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Annular flow is an important flow regime in many industrial applications. The need for a better understanding of this flow regime is driven by the desire to improve the design of many terrestrial and space-based systems. Annular two-phase flow is frequently present in the drilling, production and transportation of oil and natural gas, boilers and condensers, and in heating and refrigeration systems. The flow regime is also important for the refueling of space vehicles, and heating and refrigeration systems for space use. Past studies on annular flow have dealt with varying the gas or liquid Reynolds numbers and studying the effect of such changes on the flow regimes and pressure drops. The effect of two other relevant dimensionless groups, namely the gas-to-liquid density ratio and the gas-to-liquid viscosity ratio, on the film characteristics are noticeably absent. As well, with the increased interest in the space environment, studies on the effect of the gravitational acceleration on two-phase flow would be beneficial. The effect of the gas density and the gravitational acceleration on the annular flow average film thickness and frictional pressure drop are examined. The film thickness was measured using two-wire conductance probes. Experimental data were collected in microgravity and hypergravity aboard the Novespace Zero-G Airbus microgravity simulator and normal gravity data were collected at the University of Saskatchewan. Data were collected for a range of annular flow set points by changing the liquid and gas mass flow rates. The liquid-to-gas density ratio was examined by collecting annular flow data using helium-water and air-water. The gravitational effect on the film thickness characteristics was examined by collecting the data during the microgravity and pull-up (hypergravity) portions of each parabolic flight. A direct comparison is possible between the normal gravity data and the microgravity data, due to the matching of the liquid and gas mass flow rates and the flow regime. The reduction in gravity causes the average film thickness to increase between two and four times from the normal gravity values. The microgravity average frictional pressure drop is within approximately 20% of the normal gravity pressure drop for the same flow conditions. For all gravity levels, the air-water and the helium-water flows give similar results, for both average film thickness and frictional pressure drop, when based on the specific energy of the gas. The hypergravity average film thickness results are larger than at normal gravity for the same flow conditions. However, no flow regime map exists for the hypergravity condition, so the similarity of the flow regime cannot be confirmed. The hypergravity flow appears more chaotic, and may be in the transition from a churn type flow. The average frictional pressure drop is increased by approximately 20% due to the increase in the gravitational acceleration. New non-dimensional equations, which include the effect of the gas density, are presented for each gravity level to predict the average film thickness and the average frictional pressure drop.

Annular flow is an important flow regime in many industrial applications. The need for a better understanding of this flow regime is driven by the desire to improve the design of many terrestrial and space-based systems. Annular two-phase flow is frequently present in the drilling, production and transportation of oil and natural gas, boilers and condensers, and in heating and refrigeration systems. The flow regime is also important for the refueling of space vehicles, and heating and refrigeration systems for space use. Past studies on annular flow have dealt with varying the gas or liquid Reynolds numbers and studying the effect of such changes on the flow regimes and pressure drops. The effect of two other relevant dimensionless groups, namely the gas-to-liquid density ratio and the gas-to-liquid viscosity ratio, on the film characteristics are noticeably absent. As well, with the increased interest in the space environment, studies on the effect of the gravitational acceleration on two-phase flow would be beneficial. The effect of the gas density and the gravitational acceleration on the annular flow average film thickness and frictional pressure drop are examined. The film thickness was measured using two-wire conductance probes. Experimental data were collected in microgravity and hypergravity aboard the Novespace Zero-G Airbus microgravity simulator and normal gravity data were collected at the University of Saskatchewan. Data were collected for a range of annular flow set points by changing the liquid and gas mass flow rates. The liquid-to-gas density ratio was examined by collecting annular flow data using helium-water and air-water. The gravitational effect on the film thickness characteristics was examined by collecting the data during the microgravity and pull-up (hypergravity) portions of each parabolic flight. A direct comparison is possible between the normal gravity data and the microgravity data, due to the matching of the liquid and gas mass flow rates and the flow regime. The reduction in gravity causes the average film thickness to increase between two and four times from the normal gravity values. The microgravity average frictional pressure drop is within approximately 20% of the normal gravity pressure drop for the same flow conditions. For all gravity levels, the air-water and the helium-water flows give similar results, for both average film thickness and frictional pressure drop, when based on the specific energy of the gas. The hypergravity average film thickness results are larger than at normal gravity for the same flow conditions. However, no flow regime map exists for the hypergravity condition, so the similarity of the flow regime cannot be confirmed. The hypergravity flow appears more chaotic, and may be in the transition from a churn type flow. The average frictional pressure drop is increased by approximately 20% due to the increase in the gravitational acceleration. New non-dimensional equations, which include the effect of the gas density, are presented for each gravity level to predict the average film thickness and the average frictional pressure drop.

