Development of a DYNASWIRL Phase Separator for Space Applications

Transcription

1 Appears in 50th AIAA Aerospace Sciences Meeting Online Proceedings including the New Horizons Forum and Aerospace Exposition (01) Development of a DYNASWIRL Phase Separator for Space Applications Xiongjun Wu and Georges L. Chahine DYNAFLOW, INC., Jessup, MD, 0794 This paper describes the development of a new passive phase separator based on imparting swirl to the flow using a DYNAFLOW patented nozzle. By combining swirl, cavitation, and rectified gas diffusion, the DYNASWIRL phase separator is able to force even very low void fraction gas out of the liquid into the central core of the vortex, and the extract it from the core. A prototype that was tested on a NASA reduced gravity flight in August 009 successfully reduced the void fraction down to Data analysis and flow visualization indicated that a steady vortex core was maintained and that the gas was continuously removed from the vortex core under the reduced-gravity conditions. Development of the phase separator was guided by 3D Numerical simulations using 3DYNAFS, which couples an incompressible Navier-Stokes flow solver and a bubble tracking and dynamics code. Nomenclature = void fraction = dynamic viscosity = kinetic viscosity = medium density = rotational speed = swirl parameter = vortex strength a c = vortex core radius a g = gas extraction orifice radius c = sound speed d ij, d kl, d lk = deformation tensors dt = time step h = height p c = pressure at vortex core p e = ambient pressure at exit p g0 = initial gas bubble pressure p o = pressure at chamber boundary p v = liquid vapor pressure v a = exit velocity v t = tangential inlet velocity r = radial distance A t = area of tangential slots A o = area of exit orifice C d = discharge coefficient D 1 = chamber diameter F D = drag force coefficient P enc = average pressure over bubble surface Q t = inlet flow rate R = bubble radius u = velocity vector u enc = average velocity vector over bubble surface Subscripts:

2 b g l m = bubble = gas = liquid = mixture I. Introduction and Background HE limits on the amount of liquids and gases that can be carried to space make it imperative to recycle and Treuse these fluids. On earth, bubbles in a liquid are easily separable by buoyancy. In microgravity, other external forces, such as a centrifugal force, must be utilized to separate bubbles from liquids. Two categories of centrifugal force separators exist: one uses active rotation by mechanically spinning the tank. This is very efficient but requires shaft, bearings, and motor. In the second, the tank is fixed, and the rotation is induced by eccentric injection of the mixture. This method requires little power, is much simpler than active separators, and consumes less energy. McQuillen et al. [1,] at NASA Glenn Research center has been developing the Cascade Cyclonic Separation Device (CSD-C) since the mid 90 s. This separator has been proven to be very efficient for mid-range void fraction (from 50 to 80% of gas) with an efficiency approaching 100%. However, it is not as efficient for lower void fraction, where there is a need for further research [3,4]. A. DYNASWIRL Vortex Chamber Under on-going support from NASA, DYNAFLOW INC. has been developing a phase separator for gas-liquid mixtures based on its efficient swirling flow generation DYNASWIRL technology. The DYNASWIRL was extensively tested for various applications in previous studies and was shown to be able to simultaneously accomplish several water reclamation functions: liquid oxidation, TOC reduction, disinfection, and oxygenation [5-9]. This type of vortex generator has also been extensively used in applications such as algae removal, underwater painting and surface preparation [10]. The DYNASWIRL has also been used to develop a fine bubble generator [11]. The mechanisms enabling the accomplishment of each of these functions are different while all arise from induced cavitation bubble dynamics, growth, and collapse. The phase separation DYNASWIRL consists of two concentric cylinders as shown in Figure 1. The fluid is introduced in the space between the two cylinders and exits through orifices on the cylinder axis, with area, A o. The swirling flow inside the inner cylinder is generated with tangential slots which enable flow from outer to inner cylinder. This configuration makes the vortex core very stable even at low flow rates. The ratio between axial and tangential velocities,, or the swirl Figure 1. Cross section of a DYNASWIRL Chamber. parameter, can be controlled by changing the relative areas of the slots, A t, and the orifice, A o, respectively. With high tangential velocities, the pressure on the axis can be made low enough to make microbubbles grow on the axis and collect into a line vortex bubble. At even higher tangential velocities cavitation occurs and further enhances gas diffusion and gas transfer into the central vortex core. The main idea of the DYNASWIRL separator is to use swirl, cavitation, and rectified gas diffusion to force low void fraction gas out of the liquid into the central core of the vortex and extract the gas from the central cavitation core. The pressure drop at the center of a vortex flow is a direct function of the vortex strength, Γ, and of the radius of the vortex viscous core, a c. The flow field of a rotating fluid or vortex, can be considered with reasonable accuracy to be composed of two regions. In the innermost region, of radius a c, the fluid viscosity is predominant, and the fluid rotates en masse as a solid body. In that region the tangential velocity of the fluid increases linearly with the distance from the vortex axis where the tangential velocity is zero. At a distance r from the vortex axis the tangential velocity in this viscous region can be related to the angular velocity,, as v r (0) 1.

