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

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Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines downstream the flow. At position z 7 (z/D=4.116) and cylinder lengths (8D and 6D), the magnitude of radial velocity becomes very close to zero and then slowly starts increasing again. The mean axial vorticity has a Gaussian like profile at z 1 (z/D=0.963) i.e. for the regions at larger radial distance from the cylinder axis, except the near wall region, the vorticity is very weak and then in the vortex core region it gets very strong. With swirl decay downstream the flow, the vortex core loses its vortical strength by transferring it in outward radial directions. The rate of this vorticity transfer is higher in case of high Reynolds number thus leading to quicker decay of the weak vortical ‘annular region’ in the tangential velocity profile. The detection of vortex core position in the instantaneous PIV measurements at z 1 (z/D=0.963) show that the vortex core position is not stationary and moves in an area of r/R= ±0.2. This indicates a precessing vortex core (PVC) and LDA measurements show that the precession frequencies at Reynolds number of 65000 and 32500 are 5.8 Hz and 3.15 Hz. The precession frequency is found to increase linearly as a function of flow rate. The mean vortex core position is not aligned with the cylinder axis at all measuring positions indicating an asymmetric swirling flow with core following a helical path. The radius of the helical path is not same for all measuring positions and initially the radius of the helical path is smaller but then gradually increases. The helix rotation is in a clockwise direction similar to the swirl which is also in clockwise direction. This together with a Burgers vortex like tangential velocity profile and wake-like axial velocity profile indicate a left-handed helical vortex and having a negative pitch. For all the cylinder lengths the helical vortex core does not complete one revolution. In case of cylinder length 8D, the helical vortex path at the downstream positions, instead of rotating around the cylinder axis, re-twists at one side of the X-axis. In addition, at those far downstream positions the tangential velocity has forced vortex profile, the axial velocity and mean axial vorticity profiles become uniform showing an infinite pitch of the helical path. This indicates a transition of left handed to right handed helical vortex called as ‘L-transition’ and is one of characteristics of vortex breakdown. In the presence of a precessing vortex core (PVC), the measured values of Reynolds stresses are actually a combination of turbulence and vortex core oscillation. The Reynolds stress normal components are high in the vortex core and near wall regions. Considering the maximum values, for normal stresses (Cartesian coordinates) at z (z/D=0.963) ww uu vv . 1 Regarding the components in polar coordinates, v v and vrv r are larger near the wall and in the vortex core regions at z (z/D=0.963). The radial 1 distance, from the vortex center, where vrv r increases towards the vortex center, is larger than the v v and follows the same for both high and low Reynolds numbers. As the swirl decays downstream, the magnitude of radial and tangential fluctuations decrease but their spatial distribution increase to larger radial distances of the cylinder cross-section. 166 Summary & Conclusions
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines In general the magnitudes of shear stress components, at different measuring positions are observed to be smaller than normal stress components. Considering the maximum values, for shear stresses (Cartesian coordinates) uw vw uv indicating anisotropy of the flow. However, with swirl decay downstream at z (z/D=3.068), the maximum values become almost 5 equal for individual components of both normal and shear stress components i.e. ww uu vv and uw vw uv . The Reynolds stresses decay with the swirl and distribution of normal and shear components tends to become comparatively and gradually more uniform in the flow domain. With swirl decay downstream, the flow at high Reynolds number has higher tendency towards a more uniform spatial distribution of individual Reynolds stress components in the flow domain. At z 1 (z/D=0.963) the turbulent kinetic energy is strong in the vortex core and near wall region. The maximum value is observed in the vortex core region and the contours show an asymmetric distribution. The turbulent kinetic energy decreases downstream. The flow at low Reynolds number is less responsive or in other words more resistive to the variations in vorticity, Reynolds stresses and turbulent kinetic energy as the swirl decays along the pipe. 1.1.2 Effect of Piston Position on the Confined Swirling Flow In this experiment the length of cylinder was kept 4D but the piston is translated and adjusted to fixed positions where it closes the intake to the cylinder by 0% (Fully Open intake port), 25%, 50% and 75%. For each piston position, stereoscopic PIV measurements were conducted at the aforementioned Reynolds numbers. When the piston is partially closing the cylinder intake port, the piston serves as a forward-step facing the incoming flow into the cylinder and affects the magnitude of radial velocity in particular and also tangential velocity to some extent. This consequently increases the axial velocity magnitude. Since the flow rate is kept constant but the inlet area is reduced, therefore, the average velocity at the inlet increases. The piston also behaves as a bluff-body in the flow path generating unsteady fluctuations/ disturbances at the sharp-edge interface of the piston top and outer wall. These fluctuations result in growth of instabilities and waves and are superimposed on already precessing helical vortical flow that is observed when the port is fully open. This indicates that the resulting in-cylinder swirling flow becomes more transient. This also puts some challenge for making non-time resolved PIV measurements. At 25% intake port closure and a given Reynolds number, the peak values of tangential velocity decreases and axial velocity increases. At z 1 (z/D=0.963), 167 Summary & Conclusions
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DTU Mechanical Engineering Section
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Experimental and Numerical Study of Swirling ... - Solid Mechanics

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