Simulating unsteady conduit flows with smoothed particle hydrodynamics

Pipelines are widely used for transport and cooling in industries such as oil and gas, chemical, water supply and sewerage, and hydro, fossil-fuel and nuclear power plants. Unsteady pipe flows with large pressure variations may cause a range of problems such as pipe rapture, support failure, pipe movement, vibration and noise. The unsteady flow is generally caused by flow velocity changes due to valve or pump operation. Water hammer is the best known and extensively studied phenomenon in this respect. Fast transient may also occur in rapid pipe filling and emptying processes. Due to high driving heads, the advancing liquid column may achieve a high velocity. When this high-velocity column is blocked or restricted in its flow, high water-hammer pressures may result. Another scenario is that of slug flow, which arguably is the most dangerous type of two-phase pipe flow. Heavy isolated liquid slugs travelling at high speed behave like cannonballs. Damage is likely to happen when these slugs impact on barriers such as pumps, bends and partially closed valves. Advancing liquid columns occurring in rapid pipe filling and emptying can be seen as a special case of isolated slugs. In this thesis, we present a Lagrangian particle method for solving the Euler equations with application to water hammer, rapid pipe filling and emptying, and isolated slugs travelling in an empty pipeline. As a meshfree method, the smoothed particle hydrodynamics (SPH) used herein is suitable for problems encompassing moving boundaries and impact events, which are the common features of the concerned topics. We first present the kernel and particle approximation concepts, which are two essential steps in SPH. Based on numerical approximation rules, the SPH discrete form of the Euler and Navier-Stokes equations are derived. To treat various boundary conditions, we apply several types of image particles that are particularly designed to complete the kernels truncated by system boundaries. The global conservation of mass and linear momentum is then demonstrated. The SPH errors in the integral approximation and summation approximation are analyzed based on given particle distribution patterns. Several other problems such as particle clustering, tensile instability, particle boundary layer and lacking of polynomial reproducing abilities (incompleteness) are also discussed together with possible remedies. Before applying the implemented particle solver to the thesis topics, we first thoroughly test it against a selection of two-dimensionale benchmarks, which have close relationship with the concerned problems. They include dam-break, jet impinging onto an inclined plane, emerging jet under gravity, free overfall and flow separation at bends. Good agreements with analytical and numerical solutions in literature are found. The convergence rate of SPH is shown to be of first order, which is consistent with the theoretical analysis. For the rapid pipe filling problem, we apply the 1D SPH solver to the experiment of Liou & Hunt [114]. The velocity head at the inlet has to be taken into account to obtain a good agreement with the experiment. Water elasticity does not play a role and the friction formulation for steady state flows can be used. Head transition analysis provides deeper insight into the hydrodynamic behaviour in the filling process. As a special case of pipe filling, water hammer due to liquid impact at partially and fully closed valves is studied. The results agree well with standard MOC solutions. Similar observations are made for the rapid emptying process. For the isolated slug travelling in a voided pipeline and impacting on a bend, we apply the 1D and 2D SPH solvers to the experiments of Bozkus [24]. To obtain the arrival velocity of the slug at the elbow, a 1D model including mass loss at the slug tail is used. In the slug impact, flow separation at the bend plays a vital role, which is typical 2D flow behaviour at a geometrical discontinuity. With the flow contraction coefficient obtained from 2D SPH solutions, the improved 1D model gives good results for the reaction force, not only in magnitude but also its duration and shape. Finally, to study the evolutions of air/water interface and its possible effect on filling and emptying processes, a new experimental study is performed in a large-scale pipeline. It is found that in filling the water front tends to split into two fronts propagating with different velocities. This results in air intrusion on top of a water platform. In emptying, flow stratification occurs at the water tail. Consequently, the validated assumption of vertical air/water interfaces for small-scale system with high driving head may not be applicable to large-scale systems. The interface evolution does not play an important role in pipe filling, the overall behaviour of which can be well predicted with 1D SPH solutions. However, flow stratification largely affects the overall draining process