3 In the outermost region of the vortex, the flow is that of an ideal inviscid fluid. In that region, the circulation, which is equal to the integral of the velocity along a closed line encircling the vortex center, is everywhere constant and equal to the vortex strength, Γ. The velocity at a point located at a distance r from the vortex center in this outermost region is related to Γ by v / r. (1) At the transition between the viscous and inviscid regions, where r = a c and v 1 = v, the following relationship can be derived: a c. () Let p o be the ambient pressure at the inner chamber boundary, the pressure at the vortex center, p c, can be determined knowing Γ, and the liquid density, ρ. By applying Bernoulli's equation in the inviscid region and solving the equations of conservation of mass and momentum in the viscous region one finds: pc po ( / ac ). (3) Cavitation in the vortex occurs when p c drops locally below the liquid critical pressure, p c, or for simility the vapor pressure, p v, of the liquid at the considered temperature [1]. In order to increase the pressure drop or the degree of cavitation it is necessary to either increase Γ or decrease a c. In order to extract the gas from the central core, the core size has to be larger than the gas extraction orifice size, a g, therefore p p ( / a ). (4) v o g With DYNASWIRL, the swirling vortex is generated by tangentially directing fluid about the longitudinal axis of a cylindrical swirl chamber positioned within a nozzle. If v t is defined as the fluid tangential injection velocity into the swirl chamber, and D 1 is the diameter of the swirl chamber, Γ is directly related to v t and D 1 by the simple relation: Dv 1 t. (5) Since D 1 is a fixed geometric dimension of the swirl chamber, v t can be varied and is directly determined by the total tangential flow rate, Q t and the total tangential injection area, A t with the following relation: vt Qt / At. (6) The axial velocity component of the liquid coming out of the orifice has an average value, v a, directly related to Q t and the area, A o, of the exit orifice, by the relation: va Qt / Ao. (7) Since p o is the driving pressure that push the liquid out of the chamber through the axial orifice, it can be written as 1 po pe va, (8) where p e is the ambient pressure outside of the DYNASWIRL chamber liquid exit. And Q t and V t can be related by the relation: Qt C, d Ao va (9) where C d is the discharge coefficient of the liquid exit orifice. Combining the above equations, the following relationship is satisfied: Q t D1Q t Cd A 0 ag A t 1 pv pe This equation defines what is needed to obtain a stable cavitating core. B. 3D Coupled Numerical Simulations Numerical simulations were conducted to to gain insight of the separation process and to assist the design of an efficient separator. The bubbly mixture flow inside the swirl chamber was treated from the following two perspectives: Microscopic level: Individual bubbles are tracked in a Lagrangian fashion, and their dynamics are followed by solving the surface averaged pressure Rayleigh-Plesset equation. The bubble responds to its surrounding medium described by its mixture density, pressure, velocity, etc. Macroscopic level: Bubbles are considered collectively and define a void fraction space distribution. The mixture medium has a time and space dependant local density which is related to the local void fraction. The mixture density is provided by the microscale tracking of the bubbles and the determination of their local volume fraction. (10)

