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SEPARATIONS Experimental and Computational Fluid Dynamics Investigation of the Flow in and around Once-Through Swirl Tubes Siri Jacobsson, Trond Austrheim, and Alex C. Hoffmann* Department of Physics and Technology, UniVersity of Bergen, Allegaten 55, 5007 Bergen, Norway
The gas flow in and around once-through swirl tubessalso called axial flow cyclonessfor gas demisting has been investigated by flow visualization using a neutrally boyant tracer and by computational fluid dynamics (CFD). The neutrally boyant tracer was helium-filled soap bubbles, which were made visible by illumination with sheets of white light and recorded by a variety of photographic techniques. The flowpatterns in the inlet swirl vanes, the separation space, and the core of the vortex are shown as streak patterns. Additionally, it was possible to study the breakdown of the vortex beyond the swirl tube outlet. Using high-speed photography and dedicated software, velocity information was gleaned from the experiments. The experimental results were compared with CFD simulations wherein the renormalization group k- turbulence model was used to simulate the flowpattern in the same configuration. The simulations matched the experimental results well in the separation space itself but could not properly reproduce the complicated flow around the tube outlet. 1. Introduction This research project was carried out as part of a larger research program, HiPGaS, aimed at the improved modeling and design of natural gas scrubbers. The development of oil and gas fields moves toward smaller and more remote fields. The technology push is to carry out more processes, such as cleaning and separation, remotely and with minimal intervention. The long-term vision is to carry out most offshore gas/liquid and liquid/liquid separation subsea. Incorrect design of separation equipment would be particularly damaging for remote and subsea installations, so that a better basis for design is crucial. Separation of droplets from gas/vapor streams is a ubiquitous operation in the processing of natural gas. Liquid must be separated from natural gas at several stages, for instance to protect downstream equipment from corrosive liquids, lower the hydrocarbon or water dew point of the gas, or recover valuable liquefied petroleum gas (LPG) products from the gas stream. There are many different types of demisting equipment on the market. Often different types of equipment are combined to give optimal liquid removal efficiency. One typical arrangement is a gas-liquid scrubber that consists of three stages: a preseparation stage at the scrubber inlet section is followed by an agglomeration stage, often a mist mat, and finally by a demisting stage. The demisting stage often consists of a bank of once-through swirl tubes, also called axial flow cyclones, working in parallel. There is a large body of research literature dedicated to modeling, characterizing, and optimizing traditional reverseflow tangential-inlet cyclones, but despite their popularity, few publications exist in the open literature dealing with efficiency predictions for once-through swirl tubes. Their separation * To whom correspondence should be addressed. Tel.: 004755582876. Fax: 004755589440. E-mail:
[email protected].
efficiency has been discussed by Bu¨rkholz,1 Ramachandran et al.,2 and Brunazzi et al.,3 while different geometries have been more thoroughly discussed by, e.g., Swanborn,4 Bu¨rkholz,1 Verlaan,5 Nieuwstadt and Dirkzwager,6 and Hoffmann and Stein.7 Of these publications, only Verlaan has given a description of the geometry of a tested swirl tube with sufficient details to reproduce it. Most of the published work has been performed under nearatmospheric air-water conditions, and the design of separators for high-pressure natural gas scrubbing has, therefore, often been based on either extrapolated low-pressure data or confidential data provided by the suppliers, which also in many cases may have been produced under low-pressure conditions. In this paper, we are concerned with some outstanding questions around the flowpattern in and around once-through cyclones, which must be answered to predict their performance. One issue is the radial profile of the tangential velocity. In reverse-flow cyclones, the consensus in the literature is that the tangential velocity distribution is near solid-body rotation (i.e., with a constant angular velocity) in a narrow core, surrounded by a near loss-free roation (i.e., a decreasing tangential velocity with increasing radius); thus, the rotation resembles a Rankinetype vortex. In once-through cyclones, the picture is not so clear. Ramachandran et al.,2 while investigating a rotary-flow oncethrough cyclone, state that in contrast to a reverse-flow cyclone there are no outer and inner vortices in a rotary-flow cyclone and that the tangential velocity profile is also different: instead of the velocity decreasing from the central core to the wall in the outer part, they assumed it to increase from zero at the axis to a maximum at the wall. Also, Stenhouse and Trow8 in their model for an axial flow cyclone assume, and verify using laser Doppler anemometry, that the tangential velocity is zero at the wall and increases toward the wall in a more-or-less solid-body rotation, apart from a “boundary layer” at the wall, where the tangential velocity again decreases. Maynard,9 in his model for once-through cyclones, calculated the tangential velocity profile
10.1021/ie051200s CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006
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Figure 2. Diagram of the experimental rig. Figure 1. Diagram (adapted from ref 5) and photograph of one of the swirl tubes used. The secondary outlet is arrowed.
