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Time-resolved, 3D, laser-induced fluorescence measurements of fine-structure passive scalar mixing in a tubular reactor

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Abstract

A three-dimensional, time-resolved, laser-induced fluorescence (3D-LIF) technique was developed to measure the turbulent (liquid-liquid) mixing of a conserved passive scalar in the wake of an injector inserted perpendicularly into a tubular reactor with Re=4,000. In this technique, a horizontal laser sheet was traversed in its normal direction through the measurement section. Three-dimensional scalar fields were reconstructed from the 2D images captured at consecutive, closely spaced levels by means of a high-speed CCD camera. The ultimate goal of the measurements was to assess the downstream development of the 3D scalar fields (in terms of the full scalar gradient vector field and its associated scalar energy dissipation rate) in an industrial flow with significant advection velocity. As a result of this advection velocity, the measured 3D scalar field is artificially “skewed” during a scan period. A method to correct for this skewing was developed, tested and applied. Analysis of the results show consistent physical behaviour.

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Notes

  1. For isotropic turbulence, both the strain rate, S, and the energy dissipation, ε, can be related to the Taylor micro scale, λ g, by \(S^{2} = 2{u^{2}_{l} } \mathord{\left/ {\vphantom {{u^{2}_{l} } {\lambda ^{2}_{{\text{g}}} }}} \right. \kern-\nulldelimiterspace} {\lambda ^{2}_{{\text{g}}} } \) and \(\varepsilon = {15vu^{2}_{l} } \mathord{\left/ {\vphantom {{15vu^{2}_{l} } {\lambda ^{2}_{{\text{g}}} }}} \right. \kern-\nulldelimiterspace} {\lambda ^{2}_{{\text{g}}} } \) (Hinze 1975). Elimination of u l /λ g yields S 2=2ε/(15ν), which, in combination with λ ν =c 1(ν/S)1/2, leads to λ ν =(c 115ν 3/(2ε))1/4~η.

  2. In the present article, the width of a laser sheet will be based on the (1/e) intensity decay position, while the thickness will be based on the (1/e2) position. The definitions are in a ratio of \({\sqrt 2 } \).

  3. For every discrete grid position, x′, of the output concentration field, ζ′, the value is retrieved from the input concentration field, ζ, at the corresponding position, x, given by Eqs 12 and 13. Since x is generally located within the fixed grid, a bi-linear interpolation is applied using the values at the eight surrounding grid points to estimate ζ(x).

  4. The mixing fraction distribution is obtained from the SED pdf, f logχ (Fig. 24), by plotting the volume fraction \(1 - {\int_{ - \infty }^{s = \chi } {f\chi {\left( s \right)}} }{\text{d}}s \) against the mixing fraction \(1 - \frac{1}{{\langle \chi \rangle }}{\int_{ - \infty }^{s = \chi } {sf\chi {\left( s \right)}} }{\text{d}}s \), where the conditional distribution, f χ , is obtained via the transformation f χ =f log(χ)/(χln(10)).

Abbreviations

A :

Deformation tensor

D t, D f :

Reactor and injector diameter

L x, L y, L z :

Dimensions of the 3D-LIF measurement volume

N x, N y, N z :

Number of data samples per measurement volume

Re m :

Reynolds number based on mean velocity

Sc :

Schmidt number

f :

Focal length

f c,lens, f c,array :

Cut-off frequency for camera lens and sensor array

f θ , f φ :

Marginal probability density function for θ and φ

f θφ :

Joint probability density function of θ and φ

\(\Delta t \) :

Temporal separation of the 2D data planes

\(\Delta T \) :

Temporal resolution of the measurement volume

\(\Delta x,\;\Delta y,\;\Delta z \) :

Spatial resolution of the measurement volume

γ, α :

Deformation angle and deformation, where α=tanγ

ε :

Fluid energy dissipation rate

λ ν , λ Γ :

Strain limited vorticity and scalar diffusion layers

ζ :

Scalar concentration

η, η B :

Kolmogorov and Batchelor length scale

θ, φ :

Spherical angles of the scalar gradient vector, \(\nabla \zeta \)

ν :

Kinematic viscosity

σ e –2 :

Half-thickness (1/e2) of the laser sheet

τ, τ a :

Kolmogorov and Kolmogorov advection time scales

χ :

Scalar energy dissipation rate

Γ :

Scalar diffusivity

2D, 3D:

Two- and three-dimensional

DNS:

Direct numerical simulation

LIF:

Laser-induced fluorescence

SED:

Scalar energy dissipation rate

TR:

Tubular reactor

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Van Vliet, E., Van Bergen, S.M., Derksen, J.J. et al. Time-resolved, 3D, laser-induced fluorescence measurements of fine-structure passive scalar mixing in a tubular reactor. Exp Fluids 37, 1–21 (2004). https://doi.org/10.1007/s00348-004-0779-1

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