Pipe fractional flow theory : principles and applications

The contribution of this research is a simple, analytical mathematical modeling framework that connects multiphase pipe flow phenomena and satisfactorily reproduces key multiphase pipe flow experimental findings and field observations, from older classic data to modern ones. The proposed unified formulation presents, for the first time, a reliably accurate analytical solution for averaged (1D) multiphase pipe flow over a wide range of applications. The two new fundamental insights provided by this research are that: (a) macroscopic single-phase pipe flow fluid mechanics concepts can be generalized to multiphase pipe flow, and (b): viewing and analyzing multiphase pipe flow in general terms of averaged relative flow (or fractional flow) can lead to a unified understanding of its resultant (global) behavior. The first insight stems from our finding that the universal relationship that exists between pressure and velocity in single-phase flow can also be found equivalently between pressure and relative velocity in multiphase flow. This eliminates the need for a-priori flow pattern determination in calculating multiphase flow pressure gradients. The second insight signifies that, in general, averaged multiphase flow problems can be sufficiently modeled by knowing only the averaged volume fractions. This proves that flow patterns are merely the visual, spatial manifestations of the in-situ velocity and volume fraction distributions (the quantities that govern the transport processes of the flow), which are neatly captured in the averaged sense as different fractional flow paths in our proposed fractional flow graphs. Due to their simplicity, these new insights provide for a deeper understanding of flow phenomena and a broader capability to produce quantitative answers in response to what-if questions. Since these insights do not draw from any precedent in the prior literature, a science-oriented, comprehensive validation of our core analytical principles was performed. Model validation was performed against a diverse range of vapor-liquid, liquid-liquid, fluid-solid and vapor-liquid-liquid applications (over 74,000 experimental measurements from over 110 different labs and over 6,000 field measurements). Additionally, our analytical theory was benchmarked against other modeling methods and current industry codes with identical (unbiased), named published data. The validation and benchmarking results affirm the central finding of this research – that simple, suitably-averaged analytical models can yield an improved understanding and significantly better accuracy than that obtained with extremely complex, tunable models. It is proven that the numerous, continuously interacting (local) flow microphysics effects in a multiphase flow can be (implicitly) accounted for by just a few properly validated (global) closure models that capture their net (resultant) behavior. In essence, it is the claim of this research that there is an underlying simplicity and connectedness in this subject if looking at the resultant macroscopic (averaged) behaviors of the flow. The observed coherencies of the macroscopic, self-organizing physical structures that define the subject are equivalently present in the macroscopic mathematical descriptions of these systems, i.e., the flow-pattern-implicit, averaged-equations mixture models that describe the collective behavior of the flowing mixture.Chemical Engineerin

One-dimensional modelling of mixing, dispersion and segregation of multiphase fluids flowing in pipelines

The flow of immiscible liquids in pipelines has been studied in this work in order to formulate a one-dimensional model for the computer analysis of two-phase liquid-liquid flow in horizontal pipes. The model simplifies the number of flow patterns commonly encountered in liquid-liquid flow to stratified flow, fully dispersed flow and partial dispersion with the formation of one or two different emulsions. The model is based on the solution of continuity equations for dispersed and continuous phase; correlations available in the literature are used for the calculation of the maximum and mean dispersed phase drop diameter, the emulsion viscosity, the phase inversion point, the liquid-wall friction factors, liquid-liquid friction factors at interface and the slip velocity between the phases. In absence of validated models for entrainment and deposition in liquid-liquid flow, two entrainment rate correlations and two deposition models originally developed for gas-liquid flow have been adapted to liquid-liquid flow. The model was applied to the flow of oil and water; the predicted flow regimes have been presented as a function of the input water fraction and mixture velocity and compared with experimental results, showing an overall good agreement between calculation and experiments. Calculated values of oil-in-water and water-in-oil dispersed fractions were compared against experimental data for different oil and water superficial velocities, input water fractions and mixture velocities. Pressure losses calculated in the full developed flow region of the pipe, a crucial quantity in industrial applications, are reasonably close to measured values. Discrepancies and possible improvements of the model are also discussed. The model for two-phase flow was extended to three-phase liquid-liquid-gas flow within the framework of the two-fluid model. The two liquid phases were treated as a unique liquid phase with properly averaged properties. The model for three-phase flow thus developed was implemented in an existing research code for the simulation of three-phase slug flow with the formation of emulsions in the liquid phase and phase inversion phenomena. Comparisons with experimental data are presented

