Browsing by Subject "multiphase flow"

Now showing 1 - 5 of 5

Low differential pressure and multiphase flow measurements by means of differential pressure devices
(Texas A&M University, 2004-11-15) Justo, Hernandez Ruiz,
The response of slotted plate, Venturi meter and standard orifice to the presence of two phase, three phase and low differential flows was investigated. Two mixtures (air-water and air-oil) were used in the two-phase analysis while a mixture of air, water and oil was employed in the three-phase case. Due to the high gas void fraction (α>0.9), the mixture was considered wet gas. A slotted plate was utilized in the low differential pressure analysis and the discharge coefficient behavior was analyzed. Assuming homogeneous flow, an equation with two unknowns was obtained for the multi-phase flow analysis. An empirical relation and the differential response of the meters were used to estimate the variables involved in the equation. Good performance in the gas mass flow rate estimation was exhibited by the slotted and standard plates for the air-water flow, while poor results were obtained for the air-oil and air-water oil flows. The performance of all the flow meter tested in the analysis improved for differential pressures greater than 24.9 kPa (100 in_H2O). Due to the tendency to a zero value for the liquid flow, the error of the estimation reached values of more than 500% at high qualities and low differential pressures. Air-oil and air-water-oil flows show that liquid viscosity influences the response of the differential pressure meters. The best results for high liquid viscosity were obtained in the Venturi meter using the recovery pressure for the gas flow estimation at differential pressures greater than 24.9 kPa (100 in_H2O). A constant coefficient Cd was used for the low differential pressure analysis and results did show that for differential pressure less than 1.24 kPa (5 inH2O) density changes are less than 1% making possible the incompressible flow assumption. The average of the computed coefficients is the value of Cd.
Modeling of Multiphase Flow in the Near-Wellbore Region of the Reservoir under Transient Conditions
(2010-07-14) Zhang, He
In oil and gas field operations, the dynamic interactions between reservoir and wellbore cannot be ignored, especially during transient flow in the near-wellbore region. As gas hydrocarbons are produced from underground reservoirs to the surface, liquids can come from condensate dropout, water break-through from the reservoir, or vapor condensation in the wellbore. In all three cases, the higher density liquid needs to be transported to the surface by the gas. If the gas phase does not provide sufficient energy to lift the liquid out of the well, the liquid will accumulate in the wellbore. The accumulation of liquid will impose an additional backpressure on the formation that can significantly affect the productivity of the well. The additional backpressure appears to result in a "U-shaped" pressure distribution along the radius in the near-wellbore region that explains the physics of the backflow scenario. However, current modeling approaches cannot capture this U-shaped pressure distribution, and the conventional pressure profile cannot explain the physics of the reinjection. In particular, current steady-state models to predict the arrival of liquid loading, diagnose its impact on production, and screen remedial options are inadequate, including Turner's criterion and Nodal Analysis. However, the dynamic interactions between the reservoir and the wellbore present a fully transient scenario, therefore none of the above solutions captures the complexity of flow transients associated with liquid loading in gas wells. The most satisfactory solution would be to couple a transient reservoir model to a transient well model, which will provide reliable predictive models to link the well dynamics with the intermittent response of a reservoir that is typical of liquid loading in gas wells. The modeling work presented here can be applied to investigate liquid loading mechanisms, and evaluate any other situation where the transient flow behavior of the near-wellbore region of the reservoir cannot be ignored, including system start-up and shut-down.
Practical Aspects of the Implementation of Reduced-Order Models Based on Proper Orthogonal Decomposition
(2012-07-16) Brenner, Thomas Andrew
This work presents a number of the practical aspects of developing reduced- order models (ROMs) based on proper orthogonal decomposition (POD). ROMS are derived and implemented for multiphase ?ow, quasi-2D nozzle ?ow and 2D inviscid channel ?ow. Results are presented verifying the ROMs against existing full-order models (FOM). POD is a method for separating snapshots of a ?ow ?eld that varies in both time and space into spatial basis functions and time coe?cients. The partial di?erential equations that govern ?uid ?ow can then be pro jected onto these basis functions, generating a system of ordinary di?erential equations where the unknowns are the time coe?cients. This results in the reduction of the number of equations to be solved from hundreds of thousands or more to hundreds or less. A ROM is implemented for three-dimensional and non-isothermal multiphase ?ows. The derivation of the ROM is presented. Results are compared against the FOM and show that the ROM agrees with the FOM. While implementing the ROM for multiphase ?ow, moving discontinuities were found to be a ma jor challenge when they appeared in the void fraction around gas bubbles. A point-mode POD approach is proposed and shown to have promise. A simple test case for moving discontinuities, the ?rst order wave equation, is used to test an augmentation method for capturing the discontinuity exactly. This approach is shown to remove the unphysical oscillations that appear around the discontinuityin traditional approaches. A ROM for quasi-2D inviscid nozzle ?ow is constructed and the results are com- pared to a FOM. This ROM is used to test two approaches, POD-Analytical and POD-Discretized. The stability of each approach is assessed and the results are used in the implementation of a ROM for the Navier-Stokes equations. A ROM for a Navier-Stokes solver is derived and implemented using the results of the nozzle ?ow case. Results are compared to the FOM for channel ?ow with a bump. The computational speed-up of the ROM is discussed. Two studies are presented with practical aspects of the implementation of POD- based ROMs. The ?rst shows the e?ect of the snapshot sampling on the accuracy of the POD basis functions. The second shows that for multiphase ?ow, the cross- coupling between ?eld variables should not be included when computing the POD basis functions.
