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Experimental study on the injection performance of the gas-solid injector for large coal particles
PTEC-14969; No of Pages 10 Powder Technology xxx (2019) xxx
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental study on the injection performance of the gas-solid injector for large coal particles Daolong Yang a,b,⁎, Bangsheng Xing a,⁎, Jianping Li c, Yanxiang Wang a,b, Kuidong Gao b, Feng Zhou c, Youtao Xia a, Cong Wang a a b c
School of Mechareonic Engineering, Jiangsu Normal University, Xuzhou 221116, China Shandong Province Key Laboratory of Mine Mechanical Engineering, Shandong University of Science and Technology, Qingdao 266590, China School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, 221116, China
a r t i c l e
i n f o
Article history: Received 2 July 2019 Received in revised form 18 November 2019 Accepted 25 November 2019 Available online xxxx Keywords: Pneumatic conveying Gas-solid injector Injection experiment Injection performance Large particle
a b s t r a c t The gas-solid injector is the power source of the particle pneumatic conveying system. The injector structural parameters have a large impact on the pneumatic conveying system. To obtain the optimal structural parameters of the injector for large particles, the injection ratio in the pure flow field experiment (PFFE), the average static pressure of stable conveying (pressure index) and the particle transport time (time index) in the particle injection experiment (PIE) are used as the performance indexes. The influence of the structural factors on the injection performance in the PFFE and the PIE are obtained by the multi-index orthogonal test scheme. The optimal structural parameters of the injector in the PFFE and the PIE are compared and verified, providing significant guidance for practical applications and an experimental basis for the large particle pneumatic conveying system. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Pneumatic conveying systems are often used to convey powdery materials due to their simple structure and the absence of dust pollution [1,2]. Pneumatic conveying systems are generally composed of an air source, silo, feeder, conveying pipes, supercharger, and dust removing device [3,4]. Current researches for pneumatic conveying are almost powder pneumatic conveying [5–7]. In actual industrial production, the basic theories and equipments of powder materials pneumatic convey are developed, but the theory and equipment research of large size particles pneumatic conveying is less. However, when the particle size is larger than the Geldart D [8,9], the particles are not easily or effectively fluidized, and the dilute phase or dense phase pneumatic conveying method are used to convey large-sized particles. The most common feeder is the gas-solid injector. The gas-solid injector is the power source of particle pneumatic conveying. The first exchange of momentum and energy between the particles and the flow field happens in the gas-solid injector. The high-speed airflow is injected form the injector nozzle, and the low-pressure vacuum region is generated in the mixture part so that the outside air and the material are sucked into the mixture part from the feed part. The outside air and the material are ⁎ Corresponding author at: School of Mechareonic Engineering, Jiangsu Normal University, Xuzhou 221116, China. E-mail addresses: [email protected], [email protected] (D. Yang), [email protected] (B. Xing).
mixed with the high-speed airflow and are accelerated into the conveying part through the contraction part. Many scholars have studied the influence of the injector structure on the static pressure distribution of the internal flow field and the injection performance of small particles (size b5 mm). Chellappan [10] studied the mass flow rate of solids by varying different parameters related to the geometry of the secondary nozzle and by varying the position of the primary nozzle. Kmiec and Leschonski [11,12] presented the results of a numerical analysis of the aerodynamic model of the gas-solid pipetype injector and the dispersing injector for both single-sized particles and a mixture, and the theoretical pressure distributions along the axis in the respective parts of the injector are found to be in agreement with the experimental data of Bohnet and Wagenknecht [13] and Hutt [14]. Wang [15] developed a mathematical model to predict the airsolids performance of central air-jet pumps based on the fundamentals of fluid and particle mechanics. Xiong [16–21] used Lagrangian and Euler coupling simulation methods and a small particle (particle size 2.5 mm) injection experiment to analyze the influence of the nozzle flow field velocity, the throat distance, the mixing part size and the back pressure on the static pressure distribution and the maximum particle mass flow rate. Huang [22] measured pressure drops on different nozzles sizes and developed a new model to predict the nozzle pressure drop in the dense phase pneumatic conveying of pulverized coal based on Barth's pneumatic conveying theory. Xie [23] developed and calibrated a novel, vertical solid particle injector to naturally entrain micron-sized solid particles. Yang [24,25] used the particle trajectory
https://doi.org/10.1016/j.powtec.2019.11.087 0032-5910/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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D. Yang et al. / Powder Technology xxx (2019) xxx Table 1 Injector structural parameters.
Nomenclature d d1 d3 F(d) l1 l3 φ1 Ki d’ d2 d4 L l2 n rm R
the particle diameter, mm the diameter of feed part, mm the diameter of conveying part, mm the distribution function the length of nozzle, mm the distance of axis, mm the included angle between feed and mixture parts, ° the sum of the test results corresponding to the level i on any column the median parameter, mm the diameter of mixture part, mm the diameter of nozzle, mm the length of mixture part, mm the length of contraction part, mm the scale parameter the ejection ratio the range and the difference between the maximum test result and the minimum test result in any column
model and experimental method to obtain the influence of different nozzle positions of the gas-solid injector on injection performance and obtained an ideal nozzle position. Chen [26] simulated and analyzed the internal and external flow fields of a variety of injection nozzles by CFD simulation and the orthogonal test method to optimize the nozzle of a moist-mix shotcrete. AbdEl-Hamid [27] conducted detailed experimental investigations to study the single-phase (air-air) and two-phase (air-solid) flows through three different geometries of mixing ducts with an injector tail section. Tsuji [28] carried out Lagrangian-type numerical simulation on plug flow of cohesionless, spherical particles conveyed in a horizontal pipe. Sakai [29] proposed a coarse grain model for large-scale DEM simulations where a modeled particle whose size is larger than the original particle is used instead of a crowd of original particles. The above studies obtained the influence of the partial structural parameters on the pure flow field injection and the small-sized particle (b 5 mm) injection. However, the other injector structural parameters, such as the distance of the nozzle axis and the angle of the feeding part, also have an effect on injection performance, and the suitable injector structural parameters for large particles have not been studied. In this paper, a pure flow field experiment (PFFE) and a particle injection experiment (PIE) with multistructural factors of a gas-solid injector are carried out to reveal the influence of the structural parameters of a gas-solid injector on injection performance and to provide significant guidance for the large particle pneumatic conveying system. 2. Experiments 2.1. Experimental scheme To obtain the influence of the injector structural parameters on injection performance, the main components of the injector are designed
Parameters
Value
Diameter of the feed part d1 (mm) Diameter of the mixture part d2 (mm) Diameter of the conveying part d3 (mm) Diameter of the nozzle d4 (mm) Length of the mixture part L (mm) Length of the nozzle l1 (mm) Length of the contraction part l2 (mm) Distance of the axis l3 (mm) Angle of the feed and mixture part φ1 (°)
100 105 80 22 260 50, 90, 130, 170 100, 200, 300, 400 30, 20, 10, 0 60°, 70°, 80°, 90°
as the alternative structure, which is shown in Fig. 1. The injector is divided into the nozzle, the end cover, the feed and mixture part, the contraction part and the conveying part. The structural parameters studied in this paper are shown in Table 1, which includes the length of the nozzle l1, the length of the contraction part l2, the distance of the axis l3 and the angle of the feed and mixture part φ1. The injection experiment bed for a pure flow field and the particle pneumatic injection experiment bed are built to carry out the PFFE and the PIE. The orthogonal experiment factors and levels are shown in Table 2. The 16 groups of experiments with different injector structural parameters are shown in Table 3, with the orthogonal test scheme of L16(45). The orthogonal test factors A, B, C and D are the injector structural factors, and the factor E is set to a null factor. 2.2. Pure flow field experiment bed The injection experiment bed for a pure flow field is shown in Fig. 2 and mainly consists of an air-feeding measurement system and an injecting measurement system. The air-feeding measurement system is connected to a highpressure air source, and the high-pressure air flow enters the vortex flowmeter I through the anemostat and then enters the injecting measurement system. The injector of the injecting measurement system is the alternative structure, which is shown in Fig. 3, and the structural parameters are shown in Table 1. The vortex flowmeters I and II can be output parameters such as the vortex frequency, the air flow rate of working conditions and standard conditions, the air temperature and the static pressure. The air flow rate of working conditions indicates the actual flow rate (m3/h) under the working temperature and working pressure. The air flow rate of standard conditions indicates the flow rate (Nm3/h) at 0 °C and 1 atm. The air flow rate of standard conditions is used as experimental data and named as the flow rate in this paper. 2.3. Particle pneumatic injection experiment bed The particle pneumatic injection experiment bed, which is mainly composed of an air-feeding measurement system, an injection system and a conveying system, as shown in Fig. 4, can obtain the flow rate of the injector nozzle, the static pressure of the anemostat and the conveying pipe and the movement state of the particles in the transparent pipe. The air-feeding measurement system of the particle pneumatic injection experiment bed is similar to the injection experiment bed for a
Table 2 Orthogonal experiment factors and levels.