Here we seek to identify the conditions for the collapse of the molten metal layer surrounding a keyhole filled with vapor and liquid particles during high power density laser and electron beam welding processes. Investigating the collapse of the liquid layer is essential for a fundamental understanding of pore formation in the keyhole mode welding. We treat the collapse of the keyhole as similar to a transition between the slug and annular two-phase flows in a vertical pipe of varying cross-section. A quasi-steady, one-dimensional model for two-phase flow is developed and solved assuming that the mixture in the core is homogenous. Ignoring friction within the liquid layer and considering supersonic flow in the keyhole, the two phase flow regimes can be divided into four regions characterized by entrainment and deposition of liquid particles. Keyhole collapse occurs from entrainment, whereas the keyhole exhibits wavy shape from deposition. A condition for the formation of macro-porosity based on a fundamental understanding of annular two-phase flow is presented.

<p>The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.</p>

In this experimental and numerical investigation, the use of flow turbulation was considered as a method to increase local heat transfer coefficients in annular heat transfer passages. Experimental data was obtained for cases with and without inserted ring turbulators within a horizontal annular test section using water for average Reynolds numbers ranging from 2000 to 7500 and average Prandtl numbers ranging from 6.73 to 6.79. The test section was heated uniformly on the inner annular wall and had a hydraulic diameter of 14.8mm, a diameter ratio (inner wall diameter to outer wall diameter) of 0.648, and a length to hydraulic diameter ratio of approximately 74. A set of circular cross sectioned ring-type turbulators were used which had a thickness of 1mm, a ring diameter of 15.1mm and a pitch of 50mm. It was found that the presence of the flow turbulators increased the average Nusselt number by between 33.9% and 45.8%. The experimental tests were followed by numerical simulations to identify the response in the heat transfer coefficient by changing the geometry of the turbulators. For this, the turbulator diameters were ranged from 0.5 mm to 2 mm, and the gap size (between the inner wall and a turbulator ring) ranged from 0.125 mm to 4 mm at a pitch of 50 mm. The results showed that the use of turbulators increased the Nusselt numbers by a maximum of 34.8% and that the maximum can be achieved for a turbulator diameter of 2 mm and a gap size of 0.25 mm, for all the Reynolds numbers tested. From the numeric determined pressure drop values it was found that the smaller gap size had the lowest pressure drop and the smallest turbulators also produced the lowest pressure drop. / Dissertation (MEng)--University of Pretoria 2020 / South African Centre for High Performance Computing (CHPC) / Mechanical and Aeronautical Engineering / MEng / Unrestricted

This master thesis aim to model the annular flow that occurs in the final section between the fuel rods inside Boiling Water Reactors, by approximating the geometry to a cylindrical pipe. Simulations were performed in the software ANSYS Fluent, as a step in the development of replacing the 1D correlations currently used in the nuclear industry with CFD models in 3D. An Eulerian-Lagrangian approach was used for the three fields of steam, liquid film and liquid droplets in the model. Entrainment was modeled based on 1D correlations from Okawa [7] and deposition with the built in Discrete Phase Model in ANSYS Fluent. The work focused on making the process less time consuming, and increasing accuracy of the model by comparing the results with empirical data based on experimental values. A transverse velocity was applied on the droplets at the point of entrainment with better correlating results with the Okawa model.

<p>To fully realize the advantages of the two-fluid model, the interfacial area concentration (IAC) should be properly given by a constitutive model. The conventional flow-regime-based IAC correlations intrinsically cannot predict the dynamic flow structure change and would introduce a discontinuity and numerical instability to system codes. As a promising alternative, the interfacial area transport equation (IATE) is developed to model the interface structure mechanistically. Progress has been achieved for IATE modeling in bubbly, slug, and churn-turbulent flow during the past two decades. Aiming at a comprehensive flow structure predictor for all flow regimes, further development in two directions is highly desirable. First is extending the current experiment and modeling capability from churn-turbulent to annular flow. In this study, an advanced four-sensor droplet capable conductivity probe (DCCP-4) is developed to capture all interfaces in churn-turbulent and annular flow, including liquid film, liquid droplet, gas core, and gas bubble. A first of a kind experimental database in churn-turbulent, annular, and wispy annular flow with two-dimensional spatial distributions is established, which provides the experimental basis for the multi-field two-phase flow model development. The measured parameters include local time-averaged volume faction, IAC, and velocity for various fields of annular flow. In addition, a new constitutive model to quantify the interfacial area between the gas core and liquid film of annular flow is developed, which fills the last theoretical gap of interfacial area modeling. The other important direction is improving the current IATE model to fulfill the dynamic prediction of developing flow, especially the bubbly to slug transition flow. Vertical-upward air-water two-phase flow experiments are performed. The state-of-the-art IATE model is evaluated against the newly collected data at bubbly and slug flow, and the result shows unsatisfactory performance in predicting the developing flow with intensive bubble coalescence. A new bubble coalescence model is derived by using the log-normal bubble size distribution, which significantly improves the model prediction capability.</p>