Separator Design for Use in High GVF Multiphase Flow
(2012-10-19) Cihak, Daniel
The requirement of bringing an outside coolant source to run through the seals of a multiphase pump has always been a costly endeavor. Using a separator to extract liquid from the exhaust of the pump to use as a coolant is often more expensive than providing an outside source of coolant. This research proposes a cost effective separator design which efficiently separates the liquid from gas, while maintaining a high enough residence time to remove any gas entrainment, and separates only the seal flush requirement by letting any excess liquids carryover with the gas. Conventional multiphase separators operate by substantially decreasing the velocity of the mixture, which reduces the drag force put forth by the gasses and allows gravity to force the liquids downward. Gas-Liquid Cylindrical Cyclones (GLCCs) operate by increasing the velocity of the mixture, using radial force to separate liquids and gasses. This technique requires a smaller diameter vessel to achieve separation. The separator in this research uses gravity as the separation force while maintaining a pipe diameter similar to the GLCC. This way, only standard pipe and pipe fittings are used. The effectiveness of this design is measured two ways. First, efficiency is studied at varying gas volume fractions (GVFs), velocities, pressures, and pipe diameters. Second, the length of air entrainment (LAE) is measured at the same varying conditions. The efficiency and air entrainment studies provide design recommendations to accommodate seal flush requirements and size limitations. The following investigation also offers further areas of research to improve the understanding and modeling of using standard pipe and pipe fittings to create more effective design equations.
Study of Kinematics of Extreme Waves Impacting Offshore and Coastal Structures by Non Intrusive Measurement Techniques
(2013-11-07) Song, Youn Kyung
Extreme wave flows associated with a large scale wave breaking during interactions with marine structures or complex coastal geography of is one of the major concerns in a design of coastal and ocean structures. In order to properly understand the impact mechanisms of breaking extreme waves, full field evaluations of impacting multiphase flow velocities should be properly conducted first. In this context, this present dissertation experimentally investigated velocity structures of turbulent, multiphase wave flow velocities during active interactions with various offshore and onshore ocean environments. First, initial inundation flow structures of tsunami-like long waves interacting with complex coastal topography are experimentally investigated. Turbulent wave surface velocities were effectively measured by introducing a non-intrusive video imagery technique, the ?wave front tracing method?. Three distinctive configurations for patch layouts that vary either in characteristic patch diameter (D) or in center-to-center spacing between patches (?S) were employed. That is, patch layouts consisted of six (G1) and twelve (G2), ?small? circular macro roughness patches of D =1.2 m and six, ?large? circular macro roughness patches (G3) of D = 1.7 m were employed, respectively. A patch layout employed for G1 appears to be effective in reducing the u velocities along the centerlines of the reference patch that consistently decreased to 85% of a convergence velocity U = 2m/s and to 45% of U. However, in the channel, u velocities hardly reduced below the convergence velocity. On the other hand, the patch layout G2 is observed as rather effective in uniformly reducing the u velocities alongshore. The hand, the patch layout G3 is observed as effective in suppressing the alongshore variability in flow behind the frontal patches. This may be due to the "holding-up" effects produced by the large patches holding the flow within the patch for a longer duration. Furthermore, such a "holding-up" effect from G3 appears to induce a large inundation depth in the flow along the opening. Next, green water velocities and dynamic impacts of the extreme ocean waves on a fixed offshore deck structure are investigated. The experiments focused on the impacting waves generated in a large-scale, three-dimensional ocean wave basin. Using the BIV technique, overall flow structures and temporal and spatial distributions of the maximum velocities were successfully evaluated. The most significant spatial variability in mean velocities in the propagating direction was found from the protruding wave front near the center of the deck during early stages of the wave run-up. The maximum front speed of 1.4C was first observed in the center of the deck near y = 0 at a midpoint of the deck (x = 0.5L), where C is the wave phase speed. The flow velocities started decreasing below 1C over all fields once the wave frontal flow passed the rear edge and started leaving the deck. Pressure measurements were also conducted at four different vertical positions on vertical measurement planes at three different locations on the horizontal plane. Most of measured pressures showed impulsive impact patterns with sudden rises of pressure peaks. The highest pressure was observed as 1.56pC^(2) at x = L/2. Correlations between wave kinematic energy and dynamic pressure were examined to determine the impact coefficients ci'. ci' varied within relatively narrow ranges 0.29 ? ci' ? 1.56. In the present large scale experiments, the impact pressures on the structures are strongly affected by both variability of flow structures and impulsiveness of impacting waves containing considerable air volumes. Lastly, the study is extended for more violent sloshing wave flows. The study experimentally investigated flow kinematics and impact pressures of a partially filled liquid sloshing flow during the periodic longitudinal motion of a rectangular tank. The horizontal velocities near the free surface reached 1.6C with C being the wave phase speed calculated based on the shallow water assumption. As the tank reached its maximum displacement and about to reverse, the dominant flow changed its direction rapidly to vertical upward after the breaking wave crest impinging on the side wall and forming an up-rushing jet. The vertical velocity of the rising jet reached 3.4C before it impacted the top wall. During the flip-through event as the fast moving wave crest collided with the side wall, the steep wave crest resulted in a focused impact on the side wall at the SWL. The resulting impulsive peak pressure was recorded as about 10gh? immediately followed by the evident pressure oscillation with a frequency approximately 500 Hz. After the wall impact, the multiphase up-rushing jet shot up and impacted the top wall. The magnitude of the pressure was again about 10gh?, similar to that recorded by the breaking wave impact on the side wall. Correlating the dynamic impact pressures with the corresponding local maximum flow velocities in the direction normal to the walls was performed by introducing the impact coefficient ic and the modified impact coefficient c'_(i) , defined as p_(max)=c_(i)pV^(2)= c'_(i)pC^(2) with V_(max) being the magnitude of the maximum local velocities. The average values of the modified impact coefficient c?_(i) between the side wall impacts and the top wall impacts were nearly identical, with the average value of c'_(i)=5.2.