Fig. 1. Ejector structure.
Factors
Levels
A - Length of nozzle l1(mm) B - Length of contraction part l2(mm) C - Distance of axis l3(mm) D - Angle of feed and mixture part φ1(°) E - Null
50 100 30 60 –
90 200 20 70 –
130 300 10 80 –
170 400 0 90 –
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
D. Yang et al. / Powder Technology xxx (2019) xxx
3. Experimental results and discussions
Table 3 L16(45) orthogonal experiment. No.
1 2 3 4 5 6 7 8
Factors
No.
A
B
C
D
50 50 50 50 90 90 90 90
100 200 300 400 100 200 300 400
30 20 10 0 20 30 0 10
60 70 80 90 80 90 60 70
9 10 11 12 13 14 15 16
3.1. Experimental results of PFFE
Factors A
B
C
D
130 130 130 130 170 170 170 170
100 200 300 400 100 200 300 400
10 0 30 20 0 10 20 30
90 80 70 60 70 60 90 80
pure flow field. The air-feeding measurement system is connected to the injection system. The injection system adds an aggregate hopper to the top of the funnel. There is a separation plate between the aggregate hopper and the funnel. The outlet of the injection system is connected to the transparent pipe of the conveying system to record the movement of the particles, and the high-speed camera and the reflecting plate are arranged on both sides of the transparent pipe. A pressure transducer II is installed downstream of the transparent pipe, and the dust removing tank is connected by a 3-m straight conveying pipe from the transparent pipe. The coal particles with a particle size of 5–30 mm are used in the experiment, and the particle size is consistent with the R-R distribution and the particle distribution is shown as Eq. (1) [30]. h i 0 n F ðdÞ ¼ 1− exp − d=d
3
ð1Þ
where F(d) is the cumulative distribution function expressing the particle diameter less than d, d is the characteristic particle diameter expressing the minimum mesh size which allows particle to pass through, d’ is the median diameter expressing the median particle diameter when the particle size at a mass fraction of 0.5 oversize (d' = 14.60 mm), n is the scale parameter describing the width of distribution (n = 1.56). In the PIE, the separation plate is closed, and 5 kg of 5–30-mm coal particles are poured into the aggregate hopper. Then, the data acquisition system and the air supply compressor are turned on. When the signal value of the pressure transducer I is stable at 0.4 MPa, data is collected, and the valve of the air-feeding measurement system is opened. After the signal value of the vortex flowmeter I and II is stabilized, the high speed camera is turned on, and the separation plate is pulled away to allow 5 kg of coal particles to enter the injector at one time. When there are no coal particles passing through the transparent pipe, the high-speed camera is stopped, the valve of the air-feeding measuring system is closed, the experimental data is saved and the next experiment is carried out.
3.1.1. Result analysis According to the orthogonal test scheme of Table 3, four air supply pressures (0.10, 0.20, 0.30 and 0.40 MPa) are used for the PFFE. Four groups of the flow rate from the vortex flowmeters I and II are obtained and are shown in Fig. 5. The flow rate from the vortex flowmeters I under four air supply pressures are substantially constant, while the values from the vortex flowmeter II show the same change trend as the orthogonal experiment schemes. The injection ratio (the ratio of the flow rate from the vortex flowmeter II to that from the vortex flowmeter I) is taken as the injection performance index of the PFFE. The experimental results of the injection ratio are shown in Table 4. 3.1.2. Optimal injector structural parameters for PFFE The factor order and optimal solution of the influence of the injector structural factors on the injection performance of the PFFE under different air supply pressures are obtained by range analysis, which are shown in Table 5. The R value indicates the influence of the level change of the factor on the experimental results. The maximum R value shows that the factor has the greatest effect on the experimental results. The factor order is the order of the R value. The optimal solution refers to the combination of the optimal level of each factor, and the determination of the optimal level of each factor is related to the experiment index. According to the factor order in Table 5, the length of the nozzle has the most significant influence on the injector rate. The influence of the length of the contraction part and the distance of the axis is basically the same when the air supply pressure is high. The influence of the angle of the feed and mixture part is the smallest. The K values of the injector structural factors under the four air supply pressures are shown in Fig. 6. It can be seen from Fig. 6 that the K value increases with the length of the nozzle and the distance of the axis and decreases with the length of the contraction part and the angle of the feed and mixture part. The K value curve under the air supply pressure of 0.1 MPa is the same as the other curves, but there is some difference in the values; the K value curves under the air supply pressures of 0.2, 0.3, and 0.4 MPa are basically coincident. In the same ordinate of the K value, the change range of the length of the nozzle is the largest, and the length of the contraction part and the distance of the axis is basically the same (except 0.1 MPa), and the angle of the feed and mixture part is the smallest, which indicates that different factors have different influences on the K and R values; the larger the change range is, the larger the R value is, and the greater the influence on the injector rate. According to the analysis results of Fig. 6, the trend of K values with the injector structural factors indicates that the influence of the injector
Fig. 2. Injection experiment bed for pure flow field.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 3. Injector with different structural parameters.
structure on the K value is monotonously increasing or monotonously decreasing. Therefore, the optimal level of the injector structure for the PFFE is A4B1C1D2, which is based on the maximum number of occurrences in the optimal solution under different air supply pressures. The verification experiment is carried out, and the injector rates under four air supply pressures compared with the results of 16 orthogonal experiments are shown in Fig. 7. In Fig. 7, the injector rate of the optimal level is higher than the 16 orthogonal experimental results. Therefore, the optimal injector structure for the PFFE is the length of the nozzle 170 mm (A4), the length of the contraction part 100 mm (B1), the distance of the axis 30 mm (C1) and the angle of the feed and mixture part 70° (D2). 3.2. Experimental results of PIE 3.2.1. Result analysis According to the results of the PFFE, when the air supply pressure is large, the influence of the injector structure on the injector rate is basically the same. To simplify the experiment and prevent the particles from clogging the injector, an air supply pressure of 0.40 MPa is used in the PIE. The injector structural factors and the experiment scheme in the PIE are the same as those in the PFFE. In the 16 groups of the PIE, the static pressure average and the flow rate average of the vortex flowmeter I are shown in Fig. 8. The pressure without particles indicates the static pressure average of the vortex flowmeter I in the PFFE; the pressure with particles indicates the static pressure average of the vortex flowmeter I in the PIE; the flow rate
without particles indicates the flow rate average of the vortex flowmeter I in the PFFE; and the flow rate with particles indicates the flow rate average of the vortex flowmeter I in the PIE. In Fig. 8, the static pressure average of the PIE is larger than that of the PFFE, and the flow rate average of the PIE is smaller than that of the PFFE, which indicates that the injection system produces a certain back pressure due to the entry of particles. This back pressure makes the flow rate average of the PIE decrease slightly and the static pressure average of the PIE increase slightly. The differences of the flow rate average and the static pressure average between the results of the PFFE and the PIE are b5%; the differences of the flow rate average and the static pressure average between the 16 results in the PIE are also small, which indicates that the different injector structures in the PIE have little effect on the static pressure average and the flow rate average. Therefore, this paper assumes that the static pressure average and the flow rate average from the air-feeding measurement system in the PIE are substantially constant and only analyzes the pressure changes of the pressure transducer II and the particle motion obtained by the high-speed camera. The pressure curve obtained by the pressure transducer II of the first group in the PIE is shown in Fig. 9(a). The blue curve is the measured value, and the black curve is the smooth value. The movement of the particles in the transparent pipe corresponding to the pressure curve is shown in Fig. 9(b)-(e). The pressure curve in Fig. 9(a) is the static pressure curve. According to the particle conveying state and the pressure change curve, the coal particle injection process can be divided into 4 parts: the pure flow
Fig. 4. Particle pneumatic injection experiment bed.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 5. Air flow rate of two Vortex flowmeter.