4 The two levels are fully coupled: the bubble dynamics are in response to the variations of the mixture flow field characteristics, and the flow field depends directly on the bubble density variations. This is achieved through a twoway coupling between the unsteady Navier Stokes solver 3DYNAFS-VIS and the bubble dynamics code 3DYNAFS-DSM. 3DYNAFS-VIS is an incompressible Navier-Stokes flow solver, it can includes bubbles, cavities, and large free surface deformation effects and uses moving overset grids and dynamic grid generation schemes. 3DYNAFS-VIS enables direct numerical solution in addition to RANS. The code is based on the artificial-compressibility method. The mixture medium satisfies the following general continuity and momentum equations: m mu m 0, (11) t Dum m pm mij m u m, (1) Dt 3 where, the subscript m represents the mixture medium, and ij is the Kronecker delta. The mixture density and the mixture viscosity for a void volume fraction can be expressed as: 1, (13) m g 1, (14) m where the subscript represents the liquid and the subscript g represents the gaseous bubbles. The medium has a variable density because the void fraction varies in space and in time. Lagrangian bubble tracking is accomplished by 3DYNAFS-DSM. It is a multi-bubble dynamics code for tracking and describing the dynamics of bubble nuclei present in a flow field. The user can select a bubble dynamics model; either the incompressible Rayleigh-Plesset equation [13] or the compressible Keller-Herring equation [14-16]. In the first option, the bubble dynamics is solved by using a modified Rayleigh-Plesset equation improved with a Surface- Averaged Pressure (SAP) scheme: 3k 3 1 R0 4mR uenc ub RR R pv pg 0 Penc, m R (15) R R 4 where R is the bubble radius at time t, R 0 is the initial or reference bubble radius, is the surface tension parameter, µ m is the medium viscosity, m is the density, p v is the liquid vapor pressure, p g0 is the initial gas pressure inside the bubble, k is the polytropic compression law constant, u enc is the liquid convection velocity vector and u b is the bubble travel velocity vector, and P enc is the ambient pressure seen by the bubble during its travel. With the Surface Averaged Pressure (SAP) model, P enc and u enc are the average of the pressure over the surface of the bubble. If the second option is adopted, the effect of liquid compressibility is accounted for by using the following Keller-Herring equation. 3k R 3 R 1 R R d R0 4 mr uenc ub 1 RR 1 R 1 pv pg 0 Penc, cm 3cm m cm cm dt R (16) R R 4 where c m is the speed of sound in the mixture medium. The bubble trajectory is obtained from the bubble motion equation derived by Johnson and Hsieh [17]: 1/ du ( ) b m 3R m u 1 m du dub b m Kv mdij FD u ub u ub ub g 1/4 b. dt R x dt dt u u (17) R( d d ) b b b b b The first term in Equation (17) accounts for the drag force effect on the bubble trajectory. The drag coefficient F D is determined by the empirical equation of Haberman and Morton [18]. The second and third term in Equation (17) account for the effect of change in added mass on the bubble trajectory. The fourth term accounts for the effect of pressure gradient, and the fifth term accounts for the effect of gravity. The last term in Equation (17) is the Saffman [19] lift force due to shear as generalized by Li & Ahmadi [0]. The coefficient K is.594, is the kinematic viscosity, and d ij is the deformation tensor. The unsteady two-way interactions can be described as follows: The bubbles in the flow field are influenced by the local densities, velocities, pressures, and pressure gradients of the mixture medium. The dynamics of individual bubbles and Lagrangian tracking of them are based on these local flow variables as described in Equations (15) to (17). g lk kl

