Research Article Experimental Investigation of Atomizing Performance ...

Hindawi Publishing Corporation Advances in Mechanical Engineering Volume 2014, Article ID 782064, 10 pages http://dx.doi.org/10.1155/2014/782064

Research Article Experimental Investigation of Atomizing Performance of Low Pressure Swirl Nozzle Yunfei Yan,1,2 Li Zhang,1,2 WenLi Pan,2 and Ge Pu1,2 1

Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, China 2 College of Power Engineering, Chongqing University, Chongqing 400030, China Correspondence should be addressed to Yunfei Yan; [email protected] Received 27 August 2013; Accepted 4 December 2013; Published 9 January 2014 Academic Editor: Hakan F. Oztop Copyright © 2014 Yunfei Yan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The lime slurry nozzle is a key equipment component in the flue gas drying desulfurization system. The atomizing performance of lime slurry nozzles with different structure parameters under low pressure conditions was experimentally studied by using the laser diffraction/scattering particle size distribution analyzer (Win212-2), and the optimized structure of nozzle was obtained. Experimental results indicate that there is a relation between the average granularity and the fluid pressure and that the spray angle increases with increasing pressure. As the solid percentage of lime slurry increases, the Sauter Mean Diameter (SMD) of droplets also increases, the atomizing performance of the nozzle slightly decreases, and the spray angle of nozzle also decreases. The flux of lime slurry is uniformly distributed along the radial direction of the nozzle, and then the flux characteristic of the spray nozzle is obtained. In order to meet the size of droplets for the semidry desulphurization process, the performance of an optimum nozzle was experimentally tested. The proper nozzle type and optimal parameters for low pressure swirl nozzles are suggested.

1. Introduction To date, secondary pollution is still a serious problem for our environment caused by some industries, such as power stations and waste incineration plants. Therefore, flue gas is necessarily purified before being discharged into the atmosphere. In the semidry flue gas desulfurization process, the lime slurry spray nozzle is an important component for the spray-drying flue gas desulfurization device. The atomizing performance of the spray nozzle, which has significant effects on the flue gas desulphurization as well as the security and economy of the semidry flue gas desulphurization system, mainly depends on the Sauter Mean Diameter (SMD) of droplets, spray angle, atomization uniformity, and so on. The effects of water-coal-slurry spray nozzle structure on the distribution of atomizing particles, SMD of droplets, and spray angle were thoroughly analyzed by Yu et al. [1]. It was found that the atomization quality can be enhanced by using a new method of multistage atomization as discussed by Huang et al. [2]. In order to take full advantage of the atomizing gas momentum, the atomizing gas is brought into the nozzle by

multiple stages and the next stage atomizing air is guided into the nozzle until the atomizing air at previous stage is completely consumed. Zeng et al. [3] studied the behaviour of collared jets under different expansion ratios and the use of indeterminate-origin collars comprising of V- and A-shaped notches as well as a circular nozzle. However, the results showed that for the geometric configurations used in that study, the influences on the unaltered jet were favourable though limited. Datta and Som [4] studied the influence of flux and structure parameters on the discharge coefficient and air core by using the standard 𝜅-𝜀 model. On the other hand, the spray nozzle with expanded tangent channel structure was first proposed and studied by Ran et al. [5, 6], and the atomization and flow of two phases have become an attractive topic for decades, as discussed in [7–11]. Khavkin [12] proposed that the droplets sizes in swirl atomizers depend only on the specific energy of the liquid drops and on viscosity. The new theory, based only on two parameters, is shown to be far simpler and in better agreement with experimental data than any previous presentations. It is also found by Khavkin [12] that the swirl nozzles have strong

