Abstract
Mixing by secondary flow is studied by particle image velocimetry (PIV) in a developing laminar pulsating flow through a circular curved pipe. The pipe curvature ratio is η = r 0/r c = 0.09, and the curvature angle is 90°. Different secondary flow patterns are formed during an oscillation period due to competition among the centrifugal, inertial, and viscous forces. These different secondary-flow structures lead to different transverse-mixing schemes in the flow. Here, transverse mixing enhancement is investigated by imposing different pulsating conditions (Dean number, velocity ratio, and frequency parameter); favorable pulsating conditions for mixing are introduced. To obviate light-refraction effects during PIV measurements, a T-shaped structure is installed downstream of the curved pipe. Experiments are carried out for the Reynolds numbers range 420 ≤ Rest ≤ 1,000 (Dean numbers 126.6 ≤ Dn ≤ 301.5) corresponding to non-oscillating flow, velocity component ratios 1 ≤ (β = U max,osc/U m,st) ≤ 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters 8.37 < (α = r 0(ω/ν)0.5) < 24.5, where α2 is the ratio of viscous diffusion time over the pipe radius to the characteristic oscillation time. The variations in cross-sectional average values of absolute axial vorticity (|ζ|) and transverse strain rate (|ε|) are analyzed in order to quantify mixing. The effects of each parameter (Rest, β, and α) on transverse mixing are discussed by comparing the dimensionless vorticities (|ζ P |/|ζ S |) and dimensionless transverse strain rates (|ε P |/|ε S |) during a complete oscillation period.
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Abbreviations
- Amixed :
-
Area of fully mixed core
- D:
-
Coefficient of diffusion
- K:
-
Concentration of fluid
- r o :
-
Pipe cross-sectional radius
- r c :
-
Curvature radius
- u :
-
Velocity in x direction
- \( v \) :
-
Velocity in y direction
- Dn:
-
Dean number, \( {\text{Dn}} = {\frac{{U_{m} (2r_{0} )}}{\upsilon }}\sqrt {{\frac{{r_{0} }}{{r_{c} }}}} \)
- \( M_{a} \) :
-
Augmentation in fuel consumption rate
- SD:
-
Standard deviation
- Re:
-
Reynolds number, \( \text{Re} = {\frac{{U_{m} (2r_{0} )}}{\upsilon }} \)
- RSD:
-
Relative standard deviation
- α:
-
Womersley number, \( r_{o} \left( {\omega /\upsilon } \right)^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}} \)
- β:
-
Velocity component ratio
- \( \upsilon \) :
-
Kinematic viscosity
- η:
-
Curvature ratio of curved pipe, \( {{r_{o} } \mathord{\left/ {\vphantom {{r_{o} } {r_{c} }}} \right. \kern-\nulldelimiterspace} {r_{c} }} \)
- ω:
-
Angular frequency
- \( \zeta (x,y) \) :
-
Axial vorticity at position (x, y) in the curved pipe cross section:\( \;{\frac{\partial v}{\partial x}} - {\frac{\partial u}{\partial y}} \)
- \( \left| {\zeta_{P} } \right| \) :
-
Cross-sectional average value of absolute vorticity in a pulsatile flow
- \( \left| {\zeta_{S} } \right| \) :
-
Cross-sectional average value of absolute vorticity in a steady flow
- \( \varepsilon (x,y) \) :
-
Transverse strain rate at position (x, y) in the curved pipe cross section: \( \frac{1}{2}\left( {\;{\frac{\partial v}{\partial x}} + {\frac{\partial u}{\partial y}}} \right) \)
- \( \left| {\varepsilon_{P} } \right| \) :
-
Cross-sectional average value of absolute transverse strain rate in a pulsatile flow
- \( \left| {\varepsilon_{S} } \right| \) :
-
Cross-sectional average value of absolute transverse strain rate in a steady flow
- Γ:
-
Circulation
- m:
-
Mean value
- B:
-
Blue fluid
- P:
-
Pulsating flow
- R:
-
Red fluid
- S:
-
Steady flow
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Jarrahi, M., Castelain, C. & Peerhossaini, H. Secondary flow patterns and mixing in laminar pulsating flow through a curved pipe. Exp Fluids 50, 1539–1558 (2011). https://doi.org/10.1007/s00348-010-1012-z
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DOI: https://doi.org/10.1007/s00348-010-1012-z