assuming the flow to be plug flow in a helicoidal channel. This also results in a tangential velocity that increases from zero at the axis to a maximum at the wall. On the other hand, Verlaan5 found that the tangential velocity profile follows a modified Rankine-type vortex profile. An added complexity is that many once-through cyclones are equipped with vertical slits to aid in the liquid separation. These slits may create extra turbulence in the vortex and cause the tangential velocity distribution to be different from that in conventional cyclones. Another outstanding issue is the flow at the exit from the axial flow cyclones. Does the vortex break down rapidly, or does it persist in the space above the axial flow cyclones, attaching to some solid wall in the scrubber space above the cyclone bank? Little is known about this issue at present. 2. Objective The objective of this work was to answer the two questions outlined above: Is the tangential velocity pattern essentially different from that of reverse-flow cyclones? Does the vortex break down immediately as it exits the tube? The object was also to gain insight into the detailed mechanisms of liquid separation and possible reentrainment and to assess the potential and shortcomings of computational fluid dynamics (CFD) simulation of the flow. 3. Experimental Section 3.1. Swirl Tubes. Figure 1 shows a diagram and a photograph of the swirl tube tested. The swirl tube is configured in accordance with the tube of Verlaan.5 Two different sizes of the swirl tube were made, with lengths of 250 and 400 mm, respectively, and diameters of 50 and 80 mm. The material of construction is perspex, except for the vane arrangement, which is of stainless steel (SS 316). The swirl vanes have a blade exit angle of 45° and contain six vane blades. The central body diameters are 30 and 48 mm, respectively, and the total lengths, 125 and 200 mm. The axial length of the blades themselves are 50 and 80 mm. Four vertical drainage slits in the cyclone tube wall, 111 and 176 mm long, respectively, and spanning the separation section between the swirl device and the exit are used, as shown in Figure 1. These slits were sharp-edged and expanding through the tube wall, having inner diameters of 5 and 8 mm in the two models, respectively, and outer diameters of 7.5 and 12 mm, respectively. One possible modification to make the liquid drainage more efficient is to draw off a secondary gas flow from the surrounding collection chamber to help the liquid through the
Figure 3. Diagram showing the positions of horizontal light sheets used. Measurements on the left are for the 80 mm diameter tube, and those on the right are for the 50 mm diameter tube.