converted into static pressure, so the pressure curve rises to meet the power required by the conveying of the coal particle dunes. When the dune or plug of coal particles passes through the conveying pipe, some coal particles remain in the pipe bottom. The dune or plug of coal particles gradually shrinks, and the space for flow field gradually increases, which leads to some static pressure being converted into dynamic pressure again. The pressure curve shows a downward trend at this moment and then gradually stabilizes, corresponding to the stable conveying part shown in Fig. 9(d). It can be found from the high-speed video that most of the coarse-grained coal particles are deposited at the pipe bottom in the stable conveying part. The transparent pipe in the high-speed video shows a layered state consisting of a fluid layer, a conveying layer and a deposition layer. The fluid layer is mainly a high-speed air flow field, while the conveying layer is a mixed area of particles and flow field. There are more small-sized particles in the conveying layer, and the coal particle velocities decrease under the resistance action of the deposition layer. The deposition layer mainly consists of coarse-grained particles that have a lower velocity or are static. The fluidity of the deposition layer is poor, and it is difficult to promote the movement of coal particles. When the coal particles leave the conveying pipe, the fluid layer gradually increases, and the conveying layer gradually moves downward. The coal particles in the deposition layer gradually enter the flow field in the remaining conveying part, which is shown in Fig. 9(e). Because no more coal particles enter the injector, the coal particles in the deposition layer
part, the feeding part, the conveying part and the remaining conveying part. There are no particles in the injector or the conveying pipe in the pure flow part, and the flow field is in a stable state. Because the conveying pipe is connected to the dust removing tank (the back pressure is almost zero), the measured value should be substantially near the zero position. However, the smooth value of the flow field part is slightly lower than the zero point, which indicates that the pressure transducer II has a certain zero drift (the zero drift value is excluded when the data are analyzed). The pressure curve of the feeding part has a certain fluctuation, and the time of the feeding part is short. When the separation plate is pulled away from the aggregate hopper, the first falling particles enter the conveying pipe with a high velocity, which is shown in Fig. 9 (b). To convey the coal particles, part of the dynamic pressure is converted into static pressure to push the particle movement, which leads to the fluctuation of the pressure curve. However, the measured value is small due to the small amount of coal particles. Moreover, it can be found from high-speed video that the coal particles are basically in suspension and the particle velocity is high. The pressure curve first rises to a peak, then falls and gradually stabilizes when a large number of particles enter the injector. Because a large amount of coal particles enter the injector and the conveying pipe simultaneously, the coal particles first pass through the conveying pipe in the form of dunes or plugs, as shown in Fig. 9(c). This event corresponds to the high peak of the pressure curve. The space for flow field is small in the pipe, and more dynamic pressure of the flow fluid is Table 4 The ejection ratio under the four air supply pressures. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.1 MPa 0.2 MPa 0.3 MPa 0.4 MPa
2.7 2.71 2.72 2.72
2.68 2.69 2.69 2.69
2.34 2.44 2.47 2.48
2.4 2.41 2.41 2.41
2.87 2.88 2.84 2.86
2.76 2.77 2.78 2.79
2.56 2.57 2.56 2.57
2.47 2.61 2.6 2.63
2.86 2.81 2.81 2.82
2.72 2.76 2.76 2.74
2.84 2.86 2.87 2.88
2.77 2.78 2.79 2.77
2.88 2.87 2.87 2.87
2.9 2.87 2.92 2.9
2.79 2.84 2.84 2.83
2.82 2.83 2.84 2.84
Table 5 Range analysis of ejection ratios 0.1 MPa A K1 K2 K3 K4 R Factor order Optimal solution
B
10.12 11.3 10.66 11.06 11.19 10.54 11.39 10.46 1.26 0.84 ANBNCNEND A4B1C1E4D1
0.2MPa
0.3MPa
C
D
E
A
B
11.12 11.11 10.57 10.56 0.55
10.98 10.87 10.75 10.8 0.23
10.69 10.91 10.75 11.02 0.33
10.25 11.27 10.83 11.09 11.21 10.71 11.41 10.62 1.17 0.65 ANBNCNDNE A4B1C2D2E4
0.4MPa
C
D
E
A
B
11.18 11.19 10.72 10.61 0.58
11.01 11.03 10.92 10.83 0.2
10.92 10.89 10.87 11.02 0.15
10.29 11.24 10.78 11.16 11.23 10.75 11.48 10.63 1.19 0.61 ANCNBNDNE A4C1B1D2E4
C
D
E
A
11.22 11.15 10.81 10.6 0.61
11.03 11.04 10.91 10.84 0.2
10.92 10.91 10.92 11.04 0.13
10.31 11.28 10.85 11.13 11.21 10.75 11.45 10.65 1.14 0.64 ANBNCNDNE A4C1B1D2E4
B
C
D
E
11.23 11.15 10.83 10.6 0.63
11.05 11.07 10.92 10.85 0.22
10.92 10.92 10.91 11.06 0.15
Note: Ki is the sum of the test results corresponding to the level i on any column; R is the range and the difference between the maximum test result and the minimum test result in any column, R=max{K1, K2, ... Ki}-min{K1, K2, ... Ki}.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 6. K values of injector structure factors.
re-enter the flow field. At this time, it is obvious that there is a boundary line in the fig. There is a layered state on the left side of the boundary line, but the right side is the no-layered area. Due to the reduction of coal particles in the injector, the particles can obtain more kinetic energy in the flow field, and the excess kinetic energy can cause the coal particles to be in suspension or hop into the conveying pipe.
3.2.2. Optimal injector structural parameters for PIE The pressure average of the stable conveying part and the time of the conveying part are used as evaluation indexes. The larger the pressure average of the stable conveying part (hereinafter referred to as the pressure index), the greater the change from dynamic pressure to static pressure, which is more conducive to particle conveying. The shorter the time of the conveying part (hereinafter referred to as the time
index), the faster the movement of the coal particles and the greater the mass flow rate of particle conveying in the pipe. The results of the PIE are shown in Table 6. Because the injector performances of the pressure index and the time index are different, the two groups of indexes have different trends for the 16 groups of injector structures. The range analysis of the experimental results in Table 6 is carried out, which is shown in Table 7. The optimal solution of the pressure index is the maximum value of each level, and the optimal solution of the time index is the minimum value of each level. In the pressure index of Table 7, the R values of factor A and factor D are large, which indicates that these two factors are the significant factors and that the length of the nozzle and the angle of the feed and mixture part have a great influence on the pressure index. The R values of factor B, factor C and the null factor E are similar, which indicates that
Fig. 7. Pure flow experimental results of optimum level of injector structure.
Fig. 8. Stable output value of pressure transducer I and vortex flowmeter I.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 9. Pressure change curve and particle movement.
and the smaller the time index, the greater the mass flow rate of the particles conveying. The trends of change of the K value of pressure index and the time index with the injector structure are different, but the trends are almost opposite. It is because that the static pressure in the flow field of injector increases when the pressure index increases, and the thrust that particles obtain from the flow field also increases, which make the acceleration of particles grow. As a result, the particles move through the injector quickly, which reduces the index of time. On the contrary, when the injector structure is not conducive to increase the static pressure in flow field, the thrust and the acceleration of particles is small and the particles speed are slow, which increase the movement time, and the time index increases correspondingly. According to Fig. 10(a), the K value first increases and then decreases with the length of the nozzle under the pressure index, and the maximum position is 90 mm (A2); the K value first decreases and then increases with the length of the nozzle under the time index, and the minimum position is 130 mm (A3). Because the maximum position of the pressure index and the minimum position of the time index are noncoincident, there may be a suitable length of the nozzle that can balance the pressure index and the time index.
