Riser Flow Research Articles

CFD-DEM simulations of riser geometry effect and cluster phenomena

In this paper the influence of the inlet and outlet configurations of a pseudo-2D fluidized riser on the hydrodynamics (i.e. flow pattern and cluster characteristics) are studied. A detailed comparison between experimental data and full 3D CFD-DEM simulation results is performed. Solids volume fraction and mass flux are characterized and particle clusters are detected in our CFD-DEM simulations and compared to experimental data. It is shown that a correct representation of the outlet is very important for a correct simulation of the solids holdup in the top section of the riser. The core-annulus flow behavior is generally well predicted by the model and cluster characteristics such as cluster occurance and velocity are also in good agreement with experimental data. Finally, an outlook is given for the future use of CFD-DEM simulation approaches to study riser flows.

Two-Phase Flow Mass Transfer Analysis of Airlift Pump for Aquaculture Applications

Airlift pumps can be used in the aquaculture industry to provide aeration while concurrently moving water utilizing the dynamics of two-phase flow in the pump riser. The oxygen mass transfer that occurs from the injected compressed air to the water in the aquaculture systems can be experimentally investigated to determine the pump aeration capabilities. The objective of this study is to evaluate the effects of various airflow rates as well as the injection methods on the oxygen transfer rate within a dual injector airlift pump system. Experiments were conducted using an airlift pump connected to a vertical pump riser within a recirculating system. Both two-phase flow patterns and the void fraction measurements were used to evaluate the dissolved oxygen mass transfer mechanism through the airlift pump. A dissolved oxygen (DO) sensor was used to determine the DO levels within the airlift pumping system at different operating conditions required by the pump. Flow visualization imaging and particle image velocimetry (PIV) measurements were performed in order to better understand the effects of the two-phase flow patterns on the aeration performance. It was found that the radial injection method reached the saturation point faster at lower airflow rates, whereas the axial method performed better as the airflow rates were increased. The standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) were calculated and were found to strongly depend on the injection method as well as the two-phase flow patterns in the pump riser.

Full‐field PIV/DIA and local concentration measurements of ozone decomposition in riser‐flow

AbstractA full‐field PIV‐DIA technique has been coupled to concentration measurements in order to characterize the riser performance at different gas superficial velocities and reaction rates for ozone decomposition. Experiments under riser flow conditions were performed with the use of a compact pseudo‐2D lab‐scale riser reactor. The purpose of this work is to generate reliable experimental data to validate computational models for mass transfer in riser flow conditions. The catalytic activity of the particles was characterized. To analyze the reaction rate effect over the performance of a riser reactor, two catalyst batches with different activities were used and their kinetics were measured. The hydrodynamics of the gas‐solid flow was determined using the PIV‐DIA technique. Particle clusters were identified and characterized. By correlating the cluster information and mass concentration measurements, it was shown that a higher frequency of local flow heterogeneities near the walls led to higher conversion rates than in the core of the riser. Reaction rate and gas superficial effects are analyzed.

Gas-solid two phase flow simulation in rectangular riser

The numerical studies of gas-solid two phase flow in a rectangular riser are carried out and evaluated. Eulerian-Eulerian approach using kinetic theory of granular flow is utilised in the simulation. Syamlal et al. drag model is utilised to calculate the energy exchange between gas and solid phase. Radial distribution model of Syamlal et al. and the restitution coefficient of 0.7 are used to predict the hydrodynamics flow in the riser. The simulation is carried out from 0 s to 40 s using time steps of 0.00015 s and on a single computer. The comparisons of numerical results with experimental data are made using lateral profiles of the solid phase velocity and the gas volume fraction. The simulation result overestimates the experimental value due to the coarse nature of the grid and probably due to uncertainties of the inlet flow properties.

