Proceedings of ETCE2002 ASME Engineering Technology Conference on Energy February 4–6, 2002, Houston, TX
ETCE 2002/MANU-29110 EFFECT OF INLET CONFIGURATION ON FLOW BEHAVIOR IN A CYLINDRICAL CYCLONE SEPARATOR Ferhat M. Erdal INTEC Engineering 15600 JFK BLVD Houston, TX 77032 Phone: (281) 925-2435 – Fax: (281) 925-2379 e-mail:
[email protected] ABSTRACT The use of Gas-Liquid Cylindrical Cyclone (GLCCã) separators for gas-liquid separation is a new technology for oil and gas industry. Consequently, it is important to understand the flow behavior in the GLCC© and effect of different geometrical configurations to enhance separation. The main objective of this study is to address the effect of different inlet configurations on flow behavior in the GLCC© by measuring velocity components and turbulent kinetic energy inside the GLCC© using a Laser Doppler Velocimeter (LDV). Three different inlet configurations are constructed, namely: one inclined inlet, two inclined inlets and a gradually reduced inlet nozzle. Axial and tangential velocities and turbulent intensities across the GLCC© diameter were measured at 24 different axial locations (12.5″ to 35.4″ below the inlet) for each inlet configuration. Flow rates of 72 and 10 gpm are selected to investigate the effect of flowrate (Reynolds number) on the flow behavior. Measurements are used to create color contour plots of axial and tangential velocity and turbulent kinetic energy. Color contour maps revealed details of the flow behavior. INTRODUCTION Separation based on density difference (buoyancy) and gravity by using vessel type separators is conventional technology in oil and gas production operations. However, only thirty percent of the deep-water oil and gas fields have been developed in the world. To be able to develop more fields in deep-waters economically, compact separation technology with high efficiency and low residence time has to be developed. Gas Liquid Cylindrical Cyclone (GLCC©) separation technology promises smaller footprint, high efficiency, low residence time with wide variety of applications from partial separation to multiphase metering. Thus, it is important to
Siamack A. Shirazi Department of Mechanical Engineering The University of Tulsa Tulsa, OK, 74104 Phone: (918) 631-3001 - Fax: (918) 631-2397 e-mail:
[email protected] understand the flow behavior in the GLCC© and effect of different geometrical configurations to enhance separation. The GLCC© configuration is shown schematically in Fig. 1. The gas and liquid mixture flows through an inclined inlet section, to enhance stratification, before reaching a tangential inlet slot. As a result of the tangential inlet, a vortex is formed causing the gas and liquid to separate due to the centrifugal/buoyancy forces. The liquid moves toward the wall and downward, while the gas flows to the center and exits from the top. For certain operating conditions, some liquid flows with the gas and moves up toward the gas leg. This phenomenon is referred to as liquid carry-over. On the other hand, some gas may be entrained with the liquid and exit from the bottom of the GLCC©, namely, gas carry-under. To investigate the effect of different inlet configurations on the flow behavior and the efficiency of the separator, information about details of flow such as velocity profiles and turbulent intensity are required. Several experimental studies on single-phase turbulent swirling flow in pipes, generated with tangential injection or guide vanes, have been reported. One of the first experimental studies in this area is by Nissan and Bressan [1]. To generate the swirling flow, water was injected through two horizontal tangential inlets. The flow field was measured with impact probes. The axial velocity distribution showed a region of flow reversal near the center of the tube. It was reported that under some circumstances there was visual evidence of double flow reversal, with water flowing forward near the wall and in the center of the cylinder and moving backwards in the region in between. Ito et al. [2] investigated swirl decay in a tangentially injected swirling flow. They used water as the working fluid with a high ratio of tangential momentum to axial momentum, namely, 50. Water was injected through two horizontal
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Gas
Liquid Multiphase Flow