the significance of these factors is low. The R value of the null factor E is the smallest, which indicates that there is no correlation between the four structural factors of the injector, and there is no other factor that affects the pressure index in the PIE. In the time index of Table 7, the R value of factor A is the largest, and the R values of factor B, factor C, and factor D decrease sequentially. The R value of factor E is the smallest. This indicates that the significance of the length of the nozzle, the length of the contraction part, the distance of the axis and the angle of the feed and mixture part on the time index decreases sequentially, and no other factors have an influence on the time index in the PIE. The optimal solutions under the two indexes in Table 7 are A2B1C1D1 and A3B1C2D1. The K values in Table 6 are compared and shown in Fig. 10. The trends of the K values with the injector structural factors are obtained. In Fig. 10, pressure represents the pressure index, and time represents the time index. In Fig. 10, the larger the range occupied by the K value curve under the same ordinate, the larger the R value is and the more significant the influence of the injector structural factor on the two indexes is. The length of the nozzle (factor A) is in the prominent position under both the pressure and time indexes. The higher the pressure index, the greater the static pressure obtained by the particles conveying; Table 6 The particle injection experiment results. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Pressure Time
2309 4.02
1413 4.27
764 4.49
897 4.93
2780 2.56
3233 3.15
3512 4.1
3224 4.64
1949 2.8
1710 3.05
1791 2.82
2132 2.93
1782 4.76
1872 4.9
1511 4.34
1421 5.5
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Table 7 Range analysis of particle injection experiment results. Pressure of stable conveying part A K1 K2 K3 K4 R Factor order Optimal solution
B
5383 8820 8749 8229 7583 7579 6587 7675 3366 1241 ANDNBNCNE A2D1B1C1E4
Time of conveying part
C
D
E(empty)
A
8754 7836 7610 7702 1145
9825 8211 6676 7590 3150
8755 8296 7911 7640 1115
17.72 14.13 14.45 15.38 11.60 15.75 19.50 18.01 7.90 3.87 ANBNCNDNE A3B1C2D1E4
According to Fig. 10(b), the K value shows a monotonous decrease with the length of the contraction part under the pressure index and a monotonous increase with the length of the contraction part under the time index. The maximum position of the pressure index and the minimum position of the time index are the same position at 100 mm (B1). Therefore, the optimal length of the contraction part is 100 mm (B1). According to Fig. 10(c), the K value decreases and then increases with the distance of the axis under the pressure index, and the maximum position is 30 mm (C1); the K value is steady, then decreases and finally increases with the distance of the axis under the time index, and the minimum position is 20 mm (C2). Because the maximum position of the pressure index and the minimum position of the time index are non-coincident, there may be a suitable distance of the axis that can balance the pressure index and the time index. According to Fig. 10(d), the K value first decreases and then increases with the angle of the feed and mixture part under the pressure index, and the maximum position is 60° (D1); the K value first increases and then decreases with the angle of the feed and mixture part under the time index, and the minimum position is 60° (D1). The maximum position of the pressure index and the minimum position of the time index are the same position. Therefore, the optimal angle of the feed and mixture part is 60° (D1). The K values of factor A and factor C are fitted to obtain four fitting curves, which are shown in Fig. 11.
B
C
D
E(empty)
15.50 14.09 16.83 16.84 2.75
15.15 16.49 15.60 15.22 1.34
16.05 16.07 15.33 15.21 0.85
In Fig. 11(a), the relative coefficients R2 of the fitting curves for the length of the nozzle under the pressure index and the time index are 0.855 and 0.821, respectively, and the fitting degree is normal. Although the extreme points of the two fitting curves are different, the difference is small. In the case in which both indexes and the manufacturing processes are considered, the two extreme data points can be rounded. Thus, the length of the nozzle is 110 mm. In Fig. 11(b), the relative coefficient R2 of the fitting curve of the distance of the axis under the pressure index is 0.992, and the fitting degree is good, but the relative coefficient R2 under the time index is 0.541, and the fitting degree is poor. Although the extreme points of the fitting curve under the two indexes have a large difference, the maximum value of the pressure index is similar to the extreme point of the time index. In the case in which both indexes and the experimental results are taken into consideration, the optimal axis distance is 30 mm. According to the analysis results of Fig. 11, the optimal levels of the injector structural parameters are a nozzle length of 110 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60°. The verification experiment is carried out, and the experimental results are shown in Fig. 12. The best shape expresses the experimental results of the optimal levels. In Fig. 12, the verification results of the PIE with the optimal levels show that the pressure index is slightly smaller than the maximum result (No. 7) and the time index is slightly higher than the minimum result (No. 5). Therefore, a nozzle length of 110 mm, a contraction part length
Fig. 10. K values of injector structural factors for PIE.
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D. Yang et al. / Powder Technology xxx (2019) xxx
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Fig. 11. Fitting of K value of factors A and C.
of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60° can be used as the optimal injector structure parameters for the PIE.
3.3. Discussions 3.3.1. Comparison of optimal injector structure in PFFE and PIE In the PFFE, the optimal structural parameters of the injector are a nozzle length of 170 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 70°. In the PIE, the optimal structural parameters of the injector are a nozzle length of 110 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60°. In the two experiments, the optimal parameters of the length of the nozzle and the angle of the feed and mixture part are different, which indicates that the particles have an effect on these factors. The optimal parameters of the length of the contraction part and the distance of the axis are the same, which indicates that the particles have no effect on these factors. It is foreseeable that the optimal structural parameters of the injector will change with the change of particle size. When the particle size is small and the particles can be suspended in air or in a fluidized state, the optimal injector structural parameters obtained by the PFFE are suitable. As the particle size increases, the particles are difficult to fluidize. The effect of particle size on the injector structure is gradually apparent until the optimal structural parameters obtained by the PIE are achieved. Therefore, further experiments will be carried out using different particle sizes to obtain the optimal structural parameters of injectors under different particle sizes to meet the requirements of pneumatically conveying different materials.
3.3.2. Significance analysis of injector structures in PFFE and PIE Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the length of the nozzle on injection performance is the most prominent, indicating that the length of the nozzle is the main factor affecting injection performance, and a reasonable nozzle length is critical to injection performance. Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the length of the contraction part and the distance of the axis on injection performance is basically the same in the experiments, and the factor order of these structures is nearly in the middle position. The optimal structural parameters are basically unchanged, indicating that the two structural parameters have a relatively stable effect on injector performance, but the effect of particle addition on the optimal structural parameters is small. Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the angle of the feed and mixture part on injection performance is prominent in the PIE. The feeding time of particles is the shortest when the feed and mixture part is vertical, but the initial velocity direction of particles is perpendicular to the flow field direction. The flow field needs to provide more energy to initially accelerate the particles. When the angle of the feed and mixture part is b90°, the feeding time is prolonged, but the initial velocity of the particle has a certain component in the flow field direction. It is easier for the particles to move with the flow field when entering the injector. However, if the angle of the feed and mixture part is further reduced, for large particle sizes, it can be predicted that the probability of particle blocking will increase due to the irregular shape and the large repose angle of the particles. Therefore, a suitable angle for the feed and mixture part will facilitate the initial acceleration process of the particles and prevent injector blocking.
4. Limitation of experimental results on contraction part In the PFFE and the PIE, the distance between the flow field inlet and the measuring point is different due to the difference in the length of the nozzle and the length of the contraction part. In the PFFE, the experimental data with a lower supply pressure are slightly different from the other experiment data due to the pressure loss. In the PIE, due to the addition of particles, the interactions between the particles and between the particles and the pipe wall consume a large amount of the flow field energy. The farther the distance from the flow field inlet to the measuring point, the larger the energy consumption of the flow field is. This distance also affects injection performance, which is more obvious that the improvement in injection performance under the two indexes when the length of the contraction part was decreased. However, it is shown that the length of the contraction part is not the main factor affecting injection performance in both the PFFE and the PIE. Therefore, the experiments have some limitations, but the results Fig. 12. Particle injection experimental results of optimum level of injector structure.