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Advances in Mechanical Engineering

advantages over other types of nozzles such as cross-coaxial nozzles, shear coaxial nozzles, rotary nozzles, and jet nozzles. No matter which type of atomizers is used, the flow field with spray is vitally affected by the inner geometry configuration of the atomizers. The design of suitable geometry can get good atomization. Lefebvre [13] reported that the orifice length and diameter were the key geometric variables for plain-orifice injectors and that the exit orifice diameter was also primarily important for pressure swirl atomizers. It is also concluded that when viscous effects were ignored, charge coefficients were not affected by injection pressure. The quality of the spray depends on parameters such as the atomizer geometry, flow, and properties of the liquid and surroundings, typically air. A semiempirical relationship for droplets mean diameter in air-assisted atomizers, derived by Wigg [14], suggests that droplets size is dependent on atomizing gas velocity, density, and mass flow rate. The method measuring the atomization uniformity by volumetric cylinders in the radial direction is simple and feasible. The atomization properties of low percentage solids of lime slurry under low pressures can be found in [15–20]. In order to further improve atomizing performance of low pressure swirl nozzles and adapt the semidry flue gas desulfurization technique in engineering applications, it is necessary to investigate the atomizing performance of low pressure swirl nozzles. In this paper, a series of experimental devices and various types of swirl nozzles were constructed. The particle size and its distribution of atomized lime slurry in the spray are measured by a laser diffraction/scattering particle size distribution analyzer (Win212-2). The laser particle size analyzer measures the particle size on the basis of the phenomenon that the laser beam will be diffracted by particles. When the light beam irradiates on the particles, part will be diffracted and an angle will be formed between the diffracted light and the main beam. Both diffraction theory and results from experiment show that this angle is related to the size of the particles. The larger the particle is, the smaller the angle is. The intensity of the diffracted light represents the number of the particles. Thus particle size of the sample is obtained through measuring the intensity of the diffracted light at different angle. In this study, the influence of some factors on atomizing performance of lime slurry nozzles is analyzed, and the optimal type of nozzle and optimum operating parameters are proposed.

2. Experimental Apparatus and Method The structural dimensions of a low pressure swirl spray nozzle are shown in Figure 1. It is composed of an atomizing component, a rotational component, and a flow homogenizing component. The liquid flows into the expanded tangent channel after being split by the flow homogenizing component and then tangentially enters the swirl chamber. The liquid revolves at high speed in the atomization coneshape channel and then is atomized and emitted from the nozzle hole. The injection pressure is the pressure of lime slurry at the inlet of the nozzle. 𝛼 is chamber angle, and the nozzle channel angle, 𝛽, is a physical parameter of channel on

Table 1: Density of lime slurry. Percent solids 0% water (10∘ C) 10% 15% 20%

Density (kg/m3 ) 1000 1232 1348 1464

the rotational component in Figure 1. 𝛾 is the spray angle of the nozzle in Figure 2. The experimental system is shown in Figure 2. Gao [21] set up a system composed of two subsystems, that is, the lime slurry system and the measuring system. The lime slurry system mainly includes a water tank, a screw pump, a pressure gauge, and a nozzle. To prevent debris from entering the nozzle, a filter is placed in front of the screw pump. The lime slurry is injected into the spray nozzle by the screw pump and then atomized. The measuring system includes a laser particle size analyzer, a camera, and liquid collectors. The pressures of the lime slurry are recorded by two pressure gauges. The SMD of droplets is measured by the laser particle size analyzer, based on the principle of laser diffraction/scattering. The spray angle of the nozzle is measured after being photographed by the camera. Taking advantage of the powerful image processing function in MATLAB, the spray image taken by a video camera is first preprocessed with denoise and binarization procedures, and the image edges are extracted. Then the irregular edges of spray image were fitted to regression lines using a least square fitting function. Two intersecting straight lines closest to the edges are obtained. The angle between the two lines is just the required spray angle. The uniformity of the spray is measured after the liquid has entered the liquid collectors. The 10 collection tubes are uniformly and simultaneously arranged along the horizontal and vertical directions on the circular section of fog. In order to optimize the nozzle structure, the performance of the spray nozzle is thoroughly studied using different structure parameters. Once the optimized structure of the nozzle is obtained, the atomizing performance of nozzle is tested at different atomizing pressures and solid percentages of lime slurry. The mass flow rate, SMD of droplets, spray angle, spray uniformity, and the shape of spray are recorded, and then the comprehensive atomizing and flow characteristics of the optimized nozzle are obtained. In this study, the ambient pressure and temperature are 1.0 × 105 Pa and 293 K, respectively. The injection pressures of the nozzle are 0.4 MPa, 0.8 MPa, 1.2 MPa, and 1.4 MPa, and the solid percentages of lime slurry are 0%, 10%, 15%, and 20%. The density of the lime slurry is listed in Table 1.