5 The mixture flow field is influenced by the presence of the bubbles. The local void fraction, and accordingly the local mixture density, is modified by the migration and size change of the bubbles, i.e., the bubble population and size. The flow field is adjusted according to the modified mixture density distribution in such a way that the continuity and momentum are conserved through Equations (11) and (1). Void fractions based on -cells were used in the 3-D space to compute the void fraction [1]. II. Reduced Gravity Flight Test A prototype of the DYNASWIRL Free Vortex Phase Separator (FVS) was selected to be tested at a reduced gravity flight test under the NASA FAST program (The Facilitated Access to the Space Environment for Technology Development and Training) in August, 009. This prototype consisted of 3 chambers connected in series as shown in Figure. In the center was the DYNASWIRL chamber where the gas-liquid mixture was introduced in the space between the two cylinders. The side chambers were a vacuum chamber and a mixing chamber. An orifice between the vacuum chamber and the DYNASWIRL chamber acted as a gas finder orifice. Once the core was generated in the vortex chamber, the gas finder orifice started to extract gas from the vortex core into the vacuum chamber. This chamber was connected to a vacuum pump to insure gas removal at a faster speed. The mixing chamber was also connected to the vortex chamber, but through a larger orifice. It was used to receive a gas-liquid mixture obtained with a DYNASWIRL bubble generator before the test. During the test it was used as a reservoir to collect the gas reduced mixture (which exited from the larger orifice of the vortex chamber). This mixture of gas and liquid was then pumped back to the vortex chamber for further gas-liquid separation. Figure. Schematic of the prototype of the DYNASWIRL Phase Separator, which was tested at a reduced gravity flight test under the NASA FAST program in August, 009. A. Experiment Setup The DYNASWIRL phase separator prototype was setup for the reduced gravity flight test in a loop sketched in Figure 3. A ¾ HP centrifugal pump was used to supply the mixture to the phase separator and a vacuum/air supply pump was used to provide vacuum during the phase separation process. A vacuum breaker was installed at the gas extraction port of the vacuum chamber to prevent liquid from getting into the gas extaction line. Additionally, a liquid catcher chamber was installed to store the water escaped from the vacuum chamber under microgravity when vacuum was applied to prevent the water from entering the vacuum pump. This water from the liquid catcher chamber was recycled back into the FVS periodically. An ABS ACOUSTIC BUBBLE SPECTROMETER [-6] with two hydrophones setup in the mixing chamber was used to monitor the void fraction during the whole flight. A high definition video camera was also used for flow visualization of the gas extraction process. In addition, the pressures in all the three chambers, as well as the accelerometer signals from the airplane, were recorded during the flight. The entire setup had a secondary confinement to prevent any possible leaks during the reduced gravity flight. The DYNASWIRL phase separator was confined in a Plexiglas box and all pipes were equipped with a secondary hose to capture any potential leaks. Table 1 shows a list of the components used in the experiment setup as shown in Figure 3.

6 Figure 3. Experimental setup for the bubble generation and air separation experiments conducted in a reduced gravity flight test under FAST program in August, 009. Item Description Dimensions (in) Weight (lbs) Pump Water pump, 3/4HP, 115V, 9 Amps FVS DYNASWIRL Chamber: Free Vortex Separator Vacuum/ Air Pump Vacuum/air pump, 115V, 4. Amps Bubble Generator DYNASWIRL Bubble Generator Laptop Dell Inspiron laptop ABS ABS ACOUSTIC BUBBLE SPECTROMETER box Frame Aluminum tubing frame with aluminum bottom and top Camcorder HD digital camcorder Misc. PVC piping, valves, tubing, water - ~115 Table 1. List of Equipment used in the Free Vortex Separator Experiment on the reduced gravity flight test under the FAST program in August, 009. B. Numerical Simulations 3DYNAFS-VIS was used to characterize the DYNASWIRL phase separator prototype. A multi-block grid, as shown in Figure 4, was generated to represent the DYNASWIRL chamber with two orifices connecting to the two external chambers. A larger orifice connected with the mixing chamber and a smaller one connected with the vacuum chamber. Figure 5 shows a sample pressure distribution at an inlet flow rate of 0 GPM, the pressure was normalized with the high pressure at the injection slot of the inner DYNASWIRL chamber. A low pressure vortex core region can be clearly observed in the swirl chamber from Figure 5. The flow field in the phase separator was unsteady and the flow near the two orifices behaved quite differently from each other. Figure 6 and Figure 7 show some details of the flow field at two time steps near the liquid extraction and gas extracton orifices respectively. Figure 4. Multi-block grid used for 3DYNAFS- VIS simulation of the DYNASWIRL phase