drainage slits. In general, there are two ways of doing this: by recycling some gas back to the low-pressure zone upstream of the swirl element; by adding small secondary outlets directly at the top of the drain chamber. In this study, only the last one was used by opening one circular secondary outlet of 15 mm, giving a cross-sectional area of 176 mm2, as arrowed in the figure. 3.2. Rig. The rig was produced in-house, and a diagram is shown in Figure 2. The swirl tubes and piping were manufactured in transparent material (perspex) to make visualization possible. The bubble generator that makes neutrally buoyant soap bubbles (see below) is connected to the inlet tube, which has a length of 1000 mm and a diameter of 80 mm. Thus, when the smallest swirl tube was installed, the inlet tube went from 80 to 50 mm before the swirl device. The gas with tracer bubbles enters the swirl device and moves, while spinning, through the separation section. At the outlet of the tube, the tracer-laden gas exits into a rectangular chamber with a cross section of 150 mm × 150 mm. The outlet from the box was higher up, at one side, as indicated in the diagram. The flow is regulated using a hand valve and then passed through a venturimeter and a pump. 3.3. Flow Visualization Equipment. The bubble generator, supplied by Sage Action Inc., Ithaca, NY, produces heliumfilled neutrally buoyant bubbles (soap bubbles) of a controlled size of 1-2 mm. At the core of the generator, there are two concentric stainless steel hypodermic tubes. Helium passes through the inner tube and bubble film solution through the outer one. An air flow blows the bubbles off the tip of the tube. A mini vortex filter removes bubbles that deviate from neutral
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Figure 4. Horizontal light sheet, placed 36 cm above the inlet of the 50 mm swirl tube, i.e., in the vessel of square cross section above the tube, photographed from above. The inlet velocity is 3.8 m/s. (left) Vector plot of the averaged flow pattern. The scale seen in the top left-hand corner corresponds to 0.89 m/s. (right) Trackings that are included in the calculations.
density. The bubbles follow the flow streamlines without bursting or impacting on objects within the flow. To visualize the flow pattern, continuous white light from a 300 W halogen lamp, equipped with cooling fan, in combination with either digital film/image cameras or a high-speed camera was used. When required, pulsed light was created using a fan (a plate with slits) in front of the lamp. 4. Experimental Method and Analysis 4.1. Flow Visualization and Analysis. The flow was visualized using continuous light combined with either image or video photography. The streak length and shape gave a good impression of the flowpattern and the local velocity distribution. The light source and camera were always at 90° to each other for optimal clarity. The light source was placed both at the top and bottom of the device, and horizontal light sheets were also used at a range of axial positions, as shown in Figure 3. For velocity measurements, two different methods were tried: (1) the use of continuous light together with a high-speed camera and (2) pulse illumination in combination with a digital image camera, using a long exposure time compared to the illumination pulses. For the second of the two methods, the above-mentioned fan was used to create pulsed light. The slits in the fan were of different angular sizes to “code” the streak pattern to obtain not only the velocity from the length of the streaks but also the direction of flow from the pattern of streaks. Although we managed to create software to make this method work, the first of the two methods gave the best results with the software as it is developed at the present time, and we will therefore only describe the first method below. DiaTrack, a commercial software package, was used for extracting velocity information. The principle is to recognize individual tracer particles on successive frames and calculate velocities from the distance between images on successive frames and the known interval between the frames. To do this successfully, the high-speed camera needed to record as much as 2000 frames per s when high-velocity regions, such as the region within the tube itself, were analyzed. Of the 64 films taken, 33 films were analyzed: 24 films from the 50 mm cyclone and 7 films from the 80 mm cyclone. Most films were from the 50 mm cyclone, since this was the tube configuration used for the CFD simulations. Films were obtained in the 50 mm diameter tube at three inlet velocities: 3.8, 4.6, and 5.8 m/s.
Figure 5. Swirl vanes with the computational grid indicated. Table 1. Model Settings for the CFD Simulations Cmu
C1-epsilon
C2-epsilon
swirl factor
0.0845
1.42
1.68
0.07
In principle, threshold and filters could be set in the software to recognize the tracer particles automatically. Despite this, recognition sometimes had to be done manually due to variations in the strength of the light source, making the frames unevenly exposed. Reflections also caused some problems. Once the trackings had been analyzed, the software extracted velocity information that could be displayed either as a vector plot or in color coding. Figure 4 shows an example of a set of tracks and the resulting vector plot of the average velocity 36 cm above the inlet of the 50 mm tube, i.e., in the vessel of square cross section above the tube (see Figure 3). 4.2. CFD. 3-D CFD simulations of the flowpattern in the 50 mm swirl tube, at the three inlet velocities used experimentally, were carried out using the commercial finite-volume software package FLUENT 6.1, having built the computational grid using the associated package GAMBIT. The turbulence model used was the RNG k- turbulence model,10 with the swirl-dominated flow option. The computational grid consisted of 502 000 unstructured tetrahedral cells. Figure 5 gives an impression of the grid resolution relative to the equipment.
