at various gas and liquid flow rates.
uids at various impingement angles and liquid jet velocity while the comparison of spray angles across different liquids at an impingement angle of 90° is shown in Fig. 16. The values for water and ethanol at different impingement angles are shown in Fig. 14. It is observed that the spray angle is maximum at an impingement angle of 120° and minimum at an impingement angle of 60°. For a fixed impingement angle, increasing the liquid flow rate increases the spray angle. This increase is steep at low liquid jet velocity. Also, an increase in the spray angle is higher for the 120° impingement angle. Similar trends are observed for the water–ethanol mixture. For water–glycerol mixtures, it is observed that the spray angles at 60° and 90° impinging angles are mostly similar upto a liquid jet velocity of 5.5 m/s. The spray angles for water–glycerol mixtures (30:70, 20:80) are shown in Fig. 15(a and b), respectively. Spray angle comparison between various liquids at a fixed impingement angle of 90° shown in Fig. 16. Results showed that reducing the surface tension or increasing the viscosity reduces the spray angle. The reduction in the spray angle with a change in viscosity is higher when compared to a change in the surface tension. The variation of the spray angle with liquid properties is
lower at low liquid jet velocity and increases with an increase in the liquid jet velocity. This is due to the fact that liquid properties start affecting the spread of the spray when the two liquid jets start to interact, which happens at higher liquid jet velocity. A similar observation is made at impingement angles of 60° and 120°. A similar calculation is performed for the spray images obtained with the smaller gas orifice diameter at an impinging angle of 90°. A comparison of the spray angles obtained with a 1.1 mm gas orifice and a 0.76 mm gas orifice for water and water–glycerol mixture (20:80) are shown in Fig. 17. The gas flow rate is maintained at 12 gpm for all cases. For water, the spray angle is mostly similar with both gas orifices at low liquid jet velocity. This is possibly due to the fact that the prompt mode of atomization is dominant at these conditions. However, the smaller gas orifice results in a higher spray angle at higher liquid jet velocities. The difference in the spray angle for the two orifice diameters at higher liquid jet velocity stems from the fact that the larger orifice diameter case undergoes the classical mode of atomization, whereas the smaller orifice diameter undergoes the prompt mode of atomization. The difference in the spray angle is observed even at lower liquid jet velocity with water–glycerol mixtures. Fig. 18 compares the spray angles
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Fig. 13. Effect of gas orifice diameter on spray structure for water–glycerol (20:80) mixture at various gas and liquid flow rates.
Fig. 14. Spray angles for at different impinging angles (a) water, (b) ethanol.
for different liquids with the 0.76-mm orifice. For this orifice diameter, the difference in the spray angles between the liquids is small, especially between water and water–glycerol (50:50) mixture. However, the trend of higher viscosity liquids having smaller spray angles is observed with the 0.76 mm orifice also.
3.3. Droplet sizing The droplet sizing measurements are carried out using the Particle/Droplet Imaging Analysis (PDIA) technique. Measurements are taken at an axial location of 75 mm downstream of the imping-
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(a)
(b)
Fig. 15. Spray angles at different impinging angles for (a) water–glycerol (50:50). (b) Water–glycerol (20:80).
°
Spary angle ( )
150
Water Ethanol Water−ethanol(90:10) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
100
50
0
0
2
4
6
8
10
Liquid jet velocity (m/s) Fig. 16. Spray angle variation for different liquids at an impingement angle of 90°.
ing point and at different radial locations. Since the spray structure images show that changing the impingement angle does not change the atomization mode (classic vs prompt), droplet sizing is carried out only for an impingement angle of 90°. The gas flow rate is maintained at 12 gpm while the liquid jet velocity is varied between 1.8 m/s and 7.3 m/s. A 5-mm 5-mm region of the spray has been imaged, which gives a pixel resolution of 2.4 lm per pixel. To reduce the error in SMD estimation, droplets which are significantly below 10-lm in size are ignored. A total of 400 images, at a rate of 10 frames per second, were acquired at each condition to obtain a statistically sufficient number of droplets. The number of droplets typically used for calculating SMD is between 7000 and 20,000. The acquired images were analyzed using the LaVision Par-
ticle Master Shadow module to obtain the droplet size information. The depth of focus calibration was performed using a plate with predefined opaque circles of size ranging from 20 lm to 400 lm. For the settings used in the current study, it was observed that the depth of focus varies from 0.1 mm for 20 lm droplets to 3 mm for 200 lm droplets, and varies linearly between these limits. The results of droplet sizing experiments with 0.76 mm gas orifice diameter case are presented in Figs. 19–21. The measurements for the water spray are shown in Fig. 19(a). At a liquid jet velocity of 1.8 m/s, the SMD is minimum at the center and maximum at r = 30 mm. The variation is of the order of 120-lm between the center and the outer edge of the spray. Increasing the liquid velocity increases the SMD marginally at the center and reduces significantly at the outer edge of the spray. Hence, increasing the liquid velocity reduces the difference in the SMD between the center and radial locations. The reason could be that at low liquid velocity, the higher momentum of gas jet as compared to liquid jets causes the gas jet penetrate through the liquid jets and results in larger droplets towards the outer periphery of the spray. This effect is reduced with increase in liquid jet velocity thereby increases the liquid jet momentum. A minimum variation for water is observed at a liquid velocity of 7.8 m/s. At this condition, there might be a balance between breakup due to liquid impingement and breakup due to the gas jet. The results for ethanol and water–glycerol mixtures (30:70 and 20:80) are shown in Fig. 19(b–d) respectively. The variation between ethanol and water is not significant implying that the effect of surface tension is not significant in this mode of atomization. Results obtained with water–glycerol mixture show that increasing the liquid jet velocity up to 5.8 m/s reduces the radial SMD variation. However, a liquid jet velocity of 7.8 m/s resulted
150
150
100
100
50
50
0
0
(a)
(b)
Fig. 17. Spray angle comparison for (a) Water, and (b) water–glycerol (20:80) mixture.
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150
°
Spary angle ( )
Water Water−glycerol (50:50) Water−glycerol (20:80) 100
50
0
0
2
4
6
8
10
Liquid jet velocity (m/s)
parameters. Hence, the variation in non-dimensional SMD values as a function of non-dimensional parameters such as the GLR and momentum flux ratio (gas jet to liquid jet) in Figs. 20 and 21, respectively. The momentum flux ratio is calculated considering the momentum flux at the exit of the orifice which in turn is estimated from the flow rates. Interestingly, the results show that the non-dimensional SMD values for the various liquids show a definite trend with both GLR and momentum flux ratio. More importantly, the liquid properties do not affect the atomization significantly after a critical GLR or momentum flux ratio. In other words, the non-dimensional SMD is around 0.8 beyond a critical GLR of 0.05 and a critical momentum ratio of around 4. Moreover, this SMD value is constant over a viscosity range from 1 to 39 mPa s and surface tension ranging from 22 to 72 mN/m. This is a very useful finding and has interesting ramifications for fuelflexible systems.
Fig. 18. Spray angle variation at an impinging angle of 90° with 0.76 mm gas orifice.