Figure 1. Schematic of GLCCã Configuration. tangential inlets. The measurements were made with a multielectrode probe. The tangential velocity distribution showed that there were two flow regions: a region of forced-vortex flow near the center of the tube and a surrounding region of freevortex flow. The swirl decayed with the axial distance, resulting in a decrease in the extent of the solid rotational flow (forced vortex). Chang and Dhir [3] studied turbulent flow field in a tube, where air is injected tangentially trough four horizontal tangential inlets, using a single rotated straight hot wire and single rotated slanted hot wire anemometers. Profiles for mean velocities in the axial and tangential directions, as well as the Reynolds stresses, were obtained. The axial velocity profile shows the existence of a flow reversal region in the central line of the tube and an increased axial velocity near the wall. Tangential velocity profiles have a local maximum, the location of which moves radially inwards with axial distance. The swirl intensity, defined as the circulation over a cross sectional area, was found to decay exponentially with axial distance. In a study by Kurokawa [4], the existence of a complex velocity profile was confirmed by accurate measurements in single-phase liquid flow. The study distinguishes three regions, namely, a forced vortex, generating a jet region with extremely high swirl velocity around the pipe center, a second swirl region formed by a free vortex, and an intermediate region of back flow with high swirl velocity. Wang [5] suggested using a gradually reduced inlet nozzle configuration for wider ranges of operational envelop for liquid carry-over in the GLCC©. However, effect of inlet
configuration on the flow behavior or gas carry under was not addressed. Based on experimental data and theoretical studies performed at The University of Tulsa, Gomez et al. [6], a design program has been developed which has mechanistic models to predict the operational envelop for liquid carry-over, bubble trajectories, and simple 2-D axial and tangential velocities. The models for axial and tangential velocities were developed by Mantilla et al. [7]. However, Mantilla et al. correlations were based on data for horizontal tangential inlets. Furthermore, Mantilla et al. [7] model was based on literature data for multiple (two and more) horizontal tangential inlets. Several flow conditions and inlet geometries were used for the previous experiments in the literature. Erdal and Shirazi. [8-9] presented extensive LDV data on swirling flow in a cylinder with one tangential inclined inlet. Therefore, the main objective of this study is to address the effect of different inlet configurations on flow behavior in the GLCC© by measuring velocity components and turbulent kinetic energy inside the GLCC© using a LDV. EXPERIMENTAL FACILITY AND MEASUREMENTS Since the flow is mostly liquid below the inlet of GLCC©, an experimental facility for single-phase flow has been designed and constructed for obtaining measurements of axial and tangential velocities and turbulence quantities below the inlet of the GLCC©. A schematic of the experimental facility is shown in Fig. 2. The experimental facility meets following requirements: • Single-phase, closed liquid flow loop. • Test section accommodates the Laser Velocimeter (LDV) measurement technique.
Doppler
• Easy and quick change of different inlet configurations. • Three phase, 2 HP centrifugal pump, capable of producing 80 GPM (at max. head of 27 m). • Two rotameters (5-40 GPM) and flow rate measurement tree to measure flow rates for different inlet configurations. • A bypass line with a gate valve to control the flow rate through the test section. • A gate valve on the outlet of the GLCC© to increase pressure and to release air from the system. • A 200 gallon reservoir to accommodate high flow rates.
• An air release bleed valve at the top of the GLCC©
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P
Bypass Line
Inlet Section
Cap for Two Inlet Configuration
Valve
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Figure 2. Schematic of Experimental Facility.
Figure 3. Three Different GLCCã Inlet Configurations. Inclined Inlet Inlet 1.25 ″
Inclined Inlet Pipe
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Figure 4. GLCCã Test Section and LDV Measurement Plane.
