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Convective heat transfer under unsteady impinging jets: the effect of the shape of the unsteadiness

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Abstract

Unsteady impinging jets are systematically controlled with respect to their time dependence in order to investigate the influence of unsteadiness on the heat transfer performance. This is achieved by a special mass flow control device, which allows almost arbitrary shapes of unsteadiness to be imposed onto the impinging jet. Three different standard signals (sinusoidal, triangular, rectangular) and two specially designed signals are applied and their influence on heat transfer is determined in terms of an enhancement factor. Heat transfer augmentation up to 30% was found and could be physically explained with the help of PIV and hot-wire measurements of the flow field.

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Abbreviations

A :

Area (m2)

D :

Nozzle diameter (m)

f :

Frequency (Hz)

F :

Dimensionless function (–)

H :

Nozzle-to-plate distance (m)

I :

Electrical current (A)

k :

Thermal conductivity (W/mK)

\( \dot{m} \) :

Mass flow rate (kg/s)

n :

Counter (–)

N :

Number of samples (–)

Nu :

Nusselt number (–)

Pr :

Prandtl number (–)

\( \dot{q} \) :

Heat flux density (W/m2)

r :

Radial distance, radius (m)

R :

Electrical resistance (Ω)

Re :

Reynolds number (–)

s :

Input signal (V)

S :

Material thickness (m)

T :

Temperature (K)

u :

Velocity (m/s)

U :

Voltage (V)

α :

Electrical resistance coefficient (1/K)

Λ:

Enhancement coefficient (–)

μ :

Dynamic viscosity (Pa s)

ρ :

Density (kg/m3)

τ :

Time (s)

τ s :

Period length (s)

\( \hat{\tau } \) :

Integration time (s)

Φ:

Phasing (–)

A :

Area

AMB:

Ambient

c:

Conduction

cl:

Centerline

conv:

Convection

e:

Electrical

F:

Fluid

i :

Isolation

m:

Area mean

R:

1. Rectangular

2. Reference

S:

1. Surface

2. Sinusoidal

SF:

Single field

st:

Steady

T:

Triangular

0:

At 0°C

–:

Time mean

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Acknowledgments

The authors wish to thank the DFG (Deutsche Forschungsgemeinschaft) for their support of this study. They also want to thank Dr. Sid Becker, North Carolina State University and Humboldt Fellow at TU Hamburg-Harburg, for his assistance in preparing the final version of this paper.

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Authors and Affiliations

  1. Institute of Thermo-Fluid Dynamics, Hamburg University of Technology, Denickestraße 17, 21071, Hamburg, Germany

    G. Middelberg & H. Herwig

Authors
  1. G. Middelberg
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  2. H. Herwig
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Correspondence to H. Herwig.

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Middelberg, G., Herwig, H. Convective heat transfer under unsteady impinging jets: the effect of the shape of the unsteadiness. Heat Mass Transfer 45, 1519–1532 (2009). https://doi.org/10.1007/s00231-009-0527-4

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  • DOI: https://doi.org/10.1007/s00231-009-0527-4

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