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Effect of liquid viscosity on atomization in an internal-mixing twin-fluid atomizer
Fuel 103 (2013) 486–494
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Effect of liquid viscosity on atomization in an internal-mixing twin-fluid atomizer Zhouhang Li a, Yuxin Wu a, Hairui Yang a,⇑, Chunrong Cai a, Hai Zhang a, Kazuaki Hashiguchi b, Keiji Takeno b, Junfu Lu a a b
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Nagasaki R&D Center, Mitsubishi Heavy Industries, Ltd., Japan
h i g h l i g h t s " SMD was nearly independent of viscosity and pressure at small axial distances. " SMD increased with increase of viscosity at high pressures and large axial distances. " A high viscosity of 120 mPa s changed Volume-based Droplet Size Distribution markedly. " The decay of droplet velocities along the axis was stronger at a larger viscosity. " A larger viscosity made radial distributions of both SMD and velocities flatter.
a r t i c l e
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Article history: Received 21 March 2012 Received in revised form 18 June 2012 Accepted 27 June 2012 Available online 20 July 2012 Keywords: Internal-mixing twin-fluid atomizer Viscosity SMD Velocity Volume-based droplet size distribution
a b s t r a c t Experimental studies on atomization in an internal-mixing twin-fluid atomizer are reported over a wide range of liquid viscosity, gas supply pressure and Gas to Liquid mass Ratio (GLR). Among all test conditions, the finest sprays were obtained at an axial distance of 150 mm and Sauter Mean Diameter (SMD) was dominated by GLR while being nearly independent of viscosity and pressure. However, droplet size distributions changed obviously when viscosity increased to 120 mPa s. More large droplets were produced and large droplets always took up a large proportion, and spray quality deteriorated, though SMD changed little. At large axial distances (250 or 400 mm) SMD increased with the increase of viscosity when the atomizer was operated at high pressure and GLR. The decay of axial velocities along the spray centerline was stronger at a larger viscosity. In the radial direction, an increase in viscosity made distributions of both SMD and velocity flatter. An increase in GLR generated more small droplets in the expense of large droplets and hence improved spray quality. Pressure had a weaker promotion on the droplet size distribution when comparing to GLR. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Heavy fuel oil has been used as the substitute of high quality oils due to its more reserves and less cost [1]. Atomizing this high-viscosity liquid adequately is challenging. The Y-jet atomizer, which is a semi-internal-mixing twin-fluid atomizer, is most commonly used for atomization of heavy fuel oil in industrial burners [2,3]. In order to acquire finer droplets at lower pressure with less gas, one alternative for the Y-jet atomizer is the effervescent atomizer, which was first developed by Lefebvre et al. [4]. In effervescent atomizers, gas is injected directly into liquid and a bubbly flow or annular flow is formed after sufficient two-phase mixing. There are two basic types of injecting atomizing gas into liquid, referred as outside-in/inside-out gas injection geometry. Sovani et al. [5] made a state-of-the-art review and concluded that com⇑ Corresponding author. Tel./fax: +86 10 62781743. E-mail address: [email protected] (H. Yang). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.097
paring to Y-jet atomizers, both kinds of effervescent atomizers attain a comparative spray quality with a less expense of gas. Another alternative for the Y-jet atomizer is an internal-mixing twin-fluid (IMTF) atomizer with coaxial gas/liquid feed. This kind of atomizer is also regarded as the effervescent atomizer [6] despite of less formation of gas bubbles. IMTF atomizers allow independent controls of gas and liquid input velocity, direction and distribution. Barreras et al. [7–9] and Li et al. [10] have conducted a series of researches and they found that IMTF atomizers can produce smaller droplets with less gas. Moreover, the atomizing gas in both an effervescent atomizer and an IMTF atomizer helps protect the heavy fuel oil in the atomizer from attack of hot flue gases especially near the nozzle exit [11,12], which extends the atomizer service life. Influence of liquid viscosity on atomization must be considered in heavy fuel oil atomization because of its large viscosity even if the atomizer is operated at a high operating temperature. Buckner and Sojka [13] studied sprays of glycerin-water solutions
Z. Li et al. / Fuel 103 (2013) 486–494
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Nomenclature Cdv Dv0.5 d0 dave di GLR IMTF ml PDA p r SMD VDSD
vave
correlation coefficient between droplet size and velocity () 50% fractional volume diameter, volume median diameter (lm) diameter of atomizer exit orifice (mm) average droplet diameter (lm) diameter of the ith droplet (lm) Gas to liquid mass Ratio (-) Internal-mixing twin-fluid liquid mass flow rate (g/s) Phase Doppler Anemometry gauge pressure at the gas inlet (MPa) radial distance from spray axis(mm) Sauter Mean Diameter, D32 (lm) Volume-based Droplet Size Distribution average droplet axial velocity (m/s)
(400–970 mPa s) and found SMD (Sauter Mean Diameter) was nearly independent of liquid viscosity at a measuring axial distance (x) of 150 mm. Sutherland et al. [14] studied liquids with viscosity of 1–80 mPa s and observed only a small effect of viscosity on droplet size at an x of 150 mm. Lund et al. [15] also measured droplet size at an x of 150 mm and found that SMD increased only about 15% when viscosity increased from 20 to 80 mPa s. However, other researchers drew different conclusions. Loebker and Empie [16] sprayed liquids with viscosity up to 7 Pa s and found that at a very far measuring position (x is approximately 1300 mm), droplet mean size increased sharply with viscosity. Broniarz-Press et al. [17] measured droplet sizes of different liquids (1.92–26.4 mPa s) at a large x of about 600 mm and they also observed a considerable increase in SMD with the increase of viscosity. Once again, results of Sataphthy et al. [18] showed that SMD increased markedly with viscosity at very high pressure (11–33 MPa). Chen and Lefebvre [19,20] elevated the ambient air pressure to above 0.6 MPa and a considerable increase in SMD was observed with the increase of viscosity (1–100 mPa s). It should be noted here that all the above results are measured on the spray centerline, but at different x. The droplet radial distribution is also an important index of spray quality and some researchers have already paid attention to it. Ferreira et al. [21] conducted experiments on two liquids (24 mPa s and 196 mPa s) at various x (20, 50, 100 and 200 mm) and found that as viscosity decreased the droplet size near the centerline also decreased, while droplet size at larger radial positions gradually increased. Droplet size radial distributions were much steeper at a lower viscosity, and influence of viscosity on the droplet size radial distribution became less as x increased. Ejim et al. [22] studied the effect of viscosity (1 mPa s and 67 mPa s) at x of 100 mm and 202 mm. They found that a lower viscosity produced a steeper droplet size radial distribution in the relatively far region (x = 202 mm), the same result with that of Ferreira et al. [21] when x = 20 mm or 50 mm. Droplet size radial distribution at an x of 100 mm was influenced less by viscosity. These observations have not been explained in either study, but according to the velocity profiles given by Ferreira et al., viscosity influenced radial distributions of droplet axial velocities as well, and there may be some relationship between SMD and velocities. However, since Ejim et al. did not present their droplet axial velocities, this finding needs to be validated. Publications reported influences of viscosity in the very nearnozzle region are relatively less. High-speed flash photographs in the near-nozzle region were obtained by Buckner and Sojka [13].
