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Research Papers: Flows in Complex Systems

# Pulsating Flow for Mixing Intensification in a Twisted Curved Pipe

[+] Author and Article Information
B. Timité, M. Jarrahi, C. Castelain

Thermofluids, Complex Flows and Energy Group, Laboratoire de Thermocinétique, UMR CNRS 6607, Ecole Polytechnique-Université de Nantes, La Chantrerie, BP 50609, F-44306 Nantes, France

H. Peerhossaini

Thermofluids, Complex Flows and Energy Group, Laboratoire de Thermocinétique, UMR CNRS 6607, Ecole Polytechnique-Université de Nantes, La Chantrerie, BP 50609, F-44306 Nantes, Francehassan.peerhossaini@univ-nantes.fr

J. Fluids Eng 131(12), 121104 (Nov 24, 2009) (10 pages) doi:10.1115/1.4000556 History: Received October 14, 2008; Revised August 28, 2009; Published November 24, 2009; Online November 24, 2009

## Abstract

This work concerns the manipulation of a twisted curved-pipe flow for mixing enhancement. Previous works have shown that geometrical perturbations to a curved-pipe flow can increase mixing and heat transfer by chaotic advection. In this work the flow entering the twisted pipe undergoes a pulsatile motion. The flow is studied experimentally and numerically. The numerical study is carried out by a computational fluid dynamics (CFD) code (FLUENT 6 ) in which a pulsatile velocity field is imposed as an inlet condition. The experimental setup involves principally a “Scotch-yoke” pulsatile generator and a twisted curved pipe. Laser Doppler velocimetry measurements have shown that the Scotch-yoke generator produces pure sinusoidal instantaneous mean velocities with a mean deviation of 3%. Visualizations by laser-induced fluorescence and velocity measurements, coupled with the numerical results, have permitted analysis of the evolution of the swirling secondary flow structures that develop along the bends during the pulsation phase. These measurements were made for a range of steady Reynolds number $(300≤Rest≤1200)$, frequency parameter $(1≤α=r0⋅(ω/υ)1/2<20)$, and two velocity component ratios $(β=Umax,osc/Ust)$. We observe satisfactory agreement between the numerical and experimental results. For high $β$, the secondary flow structure is modified by a Lyne instability and a siphon effect during the deceleration phase. The intensity of the secondary flow decreases as the parameter $α$ increases during the acceleration phase. During the deceleration phase, under the effect of reverse flow, the secondary flow intensity increases with the appearance of Lyne flow. Experimental results also show that pulsating flow through a twisted curved pipe increases mixing over the steady twisted curved pipe. This mixing enhancement increases with $β$.

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## Figures

Figure 1

(a) Dean cells (steady flow), (b) deformed Lyne instability (pulsatile flow), and (c) Lyne instability (pulsatile flow)

Figure 2

Scotch-yoke mechanism

Figure 3

Photograph of the test section

Figure 6

Cross section meshing

Figure 7

Second convergence criterion: evolution of the instantaneous velocity amplitude for the same moment of ω⋅t=90 deg versus time

Figure 8

Experimental and numerical results for secondary flow in the first bend versus the parameter (α) and the steady Reynolds number when β=1

Figure 9

Experimental and numerical results for secondary flow in the first bend versus the parameter (α) and the steady Reynolds number when β=2

Figure 10

(a) Numerical simulations (—) compared with experimental axial velocity profiles (●) in the first bend for various moments (ω⋅t) at Rest=600, β=2, and α=10.26. (b) Numerical secondary velocity vectors in the first bend for various moments (ω⋅t).

Figure 11

Secondary flow at the exit of the bends at various moments (ω⋅t) for Rest=600, β=2, and α=10.26: (a) experimental visualization and (b) numerical velocity vectors

Figure 12

Particle trajectories at the exit of the first bend for a period: (a) injection at the center of the inlet section and (b) injection near the wall

Figure 13

Divergence of trajectories: (a) second bend in steady flow, (b) second bend in pulsatile flow (Rest=620, β=2, and α=10.26), (c) sixth bend in steady flow, and (d) sixth bend in pulsatile flow (Rest=620, β=2, and α=10.26)

Figure 14

Evolution of the deformation of a dye injection at the exit bend versus the amplitude velocity ratio β (Rest=430 and α=10.26; injection: x/r0=0; y/r0=0.7)

Figure 15

Streamwise evolution of the cross-sectional area of tracer filament injected at the entrance to the first bend

Figure 4

Visualization setups: (a) LDV system and (b) LIF system

Figure 5

(a) Instantaneous velocity at r/r0=2/5, and (b) average velocity, experimental (●, ◆, and ○) and analytical (—) for Rest=280, α=11.24, and β=1 at (1) r/r0=4/5, (2) r/r0=2/5, and (3) r/r0=0

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