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Diagnosis of oscillating pressure-driven flow in a microdiffuser using micro-PIV

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Abstract

In this study, the characteristics of oscillating pressure-driven flow in a microdiffuser are examined by μPIV (micro Particle Image Velocimetry) diagnostics. Utilizing a cam-follower system, a dynamic pressure generator is built in-house to provide a time-varying sinusoidal pressure source. Three parameters are examined experimentally: the excitation frequency, the cam size, and the half-angle of the microdiffuser. Driven by oscillating pressure, we find that there exists an optimal half-angle such that maximum net flow is attained in the expansion direction. Contrarily to the prediction of hydraulics theory which only considers steady flow, flow in the microdiffuser of the optimal half-angle does not necessarily remain attached. Rather, maximum net flow can also occur in microdiffusers where vortices retain a slender shape. When vortex bubbles are slender, the μPIV results reveal that the core flow accelerates to a higher forward velocity during the first half of the cycle and flow rectification is actually enhanced. Due to the three-dimensional flow structure, fluid is drawn out of the vortices near the reattachment point to join the core flow and consequently magnifies the forward flow. As the half-angle increases, vortices become rounder and the core flow is drastically narrowed to reduce flow rectification.

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Abbreviations

d t :

Inner diameter of the Teflon tube

D :

Microchannel depth

D h :

Hydraulic diameter of the throat

d p :

Diameter of seeding particles

e :

Spatial resolution

f :

Excitation frequency

M:

Magnification

n :

Refractive index

NA:

Numerical aperture

P :

Instantaneous pressure

P 0 :

Pressure amplitude

Q :

volume flow rate

R :

Tensor invariant

s m :

Net displacement of the meniscus in the Teflon tube

∆t :

Circulation lifetime

t :

Time

t m :

Time interval

T :

Period

u :

Streamwise velocity

W inlet :

Width of the inlet

w :

Corresponding microchannel width at x

x :

Streamwise coordinate

y :

Transverse coordinate

ε:

Threshold value (ε = 0.01)

δDOC :

Depth of correlation

δDOF :

Depth of field

δm :

Depth of measurement

∆:

Discriminant of velocity gradient tensor

λ:

Wavelength of incident light

μ:

Dynamic viscosity of the working fluid

θ:

Half-angle of the microdiffuser

ρ:

Density of the working fluid

inlet:

Inlet

opt:

Optimal

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Acknowledgments

This work is supported by the National Science Council of Taiwan under grant number NSC 93-2212-E-011-017 and NSC 94-2212-E-011-039.

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Authors and Affiliations

  1. Department of Mechanical Engineering, National Taiwan University, 1 Sec. 4 Roosevelt Road, Taipei, 10617, Taiwan

    Chen-li Sun

  2. Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Sec. 4 Keelung Road, Taipei, 10607, Taiwan

    Hung-Chien Lee & Ran-Xing Kao

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  1. Chen-li Sun
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  2. Hung-Chien Lee
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  3. Ran-Xing Kao
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Correspondence to Chen-li Sun.

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Sun, Cl., Lee, HC. & Kao, RX. Diagnosis of oscillating pressure-driven flow in a microdiffuser using micro-PIV. Exp Fluids 52, 23–35 (2012). https://doi.org/10.1007/s00348-011-1204-1

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  • DOI: https://doi.org/10.1007/s00348-011-1204-1

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