vi x x⁄
axial velocity of the ith droplet (m/s) axial distance downstream from the atomizer exit orifice (mm) dimensionless axial distance, x⁄ = x/d0
Greek letters a spray half angle, a = tan1 (r/x) (degrees) Dpgl pressure differential between gas and liquid at the atomizer inlet (kPa) r surface tension (m N/m) l liquid viscosity (mPa s, 103 kg m1 s1) q liquid density (kg/m3) Subscripts g gas l liquid
Their shadow photographs showed that very large bubbles (termed macrobubbles) existed in the near-nozzle region, while disappeared when the axial distance increased. They claimed that macrobubbles play a consequential role in the spray formation process. A holographic investigation of near-nozzle spray structure was carried out by Santangelo and Sojka [23], using liquids spanning a range of viscosity. Two distinct near-nozzle flow patterns, bubbly flow and annular flow, were observed under different GLR, and viscosity had only a minimal effect on near-nozzle two-phase flow. In the near-nozzle region, the laser light attenuation caused by the very dense spray makes it difficult to take an accurate measurement on the droplet size. Ferreira et al. [21] measured sprays at a very near-nozzle position (x = 20 mm) and found that SMD was sensitive to liquid viscosity especially at positions far away from the centerline. Unfortunately, they didn’t specify the accuracy of their measurements. Besides, their atomized liquids were the same kind of oil at different temperatures and hence had different surface tensions which were not specified either. More studies are needed on the independent influence of viscosity on droplet size in the near-nozzle region. Although a large volume of information is already available in the open literature, reported results are quite different and the influence of liquid viscosity on atomization is not well understood yet, especially not for the IMTF atomizer. The objective of this work is to investigate the independent influence of liquid viscosity on atomization in an IMTF atomizer by analyzing both SMD and the Volume-based Droplet Size Distribution (VDSD) over a wide range of operating conditions. Especially, effects of viscosity on the VDSD were analyzed at different measuring positions and operating conditions, since VDSD has a direct relationship to the burner performance for heavy fuel oil. Influence of viscosity on the droplet axial velocity is also studied. 2. Experimental apparatus 2.1. Atomizer and test rig The designed IMTF atomizer, presented in Fig. 1a, is made by acrylic glass for visualization study on the internal two-phase mixing. The overall length of the atomizer is 600 mm. Liquid inlet of the mixing chamber has a diameter of 4 mm. Liquid inlet is surrounded by four 1 mm air inlet holes with a slant angle of 45°. The length of mixing chamber is 55 mm. Atomizer has an exit orifice with a diameter (d0) of 3 mm, which enables the atomizer
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air
liquid
(a)
1-pressurized liquid tank 2-control valv e 3-regulating valve 4-flow meter 5- pressure gauge 6-tested atomizer 7-collecting container 8-circulating pump
(b) Fig. 1. (a) Schematic of the IMTF atomizer, (b) schematic of the test rig.
to spray liquid at a high mass flow rate (up to 110 g/s among the studied sprays). The test rig is depicted in Fig. 1b. A liquid tank is pressurized by compressed air up to 1.5 MPa. A needle valve and a ball valve were installed on the liquid/gas supply line to adjust gas/liquid flow rates precisely to the desired values. Gas flow meter can measure a flow rate up to 250 Nm3/h, with an uncertainty of 2%. The liquid flow meter is a modified one which is fitted for measurement of high-viscosity liquids and was first calibrated by weighing liquids with different viscosities (1 mPa s, 30 mPa s, 120 mPa s and 200 mPa s). The uncertainty of liquid flow rates is within 3% over the test range. Pressure gauges were located just before inlets of gas/liquid with a precision of 2 kPa. Liquid is collected in a container and pumped back to the liquid tank after the high pressure air in the liquid tank is discharged. 2.2. Atomizer operation Desired liquids with different viscosities and almost the same surface tension were formulated by changing the mass ratio of industrial glycerin to water. Liquid viscosity was measured by a
Physica MCR300 rheometer of Anton Paar. The rheometer was calibrated using deionized water (1 mPa s) and two standard oils (10 mPa s and 100 mPa s). The uncertainty of viscosity is less than 2%. Surface tension measurements were made on a Kruss K12 surface tensiometer by the Wihelmy plate technique. The uncertainty is less than 2% over the experimental range after the tensiometer was calibrated using deionized water. Moreover, an Abbe refractometer (Model G, Carl Zeiss, Jena) with an uncertainty of 1% was used to measure the refraction index of tested liquids. All of these properties were measured at 11 ± 0.1 °C, matching the room temperature (11 ± 1 °C) of test conditions. Measured results are listed in Table 1, where liquid densities were obtained by weighing method. The lower three liquids in Table 1 have surface tensions spanning from 61.3 mN/m to 74.1 mN/m. This small variation in surface tension has only little impact on the spray droplet size [14,15,22], therefore these three liquids were sprayed to investigate the independent influence of viscosity on atomization over a range of 1.3–120 mPa s (120 mPa s is comparative to the viscosity of heavy fuel oil under operating conditions). Considering the variation of room temperature, viscosity has an additional uncertainty of 5% for the higher viscosity solution.
Table 1 Physical properties of liquids (at room temperature of 11 °C). Liquid
l (mPa s)
r (mN/m)
q (kg/m3)
Refractive index
Industrial glycerin Water Glycerin/water (62/38 wt%) Glycerin/water (79/21 wt%)
2312 ± 24 1.3 ± 0.04 30 ± 0.2 120 ± 2.7
57.5 ± 0.03 74.1 ± 0.01 61.3 ± 0.04 66.3 ± 0.06
1260 998 1148 1196
1.48 1.33 1.42 1.44
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Z. Li et al. / Fuel 103 (2013) 486–494 Table 2 Experimental operating conditions.
l (mPa s)
p (MPa)
GLR 0.02
Dpgl (kPa)
0.05
Dpgl (kPa)
0.10 ml (g/s)
0.20
Dpgl (kPa)
ml (g/s)
8 13 –
34.7 66.7 –
11 5 7
23.1 42.5 56.9
4 9 1
15.3 26.4 38.3
14 4 6
12.2 21.9 33.3
10 7 13
11.4 19.7 27.8
30
0.1 0.3 0.5
9 15 12
34.4 77.6 102.8
8 5 6
26.3 46.6 62.4
5 6 4
17.4 31.6 42.8
3 5 7
13.6 25.2 34.3
9 10 14
11.3 21.5 28.3
120
0.1 0.3 0.5
19 10 8
36.9 82.4 110.1
8 7 12
25.4 48.4 69.5
6 10 8
18.8 31.7 45.2
4 4 9
15.6 24.4 34.9
11 7 14
13.6 20.2 28.8
This study spans a GLR range of 0.02–0.20. The gauge pressure at the gas inlet, p, is 0.1–0.5 MPa. Detailed operating information, including liquid mass flow rate (ml) and pressure differential between gas and liquid at the atomizer inlet (Dpgl), is listed in Table 2.
Dpgl (kPa)
0.15
0.1 0.3 0.5
1.3
ml (g/s)
ml (g/s)
Dpgl (kPa)
ml (g/s)
droplet mean size of selected measurements can well represent the real droplet size distribution in the PDA measuring volume. Specific criteria have been mentioned in Section 2.3. Moreover, error bars haven’t been included in the SMD and velocity profiles for the sake of clarity, but uncertainties have also been discussed earlier.
