Enhancing effect of an open pipe exit on the precessing vortex core occurring in confined swirling flows
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RESEARCH ARTICLE
Enhancing effect of an open pipe exit on the precessing vortex core occurring in confined swirling flows Arthur Favrel1 · Zhihao Liu2 · Kazuyoshi Miyagawa3 Received: 24 April 2020 / Revised: 31 July 2020 / Accepted: 28 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract This paper investigates the effect of an open pipe exit on the dynamic behaviour of the helical precessing vortex core (PVC) occurring in a confined swirling flow after vortex breakdown. This is achieved by carrying out extensive particle image velocimetry and dynamic pressure measurements downstream of an air swirling flow generator in two pipes configurations at two values of swirl number S = 0.7 and S = 1.1 . The first pipe configuration features a diffuser followed by a straight pipe and an open exit. In the second configuration, the open exit is placed closer to the outlet of the swirl generator by removing the straight part. In both configurations, the PVC structural parameters are properly determined by phase-averaging of the instantaneous velocity fields. In the first configuration, a sudden change in the pitch of the helical vortex is observed at both swirl number values downstream of the connection between the diffuser and the straight part. Beyond this transition, the vortex trajectory starts widening as it is getting closer to the open exit of the pipe. The results in the second configuration confirm this enhancing effect of the open exit on the PVC: at both swirl numbers, the vortex trajectory is strongly widened and its radius continuously increases from the swirl generator outlet to the pipe exit, resulting in an increase in the Strouhal number and pressure fluctuations amplitude by 20% and a factor up to 9, respectively, compared with the first configuration. Finally, based on a dimensional analysis, a linear relation between two dimensionless parameters including the vortex parameters and the pressure fluctuations amplitude is derived and supported by the data obtained in both pipe configurations.
* Arthur Favrel [email protected] 1
Waseda Research Institute for Science and Engineering, Waseda University, Tokyo, Japan
2
Department of Applied Mechanics and Aerospace Engineering (FSE Graduate School), Waseda University, Tokyo, Japan
3
Department of Applied Mechanics and Aerospace Engineering, Waseda University, Tokyo, Japan
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Vol.:(0123456789)
211
Page 2 of 19
Experiments in Fluids
(2020) 61:211
Graphic abstract
configuration 1 configuration 2
Reservoir Blower f = [0-60] Hz
Flow direction
Window for LDV measurements
1 1
+ +
+
2
PVC position at t = TPVC
3
2
ωPVC
1
y/R (-)
ωPVC 0
y x
2
3 y
Flow direction
PVC
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High-speed camera z
Pressure signals
Photo-detector
Acquistion system
(a) Experimental set-up
1 Introduction Swirling flows are encountered in a wide range of industrial applications, including combustion chambers, cyclone separators and turbomachines among others (Gupta et al. 1984). Beyond a certain swirl intensity,
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