Effects of Expansion Chamber on the Flow Pattern and Performance of Cyclones

Han Xiao; Sun Guogang,2; Sun Zhanpeng; Han Xiaopeng

(1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249; 2. Beijing Key Laboratory of Process Fluid Filtration and Separation, Beijing 102249)

Effects of Expansion Chamber on the Flow Pattern and Performance of Cyclones

Han Xiao1; Sun Guogang1,2; Sun Zhanpeng1; Han Xiaopeng1

(1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249; 2. Beijing Key Laboratory of Process Fluid Filtration and Separation, Beijing 102249)

To improve the performance of the Stairmand cyclone, the effects of an expansion chamber on the fow feld, the pressure drop and the separation efficiency were investigated numerically and experimentally. The experimental results showed that compared with the Stairmand cyclone, the cyclone with an upper expansion chamber worked better at low inlet velocity (less than 14 m/s in this study), while the cyclone with a lower expansion chamber achieved higher effciency at a relatively high inlet velocity (14-20 m/s). The presence of an expansion chamber can generally result in a slight decrease in the cyclone pressure drop. The simulated results, which were used to further analyze the reason behind the experimental phenomena, suggested that the expansion chamber had insignifcant effects on the tangential velocity profles in the cylindrical part of cyclones. While in the cone part, the expansion chamber and the dipleg, the tangential velocity slightly decreased. Nevertheless, the expansion chamber decreased the possibility of the vortex end to sweep the wall and then reduce the particle re-entrainment. Therefore, installing the expansion chamber at a proper position could improve the separation performance of Stairmand cyclones. Both the experimental and simulated results represent a potential improvement of the Stairmand cyclone performance.

expansion chamber; numerical simulation; fow feld; separation effciency

1 Introduction

Cyclones are typical dust collection devices that utilize the centrifugal force to capture particles. Because of their simplicity in construction, low cost in operating and maintenance, as well as adaptability to harsh circumstance, cyclones are widely used in petrochemical industry[1].

The cyclone performance could be optimized by adjusting different geometrical parameters including the cyclone length, the inlet, and the vortex fnder geometries. Some researchers have paid their attention to effects of the lower part of cyclones. Obermair, et al.[2]tested five cyclones with different dust outlet geometries, and proved that changing the dust outlet geometry could influence the separation effciency signifcantly. Besides, by using a downcomer tube between the cyclone cone and the dustbin, a high separation efficiency could be obtained with an acceptable pressure drop. Also, Obermair, et al.[3]reaffirmed that the best separation efficiency could be achieved by a cyclone with a downcomer tube and bin, due to the “additional” separation process taking place within it. Elsayed and Lacor[4]examined the effect of the dustbin on the flow pattern and the performance of cyclones, and reported that the maximum tangential velocity was almost unchanged while the axial velocity could exhibit an inverted V or inverted W profile. Gao Cuizhi[5]made the comparison between four cases, viz.: the cyclone without dustbin, the cyclone with half a dustbin, with one dustbin, and with double dustbins. The results showed that at high velocity the cyclone with half a dustbin could obtain the best separation performance.

The particle re-entrainment from the dustbin is an important factor infuencing the separation effciency[6-7]. Mothes[8]found that higher particle concentration was near the cyclone bottom rather than at the inlet, which indicated a requirement for control of particle reentrainment from the dustbin. Researchers have optimizedcyclones for installing the apex cone and down-comer tubes for the purpose of reducing particle re-entrainment. Yoshida[7,9]investigated the effect of apex cone shape and height on the separation performance. Sakura, et al.[10]investigated the performance of cyclones modified with down-comer tubes and proved the effectiveness of downcomer tubes in reducing the particle re-entrainment. Qian Fuping, et al.[6]examined the vertical tube length and found that the tangential velocity, axial velocity and the turbulent kinetic energy in the dustbin reduced greatly when the vertical tube was connected to the dust outlet, which could avoid the re-entrainment effectively.

In order to decrease the particle re-entrainment, the expansion chamber has been introduced in this paper to study the effect of the expansion chamber on the flow pattern, velocity profles, and separation performance and to compare different axial positions of expansion chamber at different inlet velocities. This approach serves two purposes: (1) It gives information for designers about the effect of the expansion chamber on the fow pattern and separation performance of the Stairmand high-effciency cyclones. (2) It determines the proper working range of the Stairmand high-effciency cyclones with and without an expansion chamber.