(a) Water
(c) Water-glycerol (30:70) mixture
0.5 Water Ethanol Water−ethanol (90:10) Water−ethanol (75:25) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
0.45 0.4 0.35 SMD/dg
in a spray with larger SMD at the center and smaller SMD towards the outer edge. This is due to fact that at this velocity, the two liquid jets interact resulting in the classical mode of atomization. It is also observed that the absolute values of the SMD are comparable for all the liquids up to 5.8 m/s liquid jet velocity which implies that effect of liquid properties is not significant at these conditions. It is also observed that SMD of the order 40 lm can be achieved even with the liquid of viscosity of 39 mPa s at GLR below 0.1 using this method of atomization. To compare results across the various conditions, the overall SMD is calculated by combining the droplet statistics obtained at all radial locations at a particular axial location. To eliminate the effect of gas orifice diameter, a non-dimensional SMD is calculated by dividing the overall SMD with gas orifice diameter. Since there is extensive data on droplet diameters for a range of liquid viscosity and surface tension, it was decided to explore possible trends of the non-dimensional SMD in terms of other non-dimensional
0.3 0.25 0.2 0.15 0.1 0.05 0 0
0.05 0.1 Gas to liquid ratio (GLR)
0.15
Fig. 20. Variation of non-dimensional SMD with GLR (closed symbols represents 1.1 mm gas orifice while open symbols are for 0.76 mm gas orifice).
(b) Ethanol
(d) Water-glycerol (20:80) mixture
Fig. 19. Variation of SMD with radial location for various liquids with 0.76 mm gas orifice.
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El ¼ V l ðPl2 P1 Þ
0.5 Water Ethanol Water−ethanol (90:10) Water−ethanol (75:25) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
0.45 0.4
SMD/dg
0.35 0.3
The total energy supplied is the sum of the gas energy and the liquid energy and is given by:
Ei ¼ Eg þ El
e¼
0.2 0.15 0.1 0.05 5
10 Momentum flux ratio
15
20
Fig. 21. Variation of non-dimensional SMD with momentum flux ratio (closed symbols represents 1.1 mm gas orifice while open symbols are for 0.76 mm gas orifice).
According to Bayvel and Orzechowski (1993), energy efficiency of atomization is defined as the ratio of the energy used for the development of new liquid surface to the total energy used in the process. The energy efficiency of the pressure atomizers falls in the range of 0.05–0.07%, while the twin-fluid atomization process results in even lower efficiencies (Bayvel and Orzechowski, 1993). Lefebvre (1992) used an atomization efficiency value of 0.7 % for his model to predict droplet sizes in twin-fluid atomization. Recent studies on effervescent atomizer by Jedelsky and Jicha (2012) showed that atomization efficiency is of the order 0.2% and varies with GLR. Energy efficiency of the present method of atomization is analyzed by calculating the ratio of surface energy change of the liquid to the total energy input. The total energy input in air-assisted atomization process is the sum of atomizing gas energy (Eg) and the energy of supplied liquid (El) (Jedelsky and Jicha, 2012). Initial bulk liquid surface energy of the liquid is calculated by assuming liquid as a cylinder with a diameter equal to the liquid injection orifice. Initial surface energy of the liquid is given by:
SEi ¼ r p do L
ð7Þ
The efficiency of the atomization process is given by:
0.25
0 0
ð6Þ
DSE Ei
ð8Þ
Efficiency of the impinging jet air-assisted method is calculated for various liquids at several GLRs using the above methodology. It is assumed that the bulk liquid is transformed into spray with the droplet sizes of overall SMD mentioned in the previous section at the corresponding conditions. Atomization efficiency is computed for both 1.1-mm and 0.76-mm gas orifice cases and the results are presented in Fig. 22 as a function of GLR. In the case of 1.1mm gas orifice, atomization efficiency first increases with GLR, reaches a maximum and then reduces for all the liquids. This variation is due to that fact that at higher GLRs, the atomizing gas carries much more energy than required and hence the efficiency is low. Also, at lower GLRs, the atomization happens predominantly due to impingement of liquid jets and hence the utilization of gas energy is lower. A balance between the two modes of atomization occurs at a mid-range of GLRs, hence the atomization efficiency peaks at these GLRs. The maximum efficiency occurs around a GLR of 0.05 for all the liquids. The maximum value of atomization efficiency is of the order 0.25 which is similar to the values reported in the literature (Jedelsky and Jicha, 2012). A var-
ð1Þ
where r is surface tension of the liquid, do is the liquid orifice diameter and L is the length of the liquid jet. Due to the atomization process, initial bulk liquid is transformed to droplets of diameter D. The total liquid surface energy after atomization is obtained as a sum of surface energies of all the droplets, and is given by:
SEf ¼ n r p D2
ð2Þ
(a) Atomization efficiency with 1.1 mm gas orifice
where n is the total number of droplets which can be calculated by mass conservation of the liquid before and after the atomization process as shown below.
n¼
4ml
ql pD2
ð3Þ
Hence, change in the surface energy of the liquid is calculated as:
DSE ¼ SEf SEi
ð4Þ
Energy introduced by the atomizing gas is calculated as isothermal compression energy required to pressurize the gas mass mg from ambient pressure P1 to the pressure Pg2 upstream of the air injection orifice (Jedelsky and Jicha, 2012).
Eg ¼ P1 V 1 lnðP g2 =P1 Þ
ð5Þ
where V1 is the volume of gas at ambient pressure. The energy contained in the liquid supplied El is given by:
(a) Atomization efficiency with 0.76 mm gas orifice Fig. 22. Energy efficiency of the proposed method at various conditions for different liquids.