2.3. Phase doppler anemometry (PDA) The droplet size and velocity distributions were measured using a Dantec Dynamics fiber PDA. An argon-ion laser generator and optical splitter were used to generate two laser beams with a spacing of 60 mm. The wavelength was 514.5 nm for beam U1 (green) and 488 nm for beam U2 (blue). The beam diameters were 1.35 mm and beam expander ratios were set to 1. The shift frequency of one of the two beams is 40 MHz. The focal length for the transmitting lens was 800 mm which resulted in a beam intersection angle of 4.3°. The fringe spacing was 6.87 lm and 6.51 lm for U1 and U2, respectively. Fringes moved in a negative direction. For data collection, the PDA was operated in 2nd order refraction mode. The receiver focal length was 600 mm. Depending on the refraction index of sprayed liquids, scattering angle was altered over a range of 151–163° to ensure that 2nd order refracted light is dominant. Aperture mask A was chosen here based on the droplet size. A 3D auto-controlled traversing system allows measurements conducted at different spatial locations. PDA measurements were performed at axial distances (x) of 50, 100, 150, 200, 250 and 400 mm downstream from the exit orifice of the atomizer. The spherical validation band was set to 10% to ensure a high sphericity of recorded droplets and a high accuracy of droplet size. The laser power was varied in 130–170 mW at different x to ensure a high validation rate of measurements. These optical selections enable PDA to measure droplet size up to 250 lm. The sample size at every measuring position was set to 10000 and sampling time was 30 s which enable estimation of droplet size and velocity statistics with confidence. Repeated measurements were taken at every operating condition and only a selection of measurements has been employed to analyze spray quality. Measurements used to plot figures have a validation rate about 85%, and the spherical validation rate of selected measurements is around 75%. Using these chosen data, uncertainties (relative standard deviation) for SMD and velocity are 5% and 6%, respectively. 3. Results and discussion Measurements used to present the following figures have a high validation rate to ensure that results were only disturbed little by the background noise. A high spherical validation rate is another criterion for choosing plotted data, which makes us confident that
3.1. Effects of viscosity on SMD and droplet velocity on the spray centerline In this section, only a part of measurements on the atomizer centerline are chosen and presented below to study the effect of viscosity at different GLR and p. Fig. 2 shows variations of SMD with GLR for different viscosities and pressures. In all study cases for a constant viscosity l, SMD decreases with an increase in GLR when other conditions are unchanged. This general rule was also observed by many researchers [5]. Measurements at different x⁄ (x⁄ = x/d0) indicate that the finest atomization can be obtained at an x⁄ of around 50, i.e., x = 150 mm, which means secondary atomization is completed at this distance. This is in accordance with results of Whitlow and Lefebvre [24] and Jedelsky et al. [6]. Meanwhile, droplet sizes at x larger than 150 mm are less concerned for heavy fuel oil combustion applications, because droplets there will interact with the combustor flow field and evaporate. Therefore, droplet size at an x of 150 mm is of great importance, as studied by quite a number of researchers [6,24–26]. As shown in Fig. 2, at an x of 150 mm the designed atomizer produces sprays with SMD less than 70 lm at a low operating condition of p = 0.1 MPa and GLR = 0.1 even for liquids with high viscosities. These results prove that the tested IMTF atomizer has a good performance at all test conditions. In most cases, SMD was dominated by GLR and nearly independent of l, especially at low GLR or p. While at high p and GLR, i.e. p = 0.3 or 0.5 MPa and GLR = 0.15 or 0.20, SMD tended to increase with the increase of l. Measurements show that and an increase in l from 1.3 mPa s to 120 mPa s produced a maximum increase in SMD of about 27%, which is much higher than the uncertainty. According to Fig. 2b and c, the negative effects of viscosity become stronger as GLR increases. Since the influence of viscosity becomes obvious at high GLR, SMD distributions along x⁄ when GLR is 0.2 are presented in Fig. 3. At a low p of 0.1 MPa, l affected SMD little at any x⁄. However, an increase in SMD with the increase of l was observed at a higher p of 0.5 MPa and SMD differences among different l increased with the increase of x⁄ as well. SMD increased up to 49% as l increased from 1.3 mPa s to 120 mPa s at a faraway distance of x⁄ = 133. As mentioned in the Introduction, some previous works [13–15] indicate that viscosity has little influence on SMD while others [16–18] show an obvious increase in SMD with viscosity. Results in this study explain this discrepancy well and as a conclu-
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Fig. 2. SMD versus GLR for different l and p when x⁄ = 50: (a) p = 0.1 MPa, (b) p = 0.3 MPa, (c) p = 0.5 MPa.
Fig. 3. SMD versus x⁄ for different l: (a) p = 0.1 MPa, (b) p = 0.5 MPa.
sion, the negative influence of viscosity on SMD becomes obvious only at high operating pressures or large x⁄. Since both droplet size and velocity are influenced by viscosity, a study of the correlations between them is helpful for understanding the effect of viscosity on atomization. Relationship of the single particle size and the single particle velocity shall be investigated. Here the correlation coefficient Cdv was introduced to correlate particle velocity and size:
C dv ¼
X i
," # X X ðdi dav e Þ ð v i v av e Þ
ðdi dav e Þðv i v av e Þ
i
i
ð1Þ where di and vi are the diameter and velocity of the ith particle, dave and vave are the algebraic average value of particle diameter and velocity. Cdv close to 0 indicates the little correlation between particle size and velocity, and a positive value of Cdv means that a larger particle has a higher velocity and a smaller particle has a lower velocity. Plots of Cdv on the spray centerline versus x⁄ when
p = 0.3 MPa and GLR = 0.1, are depicted in Fig. 4. Over the measuring range Cdv was always less than 0.3, indicating a very weak positive correlation between particle sizes and velocities on the spray centerline. At the near-nozzle region (x⁄ = 17), Cdv is close to 0.0 for low viscosity liquids. This result reveals that particle sizes are not related with particle velocities. Since small droplets correspond with the local gas phase very well, it’s proved that a complete momentum exchange occurs between the gas and liquid at the near-nozzle region for the low viscosity solutions. As the viscosity increases, Cdv at an x⁄ of 17 is 0.14 and is larger than Cdv of low viscosity solutions. As x⁄ increases, Cdv increases as well and keeps almost constant when x⁄ is larger than 50. The increase in Cdv is reasonable since the negative drag force on small droplets is larger as gas velocity decreases with increase of x⁄. As a result, small droplets have smaller axial velocities than axial velocities of the large droplets, although the difference is not large. The weak correlation between droplet size and velocity indicates that measured velocities are more likely to be the velocity distribution of the flow field within spray, and number averaged
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Fig. 4. Cdv versus x⁄ when p = 0.3 MPa, GLR = 0.10.
droplet velocity can well represent characteristics of the flow field on the centerline. Thus, velocity distributions of different l along x⁄ are presented in Fig. 5. The velocity distributions at various operating conditions show that a larger l results in a sharper decrease in velocity with the increase of x⁄. At an x⁄ of 17 where is relatively near the atomizer exit, larger l tended to have a higher velocity, especially at high GLR or p. This can be attributed to the increase of the liquid mass flow rate (shown in Table 2) with the increase of l. Velocities of higher l were lower as x⁄ increased due to the sharper decrease. Fig. 5 also shows the promotion of GLR or p on velocity. Thus, a lower p leads to a decrease in spray velocities while only a small change in SMD, which is desired in some cases where velocities are too high to result in the local flame extinction [27]. In summary, there is a strong interaction between the gas phase and liquid phase at the near nozzle region of an IMTF atomizer. As liquid viscosity increases, a sharper velocity decay reveals a stronger gas–liquid interaction. 3.2. Effects of viscosity on volume-based droplet size distribution (VDSD) In heavy fuel oil combustion, burning out large droplets generally spends a long time, which is proportional to the droplet diameter. As a result, an increase in the amount of large droplets reduces the combustion efficiency and is undesired during the atomizer operation. Since the proportion of large droplets may be different even though SMD are the same, a detailed investigation on the VDSD is necessary. The distribution can also help us further understand the roles of viscosity, GLR and pressure in spray formation. Fig. 6 shows typical VDSD of different l observed in this study and SMD are noted in these figures. Bimodal distributions were observed when x⁄ = 17 for all three l and x⁄ = 50 for 120 mPa s. The large liquid sheet is preliminarily broken up into fragments at the nozzle exit, due to the pressure drop and gas expansion, while the breakup is not sufficient and new-formed fragments are still large. Meanwhile, the intense gas–liquid interaction on the twophase interface strips some small drops out of liquid fragments. The combination effect forms bimodal distributions at an x⁄ of 17, where is relatively near the nozzle exit. Comparing VDSD of different viscosity solutions at this position, the large droplet volume fraction is pretty large for the high viscosity solution. This difference shows that the primary atomization for the high viscosity solution is worse than low viscosity solution. As atomization proceeds, fragments break up further and small drops begin to merge. The distribution changes to unimodal when x⁄ = 50 for two lower l. For 120 mPa s, note that the volume percent of large droplets is much larger than that of small droplets when x⁄ = 17, so the distribution when x⁄ = 50 is still bimodal, despite of the lower peak
Fig. 5. Average droplet axial velocity distributions along x⁄ for different l: (a) p = 0.1 MPa, GLR = 0.2, (b) p = 0.3 MPa, GLR = 0.2, (c) p = 0.3 MPa, GLR = 0.1.
value in the large size region. With a further increase in x⁄ to 133, right shifts of the peaks are observed, suggesting the dominance of droplet coalescence in the spray. Distributions for all three l are unimodal. Remarkably, when l = 120 mPa s, no peak appears in the small-diameter region and a high peak is formed at a large diameter. In general, an increase in l from 1.3 mPa s to 30 mPa s changes distributions little at all x⁄, while VDSD of 120 mPa s is quite different from VDSD of two lower l and spray quality of 120 mPa s is worse when x⁄ is 50, though SMD of three liquids are almost the same. One interesting observation is that the range of droplet size shrinks when l increases. For the high viscosity liquid, the measured largest particle size is only 180 lm, which is 20 lm less than the other two solutions. This observation shows that atomization process of a higher l may be steadier than that of lower l. Based on Fig. 6, it is reasonable to consider that a high l of 120 mPa s produces much more large droplets than the lower l at the nozzle exit due to the larger viscous force, which indicates that viscosity has an important influence on the primary atomization in an IMTF atomizer. Comparing Fig. 6a and b, as x⁄ increases to 50 where secondary atomization is completed, change trends of VDSD are the same for all three l, and changes in peak values are
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Fig. 6. VDSD and cumulative VDSD for different l (p = 0.5 MPa and GLR = 0.10): (a) x⁄ = 17, (b) x⁄ = 50, (c) x⁄ = 133.
almost the same, and changes in SMD are also very similar (about 5 lm for all three l). These observations indicate that viscosity has only a little influence on secondary atomization, and the differences among VDSD in Fig. 6b are the subsequent result of viscosity effect on primary atomization. Comparison between Fig. 6b and c shows that when l = 120 mPa s, VDSD shifts right more obviously and very large drops take up a larger fraction. This indicates that in the developed region (x⁄ > 50) l either strengthens the droplet coalescence or restrains breakup of large droplets. The possible reason is that high viscosity liquid experiences larger drag forces, owing to a greater gas–liquid slip ratio since the drop velocities decay faster (as shown in Fig. 5). In conclusion, viscosity affects primary atomization a lot, has only a little influence on secondary atomization and affects droplet characteristics after the atomization is completed (x⁄ > 50). Fig. 6 also shows that in the fully-developed jet flow region (x⁄ = 133), cumulative volume curves of the lower l are typical Rosin–Rammler distributions, while the curve of l = 120 mPa s deviates a lot.
