Chapter_LC67_MKT p1

Chapter_LC67_MKT p1 - Week 6&amp 7 Flow in Conduits Part 1 HES 2340 Fluid Mechanics 1 School of Engineering Computing&amp Science

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Unformatted text preview: Week 6 & 7 Flow in Conduits Part 1 HES 2340: Fluid Mechanics 1 School of Engineering, Computing & Science Sarawak Campus This chapter is devoted to an important practical fluids engineering problem: flow in ducts with various velocities, various fluids, and various duct shapes. Piping system are encountered in almost every engineering design and thus have been studied extensively. One of the basic piping problems is this: Given the pipe geometry and its added components (such as fittings, valves, bends, and diffusers) plus the desired flow rate and fluid properties, what pressure drop is needed to drive the flow? Average velocity in a pipe Recall - because of the no-slip condition, the velocity at the walls of a pipe or duct flow is zero We are often interested only in V avg , which we usually call just V (drop the subscript for convenience) Keep in mind that the no-slip condition causes shear stress and friction along the pipe walls Friction force of wall on fluid Introduction For pipes of constant diameter and incompressible flow V avg stays the same down the pipe, even if the velocity profile changes Why? Conservation of Mass V avg V avg Introduction For pipes with variable diameter, is still the same due to conservation of mass, but V 1 V 2 D 2 V 2 2 1 V 1 D 1 m m Introduction Laminar and turbulent flows If the flow is laminar, there may be occasional natural disturbances which damp out quickly (a). If transition is occurring, there will be sharp bursts of turbulent fluctuation as the increasing Reynolds number causes a breakdown or instability of laminar motion (b). At sufficiently large Re , the flow will fluctuate continually and is termed fully turbulent (c). The fluctuations are random and encompass a continuous spectrum of frequencies. Flow in a conduit is classified as being either laminar or turbulent, depending on the magnitude of the Reynolds number. Reynolds in the 1880s injected dye into the center of the tube and observed the following: When the velocity was low, the streak of dye flowed down the tube with little expansion (b). However, if the water in the tank was disturbed, the streak would shift about in the tube. If velocity was increased, at some point in the tube, the dye would all at once mix with the water (c). When the dye exhibited rapid mixing, illumination with an electric spark revealed eddies in the mixed fluid (d). Laminar flow- characterised by smooth streamlines and highly ordered motion Turbulent flow characterised by velocity fluctuations and highly disordered motion Transition flow laminar and turbulent flow does not occur suddenly, it fluctuates between laminar and turbulent flow before it become fully turbulent. Reynolds showed that the onset of turbulence was related to a -group that is now called the Reynolds number Re in honor of Reynolds' pioneering work....
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This note was uploaded on 11/19/2011 for the course HES 2340 taught by Professor Tomedwards during the Three '09 term at Swinburne.

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Chapter_LC67_MKT p1 - Week 6&amp 7 Flow in Conduits Part 1 HES 2340 Fluid Mechanics 1 School of Engineering Computing&amp Science

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