3. Results and Discussion 3.1. Influence of Nozzle Structure Parameters on Atomizing Performance. This part of experiment is tested at 0.8 MPa injection pressure and with 15% solid percentage of lime slurry.



3

L

Atomizing component

b

D𝛼 d 𝛽

Rotational component


Flow-homogenizing component

Figure 1: Configuration of low pressure swirl spray nozzle. Table 2: Structure parameters of nozzle when changing channel angle. Diameter of rotational chamber 𝐷/mm 7

Atomizing component

Diameter of nozzle hole 𝑑/mm 1.6 Diameter of rotational chamber 𝐷/mm 7 7 7 7

Rotational channel numbers Rotational component

4 4 4 4 Outside diameter/mm 20

Flow homogenizing component

1

8

9

4 5

Angular channel 𝛽/(∘ )

1 × 1.5 0.9 × 1.5 0.78 × 1.5 0.7 × 1.5

0 3 6 8

2.5

5

11 12 𝛾

7

13

6 P

(1) Motor (2) Water tank (3) Impeller (4) Strainer (5) Water inlet pipe (6) Screw pump

Dimension of channel 𝑏 × ℎ/mm

Channel length 𝐿/mm

2 3

Length of nozzle hole 𝐿/mm 1

Hole diameter/mm

10 P

Convergence angle of rotational chamber 𝛼 90∘

(7), (10) Pressure gauge (0–1.5 MPa) (8), (9) Valve (11) Nozzle (12) Win212-2 laser particle size analyzer (13) Liquid collector

Figure 2: Atomization performance test system of nozzle.

3.1.1. Influence of Channel Angle on Atomizing Performance. The structure parameters of the nozzle with changing channel angle are listed in Table 2. The influences of nozzle channel angle on SMD of droplets are shown in Figure 3. Four kinds

of the nozzle with channel angles (i.e., 0∘ , 3∘ , 6∘ , and 8∘ ) were studied and the SMD of droplets from all these nozzles is less than 100 𝜇m, which can meet the requirement for a semidry flue gas desulphurization process. From Figure 3, it is clearly observed that the SMD of droplets reaches a minimum value when the channel angle of the nozzle is 6∘ . It can be calculated that the percentages of droplets diameter less than 120 𝜇m for these four nozzles are 74%, 73.75%, 79%, and 73.2%, respectively. It is obvious that the droplets of nozzle with 6∘ channel angle are more uniform. The channel angle plays an important role for the fluid characteristics. The counterflow and energy loss in the nozzle would be obviously decreased when the channel angle increases. The energy loss is the least and the counterflow at the exit of the nozzle is not exactly formed while the channel angle is 6∘ . The energy loss would increase with increasing channel angle above 6∘ . Therefore, the SMD of droplets reaches the minimum value when the channel angle of nozzle is 6∘ . The relationship between energy loss, velocity, and channel angle obtained by Ran et al. [22] indicates also the energy loss is the least and the counterflow at the exit of the nozzle disappears while the channel angle is 6∘ . Figure 4 shows the influence of nozzle channel angle on spray angle. It can be obviously found that the spray angle proportionately increases with increasing


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Advances in Mechanical Engineering Table 3: Structure parameters of nozzle when changing swirl chamber diameter and entrance width ratio. Diameter of rotational chamber 𝐷/mm 7 9

