Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figure e 5.1: Tangenti ial Velocity for Intake po ort Fully Open. Figu ure 5.2: Axial Velocity V for Intak ke port Fu ully Open. Chapter 5 The axi ial velocity V exhibits a ‘wwake-like’ proofile throughout the cylindeer. z Howeve er, no reverse e flow is obseerved at the vortex core ( Figure 5.2). AAt position ns close to cylinder inlet, loow velocities eexist in the vorrtex core regioon and the e near-wall reg gion. The axiaal velocity proofile in the reggion in betweeen the core e and near-wall has near flatt profile and hhas peak V vvalue for a giveen z cross-sectional plane. With swirl decay along the flow, gradually smalller velocity y gradients are e observed at ddownstream crross-sections. TThe contractioon due to smaller diam meter of the ccylinder exit aaccelerates thee flow at crosss- sections s close to it. 110 Effect of Piston Position
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 5 Thus for a fully open cylinder inlet, the in-cylinder swirling flow is similar to concentrated vortex type. The experiments conducted by Escudier et al. (1982) suggest that similar type of confined swirling flow at higher Reynolds number is actually a result of upstream effect of vortex break down in the exhaust pipe. However, in the current experiment, this reason cannot be verified since no measurements are conducted in the exhaust pipe. 25% Port Closure When the piston is translated to partially close the intake port by 25%, the piston serves as a small forward-step facing the incoming flow in the cylinder. This step will affect the intensity of the radial component of incoming velocity and transform a part of it into axial direction at the cylinder inlet interface. The tangential velocity may also reduce due to friction with the piston outer wall surface exposed to the flow. The decrease in the radial and tangential components is compensated by the increase in axial velocity component due to continuity. The intake port area reduces and piston also behaves like a bluff-body in the flow path. Unsteady fluctuations/ disturbances are generated at the sharp-edge interface of the piston top and outer wall and recirculation regions are formed at the side of piston wall and piston surface (Figure 5.3). These fluctuations result in growth of instabilities and waves and are superimposed on already precessing helical vortical flow that exists when the port is fully open. Thus the resulting in-cylinder swirling flow becomes comparatively more transient and chaotic. In order to get a better average, for most of the measurements with piston partially covering the intake port, more number of instantaneous PIV snapshots has been taken compared to measurements for fully open port. From the point of view of Stereoscopic PIV experimentation, a significant aspect to be considered during the measurements is that the general method of setting the time between two laser pulses or PIV images by accepting in-plane particle displacements by 25-30% of the side of interrogation area is no more a good estimation. In stereoscopic PIV the third component of velocity is obtained mathematically after processing the snapshots of two cameras viewing inplane displacement of particles. The increase in magnitude of the axial component and large variations in out-of-plane component (in this experiment is axial velocity) require the time between pulses to be chosen by considering the maximum displacement of particles in the axial direction by 25% of the laser sheet thickness. In the presence of a large axial velocity component, this axial displacement limit for the particles is achieved by setting the time between pulses in a way that allows the particles to have an in-plane displacement less than 25-30% of the side of interrogation area. Otherwise, the magnitude of particles with high axial velocity magnitude will not be measured correctly. Thus there will be a trade-off between the error/ uncertainty in the measurement of in-plane components (radial and tangential components in this case) and the out-of-plane component (axial velocity). For this reason, the measurements for some positions have been conducted with different times between pulses and only the selected ones are presented. 111 Effect of Piston Position
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DTU Mechanical Engineering Section
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Experimental and Numerical Study of Swirling ... - Solid Mechanics

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