<p>The main purpose of this research is to investigate liquid entrainment mechanisms of annular flow by computational fluid dynamics (CFD) techniques. A numerical model is developed. The model is based on the physics of an upward annular flow. In the modeling, a transient renormalization group (RNG) k-å model in conjunction with enhanced wall treatment method was employed. In order to reconstruct the two-phase interface, the geometric reconstruction scheme of volume of fluid (VOF) model was adopted. Fluent® 6.18 was used as the solution tool. Simulation results indicated that disturbance waves were generated first on the two-phase interface and their evolution eventually resulted in the liquid entrainment phenomena. The most significant accomplishment of this work is that details of the entrainment mechanisms are well described by the numerical simulation work. In addition, two new entrainment mechanisms are presented. One entrainment mechanism demonstrates that the evolution of individual waves causes the onset of liquid entrainment; the other mechanism shows that the coalescence of two adjacent waves (during the course of their evolution) plays an important role in the progression of liquid entrainment. The newly developed entrainment mechanisms are based on conservation laws. In order to explore the flow physics of the targeted annular flow, the law of the wall, in conjunction with an analytical model based on a force balance, was applied to previously collected experimental data. Results indicated that the film flow had strong features of near-wall flow. In addition, based on prior experimental work and a newly developed physical wave model by researchers in the Microgravity Research Group, University of Saskatchewan, a steady RNG k-å model, in conjunction with the enhanced wall treatment method, was applied to the gas core. The simulation results showed turbulent flow features in the gas core and strong effects of the interfacial waves on the simulation results. The above information forms the physical foundation for the simulation work on the entrainment mechanism.</p><p>One significant contribution to the authors research group is the liquid entrainment fraction data. A new method was introduced to make the measurements. The method combined a chemically-based titration method with a newly-designed instrument, a separator, to effectively measure the entrainment fraction. Experiments were conducted at low system pressure (~ 1 atm) and relatively low gas and liquid superficial velocities (Vsg = 25.8 m/s to 45.5 m/s, and Vsl = 0.15 m/s to 0.30 m/s, respectively). The entrainment fraction was found to be under 7 %, with a maximum uncertainty of 0.26 % for all the experimental set points. Repeatability test results and comparisons with previous entrainment data indicated that the new technique can perform as well as other measurement techniques.</p>

<p>The main purpose of this research is to investigate liquid entrainment mechanisms of annular flow by computational fluid dynamics (CFD) techniques. A numerical model is developed. The model is based on the physics of an upward annular flow. In the modeling, a transient renormalization group (RNG) k-å model in conjunction with enhanced wall treatment method was employed. In order to reconstruct the two-phase interface, the geometric reconstruction scheme of volume of fluid (VOF) model was adopted. Fluent® 6.18 was used as the solution tool. Simulation results indicated that disturbance waves were generated first on the two-phase interface and their evolution eventually resulted in the liquid entrainment phenomena. The most significant accomplishment of this work is that details of the entrainment mechanisms are well described by the numerical simulation work. In addition, two new entrainment mechanisms are presented. One entrainment mechanism demonstrates that the evolution of individual waves causes the onset of liquid entrainment; the other mechanism shows that the coalescence of two adjacent waves (during the course of their evolution) plays an important role in the progression of liquid entrainment. The newly developed entrainment mechanisms are based on conservation laws. In order to explore the flow physics of the targeted annular flow, the law of the wall, in conjunction with an analytical model based on a force balance, was applied to previously collected experimental data. Results indicated that the film flow had strong features of near-wall flow. In addition, based on prior experimental work and a newly developed physical wave model by researchers in the Microgravity Research Group, University of Saskatchewan, a steady RNG k-å model, in conjunction with the enhanced wall treatment method, was applied to the gas core. The simulation results showed turbulent flow features in the gas core and strong effects of the interfacial waves on the simulation results. The above information forms the physical foundation for the simulation work on the entrainment mechanism.</p><p>One significant contribution to the authors research group is the liquid entrainment fraction data. A new method was introduced to make the measurements. The method combined a chemically-based titration method with a newly-designed instrument, a separator, to effectively measure the entrainment fraction. Experiments were conducted at low system pressure (~ 1 atm) and relatively low gas and liquid superficial velocities (Vsg = 25.8 m/s to 45.5 m/s, and Vsl = 0.15 m/s to 0.30 m/s, respectively). The entrainment fraction was found to be under 7 %, with a maximum uncertainty of 0.26 % for all the experimental set points. Repeatability test results and comparisons with previous entrainment data indicated that the new technique can perform as well as other measurement techniques.</p>