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Figure 6. Photos showing streak-lines of He-bubble tracer particles around the inlet swirl vanes. Each line is double due to the two reflection points on a bubble.
5. Results and Discussion
Figure 7. Photo showing the flowpattern in the separation space of the 80 mm tube. The inlet velocity is 2.9 m/s. The light is from the top. The shutter speed of the camera is 1/100 s. This photo is meant to show the core of the vortex clearly; by fine-tuning the bubble generator, a much higher bubble concentration in the outer part of the vortex could be obtained.
The model settings are given in Table 1. At the inflow boundary, the turbulence intensity was taken as 5%; this is slightly more than the 4.3-4.5% estimated using the well-known empirical expression: 1.6Re-1/8. However, we considered that the flow had been through a sharp turn just before arriving at the inflow boundary. The turbulence length scale was taken as 0.05 m, i.e., of the order of magnitude of the container, as is often done.
We first show the most interesting results from the flowvisualization studies and then the calculated velocity fields, comparing them with the results of the CFD simulations. 5.1. Flow-Visualization. Figure 6 shows the flowpattern around the swirl Vanes. Due to reflection from the vanes, it was difficult to obtain streak-lines of the flow in the swirl vanes themselves, but the images clearly show the swirling flowpattern generated by the vanes and how the shaping of the central body helps in avoiding turbulence creation in the flow exiting the vanes. These are. to our knowledge, the first pictures in the published literature to show the flowpattern in a swirl-vane assembly with neutral-density tracer. The method is clearly suitable for studying recirculation or turbulence in, and around, swirl-vane designs. In the separation space, the tracer bubbles tended to concentrate in the center of the vortex, although some remained in the outer regions of the vortex. This issue, and the wider issue of the faithfulness with which the tracer bubbles follow the flowpattern, including the turbulent eddies, is discussed in ref 11. The conclusion is that evaporation from the bubble surface may cause them to become slightly less than neutrally boyant as they move through the flow, resulting in them tending to centralize in the strongly swirling flow in the separation space. The advantage of this is that the core is clearly visible. The strongly swirling flow will exacerbate any deviation from neutral boyancy of the tracer. Those tracer bubbles that are too dense will be centrifuged to the wall and vanish as they are crushed on the wall, while those that are too light will assemble in the core, where they will form a highly visible band. Thus under all circumstances, one is likely to see the core very clearly using this type of tracing. We would like to stress, however, that the photos we are showing here are meant to show the core. By fine-tuning the
Figure 8. Photos of the 80 mm cyclone 41 cm above the inlet, just above the outlet. The inlet velocity is 2.9 m/s. The camera exposure times are (left) 1/80 and (right) 1/50 s.
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Figure 9. Pathlines of the simulated flow in the 50 mm swirl tube. The inlet velocity is 3.8 m/s.
