Numerical Simulation of Aspect Ratio Effect on Turbulent Annular Flows

The control of energy in various industrial applications passes by the comprehension of the phenomena of transfers especially in complex flows. The structure of the turbulent flow in the cavities in rotation depends on several parameters like the Reynolds number of rotation Ra and the aspect ratio of the cavity. The purpose of this work is to simulate numerically the effect of the aspect ratio on the level of turbulence in the annular steady flow with an incompressible fluid for three different configurations. In the first, the interior cylinder is fixed and the external is moving. The second configuration is the reverse. The third is the contra-rotating cylinders. For all these configurations, we varied the aspect ratio from 0.5 to 2.5. The numerical tool is based on a statistical model in a point using the closing of the second order of the transport equations of the Reynolds stresses （Reynolds Stress Model： RSM）. The results of our numerical simulation show that this geometrical parameter can be an interesting factor to increase the level of turbulence that is often required in several industrial applications where the economy and the control of energy are always required.

Annular multiphase flow behavior during deep water drilling and the effect of hydrate phase transition

It is very important to understand the annular multiphase flow behavior and the effect of hydrate phase transition during deep water drilling. The basic hydrodynamic models, including mass, momentum, and energy conservation equations, were established for annular flow with gas hydrate phase transition during gas kick. The behavior of annular multiphase flow with hydrate phase transition was investigated by analyzing the hydrate-forming region, the gas fraction in the fluid flowing in the annulus, pit gain, bottom hole pressure, and shut-in casing pressure. The simulation shows that it is possible to move the hydrate-forming region away from sea floor by increasing the circulation rate. The decrease in gas volume fraction in the annulus due to hydrate formation reduces pit gain, which can delay the detection of well kick and increase the risk of hydrate plugging in lines. Caution is needed when a well is monitored for gas kick at a relatively low gas production rate, because the possibility of hydrate presence is much greater than that at a relatively high production rate. The shut-in casing pressure cannot reflect the gas kick due to hydrate formation, which increases with time.

Mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device

Mass transfer performance of gas–liquid two-phase flow at microscale is the basis of application of microreactor in gas–liquid reaction systems.At present,few researches on the mass transfer property of annular flow have been reported.Therefore,the mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device are studied in the present study.We find that the main factors,i.e.,flow pattern,liquid film thickness,liquid hydraulic retention time,phase interface fluctuation,and gas flow vorticity,which influence the flow mass transfer property,are directly affected both by gas and liquid flow velocities.But the influences of gas and liquid velocities on different mass transfer influencing factors are different.Thereout,the fitting relationships between gas and liquid flow velocities and mass transfer influencing factors are established.By comparing the results from calculations using fitting equations and simulations,it shows that the fitting equations have relatively high degrees of accuracy.Finally,the Pareto front,namely the Pareto optimal solution set,of gas and liquid velocity conditions for the best flow mass transfer property is obtained using the method of multi-objective particle swarm optimization.It is proved that the mass transfer property of the gas–liquid two-phase flow can be obviously enhanced under the guidance of the obtained Pareto optimal solution set through experimental verification.

Steady thermocapillary-buoyant convection in a shallow annular pool. Part 1:Single layer fluid

This paper examines the steady thermocapillarybuoyant convection in a shallow annular pool subjected to a radial temperature gradient. A matched asymptotic theory is used to obtain the asymptotic solutions of the flow and thermal fields in the case of small aspect ratios,which is defined as the ratio of the layer thickness to the gap width. The flow domain is divided into the core region away from the cylinder walls and two end regions near each cylinder wall. Asymptotic solutions are obtained in the core region by solving the core and end flows separately and then joining them through matched asymptotic expansions. For the system of silicon melt,the asymptotic solutions are compared with the results of numerical simulations. It is found that the two kinds of solutions have a good agreement in the core region for a small aspect ratio. With the increase of aspect ratio,the applicability of the present asymptotic solutions decreases gradually.

Unsteady Rotative Flow of Non-Newtonian Fluid in an Annular Pipe

This paper .Studies power law no-Newtonian fluid rotative flow. in an annularpipe. The governing equation is nonlinear one, we linearized the governing equationby assuming that partial factor is at state. With Laplace transform we obtain ananalytical solution of the problem In the paper several groups of curves are given.these curves reflect the temporal change law and. spatial distribution of fluid velocity.In addition.we study the effection of power law index on the flow field the resultindicates that when the power law index n < l. the flow velocity is highly sensitive tothe index. and this fact is importanl in related engineering decisions.