Atomizing component

Rotational channel numbers Rotational component

4 4 4 4 Outside diameter/mm 20 22

Convergence angle of rotational chamber 𝛼 90∘ 90∘

Length of nozzle hole 𝐿/mm 1 1



1.3 × 1.5 1 × 1.5 1 × 1.5 0.7 × 1.5

0 0 0 0

Hole diameter/mm


2.5 2.5

5 5

98

108

97

104 SMD (𝜇m)

SMD (𝜇m)


Diameter of nozzle hole 𝑑/mm 1.6 1.6 Diameter of rotational chamber 𝐷/mm 7 7 9 7

96

96

95

94

100

92

0

1

2

3

4

5

6

7

8

5

6

7

8

9

10

D/b

∘

Channel angle ( )

Figure 3: Influence of the nozzle channel angle on SMD of droplets.

Figure 5: Influence of D/b on SMD of droplets.

71

Spray angle (∘ )

70 69 68 67 66 65 64

0

2

4

6

8

∘

Channel angle ( )

Figure 4: Influence of channel angle on spray angle.

channel angle. Therefore, a moderate spray angle should be chosen in real industrial applications. 3.1.2. Influence of Swirl Chamber Diameter and Entrance Width Ratio (D/b) on Atomizing Performance. The structure

parameters of the nozzle when changing swirl chamber diameter and entrance width ratio are listed in Table 3. In Figure 5, the relationship between SMD of droplets and D/b is presented. It is clearly shown that the parameter of D/b plays an important role in the atomizing performance. When D/b is smaller than 7.0, the SMD of droplets decreases with increasing D/b; however, as D/b increases beyond 7.0, the SMD of droplets increases as well. Lower swirl intensity, caused by smaller D/b, and lower axial kinetic energy, caused by bigger D/b, are both disadvantageous for atomization of droplets. Both swirl intensity and axial kinetic energy are maximum at D/b = 7. Therefore, the minimum SMD of droplets is reached when D/b is around 7.0. Based on this figure, it can be also seen that the SMD of droplets is below 100 𝜇m when D/b is between 6.0 and 8.0; however, the SMD of droplets is greater than 100 𝜇m when D/b is beyond this range. Therefore, a conclusion can be made from Figure 5 that the most suitable range of D/b is between 6.0 and 8.0, which is consistent with previous studies [23, 24].



5

102

80

101

78 Spray angle (∘ )

SMD (𝜇m)

100 99 98 97

76 74 72

96 70 95

94 2.5

2.7

2.9

3.1

3.3

3.5

3.7

68 2.5

3.9

3

3.5

4

L/b

L/b

Figure 7: Influence of L/b on spray angle.

Figure 6: Influence of L/b on SMD of droplets. 130

120 SMD (𝜇m)

3.1.3. Influence of Tangential Channel Length and Width Ratio (L/b) on Atomizing Performance. In order to preferably cause rotary flow in a nozzle, the entrance of the swirl chamber should usually be tangential with the chamber. Although the cross-section of the entrance can be either circular or rectangular, it is generally believed that the entrance length and width ratio L/b has little influence on the flow coefficient of the nozzle when the ratio is between 0.9 and 7.0 [23]. In order to obtain a lower pressure drop and the most uniform rotary flow in a nozzle, the entrance cross section of the nozzle in this study is rectangular, and three different ratios of L/b (i.e., 2.7, 3.3, and 3.9) were experimentally studied in terms of the atomizing performance of the nozzle. The structure parameters of the nozzle with changing tangential channel length and width ratio are listed in Table 4. The relationship between L/b and SMD of droplets is shown in Figure 6. It can be seen that with increasing L/b (i.e., from 2.7 to 3.3) the SMD of droplets decreases at first, and then the SMD of droplets gradually reaches a certain value (i.e., around 97 𝜇m) with further increase of L/b (from 3.3 to 3.9). The greater L/b would result in greater pressure loss, while the lesser L/b would cause a kind of scattered flow and asymmetrical rotation, and both would cause worse atomization. The relationship between L/b and spray angle is shown in Figure 7. It can be seen that the trend of L/b versus SMD of droplets is similar to that of L/b versus spray angle, and the spray angle gradually reaches 70∘ with increasing L/b. Combining Figures 6 and 7, it can be concluded that the atomizing performance of the nozzle reaches optimum status when the ratio of L/b is between 3.0 and 4.0. The nozzle structure is optimized based on the combined consideration of the effects of structural parameters (channel angle, swirl chamber diameter and entrance width ratio, and tangential channel length and width ratio) on the atomizing performance of the nozzle. The optimum chamber angle and ideal range of D/d0 gained by Yan et al. [25] should be around 90∘ and 4.0 ∼ 6.5, respectively. In Table 5, the structure parameters of optimized low pressure and swirl spray nozzle are listed. The atomizing performance of optimum nozzle is tested at different atomizing pressures and solid percentages of lime slurry.