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50.8 mm ID PVC pipes are used for piping. Flow metering section is designed to allow flow rate measurements by rotameters for different inlet (one and two) configurations. Omega Fl75-F variable area flow meter is used which has an accuracy of ±2.5%. GLCC© configuration is divided in three sections, namely, Inlet Section, Test (Measurement) Section, and Outlet Section as shown in Fig. 2. These three sections are connected to each other with flanges. Inclined inlet configurations are used in all experiments that are inclined 27° with respect to the horizontal plane. GLCC© test section is made out of 88.9 mm ID clear acrylic pipe as shown in Fig. 3. The temperature of the liquid in the flow loop is measured by a thermometer and is maintained to be around 20-25 °C. Three different inlet configurations are constructed to investigate the flow field: one inclined inlet, two inclined inlets and a gradually reduced inlet nozzle, as shown in Fig. 3. These inlet configurations can be easily changed to conduct local measurements. The inlet pipe ID is 31.75 mm for the one inclined inlet. This provides a test section area to the total inlet area ratio of 7.84. This ratio approximately represents the tangential momentum flux at the inlet to the total momentum flux. Two inclined inlets configuration has two 19.05 mm ID inlet pipes, which have a test section area to the total inlet area of 10.9. Therefore, the two inlets configuration has a higher ratio of the tangential momentum flux at the inlet to the total momentum flux than the one inlet configuration considered above because of manufacturing difficulties. The third inlet geometry investigated is the gradually reduced inlet nozzle configuration. This configuration has a large inlet pipe diameter, 76.2 mm ID. Then, this pipe area is gradually reduced by placing a plate within the inlet pipe to reduce the inlet area to about 25% of the area of the cylinder. Because of the inlet area ratio, the tangential momentum flux at the inlet to the total momentum flux is equal to 4 for this gradually reduced inlet nozzle. The measurement plane is one plane between 318 to 900 mm below the inlet, as shown in Fig. 4 with one inclined inlet configuration. 24 measurement locations are selected in the measurement plane. Locations of these measurement lines are listed in Table-1. At each measurement location, axial velocity, tangential velocity and turbulent quantities are measured along the diameter by the LDV. Local measurements are conducted for two flow rates. Selected flow rates, corresponding average axial velocities and Reynolds numbers in the GLCC© are given in Table-2. These flow rates represent one high Reynolds number case and one low Reynolds number case. The data is obtained using a two-component LDV system developed by TSI, Inc., that includes the following components: a 3-Watt Argon-ion laser, a multicolor beam separator, a fiber optic probe for transmitting and receiving, a multicolor receiver, a digital processor, data storage and analysis software, and a two-axis traverse. For this study, a lens with a focal length of 362.6 mm and a focal distance of 350 mm is used with
a probe beam spacing of 50.0 mm. Thus, the half-angle of the beams, κ, equals 3.95°. Table 1 - Measurement Locations # 1 2 3 4 5 6 7 8 9 10 11 12
L (in.) 12.5 13.0 13.5 13.9 14.4 14.9 15.4 15.9 16.4 16.9 17.4 17.7
L (mm) 317.5 330.2 342.9 352.4 365.1 377.8 390.5 403.2 415.9 428.6 441.3 449.3
L/D 3.6 3.7 3.9 4.0 4.1 4.3 4.4 4.5 4.7 4.8 5.0 5.1
# 13 14 15 16 17 18 19 20 21 22 23 24
L (in.) 18.4 18.8 19.6 20.6 21.5 22.4 23.4 24.2 27.0 29.8 32.6 35.4
L (mm) 466.7 477.8 498.5 522.3 546.1 569.9 594.4 614.4 685.8 757.2 828.7 900.1
L/D 5.3 5.4 5.6 5.9 6.1 6.4 6.7 6.9 7.7 8.5 9.3 10.1
Table 2 – Flow Conditions gpm 72 10
m3/s 0.00454 0.00063
Uav (m/s) 0.7310 0.102
Re 66900 9290
To reduce the effects of the pipe curvature, a water-filled, clear-acrylic surrounding box is used. This enables the location of the measurement volume for each component (axial and tangential) to be approximately equal. The flow is seeded using silicon carbide particles with a mean diameter of 2 µm. 3 Approximately 1 cm of particles are used for a 200-gallon reservoir of water. 50,000 data points are obtained at each measurement point of all locations across the pipe, and the data points that lie outside of a three-standard deviation range from average are eliminated. Turbulent kinetic energy (k) is a measure of the kinetic energy per unit mass associated with the velocity fluctuations in three orthogonal directions is defined by Equation (1).
