Shear dispersion along circular pipes is affected by bends, but the torsion of the pipe is negligible

The flow of a viscous fluid along a curving pipe of fixed radius is driven by a pressure gradient. For a generally curving pipe it is the fluid flux which is constant along the pipe and so I correct fluid flow solutions of Dean (1928) and Topakoglu (1967) which assume constant pressure gradient. When the pipe is straight, the fluid adopts the parabolic velocity profile of Poiseuille flow; the spread of any contaminant along the pipe is then described by the shear dispersion model of Taylor (1954) and its refinements by Mercer, Watt et al (1994,1996). However, two conflicting effects occur in a generally curving pipe: viscosity skews the velocity profile which enhances the shear dispersion; whereas in faster flow centrifugal effects establish secondary flows that reduce the shear dispersion. The two opposing effects cancel at a Reynolds number of about 15. Interestingly, the torsion of the pipe seems to have very little effect upon the flow or the dispersion, the curvature is by far the dominant influence. Lastly, curvature and torsion in the fluid flow significantly enhance the upstream tails of concentration profiles in qualitative agreement with observations of dispersion in river flow

On the effects of centrifugal forces in air-water two-phase flow regime transitions of an adiabatic helical geometry

Two-phase flow in helical conduits is important in many industries where reaction between components, heat transfer, and mass transport are utilized as processes. The helical design is chosen for the effects of secondary flow patterns that reduce axial dispersion, increased heat transfer, and also their compact design. The first is a result of the secondary flow, which continually transports fluid from the near wall region to the bulk of the flow. In single-phase chemical reactor design this secondary flow increases radial mixing and reduces axial dispersion. In heat exchanger design it increases laminar heat transfer while extending the Reynolds number range of laminar flow. A literature review of the work on helical pipe flow shows that the vast majority of the work is on toroidal single-phase flow, and analyses of two-phase flow are sparse. This dissertation addresses this void by presenting an analytical model of the stratified and annular flow regime transitions in helical conduits, by consideration of the governing equations and mechanisms for transition in the toroidal geometry including the major impact of pitch. Studies have taken a similar approach for straight inclined horizontal and vertical geometries, but none have been found which resolve two-phase flow in the curved geometry of a helix. The main issue in resolving the flow in this geometry is that of determining appropriate inter-phase momentum transfer, and the appropriate friction correlations for wall interaction. These issues are resolved to yield a novel attempt at modeling helical two-phase flow. Pitch is considered negligible in introduction of torsion, while the dominating influence of the centrifugal force is retained. The formulation of the governing equations are taken from a general vector form that is readily extended to a true helix that includes torsion. The predictive capability of the current model is compared to the data and observations of the two-phase helical flow studies available in the open literature. The new model is found to be accurate in the linear asymptote, and to correctly predict the trends of increased liquid hold-up, a shift in the transition boundary between non-stratified and stratified flows such that the non-stratified regimes are favored, and the new liquid equilibrium height calculations shift the transition between annular and intermittent flows such that the intermittent regime is favored. The current model is an improvement over the previous methods in that it has the same accuracy of prediction of linear flowing inclined flows as methods developed for the linear flow condition, and improves the prediction of curved flow regimes by correctly shifting the boundaries as described above

Swirling pipeflow of non-Newtonian and particle-laden fluids

This thesis describes the application of novel swirl inducing pipe to various pipe configurations, when pumping a range of fluids and fluid / particle mixtures. An extensive experimental programme, incorporating particle image velocimetry and photography, was implemented using a pipe flow loop designed specifically for the purpose. Experimental data was obtained on the effect of a 4-lobe near-optimal swirl pipe on coal-water, sand-water and magnetite-water slurries of various particle size. Results indicated that swirl induction produced greater benefit for denser slurries and higher concentrations, and that swirl induced into slurries containing larger and denser particles decayed more rapidly. At low velocity, experimental data highlighted a reduction in the total pressure drop experienced across a 3.0m horizontal pipe section, a downward sloping section and vertical pipe bends, when the swirl-inducing pipe was present. PIV was used to measure the axial and tangential velocity of swirling flows downstream of a near-optimal swirl-inducing pipe. It was confirmed that a significant tangential velocity was generated when pumping water in the turbulent regime, however, when the fluid viscosity was increased, leading to laminar flow, no significant tangential velocity was detected

