Secondary flow patterns and mixing in laminar pulsating flow through a curved pipe
EXPERIMENTS IN FLUIDS
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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 eta = r (0)/r (c) = 0.09, and the curvature angle is 90A degrees. 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 a parts per thousand currency sign Re-st a parts per thousand currency sign 1,000 (Dean numbers 126.6 a parts per thousand currency sign Dn a parts per thousand currency sign 301.5) corresponding to non-oscillating flow, velocity component ratios 1 a parts per thousand currency sign (beta = U (max,osc)/U (m,st)) a parts per thousand currency sign 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters 8.37 < (alpha = r (0)(omega/nu)(0.5)) < 24.5, where alpha(2) 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 (|zeta|) and transverse strain rate (|epsilon|) are analyzed in order to quantify mixing. The effects of each parameter (Re-st, beta, and alpha) on transverse mixing are discussed by comparing the dimensionless vorticities (|zeta (P) |/|zeta (S) |) and dimensionless transverse strain rates (|epsilon (P) |/|epsilon (S) |) during a complete oscillation period.