Computational methods applied in the identification of slug flow in marine risers

Computational methods applied in the identification of slug flow in marine risers

Parametric study of particles homogenization in cold-flow riser reactors

Fluid Catalytic cracking (FCC) process are frequently used for the upgrading of heavy crude oil into more valuable products. Traditionally, the FCC process occurs in a riser reactor, where both vaporised hydrocarbon feed-stock and solid catalyst flow in the direction against gravity. This kind of reactor presents heterogeneous flow distributions that affect the axial and radial flow structures, as well as the product quality. To understand the effect of operational variables over riser flow distribution, a numerical study of the two-phase cold-flow in the pre-acceleration zone of a riser reactor fed with catalyst particles used in this kind of reactors is presented in this work considering a two-dimensional model. The Computational Fluids Dynamics (CFD) software ANSYS® Fluent® is used to study two-dimensional gas (air) and solid (catalyst particle) flow in a riser section of a cold-flow Circulation Fluidized Bed (CFB) system under different flow inlet conditions (i.e., either for the gas and solid flux), as well the particle diameter size. An Eulerian-Eulerian approach, including the turbulence ( κ − ε model) and the Kinetic Theory of Granular Flow (KTGF) models, are solved for the gas phase and the solid particles treated as a dispersed phase. The implemented computational model is validated by comparing numerical results for solid velocity and volume fraction distribution to values reported by peers. As expected, a higher concentration towards the axis of the reactor is obtained for the solid volume fraction values. The R N I index estimated for gas velocities of 3 m/s and 4 m/s are significantly lower than the others, indicating that as the inlet gas velocity is reduced, a more homogeneous profile is obtained. Finally, the numerical results obtained shown closer to the experimental data used for the validation as the steady-state is attained.

Tuning of oil well models with production data reconciliation

The daily optimization of oil production systems relies on models to simulate phenomena of interest, such as fluid flow in wells and risers. Keeping models up to date is no easy task, due to the complex nature of the processes, and the need of human intervention to tune simulators. This work contributes by taking advantage of real-time measurements, to adjust process parameters that play a key role in steady-state simulators, e.g. the basic sediment and water and gas-oil ratio, and thereby optimize production for the prevailing conditions. The methodology consists of: (i) a strategy to identify steady-state of process variables (i.e., flows and pressures); (ii) an optimization formulation to adjust the key parameters such that the predicted total flows from the wells match the measured platform streams, while honoring pressure and flow measurements at critical points.

Identification of Oil-Gas Two Phase Flow in a Vertical Pipe using Advanced Measurement Techniques

The characteristics of flow configuration in pipes are very important in the oil industry due to its role in governing equipment design. In vertical risers, many flow configurations could be observed such as bubbly, slug, churn, and annular flow. In this project, two tomographic techniques have been applied simultaneously to the flow in a vertical riser: the Electrical Capacitance Tomography (ECT) technique and the Capacitance Wire Mesh Sensor (WMS) technique. The employed pipe diameter was 50mm and the superficial studied velocities were 0.06-3.0m/s for gas and 0.06-0.4m/s for oil. Several techniques have been used to analyze the output data of the two tomography techniques such as time series of cross-sectional averaged void fraction, Probability Density Function (PDF), image reconstruction, and liquid hold-up profile. The averaged void fractions were calculated from the output signal of the two measurement techniques and plotted as functions of the superficial velocity of the gas. The flow patterns were identified from the PDF of the averaged void fraction. In addition, it was found that both tomographic techniques are reliable in identifying the flow regimes in pipes.

Characteristics of clusters in the jet influence zone of riser

By analyzing solid holdup signals in the jet influence zone of risers, the characteristics of clusters were studied. The solid holdups inside clusters and their distribution ranges were calculated under the cases of both upward and downward feed jets. Moreover, the effects of the jet velocity on the cluster characteristics were analyzed. The solid holdups inside clusters have higher values and wider distribution ranges in the upward feed injection zone than in the downward zone, implying that the number of individual particles in a cluster is unstable under the influence of upward jets. For the case of downward jets, cluster formations in the jet influence zone of risers can more easily maintain stability, offering a better mixing environment for jets and particles. As the jet velocity increases, the solid holdups inside clusters in the riser wall of the upward injection zone increase accordingly, while the distributions of solid holdups inside clusters have no significant changes if the feed jets are downward. This phenomenon confirms that improved operational flexibility can be obtained if downward jets are mixed with the prelift gas–solid flow in risers.