Experimental measurement and modelling of heat transfer in spiral and curved channels

Heat transfer enhancement is desired in most thermal applications. In general, there are two methods to improve the heat transfer rate: active and passive techniques Active techniques are based on external forces such as electro-osmosis, magnetic stirring, etc. to perform the augmentation. Active techniques are effective; however, they are not always easy to implement with other components in a system. They also increase the total cost of the system manufacturing. On the other hand, passive techniques employ fluid additives or special surface geometry. Using the surface geometry approach is easier, cheaper and does not interfere with other components in the system. Surface modification or additional devices incorporated in the stream are two passive augmentation techniques. With these techniques, the existing boundary layer is disturbed and the heat transfer performance is improved. However, pressure drop is also increased. Curved geometry is one of the passive heat transfer enhancement methods that fit several heat transfer applications such as: compact heat exchangers, steam boilers, gas turbine blades, electronics cooling, refrigeration and etc. This dissertation contains eight chapters.. Chapter one is the introduction and shows the originality, novelty and importance of the work. Chapter two reviews the literatures on the heat transfer and the pressure drop correlations in curved circular tubes. In chapters three and four, two heat sinks having spiral and straight channel geometry engraved on them are examined experimentally. Heat transfer and pressure drop inside them are measured, and reduced to apply two existing correlations to predict their behaviour analytically. In chapters five, six and seven, thermal and flow behaviour inside curved geometry are studied experimentally. The calculated heat transfer coefficient and pressure drop are compared to the existing models. Comparing the predicted Nusselt number from the existing models, poor accuracy was observed in the region of 5 < Pr < 15. Finally, in chapters six and seven two new asymptotic correlations are proposed to calculate the heat transfer and the pressure drop inside mini scale curved and coiled tubes

Models for designing pipe-grade polyethylenes to resist rapid crack propagation

Plastic pipeline systems have now become dominant for fuel-gas and water distribution networks. Although they have an impressive service record failures do occur, with Rapid Crack Propagation being characterised as the least probable but most potentially catastrophic one. This study investigates the effect of structural morphology and bulk residual strains on the RCP performance of polyethylene pipes, and proposes a new methodology for predicting a safe service envelope. During crack propagation in PE pipes, the fracture surface has two distinct regions; plane strain and plane stress. In addition to the Instrumented Charpy, Reversed Charpy, High Speed Double Torsion, Dynamic Mechanical Analysis and uniaxial tensile testing, S4 tests of extruded pipe specimens were employed in order to evaluate the structural and fracture parameters of pipe grade resins in these two fracture modes on pipe. A new experimental technique, which modified the pipe bore crystallinity without altering the residual strain field (as evaluated from slit ring tests) showed that the bore surface layer properties had much less influence on RCP than previously thought. Parallel with the experimental work, modeling of the fracture mechanisms was also undertaken. Using previous models in the field, such as the adiabatic decohesion model, the plane strain fracture toughness was evaluated while the plane stress fracture toughness was evaluated either from the Reversed Charpy or from the stability of adiabatic drawing in a tensile test. A mixed mode, temperature sensitive toughness was finally evaluated, leading to an overall fracture properties assessment for polyethylene pipes which could be compared directly to the crack driving force during RCP in pipe. By employing a new mathematical approach, which incorporated both the effects of residual strains and pipe stiffness behind the pressure decay length, a previous basic analytical RCP model was further developed and compared to more elaborate finite element and finite volume solutions. The new results were also compared to S4 experiments using high-speed photography and showed that the new methodology could be employed by the end user even when testing facilities are not directly availabl

Fluid-structure interaction during hydraulic transients in pressurized pipes:experimental and numerical analyses

