垂直撞击平面射流的实验研究

2019/08/06   下载量： 1 次

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采用LaVision的DaVis图像采集和处理软件平台，高速相机，可编程同步时间控制器构成一套时间分辨PIV系统。研究了垂直撞击平面射流。

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Impinging jets are commonly employed in engineering applications, such as cooling and
drying processes, requiring relatively high local transport properties. Flow development of a
planar impinging jet and the effect of varying jet parameters are investigated experimentally
in a jet facility that is specifically designed, fabricated, and characterized as part of the
current study. The velocity field is measured using time-resolved, planar, two-component
Particle Image Velocimetry. The investigation focuses on two jet parameters: Reynolds
number and nozzle-to-plate spacing. Four test cases are investigated, including two Reynolds
numbers 3000 and 6000, and two nozzle-to-plate spacings, 2B and 4B, where B is the jet
width.
Primary vortices form in the shear layer just downstream of the nozzle exit due to
amplification of disturbances through the Kelvin-Helmholtz instability. Vortex shedding
exhibits higher periodicity with increasing Reynolds number but is not significantly affected
by changes to the nozzle-to-plate spacing. Further, vortex shedding location shifts upstream
with increasing Reynolds number. The Strouhal number based on vortex shedding frequency
varies between 0.4  StB  0.5 and the vortices are convected downstream at an average
convective velocity of 57% of the jet centerline velocity for all cases examined. For
ReB = 6000, the characteristic wavelength of the primary vortices is 1.1B for both nozzleto-
plate spacings. At the lower Reynolds number, the wavelengths increase to approximately
1.3B and 1.5B, for nozzle-to-plate spacings of 2B and 4B, respectively. Local deceleration
of primary vortices, due to the impinging surface, causes consecutively shed vortices to
merge, with vortex merging observed in all cases investigated.
As the primary vortices pass in the outer shear layer of the wall jet region, secondary
vortices of opposingly signed vorticity form due to roll-up of the wall bound vorticity in the
inner shear layer. The secondary vortex shedding is marked by amplification of the surface
normal velocity fluctuations in the inner shear layer when x/B > 3. The secondary vortex
shedding shows a lower periodicity compared to the primary vortices but in general have
convective velocities that are larger than that of the primary vortices. As the Reynolds
number is increased, the secondary vortex formation is suppressed, and, as a result, the wall
jet develops more parallel to the impinging surface. With increasing Reynolds number, the
convective velocities of the secondary vortices also decrease. In all cases, pairing between
a primary and secondary vortex is observed and the paired vortices are convected in the
streamwise direction away from the surface. At the higher Reynolds number, the pairing is
immediately followed by vortex breakdown.

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北京欧兰科技发展有限公司
金牌会员 丨 第17年
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