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D. Yang et al. / Powder Technology xxx (2019) xxx
obtained in the PFFE and the PIE are still important for guiding the design of injectors for large particles. 5. Conclusion (1) To obtain the optimal injector structural parameters for large particle pneumatic conveying, the PFFE and the PIE are carried out by using the injection rate, the pressure index and the time index as the injection performance indexes. The results show the influence of the injector structural factors on the injection performance of the PFFE and the PIE. (2) The experimental results of the PFFE show that the length of the nozzle is the most significant factor. The K value of the injection performance index increases with the length of the nozzle and the distance of the axis and decreases with the length of the contraction part and the angle of the feed and mixture part. The optimal structural parameters of the injector are 170 mm for the length of the nozzle, 100 mm for the length of the contraction part, 30 mm for the distance of the axis and 70° for the angle of the feed and mixture part. (3) The experimental results of the PIE show that the curve change of static pressure can reflect the particle conveying state. The length of the nozzle is the most significant under the pressure index and time index. The optimal structural parameters of the injector are 110 mm for the length of the nozzle, 100 mm for the length of the contraction part, 30 mm for the distance of the axis and 60° for the angle of the feed and mixture part. (4) The optimal structural parameters of the injector in the PFFE and the PIE are compared and verified, which provides significant guidance for practical applications and an experimental basis for the large particle pneumatic conveying system.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2019.11.087. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The paper is funded by the National Natural Science Foundation of China (51705222), the National Natural Science Foundation of Jiangsu Province (BK20170241), the National Natural Science Foundation of China (51675521), the Open Foundation of Shandong Province Key Laboratory of Mine Mechanical Engineering (2019KLMM105) and the Natural Science Foundation of Jiangsu Normal University (17XLR028). References [1] K. Li, S.B. Kuang, R.H. Pan, A.B. Yu, Numerical study of horizontal pneumatic conveying: effect of material properties, Powder Technol. 251 (2014) 15–24, https://doi. org/10.1016/j.powtec.2013.10.013. [2] K. Takabatake, Y. Mori, J.G. Khinast, M. Sakai, Numerical investigation of a coarsegrain discrete element method in solid mixing in a spouted bed, Chem. Eng. J. 346 (2018) 416–426, https://doi.org/10.1016/j.cej.2018.04.015. [3] D.L. Yang, J.P. Li, B.S. Xing, Y.X. Wang, Recent patents on gangue pneumatic filling for coal auger mining method, Recent Patents on Mechanical Engineering. 11 (2018) 31–40, https://doi.org/10.2174/2212797611666180118094926.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental study on the injection performance of the gas-solid injector for large coal particles Daolong Yang a,b,⁎, Bangsheng Xing a,⁎, Jianping Li c, Yanxiang Wang a,b, Kuidong Gao b, Feng Zhou c, Youtao Xia a, Cong Wang a a b c
School of Mechareonic Engineering, Jiangsu Normal University, Xuzhou 221116, China Shandong Province Key Laboratory of Mine Mechanical Engineering, Shandong University of Science and Technology, Qingdao 266590, China School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, 221116, China
a r t i c l e
i n f o
Article history: Received 2 July 2019 Received in revised form 18 November 2019 Accepted 25 November 2019 Available online xxxx Keywords: Pneumatic conveying Gas-solid injector Injection experiment Injection performance Large particle
a b s t r a c t The gas-solid injector is the power source of the particle pneumatic conveying system. The injector structural parameters have a large impact on the pneumatic conveying system. To obtain the optimal structural parameters of the injector for large particles, the injection ratio in the pure flow field experiment (PFFE), the average static pressure of stable conveying (pressure index) and the particle transport time (time index) in the particle injection experiment (PIE) are used as the performance indexes. The influence of the structural factors on the injection performance in the PFFE and the PIE are obtained by the multi-index orthogonal test scheme. The optimal structural parameters of the injector in the PFFE and the PIE are compared and verified, providing significant guidance for practical applications and an experimental basis for the large particle pneumatic conveying system. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Pneumatic conveying systems are often used to convey powdery materials due to their simple structure and the absence of dust pollution [1,2]. Pneumatic conveying systems are generally composed of an air source, silo, feeder, conveying pipes, supercharger, and dust removing device [3,4]. Current researches for pneumatic conveying are almost powder pneumatic conveying [5–7]. In actual industrial production, the basic theories and equipments of powder materials pneumatic convey are developed, but the theory and equipment research of large size particles pneumatic conveying is less. However, when the particle size is larger than the Geldart D [8,9], the particles are not easily or effectively fluidized, and the dilute phase or dense phase pneumatic conveying method are used to convey large-sized particles. The most common feeder is the gas-solid injector. The gas-solid injector is the power source of particle pneumatic conveying. The first exchange of momentum and energy between the particles and the flow field happens in the gas-solid injector. The high-speed airflow is injected form the injector nozzle, and the low-pressure vacuum region is generated in the mixture part so that the outside air and the material are sucked into the mixture part from the feed part. The outside air and the material are ⁎ Corresponding author at: School of Mechareonic Engineering, Jiangsu Normal University, Xuzhou 221116, China. E-mail addresses: [email protected], [email protected] (D. Yang), [email protected] (B. Xing).
mixed with the high-speed airflow and are accelerated into the conveying part through the contraction part. Many scholars have studied the influence of the injector structure on the static pressure distribution of the internal flow field and the injection performance of small particles (size b5 mm). Chellappan [10] studied the mass flow rate of solids by varying different parameters related to the geometry of the secondary nozzle and by varying the position of the primary nozzle. Kmiec and Leschonski [11,12] presented the results of a numerical analysis of the aerodynamic model of the gas-solid pipetype injector and the dispersing injector for both single-sized particles and a mixture, and the theoretical pressure distributions along the axis in the respective parts of the injector are found to be in agreement with the experimental data of Bohnet and Wagenknecht [13] and Hutt [14]. Wang [15] developed a mathematical model to predict the airsolids performance of central air-jet pumps based on the fundamentals of fluid and particle mechanics. Xiong [16–21] used Lagrangian and Euler coupling simulation methods and a small particle (particle size 2.5 mm) injection experiment to analyze the influence of the nozzle flow field velocity, the throat distance, the mixing part size and the back pressure on the static pressure distribution and the maximum particle mass flow rate. Huang [22] measured pressure drops on different nozzles sizes and developed a new model to predict the nozzle pressure drop in the dense phase pneumatic conveying of pulverized coal based on Barth's pneumatic conveying theory. Xie [23] developed and calibrated a novel, vertical solid particle injector to naturally entrain micron-sized solid particles. Yang [24,25] used the particle trajectory
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Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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D. Yang et al. / Powder Technology xxx (2019) xxx Table 1 Injector structural parameters.
Nomenclature d d1 d3 F(d) l1 l3 φ1 Ki d’ d2 d4 L l2 n rm R
the particle diameter, mm the diameter of feed part, mm the diameter of conveying part, mm the distribution function the length of nozzle, mm the distance of axis, mm the included angle between feed and mixture parts, ° the sum of the test results corresponding to the level i on any column the median parameter, mm the diameter of mixture part, mm the diameter of nozzle, mm the length of mixture part, mm the length of contraction part, mm the scale parameter the ejection ratio the range and the difference between the maximum test result and the minimum test result in any column
model and experimental method to obtain the influence of different nozzle positions of the gas-solid injector on injection performance and obtained an ideal nozzle position. Chen [26] simulated and analyzed the internal and external flow fields of a variety of injection nozzles by CFD simulation and the orthogonal test method to optimize the nozzle of a moist-mix shotcrete. AbdEl-Hamid [27] conducted detailed experimental investigations to study the single-phase (air-air) and two-phase (air-solid) flows through three different geometries of mixing ducts with an injector tail section. Tsuji [28] carried out Lagrangian-type numerical simulation on plug flow of cohesionless, spherical particles conveyed in a horizontal pipe. Sakai [29] proposed a coarse grain model for large-scale DEM simulations where a modeled particle whose size is larger than the original particle is used instead of a crowd of original particles. The above studies obtained the influence of the partial structural parameters on the pure flow field injection and the small-sized particle (b 5 mm) injection. However, the other injector structural parameters, such as the distance of the nozzle axis and the angle of the feeding part, also have an effect on injection performance, and the suitable injector structural parameters for large particles have not been studied. In this paper, a pure flow field experiment (PFFE) and a particle injection experiment (PIE) with multistructural factors of a gas-solid injector are carried out to reveal the influence of the structural parameters of a gas-solid injector on injection performance and to provide significant guidance for the large particle pneumatic conveying system. 2. Experiments 2.1. Experimental scheme To obtain the influence of the injector structural parameters on injection performance, the main components of the injector are designed
Parameters
Value
Diameter of the feed part d1 (mm) Diameter of the mixture part d2 (mm) Diameter of the conveying part d3 (mm) Diameter of the nozzle d4 (mm) Length of the mixture part L (mm) Length of the nozzle l1 (mm) Length of the contraction part l2 (mm) Distance of the axis l3 (mm) Angle of the feed and mixture part φ1 (°)
100 105 80 22 260 50, 90, 130, 170 100, 200, 300, 400 30, 20, 10, 0 60°, 70°, 80°, 90°
as the alternative structure, which is shown in Fig. 1. The injector is divided into the nozzle, the end cover, the feed and mixture part, the contraction part and the conveying part. The structural parameters studied in this paper are shown in Table 1, which includes the length of the nozzle l1, the length of the contraction part l2, the distance of the axis l3 and the angle of the feed and mixture part φ1. The injection experiment bed for a pure flow field and the particle pneumatic injection experiment bed are built to carry out the PFFE and the PIE. The orthogonal experiment factors and levels are shown in Table 2. The 16 groups of experiments with different injector structural parameters are shown in Table 3, with the orthogonal test scheme of L16(45). The orthogonal test factors A, B, C and D are the injector structural factors, and the factor E is set to a null factor. 2.2. Pure flow field experiment bed The injection experiment bed for a pure flow field is shown in Fig. 2 and mainly consists of an air-feeding measurement system and an injecting measurement system. The air-feeding measurement system is connected to a highpressure air source, and the high-pressure air flow enters the vortex flowmeter I through the anemostat and then enters the injecting measurement system. The injector of the injecting measurement system is the alternative structure, which is shown in Fig. 3, and the structural parameters are shown in Table 1. The vortex flowmeters I and II can be output parameters such as the vortex frequency, the air flow rate of working conditions and standard conditions, the air temperature and the static pressure. The air flow rate of working conditions indicates the actual flow rate (m3/h) under the working temperature and working pressure. The air flow rate of standard conditions indicates the flow rate (Nm3/h) at 0 °C and 1 atm. The air flow rate of standard conditions is used as experimental data and named as the flow rate in this paper. 2.3. Particle pneumatic injection experiment bed The particle pneumatic injection experiment bed, which is mainly composed of an air-feeding measurement system, an injection system and a conveying system, as shown in Fig. 4, can obtain the flow rate of the injector nozzle, the static pressure of the anemostat and the conveying pipe and the movement state of the particles in the transparent pipe. The air-feeding measurement system of the particle pneumatic injection experiment bed is similar to the injection experiment bed for a
Table 2 Orthogonal experiment factors and levels.