110

100

90 0.4

0.5

0.6

0.7

0.8 0.9 1 Pressure (MPa)

1.1

1.2

1.3

1.4

Figure 8: Influence of pressure on SMD of droplets.

3.2. Influence of Injection Pressure on Atomizing Performance. In this study, the atomizing performance of optimized nozzle at different solid percentages of lime slurry was carried out at ambient temperature and 0.8 MPa injection pressure, as listed in Table 7. In Table 5, the structure parameters of optimized low pressure and swirl spray nozzle are listed. This part of the experiment is tested under the injection pressure from 0.4 MPa to 1.4 MPa and 15% solid percentage of lime slurry. The influence of injection pressure on spray angle, fog length, and SMD of droplets is correspondingly shown in Table 6. The fog length can be directly measured by ruler. It can be defined that the fog length is the distance between the farthest section the spray droplets can reach and the outlet section of the nozzle when it is oriented horizontally. It is clearly observed that, with increasing injection pressure from 0.4 MPa to 1.4 MPa, the spray angle approaches a constant value, 66∘ , and the fog length gradually increases. Therefore, it can be concluded that the atomizing capability of the optimum nozzle is significantly improved as increasing fluid injection pressure. The injection pressure plays an important role in the SMD of droplets. The relationship between injection pressure and the SMD of droplets is shown in Figure 8. It can be


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Advances in Mechanical Engineering Table 4: Structure parameters of nozzle when changing tangential channel length and width ratio. Diameter of rotational chamber 𝐷/mm 7

Atomizing component

Rotational channel numbers


4 Outside diameter/mm 20 20 20


Diameter of nozzle hole 𝑑/mm 1.6 Diameter of rotational chamber 𝐷/mm 9





1 × 1.5

0

Hole diameter/mm


2.5 3.0 2.0

5 6 4

Table 5: Measurements of low pressure swirl spray nozzle for optimization. Diameter of rotational chamber 𝐷/mm 7

Atomizing component

Rotational channel numbers


4 Outside diameter/mm 20


Diameter of nozzle hole 𝑑/mm 1.6 Diameter of rotational chamber 𝐷/mm 7

0.4 0.8 1.2 1.4

0% 10% 15% 20%

0.78 × 1.5

6

5

Spray angle

Fog length (m)

SMD of droplets (𝜇m)

21.6 31.3 37.3 39

57 66.7 66 66

2.3 2.6 2.74 2.85

126.3 94.94 92 90.7

0.8 0.8 0.8 0.8


2.5

Flow (mL/s)

Solids Pressure percentage. (MPa)



Table 7: Influence of solids percentage. Condition


Hole diameter/mm

Table 6: Influence of pressure. Condition Pressure (MPa)


Spray angle (∘ )

SMD of droplets (𝜇m)