k=
1é 2 (u′) + (v′)2 + (w′)2 ùú ê 2ë û
(1)
The LDV system employed in the study is a twocomponent system. Therefore, axial and tangential velocity fluctuations can be measured. Measurements showed that fluctuations in axial and tangential directions have the same order-of- magnitude. In order to get an estimate of k, radial velocity fluctuations are assumed to be average of the axial and tangential velocity fluctuations. Measurements of turbulent velocity fluctuations in a swirling flow by Chang and Dhir [3] indicate that the velocity fluctuations have the same order-ofmagnitude in the axial, radial and tangential directions. Thus, as a first-order approximation, the radial velocity fluctuations are calculated by using equation (2).
ö 1æ ( v′) 2 ≈ çç (u′)2 + ( w′) 2 ÷÷ 2è ø
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(2)
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EXPERIMENTAL RESULTS Experiments are conducted for three different inlet configurations. Axial and tangential velocities and turbulence quantities are measured with LDV for flow rate of 72 and 10 gpm.
case shows a downward flow at the center, which is surrounded by a narrow upward flow region. There is a downward flow near the walls also. Upward flow maximum velocity is about 3 times stronger than the upward flow maximum velocity observed for the one inclined inlet. This behavior is certainly very complicated and may not be desirable for GLCC design. Downward flow region at the center is not a desired flow field. This might contribute to more gas carry under. Complex flow field for 72 gpm case for two inclined inlets configuration changes to a much more symmetrical flow field for the 10 gpm case as shown in Fig. 6, which has a lower Reynolds number. This indicates that flow field (axial velocity shape/profile) changes as the Reynolds number changes for this inlet configuration. The 10 gpm case has a wider upward flow region. In GLCC design, this means that there is more room to capture bubbles at the center and elevate them to gas liquid interface for separation. Axial velocity contours for gradually reduced inlet nozzle configuration are also presented in Figs. 5 and 6. This flow behavior is very similar to the one observed in one inclined inlet case, with swirling downward flow near the walls of the test section and helical upward flow at the center. This inlet configuration has a relatively stronger upward flow than the one inclined inlet configuration (about two times stronger). In addition, the helical vortex center and upward flow region are closer to center of the cylinder. This might be due to low ratio of tangential momentum flux at the inlet to the total momentum flux. Similar flow behavior also is observed in low flow rate (Reynolds number) case, as seen in Fig. 6. Again, change of the wavelength of the helical vortex center and upward region can be seen in comparison with the 72 gpm case.
Axial Velocity Measured axial velocity profiles at 24 locations in the measurement plane are used to obtain the color contour plot of axial velocity. Figs. 5 and 6 show color contour plots of axial velocities that are generated from the measured axial velocity profiles for the 72 and 10 gpm. Details of the axial flow behavior can be learned form this contour plots. Axial velocity contour plots clearly show regions of high and low axial velocities and upward flow (negative axial velocity) regions. The blue and purple regions represent the regions of low and negative axial velocities. Upward flow region for one inclined inlet has a helical (spiral) shape, as show in Figs. 5 and 6. In this helical shape region, appearance and disappearance of upward flow in the measurement plane can be observed. Downward spiral flow near the wall has high axial velocities. Strengths of both upward and downward flow decay as the flow moves toward the outlet. This decay appears to cause a stretch on the vortex. This means that the wavelength of the helical vortex increases with axial distance. Similar observations can be made for low flowrate or Reynolds number case in Figure 6. However, wavelength of the helical upward flow is increasing when the flow rate (Re) is decreasing. Axial velocity contours for flow rates of 72 and 10 gpm for the two inclined inlets configuration also shown in Figs. 5 and 6 show a nearly axisymmetric flow field. Surprisingly, 72 gpm
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(Not to Scale) Figure 5. Axial Velocity Contours for 72 gpm (Re = 66900).