Discrete particle simulation of heterogeneous gas-solid flows in riser and downer reactors

The heterogeneous structures of the gas-solid flow in riser and downer reactors determine reactor performance but are not fully understood. This paper presents a numerical study of the hydrodynamic characteristics of cohesive particles in a riser and a downer. This is done by the combined approach of discrete element method (DEM) for particles and computational fluid dynamics (CFD) for the gas phase. The numerical results show that this CFD-DEM model can reproduce different annular dense distributions of particles in the fully developed sections of the riser and downer. This realization requires suitable periodic boundary conditions applied to both the gas and solid phases in the flow direction as well as adequately fine meshes. The flow behaviors are analyzed in detail in terms of particle flow pattern, cluster behaviors, gas and solid velocities and forces acting on particles. Also, the dependence of the dense solid ring in the downer on some pertinent variables related to pipe geometries, material properties, and operational conditions is examined.

A unified EMMS-based constitutive law for heterogeneous gas-solid flow in CFB risers

Energy minimization multi-scale (EMMS) drag model has been demonstrated to be very successful in quantifying the effect of mesoscale structure on the effective interphase drag force of heterogeneous gas-solid flow in circulating fluidized bed (CFB) risers, however, a corresponding model for the effective particle phase stress is not yet available. To this end, an EMMS drag model was extended to predict the granular temperature of particles inside the dilute phase and the dense phase as well as the granular temperature of clusters. A model for the effective particle phase stress was then developed based on the concept of multiscale analysis in EMMS model. It was shown that the experimentally measured granular temperature, granular pressure and granular viscosity as a function of mean solid concentration can be predicted reasonably well. The agreement demonstrates its effectiveness in the quantification of the effect of mesoscale structure on the particle phase stress, thus establishing a unified EMMS-based model for the constitutive law of heterogeneous gas-solid flow in CFB risers.

Effect of nonuniform isoflux heating on natural convection heat transfer in a prismatic modular reactor

A high temperature and pressure dual-channel facility has been designed and developed at Multiphase Reactors Engineering and Applications Laboratory (mReal) at Missouri University of Science and Technology (Missouri S&T) to investigate the thermal hydraulics of natural circulation under the loss of flow accident scenario (LOFA) in the core of a prismatic modular reactor (PMR). An advanced heat transfer measurement technique consisting of a series of heat flux foil sensors and thermocouples is adapted and implemented along the coolant flow channels (riser and downcomer channels). This study presents the effect of nonuniform isoflux heating (1.432 kW/m2 to 3.152 kW/m2, in form of stepwise-reducing and stepwise-increasing) on natural circulation heat transfer using air as a coolant at 413.7 kPa. The results showed a reversal in the direction of heat transfer and reduction in temperature fields as well as observed within the riser flow channel close to the exit (Z/L = 0.773) due to the end effect. A significant reduction of the average field temperatures along the riser channel is observed to be 24.5% and 22.4% for inner surface and air centerline temperatures, respectively for stepwise-reducing. Moreover, for stepwise-increasing, a significant reduction of the average riser channel temperature fields is observed with decreasing stepwise-increasing heating by 25.6% and 23.1% for the inner surface and air centerline temperatures, respectively. The average values of the heat transfer coefficient(h¯L) and the average Nusselt number (Nu¯L) along the riser channel were decreased with reducing the heating by 38% and 19% for stepwise-reducing heating, respectively, and by 32% and 13% for the stepwise-increasing heating, respectively.

Analysis of multiphase flow characteristics in a deepwater riser

Oil and gas mixed transportation is an important technology in oil and gas production. Multiphase fluid is transported from the wellhead to the floating platform through a riser system. Based on computational fluid dynamics, a numerical model of multiphase flow in a deepwater riser by considering hydrate phase transformation and sand production was established, which was then solved by the finite-difference method. The results show that under constant inlet pressure, the pressure drop decreases with increasing gas–liquid two-phase flow rate, and increases with increasing sand production rate. Furthermore, the hydrate phase transition decreases the pressure. The effect of liquid on the temperature in the riser is apparent: the heat transfer between the fluid and seawater is mitigated by the insulation layer, thus raising the critical point of the hydrate phase transition. Finally, the velocity and volume fraction of the gas phase increase from the inlet to the outlet of the riser. At the same time, the velocity of the liquid–solid phase increases, whereas the volume fraction decreases.