Fig. 1. Ejector structure.
Factors
Levels
A - Length of nozzle l1(mm) B - Length of contraction part l2(mm) C - Distance of axis l3(mm) D - Angle of feed and mixture part φ1(°) E - Null
50 100 30 60 –
90 200 20 70 –
130 300 10 80 –
170 400 0 90 –
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
D. Yang et al. / Powder Technology xxx (2019) xxx
3. Experimental results and discussions
Table 3 L16(45) orthogonal experiment. No.
1 2 3 4 5 6 7 8
Factors
No.
A
B
C
D
50 50 50 50 90 90 90 90
100 200 300 400 100 200 300 400
30 20 10 0 20 30 0 10
60 70 80 90 80 90 60 70
9 10 11 12 13 14 15 16
3.1. Experimental results of PFFE
Factors A
B
C
D
130 130 130 130 170 170 170 170
100 200 300 400 100 200 300 400
10 0 30 20 0 10 20 30
90 80 70 60 70 60 90 80
pure flow field. The air-feeding measurement system is connected to the injection system. The injection system adds an aggregate hopper to the top of the funnel. There is a separation plate between the aggregate hopper and the funnel. The outlet of the injection system is connected to the transparent pipe of the conveying system to record the movement of the particles, and the high-speed camera and the reflecting plate are arranged on both sides of the transparent pipe. A pressure transducer II is installed downstream of the transparent pipe, and the dust removing tank is connected by a 3-m straight conveying pipe from the transparent pipe. The coal particles with a particle size of 5–30 mm are used in the experiment, and the particle size is consistent with the R-R distribution and the particle distribution is shown as Eq. (1) [30]. h i 0 n F ðdÞ ¼ 1− exp − d=d
3
ð1Þ
where F(d) is the cumulative distribution function expressing the particle diameter less than d, d is the characteristic particle diameter expressing the minimum mesh size which allows particle to pass through, d’ is the median diameter expressing the median particle diameter when the particle size at a mass fraction of 0.5 oversize (d' = 14.60 mm), n is the scale parameter describing the width of distribution (n = 1.56). In the PIE, the separation plate is closed, and 5 kg of 5–30-mm coal particles are poured into the aggregate hopper. Then, the data acquisition system and the air supply compressor are turned on. When the signal value of the pressure transducer I is stable at 0.4 MPa, data is collected, and the valve of the air-feeding measurement system is opened. After the signal value of the vortex flowmeter I and II is stabilized, the high speed camera is turned on, and the separation plate is pulled away to allow 5 kg of coal particles to enter the injector at one time. When there are no coal particles passing through the transparent pipe, the high-speed camera is stopped, the valve of the air-feeding measuring system is closed, the experimental data is saved and the next experiment is carried out.
3.1.1. Result analysis According to the orthogonal test scheme of Table 3, four air supply pressures (0.10, 0.20, 0.30 and 0.40 MPa) are used for the PFFE. Four groups of the flow rate from the vortex flowmeters I and II are obtained and are shown in Fig. 5. The flow rate from the vortex flowmeters I under four air supply pressures are substantially constant, while the values from the vortex flowmeter II show the same change trend as the orthogonal experiment schemes. The injection ratio (the ratio of the flow rate from the vortex flowmeter II to that from the vortex flowmeter I) is taken as the injection performance index of the PFFE. The experimental results of the injection ratio are shown in Table 4. 3.1.2. Optimal injector structural parameters for PFFE The factor order and optimal solution of the influence of the injector structural factors on the injection performance of the PFFE under different air supply pressures are obtained by range analysis, which are shown in Table 5. The R value indicates the influence of the level change of the factor on the experimental results. The maximum R value shows that the factor has the greatest effect on the experimental results. The factor order is the order of the R value. The optimal solution refers to the combination of the optimal level of each factor, and the determination of the optimal level of each factor is related to the experiment index. According to the factor order in Table 5, the length of the nozzle has the most significant influence on the injector rate. The influence of the length of the contraction part and the distance of the axis is basically the same when the air supply pressure is high. The influence of the angle of the feed and mixture part is the smallest. The K values of the injector structural factors under the four air supply pressures are shown in Fig. 6. It can be seen from Fig. 6 that the K value increases with the length of the nozzle and the distance of the axis and decreases with the length of the contraction part and the angle of the feed and mixture part. The K value curve under the air supply pressure of 0.1 MPa is the same as the other curves, but there is some difference in the values; the K value curves under the air supply pressures of 0.2, 0.3, and 0.4 MPa are basically coincident. In the same ordinate of the K value, the change range of the length of the nozzle is the largest, and the length of the contraction part and the distance of the axis is basically the same (except 0.1 MPa), and the angle of the feed and mixture part is the smallest, which indicates that different factors have different influences on the K and R values; the larger the change range is, the larger the R value is, and the greater the influence on the injector rate. According to the analysis results of Fig. 6, the trend of K values with the injector structural factors indicates that the influence of the injector
Fig. 2. Injection experiment bed for pure flow field.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 3. Injector with different structural parameters.
structure on the K value is monotonously increasing or monotonously decreasing. Therefore, the optimal level of the injector structure for the PFFE is A4B1C1D2, which is based on the maximum number of occurrences in the optimal solution under different air supply pressures. The verification experiment is carried out, and the injector rates under four air supply pressures compared with the results of 16 orthogonal experiments are shown in Fig. 7. In Fig. 7, the injector rate of the optimal level is higher than the 16 orthogonal experimental results. Therefore, the optimal injector structure for the PFFE is the length of the nozzle 170 mm (A4), the length of the contraction part 100 mm (B1), the distance of the axis 30 mm (C1) and the angle of the feed and mixture part 70° (D2). 3.2. Experimental results of PIE 3.2.1. Result analysis According to the results of the PFFE, when the air supply pressure is large, the influence of the injector structure on the injector rate is basically the same. To simplify the experiment and prevent the particles from clogging the injector, an air supply pressure of 0.40 MPa is used in the PIE. The injector structural factors and the experiment scheme in the PIE are the same as those in the PFFE. In the 16 groups of the PIE, the static pressure average and the flow rate average of the vortex flowmeter I are shown in Fig. 8. The pressure without particles indicates the static pressure average of the vortex flowmeter I in the PFFE; the pressure with particles indicates the static pressure average of the vortex flowmeter I in the PIE; the flow rate
without particles indicates the flow rate average of the vortex flowmeter I in the PFFE; and the flow rate with particles indicates the flow rate average of the vortex flowmeter I in the PIE. In Fig. 8, the static pressure average of the PIE is larger than that of the PFFE, and the flow rate average of the PIE is smaller than that of the PFFE, which indicates that the injection system produces a certain back pressure due to the entry of particles. This back pressure makes the flow rate average of the PIE decrease slightly and the static pressure average of the PIE increase slightly. The differences of the flow rate average and the static pressure average between the results of the PFFE and the PIE are b5%; the differences of the flow rate average and the static pressure average between the 16 results in the PIE are also small, which indicates that the different injector structures in the PIE have little effect on the static pressure average and the flow rate average. Therefore, this paper assumes that the static pressure average and the flow rate average from the air-feeding measurement system in the PIE are substantially constant and only analyzes the pressure changes of the pressure transducer II and the particle motion obtained by the high-speed camera. The pressure curve obtained by the pressure transducer II of the first group in the PIE is shown in Fig. 9(a). The blue curve is the measured value, and the black curve is the smooth value. The movement of the particles in the transparent pipe corresponding to the pressure curve is shown in Fig. 9(b)-(e). The pressure curve in Fig. 9(a) is the static pressure curve. According to the particle conveying state and the pressure change curve, the coal particle injection process can be divided into 4 parts: the pure flow
Fig. 4. Particle pneumatic injection experiment bed.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 5. Air flow rate of two Vortex flowmeter.