M.M. Avulapati, R.V. Ravikrishna / International Journal of Multiphase Flow 57 (2013) 88–101
iation in the efficiency with liquid properties is observed with both the gas orifices. It is observed that atomization efficiency is lower for liquids with lower surface tension. Although the droplet sizes are similar for liquids with varying surface tension which implies that the change in surface area is similar, the surface energy change due to atomization is lower for lower surface tension liquids. This results in a lower efficiency value for the same amount of atomizing gas energy. It is also observed that increase in the liquid viscosity also reduces the atomization efficiency. This is attributed to two reasons, the first being inferior atomization due to higher viscosity and hence lesser surface energy change, and secondly, higher viscosity liquids requiring higher pressure to pump same volume of liquid and hence higher energy input. It is also observed that atomization efficiency is higher for the 0.76mm orifice at lower GLR compared to the 1.1-mm orifice. The opposite trend is observed at higher GLRs. This is due to the fact that improvement in atomization is significant at lower GLRs when the gas orifice is changed from 1.1-mm to 0.76-mm, while the change in SMD is less at higher GLRs. 4. Conclusions A novel, air-assisted impinging jet atomization strategy in which a gas jet is directed on to the impinging point of two liquid jets is proposed and demonstrated experimentally. Specifically, the effect of liquid properties such as viscosity and surface tension, liquid jet impingement angle and gas jet orifice diameter is studied. Depending on the liquid and gas flow rates, two modes of atomization are observed viz., classical and prompt. Critical values of nondimensional parameters such as GLR and momentum flux ratio have been identified at which the mode of atomization changes from ‘classical mode’ to ‘prompt mode’. The spray structure images showed that changing the impingement angle between the liquid jets does not change the atomization mode, however, it only affects the spray angle. The effect of surface tension is observed to be lesser as compared to the effect of viscosity. A delay in the liquid breakup is observed with increase in the liquid viscosity, especially in the classical mode of atomization. Reducing the gas orifice diameter led to increased gas jet momentum which in turn caused improved atomization at all conditions. This is attributed to the change in atomization mode from classical to prompt. Droplet size measurements are conducted at a gas flow rate of 12 gpm using both 1.1-mm and 0.76-mm gas orifices. It is observed that SMD of the order 40 lm can be achieved even with a liquid of viscosity of 39 mPa s at GLR below 0.1. It is also observed that beyond the critical values of GLR of 0.05 and momentum flux ratio of 4, the liquid properties do not affect the atomization significantly. This implies that ‘‘irrespective of the liquid used over a large range of viscosity or surface tension, the resulting SMD is same with the
101
proposed method’’. Specifically, the SMD of the spray obtained through the proposed method is constant over a viscosity range from 1 to 39 mPa s and surface tension ranging from 22 to 72 mN/m. This is a very important point and has interesting ramifications for flexible fuel systems in combustors. References Ashgriz, N., 2011. Handbook of Atomization and Sprays. Springer. Bailardi, G., Negri, M., Ciezki, H.K., 2010. Several aspects of the atomization behavior of various Newtonian fluids with a likeon-like impinging jet injector. In: Proc. 23rd European Conf. on Liquid Atomization and Spray Systems, Brno, Czeck Republics. Bayvel, L., Orzechowski, Z., 1993. Liquid Atomization. Taylor & Francis Inc. Beck, J.E., Lefebvre, A.E., Koblish, T.R., 1991. Liquid sheet disintegration by impinging air streams. Atomization Sprays 1, 155–170. Bremond, N., Villermaux, E., 2006. Atomization by jet impact. J. Fluid Mech. 549, 273–306. Dombrowski, N., Hooper, P., 1964. A study of the sprays formed by impinging jets in laminar and turbulent flow. J. Fluid Mech. 18, 392–400. Bush, J., Hasha, A., 2004. On the collision of laminar jets: fluid chains and fishbones. J. Fluid Mech. 511, 285–310. Choo, Y., Kang, B., 2003. A study on the velocity characteristics of the liquid elements produced by two impinging jets. Exp. Fluids 34, 655–661. Blaisot, J.B., Yon, J., 2005. Droplet size and morphology characterization for dense sprays by image processing: application to the diesel spray. Exp. Fluids 39, 977– 994. Esmail, M., Kawahara, N., Tomita, E., Sumida, M., 2010. Direct microscopic image and measurement of the atomization process of a port fuel injector. Meas. Sci. Technol. 2, 075403. Feras, Z.B., Roisman, I.V., Tropea, C., 2010. Characterization of spray generated by an airblast atomizer with prefilmer. Atomization Sprays 20, 887–903. Ibrahim, E.A., Przekwas, A., 1991. Impinging jets atomization. Phys. Fluids A 3, 2981–2987. Jedelsky, J., Jicha, M., 2012. Energy conversion in effervescent atomization. In: Proc. 12th Triennial Conf. on Liquid Atomization and Spray Systems, Heidelberg, Germany. Kashdan, J., Shrimpton, J., Whybrew, A., 2004. Two-phase flow characterization by automated digital image analysis. Part 2: Application of PDIA for sizing sprays. Part. Part. Syst. Charact. 21, 15–23. Kim, K.S., Kim, S.S., 1994. Drop sizing and depth-of-field correction in TV imaging. Atomization Sprays 4, 65–78. Koh, K.U., Kim, J.Y., Lee, S.Y., 2001. Determination of in-focus criteria and depth of field in image processing of spray particles. Atomization Sprays 11, 317–333. Lai, W., Haung, W., Jaing, T., 1999. Characteristic study on the like-doublet impinging jets atomization. Atomization Sprays 3, 277–289. Lee, S.Y., Kim, Y.D., 2003. Sizing of spray particles using image processing technique, ICLASS-2003, Sorrento, Italy. Lefebvre, A.H., 1989. Atomization and Sprays. Hemisphere Publishing Corporation. Lefebvre, A.H., 2000. Fifty years of gas turbine fuel injection. Atomization Sprays 10, 251–276. Lefebvre, A.H., 1992. Energy considerations in twin-fluid atomization. J. Eng. Gas Turbine Power 114, 89–96. Ren, N., Marshall, A.W., 2012. Characterizing the initial spray from large Weber number impinging jets. Int. J. Multiph. Flow, http://dx.doi.org/10.1016/ j.ijmultiphaseflow.2012.08.004. Shen, Y., Mitts, C., Poulikakos, D., 1997. Holographic investigation of the effect of elevated ambient temperature on the atomization characteristics of impinging jet sprays. Atomization Sprays 7, 123–142. Yang, L., Fu, Q., Qu, Y., Gu, B., Zhang, M., 2012. Breakup of a power-law liquid sheet formed by an impinging jet injector. Int. J. Multiph. Flow 39, 37–44.
Contents lists available at ScienceDirect
International Journal of Multiphase Flow j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j m u l fl o w
Experimental studies on air-assisted impinging jet atomization Madan Mohan Avulapati ⇑, Ravikrishna Rayavarapu Venkata Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 11 March 2013 Received in revised form 24 June 2013 Accepted 25 July 2013 Available online 7 August 2013 Keywords: Air-assisted impinging jet atomization Spray characterization PDIA Twin-fluid atomization
a b s t r a c t In the present study, a novel air-assisted impinging jet atomization is demonstrated. A configuration in which a gas jet is directed on to the impinging point of two liquid jets is used to improve the atomization. The effect of liquid properties such as viscosity and surface tension, angle between liquid jets and gas injection orifice diameter on spray characteristics has been experimentally studied. Backlit imaging and particle/droplet imaging and analysis techniques are utilized to characterize the sprays. The experimental results indicate that the effect of liquid viscosity is significant on the liquid sheet break up formed by the impinging jets. However, surface tension does not affect the spray structure significantly in this mode of atomization. At low liquid jet velocity, the prompt mode of atomization is observed where as atomization occurs in classical mode at higher liquid jet velocity. Results showed that variation in the angle between liquid jets do not affect the breakup phenomenon significantly. The spray angle is computed by finding the angle between the lines joining the impinging point and spray edge at an axial distance of 15 mm downstream of the impinging point from the ensemble-averaged data over 100 spray images. It was observed that effect of liquid jets impinging angle on the spray angle is higher at higher liquid velocity. Higher viscosity liquids exhibit lower spray angles. Droplet size measurements indicate a radial variation in the spray. An overall Sauter Mean Diameter (SMD) value is obtained by combining the droplet statistics at all radial locations at a fixed axial location. A very interesting trend is that the SMD is constant beyond a critical Gas to Liquid Ratio (GLR) and momentum ratio for a large variation in liquid viscosity and surface tension. This observation has important ramifications for fuel flexible systems. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Liquid atomization is an important process in many applications such as IC engines, gas turbines, spray-painting, and chemical reactions. Various liquid atomization strategies are employed as reported in the literature to obtain desired performance. Twinfluid atomization is one such method where the gas energy is used to assist liquid atomization. Air-assisted atomization and airblast atomization are examples of twin-fluid atomization methods, in which kinetic energy of air is used to aid liquid breakup. The fundamental difference between air-assisted and airblast atomization relates to the quantity of air used (Lefebvre, 1989). Air-assisted atomization uses a relatively less amount of air at high pressure while relatively large quantity of air at low pressure is used in airblast atomization. These methods have been used in the literature extensively to atomize various kinds of liquids (Lefebvre, 1989; Ashgriz, 2011). Various designs of air-assist and airblast atomizers used in the literature are discussed in a review article by Lefebvre ⇑ Corresponding author. Tel.: +91 8022932352. E-mail addresses: [email protected] (M.M. Avulapati), [email protected] (R. Rayavarapu Venkata). 0301-9322/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2013.07.007
