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 Figu ure 5.36: Averag ge Static Pressure Drop at a z with intake 2 port cl losure % @ Re = 6.5 6 x 10 4 e e . that wit th the partial closure of cyylinder intake port, the cylinder static waall pressure e drop increa ase follows a behavior thatt can be apprroximated by a power law. l 5.3 Discussi ion Chapter 5 This ex xperiment stu udied the effeect of piston position on the incylindder confine ed swirling fl low. The expperiment, howwever, did nnot include thhe dynami ic effect of the e piston and innstead the meeasurements are conducted at fixed pi iston position ns. The designn swirl parameeter remains tthe same for aall piston positions. p The ma agnitudes of ta angential and axial velocitiees decay downstream the floow due to friction f with cylinder c internnal wall. The tangential vellocity profile, at fully op pen port, is sim milar to a "Burrger Vortex" i. e. an inner forrced vortex coore and fre ee vortex oute er region. Thhe higher veloocities are observed at somme interme ediate radial position p betweeen cylinder wwall and the geometric centter where force and free fr vortex reegions meet. With the ddecay in swiirl downstream the flow w, the size of the forced voortex region inncreases. As thhe piston is i positioned to t close the inntake port by 225%, both thee tangential annd axial ve elocity profiles changes at z . The tangeential velocity becomes moore 1 asymme etric and for the axial vellocity the wake effect of tthe vortex coore decrease es. The dow wnstream decaay of the swwirl additionaally affects thhe tangent tial and axial velocity proffiles. The tanggential velocitty profile at ffar downstream position ns starts changging to a forceed vortex i.e. hhigher velocitiies are obse erved near the e cylinder wallls and the wakke like profile of axial velociity diminis shes until it develops a jeet like profilee at z . This shows that thhe 6 downstream decay in n the swirl is decreasing thhe wake effect by transferrinng 144 Effect of Piston Position
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 5 more mass into the wake region. This change in velocity profile begins from cross-sectional positions near the cylinder outlet and moves to upstream positions as the piston gradually closes the port. However, in case of piston position with 50% port closure, at cross-sectional positions close to cylinder inlet, the tangential velocity profiles resembled more towards a wall-jet like profile rather than the forced vortex. The axial velocity in the whole cylinder exhibits a jet-like profile with a broad peak. For the cross-sectional positions close to the intake port, the partial closure of the intake port introduces asymmetry and variation in the mean tangential velocity profile. At 75% port closure the tangential velocity in the cylinder has a forced vortex like profile indicating high velocities in the near wall region. At z 1 , the axial velocity still has a jet like profile but with a sharp peak. This jet-like mean axial velocity profile changes back to wake-like at the adjacent downstream cross-sectional position z 2 . This indicates a vortex breakdown like characteristic. The mean axial velocity profile then continues to have the wake-like profile at the remaining downstream positions. The wake effect however is small compared to when the port was fully open. The mean axial vorticity of the mean velocity field has a Gaussian like profile when the intake port is fully open. However with the decay in the swirl downstream as well as the partial closure of the intake port, the vorticity confined in the vortex core region is transmitted to the outer regions. This results in a comparatively uniform mean axial vorticity distribution throughout the cylinder for 75% port closure. The increase in port closure increases the magnitude of Reynolds stress components as well as the anisotropy. The distribution of normal Reynolds stress components can be understood and summarized from the profiles of axial and tangential velocity components. In case of tangential velocity having the burgers vortex like profile, the u and v velocity components have high fluctuations in the vortex core region and the near wall region. For a forced vortex like profile, uu and vv attain higher values in the region close to vortex center than to region with higher tangential velocities. In case of axial velocity with wake like profile, the fluctuations w is high in the vortex core region and near wall region. As the axial velocity profile attains a jet like profile, the ww in the jet center seems to be very low and increase as the radial distance from the jet center increases. In general, the turbulent kinetic energy increases with the partial closure of the intake port. For the incylinder flow at all piston positions, the Reynolds shear stresses have lower magnitude compared to normal components. Figure 5.37 give profile of Reynolds shear stress components (in polar coordinates) at z and 1 Re for each piston position. An important thing to be noted is the rise in the A magnitude of vrvz with percentage of piston closure especially from the region at X/R=± 0.6. At 75% port closure, increase in values of vrvz and vrvz show that the covariance of axial velocity with both tangential and radial velocity components is large. 145 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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