A microscopic gas-solid drag model considering the effect of interface between dilute and dense phases

Particle-resolved direct numerical simulations (PR-DNSs) are employed to simulate flows past various stationary clusters of spheres in low-particle-Reynolds-number regime. The results show that the drag force at the interface between the dilute and dense phases is significantly different from the prediction of the traditional homogeneous BVK law (AIChE Journal, 2007, 53(2):489-501). The drag force at the interface is found to be the function of the solid volume fraction gradient, the superficial velocity gradient and the size of the grid on which the drag force is computed. Based on the PR-DNS data, a new microscopic drag model considering the interface effect is formulated. The effectiveness of the new drag models is demonstrated in flows past ellipsoidal clusters of particles. The results show that, in terms of the global drag force on the clusters, the difference between the prediction of the new model and the PR-DNS data is generally less than 17%. While the prediction of the BVK law is more than 200% larger than the PR-DNS data. The comparison is made at the grid size of three times of the particle diameter, which is a commonly adopted size in fine-grid two fluid model as well as computational fluid dynamics-discrete element method simulations. Therefore, the new drag model is very promising in resolving the detailed interactions between the gas and solid phases in flows with complex fine structures such as riser flows.

Study on mud discharge after emergency disconnection of deepwater drilling risers

Mud is often discharged into seawater after emergency disconnection of risers in offshore oil and gas drilling engineering. The discharged mud needs to be simulated accurately since it can produce a huge load on risers. Several mud discharge models have been proposed, such as Young's slug model, SFM model, WFC model and one-dimensional finite volume model. However, the mud is taken as a one-dimensional rigid body in existing mud discharge models. As a consequence, the radial velocity of mud, the mixing interface between mud and seawater, and mud discharge in complicated riser flow channels cannot be simulated. In this paper, a more accurate and applicable three-dimensional (3D) computational fluid dynamics (CFD) model is established to simulate the mud discharge process. The proposed method is demonstrated by its application to a case. It turns out that the actual mud discharge process can be simulated based on the 3D CFD model. Characteristics of the mud discharge including the radial velocity, axial velocity and the interface between two fluids are calculated based on the 3D CFD model, which improves the understanding of the mud discharge. Besides, the applicability of each mud discharge model is determined through model comparison. The influence of key factors including drill pipes with pipe joints, riser inner diameter, mud density and mud dynamic viscosity on the mud discharge is also analyzed based on the 3D CFD model.

High-Viscosity Liquid/Gas Flow Pattern Transitions in Upward Vertical Pipe Flow

Summary High-viscosity liquid two-phase upward vertical flow in wells and risers presents a new challenge for predicting pressure gradient and liquid holdup due to the poor understanding and prediction of flow pattern. The objective of this study is to investigate the effect of liquid viscosity on two-phase flow pattern in vertical pipe flow. Further objective is to develop new/improve existing mechanistic flow-pattern transition models for high-viscosity liquid two-phase-flow vertical pipes. High-viscosity liquid flow pattern two-phase flow data were collected from open literature, against which existing flow-pattern transition models were evaluated to identify discrepancies and potential improvements. The evaluation revealed that existing flow transition models do not capture the effect of liquid viscosity, resulting in poor prediction. Therefore, two bubble flow (BL)/dispersed bubble flow (DB) pattern transitions are proposed in this study for two different ranges of liquid viscosity. The first proposed transition model modifies Brodkey's critical bubble diameter (Brodkey 1967) by including liquid viscosity, which is applicable for liquid viscosity up to 100 mPa·s. The second model, which is applicable for liquid viscosities above 100 mPa·s, proposes a new critical bubble diameter on the basis of Galileo's dimensionless number. Furthermore, the existing bubbly/intermittent flow (INT) transition model on the basis of a critical gas void fraction of 0.25 (Taitel et al. 1980) is modified to account for liquid viscosity. For the INT/annular flow (AN) transition, the Wallis transition model (Wallis 1969) was evaluated and found to be able to predict the high-viscosity liquid flow pattern data more accurately than the existing models. A validation study of the proposed transition models against the entire high-viscosity liquid experimental data set revealed a significant improvement with an average error of 22.6%. Specifically, the model over-performed existing models in BL/INT and INT/AN pattern transitions.