3.3. Effects of p and GLR on atomization of high viscosity liquids Effects of p and GLR on spray formation can also be seen from the cumulative VDSD, as shown in Fig. 7. Since the promotion of p or GLR on VDSD in water spray has been reported by Karnawat and Kushari [28,29], here we care only about liquid with a high viscosity of 120 mPa s at an x⁄ of 50. Fig. 7a shows the independent influence of GLR on the droplet size distribution. As GLR increased from 0.05 to 0.20, more small droplets were produced and the cumulative curve tended to move left, resulting in a decrease in SMD from 74.4 lm to 64.2 lm. When p increased independently, as Fig. 7b shows, the volume fraction of very large droplets (with a diameter > 140 lm) was almost the same at different p, thereby generating little change in SMD (from 67.9 lm to 65.4 lm). However, Dv0.5 (volume median diameter) decreased considerably as p increased to 0.5 MPa. Thus an increase in p improves spray quality, though the pressure-induced promotion on spray quality is weaker than that induced by the increase of GLR.
Fig. 7. Influence of operating conditions on droplet size distribution: (a) influence of GLR, (b) influence of p.
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Fig. 8. Radial variations of SMD and velocity: (a) p = 0.1 MPa, GLR = 0.1, (b) p = 0.1 MPa, GLR = 0.2.
3.4. Effects of viscosity on spray radial distributions Radial variations of SMD versus the spray half angle (a) are presented in Fig. 8, where dotted/dashed lines were plotted to highlight differences in profiles. Measurements for a constant l are in accordance with the result reported by previous researchers [30,31] that SMD increases as the measuring position was moved radially away from the centerline. For different l, SMD of 1.3 mPa s was slightly lower than that of 30 mPa s near the centerline. When the radial location was moved away, SMD of lower l became larger. The corresponding velocity variations are just opposite to changes of SMD. Comparing with velocities of 30 mPa s, velocities of 1.3 mPa s were higher near the centerline but lower at larger radial positions. Both the velocity curve and the SMD curve of a lower l are steeper than those of a higher l. The comparison reveals that liquids with higher viscosities have a stronger effect on local gas and lead to a stronger entrainment of the surrounding atmosphere. It’s also found that at different operating conditions, the smallest SMD always appear on the centerline where the highest velocities appear. Besides, the smaller SMD always corresponds to the higher velocities regardless of viscosities. These observations show that there is a close relationship between SMD and velocities. 4. Conclusion Sprays in an IMTF atomizer were measured using PDA to study effects of liquid viscosity on atomization. The finest sprays were observed at an axial distance of 150 mm in the experimental range. At this distance the tested atomizer can produce droplets with SMD less than 70 lm at low p and GLR for liquids with viscosity up to 120 mPa s, and SMD is dominated by GLR while being nearly independent of liquid viscosity (l) and gas supply pressure (p). Though SMD changes little when l increases to 120 mPa s, VDSD changes a lot. Large droplets always take up a large proportion and spray quality deteriorates. Therefore, only SMD is not enough
to represent the spray quality of high-viscosity liquids when comparing with low-viscosity liquids. At large axial distances (250 or 400 mm) SMD increases with the increase of l when the operating p and GLR are high. A higher l produces a sharper decay of axial velocities along the centerline. In the radial direction, l also affects axial velocity distribution and a higher l makes the distribution flatter, meanwhile, radial SMD distribution is flatter as well. A close relationship between SMD and velocities is observed. Acknowledgements This material is based upon work supported by National Nature Science Foundation of China Grant (No. 51076081) and Mitsubishi Heavy Industries, Ltd. References [1] Saario A, Rebola A, Coelho P, Costa M, Oksanen A. Heavy fuel oil combustion in a cylindrical laboratory furnace. Measurements and modeling. Fuel 2005;84:359–69. [2] Gong JS, Fu WB. The experimental study on the flow characteristics for a swirling gas-liquid spray atomizer. Appl Therm Eng 2007;27:2886–92. [3] Zhou YG, Zhang MC, Yu J, Zhu X, Peng J. Experimental investigation and model improvement on the atomization performance of single-hole y-jet nozzle with high liquid flow rate. Powder Technol 2010;199(3):248–55. [4] Lefebvre A, Wang X, Martin C. Spray characteristics of aerated-liquid pressure atomizers. AIAA J Prop Power 1988;4(4):293–8. [5] Sovani SD, Sojka PE, Lefebvre AH. Effervescent atomization. Prog Energ Combust 2001;27(4):483–521. [6] Jedelsky J, Jicha M, Slama J, Otahal J. Development of an effervescent atomizer for industrial burners. Energ fuel 2009;23:6121–30. [7] Barreras F, Lozano A, Barroso J, Lincheta E. Experimental characterization of industrial twin-fluid atomizers. Atomization Spray 2006;16(2):127–45. [8] Ferreira G, Barreras F, Lozano A, Garcia JA, Lincheta E. Effect of the inner twophase flow on the performance of an industrial twin-fluid nozzle with an internal mixing chamber. Atomization Spray 2009;19(9):873–84. [9] Ferreira G, Garciia JA, Barreras F, Lozano A, Lincheta E. Design optimization of twin-fluid atomizers with an internal mixing chamber for heavy fuel oils. Fuel Process Technol 2009;90(2):270–8.
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[10] Li ZH, Wu YX, Cai CR, Zhang H, Gong YL, Takeno K, et al. Mixing and atomization characteristics in an internal-mixing twin-fluid atomizer. Fuel 2012;97:306–14. [11] Wu SR, Chang WC, Chiao J. Low NOx heavy fuel oil combustion with high temperature air. Fuel 2007;86:820–8. [12] Wu SR, Chen CH, Chung IL, Lee HT. Combustion of low-calorific waste liquids in high temperature air. Fuel 2011;90:2639–44. [13] Buckner HN, Sojka PE. Effervescent atomization of high-viscosity fluids. Part 1: Newtonian liquids. Atomization Spray 1991;1(3):239–52. [14] Sutherland JJ, Sojka PE, Plesniak MW. Ligament-controlled effervescent atomization. Atomization Spray 1997;7(4):383–406. [15] Lund MT, Sojka PE, Lefebvre AH, Gosselin PG. Effervescent atomization at low mass flow rates. Part I: The influence of surface tension. Atomization Spray 1993;3(1):77–89. [16] Loebker D, Empie HJ. High mass flowrate effervescent spraying of a high viscosity Newtonian liquid. In: Proceedings of the 10th annual conference on liquid atomization and spray systems, Ottawa, Canada; 1997. [17] Broniarz-Press L, Ochowiak M, Rozanski J, Woziwodzki S. The atomization of water-oil emulsions. Exp Therm Fluid Sci 2009;33(6):955–62. [18] Sataphthy MR, Sovani SD, Sojka PE, Gore JP, Eckerle WA. The effect of ambient density on the performance of an effervescent atomizer operating in the MPa injection pressure range. In: Proceedings of the technical meeting of the central states section of the combustion institute, Pittsburgh, PA; 1998. pp. 76–80. [19] Chen SK. Influence of ambient air pressure on effervescent atomizer. Ph.D. thesis. Purdue University; 1992. [20] Lefebvre AH. Some recent developments in twin-fluid atomization. Part Part Syst Char 1996;13(3):205–16.
