Hong-Quan Zhang

On the transition of the cylinder wake

The transition of the cylinder wake is investigated experimentally in a water channel and is computed numerically using a finite‐difference scheme. Four physically different instabilities are observed: a local ‘‘vortex‐adhesion mode,’’ and three near‐wake instabilities, which are associated with three different spanwise wavelengths of approximately 1, 2, and 4 diam. All four instability processes can originate in a narrow Reynolds‐number interval between 160 and 230, and may give rise to different transition scenarios. Thus, Williamson’s [Phys. Fluids 31, 3165 (1988)] experimental observation of a hard transition is for the first time numerically reproduced, and is found to be induced by the vortex‐adhesion mode. Without vortex adhesion, a soft onset of three‐dimensionality is numerically and experimentally obtained. A control‐wire technique is proposed, which suppresses transition up to a Reynolds number of 230.

Unified model for gas-liquid pipe flow via slug dynamics: Part 1: Model development

A unified hydrodynamic model is developed for predictions of flow pattern transitions, pressure gradient, liquid holdup and slug characteristics in gas-liquid pipe flow at all inclination angles from -90° to 90° from horizontal. The model is based on the dynamics of slug flow, which shares transition boundaries with all the other flow patterns. By use of the entire film zone as the control volume, the momentum exchange between the slug body and the film zone is introduced into the momentum equations for slug flow. The equations of slug flow are used not only to calculate the slug characteristics, but also to predict transitions from slug flow to other flow patterns. Significant effort has been made to eliminate discontinuities among the closure relationships through careful selection and generalization. The flow pattern classification is also simplified according to the hydrodynamic characteristics of two-phase flow.

Unified Model for Gas-Liquid Pipe Flow via Slug Dynamics—Part 2: Model Validation

In Zhang et al. [1], a unified hydrodynamic model is developed for prediction of gas-liquid (co-current) pipe flow behavior based on slug dynamics. In this study, the new model is validated with extensive experimental data acquired with different pipe diameters, inclination angles, fluid physical properties, gas-liquid flow rates and flow patterns. Good agreement is observed in every aspect of the two-phase pipe flow.

A unified mechanistic model for slug liquid holdup and transition between slug and dispersed bubble flows

Abstract A unified mechanistic model for slug liquid holdup is developed based on a balance between the turbulent kinetic energy of the liquid phase and the surface free energy of dispersed spherical gas bubbles. The turbulent kinetic energy is estimated by use of the shear stress at the pipe wall and the momentum exchange (mixing term or acceleration term) between the liquid slug and the liquid film in a slug unit. The momentum exchange term varies significantly with pipe inclination and enables the model to give an accurate prediction of slug liquid holdup for the entire range of pipe inclination angle. The model has been compared with experimental data acquired at TUFFP for slug flows at all inclinations and good agreement has been observed. The model can also be used to predict the slug–dispersed bubble flow pattern transition boundary over the whole range of inclination angles. From comparison with previous experimental results, the model predictions are accurate for gas superficial velocities larger than 0.1 m/s.

Investigation of Paraffin Deposition During Multiphase Flow in Pipelines and Wellbores—Part 2: Modeling

Results are presented from two-phase flow wax deposition tests using a state-of-the-art, high-pressure, multiphase flow test facility. Wax deposition was found to be flow pattern specific and dependent on the flow velocities of the two-phase fluids. Wax deposition occurs only along the pipe wall in contact with a waxy crude oil. An increase in mixture velocity results in harder deposits, but with a lower deposit thickness. The wax buildup trend at low mixture velocities is similar to that observed in laminar single-phase flow tests. The wax buildup trend at high mixture velocities is similar to that observed in turbulent single-phase flow tests. Thinner and harder deposits at the bottom than at the top of the pipe were observed in horizontal and near-horizontal intermittent flow tests. For annular flow tests, thicker and harder deposits were observed at low superficial liquid velocity than at high superficial liquid velocity. In stratified flow tests, no wax deposition was observed along the upper portion of the pipe.