Investigation of gas-solids flow characteristics in a pressurised circulating fluidised bed by experiment and simulation

Pressurised circulating fluidised bed (PCFB) is a new technology for coal/biomass combustion and gasification. To design, optimise and scale-up of PCFBs, it is necessary to understand the complex gas-solids flow characteristics. In this research, electrical capacitance tomography (ECT) technique, pressure measurement, and computational particle fluid dynamic (CPFD) simulation were combined for the first time to investigate the hydrodynamic behaviour of a pilot-scale PCFB test rig. The custom single- and dual-plane ECT sensors in the riser bottom and cyclone dipleg, provided the real-time cross-sectional particle distribution and circulation flux in a non-intrusive way. Pressure measurements in low and high frequency, revealed the full-loop pressure distribution and local flow regime respectively. In addition, CPFD simulations based on Wen-Yu/Ergun and EMMS (energy-minimization multi-scale) drag model, were performed and evaluated by the experiments. The results demonstrate that (i) gas-solids flow in the PCFB riser shows the non-uniform distribution both axially and radially; (ii) EMMS based simulation predicts flow behaviour in agreement with the experiments, while Wen-Yu/Ergun model significantly over-estimates the particle dispersion and circulation flux; (iii) the elevated operating pressure enhances the gas-solids mixing, by demonstrating the more uniform particles axial distribution and development of radial “core-annular” structure towards the riser bottom and (iv) the elevated operating pressure improves the particle circulation flux, with the improvement limited by the saturated carrying capacity of the PCFB test rig.

Application of machine learning methods to understand and predict circulating fluidized bed riser flow characteristics

Machine learning methods were applied to circulating fluidized bed (CFB) riser data. The goals were to (i) provide insights on various fluidization phenomena through determining the relative dominance of the process variables, and (ii) develop a model to provide predictive capability in the absence of first-principles understanding that remains elusive. The Random Forest results indicate radial position had the most dominant influence on local mass flux and species segregation, overall mass flux was the most dominant for local particle concentration, while no variable was particularly dominant or negligible for the local clustering characteristics. Furthermore, the Neural Network can be trained to provide good predictive capability, without any mechanistic understanding needed, if a sufficiently large dataset is used for training and if the input variables fully account for all the effects at play. This study underscores the value of machine learning methods in fluidization to advance understanding and provide adequate predictions.

Scaling method of CFD-DEM simulations for gas-solid flows in risers

In this paper a scaling method is proposed for scaling down the prohibitively large number of particles in CFD-DEM simulations for modeling large systems such as circulating fluidized beds. Both the gas and the particle properties are scaled in this method, and a detailed comparison among alternative mapping strategies is performed by scaling both the computational grid size and the riser depth. A series of CFD-DEM simulations has been performed for a pseudo-2D CFB riser to enable a detailed comparison with experimental data. By applying the scaling method, the hydrodynamic flow behavior could be well predicted and cluster characteristics, such as cluster velocity and cluster holdups agreed well with the experimental data. For a full validation of the scaling method, four mapping conditions with different ratios of the grid size and particle volume and of modified ratio of riser depth to particle size were analyzed. The results show that in addition to hydrodynamic scaling of the particle and fluid properties, scaling of the dimensions for the interphase mapping is also necessary.

Capability assessment of coarse-grid simulation of gas-particle riser flow using sub-grid drag closures

Our prior work developed an effective material-property-dependent sub-grid model (SGM) for efficient coarse-grid riser flow simulations. To apply this SGM for the purpose of designing and scaling-up riser reactors reliably, quantitative assessment in its predictive capability is required. Here, we quantify the influence of various model parameters with respect to time-averaging, grid resolution, time step, discretization scheme for convection terms, and solid-wall slip condition by implementing extensive three-dimensional coarse-grid simulations. Moreover, we compare several crucial SGM-based coarse-grid and coarse-grained methods to reveal their advantages and disadvantages. Detailed parametric sensitivity analyses demonstrate that the accurate determination of SGM appears to play a dominant role in successfully predicting hydrodynamics. The optimal selection of the other model parameters is suggested to realize a compromise between computation speed and accuracy. Overall, this fundamental study helps to quantitatively understand the predictability of SGMs for coarse-grid simulations of large-scale riser reactors.