71 69.5 66.7 62.3

87.48 90.2 94.94 101.3

seen that the SMD of droplets decreases at first and then approaches a constant value, around 90 𝜇m, when increasing the injection pressure from 0.4 MPa to 1.4 MPa. When the injection pressure is between 0.4 MPa and 0.8 MPa, the SMD of droplets linearly and sharply decreases with increasing injection pressure. However, once the injection pressure is greater than 0.8 MPa, the SMD of droplets slowly decreases. This indicates that the atomization capability of the nozzle can

be significantly improved by simply increasing the injection pressure when this injection pressure is less than a certain value; however, once the injection pressure is greater than a certain threshold (i.e., 0.8 MPa in this example), the atomization capability of nozzle cannot be significantly improved with further increase of injection pressure. The reason may lie in the relation between liquid atomization and wave motion for the atomization process at middle Reynolds number. Because of the existence of the surface tension and viscous force, the supporting liquid film, the breaking time, and length of the liquid film increase with increasing pressure and the thickness of the liquid film decreases, based on continuity equation for a fluid. Consequently, the SMD of droplets sharply decreases with increasing injection pressure at low Reynolds number in the first phase. On the contrary, the inertia force has become the main factor and the influences of surface tension and viscous force decrease, so that the SMD of droplets slowly decreases at higher Reynolds number (higher pressure) in the second phase. Therefore, in this situation, it is not a good way to improve the atomization capability by indefinitely increasing the nozzle injection pressure. The structure optimization of the nozzle might be the most important factor to further improve the atomizing performance for a nozzle. Based on the experimental data as shown in Figure 8, the relationship between the SMD of droplets and injection pressure can be exponentially fitted as follows: 𝐷vs = 90.7 + 35.47𝑒−(𝑃−0.4)/0.2285 ,


(1)


7

70

105

67 64

SMD (𝜇m)

Spray angle (∘ )

100

61

95

90

58 55 0.4

0.6

0.8

1

1.2

1.4

Pressure (MPa)

85

0

5

10

15

20

25

Solid percentages (%)

Figure 9: Influence of pressure on spray angle.

where 𝐷vs is the particle diameter (𝜇m) and 𝑃 is the nozzle injection pressure, (MPa) The fitting error of the above formula is only 0.9751. In Figure 9, the relationship between nozzle injection pressure and spray angle is presented. It is clearly shown that the spray angle increases with increasing injection pressure in the range of 0.4 MPa and 0.8 MPa; however, once the injection pressure is greater than a certain value, 0.8 MPa, the spray angle gradually approaches a constant, 66.7∘ . The reason for the above trend between injection pressure and spray angle is that the flux and velocity of droplets increase with increasing injection pressure, and then the effect of air entrainment and swirl centrifugation is strengthened simultaneously; therefore, with increasing injection pressure, the spray angle will significantly increase. However, when the injection pressure is greater than a certain value, the effect of air entrainment is strengthened but the drag force also increases. In addition, the effect of droplet coalescence is strengthened. As a result, the spray angle gradually approaches a constant under higher pressure. Consequently, the spray angle will not significantly increase with further increase of injection pressure. The relationship between injection pressure and spray angle is consistent with the results obtained by Li et al. [26]. 3.3. Influence of Solid Percentages of Lime Slurry on Atomizing Performance. Since different solid percentages of lime slurry have different viscosity and surface tension, they play an important role in the atomizing performance of the nozzle. Generally speaking, the higher solid percentages of lime slurry, the higher viscosity and surface tension for the lime slurry. Therefore, the higher percent solid of lime slurry has negative influence on the SMD of droplets. In this section, the atomizing performance of the optimized nozzle at different solid percentages (0%∼20%) of lime slurry was carried out at ambient temperature and 0.8 MPa injection pressure, as listed in Table 7. As shown in Table 5, the structure parameters of optimized low pressure and swirl spray nozzle are listed. Figure 10 gives the relationship between solid percentages of lime slurry and SMD of droplets for the optimized nozzle at 0.8 MPa injection pressure. It can be clearly seen that the

Figure 10: Influence of solid percentages on SMD of droplets.