(2000). In most of these studies, either a liquid sheet is blasted with air jets, or gas is allowed to mix with the liquid inside the atomizer to form a spray. These methods require a large amount of gas to achieve droplet sizes of the order of 50 lm. Beck et al. (1991) achieved a Sauter Mean Diameter (SMD) of 75 lm with water at a gas-to-liquid ratio (GLR) of the order of 1.0. Experimental studies on swirl airblast atomizers by Feras et al. (2010) showed that an SMD of the order 50 lm can be achieved at a GLR of 8. On the other hand, impinging jet atomization has been studied in applications to rocket engines and chemical processes due to its good atomization and mixing characteristics (Dombrowski and Hooper, 1964; Ibrahim and Przekwas, 1991; Bailardi et al., 2010; Yang et al., 2012). Bremond and Villermaux (2006) studied the dynamics of impinging jet atomization mechanism and effect of impinging angle on the liquid sheet formation and resulting droplet size using water and ethanol as liquids. Choo and Kang (2003) studied the effect of impinging angle on droplet velocities. Bailardi et al. (2010) conducted experiments with various liquids and identified seven different breakup patterns and corresponding Reynolds and Weber numbers. Bush and Hasha (2004) also studied various patterns produced by impingement of laminar liquid jets. Ren and Marshall (2003) studied the sheet breakup at high liquid
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jet Weber numbers. Impinging jet experiments with water by Shen et al. (1997) showed that a jet velocity of the order of 12.5 m/s was required to obtain an SMD in the range of 300 lm. Studies by Lai et al. (1999) showed that an SMD of 50 lm can be achieved with impingement liquid jets with a velocity of the order of 40 m/s. At such a high velocity, the radial spread of the spray is higher which results in droplet impingement on the walls. In the present study, a novel strategy of combining the effect of impinging jets and air assisted atomization is used to achieve smaller droplets at low liquid injection velocities. In this method of atomization, a liquid sheet is formed using two impinging liquid jets, and an air jet is directed on to this sheet to aid the atomization process. The present study focuses on the effect of liquid properties, specifically viscosity and surface tension, angle between the liquid jets, and gas injection orifice diameter on spray characteristics such as spray structure, spray angle and the droplet sizes. According to Lefebvre (1989), there exist two fundamental atomization modes in case of air blast/air-assist atomizers, viz. classical mode and prompt mode of atomization. The classic mode of atomization is a mechanism where small disturbances, either within or on the surface of a liquid jet or sheet promote the formation of waves that eventually lead to disintegration into ligaments and then droplets. The mechanism is called prompt atomization when the breakup takes place very rapidly. Under these conditions, the jet or sheet has no time to develop a wavy structure, but is immediately torn into fragments by its vigorous interaction with the surrounding air. The spray structure images and droplet sizing data are analyzed to find the conditions at which these processes occur.
2. Experimental setup The experimental setup consists of two parts, (i) liquid and gas supply system to the atomizer and (ii) optical setup to measure spray characteristics. In the supply system, high pressure nitrogen from a gas bottle is regulated to the gas reservoir tank and a part of it is used to compress the liquid in the reservoir. The liquid and gas flow rates are controlled using needle valves in their respective lines. The gas flow rate is measured using a thermal mass flow meter while the liquid flow is measured using a gear flow meter. Pressure gages are placed closer to the atomizer for injection pressure measurements. The spray is injected in a quiescent ambient condition. The optical setup for spray characteristic measurements is based on backlit imaging method. In this setup, a pulsed Nd:YAG laser along with a fluorescent diffuser is used as a light source to provide back-illumination for the spray along with a CCD camera with a resolution of 2048 2048 pixels to record spray images. A laser pulse duration of 10 ns is used for the experiments. A schematic of the experimental setup is shown in Fig. 1. In the present study, two types of image based measurements are performed, one to study the spray structure and the other for droplet size measurements. For the spray structure measurements, images span a distance from the exit of the orifice to 80-mm downstream. The relevant portion of the spray is then imaged and analyzed using the using Particle/Droplet Imaging Analysis (PDIA) technique for droplet size measurements. Specifically, this method involves the use of a long distance microscope (Questar: QM1). The atomizer configuration used in the current study has two identical liquid injection orifices and a gas injection orifice as shown in Fig. 2. Two different gas orifices of diameter 0.76-mm and 1.1-mm are used while the liquid injection orifices of the diameter 0.76-mm each. The length of liquid injection orifice is about 50 mm which gives an L/D ratio of 65 and ensures fully developed flow profile at the exit. The angle between the liquid jets is varied between 60° and 120°. The gas jet is always placed above the impinging point of liquid jets making equal angles with the
89
both liquid jets. The exit of the gas jet is placed about 10 mm above the impinging point of the liquid jet. The free length of liquid jet, which is defined as the distance from the exit of the liquid orifice to the point of impingement is maintained at about 8 mm. An image-based, non intrusive droplet size measurement technique, Particle/Droplet Imaging Analysis (PDIA) is used for droplet size measurements. In this method, back-lit spray images obtained in a region typically 5 mm 5 mm and analyzed using image processing algorithms. Significant improvements to this technique have been reported by Kim and Kim (1994) and Koh et al. (2001). This technique has been used extensively for particle/droplet size measurements in recent times due to developments in computational algorithms and technological improvements in digital imaging equipment (Blaisot and Yon 2005; Esmail et al. 2010). Use of short duration laser illumination with pulse-width of the order 10 ns has extended the applicability of this method to high velocity diesel sprays (Blaisot and Yon 2005) and gasoline sprays (Esmail et al., 2010). Studies by Kashdan et al. (2004) and Esmail et al. (2010) showed that PDIA compares well with Phase Doppler Anemometry (PDA). Also, PDIA has additional advantages of providing particle shape information and characterizing non-spherical droplets accurately (Kashdan et al., 2004; Lee and Kim, 2003). Since PDIA is a planar imaging technique, the depth-of-focus (DOF) for larger droplets is higher compared to that for smaller droplets. According to Kim and Kim (1994), the depth of focus varies linearly with droplet size. A DOF correction can be applied to droplet statistics to minimize the error due to variation in depth of focus (Kashdan et al., 2004). To study the effect of liquid properties on this mode of atomization, a range of liquids with variation in viscosity and surface tension have been used. Water and glycerol are mixed in different proportions to get the variation in viscosity while the variation in surface tension is achieved by mixing water and ethanol. The properties of the various liquids used in the study at room temperature are shown in Table 1. A rotational viscometer is used to measure the viscosity of the liquids while the surface tension is measured using Du-Nouy ring type tensiometer. The accuracy of the instruments is verified by measuring the properties of standard fluids viz. water and ethanol. Densities of the liquids are calculated using the linear mixing rule. The uncertainty in the measurement of spray cone angle is estimated by varying the threshold value used in the image processing algorithm. The uncertainty in the spray cone with threshold is of the order ±5% of the mean value. The accuracy of the PDIA technique and the processing was evaluated by performing droplet sizing measurements on a stream of monodisperse droplets created by a mono-disperse droplet generator (Artium Technologies Inc.: MDG 100). The pre-set value of the Sauter Mean Diameter (SMD) in the droplet generator was 135 lm. Using the PDIA technique, the value of SMD obtained was 136.8 lm with an arithmetic mean diameter of 136.7 lm. Thus, the accuracy of the technique in measuring the SMD can be observed to be within 1% for the mono-disperse droplets studied. For the spray, the uncertainty is estimated to be <±5% (which is the cumulative error considering precision, accuracy and repeatability).