2 Materials and Methods

2.1 Experimental setup

Three tested cyclones are shown in Figure 1. The S cyclone is a standard Stairmand cyclone with a dipleg underneath, while both the SA cyclone and the SB cyclone have an expansion chamber respectively located above and below the connecting line (CL) of the Stairmand cyclone between the cone and the dipleg. In the SA cyclone, the expansion chamber is located above CL while the SB cyclone is located below CL. The Stairmand cyclone is used as the benchmark. Table 1 gives the dimensions of cyclones. The test facility is illustrated in Figure 2. The experiments were carried out under negative pressure by a suction fan. Standard pitot tubes were used to measure the inlet velocity and the pressure drop between the inlet and the outlet was tested by a U-tube. The separation effciency could be achieved by weighing the powder before and after the tests. The solid particles used were talcum powder, with a physical density of 2 700 kg/m3and a volume median diameter of 13.59 μm. The feed material concentration was fxed at 20 g/m3.

Figure 1 Schematic diagram of three tested cyclones

Table 1 Geometrical dimensions of the three cyclones

Figure 2 Schematic diagram of experiment setup

2.2 CFD simulation

Computational Fluid Dynamics (CFD) simulations were performed to investigate the effects of the expansion chamber on fow feld using the commercial CFD code Fluent 14.0. The Gambit 2.4.6 was used as the preprocessing tool of Fluent for establishing the solid model and mesh generation. Pure-phase simulation has been proved to be an effective way to analyze the flow field, from the results of which the velocity profiles and the static pressure distribution could be observed and this simulation method has been employed in many studies to support the experimental results[6,11-13]. Therefore, the pure-phase simulation was employed in this paper.

For the three-dimensional turbulent fow in cyclones, the key to the success of CFD lies in the accurate description of the turbulent behavior of the flow. The Reynolds stress turbulence model (RSM) has been proved to be an accurate prediction on the tangential velocity, the axial velocity and the pressure drop in the cyclone[14]. It is capable of predicting the combined vortex in accordance with the experimental data[15-16]. In this study, the Reynolds stress turbulence model (RSM) was adopted to simulate the fow feld in cyclones.

The boundary condition at inlet section was the velocityinlet with an inlet velocity of 18 m/s. An outflow boundary condition was used at the outlet. The noslip boundary condition was used at other boundaries. The PRESTO scheme has been used for the pressure interpolation, and the SIMPLE algorithm has been used for pressure velocity coupling, while the QUICK scheme has been used to deal with the momentum equations, the turbulent kinetic energy, the turbulent dissipation rate, and the Reynolds stresses.

Figure 3 shows the surface meshes of the three cyclones. In our preliminary computations, we have run the same simulation on three progressively finer grids with 129 840, 182 730 and 269 565 cells used for SA cyclone, respectively. The computational results on the three grids are shown in Table 2. Since the maximum difference between the results is less than 5%, so it could be argued that the simulations are grid independent.

Furthermore, to verify the precision of simulation computation, comparison with the tangential velocity distribution measured by Hu Liyuan[17]with LDV is made at the sectionZ=340 mm. As Figure 4 indicates a good agreement between the experimental and the numerical results, it is concluded that the numerical simulation could be used with acceptable accuracy.

Figure 3 Surface meshes for three cyclones

Table 2 Results of grid independent study for SA cyclone

Figure 4 Comparison of the tangential velocity distribution between experimental and simulated results

3 Results and Discussion

3.1 Experimental results

As shown in Figure 5 (a), the expansion chamber improved the separation efficiency by a maximum of 1.37%, and decreased the pressure drop by a maximum of 16%. For ease of clear expression, we separate the efficiency curve into three regions. Separation effciencies generally increase to a maximum frst and then decrease with the increase of inlet velocity. It is of interest to note that the value of optimal inlet velocity for different cyclones differs greatly. For Region 1, SA cyclone has a higher effciency. With further increasing the inlet velocity, the effciency of the SA cyclone begins to decrease, while the effciency of other two types of cyclones continues to rise. When the inlet velocity is between 15- 20 m/s (Region 2), the SB cyclone behaves better in terms of the particle separation. But once the inlet velocity exceeds 20 m/s (Region 3), the efficiency of SB cyclone decreases dramatically and the S cyclone obtains a highest effciency. The above results indicate that we should move upward the expansion chamber in the region of low inlet velocity (<15 m/s). On the other hand, when the inlet velocity varies from 14 m/s to 20 m/s, it is suitable to move downward the expansion chamber. But if the cyclone works at a high inlet velocity (>20 m/s), the expansion chamber causes negative effect on efficiency and we would better not install the expansion chamber under such condition.

Figure 5 Effect of an expansion chamber on separation efficiency (a) and pressure drop (b)

Figure 5 (b) shows the effect of an expansion chamber on the pressure drops. When the inlet velocity is less than 18 m/s, the variation of pressure drop caused by expansion chamber is almost negligible. However, when the inlet velocity exceeds 20 m/s, moving up the expansion chamber can decrease the pressure drop; otherwise, the pressure drop would increase.