[21] Ferreira ME, Teixeira JCF, Bates CJ, Bowen PJ. Detailed investigation of the influence of fluid viscosity on the performance characteristics of plain-orifice effervescent atomizers. Atomization Spray 2001;11(2):107–24. [22] Ejim CE, Rahman MA, Amirfazli A, Fleck BA. Effects of liquid viscosity and surface tension on atomization in two-phase, gas/liquid fluid coker nozzles. Fuel 2010;89:1872–82. [23] Santangelo PJ, Sojka PE. A holographic investigation of the near-nozzle structure of an effervescent atomizer-produced spray. Atomization Spray 1995;5(2):137–55. [24] Whitlow JD, Lefebvre AH. Effervescent atomizer operation and spray characteristics. Atomization Spray 1993;3(2):137–55. [25] Chen SK, Lefebvre AH, Rollbuhler J. Influence of ambient air pressure on effervescent atomization. J Propul Power 1993;9(1):10–5. [26] Schmidt UT, Sojka PE. Air-assist pressure-swirl atomization. Atomization Spray 1999;9(2):173–92. [27] Rasbash DJ, Rogowski ZW, Stark GWV. Mechanisms of extinction of liquid fires with water sprays. Combust Flame 1960;4:223–34. [28] Karnawat J, Kushari A. Controlled atomization using a twin-fluid swirl atomizer. Exp Fluids 2006;41(4):649–63. [29] Kushari A, Karnawat J. Spray evolution in a twin-fluid swirl atomizer. Atomization Spray 2008;18(5):449–70. [30] Lund MT, Sojka PE. Effervescent atomization at low mass flow-rates. Part 2: The structure of the spray. In: Proceedings of the 5th annual conference on liquid atomization and spray systems, San Ramon, CA; 1992. pp. 233–7. [31] Sutherland JJ, Sojka PE, Plesniak MW. Entrainment by ligament-controlled effervescent atomizer-produced sprays. Int J Multiphas Flow 1997;23(5): 865–84.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Effect of liquid viscosity on atomization in an internal-mixing twin-fluid atomizer Zhouhang Li a, Yuxin Wu a, Hairui Yang a,⇑, Chunrong Cai a, Hai Zhang a, Kazuaki Hashiguchi b, Keiji Takeno b, Junfu Lu a a b
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Nagasaki R&D Center, Mitsubishi Heavy Industries, Ltd., Japan
h i g h l i g h t s " SMD was nearly independent of viscosity and pressure at small axial distances. " SMD increased with increase of viscosity at high pressures and large axial distances. " A high viscosity of 120 mPa s changed Volume-based Droplet Size Distribution markedly. " The decay of droplet velocities along the axis was stronger at a larger viscosity. " A larger viscosity made radial distributions of both SMD and velocities flatter.
a r t i c l e
i n f o
Article history: Received 21 March 2012 Received in revised form 18 June 2012 Accepted 27 June 2012 Available online 20 July 2012 Keywords: Internal-mixing twin-fluid atomizer Viscosity SMD Velocity Volume-based droplet size distribution
a b s t r a c t Experimental studies on atomization in an internal-mixing twin-fluid atomizer are reported over a wide range of liquid viscosity, gas supply pressure and Gas to Liquid mass Ratio (GLR). Among all test conditions, the finest sprays were obtained at an axial distance of 150 mm and Sauter Mean Diameter (SMD) was dominated by GLR while being nearly independent of viscosity and pressure. However, droplet size distributions changed obviously when viscosity increased to 120 mPa s. More large droplets were produced and large droplets always took up a large proportion, and spray quality deteriorated, though SMD changed little. At large axial distances (250 or 400 mm) SMD increased with the increase of viscosity when the atomizer was operated at high pressure and GLR. The decay of axial velocities along the spray centerline was stronger at a larger viscosity. In the radial direction, an increase in viscosity made distributions of both SMD and velocity flatter. An increase in GLR generated more small droplets in the expense of large droplets and hence improved spray quality. Pressure had a weaker promotion on the droplet size distribution when comparing to GLR. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Heavy fuel oil has been used as the substitute of high quality oils due to its more reserves and less cost [1]. Atomizing this high-viscosity liquid adequately is challenging. The Y-jet atomizer, which is a semi-internal-mixing twin-fluid atomizer, is most commonly used for atomization of heavy fuel oil in industrial burners [2,3]. In order to acquire finer droplets at lower pressure with less gas, one alternative for the Y-jet atomizer is the effervescent atomizer, which was first developed by Lefebvre et al. [4]. In effervescent atomizers, gas is injected directly into liquid and a bubbly flow or annular flow is formed after sufficient two-phase mixing. There are two basic types of injecting atomizing gas into liquid, referred as outside-in/inside-out gas injection geometry. Sovani et al. [5] made a state-of-the-art review and concluded that com⇑ Corresponding author. Tel./fax: +86 10 62781743. E-mail address: [email protected] (H. Yang). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.06.097
paring to Y-jet atomizers, both kinds of effervescent atomizers attain a comparative spray quality with a less expense of gas. Another alternative for the Y-jet atomizer is an internal-mixing twin-fluid (IMTF) atomizer with coaxial gas/liquid feed. This kind of atomizer is also regarded as the effervescent atomizer [6] despite of less formation of gas bubbles. IMTF atomizers allow independent controls of gas and liquid input velocity, direction and distribution. Barreras et al. [7–9] and Li et al. [10] have conducted a series of researches and they found that IMTF atomizers can produce smaller droplets with less gas. Moreover, the atomizing gas in both an effervescent atomizer and an IMTF atomizer helps protect the heavy fuel oil in the atomizer from attack of hot flue gases especially near the nozzle exit [11,12], which extends the atomizer service life. Influence of liquid viscosity on atomization must be considered in heavy fuel oil atomization because of its large viscosity even if the atomizer is operated at a high operating temperature. Buckner and Sojka [13] studied sprays of glycerin-water solutions
Z. Li et al. / Fuel 103 (2013) 486–494
487
Nomenclature Cdv Dv0.5 d0 dave di GLR IMTF ml PDA p r SMD VDSD
vave
correlation coefficient between droplet size and velocity () 50% fractional volume diameter, volume median diameter (lm) diameter of atomizer exit orifice (mm) average droplet diameter (lm) diameter of the ith droplet (lm) Gas to liquid mass Ratio (-) Internal-mixing twin-fluid liquid mass flow rate (g/s) Phase Doppler Anemometry gauge pressure at the gas inlet (MPa) radial distance from spray axis(mm) Sauter Mean Diameter, D32 (lm) Volume-based Droplet Size Distribution average droplet axial velocity (m/s)
(400–970 mPa s) and found SMD (Sauter Mean Diameter) was nearly independent of liquid viscosity at a measuring axial distance (x) of 150 mm. Sutherland et al. [14] studied liquids with viscosity of 1–80 mPa s and observed only a small effect of viscosity on droplet size at an x of 150 mm. Lund et al. [15] also measured droplet size at an x of 150 mm and found that SMD increased only about 15% when viscosity increased from 20 to 80 mPa s. However, other researchers drew different conclusions. Loebker and Empie [16] sprayed liquids with viscosity up to 7 Pa s and found that at a very far measuring position (x is approximately 1300 mm), droplet mean size increased sharply with viscosity. Broniarz-Press et al. [17] measured droplet sizes of different liquids (1.92–26.4 mPa s) at a large x of about 600 mm and they also observed a considerable increase in SMD with the increase of viscosity. Once again, results of Sataphthy et al. [18] showed that SMD increased markedly with viscosity at very high pressure (11–33 MPa). Chen and Lefebvre [19,20] elevated the ambient air pressure to above 0.6 MPa and a considerable increase in SMD was observed with the increase of viscosity (1–100 mPa s). It should be noted here that all the above results are measured on the spray centerline, but at different x. The droplet radial distribution is also an important index of spray quality and some researchers have already paid attention to it. Ferreira et al. [21] conducted experiments on two liquids (24 mPa s and 196 mPa s) at various x (20, 50, 100 and 200 mm) and found that as viscosity decreased the droplet size near the centerline also decreased, while droplet size at larger radial positions gradually increased. Droplet size radial distributions were much steeper at a lower viscosity, and influence of viscosity on the droplet size radial distribution became less as x increased. Ejim et al. [22] studied the effect of viscosity (1 mPa s and 67 mPa s) at x of 100 mm and 202 mm. They found that a lower viscosity produced a steeper droplet size radial distribution in the relatively far region (x = 202 mm), the same result with that of Ferreira et al. [21] when x = 20 mm or 50 mm. Droplet size radial distribution at an x of 100 mm was influenced less by viscosity. These observations have not been explained in either study, but according to the velocity profiles given by Ferreira et al., viscosity influenced radial distributions of droplet axial velocities as well, and there may be some relationship between SMD and velocities. However, since Ejim et al. did not present their droplet axial velocities, this finding needs to be validated. Publications reported influences of viscosity in the very nearnozzle region are relatively less. High-speed flash photographs in the near-nozzle region were obtained by Buckner and Sojka [13].