bubble generator, it was possible to obtain a much higher bubble concentration in the outer part of the vortex. The flowpattern is seen in Figure 7. The core of the vortex (there are separation slits just in front and behind the core in the photo) is seen to be almost straight, right from the swirl vanes to the exit from the tube. Some streaks from tracer particles further out in the vortex can be seen. Above the outlet, the vortex dissipates. Some of the present authors recently published a study of the “end of the vortex” phenomenon in reverse-flow cyclones and swirl tubes.12 They
found that this phenomenon bore no relation to vortex breakdown internally in the fluid but was due to the vortex core bending to the wall, attaching to it, and rotating around it. In the present case, however, in a once-through configuration, we think that we are seeing vortex breakdown above the tube. The flow configuration and the tube expanding to the containing vessel is very similar to the configuration in which studies of vortex breakdown are carried out, where the breakdown is normally induced by insertion of a diverging section in the tube. Faler and Leibovich, in a classic paper,13 discuss a number of different vortex breakdown modes, and Lucca-Negro and O’Doherty14 review the issue of vortex breakdown in a recent paper. Figure 8 shows the flow pattern just above the outlet of the 80 mm swirl tube. It can clearly be seen that the individual tracer bubbles spread out from their centered position, probably around a recirculatory flow (a “bubble”), such as in the axisymmetric type “0” breakdown discussed by Faler and Leibovich. The flow at the stations further above the outlet showed a high degree of turbulence, and a pattern was not immediately evident from this type of photo, although, as Figure 4 shows, on the average the flowpattern did exhibit swirl. We can compare the streakline patterns shown in the previous figures with the pattern obtained from the CFD simulations. Figure 9 shows pathlines from the simulations of the 50 mm swirl tube. Obviously, the flow around the swirl element and the swirling flow with a straight vortex core in the separation section are matched by the CFD results, but the vortex breakdown just after the outlet does not seem to be well reproduced in the simulations. One pathline is seen to escape through the drainage slits. The reason that the vortex breakdown was not reflected in the simulation was probably that the turbulence model was too simple and/or the grid too rough. As many recent papers show,
Figure 10. Distribution of tangential velocity in a horizontal light sheet placed 13 cm above the inlet in the 50 mm swirl tube with an inlet velocity of 3.8 m/s. (Left) Experimental. (right) CFD simulation.
Figure 11. Distribution of tangential velocity in a horizontal light sheet placed 19 cm above the inlet in the 50 mm swirl tube with an inlet velocity of 3.8 m/s. (left) Experimental. (right) CFD simulation.
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• The vortex appears to suffer axisymmetric, bubble-type breakdown at the exit of the tube and does not persist above the tube. • The CFD simulations reflected the velocity distribution in the tube just after the swirl vanes reasonably well but less well further on in the tube, and they failed to match the breakdown of the vortex at the tube exit. A more sophisticated simulation is clearly required, a complicating factor being the presence of the vertical slits.
Figure 12. Sketch of the tangential velocity distribution in Burgers’ vortex together with the data in Figure 11.
for instance, refs 14 and 15, CFD simulations can reproduce even the fine details of type 0 vortex breakdown. The paper of Derksen15 is interesting in showing such a breakdown just after a sudden constriction, as in the vortex finder in a reverse-flow cyclone or swirl tube. 5.2. Velocity Distribution. The left plate of Figure 10 shows the measured tangential velocity distribution in the separation section of the swirl tube, just over the swirl element. The velocity is clearly low in the core, as one would expect, and it is fairly uniform over the rest of the cross section. The right plate shows the CFD simulations. The ranges of the color scales in the two plates are the same. The two flows are somewhat similar, although the CFD simulations seem to indicate an increasing velocity with an increasing radius, while this is not so in the experiments. Figure 11 shows the measured velocity distribution higher up in the tube. Clearly, the flow is changing into the familiar pattern of a near solid-body rotation surrounded by a near lossfree vortex motion, with the velocity going through a maximum close to the core of the vortex. This is similar to the tangential velocity distribution in Burgers’ vortex (sketched in in Figure 12 together with the data from Figure 11). As seen in the figure, the CFD simulations at this level reflect this transition somewhat less well, and we must conclude that the experiment tangential velocity distribution was not very well matched by the CFD simulations. 6. Conclusions This project answered the questions asked at the outset. Some itemized conclusions are the following: • Studying the flowpattern in once-through swirl tubes using neutral boyancy tracer has revealed features of the flow not yet seen. • The swirl tubes have been shown to possess an almost axisymmetric flow with the core of the vortex almost straight, right from the swirl vanes to the outlet of the tube. • The experiments show that the cross-sectional distribution of tangential velocity is somewhat flat just after the swirl element, developing into the familiar tangential velocity distribution, resembling Burgers’ vortex and the Rankine vortex further from the swirl element.