Unified Model of Heat Transfer in Gas-Liquid Pipe Flow

A unified model of multiphase heat transfer is developed for different flow patterns of gas-liquid pipe flow at all inclinations from –90 to +90 from horizontal. The required local flow parameters are predicted by use of the unified hydrodynamic model for gas-liquid pipe flow recently developed by Zhang et al. 2 The model prediction of the pipe inside convective heat transfer coefficients are compared with experimental measurements for a crude oil/natural gas system in horizontal and upward vertical flows, and good agreement is observed. Introduction As oil and gas production moves to deep and ultra-deep waters, flow assurance issues such as wax deposition, hydrate formation, and heavy oil flow become very crucial in transportation of gas, oil and water to processing facilities. These flow assurance problems are strongly related to both the hydraulic and thermal behaviors of the multiphase flow. Therefore, multiphase hydrodynamics and heat transfer need to be modeled properly to optimize the design and operation of the flow system. Compared to experimental and modeling studies of multiphase hydrodynamics, very limited research results can be found in the open literature for multiphase heat transfer. Davis et al. presented a method for predicting local Nusselt numbers for stratified gas-liquid flow under turbulent liquid/turbulent gas conditions. A mathematical model based on the analogy between momentum transfer and heat transfer was developed and tested, using heat transfer and flow characteristics data taken for air/water flow in a 63.5-mm inside diameter (ID) tube. Shoham et al. measured heat transfer characteristics for slug flow in a horizontal pipe. The time variation of temperature, heat transfer coefficients, and heat flux were reported for the different zones of slug flow. Substantial difference in heat transfer coefficient was found to exist between the bottom and top of the slug. Most previous modeling studies were aimed at developing heat transfer correlations for different flow patterns. Kim et al. evaluated 20 heat transfer correlations against experimental data collected from the open literature, and made recommendations for different flow patterns and inclination angles. However, these recommended correlations did not give satisfactory predictions when compared with experimental results by Matzain. Manabe developed a comprehensive mechanistic model for heat transfer in gas-liquid pipe flow. The overall performance was better than previous correlations in comparison with experimental data. However, some inconsistencies in the hydrodynamic model and the heat transfer formulations for stratified (annular) and slug flows need to be improved. A unified hydrodynamic model has been developed for gas-liquid pipe flow at the Tulsa University Fluid Flow Projects (TUFFP). The major advantage of this model compared with previous mechanistic models is that the predictions for both flow pattern transition and flow behavior are incorporated into a single unified model based on slug dynamics. Multiphase heat transfer depends on the hydrodynamic behavior of the flow. The objective of this study is to develop a unified heat transfer model for gas-liquid pipe flow that is consistent with the unified hydrodynamic model.

Slug Dynamics in Gas-Liquid Pipe Flow

The continuity and momentum equations for fully developed and spatially developing slug flows are established by considering the entire film zone as the control volume. They are used for the calculations of pressure gradient, slug frequency, liquid holdup in the film flow pattern transition, slug dissipation, and slug tracking. Comparison with available experimental results shows that these equations correctly describe the slug dynamics in gas-liquid pipe flow.

Modeling of Slug Dissipation and Generation in Gas-Liquid Hilly-Terrain Pipe Flow

Hilly-terrain pipelines consist of interconnected horizontal, uphill and downhill sections. Slug flow experiences a transition from one state to another as the pipe inclination angle changes. Normally, slugs dissipate if the upward inclination becomes smaller or the downward inclination becomes larger, and slug generation occurs vice versa. Appropriate prediction of the slug characteristics is crucial for the design of pipeline and downstream facilities. In this study, slug dissipation and generation in a valley pipeline configuration (horizontal-downhill-uphill-horizontal) were modeled by use of the method proposed by Zhang et al. [1]. The method was developed from the unsteady continuity and momentum equations for two-phase slug flow by considering the entire film zone as the control volume. Computed results are compared with experimental measurements at different air-mineral oil flow rate combinations. Good agreement is observed for the change of slug body length to slug unit length ratio.

Experimental Study of Low Liquid Loading Gas-Liquid Flow in Near-Horizontal Pipes

Low liquid loading flow in wet gas pipelines is a common occurrence in the transport of unprocessed raw gas. The most important parameters governing the flow behavior are pipe geometry (inclination angle and diameter), operating conditions (flow rate, pressure, and temperature), and physical properties of the gas and liquid (density, viscosity, and surface tension). In this study, extensive experiments were conducted using a test loop made of 50.1-mm diameter acrylic pipe with inclination angles from the horizontal of -2°, -1°, 0°, 1°, and 2°. Gas and liquid superficial velocities ranged from 5 to 25 m/s and from 0.001 to 0.053 m/s, respectively. In-situ liquid loading ranged from 150 to 1800 m 3 /MMm 3 . Flow patterns studied were stratified (smooth and wavy) and annular. Measured parameters included gas and liquid volumetric flow rates, liquid film flow rate, pressure drop, temperature, liquid holdup and droplet deposition rate. The experimental results show that, at low liquid loading, there is a broad transition range from stratified wavy flow to intermittent flow. Entrainment can occur in the gas core at relatively low gas velocities, and droplet deposition occurs simultaneously with entrainment. An increase in gas velocity did not increase liquid entrainment fraction over a relatively broad range of gas velocities. However, an increase in liquid flow rate did increase the liquid entrainment fraction. Surprisingly, in the annular flow region, a new phenomenon was observed at certain superficial gas velocities: an increase in liquid flow rate decreased the liquid film flow rate and liquid holdup, and increased the entrainment flow rate. This phenomenon may have significant impact on operations to sweep liquids from wet gas pipelines. Seven correlations of liquid entrainment onset point and entrainment fraction in the gas core are evaluated and modified to better fit the data. A new interfacial friction factor closure relationship is proposed. Based on this new information, a procedure is proposed for calculating low liquid loading in wet gas pipelines. The data obtained, the interfacial correlation developed and the proposed procedure to estimate liquid loading can be used in designing wet gas pipelines, determining pigging frequency, and predicting hydrate formation in deep water wet gas pipelines.

Observations of Slug Dissipation in Downward Flow

Detailed observations were made for slug flow in an upward to downward pipeline configuration with 13 capacitance sensors. The results presented in this study are based on four typical tests, which demonstrate the phenomena encountered in slug dissipation. In downward flow, slug frequency decreases with different extent at different flow rates. There is a reduction of slug length after the turning over from upward to downward flow. However, the slug tends to recover its stable length in further downstream development, except for flow that quickly becomes stratified. Growth of the slug length is accomplished by picking up liquid left by the downstream dissipating slug before being fully absorbed by the liquid film.