SMD of water droplets (i.e., the solid percentage of lime slurry is 0.0) is about 87 𝜇m and that the atomizing performance of nozzle is quite good. As the solid percentages of lime slurry increase, the SMD of droplets gradually increases. The increasing of solid percentages of lime slurry would affect the physical properties of the liquid, especially its viscosity. The higher solid percentages of lime slurry is, the easier some particles adsorb, this phenomenon makes more difficult for the atomization of liquid and results in an increase of the droplet diameter. It is concluded that the pressure nozzle is not suitable for atomizing very high viscosity liquids. The SMD of droplets is approximately 102 𝜇m when the solid percentage of lime slurry reaches 20%. Usually, in order to produce droplets with diameter less than 100 𝜇m by using a regular swirl nozzle, the fluid injection pressure is required to be not less than 3.0 MPa. As compared to the regular swirl nozzle, the optimized low pressure swirl nozzle not only satisfies the SMD of droplets for the semidry flue gas desulfurization process but also significantly reduces the fluid injection pressure, thereby saving pump power and enhancing the competitiveness of the semidry flue gas desulfurization process. Figure 11 shows the relationship between the solid percentages of lime slurry and the spray angle of nozzle. With increasing solid percentages of lime slurry, the spray angle of the nozzle slowly decreases, which is consistent with the simulation results obtained by Yan et al. [25], and the error is less than 5%. Moreover, increasing of solid percentages of lime slurry leads to an increase the liquid viscosity and the spray angle decreases as the liquid viscosity increases, which is consistent with the experimental results obtained by Yao et al. [27] 3.4. Atomizing Uniformity of Nozzle. Since the desulfurization efficiency can be highly influenced by the atomizing uniformity of the nozzle, this uniformity property in the radial direction is another important parameter to evaluate the atomization quality of the nozzle. The more uniform the atomization in the radial direction, the better the atomizing performance for a nozzle. Usually, a good design of the


Advances in Mechanical Engineering 71

25

69

20 Fluid volume (mL)

Spray angle (∘ )

8

67 65 63 61

15 10 5 0

0

5

10 15 Solid percentages (%)

20

25

−50 −40 −30 −20 −10

Figure 11: Influence of solid percentages on spray angle.

3.5. Flux Characteristic of Nozzle. At ambient temperature, the flux of nozzles under different injection pressures is an important parameter to evaluate the atomization quality of a nozzle, because it reflects the adjustability of a nozzle to some extent. The flux characteristic of a low pressure swirl nozzle was experimentally investigated at ambient temperature, 15% solid percentage of lime slurry, and fluid injection pressures from 0.4 MPa to 1.4 MPa, as shown in Figure 13. The data used were the average of three experiments, and the maximum standard error is 1.53%. It can be seen that the volume flux of the nozzle increases nonlinearly and this trend gradually decreases with increasing injection pressure. At ambient temperature, the relationship between volume flux of nozzle and injection pressure can be obtained by a fitting method as follows: 2𝑃 , 𝜌

10

20

30

40

50

Level position Vertical position

(2)

where 𝑉 is the volume flux, P is the injection pressure, and 𝜌 is the density of fluid. The discharge coefficient is another important parameter of nozzles and there exist some factors which influence the discharge coefficient such as flow pattern in a nozzle,

Figure 12: Distribution of fluid volume at different semispray angles in a horizontal plane. 40

30 Volume flux (mL/s)

nozzle will not only improve the desulfurization efficiency but also enhance the utilization of lime slurry. In this study, the atomizing uniformity of the optimized nozzle was carried out at ambient temperature and 0.8 MPa injection pressure with 15% solid percentages of lime slurry. Figure 12 gives the fluid volume distribution along both horizontal and vertical directions in the liquid collector. It can be clearly observed that the volume distribution of lime slurry along the central line has two symmetric peaks and that the fluid volume approaches a maximum value when the semispray angle is 33.5∘ . Since the volume distribution difference between horizontal and vertical directions is less than 2.77%, the fog of the nozzle is in the shape of a hollow cone distribution. Therefore, it can be concluded that the flux of lime slurry along the radial direction is uniform for this optimized swirl nozzle.