3.2 CFD results

3.2.1 The effect of expansion chamber on the velocity profiles

The tangential and axial velocities make a big difference on the separation efficiency. Especially, the tangential velocity is the dominant component of the gas flow in cyclones, which results in the centrifugal force for particle separation[18]. To make it convenient to signal different sections, Figure 6 is the sketch on different sections of the cyclone. Figure 7 and Figure 8 show the tangential and axial velocity profles in different sections at an inlet velocity of 18 m/s, and it is found that there is no big difference among tangential velocities in the cylinder and the conic space, and the highest tangential velocity value is almost the same. This conclusion is consistent with the literature report of Elsayed and Lacor[19].

Figure 6 Different sections of cyclone for capturing the velocity field

Figure 7 Radial profiles for tangential velocity in different sections operating at an inlet velocity of 18 m/s

Figure 8 Radial profiles for axial velocities in different sections operating at an inlet velocity of 18 m/s

The tangential velocities of SA cyclone are inferior to those of the other two cyclones entering the cone, illustrating that the expansion chamber can decrease the tangential velocity subtly and lower the tangential velocity, resulting in smaller centrifugal force making fewer particles captured by the wall, which in turn would lead to lower separation efficiency. This explains well why SA cyclone has inferior separation effciency at an inlet velocity of 18 m/s. For the case of SB cyclone, owing to the low velocity, the tangential velocity decreases dramatically after entering the expansion chamber. Meanwhile, it is obvious to find that SB cyclone features a better symmetry for tangential velocity than the other two cyclones atZ=-1 400 mm. This situation can to some extent help reduce the dust-entrainment and enhance the separation effciency.

As for the axial velocity, it is found that the axial velocity varies between each other. The axial velocity of S cyclone appears to be the shape of an inverted V profile, and the SA and SB cyclones have the shape of inverted W. The expansion chamber has provided SA cyclone with a higher downward velocity at outer vortex, thus making downward flow as a main factor causing severe dust re-entrainment. So the SA cyclone shows an inferior performance at high inlet velocity. In a similar way, the main rotational flow extends downwards with a growing inlet velocity, while the same axial profles turns up for the SB cyclone, which can also explain why SB cyclone is inferior to S cyclone at higher inlet velocity.

As the section extends downwards, it is clear that SB cyclone has the lowest axial velocity along the central axis, which reveals that the central upward fow has the longest residence time for the secondary separation. Besides, for S cyclone and SA cyclone, the axial velocities turn up to change from double-peak to the single-peak and then weaken in symmetry, leading to instability of the fow feld. As for the SB cyclone, the axial velocity profiles maintain the appearance of double-peak, which is quite good in symmetry until entering the dipleg. Therefore, the SB cyclone is superior to the other two types of cyclones at an inlet velocity of 18 m/s.

3.2.2 The effect of expansion chamber on the static pressure

Figure 9 Contours of static pressure atVin=18 m/s andVin=25 m/s

Figure 9 shows the contour of static pressure at an inlet velocity(Vin) of 18 m/s and 25 m/s, respectively. It can be seen from Figure 9 that the vortex end of S cyclone falls between the other two cyclones at an inlet velocity of 18 m/s. In comparison with the S cyclone, the vortex achieves its longest length in the SB cyclone. On onehand, the SB cyclone makes its vortex end located in the dipleg, which reduces the dust re-entrainment. On the other hand, the SB cyclone obtains longer residence time for particles, resulting in a better separation performance. This phenomenon matches the experimental results well. In a similar way, when the inlet velocity increases to a relatively high value, the vortex end goes through the expansion chamber in SB cyclone and the expansion chamber weakens the swirl of vortex thus shortening the natural vortex length, resulting in shorter natural vortex length and low separation effciency. This fact has been proved by simulated results depicted in Figure 9. When the inlet velocity reaches 25 m/s, the vortex achieves its longest length in S cyclone and a shortest length in SB cyclone. Therefore, the S cyclone achieves a highest separation efficiency, which matches the experimental results well.

Figure 10 shows the variations of static pressure at different inlet velocity ranging from 10 m/s to 25 m/s. It is suggested that with an increasing inlet velocity, the vortex end is approaching the bottom of dipleg, resulting in the extension of separation space. However, there appears to be swing of vortex end under such condition, resulting in instability of the fow feld and decrease of the separation effciency.