3. Results and discussion To access the merit of the proposed atomization strategy, initially, a comparison of the conventional external mixing atomization with the proposed method is conducted. The spray images at similar gas and liquid flow rates obtained with both the configurations are shown in Fig. 3. With the conventional method, an unbroken liquid jet with surface instabilities is observed. In the
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Fig. 1. Schematic of the experimental setup.
Fig. 2. Schematic of the atomizer configuration.
proposed configuration, the liquid jets breakup completely and result in a well atomized spray. The following sub-sections present details of the spray structure and droplet sizes. 3.1. Spray structure 3.1.1. Effect of liquid properties The spray structure images are taken using the back-lit direct imaging method. When the two liquid jets impinge on each other in the absence of air injection, they form a liquid sheet in the plane making a right angle to both the liquid jets. The plane in which the liquid jets are present is called the ‘plane of jets’ while the plane in which a sheet is formed is termed the ‘plane of sheet’. In the present study, images are taken mainly in the plane of sheet. The effect of liquid properties, angle between the liquid jets, gas flow rate and the gas orifice diameter on the spray structure are studied.
Fig. 4 shows the structure of the spray using water as liquid at different gas mass flow rates and liquid jet velocities. The corresponding Gas to Liquid Ratio (GLR) values are specified on each image. Gas to Liquid Ratio (GLR) is defined as the ratio of gas mass injected to the liquid mass injected. At a liquid jet velocity of 1.8 m/s, in the absence of gas injection, the two liquid jets impinge and form a sheet which is bounded by a rim. A stream of ligaments and droplets are produced at the bottom end of the sheet due to impingement of the two arms of the rim. This mode of break up is termed as ‘closed rim’ mode by Bailardi et al. (2010). Increasing the liquid jet velocity to 3.7 m/s increases the sheet width. The mode of sheet breakup shifted to ‘open rim mode’ where the lower part of liquid sheet breaks up due to Kelvin–Helmholtz-type instabilities (Bailardi et al., 2010). The shedding of droplets and ligaments from the edge of the liquid sheet is observed at this condition. Increasing the liquid velocity further to 5.5 m/s makes the liquid sheets break up rapidly due to the impact waves created by the impinging of the liquid jets. At this condition, the mode of Table 1 Properties of liquids used in the study. Liquid
Density (kg/ m3)
Viscosity (mPa s)
Surface tension (mN/m)
Water Water–glycerol (50:50) Water–glycerol (30:70) Water–glycerol (20:80) Water–ethanol (90:10) Water–ethanol (75:25) Ethanol
998 1129
1 5
72 67.6
1182
16
66.7
1208
39
66.1
978
1
55.1
944
1
37.9
780
1
22.3
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Fig. 3. Comparison of the proposed impinging jet air-assisted atomization strategy with the conventional method at similar gas and liquid flow rates.
Fig. 4. Spray structure for water at various GLRs (listed on the images).
breakup is similar to the ‘Ruffled sheet ligaments mode ‘described by Bailardi et al. (2010). A similar observation is made at a higher liquid velocity. Introducing a small amount of gas flow (6 grams per minute (gpm)) directed towards the liquid jets impinging point is observed to have a positive effect on the breakup of the liquid sheet. It was observed that the presence of gas jet eliminates the sheet formation at a liquid jet velocity of 1.8 m/s, while the liquid sheet is reduced significantly in terms of width at a liquid jet velocity of 3.7 m/s. This is due to the fact that the impact of gas jet on liquid jets reduces the momentum of the liquid jets at the impinging point and hence reduces the radial expansion of the liquid sheet. However, at liquid jet velocities of 5.5 m/s and 7.3 m/s, there is a reduced effect on the breakup except that the spray is confined to a smaller width when compared to the situation in the absence of gas flow. Increasing the gas flow rate further to 12 gpm showed a complete breakup of the liquid jets into a well atomized spray. Liquid sheet formation is not observed up to 5.5 m/s liquid jet velocity. In terms of the GLR, for a fixed gas flow rate, increasing the GLR showed improvement in the atomization. However, for a
fixed GLR, improved atomization is observed at a higher gas flow rate. Instantaneous images of the spray using a water–glycerol (50:50) mixture of viscosity 5 mPa s are shown Fig. 5. In the absence of gas jet, the sheet formation is observed to be similar to that of water. The rim bounding the liquid sheet is distinctly visible in this case. However, as the liquid velocity increases, perforations start to appear and the growth of these perforations eventually lead to the breakup of the liquid sheet. Unlike with water, a clear liquid sheet is observed even at a liquid jet velocity of 7.3 m/s. Akin to a water spray, introducing the gas jet improves the overall atomization. This improvement in the atomization is clear even at higher liquid velocity. Increasing the liquid viscosity further to 16 mPa s shows a further delay in the liquid sheet break up. Fig. 6 shows the spray images for a water–glycerol (20:80) mixture of viscosity 39 mPa s. A complete breakup of the liquid is observed at a gas flow rate of 12 gpm and a liquid jet velocity upto 3.7 m/s. It is interesting to note that the proposed strategy leads to effective atomization
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Fig. 5. Spray structure for water–glycerol (50:50) mixture (viscosity: 5 mPa s) at various GLRs (listed on the images).
Fig. 6. Spray structure for water–glycerol (20:80) mixture (viscosity: 39 mPa s) at various GLRs (listed on the images).
of high viscosity liquid at very low injection pressures (of the order 3–4 bar). The detailed dropsizing measurements are discussed in the later section.
The spray images using ethanol as the liquid are shown in Fig. 7. The effect of surface tension on this mode of atomization can be observed by comparing Figs. 4 and 7. Due to lower surface tension,
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93
Fig. 7. Spray structure for ethanol at various GLRs (listed on the images).
the sheet formed due to the impinging of the liquid jets is larger than that of water. The sheet is broken into droplets and ligaments in the presence of the gas jet. It is observed that the effect of surface tension is not significant on the spray structure in the presence of the gas jet.
3.1.2. Effect of impinging angle The effect of the liquid jet impingement angle on spray structure is studied by using three different impinging angles viz., 60°, 90° and 120°. The spray structure images for a water spray at 60°, 90° and 120° impinging angles are shown in Figs. 8, 4 and 9, respectively. It is observed from the images that changing the impinging angle does not affect the breakup phenomenon significantly. However, the spray spread is the main parameter affected by the angle. An increase in the spray spread is observed at a higher impinging angle. This is due to the fact that at higher impinging angles, impact forces are high and push the liquid radially outwards from the impinging point resulting in a wider spread. At lower angles, the forces are predominant in the vertical direction and hence the length of the liquid sheet is larger. The spray structure images at different impinging angles for a water–glycerol (20:80) mixture of viscosity 39 mPa s at 60°, 90° and 120° impinging angles are shown in Figs. 10, 6 and 11 respectively. The observations are similar for all the other liquids. Since the effect of the impinging angle has not shown any significant effect, further studies are performed at a fixed impinging angle of 90°.
3.1.3. Effect of gas orifice diameter The gas injection orifice diameter is an important parameter in this mode of atomization since it directly affects the gas jet exit velocity for the same mass flow rate. The effect of the gas jet orifice diameter is studied by using 1.1 mm and 0.76 mm gas orifices. The spray structure images are obtained at the same gas and liquid flow rates and hence at the same GLRs for both the orifices.