7 Animation with time show more significant fluctuations near the smaller orifice, where the flow can come in and out of the swirl chamber through the orifice. 3D coupled numerical simulations on the DYNASWIRL phase separator by coupling 3DYNAFS-VIS and 3DYNAFS-DSM were also performed to study the bubble dynamics in the flow field. Figure 8 shows a snapshot of the bubbles in the flow field obtained from the coupled run. Bubbles were injected from the slot of the DYNASWIRL inner chamver, as shown in the figure, bubbles were concentrated into the vortex core region and escaped into the vacuum chamber through the gas extraction orifice. Figure 5. Normalized pressure distribution in the DYNASWIRL phase separatorat flow rate of 8.5 GPM. SC MC SC MC Figure 6. Flow field near the larger orifice that connects swirl chamber (VC) and mixing chamber (MC) at two different time steps. VC SC VC SC Figure 7. Flow field near the smaller orifice that connects vacuum chamber (VC) and swirl chamber (SC) at two different time steps

8 Figure 8. A snapshot of the results from 3DYNAFS-DSM using background flow provided by 3DYNAFS-VIS. C. Flight Test Results The reduced gravity test was performed in a Zero- G Boeing 77 airplane on August 1, 009. The bubbly medium was generated before the parabolic maneuvers started. Once the void fraction reached a preset 1% value, the bubble generator was turned off and the bubbly medium were kept circulating throught the mxing chamber without going through the DYNASWIRL chamber to maintain a homogeneous bubbly medium until right before the parabolic maneuvers began. Then the bubbly medium was directed into the phase separator to remove the injected air, the phase separation process was monitored during the complete flight duration with parabolic maneuvers. The total flight time during which there were parabolic maneuvers was about 55 minutes. The DYNASWIRL phase separator was run at a flow rate of 15 GPM for the first 5 minutes for initial gas extraction, and then at an increased flow rate of 0 GPM for the final gas extraction. Data analysis and flow visualizations indicated that a steady vortex core was maintained and that the gas was continuously removed from the vortex core during the zero-gravity condition. Figure 9 shows the gas bubbles being extracted from the vortex core under zero-gravity. As the gas extraction progressed, the vortex core shrunk and the number of visible bubbles kept decreasing. It was also observed during the test that the bubbles in the vortex chamber moved towards the core as shown in Figure 10. Figure 11 shows the images of the mixing chamber taken during the initial and the final stages of the flight. As seen from these images clarity was much improved at the end as the visible amount of bubbles significantly reduced. Similar results were observed from the images of the vortex chamber and the vacuum chamber during the initial and final stages of the test. These images are shown in Figure 1. The number of visible bubbles Figure 9. Gas extraction under zero gravity. Vacuum Chamber Vortex Chamber Vortex Core Figure 10. bubbles in the vortex chamber moving towards the vortex core. Figure 11. Sanp shots of the mixing chamber during the initial (left) and final (right) stages of the flight.

9 was significantly reduced in the final stages of the flight when compared to the initial stages. These images also show that vortex core size had shrunk in the final stage when compared to that at initial stage. Figure 1. Snap shots of the vortex chamber and the vacuum chamber during the initial stage (left) and final stage (right) of the reduced gravity flight. Figure 14. Sample signals of the Pressure signal and gravity in the z-direction during the flight. Figure 13. Void fraction as a function of time. Figure 15. Comparisons of test raw signals (red ) measured by the hydrophones during the initial stage of the flight, i.e high void fraction (left) and final stage of the flight, i.e. low void fraction (right). Figure 14 shows the pressure signal amplitudes in the mixing, vacuum, and vortex chambers along with the acceleration of gravity in the z direction on the second y axis. As seen in the figure, the pressure was highest in the vortex chamber and lowest in the vacuum chamber. Since the vacuum pump was turned off to avoid water fouling the vacuum line during the reduced gravity phases, under the microgravity condition the pressure in all three chambers was higher, compared to the case when there was gravity present, however this change had no effects on the smooth operation of the phase separator.