vi x x⁄
axial velocity of the ith droplet (m/s) axial distance downstream from the atomizer exit orifice (mm) dimensionless axial distance, x⁄ = x/d0
Greek letters a spray half angle, a = tan1 (r/x) (degrees) Dpgl pressure differential between gas and liquid at the atomizer inlet (kPa) r surface tension (m N/m) l liquid viscosity (mPa s, 103 kg m1 s1) q liquid density (kg/m3) Subscripts g gas l liquid
Their shadow photographs showed that very large bubbles (termed macrobubbles) existed in the near-nozzle region, while disappeared when the axial distance increased. They claimed that macrobubbles play a consequential role in the spray formation process. A holographic investigation of near-nozzle spray structure was carried out by Santangelo and Sojka [23], using liquids spanning a range of viscosity. Two distinct near-nozzle flow patterns, bubbly flow and annular flow, were observed under different GLR, and viscosity had only a minimal effect on near-nozzle two-phase flow. In the near-nozzle region, the laser light attenuation caused by the very dense spray makes it difficult to take an accurate measurement on the droplet size. Ferreira et al. [21] measured sprays at a very near-nozzle position (x = 20 mm) and found that SMD was sensitive to liquid viscosity especially at positions far away from the centerline. Unfortunately, they didn’t specify the accuracy of their measurements. Besides, their atomized liquids were the same kind of oil at different temperatures and hence had different surface tensions which were not specified either. More studies are needed on the independent influence of viscosity on droplet size in the near-nozzle region. Although a large volume of information is already available in the open literature, reported results are quite different and the influence of liquid viscosity on atomization is not well understood yet, especially not for the IMTF atomizer. The objective of this work is to investigate the independent influence of liquid viscosity on atomization in an IMTF atomizer by analyzing both SMD and the Volume-based Droplet Size Distribution (VDSD) over a wide range of operating conditions. Especially, effects of viscosity on the VDSD were analyzed at different measuring positions and operating conditions, since VDSD has a direct relationship to the burner performance for heavy fuel oil. Influence of viscosity on the droplet axial velocity is also studied. 2. Experimental apparatus 2.1. Atomizer and test rig The designed IMTF atomizer, presented in Fig. 1a, is made by acrylic glass for visualization study on the internal two-phase mixing. The overall length of the atomizer is 600 mm. Liquid inlet of the mixing chamber has a diameter of 4 mm. Liquid inlet is surrounded by four 1 mm air inlet holes with a slant angle of 45°. The length of mixing chamber is 55 mm. Atomizer has an exit orifice with a diameter (d0) of 3 mm, which enables the atomizer
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air
liquid
(a)
1-pressurized liquid tank 2-control valv e 3-regulating valve 4-flow meter 5- pressure gauge 6-tested atomizer 7-collecting container 8-circulating pump
(b) Fig. 1. (a) Schematic of the IMTF atomizer, (b) schematic of the test rig.
to spray liquid at a high mass flow rate (up to 110 g/s among the studied sprays). The test rig is depicted in Fig. 1b. A liquid tank is pressurized by compressed air up to 1.5 MPa. A needle valve and a ball valve were installed on the liquid/gas supply line to adjust gas/liquid flow rates precisely to the desired values. Gas flow meter can measure a flow rate up to 250 Nm3/h, with an uncertainty of 2%. The liquid flow meter is a modified one which is fitted for measurement of high-viscosity liquids and was first calibrated by weighing liquids with different viscosities (1 mPa s, 30 mPa s, 120 mPa s and 200 mPa s). The uncertainty of liquid flow rates is within 3% over the test range. Pressure gauges were located just before inlets of gas/liquid with a precision of 2 kPa. Liquid is collected in a container and pumped back to the liquid tank after the high pressure air in the liquid tank is discharged. 2.2. Atomizer operation Desired liquids with different viscosities and almost the same surface tension were formulated by changing the mass ratio of industrial glycerin to water. Liquid viscosity was measured by a
Physica MCR300 rheometer of Anton Paar. The rheometer was calibrated using deionized water (1 mPa s) and two standard oils (10 mPa s and 100 mPa s). The uncertainty of viscosity is less than 2%. Surface tension measurements were made on a Kruss K12 surface tensiometer by the Wihelmy plate technique. The uncertainty is less than 2% over the experimental range after the tensiometer was calibrated using deionized water. Moreover, an Abbe refractometer (Model G, Carl Zeiss, Jena) with an uncertainty of 1% was used to measure the refraction index of tested liquids. All of these properties were measured at 11 ± 0.1 °C, matching the room temperature (11 ± 1 °C) of test conditions. Measured results are listed in Table 1, where liquid densities were obtained by weighing method. The lower three liquids in Table 1 have surface tensions spanning from 61.3 mN/m to 74.1 mN/m. This small variation in surface tension has only little impact on the spray droplet size [14,15,22], therefore these three liquids were sprayed to investigate the independent influence of viscosity on atomization over a range of 1.3–120 mPa s (120 mPa s is comparative to the viscosity of heavy fuel oil under operating conditions). Considering the variation of room temperature, viscosity has an additional uncertainty of 5% for the higher viscosity solution.
Table 1 Physical properties of liquids (at room temperature of 11 °C). Liquid
l (mPa s)
r (mN/m)
q (kg/m3)
Refractive index
Industrial glycerin Water Glycerin/water (62/38 wt%) Glycerin/water (79/21 wt%)
2312 ± 24 1.3 ± 0.04 30 ± 0.2 120 ± 2.7
57.5 ± 0.03 74.1 ± 0.01 61.3 ± 0.04 66.3 ± 0.06
1260 998 1148 1196
1.48 1.33 1.42 1.44
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Z. Li et al. / Fuel 103 (2013) 486–494 Table 2 Experimental operating conditions.
l (mPa s)
p (MPa)
GLR 0.02
Dpgl (kPa)
0.05
Dpgl (kPa)
0.10 ml (g/s)
0.20
Dpgl (kPa)
ml (g/s)
8 13 –
34.7 66.7 –
11 5 7
23.1 42.5 56.9
4 9 1
15.3 26.4 38.3
14 4 6
12.2 21.9 33.3
10 7 13
11.4 19.7 27.8
30
0.1 0.3 0.5
9 15 12
34.4 77.6 102.8
8 5 6
26.3 46.6 62.4
5 6 4
17.4 31.6 42.8
3 5 7
13.6 25.2 34.3
9 10 14
11.3 21.5 28.3
120
0.1 0.3 0.5
19 10 8
36.9 82.4 110.1
8 7 12
25.4 48.4 69.5
6 10 8
18.8 31.7 45.2
4 4 9
15.6 24.4 34.9
11 7 14
13.6 20.2 28.8
This study spans a GLR range of 0.02–0.20. The gauge pressure at the gas inlet, p, is 0.1–0.5 MPa. Detailed operating information, including liquid mass flow rate (ml) and pressure differential between gas and liquid at the atomizer inlet (Dpgl), is listed in Table 2.
Dpgl (kPa)
0.15
0.1 0.3 0.5
1.3
ml (g/s)
ml (g/s)
Dpgl (kPa)
ml (g/s)
droplet mean size of selected measurements can well represent the real droplet size distribution in the PDA measuring volume. Specific criteria have been mentioned in Section 2.3. Moreover, error bars haven’t been included in the SMD and velocity profiles for the sake of clarity, but uncertainties have also been discussed earlier.