Fig. 12 shows the spray structure for water with two different gas orifices at different liquid and gas flow rates. At a gas flow rate of 6 gpm, reducing the gas orifice diameter shows an improvement in the atomization for liquid jet velocity up to 3.7 m/s. This is attributed to the higher gas jet velocity which tends to disintegrate the liquid jets before impingement, as in prompt atomization mode. However, the improvement in the atomization is not clear at higher liquid jet velocity. The higher liquid jet momentum precludes the prompt atomization mode. The advantage of using a smaller gas orifice is significant at a gas flow rate of 12 gpm. A well atomized spray is observed at all the liquid flow rates. The results of similar experiments conducted for water–glycerol mixture (20:80) are presented in Fig. 13. In this case also, the observations are similar to that of water. A significant improvement in the atomization is observed at a 12 gpm gas flow rate. The spray structure at liquid jet velocity below 5.5 m/s is almost independent of the liquid viscosity. This might be due to the fact that the liquid jets are broken completely before they impinge and prompt atomization might occur at these conditions. A similar observation is made with two other water–glycerol mixtures. Overall, the spray structure study has shown that introducing gas jet on top of impinging point of the liquid jets improves the atomization. The effect of liquid viscosity is shown to have a significant effect on the sheet formation and breakup while the surface tension shows a much lesser effect. Changing the liquid jets impinging angle has not altered the breakup mechanism, however, it has caused spray spread in the radial direction. Using a smaller gas orifice has shown improved breakup for similar gas and liquid flow rates. 3.2. Spray angle The spray angle in the plane of sheet for different liquids is calculated from the spray structure images. Calculations are performed at a gas flow rate of 12 gpm and at different liquid jet
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Fig. 8. Spray structure for water at an impinging angle of 60° at various GLRs (listed on the images).
Fig. 9. Spray structure for water at an impinging angle of 120° at various GLRs (listed on the images).
velocities. At each condition, an ensemble average image is calculated from 100 instantaneous spray images taken at a rate of 10
frames/s. The ensemble average image is binarized using a threshold value. The spray angle is computed by finding the angle be-
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Fig. 10. Spray structure for water–glycerol (20:80) mixture at an impinging angle of 60° at various GLRs (listed on the images).
Fig. 11. Spray structure for water–glycerol (20:80) mixture at an impinging angle of 120° at various GLRs (listed on the images).
tween the lines joining the impinging point and spray edge at an axial distance of 15 mm downstream of the impinging point. An image processing software routine is used to find the spray edges and calculate the spray angle from the binarized ensemble average
image. The uncertainty in the calculation of a spray angle with the threshold value is estimated to be of the order of 5%. The results of the spray angle calculations are shown in the figures from Figs. 14– 16. The Figs. 14 and 15 give the spray angle values for different liq-
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Fig. 12. Effect of gas orifice diameter on spray structure for water at various gas and liquid flow rates.
uids at various impingement angles and liquid jet velocity while the comparison of spray angles across different liquids at an impingement angle of 90° is shown in Fig. 16. The values for water and ethanol at different impingement angles are shown in Fig. 14. It is observed that the spray angle is maximum at an impingement angle of 120° and minimum at an impingement angle of 60°. For a fixed impingement angle, increasing the liquid flow rate increases the spray angle. This increase is steep at low liquid jet velocity. Also, an increase in the spray angle is higher for the 120° impingement angle. Similar trends are observed for the water–ethanol mixture. For water–glycerol mixtures, it is observed that the spray angles at 60° and 90° impinging angles are mostly similar upto a liquid jet velocity of 5.5 m/s. The spray angles for water–glycerol mixtures (30:70, 20:80) are shown in Fig. 15(a and b), respectively. Spray angle comparison between various liquids at a fixed impingement angle of 90° shown in Fig. 16. Results showed that reducing the surface tension or increasing the viscosity reduces the spray angle. The reduction in the spray angle with a change in viscosity is higher when compared to a change in the surface tension. The variation of the spray angle with liquid properties is
lower at low liquid jet velocity and increases with an increase in the liquid jet velocity. This is due to the fact that liquid properties start affecting the spread of the spray when the two liquid jets start to interact, which happens at higher liquid jet velocity. A similar observation is made at impingement angles of 60° and 120°. A similar calculation is performed for the spray images obtained with the smaller gas orifice diameter at an impinging angle of 90°. A comparison of the spray angles obtained with a 1.1 mm gas orifice and a 0.76 mm gas orifice for water and water–glycerol mixture (20:80) are shown in Fig. 17. The gas flow rate is maintained at 12 gpm for all cases. For water, the spray angle is mostly similar with both gas orifices at low liquid jet velocity. This is possibly due to the fact that the prompt mode of atomization is dominant at these conditions. However, the smaller gas orifice results in a higher spray angle at higher liquid jet velocities. The difference in the spray angle for the two orifice diameters at higher liquid jet velocity stems from the fact that the larger orifice diameter case undergoes the classical mode of atomization, whereas the smaller orifice diameter undergoes the prompt mode of atomization. The difference in the spray angle is observed even at lower liquid jet velocity with water–glycerol mixtures. Fig. 18 compares the spray angles
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Fig. 13. Effect of gas orifice diameter on spray structure for water–glycerol (20:80) mixture at various gas and liquid flow rates.
Fig. 14. Spray angles for at different impinging angles (a) water, (b) ethanol.
for different liquids with the 0.76-mm orifice. For this orifice diameter, the difference in the spray angles between the liquids is small, especially between water and water–glycerol (50:50) mixture. However, the trend of higher viscosity liquids having smaller spray angles is observed with the 0.76 mm orifice also.
3.3. Droplet sizing The droplet sizing measurements are carried out using the Particle/Droplet Imaging Analysis (PDIA) technique. Measurements are taken at an axial location of 75 mm downstream of the imping-
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(a)
(b)
Fig. 15. Spray angles at different impinging angles for (a) water–glycerol (50:50). (b) Water–glycerol (20:80).
°
Spary angle ( )
150
Water Ethanol Water−ethanol(90:10) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
100
50
0
0
2
4
6
8
10
Liquid jet velocity (m/s) Fig. 16. Spray angle variation for different liquids at an impingement angle of 90°.
