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 2.12: Sketch h of mirror vortex x effect of wall. The wal ll is illustr rated by a thick line l at y/h h=0 (Velte et al., 2009) ). Chapter 2 Figure (2.13) shows the t sketches oof different moodels of axisymmmetric heliccal vortex filaments f where l is pitch oof the helix, V Vz axial velocity and z annd are e the axial and d tangential vvorticity compponents respectively. For thhe same Reynolds R and swirl numbeers different aaxial velocity profiles can bbe observe ed i.e. uniform m axial velocitty with rectilinnear vortex linnes and infiniite helix pi itch (Figure 2.13a), 2 jet-likee axial velocityy profile geneerated by righht handed d helical vortex x (Figure 2.13bb) and wake-liike profile gennerated by a left handed d helical vortex x (Figure 2.133c). In some ccases, the left handed heliccal vortex can result in n generating a counter fl flow regime. Practically thhe existenc ce of different vortex symmeetries indicatees that the Reyynolds and swiirl number rs are not suff ficient to charracterize the ddifferent aspeccts e.g. heat annd mass transfers for the e swirling flowws (Martemiaanov et al., 20004). In real, thhe vortices s have finite size s cores andd even in somme cases the hhelix is not axxis symmet trical. The rad dius of helix annd the pitch caan also vary att different crosss sectiona al positions along the flow and one of thhe many possibble reasons may be the geometrical g sh hape of the inllet and outlet of the vortex chamber whicch affects the t flow prope erties (a contraaction sectionn at the outlet iintroduces axiial strain etc.). e Alekseen nko et al. (20007) have alsoo discussed thhe experimenttal where double d helical l vortex filameents are observved. In non-sttationary heliccal vortices s the helix also o is a function of time in a pperiodic mannner. 26 Swirling Flows
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figu ure 2.13: Schem mes of axisym mmetrical helical vortex. . Modification of f origina al figure from (Aleks seenko et al., 1999 9). Chapter 2 Okulov v et al., (2002) have describeed the vortex breakdown ass a spontaneouus transitio on from right t to left handeed helical vorttices and vice versa under thhe same in ntegral flow parameters. p Thhis continuouss transition beetween the twwo helical vortex v types happens in twoo different wayys as shown inn figure 2.13: (1) The e pitch is posi itive for the riight handed hhelical vortex and during thhe tra ansition the pi itch becomes infinite and tthen attains a negative valuue for r left handed helical h vortex. This type of ttransition is names L (linearr) tra ansition (Figur re 2.14a). (2) In the other case e the positive ppitch for rightt handed vorteex becomes zerro du uring the trans sition before cchanging to nnegative value for left handeed vor rtex. This type e is called R (riing)-transitionn (Figure 2.14aa). 27 Swirling Flows
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DTU Mechanical Engineering Section
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Experimental and Numerical Study of Swirling ... - Solid Mechanics

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