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(Not to Scale) Figure 6. Axial Velocity Contours for 10 gpm (Re = 9285). Tangential Velocity Color contour plots of tangential velocities are shown in Figs. 7 and 8 for flow rate of 72 and 10 gpm respectively. Tangential velocity is positive on one side (left) and negative on the other side (right). This is due to the rotation of the flow. Positive velocities represent the tangential velocity out of the page and negatives represent flow into the page. Tangential velocity is high near the wall region and it decreases towards the center. There is a decay of tangential velocity in the axial direction towards the outlet or downward axial direction. Location of zero or low tangential velocity for one incline inlet configuration has a helical (spiral) shape similar to the one observed in the axial velocity contours. Tangential velocity contour plots for two inclined inlets configuration for both flow rates show similar and nearly axisymmetric flow fields, except the decay due to Reynolds number. However, maximum tangential velocities are higher than the one inclined inlet configuration. This might be due to the difference in the inlet area where one inclined inlet has about 1.39 times more inlet area (and lower tangential velocity) than the two inclined inlets configuration. Interestingly, the decay of the tangential velocity with Reynolds number and axial distance is not as drastic as in the case of one inclined inlet configuration. This might be due to axisymmetry and higher tangential velocities at the inlet. Gradually reduced inlet nozzle configuration tangential velocity contours that are also shown in Figs. 7 and 8 for both flow rates are similar to one incline inlet configuration, except more symmetrical flow field. However, maximum tangential velocities are slightly higher than the one inlet case (because the area of the inlet is decreased).
Turbulent Kinetic Energy Calculated turbulent kinetic energy (k, Equations 1 and 2) 2 , are color contour plotted in Figs. profiles, normalized with Uav 9 and 10. Relatively high k near the wall at the top is observed for one inclined inlet configuration for 72 gpm. This high k decays in axial direction in the near wall region. This is due to inlet effects and the inlet jet that is spiraling down into the cylinder. The high k region at the center, which has a helical shape, does not show a strong decay. Although axial and tangential velocities in this region are low with respect to the near wall region, k is considerably high, comparable to average axial velocity. This high turbulence shows the instability of the flow near the vortex center. This might have great impact on the separation of small bubbles below the inlet of GLCC©. Turbulent kinetic energy color contours for two inclined inlet for 72 and 10 gpm show that both flow rates have similar turbulent energy distributions in the measurement planes, except some decay due to difference in Reynolds number. Both plots show a high turbulence at the center of the GLCC©. Turbulence due to inlet effects does not appear in these plots, which was observed in one inclined inlet measurements. However, turbulent kinetic energy at the center is very high. In one inclined inlet case highest k/Uav2 was about 1.5. This high turbulent kinetic energy doubles at the center for two inclined inlets configuration. Turbulent kinetic energy color contours for gradually reduced inlet nozzle for 72 and 10 gpm show that both flow rates have similar turbulent energy distributions in the measurement plane, except the turbulence levels are lower for the lower Reynolds number case. Both plots show a high turbulence at the center of the GLCC©. A high level of turbulence near the inlet that was observed in one pipe inclined inlet measurement does not appear in these plots again.
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However, turbulent kinetic energy reduction due to change in the flow rate (Reynolds number) is more obvious and very
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Figure 7. Tangential Velocity Contours for 72 gpm (Re = 66900).
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Figure 8. Tangential Velocity Contours for 10 gpm (Re = 9285).
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Figure 9. Turbulent Kinetic Energy Contours for 72 gpm (Re = 66900). 350 400
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Figure 10. Turbulent Kinetic Energy Contours for 10 gpm (Re = 9285). the axial velocity of one inclined inlet configuration in a horizontal plane approximately at 318 to 467 mm below the Axial Velocity in Horizontal Plane In order to shed more light to the behavior of the flow and GLCC inlet for 72 and 10 gpm. The black line shown along the helical shape that is observed in the upper portion of the the diameter represents the measurement plane at 318 mm tube (318