ing point and at different radial locations. Since the spray structure images show that changing the impingement angle does not change the atomization mode (classic vs prompt), droplet sizing is carried out only for an impingement angle of 90°. The gas flow rate is maintained at 12 gpm while the liquid jet velocity is varied between 1.8 m/s and 7.3 m/s. A 5-mm 5-mm region of the spray has been imaged, which gives a pixel resolution of 2.4 lm per pixel. To reduce the error in SMD estimation, droplets which are significantly below 10-lm in size are ignored. A total of 400 images, at a rate of 10 frames per second, were acquired at each condition to obtain a statistically sufficient number of droplets. The number of droplets typically used for calculating SMD is between 7000 and 20,000. The acquired images were analyzed using the LaVision Par-
ticle Master Shadow module to obtain the droplet size information. The depth of focus calibration was performed using a plate with predefined opaque circles of size ranging from 20 lm to 400 lm. For the settings used in the current study, it was observed that the depth of focus varies from 0.1 mm for 20 lm droplets to 3 mm for 200 lm droplets, and varies linearly between these limits. The results of droplet sizing experiments with 0.76 mm gas orifice diameter case are presented in Figs. 19–21. The measurements for the water spray are shown in Fig. 19(a). At a liquid jet velocity of 1.8 m/s, the SMD is minimum at the center and maximum at r = 30 mm. The variation is of the order of 120-lm between the center and the outer edge of the spray. Increasing the liquid velocity increases the SMD marginally at the center and reduces significantly at the outer edge of the spray. Hence, increasing the liquid velocity reduces the difference in the SMD between the center and radial locations. The reason could be that at low liquid velocity, the higher momentum of gas jet as compared to liquid jets causes the gas jet penetrate through the liquid jets and results in larger droplets towards the outer periphery of the spray. This effect is reduced with increase in liquid jet velocity thereby increases the liquid jet momentum. A minimum variation for water is observed at a liquid velocity of 7.8 m/s. At this condition, there might be a balance between breakup due to liquid impingement and breakup due to the gas jet. The results for ethanol and water–glycerol mixtures (30:70 and 20:80) are shown in Fig. 19(b–d) respectively. The variation between ethanol and water is not significant implying that the effect of surface tension is not significant in this mode of atomization. Results obtained with water–glycerol mixture show that increasing the liquid jet velocity up to 5.8 m/s reduces the radial SMD variation. However, a liquid jet velocity of 7.8 m/s resulted
150
150
100
100
50
50
0
0
(a)
(b)
Fig. 17. Spray angle comparison for (a) Water, and (b) water–glycerol (20:80) mixture.
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150
°
Spary angle ( )
Water Water−glycerol (50:50) Water−glycerol (20:80) 100
50
0
0
2
4
6
8
10
Liquid jet velocity (m/s)
parameters. Hence, the variation in non-dimensional SMD values as a function of non-dimensional parameters such as the GLR and momentum flux ratio (gas jet to liquid jet) in Figs. 20 and 21, respectively. The momentum flux ratio is calculated considering the momentum flux at the exit of the orifice which in turn is estimated from the flow rates. Interestingly, the results show that the non-dimensional SMD values for the various liquids show a definite trend with both GLR and momentum flux ratio. More importantly, the liquid properties do not affect the atomization significantly after a critical GLR or momentum flux ratio. In other words, the non-dimensional SMD is around 0.8 beyond a critical GLR of 0.05 and a critical momentum ratio of around 4. Moreover, this SMD value is constant over a viscosity range from 1 to 39 mPa s and surface tension ranging from 22 to 72 mN/m. This is a very useful finding and has interesting ramifications for fuelflexible systems.
Fig. 18. Spray angle variation at an impinging angle of 90° with 0.76 mm gas orifice.
(a) Water
(c) Water-glycerol (30:70) mixture
0.5 Water Ethanol Water−ethanol (90:10) Water−ethanol (75:25) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
0.45 0.4 0.35 SMD/dg
in a spray with larger SMD at the center and smaller SMD towards the outer edge. This is due to fact that at this velocity, the two liquid jets interact resulting in the classical mode of atomization. It is also observed that the absolute values of the SMD are comparable for all the liquids up to 5.8 m/s liquid jet velocity which implies that effect of liquid properties is not significant at these conditions. It is also observed that SMD of the order 40 lm can be achieved even with the liquid of viscosity of 39 mPa s at GLR below 0.1 using this method of atomization. To compare results across the various conditions, the overall SMD is calculated by combining the droplet statistics obtained at all radial locations at a particular axial location. To eliminate the effect of gas orifice diameter, a non-dimensional SMD is calculated by dividing the overall SMD with gas orifice diameter. Since there is extensive data on droplet diameters for a range of liquid viscosity and surface tension, it was decided to explore possible trends of the non-dimensional SMD in terms of other non-dimensional
0.3 0.25 0.2 0.15 0.1 0.05 0 0
0.05 0.1 Gas to liquid ratio (GLR)
0.15
Fig. 20. Variation of non-dimensional SMD with GLR (closed symbols represents 1.1 mm gas orifice while open symbols are for 0.76 mm gas orifice).
(b) Ethanol
(d) Water-glycerol (20:80) mixture
Fig. 19. Variation of SMD with radial location for various liquids with 0.76 mm gas orifice.
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El ¼ V l ðPl2 P1 Þ
0.5 Water Ethanol Water−ethanol (90:10) Water−ethanol (75:25) Water−glycerol (50:50) Water−glycerol (30:70) Water−glycerol (20:80)
0.45 0.4
SMD/dg
0.35 0.3
The total energy supplied is the sum of the gas energy and the liquid energy and is given by:
Ei ¼ Eg þ El
e¼
0.2 0.15 0.1 0.05 5
10 Momentum flux ratio
15
20
Fig. 21. Variation of non-dimensional SMD with momentum flux ratio (closed symbols represents 1.1 mm gas orifice while open symbols are for 0.76 mm gas orifice).
According to Bayvel and Orzechowski (1993), energy efficiency of atomization is defined as the ratio of the energy used for the development of new liquid surface to the total energy used in the process. The energy efficiency of the pressure atomizers falls in the range of 0.05–0.07%, while the twin-fluid atomization process results in even lower efficiencies (Bayvel and Orzechowski, 1993). Lefebvre (1992) used an atomization efficiency value of 0.7 % for his model to predict droplet sizes in twin-fluid atomization. Recent studies on effervescent atomizer by Jedelsky and Jicha (2012) showed that atomization efficiency is of the order 0.2% and varies with GLR. Energy efficiency of the present method of atomization is analyzed by calculating the ratio of surface energy change of the liquid to the total energy input. The total energy input in air-assisted atomization process is the sum of atomizing gas energy (Eg) and the energy of supplied liquid (El) (Jedelsky and Jicha, 2012). Initial bulk liquid surface energy of the liquid is calculated by assuming liquid as a cylinder with a diameter equal to the liquid injection orifice. Initial surface energy of the liquid is given by:
SEi ¼ r p do L
ð7Þ
The efficiency of the atomization process is given by:
0.25
0 0
ð6Þ
DSE Ei
ð8Þ
Efficiency of the impinging jet air-assisted method is calculated for various liquids at several GLRs using the above methodology. It is assumed that the bulk liquid is transformed into spray with the droplet sizes of overall SMD mentioned in the previous section at the corresponding conditions. Atomization efficiency is computed for both 1.1-mm and 0.76-mm gas orifice cases and the results are presented in Fig. 22 as a function of GLR. In the case of 1.1mm gas orifice, atomization efficiency first increases with GLR, reaches a maximum and then reduces for all the liquids. This variation is due to that fact that at higher GLRs, the atomizing gas carries much more energy than required and hence the efficiency is low. Also, at lower GLRs, the atomization happens predominantly due to impingement of liquid jets and hence the utilization of gas energy is lower. A balance between the two modes of atomization occurs at a mid-range of GLRs, hence the atomization efficiency peaks at these GLRs. The maximum efficiency occurs around a GLR of 0.05 for all the liquids. The maximum value of atomization efficiency is of the order 0.25 which is similar to the values reported in the literature (Jedelsky and Jicha, 2012). A var-
ð1Þ
where r is surface tension of the liquid, do is the liquid orifice diameter and L is the length of the liquid jet. Due to the atomization process, initial bulk liquid is transformed to droplets of diameter D. The total liquid surface energy after atomization is obtained as a sum of surface energies of all the droplets, and is given by:
SEf ¼ n r p D2
ð2Þ
(a) Atomization efficiency with 1.1 mm gas orifice
where n is the total number of droplets which can be calculated by mass conservation of the liquid before and after the atomization process as shown below.
n¼
4ml
ql pD2
ð3Þ
Hence, change in the surface energy of the liquid is calculated as:
DSE ¼ SEf SEi
ð4Þ
Energy introduced by the atomizing gas is calculated as isothermal compression energy required to pressurize the gas mass mg from ambient pressure P1 to the pressure Pg2 upstream of the air injection orifice (Jedelsky and Jicha, 2012).