2.3. Phase doppler anemometry (PDA) The droplet size and velocity distributions were measured using a Dantec Dynamics fiber PDA. An argon-ion laser generator and optical splitter were used to generate two laser beams with a spacing of 60 mm. The wavelength was 514.5 nm for beam U1 (green) and 488 nm for beam U2 (blue). The beam diameters were 1.35 mm and beam expander ratios were set to 1. The shift frequency of one of the two beams is 40 MHz. The focal length for the transmitting lens was 800 mm which resulted in a beam intersection angle of 4.3°. The fringe spacing was 6.87 lm and 6.51 lm for U1 and U2, respectively. Fringes moved in a negative direction. For data collection, the PDA was operated in 2nd order refraction mode. The receiver focal length was 600 mm. Depending on the refraction index of sprayed liquids, scattering angle was altered over a range of 151–163° to ensure that 2nd order refracted light is dominant. Aperture mask A was chosen here based on the droplet size. A 3D auto-controlled traversing system allows measurements conducted at different spatial locations. PDA measurements were performed at axial distances (x) of 50, 100, 150, 200, 250 and 400 mm downstream from the exit orifice of the atomizer. The spherical validation band was set to 10% to ensure a high sphericity of recorded droplets and a high accuracy of droplet size. The laser power was varied in 130–170 mW at different x to ensure a high validation rate of measurements. These optical selections enable PDA to measure droplet size up to 250 lm. The sample size at every measuring position was set to 10000 and sampling time was 30 s which enable estimation of droplet size and velocity statistics with confidence. Repeated measurements were taken at every operating condition and only a selection of measurements has been employed to analyze spray quality. Measurements used to plot figures have a validation rate about 85%, and the spherical validation rate of selected measurements is around 75%. Using these chosen data, uncertainties (relative standard deviation) for SMD and velocity are 5% and 6%, respectively. 3. Results and discussion Measurements used to present the following figures have a high validation rate to ensure that results were only disturbed little by the background noise. A high spherical validation rate is another criterion for choosing plotted data, which makes us confident that
3.1. Effects of viscosity on SMD and droplet velocity on the spray centerline In this section, only a part of measurements on the atomizer centerline are chosen and presented below to study the effect of viscosity at different GLR and p. Fig. 2 shows variations of SMD with GLR for different viscosities and pressures. In all study cases for a constant viscosity l, SMD decreases with an increase in GLR when other conditions are unchanged. This general rule was also observed by many researchers [5]. Measurements at different x⁄ (x⁄ = x/d0) indicate that the finest atomization can be obtained at an x⁄ of around 50, i.e., x = 150 mm, which means secondary atomization is completed at this distance. This is in accordance with results of Whitlow and Lefebvre [24] and Jedelsky et al. [6]. Meanwhile, droplet sizes at x larger than 150 mm are less concerned for heavy fuel oil combustion applications, because droplets there will interact with the combustor flow field and evaporate. Therefore, droplet size at an x of 150 mm is of great importance, as studied by quite a number of researchers [6,24–26]. As shown in Fig. 2, at an x of 150 mm the designed atomizer produces sprays with SMD less than 70 lm at a low operating condition of p = 0.1 MPa and GLR = 0.1 even for liquids with high viscosities. These results prove that the tested IMTF atomizer has a good performance at all test conditions. In most cases, SMD was dominated by GLR and nearly independent of l, especially at low GLR or p. While at high p and GLR, i.e. p = 0.3 or 0.5 MPa and GLR = 0.15 or 0.20, SMD tended to increase with the increase of l. Measurements show that and an increase in l from 1.3 mPa s to 120 mPa s produced a maximum increase in SMD of about 27%, which is much higher than the uncertainty. According to Fig. 2b and c, the negative effects of viscosity become stronger as GLR increases. Since the influence of viscosity becomes obvious at high GLR, SMD distributions along x⁄ when GLR is 0.2 are presented in Fig. 3. At a low p of 0.1 MPa, l affected SMD little at any x⁄. However, an increase in SMD with the increase of l was observed at a higher p of 0.5 MPa and SMD differences among different l increased with the increase of x⁄ as well. SMD increased up to 49% as l increased from 1.3 mPa s to 120 mPa s at a faraway distance of x⁄ = 133. As mentioned in the Introduction, some previous works [13–15] indicate that viscosity has little influence on SMD while others [16–18] show an obvious increase in SMD with viscosity. Results in this study explain this discrepancy well and as a conclu-
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Fig. 2. SMD versus GLR for different l and p when x⁄ = 50: (a) p = 0.1 MPa, (b) p = 0.3 MPa, (c) p = 0.5 MPa.
Fig. 3. SMD versus x⁄ for different l: (a) p = 0.1 MPa, (b) p = 0.5 MPa.
sion, the negative influence of viscosity on SMD becomes obvious only at high operating pressures or large x⁄. Since both droplet size and velocity are influenced by viscosity, a study of the correlations between them is helpful for understanding the effect of viscosity on atomization. Relationship of the single particle size and the single particle velocity shall be investigated. Here the correlation coefficient Cdv was introduced to correlate particle velocity and size:
C dv ¼
X i
," # X X ðdi dav e Þ ð v i v av e Þ
ðdi dav e Þðv i v av e Þ
i
i
ð1Þ where di and vi are the diameter and velocity of the ith particle, dave and vave are the algebraic average value of particle diameter and velocity. Cdv close to 0 indicates the little correlation between particle size and velocity, and a positive value of Cdv means that a larger particle has a higher velocity and a smaller particle has a lower velocity. Plots of Cdv on the spray centerline versus x⁄ when
p = 0.3 MPa and GLR = 0.1, are depicted in Fig. 4. Over the measuring range Cdv was always less than 0.3, indicating a very weak positive correlation between particle sizes and velocities on the spray centerline. At the near-nozzle region (x⁄ = 17), Cdv is close to 0.0 for low viscosity liquids. This result reveals that particle sizes are not related with particle velocities. Since small droplets correspond with the local gas phase very well, it’s proved that a complete momentum exchange occurs between the gas and liquid at the near-nozzle region for the low viscosity solutions. As the viscosity increases, Cdv at an x⁄ of 17 is 0.14 and is larger than Cdv of low viscosity solutions. As x⁄ increases, Cdv increases as well and keeps almost constant when x⁄ is larger than 50. The increase in Cdv is reasonable since the negative drag force on small droplets is larger as gas velocity decreases with increase of x⁄. As a result, small droplets have smaller axial velocities than axial velocities of the large droplets, although the difference is not large. The weak correlation between droplet size and velocity indicates that measured velocities are more likely to be the velocity distribution of the flow field within spray, and number averaged
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Fig. 4. Cdv versus x⁄ when p = 0.3 MPa, GLR = 0.10.
droplet velocity can well represent characteristics of the flow field on the centerline. Thus, velocity distributions of different l along x⁄ are presented in Fig. 5. The velocity distributions at various operating conditions show that a larger l results in a sharper decrease in velocity with the increase of x⁄. At an x⁄ of 17 where is relatively near the atomizer exit, larger l tended to have a higher velocity, especially at high GLR or p. This can be attributed to the increase of the liquid mass flow rate (shown in Table 2) with the increase of l. Velocities of higher l were lower as x⁄ increased due to the sharper decrease. Fig. 5 also shows the promotion of GLR or p on velocity. Thus, a lower p leads to a decrease in spray velocities while only a small change in SMD, which is desired in some cases where velocities are too high to result in the local flame extinction [27]. In summary, there is a strong interaction between the gas phase and liquid phase at the near nozzle region of an IMTF atomizer. As liquid viscosity increases, a sharper velocity decay reveals a stronger gas–liquid interaction. 3.2. Effects of viscosity on volume-based droplet size distribution (VDSD) In heavy fuel oil combustion, burning out large droplets generally spends a long time, which is proportional to the droplet diameter. As a result, an increase in the amount of large droplets reduces the combustion efficiency and is undesired during the atomizer operation. Since the proportion of large droplets may be different even though SMD are the same, a detailed investigation on the VDSD is necessary. The distribution can also help us further understand the roles of viscosity, GLR and pressure in spray formation. Fig. 6 shows typical VDSD of different l observed in this study and SMD are noted in these figures. Bimodal distributions were observed when x⁄ = 17 for all three l and x⁄ = 50 for 120 mPa s. The large liquid sheet is preliminarily broken up into fragments at the nozzle exit, due to the pressure drop and gas expansion, while the breakup is not sufficient and new-formed fragments are still large. Meanwhile, the intense gas–liquid interaction on the twophase interface strips some small drops out of liquid fragments. The combination effect forms bimodal distributions at an x⁄ of 17, where is relatively near the nozzle exit. Comparing VDSD of different viscosity solutions at this position, the large droplet volume fraction is pretty large for the high viscosity solution. This difference shows that the primary atomization for the high viscosity solution is worse than low viscosity solution. As atomization proceeds, fragments break up further and small drops begin to merge. The distribution changes to unimodal when x⁄ = 50 for two lower l. For 120 mPa s, note that the volume percent of large droplets is much larger than that of small droplets when x⁄ = 17, so the distribution when x⁄ = 50 is still bimodal, despite of the lower peak
Fig. 5. Average droplet axial velocity distributions along x⁄ for different l: (a) p = 0.1 MPa, GLR = 0.2, (b) p = 0.3 MPa, GLR = 0.2, (c) p = 0.3 MPa, GLR = 0.1.