converted into static pressure, so the pressure curve rises to meet the power required by the conveying of the coal particle dunes. When the dune or plug of coal particles passes through the conveying pipe, some coal particles remain in the pipe bottom. The dune or plug of coal particles gradually shrinks, and the space for flow field gradually increases, which leads to some static pressure being converted into dynamic pressure again. The pressure curve shows a downward trend at this moment and then gradually stabilizes, corresponding to the stable conveying part shown in Fig. 9(d). It can be found from the high-speed video that most of the coarse-grained coal particles are deposited at the pipe bottom in the stable conveying part. The transparent pipe in the high-speed video shows a layered state consisting of a fluid layer, a conveying layer and a deposition layer. The fluid layer is mainly a high-speed air flow field, while the conveying layer is a mixed area of particles and flow field. There are more small-sized particles in the conveying layer, and the coal particle velocities decrease under the resistance action of the deposition layer. The deposition layer mainly consists of coarse-grained particles that have a lower velocity or are static. The fluidity of the deposition layer is poor, and it is difficult to promote the movement of coal particles. When the coal particles leave the conveying pipe, the fluid layer gradually increases, and the conveying layer gradually moves downward. The coal particles in the deposition layer gradually enter the flow field in the remaining conveying part, which is shown in Fig. 9(e). Because no more coal particles enter the injector, the coal particles in the deposition layer
part, the feeding part, the conveying part and the remaining conveying part. There are no particles in the injector or the conveying pipe in the pure flow part, and the flow field is in a stable state. Because the conveying pipe is connected to the dust removing tank (the back pressure is almost zero), the measured value should be substantially near the zero position. However, the smooth value of the flow field part is slightly lower than the zero point, which indicates that the pressure transducer II has a certain zero drift (the zero drift value is excluded when the data are analyzed). The pressure curve of the feeding part has a certain fluctuation, and the time of the feeding part is short. When the separation plate is pulled away from the aggregate hopper, the first falling particles enter the conveying pipe with a high velocity, which is shown in Fig. 9 (b). To convey the coal particles, part of the dynamic pressure is converted into static pressure to push the particle movement, which leads to the fluctuation of the pressure curve. However, the measured value is small due to the small amount of coal particles. Moreover, it can be found from high-speed video that the coal particles are basically in suspension and the particle velocity is high. The pressure curve first rises to a peak, then falls and gradually stabilizes when a large number of particles enter the injector. Because a large amount of coal particles enter the injector and the conveying pipe simultaneously, the coal particles first pass through the conveying pipe in the form of dunes or plugs, as shown in Fig. 9(c). This event corresponds to the high peak of the pressure curve. The space for flow field is small in the pipe, and more dynamic pressure of the flow fluid is Table 4 The ejection ratio under the four air supply pressures. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.1 MPa 0.2 MPa 0.3 MPa 0.4 MPa
2.7 2.71 2.72 2.72
2.68 2.69 2.69 2.69
2.34 2.44 2.47 2.48
2.4 2.41 2.41 2.41
2.87 2.88 2.84 2.86
2.76 2.77 2.78 2.79
2.56 2.57 2.56 2.57
2.47 2.61 2.6 2.63
2.86 2.81 2.81 2.82
2.72 2.76 2.76 2.74
2.84 2.86 2.87 2.88
2.77 2.78 2.79 2.77
2.88 2.87 2.87 2.87
2.9 2.87 2.92 2.9
2.79 2.84 2.84 2.83
2.82 2.83 2.84 2.84
Table 5 Range analysis of ejection ratios 0.1 MPa A K1 K2 K3 K4 R Factor order Optimal solution
B
10.12 11.3 10.66 11.06 11.19 10.54 11.39 10.46 1.26 0.84 ANBNCNEND A4B1C1E4D1
0.2MPa
0.3MPa
C
D
E
A
B
11.12 11.11 10.57 10.56 0.55
10.98 10.87 10.75 10.8 0.23
10.69 10.91 10.75 11.02 0.33
10.25 11.27 10.83 11.09 11.21 10.71 11.41 10.62 1.17 0.65 ANBNCNDNE A4B1C2D2E4
0.4MPa
C
D
E
A
B
11.18 11.19 10.72 10.61 0.58
11.01 11.03 10.92 10.83 0.2
10.92 10.89 10.87 11.02 0.15
10.29 11.24 10.78 11.16 11.23 10.75 11.48 10.63 1.19 0.61 ANCNBNDNE A4C1B1D2E4
C
D
E
A
11.22 11.15 10.81 10.6 0.61
11.03 11.04 10.91 10.84 0.2
10.92 10.91 10.92 11.04 0.13
10.31 11.28 10.85 11.13 11.21 10.75 11.45 10.65 1.14 0.64 ANBNCNDNE A4C1B1D2E4
B
C
D
E
11.23 11.15 10.83 10.6 0.63
11.05 11.07 10.92 10.85 0.22
10.92 10.92 10.91 11.06 0.15
Note: Ki is the sum of the test results corresponding to the level i on any column; R is the range and the difference between the maximum test result and the minimum test result in any column, R=max{K1, K2, ... Ki}-min{K1, K2, ... Ki}.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 6. K values of injector structure factors.
re-enter the flow field. At this time, it is obvious that there is a boundary line in the fig. There is a layered state on the left side of the boundary line, but the right side is the no-layered area. Due to the reduction of coal particles in the injector, the particles can obtain more kinetic energy in the flow field, and the excess kinetic energy can cause the coal particles to be in suspension or hop into the conveying pipe.
3.2.2. Optimal injector structural parameters for PIE The pressure average of the stable conveying part and the time of the conveying part are used as evaluation indexes. The larger the pressure average of the stable conveying part (hereinafter referred to as the pressure index), the greater the change from dynamic pressure to static pressure, which is more conducive to particle conveying. The shorter the time of the conveying part (hereinafter referred to as the time
index), the faster the movement of the coal particles and the greater the mass flow rate of particle conveying in the pipe. The results of the PIE are shown in Table 6. Because the injector performances of the pressure index and the time index are different, the two groups of indexes have different trends for the 16 groups of injector structures. The range analysis of the experimental results in Table 6 is carried out, which is shown in Table 7. The optimal solution of the pressure index is the maximum value of each level, and the optimal solution of the time index is the minimum value of each level. In the pressure index of Table 7, the R values of factor A and factor D are large, which indicates that these two factors are the significant factors and that the length of the nozzle and the angle of the feed and mixture part have a great influence on the pressure index. The R values of factor B, factor C and the null factor E are similar, which indicates that
Fig. 7. Pure flow experimental results of optimum level of injector structure.
Fig. 8. Stable output value of pressure transducer I and vortex flowmeter I.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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Fig. 9. Pressure change curve and particle movement.
and the smaller the time index, the greater the mass flow rate of the particles conveying. The trends of change of the K value of pressure index and the time index with the injector structure are different, but the trends are almost opposite. It is because that the static pressure in the flow field of injector increases when the pressure index increases, and the thrust that particles obtain from the flow field also increases, which make the acceleration of particles grow. As a result, the particles move through the injector quickly, which reduces the index of time. On the contrary, when the injector structure is not conducive to increase the static pressure in flow field, the thrust and the acceleration of particles is small and the particles speed are slow, which increase the movement time, and the time index increases correspondingly. According to Fig. 10(a), the K value first increases and then decreases with the length of the nozzle under the pressure index, and the maximum position is 90 mm (A2); the K value first decreases and then increases with the length of the nozzle under the time index, and the minimum position is 130 mm (A3). Because the maximum position of the pressure index and the minimum position of the time index are noncoincident, there may be a suitable length of the nozzle that can balance the pressure index and the time index.