The aim of the present research is to identify, describe and quantify the principal mechanic-hydraulic relationships during hydraulic transients in pressurized pipe flows in view of improving pipe design and reduce pipe and system failure. Phenomena affecting the transient wave, such as fluid-structure interaction, unsteady skin friction, dry friction or pipe-wall viscoelasticity are analysed from both the experimental and numerical standpoints. The main goal is the improvement of one-dimensional (1D) waterhammer modelling in the time-domain by means of the well-known method of characteristics approach. Experimental work is presented for three different experimental facilities: a straight copper pipe, a coil copper pipe and a coil polyethylene pipe. The analysis of the experimental data highlights differences in the response of each system in terms of wave shape, damping, and dispersion. The straight copper pipe behaviour is highly dependent on the pipe supports and anchoring; the coil copper pipe to the deformation in the radial direction; while the polyethylene facility to the pipe-wall viscoelasticity. In a second stage, the research focuses on the numerical modelling of hydraulic transients in pipe coils. The analysis is based on the experimental data collected in the coil copper pipe facility. First, a structural analysis is carried out for static conditions and then for dynamic. A four-equation model is implemented incorporating the main interacting mechanisms: Poisson, friction and junction coupling. The model is successfully validated for different flow rates showing a good performance of the dynamics of the coil behaviour during hydraulic transients. Finally, the research focuses on the straight copper pipe facility, for which the simplicity of the set-up allows deepening on the basic modelling assumptions in fluid-structure interaction. First, friction coupling is assessed using the basic four-equation model and unsteady skin friction and dry friction are incorporated in the solver. The analysis shows the dissipative effect of dry friction phenomenon, which complements that of skin friction. In a second approach junction coupling is tackled and the resistance to movement due to inertia and dry friction of the pipe anchor blocks is analysed. Numerical results successfully reproduce laboratory measurements for realistic values of calibration parameters. The work successfully identifies, describes and quantifies different physical phenomena related with FSI by means of experimental modelling and valid numerical reproduction of experimental results. Experimental modelling approaches are developed and data is made available for benchmark testing of numerical tools considering facilities with different set-up geometries and materials. A new standpoint based on pipe-degrees-of-freedom is suggested for facing FSI problems, the structural behaviour of pipe coils is successfully described and FSI in straight pipelines is analysed focusing on both junction and friction coupling. A new set of numerical solvers are developed, presented and thoroughly discussed, which can be readily used for the design of new industrial piping systems or the safety assessment of existing piping facilities

Continuum Mechanics

Continuum Mechanics is the foundation for Applied Mechanics. There are numerous books on Continuum Mechanics with the main focus on the macroscale mechanical behavior of materials. Unlike classical Continuum Mechanics books, this book summarizes the advances of Continuum Mechanics in several defined areas. Emphasis is placed on the application aspect. The applications described in the book cover energy materials and systems (fuel cell materials and electrodes), materials removal, and mechanical response/deformation of structural components including plates, pipelines etc. Researchers from different fields should be benefited from reading the mechanics approached to real engineering problems

Experimental characterizatin of axial dispersion in coiled flow inverters

Narrow residence time distributions (RTDs) are extremely desirable in many chemical engineering processes where plug flow behaviour is requested. However, at low Reynolds numbers the flow is laminar resulting in strong radial velocity gradients. This in turn causes spreading of fluid particles, usually referred to as hydrodynamic dispersion. Such problem is particularly relevant to microfluidic devices operated in laminar regime due to the reduced dimension and low operating flow rates. Many solutions have been proposed to reduce the hydrodynamic dispersion: static mixers, segmented flow, secondary flow, etc. The latter relies on the action of centrifugal force inducing transversal mixing in helically coiled tubes. Further mixing and therefore reduced dispersion can be achieved by introducing geometrical disturbances, generating chaotic advection. Coiled flow inverters (CFI) exploit the beneficial effects of secondary flow and chaotic advection. They consist of sections of helically coiled tubes with 90-degree bends placed at regular intervals along a cylindrical support. Despite being a very promising solution, they have not been extensively adopted. This is due to the lack of experimental data and correlations relating the design parameters and operating conditions to the reduction of hydrodynamic dispersion. In this thesis, a flexible and reliable experimental procedure was developed to investigate RTD in microfluidic devices. It resorts to step input injections and UV-vis inline spectroscopy for detecting the concentration of tracer. The procedure was validated using Taylor’s dispersion for straight tubes. The platform was then employed to perform experiments on CFIs, constructed with microfluidic capillaries, varying operating conditions and a geometrical parameter. A similar characterization was carried out on helically coiled tubes. A significant reduction of axial dispersion was observed as compared to straight pipes, confirming the available data in the literature. It was also demonstrated that the curvature ratio primarily defines the strength of radial mixing in CFIs and therefore represents a crucial design parameter