A Novel Method for Determining the Void Fraction in Gas-Liquid Multi-Phase Systems Using a Dynamic Conductivity Probe

Conventional conductivity methods for measuring the void fraction in gas-liquid multiphase systems are typically affected by accuracy problems due to the presence of fluid flow and salinity.This study presents a novel approach for determining the void fraction based on a reciprocating dynamic conductivity probe used to measure the liquid film thickness under forced annular-flow conditions.The measurement system comprises a cyclone,a conductivity probe,a probe reciprocating device,and a data acquisition and processing system.This method ensures that the flow pattern is adjusted to a forced annular flow,thereby minimizing the influence of complex and variable gas-liquid flow patterns on the measurement results;Moreover,it determines the liquid film thickness solely according to circuit connectivity rather than specific conductivity values,thereby mitigating the impact of salinity.The reliability of the measurement system is demonstrated through laboratory experiments.The experimental results indicate that,in a range of gas phase superficial velocities 5–20 m/s and liquid phase superficial velocities 0.079–0.48 m/s,the maximum measurement deviation for the void fraction is 4.23%.

VIBRATION AND STABILITY OF VERTICAL UPWARD-FLUID-CONVEYING PIPE IMMERSED IN RIGID CYLINDRICAL CHANNEL

A theoretical model is developed for the vibration and stability of a vertical pipe subjected concurrently to two dependent axial flows. The external fluid, after exiting the outer annular region between the pipe and a rigid cylindrical channel, is conveyed upwards inside the pipe. This configuration thus resembles of a pipe that aspirating fluid. The equation of planar mo- tion is solved by means of the differential quadrature method （DQM）. Calculations are conducted for a slender drill-string-like and a bench-top-size system, for different confinement conditions of the outer annular channel. It is shown that the vibrations of these two systems are closely related to the degree of confinement of the outer annular channel. For a drill-string-like system with narrow annuli, buckling instability may occur in the second and third modes. For a bench-top-size system, however, both buckling and flutter may occur in the lowest three modes. The form of instability depends on the annuli size.

Liquid film dryout model for predicting critical heat flux in annular two-phase flow

Gas-liquid two-phase flow and heat transfer can be encountered in numerous fields, such as chemical engineering, refrigeration, nuclear power reactor, metallurgical industry, spaceflight. Its critical heat flux (CHF) is one of the most important factors for the system security of engineering applications. Since annular flow is the most common flow pattern in gas-liquid two-phase flow, predicting CHF of annular two-phase flow is more significant. Many studies have shown that the liquid film dryout model is successful for that prediction, and determining the following parameters will exert predominant effects on the accuracy of this model: onset of annular flow, inception criterion for droplets entrainment, entrainment fraction, droplets deposition and entrainment rates. The main theoretical results achieved on the above five parameters are reviewed; also, limitations in the existing studies and problems for further research are discussed.

Error Analysis of Liquid Holdup Measurement in Gas-Liquid Annular Flow Through Circular Pipes Using High-Speed Camera Method

Accurate measurement of gas-liquid phase fraction is essential for the proper modelling of the pressure drop, heat transfer coefficient, mass transfer rate and interfacial area in two-phase flows. In this paper, taking the issue of optical distortion into account, an analytical model was proposed to estimate and correct the liquid holdup in gas-liquid annular flow through a circular pipe using high-speed camera method. The error in the liquid holdup measurement generated from different refractive indices among transparent circular pipe, liquid film and air core was firstly theoretically analyzed based on the geometric optics. Experimental tests were then carried out to identify the difference as well as to validate the proposed model. Results indicated that the prediction of the liquid holdup has a good performance with the experimental data(i.e., mean relative error is 4.1%) and the measured liquid holdup is larger than the real one. It was found that the measured liquid holdup is larger than the real one. Generally, when the real liquid holdup gets smaller, the discrepancy between the measured liquid holdup by image and the real liquid holdup becomes more significant. Thus, after measuring the liquid holdup from the images, the value of the measured liquid holdup must be corrected by the present model in order to obtain the real liquid holdup.

Numerical Analysis of Flow Instability in the Water Wall of a Supercritical CFB Boiler with Annular Furnace

In order to expand the study on flow instability of supercritical circulating fluidized bed(CFB) boiler,a new numerical computational model considering the heat storage of the tube wall metal was presented in this paper.The lumped parameter method was proposed for wall temperature calculation and the single channel model was adopted for the analysis of flow instability.Based on the time-domain method,a new numerical computational program suitable for the analysis of flow instability in the water wall of supercritical CFB boiler with annular furnace was established.To verify the code,calculation results were respectively compared with data of commercial software.According to the comparisons,the new code was proved to be reasonable and accurate for practical engineering application in analysis of flow instability.Based on the new program,the flow instability of supercritical CFB boiler with annular furnace was simulated by time-domain method.When 1.2 times heat load disturbance was applied on the loop,results showed that the inlet flow rate,outlet flow rate and wall temperature fluctuated with time eventually remained at constant values,suggesting that the hydrodynamic flow was stable.The results also showed that in the case of considering the heat storage,the flow in the water wall is easier to return to stable state than without considering heat storage.