Acknowledgment The authors wish to thank Dr. Carl Birger Jenssen of Statoil for setting up the configuration for the CFD, Dr Weiming Peng for helpful advice for experimental work and the calculations, and members of the research group of Prof. Hallvard Svendsen at NTNU for assistance. We also thank the HiPGaS industrial partners: NFR, Statoil, FMC Kongsberg Subsea, Conoco Philips, Norsk Hydro AS, ABB, and Kværner Process Systems for sponsorship and assistance. Literature Cited (1) Bu¨rkholz, A. Droplet separation; VCH Publishers: New York, 1989; ISBN 0-89573-879-1. (2) Ramachandran, G.; Raynor, P. C.; Leith, D. Collection efficiency and pressure drop for a rotary-flow cyclone. Filtr. Sep. 1994, 31, 631636. (3) Brunazzi, E.; Paglianti, A.; Talamelli, A. Simplified design of axialflow cyclone mist eliminators. AIChE J. 2003, 49, 41-51. (4) Swanborn, R. A. A new approach to the design of gas-liquid separators for the oil industry. Ph.D. Thesis, Delft University of Technology, Delft, The Netherlands, 1988. (5) Verlaan, C. C. J. Performance of novel mist eliminators. Ph.D. Thesis, Delft University of Technology, Delft, The Netherlands, 1991; ISBN 90370-0054-1. (6) Nieuwstadt, F. T. M.; Dirkzwager, M. A. Fluid mechanics model for an axial cyclone separator. Ind. Eng. Chem. Res. 1995, 34, 3399-3404. (7) Hoffmann, A. C.; Stein, L. E. Gas Cyclones and Swirl Tubes Principles, Design and Operation; Springer-Verlag: Heidelberg, 2002; ISBN 3-540-43326-0. (8) Stenhouse, J. I. T.; Trow, M. The behaviour of uniflow cyclones. In Proceedings of the Second World Filtration Congress; The Filtration Society: London, 1969; pp 151-155. (9) Maynard, A. D. A simple model of axial flow cyclone performance under laminar flow conditions. J. Aerosol Sci. 2000, 31, 151-167. (10) Ma, L.; Ingham, H. D. B.; Wen, X. Numerical modelling of the fluid and particle penetration through small sampling cyclones. J. Aerosol Sci. 2000, 31, 1097-1119. (11) Peng, W.; Hoffmann, A. C.; Dries, H. W. A.; Regelink, M.; Foo, K.-K. Neutrally boyant tracer in gas cleaning equipment. A case study. Meas. Sci. Technol. 2005, 16, 2405-2414. (12) Peng, W.; Hoffmann, A. C.; Dries, H. W. A.; Regelink, M.; Stein, L. E. Experimental study of the vortex end in centrifugal separators: The nature of the vortex end. Chem. Eng. Sci. 2005, 60, 6919-6928. (13) Faler, J. H.; Leibovich, S. Disrupted states of vortex flow and vortex breakdown. Phys. Fluids 1977, 20, 1385-1400. (14) Lucca-Negro, O.; O’Doherty, T. Vortex breakdown: A review. Progr. Energy Combust. Sci. 2001, 27, 431-481. (15) Derksen, J. J. Simulations of confined turbulent vortex flow. Comput. Fluids 2005, 34, 301-318.
ReceiVed for reView October 28, 2005 ReVised manuscript receiVed June 10, 2006 Accepted July 20, 2006 IE051200S