Eg ¼ P1 V 1 lnðP g2 =P1 Þ
ð5Þ
where V1 is the volume of gas at ambient pressure. The energy contained in the liquid supplied El is given by:
(a) Atomization efficiency with 0.76 mm gas orifice Fig. 22. Energy efficiency of the proposed method at various conditions for different liquids.
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iation in the efficiency with liquid properties is observed with both the gas orifices. It is observed that atomization efficiency is lower for liquids with lower surface tension. Although the droplet sizes are similar for liquids with varying surface tension which implies that the change in surface area is similar, the surface energy change due to atomization is lower for lower surface tension liquids. This results in a lower efficiency value for the same amount of atomizing gas energy. It is also observed that increase in the liquid viscosity also reduces the atomization efficiency. This is attributed to two reasons, the first being inferior atomization due to higher viscosity and hence lesser surface energy change, and secondly, higher viscosity liquids requiring higher pressure to pump same volume of liquid and hence higher energy input. It is also observed that atomization efficiency is higher for the 0.76mm orifice at lower GLR compared to the 1.1-mm orifice. The opposite trend is observed at higher GLRs. This is due to the fact that improvement in atomization is significant at lower GLRs when the gas orifice is changed from 1.1-mm to 0.76-mm, while the change in SMD is less at higher GLRs. 4. Conclusions A novel, air-assisted impinging jet atomization strategy in which a gas jet is directed on to the impinging point of two liquid jets is proposed and demonstrated experimentally. Specifically, the effect of liquid properties such as viscosity and surface tension, liquid jet impingement angle and gas jet orifice diameter is studied. Depending on the liquid and gas flow rates, two modes of atomization are observed viz., classical and prompt. Critical values of nondimensional parameters such as GLR and momentum flux ratio have been identified at which the mode of atomization changes from ‘classical mode’ to ‘prompt mode’. The spray structure images showed that changing the impingement angle between the liquid jets does not change the atomization mode, however, it only affects the spray angle. The effect of surface tension is observed to be lesser as compared to the effect of viscosity. A delay in the liquid breakup is observed with increase in the liquid viscosity, especially in the classical mode of atomization. Reducing the gas orifice diameter led to increased gas jet momentum which in turn caused improved atomization at all conditions. This is attributed to the change in atomization mode from classical to prompt. Droplet size measurements are conducted at a gas flow rate of 12 gpm using both 1.1-mm and 0.76-mm gas orifices. It is observed that SMD of the order 40 lm can be achieved even with a liquid of viscosity of 39 mPa s at GLR below 0.1. It is also observed that beyond the critical values of GLR of 0.05 and momentum flux ratio of 4, the liquid properties do not affect the atomization significantly. This implies that ‘‘irrespective of the liquid used over a large range of viscosity or surface tension, the resulting SMD is same with the
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proposed method’’. Specifically, the SMD of the spray obtained through the proposed method is constant over a viscosity range from 1 to 39 mPa s and surface tension ranging from 22 to 72 mN/m. This is a very important point and has interesting ramifications for flexible fuel systems in combustors. References Ashgriz, N., 2011. Handbook of Atomization and Sprays. Springer. Bailardi, G., Negri, M., Ciezki, H.K., 2010. Several aspects of the atomization behavior of various Newtonian fluids with a likeon-like impinging jet injector. In: Proc. 23rd European Conf. on Liquid Atomization and Spray Systems, Brno, Czeck Republics. Bayvel, L., Orzechowski, Z., 1993. Liquid Atomization. Taylor & Francis Inc. Beck, J.E., Lefebvre, A.E., Koblish, T.R., 1991. Liquid sheet disintegration by impinging air streams. Atomization Sprays 1, 155–170. Bremond, N., Villermaux, E., 2006. Atomization by jet impact. J. Fluid Mech. 549, 273–306. Dombrowski, N., Hooper, P., 1964. A study of the sprays formed by impinging jets in laminar and turbulent flow. J. Fluid Mech. 18, 392–400. Bush, J., Hasha, A., 2004. On the collision of laminar jets: fluid chains and fishbones. J. Fluid Mech. 511, 285–310. Choo, Y., Kang, B., 2003. A study on the velocity characteristics of the liquid elements produced by two impinging jets. Exp. Fluids 34, 655–661. Blaisot, J.B., Yon, J., 2005. Droplet size and morphology characterization for dense sprays by image processing: application to the diesel spray. Exp. Fluids 39, 977– 994. Esmail, M., Kawahara, N., Tomita, E., Sumida, M., 2010. Direct microscopic image and measurement of the atomization process of a port fuel injector. Meas. Sci. Technol. 2, 075403. Feras, Z.B., Roisman, I.V., Tropea, C., 2010. Characterization of spray generated by an airblast atomizer with prefilmer. Atomization Sprays 20, 887–903. Ibrahim, E.A., Przekwas, A., 1991. Impinging jets atomization. Phys. Fluids A 3, 2981–2987. Jedelsky, J., Jicha, M., 2012. Energy conversion in effervescent atomization. In: Proc. 12th Triennial Conf. on Liquid Atomization and Spray Systems, Heidelberg, Germany. Kashdan, J., Shrimpton, J., Whybrew, A., 2004. Two-phase flow characterization by automated digital image analysis. Part 2: Application of PDIA for sizing sprays. Part. Part. Syst. Charact. 21, 15–23. Kim, K.S., Kim, S.S., 1994. Drop sizing and depth-of-field correction in TV imaging. Atomization Sprays 4, 65–78. Koh, K.U., Kim, J.Y., Lee, S.Y., 2001. Determination of in-focus criteria and depth of field in image processing of spray particles. Atomization Sprays 11, 317–333. Lai, W., Haung, W., Jaing, T., 1999. Characteristic study on the like-doublet impinging jets atomization. Atomization Sprays 3, 277–289. Lee, S.Y., Kim, Y.D., 2003. Sizing of spray particles using image processing technique, ICLASS-2003, Sorrento, Italy. Lefebvre, A.H., 1989. Atomization and Sprays. Hemisphere Publishing Corporation. Lefebvre, A.H., 2000. Fifty years of gas turbine fuel injection. Atomization Sprays 10, 251–276. Lefebvre, A.H., 1992. Energy considerations in twin-fluid atomization. J. Eng. Gas Turbine Power 114, 89–96. Ren, N., Marshall, A.W., 2012. Characterizing the initial spray from large Weber number impinging jets. Int. J. Multiph. Flow, http://dx.doi.org/10.1016/ j.ijmultiphaseflow.2012.08.004. Shen, Y., Mitts, C., Poulikakos, D., 1997. Holographic investigation of the effect of elevated ambient temperature on the atomization characteristics of impinging jet sprays. Atomization Sprays 7, 123–142. Yang, L., Fu, Q., Qu, Y., Gu, B., Zhang, M., 2012. Breakup of a power-law liquid sheet formed by an impinging jet injector. Int. J. Multiph. Flow 39, 37–44.