𝑉 = 0.7757 × 10−6 ⋅ √

0

Semispray angle (∘ )

20

10

0

0

0.3

0.6

0.9

1.2

1.5

Pressure (MPa)

Figure 13: Flux character curve of lime slurry nozzle.

nozzle geometric structure, and physical properties of the liquid. Generally speaking, it is very difficult to determine the discharge coefficient by a theoretical method which considers all these factors. However, in engineering application, the combination of simplified theories and relevant experiments is a feasible approach to determine this coefficient. In this study, the discharge coefficient of an optimized nozzle is experimentally investigated. Based on our experimental investigation and the flow equation for a swirl pressure nozzle, obtained by Liu [23], the discharge coefficient can be determined by the following equation: 𝐶𝐷 =

𝑉 . 𝐴𝑜√2𝑃/𝜌

(3)

After the parameters of injection pressure P, fluid density 𝜌, and volume flow rate 𝑉 were measured, the discharge coefficient C𝐷 of nozzle can be calculated by using (3). The



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discharge coefficient of lime slurry in a low pressure swirl nozzle is about 0.2 in this study, which is smaller than that of water under the same condition. Usually, a given experiment has one particular dominant error, and the experimenter devotes the most effort to reducing that one. Often, when repeating measurements, one value appears to be spurious and we would like to reject it. In addition, when taking a series of measurements, sometimes one value appears “out of line.” In general, there are two different types of experimental data taken in a laboratory and the question of rejecting measurements is handled in slightly different ways for each. The two types of data are the following. (1) A series of measurements taken with one or more variables changes for each data point. (2) Repeated measurements of the same physical quantity are obtained, with all variables held as constant as experimentally possible. The main factors affecting the experimental stability are fluid injection pressure, volume flow rate, and size uniformity. These factors, for example, the measurements of laser particle size analyzer, spray angle, and spray uniformity, may cause considerable error. Therefore, experimental results are not acceptable until all these parameters (fluid injection pressure, volume flow rate, etc.) are stabilized. During the experimental data analyzing process, all large deviation data were removed, and then all the data used to calculate the discharge coefficient were the average of three experiments in which the maximum standard error is less than 2%.

4. Conclusions (1) SMD of droplets first decreases and then increases when channel, chamber angle, D/b, and L/b increase. Spray angle first decreases and then increases with increasing L/b. The range of spray angle is between 65∘ and 80∘ . The optimized structure of the low pressure swirl nozzle is as follows: channel angle is 6∘ , chamber angle is 90∘ , D/d0 is 4.4∼6.5, D/b is 7, and L/b is 3.5. (2) SMD of droplets and spray angle are important parameters to evaluate the atomization quality of the nozzle. There exists an exponential relation between SMD of droplets and injection pressure. When the injection pressure is less than 0.8 MPa, the SMD of droplets linearly decreases as the injection pressure increases. When the injection pressure increases beyond 0.8 MPa, the SMD of droplets decreases more gradually. When the injection pressure is small (less than 0.8 MPa), the spray angle increases with increasing injection pressure. However, once the injection pressure is greater than 0.8 MPa, the spray angle gradually approaches a constant and will no longer be influenced by the injection pressure any more. (3) With increasing solid percentages of lime slurry, the SMD of droplets increases and the atomization effect drops slightly. Spray angle slowly decreases with increasing solid percentages of lime slurry. The optimized low pressure swirl spray nozzle not only satisfies the requirements of the semidry flue gas desulfurization process but also can greatly reduce

the injection pressure. The flux of lime slurry reveals a twin peaks distribution, and then it is uniformly distributed along the radial direction. (4) The flux of lime slurry increases nonlinearly with increasing injection pressure. But at a gradually decreasing rate, the flux characteristic function of the optimized nozzle is obtained at ambient temperature. For an injection pressure of 0.8 MPa, the SMD of droplets is about 95 𝜇m, the spray angle is 66.7∘ , and the flux coefficient is 0.2.

Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment The authors are grateful for the support of the Natural Science Foundation Key Program of Chongqing (CSTC2009BA6067).

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