Figure 10 Contours of static pressure for SA cyclone operating at different inlet velocities

With the increase in inlet velocity, the vortex end of SA cyclone extends out of the expansion chamber, which would result in more possibilities of contact with dipleg wall and dust re-entrainment, which could decrease the separation efficiency. Figure 10 illustrates that a fierce swing of the vortex end happens through the whole dipleg at an inlet velocity of 25 m/s, which would bring about much dust-entrainments. This is why the SA cyclone fails to enhance efficiency in spite of a comparatively larger vortex length as compared with S cyclone at an inlet velocity of 25 m/s. Therefore, for the case of SA cyclone, an increased inlet velocity is not ideal for the separation effciency.

Besides, it is hard for SA cyclone to form turbulent fow based on a weaker cone contraction effect. Also, an upper expansion chamber would effectively extend the vortex at low inlet velocities. Therefore, the SA cyclone has a superior performance at low inlet velocities. An excessively large volume of downward gas fow at high inlet velocities would bring on severe dust re-entrainment and decreased effciency.

3.2.3 The effect of expansion chamber on the axial velocity vectors

Figure 11 Distribution of axial velocity vector for three cyclones atVin=10 m/s

Besides the tangential velocity, the dust-entrainment is also a main factor affecting the separation efficiency. It can be seen from Figure 11 that the dust-entrainment in different degree appears in three cyclones at the inlet velocity of 10 m/s. Despite the longer vortex length, the separation efficiencies of S cyclone and SB cyclone arereduced because of severe dust-entrainment appearing as eddies in the axial velocity vector diagram (as seen from the red circles), which would bring the collected dust off the wall to enter the exit tube. As a consequence, SA cyclone achieves the best separation performance with less dust-entrainment at an inlet velocity of 10 m/s, which agrees well with the experimental results.

3.2.4 The effect of expansion chamber on the central static pressure

Figure 12 Position versus static pressure at different inlet velocities

To further analyze the reason why the SB cyclone has an advantage over SA cyclone at the inlet velocity of 18 m/s but fails to assume a leading position at an inlet velocity exceeding 18 m/s, it is important to state that the main separation domain covers from the vortex fnder to the end of vortex, so the vortex length is critical for the separation effciency. At the end of vortex, there is a sharp change of static pressure. As shown in Figure 12, the vortex end of SA cyclone is far away from the other two cyclones at the inlet velocity of 18 m/s, thus obtaining the lowest effciency. While, it is hard to distinguish the break points of static pressure among S cyclone and SB cyclone at the inlet velocity of 18 m/s, but the SB cyclone demonstrates a relatively stable fow feld after the break point with a minimum static pressure variation. Upon taking the vortex length and dust re-entrainment into consideration, the SB cyclone obtains the best separation performance at the inlet velocity of 18 m/s, which matches the experimental results well. The tangential velocity, the vortex length and the dust re-entrainment are main factors for weighing the efficiency. Therefore, the tangential velocity, the vortex length and the degree of dust-entrainment are vital factors adopted to estimate the separation effciency. When the inlet velocity reaches 25 m/s, it is clear that the S cyclone obtains the longest vortex length, and besides this situation there appears to be a second jump in static pressure after the break point for SA cyclone. It brings the collected particles off the dipleg to re-enter the inner fow, thus decreasing the separation effciency.

4 Conclusions

Three different types of cyclones have been investigated using CFD and experimental method. The following conclusions have been drawn up to give more information about the effects of the expansion chamber:

(1) The tangential velocities obtained in different cyclones are very similar in the cylindrical part, but some differences appear in the conical part, the expansion chamber, and the dipleg. The expansion chamber could decrease the tangential velocity and also reduce the possibilities of the vortex end to sweep the internal wall, leading to enhancement of the separation efficiency. Therefore, the tangential velocity is not the main factor deciding the separation efficiency, which is assessed based on the gas re-entrainment.

(2) The axial velocity patterns appear to be similar in the cylindrical part for the three cyclones. For areas below the separation zone, the cyclone with the lower expansion chamber has the lowest axial velocity along the central axis, which has revealed that the central upward fow has the longest residence time for the secondary separation.

(3) Compared with the Stairmand cyclone, the cyclone with an upper expansion chamber works better at low inlet velocity (less than 14 m/s in this study), while the cyclone with a lower expansion chamber obtains higher effciency when the inlet velocity varies from 14 m/s to 20 m/s. Thus, with regard to the design of a Stairmandcyclone, we believe that it would be possible to design a cyclone with improved separation performance by considering the expansion chamber position with a proper inlet gas velocity.

Acknowledgement: The authors gratefully acknowledge the financial assistance from the National Natural Science Foundation (Grant No.21276274) and the National Key Project of Basic Research of the Ministry for Science and Technology of China (Grant No. 2014CB744304).

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date: 2017-01-18; Accepted date: 2017-03-14.

Prof. Sun Guogang, Telephone: +86-10- 89734820; E-mail: ggsunbj@163.com.