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An experimental investigation on flow structures of confined and unconfined impinging air jets


The flow characteristics of both confined and unconfined air jets, impinging normally onto a flat plate have been experimentally investigated. The mean and turbulence velocities, and surface pressures were measured for Reynolds numbers ranging from 30,000 to 50,000 and the nozzle-to-plate spacings in range of 0.2–6. Smoke-wire technique is used to visualize the flow behavior. The effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the flow structure are reported. In the case of confined jet, subatmospheric regions occur on both impingement and confinement surfaces at nozzle-to-plate spacings up to 2 for all Reynolds numbers in consideration and they lie up to nearly the same radial location at both surfaces. However, there is no evidence of the subatmospheric region in unconfined jet. It is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and the peaks in heat transfer coefficients for low spacings in impinging jets.

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C p :

Pressure coefficient

D :

Nozzle diameter, m

H :

Nozzle-plate spacing, m

ΔP :

Difference between the surface pressure and the atmospheric pressure, N/m2

r :

Radial distance measured from the stagnation point, m

Re :

Nozzle Reynolds number, U o D/ν

u :

Local radial velocity, m/s

v :

Local axial velocity, m/s


Turbulence velocity (rms) in the radial direction, m/s


Turbulence velocity (rms) in the axial direction, m/s

U 0 :

Nozzle exit velocity, m/s

z :

Axial distance measured from the nozzle exit, m


Kinematic vicosity of air, m2/s


Density of air, kg/m3


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This work was sponsored by the Turkey-State Planning Organization (DPT) under Grant No.2003.200.200.5.

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Correspondence to E. Baydar.

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Baydar, E., Ozmen, Y. An experimental investigation on flow structures of confined and unconfined impinging air jets. Heat Mass Transfer 42, 338 (2006).

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  • Jet impingement
  • Confined
  • Unconfined
  • Subatmospheric region
  • Turbulence