A study on high-viscosity oil-water two-phase flow in horizontal pipes

A study on high-viscosity oil-water flow in horizontal pipes has been conducted applying experimental, mechanism analysis and empirical modelling, and CFD simulation approaches. A horizontal 1 inch flow loop was modified by adding a designed sampling section to achieve water holdup measurement. Experiments on high-viscosity oil-water flow were conducted. Apart from the data obtained in the present experiments, raw data from previous experiments conducted in the same research group was collated. From the experimental investigation, it is found that that the relationship between the water holdup of water-lubricated flow and input water volume fraction is closely related to the oil core concentricity and oil fouling on the pipe wall. The water holdup is higher than the input water volume fraction only when the oil core is about concentric. The pressure gradient of water-lubricated flow can be one to two orders of magnitude higher than that of single water flow. This increased frictional loss is closely related to oil fouling on the pipe wall. Mechanism analysis and empirical modelling of oil-water flow were conducted. The ratio of the gravitational force to viscous force was proposed to characterise liquid-liquid flows in horizontal pipes into gravitational force dominant, viscous force dominant and gravitational force and viscous force comparable flow featured with different basic flow regimes. For viscous force dominant flow, an empirical criterion on the formation of stable water-lubricated flow was proposed. Existing empirical and mechanistic models for the prediction of water holdup and/or pressure gradient were evaluated with the experimental data; the applicability of different models is demonstrated. Three-dimensional CFD modelling of oil-water flow was performed using the commercial CFD code Fluent. The phase configurations calculated from the CFD model show a fair agreement with those from experiments and mechanism analysis. The velocity distribution of core annular flow is characterised with nearly constant velocity across the oil core when the oil viscosity is significantly higher than the water viscosity, indicating that the high-viscosity oil core flows inside the water as a solid body. The velocity profile becomes similar to that of single phase flow as the oil viscosity becomes close to the water viscosity

Microfluidic manipulation by AC Electrothermal effect

AC Electrokinetics (ACEK) has attracted much research interest for microfluidic manipulation for the last few years. It shows great potential for functions such as micropumping, mixing and concentrating particles. Most of current ACEK research focuses on AC electroosmosis (ACEO), which is limited to solutions with conductivity less than 0.02 S/m, excluding most biofluidic applications. To solve for this problem, this dissertation seeks to apply AC electrothermal (ACET) effect to manipulate conductive fluids and particles within, and it is among the first demonstration of ACET devices, a particle trap and an ACET micropump. The experiments used fluids at a conductivity of 0.224 S/m that is common in bio-applications. Pumping and trapping were demonstrated at low voltages, reaching ~100 um/s for no more than 8 Vrms at 200 kHz. The flow velocity was measured to follow a quadratic relationship with applied voltage which is in accordance with theory. This research also studies ACET effect on low ionic strength microfluidics, since Joule heating is ubiquitous in electrokinetic devices. One contribution is that our study suggested ACET as one possible reason of flow reversal, which has intrigued the researchers in ACEK field. Electrically, a microfluidic cell can be viewed as an impedance network of capacitances and resistors. Heat dissipation in those elements varies with AC frequency and fluid properties, so changes the relative importance of heat generation at the electrode/electrolyte interface and in the resistive fluid bulk, which could change the temperature gradient in the device, hence changing the flow direction. Another contribution of this dissertation is the reaction enhanced ACET micropumping. A dramatic improvement in flow rate over conventional ac micropumps is achieved by introducing a thin fluid layer of high ionic density near the electrodes. Such an ionic layer is produced by superimposing a DC offset on AC signal that induces Faradaic reaction. The velocity improvement, in some cases, is over an order of magnitude, reaching a linear velocity of up to 2.5 mm/s with only 5.4Vrms. This discovery presents an exciting opportunity of utilizing ACET effect in microfluidic applications

Development of boundary conditions for building drainage system components through novel numerical, laboratory and photogrammetric methods

Improvements in public health through better sanitary plumbing systems has been mainly due to the prevention afforded by barrier technologies to the ingress of foul air, which can contain toxic gases and pathogens, notwithstanding the nuisance of malodour. The main defence against this ingress is the ‘trap seal’ which comes in two forms; the ‘water trap seal’ and the ‘waterless trap seal’. Whilst these devices form effective barriers, they are vulnerable to, or can produce, transient air pressure fluctuations in the system which can lead to seal loss. Greater understanding of the characteristics of these devices is essential for the development of better protection strategies. The development of novel analytical techniques is central to this research as it increases computer model resolution at these important system extremities. Current methods employ a laboratory only approach, whereby a single loss co-efficient is developed. These laboratory derived boundary conditions are inherently static and in the case of the waterless trap seal, ignore structure flexibility. This research has produced new methodologies to evaluate performance and generate dynamic boundary conditions suitable for inclusion in an existing 1-D Method of Characteristics based model, AIRNET, which solves for pressure and velocity via the St. Venant equations of continuity and momentum in a finite difference scheme. The first novel technique developed uses photographic image and pressure data, transformed via photogrammetric and Fourier analysis to produce mathematical representations of the opening and closing of a waterless trap under transient pressures. The second novel technique developed focusses on the dynamic response of a water trap seal. Current boundary conditions use a steady state friction factor, ignoring separation losses. Analysis via ANSYS CFX allowed a frequency dependent dynamic representation of velocity change in the water trap seal to be developed, integrating unsteady friction and separation losses for the first time. Incorporation of these new boundary conditions in AIRNET confirms that frequency dependent whole system responses are possible and more realistic, reflecting both laboratory and on-site observations