Large eddy simulation of turbulent flow structure and characteristics in an annular jet pump

The large eddy simulation（LES） of the flow characteristics in an annular jet pump（AJP） is conducted, and the flow characteristics are systematically analyzed from both time-averaged and instantaneous aspects. The jet expansion, the velocity distribution and the energy are considered to analyze the time-averaged evolution of the flow field in the AJP. The transient flow characteristics can also be acquired from the analysis of the turbulence intensity and the Reynolds stress. The simulation demonstrates that in the time-averaged characteristics, the potential cores increase linearly with the increase of the flow ratio. With the flow development, the jet half-width gradually increases and the residual energy coefficient decreases. Compared with the distribution of the time-averaged axial velocity, that of the instantaneous velocity is more complex and disorderly. The high intensity of the axial turbulence mainly occurs in the mixing layer and the near-wall regions of the diffuser. The annular distribution of the Reynolds stress is mainly in the mixing layer and the recirculation region. There is a low-stress zone between the mixing layer and the high-stress region in the wall-boundary layer. The intensity of the spanwise vortexes is larger than that of the streamwise vortexes, and therefore, the former make greater contribution to the total vorticity. This research provides a better understanding of the flow characteristics in the AJP.

Experimental Study on the Characteristics of Liquid Layer and Disturbance Waves in Horizontal Annular Flow

The mechanism for transporting liquid from the bottom of the pipe to the top still to be established in the prediction of the film thickness distribution in horizontal annular two-phase flow. To resolve this issue, using five parallel-wire conductance probes, time records of local liquid film thickness at five circumferential positions were collected. The characteristics of circumferential liquid film thickness profiles and its variation with gas and liquid velocities were obtained. The basic features of probability distribution function, probability density function, auto-correlation, cross-correlation and power spectrum density function of the disturbance waves in angular flow were studied respectively. The characteristics of circumferential profiles of disturbance waves and its variation with gas and liquid velocities were presented.

Numerical study of elbow erosion due to sand particles under annular flow considering liquid entrainment

Sand particle erosion is always a challenge in natural gas production.In particular,the erosion in gas-liquid-solid annular flow is more complicated.In this study,a three-phase flow numerical model that couples the volume of fluid multiphase flow model and the discrete phase model was developed for prediction of erosion in annular flow.The ability of the numerical model to simulate the gas-liquid annular flow is validated through comparison with the experimental data.On the basis of the above numerical model,the phase distribution in the pipe was analyzed.The liquid entrainment behavior was reasonably simulated through the numerical model,which guaranteed the accuracy of predicting the particle erosion.Additionally,four erosion prediction models were used for the erosion calculation,among them,the Zhang et al.erosion model predicted the realistic results.Through the analysis of the particle trajectory and the particle impact behavior on the elbow,the cushion effect of the liquid film on the particles and the erosion morphology generation at the elbow were revealed.

Research on vibroacoustical characteristicsof an annular flow duct system

The vibroacoustical characteristics of an annular flow duct system, which consists of two infinite concentric cylindrical elastic thin steel shells (hereafter referred to as a double shell simply) coupled by the entrained axial uniform water flow, is investigated. By using a fully coupled model between fluid and shells, the dispersion behaviour is analyzed for the system, vibrating in the n = 0 and n = 1 circumferential modes. The impact of the flow on the dispersion curves is also discussed for the system.

Condensation Heat Transfer Inside a Tube in a Microgravity Environment

This paper introduces a method for studying condensation heat transfer inside a tube in a microgravity environment. The model assumes laminar flow in the condensate film and an annular flow pattern. The local heat transfer coefficient is then calculated by gravitational acceleration, gi from 0 to 9.8 m/s2.The model was tested indirectly by measuring condensation heat transfer inside a vertical tube in a normal gravity environment through experiments.

A simple analytical model to predict liquid unloading in the horizontal gas well

An accurate prediction of the critical gas velocity for the liquid loading is of great importance for operators to select the tubing diameter for the newly drilled gas wells,or to optimize the production rate for production wells.It is clear from previous experimental studies of the liquid entrainment rate in the gas core that the liquid is mainly carried in the form of film under the critical condition of the liquid loading onset.It is more reasonable to establish a mathematical model based on the film reversal rather than based on the droplet reversal.In our previous paper entitled“Prediction of the critical gas velocity of liquid unloading in the horizontal gas well”,a new analytical model was established based on the force balance between the gas-liquid interfacial friction force and the bottom film gravity,but the model is not very convenient to use because of the complexity of calculating the average film thickness.In the present study,a new method is proposed to calculate the average film thickness from the bubble drift velocity in the mixture,so the new analytical model becomes much easier to use.The new analytical model is evaluated against 103 sets of experimental data,the data in 124 vertical gas wells and one horizontal gas well.Meanwhile,the effect ofthe liquid loading on the production of the horizontal gas well is also analyzed.

Dispersion equation of a double shell-fluid coupled system

The dispersion behaviour of a double shell-fluid system, which consists of two thin concentric cylindrical elastic shells coupled by the entrained annular fluid, is a fundamental for investigation of its vibroacoustical characteristics. Based on Flugge's infinite shell equations,the sound wave equation for the fluid field and boundary conditions at the fluidstructure interfaces, the dispersion equationfor the system is deduced and a corresponding numerical example is given in the paper.