10 Figure 13 shows the void fraction in the mixing chamber during the flight as measured by the ABS. Note that the hydrophones deployed in this test were designed to measure only very low void fractions. In the initial stage, the void faction was above the upper limit of the measurement range, the reading was saturated at around x As the phase separation progressed, the void fraction decreased continuously with time, proving that the DYNASWIRL phase separator was working under microgravity. At the end of the test, the void fraction measured by the ABS was of the order of 10-8, while is started at the beginning of the flight over Figure 15 shows a comparison of a sample ABS raw signals at the initial and final stage, the received test signals are in red. At the beginning, the received signals were very weak due to the large amount of bubbles. With the DYNASWIRL phase separator continuously removing bubbles, the void fraction decreased, and the received test signals became very significantly stronger. From the above described reduced gravity test results, identified areas for improvement included: Improve gas extraction efficiency. Reduce or eliminate the remixing of liquid and gas Expand the operation ranges of void fraction and flow rate Enable a continuous feed of two-phase content We are addressing these identified areas for improvement through ground based tests and numerical simulations prior to the next scheduled zero gravity flight tests in 01. III. Conclusion A new phase separator based on DYNAFLOW patented DYNASWIRL technology was developed and tested during a NASA reduced gravity flight. During the test, the phase separator successfully reduced the mixture void fraction down to Data analysis and flow visualization indicated that a steady vortex core was maintained and that the gas was continuously removed from the vortex core under the reduced-gravity conditions. 3D Numerical simulations using a Navier-Stokes flow solver, 3DYNAFS-VIS, coupled with a bubble dynamics solver, 3DYNAFS-DSM were used to guide the design. 3DYNAFS provided us with a powerful numerical tool to help optimize the design and ultimately develop a fully operational DYNASWIRL Phase Separator to be tested by NASA on the International Space Station (ISS). Acknowledgments This work is supported by NASA under Grant No. NNX09AI35G, the authors would like to thank Mr. McQuillen, program monitor, for his support and advice. References 1 McQuillen, J. B., Neumann, E. S., Two-Phase Flow Research Using the Laserjet Apparatus, NASA Technical Memorandum, NASA, Cleveland, OH , 1995, pp Hoyt, N. C., Kamotani, Y., McQuilen, J. B., Sankovics, J. M., Computational Investigation of the NASA Cascade Cyclonic Separation Device, 46 th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Shoemaker, J. M., Schrage, D. S., Microgravity Fluid Separation Physics Experimental and Analytical Results, 35 th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Schrage, D. S., Shoemaker, J. M., McQuillen, J., Passove Two-phase Fluid Separation A Dynamic Simulation Model, 36 th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Chahine, G. L., Hsiao, C. T., Choi, J. K., Kalumuck, K. M., Loraine, G., Aley, P. D., Frederick, G. S., Development of a DYNAJETS Cavitation System for High Rate Disinfection of Combined Sewer Overflows, DYNAFLOW, INC, Report M303-1, Chahine, G. L., Kalumuck, K. M., Hsiao, C. T., Choi, J. K., Loraine, G., Aley, P. D., Frederick, G. S., Water Reclamation Using the DYNAJETS Cavitating Jets for the Advanced Life Support System, DYNAFLOW, INC, Report M001-1-NASA, Loraine, G., Chahine, G. L., Hsiao, T. T., Choi, J. K., Jain, A., Aley, P. D., Frederick, G. S., Cavitating Jets for Aquaculture Wastewater Treatment and Recycling, DYNAFLOW, INC, Report M5016-NOAA-01, Chahine, G. L., Hsiao, C. T., Choi, J. K., Loraine, G., Frederick, G. S., Aley, P. D., Reduction of Chemical and Biological Containments Using the DYNAJETS Cavitating Jets: Post-or Pre treatment for Desalination, DYNAFLOW, INC, Report M ONR, Loraine, G., Choi, J. K., Seawater Remediation Technology Using DynaJets, DYNAFLOW, INC, Report M60-1- NSWC, Chahine, G. L., Choi, J. K., Loraine, G., Aley, P., Development of a DYNASWIRL Cavitating Jet Nozzle for Underwater Hull Grooming DYNAFLOW, INC, Report M7036-NSWCCD-1, Chahine, G. L., Barbier, C., Loraine, G., Choi, J. K., "Development of a Bubble Generator Suitable for Spallation Neutron Source Shock Mitigation Applications", DYNAFLOW, INC, Report M70-DOE-Bub-1, May 008.

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