PLIF investigation on effects of chamber aspect ratio on flow and mixing in cross-shaped mixers
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摘要: 在10<Re<500时，采用平面激光诱导荧光技术（PLIF）研究了4种腔室宽高比r（r=0.5、1.0、1.5和2.0）的十字型混合器内的流动与混合特征。结果表明：不同腔室宽高比的十字型混合器内均出现了分离流、稳态吞噬流、脉动流和非稳态吞噬流等4种流型。对于稳态吞噬流，r<1.0的腔室内流场由3个共旋涡主导，而r≥1.0的腔室内中心涡和卫星涡旋转方向相反。对于脉动流，在r>1.0的腔室内，中心涡周期性收缩和扩张，且整个腔室内流体脉动，而在r=0.5和1.0的腔室下游出现涡环脱落特征。对于非稳态吞噬流，r=1.0的腔室内发生了旋涡合并和破碎现象，而r=0.5的腔室内中心涡和一侧卫星涡发生周期性合并，未观察到涡破碎；对于r>1.0的腔室，腔室内中心涡经历增长、变形和破碎过程。基于时间平均离析强度（IOS）对腔室内流体混合效果进行了量化，并揭示了混合机制。腔室宽高比的增大，促使吞噬流和脉动流的临界雷诺数显著降低，导致低雷诺数下腔室内流体混合强化。Abstract: Planar Laser-induced Fluorescence （PLIF） was used to study flow and mixing characteristics in cross-shaped mixers with four chamber aspect ratios r（r=0.5, 1.0, 1.5 and 2.0） at 10<Re<500. Results show that, there are four flow regimes in the mixers with different depths, including the segregated flow, steady engulfment flow, pulsation flow and unsteady engulfment flow. For the steady engulfment flow, the flow field is dominated by three co-rotating vortices for r<1.0, but the center and satellite vortices rotate in opposite directions for r≥1.0. For the pulsation flow, the center vortex shrinks and expands periodically, and the fluid oscillates throughout the chamber for r>1.0. For r=1.0 and 0.5, the shedding of vortex rings emerges downstream. For the unsteady engulfment flow, periodical vortex merging and breakup is observed for r=1.0. For r=0.5, vortex breakup is invisible, and instead, the center vortex merges with a satellite vortex periodically. For r>1.0, the center vortex experiences growth, deformation, and breakup processes. Mixing in cross-shaped mixers was evaluated by the time-averaged intensity of segregation （IOS）, and the mixing mechanism is revealed. An increase in chamber aspect ratios decreases the critical Reynolds number for the engulfment flow and pulsation flow, which causes the mixing enhancement in the chamber at low Re.
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Key words:
- cross-shaped mixer /
- engulfment flow /
- pulsation flow /
- mixing /
- PLIF
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图 1 PLIF实验流程图及十字型混合器结构示意图
Figure 1. Schematic diagram of the experimental system and geometry of cross-shaped mixer
图 2 不同腔室宽高比的十字型混合器内y=0平面上的PLIF图片
Figure 2. Flow regimes in the cross-shaped mixer with various chamber aspect ratios
图 3 十字型混合器的Re–r流型相图
Figure 3. The Re–r space map describing different flow regimes in cross-shaped mixers
图 4 不同腔室宽高比的十字型混合器内不同xOy平面上的瞬时PLIF图片
Figure 4. Instantaneous PLIF images of several xOy planes in cross-shaped mixers for various chamber aspect ratios
图 5 十字型混合器内稳态吞噬流型下平面z=0上的中心涡尺度随雷诺数的变化
Figure 5. Size of center vortex on plane z=0 for steady engulfment flows in cross-shaped mixers
图 6 Re=125、r=2.0时十字型混合器内z=0平面上一个周期内不同时刻的PLIF图片
Figure 6. PLIF images on plane z/h=0 at various moments during a period in the cross-shaped mixer for Re=125 and r=2.0
图 7 Re=125、r=2.0时十字型混合器内y=0平面上瞬时PLIF图片和区域（−0.25≤x/h≤0.25和−8.75≤z/h≤8.75）时空图
Figure 7. Instantaneous PLIF image on plane y=0 and spatiotemporal map on region （−0.25≤x/h≤0.25 and −8.75≤z/h≤8.75） in cross-shaped mixer for Re=125 and r=2.0
图 8 不同腔室宽高比的十字型混合器内非稳态吞噬流型下z=0平面上一个周期内不同时刻的PILF图片
Figure 8. PILF images at different moments in a period of the unsteady engulfment flow in cross-shaped mixers for various chamber aspect ratios
图 9 不同雷诺数和腔室宽高比时十字型混合器内脉动流和非稳态吞噬流的无量纲振荡频率
Figure 9. Normalized oscillation frequencies of the pulsation flow and unsteady engulfment flow in cross-shaped mixers for different Re and r
图 10 不同雷诺数和腔室宽高比下十字型混合器内z/h=5平面上的时间平均离析强度
Figure 10. The intensity of segregation of plane z/h=5 in cross-shaped mixer for various Reynolds numbers and chamber aspect ratios
表 1 十字型混合器结构参数
Table 1. Geometrical parameters of the cross-shaped mixer
Mixer No. h/mm wi /mm w/mm I 1000 500 1000 II 1000 1000 1000 III 1000 1500 1000 IV 1000 2000 1000 
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