the significance of these factors is low. The R value of the null factor E is the smallest, which indicates that there is no correlation between the four structural factors of the injector, and there is no other factor that affects the pressure index in the PIE. In the time index of Table 7, the R value of factor A is the largest, and the R values of factor B, factor C, and factor D decrease sequentially. The R value of factor E is the smallest. This indicates that the significance of the length of the nozzle, the length of the contraction part, the distance of the axis and the angle of the feed and mixture part on the time index decreases sequentially, and no other factors have an influence on the time index in the PIE. The optimal solutions under the two indexes in Table 7 are A2B1C1D1 and A3B1C2D1. The K values in Table 6 are compared and shown in Fig. 10. The trends of the K values with the injector structural factors are obtained. In Fig. 10, pressure represents the pressure index, and time represents the time index. In Fig. 10, the larger the range occupied by the K value curve under the same ordinate, the larger the R value is and the more significant the influence of the injector structural factor on the two indexes is. The length of the nozzle (factor A) is in the prominent position under both the pressure and time indexes. The higher the pressure index, the greater the static pressure obtained by the particles conveying; Table 6 The particle injection experiment results. No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Pressure Time
2309 4.02
1413 4.27
764 4.49
897 4.93
2780 2.56
3233 3.15
3512 4.1
3224 4.64
1949 2.8
1710 3.05
1791 2.82
2132 2.93
1782 4.76
1872 4.9
1511 4.34
1421 5.5
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Table 7 Range analysis of particle injection experiment results. Pressure of stable conveying part A K1 K2 K3 K4 R Factor order Optimal solution
B
5383 8820 8749 8229 7583 7579 6587 7675 3366 1241 ANDNBNCNE A2D1B1C1E4
Time of conveying part
C
D
E(empty)
A
8754 7836 7610 7702 1145
9825 8211 6676 7590 3150
8755 8296 7911 7640 1115
17.72 14.13 14.45 15.38 11.60 15.75 19.50 18.01 7.90 3.87 ANBNCNDNE A3B1C2D1E4
According to Fig. 10(b), the K value shows a monotonous decrease with the length of the contraction part under the pressure index and a monotonous increase with the length of the contraction part under the time index. The maximum position of the pressure index and the minimum position of the time index are the same position at 100 mm (B1). Therefore, the optimal length of the contraction part is 100 mm (B1). According to Fig. 10(c), the K value decreases and then increases with the distance of the axis under the pressure index, and the maximum position is 30 mm (C1); the K value is steady, then decreases and finally increases with the distance of the axis under the time index, and the minimum position is 20 mm (C2). Because the maximum position of the pressure index and the minimum position of the time index are non-coincident, there may be a suitable distance of the axis that can balance the pressure index and the time index. According to Fig. 10(d), the K value first decreases and then increases with the angle of the feed and mixture part under the pressure index, and the maximum position is 60° (D1); the K value first increases and then decreases with the angle of the feed and mixture part under the time index, and the minimum position is 60° (D1). The maximum position of the pressure index and the minimum position of the time index are the same position. Therefore, the optimal angle of the feed and mixture part is 60° (D1). The K values of factor A and factor C are fitted to obtain four fitting curves, which are shown in Fig. 11.
B
C
D
E(empty)
15.50 14.09 16.83 16.84 2.75
15.15 16.49 15.60 15.22 1.34
16.05 16.07 15.33 15.21 0.85
In Fig. 11(a), the relative coefficients R2 of the fitting curves for the length of the nozzle under the pressure index and the time index are 0.855 and 0.821, respectively, and the fitting degree is normal. Although the extreme points of the two fitting curves are different, the difference is small. In the case in which both indexes and the manufacturing processes are considered, the two extreme data points can be rounded. Thus, the length of the nozzle is 110 mm. In Fig. 11(b), the relative coefficient R2 of the fitting curve of the distance of the axis under the pressure index is 0.992, and the fitting degree is good, but the relative coefficient R2 under the time index is 0.541, and the fitting degree is poor. Although the extreme points of the fitting curve under the two indexes have a large difference, the maximum value of the pressure index is similar to the extreme point of the time index. In the case in which both indexes and the experimental results are taken into consideration, the optimal axis distance is 30 mm. According to the analysis results of Fig. 11, the optimal levels of the injector structural parameters are a nozzle length of 110 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60°. The verification experiment is carried out, and the experimental results are shown in Fig. 12. The best shape expresses the experimental results of the optimal levels. In Fig. 12, the verification results of the PIE with the optimal levels show that the pressure index is slightly smaller than the maximum result (No. 7) and the time index is slightly higher than the minimum result (No. 5). Therefore, a nozzle length of 110 mm, a contraction part length
Fig. 10. K values of injector structural factors for PIE.
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Fig. 11. Fitting of K value of factors A and C.
of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60° can be used as the optimal injector structure parameters for the PIE.
3.3. Discussions 3.3.1. Comparison of optimal injector structure in PFFE and PIE In the PFFE, the optimal structural parameters of the injector are a nozzle length of 170 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 70°. In the PIE, the optimal structural parameters of the injector are a nozzle length of 110 mm, a contraction part length of 100 mm, an axis distance of 30 mm and a feed and mixture part angle of 60°. In the two experiments, the optimal parameters of the length of the nozzle and the angle of the feed and mixture part are different, which indicates that the particles have an effect on these factors. The optimal parameters of the length of the contraction part and the distance of the axis are the same, which indicates that the particles have no effect on these factors. It is foreseeable that the optimal structural parameters of the injector will change with the change of particle size. When the particle size is small and the particles can be suspended in air or in a fluidized state, the optimal injector structural parameters obtained by the PFFE are suitable. As the particle size increases, the particles are difficult to fluidize. The effect of particle size on the injector structure is gradually apparent until the optimal structural parameters obtained by the PIE are achieved. Therefore, further experiments will be carried out using different particle sizes to obtain the optimal structural parameters of injectors under different particle sizes to meet the requirements of pneumatically conveying different materials.
3.3.2. Significance analysis of injector structures in PFFE and PIE Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the length of the nozzle on injection performance is the most prominent, indicating that the length of the nozzle is the main factor affecting injection performance, and a reasonable nozzle length is critical to injection performance. Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the length of the contraction part and the distance of the axis on injection performance is basically the same in the experiments, and the factor order of these structures is nearly in the middle position. The optimal structural parameters are basically unchanged, indicating that the two structural parameters have a relatively stable effect on injector performance, but the effect of particle addition on the optimal structural parameters is small. Comparing the experimental results of the PFFE and the PIE, it is known that the influence of the angle of the feed and mixture part on injection performance is prominent in the PIE. The feeding time of particles is the shortest when the feed and mixture part is vertical, but the initial velocity direction of particles is perpendicular to the flow field direction. The flow field needs to provide more energy to initially accelerate the particles. When the angle of the feed and mixture part is b90°, the feeding time is prolonged, but the initial velocity of the particle has a certain component in the flow field direction. It is easier for the particles to move with the flow field when entering the injector. However, if the angle of the feed and mixture part is further reduced, for large particle sizes, it can be predicted that the probability of particle blocking will increase due to the irregular shape and the large repose angle of the particles. Therefore, a suitable angle for the feed and mixture part will facilitate the initial acceleration process of the particles and prevent injector blocking.
4. Limitation of experimental results on contraction part In the PFFE and the PIE, the distance between the flow field inlet and the measuring point is different due to the difference in the length of the nozzle and the length of the contraction part. In the PFFE, the experimental data with a lower supply pressure are slightly different from the other experiment data due to the pressure loss. In the PIE, due to the addition of particles, the interactions between the particles and between the particles and the pipe wall consume a large amount of the flow field energy. The farther the distance from the flow field inlet to the measuring point, the larger the energy consumption of the flow field is. This distance also affects injection performance, which is more obvious that the improvement in injection performance under the two indexes when the length of the contraction part was decreased. However, it is shown that the length of the contraction part is not the main factor affecting injection performance in both the PFFE and the PIE. Therefore, the experiments have some limitations, but the results Fig. 12. Particle injection experimental results of optimum level of injector structure.
Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087
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obtained in the PFFE and the PIE are still important for guiding the design of injectors for large particles. 5. Conclusion (1) To obtain the optimal injector structural parameters for large particle pneumatic conveying, the PFFE and the PIE are carried out by using the injection rate, the pressure index and the time index as the injection performance indexes. The results show the influence of the injector structural factors on the injection performance of the PFFE and the PIE. (2) The experimental results of the PFFE show that the length of the nozzle is the most significant factor. The K value of the injection performance index increases with the length of the nozzle and the distance of the axis and decreases with the length of the contraction part and the angle of the feed and mixture part. The optimal structural parameters of the injector are 170 mm for the length of the nozzle, 100 mm for the length of the contraction part, 30 mm for the distance of the axis and 70° for the angle of the feed and mixture part. (3) The experimental results of the PIE show that the curve change of static pressure can reflect the particle conveying state. The length of the nozzle is the most significant under the pressure index and time index. The optimal structural parameters of the injector are 110 mm for the length of the nozzle, 100 mm for the length of the contraction part, 30 mm for the distance of the axis and 60° for the angle of the feed and mixture part. (4) The optimal structural parameters of the injector in the PFFE and the PIE are compared and verified, which provides significant guidance for practical applications and an experimental basis for the large particle pneumatic conveying system.
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Please cite this article as: D. Yang, B. Xing, J. Li, et al., Experimental study on the injection performance of the gas-solid injector for large coal particles, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.087