Stability, Dynamics And Change-Of-State In Liquid Drop-Bridge Systems

A capillary based adhesion device motivates the study of coupled free-interface shapes and the transition from the drop to bridge shape. When a large number of drops, pinned at circular contact lines, are touched to a surface they form liquid bridges, and these bridges create an adhesive force. Alternatively, if the drops are not brought to the surface quickly enough the drops will coarsen, forming instead one large drop. Consider first the coarsening process. The dissipation occurs primarily in the conduits, the drop retain their equilibrium shape - the spherical cap. Drops scavenge volume from one another based on pressure differences, proportional to the surface tension, and arising from curvature differences. This process minimizes the total surface energy. All fixed points and their linear stabilities, obtained analytically, are found to be independent of connectivity. The system coarsens in the sense that, with time, volume is increasingly localized and ends up in a single 'winner' drop. To determine which of the stable fixed points will be the winner, manifolds separating the attracting regions are found using a method which combines local information (eigenvectors at fixed points) with global information (invariant manifolds due to symmetry). The coarsening rate is predicted heuristically, with the Lifshitz-Slyozov-Wagner (LSW) model and compared against numerical simulations for a variety of networks. Distributions of large drop volumes from LSW are independent of network topology; in contrast, simulation results depend weakly on the network dimension. When a pinned drop touches a solid surface it forms a liquid bridge; here the energy is dissipated within the bridge. The dissipated energy is equal to the loss of surface energy, which can also be expressed in terms of forces along the interface using a geometric relation. This energy balance provides an extra relation which determines the microscopic nature of the contact line. Boundary integral method simulations are used to compute the flow field and viscous bending of the free interface. The energy balance is applied to simulations to find slip lengths. The energy balance is used to bound the microscopic contact angle analytically

Analytical and numerical investigation of heat and mass transport in catalytic microreactors

Microreactors for chemical synthesis and combustion have attracted increased attention in recent years. Due to the high degree of thermal control exhibited by microreactors, exothermic catalytic activity features heavily in these devices and thus advective-diffusive transport is of key importance in their analyses. An analytical study of the transport phenomena in a single microchannel microreactor filled with a porous medium is presented, first as a one-dimensional, then expanded to a two-dimensional model with catalytic activity. The systems under investigation include fluid and porous solid phases inside the microchannel under local thermal non-equilibrium (LTNE), in addition to the enclosing structure of thick walls subject to distinct thermal loads. The thermal diffusion of mass, viscous dissipation of the fluid flow is examined for both a heterogeneous catalyst placed on the channel wall and for a homogeneous reaction process. The axial and transverse variations of heat and mass transfer processes are considered to provide two-dimensional solutions of both the temperature and concentration fields. These are then used to calculate the local and total entropy generation within the system. A novel extension of an existing LTNE model capable of taking into account the enclosing structure as well as the porous solid and fluid phases is presented. The thickness of this enclosing structure is shown to have a major influence on heat and mass transport within the system, particularly the Nusselt number. Irreversibilities in the system are found to be dominated by the mass transfer contributions and the influence of the Soret effect as well as the Damköhler number. A numerical investigation is undertaken to examine the effect of hydrodynamics upon the activity of a heterogeneous catalyst. Three corrugated wall channel configurations with varying phase difference between upper and lower walls were generated. Low Reynolds number flows of fuel lean methane in air were catalytically combusted over platinum in the numerical model. Hydrodynamic reflux features were observed in the out of phase cases. Coinciding with these zones, the site surface concentration of carbon dioxide was observed to vary significantly as compared to the base case. The extension of the LTNE model significantly increases the capability of modelling the interface between thick walls and a porous medium under thermal load, permitting more accurate modelling of microreactors. The effect of the reflux feature on the surface site fraction of product for the corrugated channels reveals the complex interaction between hydrodynamics and catalytic activity

Microgravity Science and Application Program tasks, 1989 revision

The active research tasks, as of the fiscal year 1989, of the Microgravity Science and Applications Program, NASA Office of Space Science and Applications, involving several NASA Centers and other organizations are compiled. The purpose is to provide an overview of the program scope for managers and scientists in industry, university, and government communities. The scientists in industry, university, and government communities. An introductory description of the program, the strategy and overall goal, identification of the organizational structures and people involved, and a description of each task are included. Also provided is a list of recent publications. The tasks are grouped into several major categories: electronic materials, solidification of metals, alloys, and composites; fluids, interfaces, and transport; biotechnology; glasses and ceramics; combustion science; physical and chemistry experiments (PACE); and experimental technology, facilities, and instrumentation