value in the large size region. With a further increase in x⁄ to 133, right shifts of the peaks are observed, suggesting the dominance of droplet coalescence in the spray. Distributions for all three l are unimodal. Remarkably, when l = 120 mPa s, no peak appears in the small-diameter region and a high peak is formed at a large diameter. In general, an increase in l from 1.3 mPa s to 30 mPa s changes distributions little at all x⁄, while VDSD of 120 mPa s is quite different from VDSD of two lower l and spray quality of 120 mPa s is worse when x⁄ is 50, though SMD of three liquids are almost the same. One interesting observation is that the range of droplet size shrinks when l increases. For the high viscosity liquid, the measured largest particle size is only 180 lm, which is 20 lm less than the other two solutions. This observation shows that atomization process of a higher l may be steadier than that of lower l. Based on Fig. 6, it is reasonable to consider that a high l of 120 mPa s produces much more large droplets than the lower l at the nozzle exit due to the larger viscous force, which indicates that viscosity has an important influence on the primary atomization in an IMTF atomizer. Comparing Fig. 6a and b, as x⁄ increases to 50 where secondary atomization is completed, change trends of VDSD are the same for all three l, and changes in peak values are
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Fig. 6. VDSD and cumulative VDSD for different l (p = 0.5 MPa and GLR = 0.10): (a) x⁄ = 17, (b) x⁄ = 50, (c) x⁄ = 133.
almost the same, and changes in SMD are also very similar (about 5 lm for all three l). These observations indicate that viscosity has only a little influence on secondary atomization, and the differences among VDSD in Fig. 6b are the subsequent result of viscosity effect on primary atomization. Comparison between Fig. 6b and c shows that when l = 120 mPa s, VDSD shifts right more obviously and very large drops take up a larger fraction. This indicates that in the developed region (x⁄ > 50) l either strengthens the droplet coalescence or restrains breakup of large droplets. The possible reason is that high viscosity liquid experiences larger drag forces, owing to a greater gas–liquid slip ratio since the drop velocities decay faster (as shown in Fig. 5). In conclusion, viscosity affects primary atomization a lot, has only a little influence on secondary atomization and affects droplet characteristics after the atomization is completed (x⁄ > 50). Fig. 6 also shows that in the fully-developed jet flow region (x⁄ = 133), cumulative volume curves of the lower l are typical Rosin–Rammler distributions, while the curve of l = 120 mPa s deviates a lot.
3.3. Effects of p and GLR on atomization of high viscosity liquids Effects of p and GLR on spray formation can also be seen from the cumulative VDSD, as shown in Fig. 7. Since the promotion of p or GLR on VDSD in water spray has been reported by Karnawat and Kushari [28,29], here we care only about liquid with a high viscosity of 120 mPa s at an x⁄ of 50. Fig. 7a shows the independent influence of GLR on the droplet size distribution. As GLR increased from 0.05 to 0.20, more small droplets were produced and the cumulative curve tended to move left, resulting in a decrease in SMD from 74.4 lm to 64.2 lm. When p increased independently, as Fig. 7b shows, the volume fraction of very large droplets (with a diameter > 140 lm) was almost the same at different p, thereby generating little change in SMD (from 67.9 lm to 65.4 lm). However, Dv0.5 (volume median diameter) decreased considerably as p increased to 0.5 MPa. Thus an increase in p improves spray quality, though the pressure-induced promotion on spray quality is weaker than that induced by the increase of GLR.
Fig. 7. Influence of operating conditions on droplet size distribution: (a) influence of GLR, (b) influence of p.
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Fig. 8. Radial variations of SMD and velocity: (a) p = 0.1 MPa, GLR = 0.1, (b) p = 0.1 MPa, GLR = 0.2.
3.4. Effects of viscosity on spray radial distributions Radial variations of SMD versus the spray half angle (a) are presented in Fig. 8, where dotted/dashed lines were plotted to highlight differences in profiles. Measurements for a constant l are in accordance with the result reported by previous researchers [30,31] that SMD increases as the measuring position was moved radially away from the centerline. For different l, SMD of 1.3 mPa s was slightly lower than that of 30 mPa s near the centerline. When the radial location was moved away, SMD of lower l became larger. The corresponding velocity variations are just opposite to changes of SMD. Comparing with velocities of 30 mPa s, velocities of 1.3 mPa s were higher near the centerline but lower at larger radial positions. Both the velocity curve and the SMD curve of a lower l are steeper than those of a higher l. The comparison reveals that liquids with higher viscosities have a stronger effect on local gas and lead to a stronger entrainment of the surrounding atmosphere. It’s also found that at different operating conditions, the smallest SMD always appear on the centerline where the highest velocities appear. Besides, the smaller SMD always corresponds to the higher velocities regardless of viscosities. These observations show that there is a close relationship between SMD and velocities. 4. Conclusion Sprays in an IMTF atomizer were measured using PDA to study effects of liquid viscosity on atomization. The finest sprays were observed at an axial distance of 150 mm in the experimental range. At this distance the tested atomizer can produce droplets with SMD less than 70 lm at low p and GLR for liquids with viscosity up to 120 mPa s, and SMD is dominated by GLR while being nearly independent of liquid viscosity (l) and gas supply pressure (p). Though SMD changes little when l increases to 120 mPa s, VDSD changes a lot. Large droplets always take up a large proportion and spray quality deteriorates. Therefore, only SMD is not enough
to represent the spray quality of high-viscosity liquids when comparing with low-viscosity liquids. At large axial distances (250 or 400 mm) SMD increases with the increase of l when the operating p and GLR are high. A higher l produces a sharper decay of axial velocities along the centerline. In the radial direction, l also affects axial velocity distribution and a higher l makes the distribution flatter, meanwhile, radial SMD distribution is flatter as well. A close relationship between SMD and velocities is observed. Acknowledgements This material is based upon work supported by National Nature Science Foundation of China Grant (No. 51076081) and Mitsubishi Heavy Industries, Ltd. References [1] Saario A, Rebola A, Coelho P, Costa M, Oksanen A. Heavy fuel oil combustion in a cylindrical laboratory furnace. Measurements and modeling. Fuel 2005;84:359–69. [2] Gong JS, Fu WB. The experimental study on the flow characteristics for a swirling gas-liquid spray atomizer. Appl Therm Eng 2007;27:2886–92. [3] Zhou YG, Zhang MC, Yu J, Zhu X, Peng J. Experimental investigation and model improvement on the atomization performance of single-hole y-jet nozzle with high liquid flow rate. Powder Technol 2010;199(3):248–55. [4] Lefebvre A, Wang X, Martin C. Spray characteristics of aerated-liquid pressure atomizers. AIAA J Prop Power 1988;4(4):293–8. [5] Sovani SD, Sojka PE, Lefebvre AH. Effervescent atomization. Prog Energ Combust 2001;27(4):483–521. [6] Jedelsky J, Jicha M, Slama J, Otahal J. Development of an effervescent atomizer for industrial burners. Energ fuel 2009;23:6121–30. [7] Barreras F, Lozano A, Barroso J, Lincheta E. Experimental characterization of industrial twin-fluid atomizers. Atomization Spray 2006;16(2):127–45. [8] Ferreira G, Barreras F, Lozano A, Garcia JA, Lincheta E. Effect of the inner twophase flow on the performance of an industrial twin-fluid nozzle with an internal mixing chamber. Atomization Spray 2009;19(9):873–84. [9] Ferreira G, Garciia JA, Barreras F, Lozano A, Lincheta E. Design optimization of twin-fluid atomizers with an internal mixing chamber for heavy fuel oils. Fuel Process Technol 2009;90(2):270–8.
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