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Buoyancy and cross flow effects on heat transfer of multiple impinging slot air jets cooling a flat plate at different orientations

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Abstract

The present article reports on heat transfer characteristics associated with multiple laminar impinging air jet cooling a hot flat plat at different orientations. The work aims to study the interactions of the effects of cross flow, buoyancy induced flow, orientation of the hot surface with respect to gravity, Reynolds numbers and Rayleigh numbers on heat transfer characteristics. Experiments have been carried out for different values of jet Reynolds number, Rayleigh number and cross flow strength and at different orientations of the air jet with respect to the target hot plate. In general, the effective cooling of the plate has been observed to be increased with increasing Reynolds number and Rayleigh number. The results concluded that the hot surface orientation is important for optimum performance in practical applications. It was found that for Re ≥ 400 and Ra ≥ 10,000 (these ranges give 0.0142 ≤ Ri ≤ 1.59 the Nusselt number is independent on the hot surface orientation. However, for Re ≤ 300 and Ra ≥ 100,000 (these ranges give 1.59 ≤ Ri ≤ 42.85): (i) the Nusselt number for horizontal orientation with hot surface facing down is less that that of vertical orientation and that of horizontal orientation with hot surface facing up, and (ii) the Nusselt number of vertical orientation is approximately the same as that of horizontal orientation with hot surface facing up. For all surfaces orientations and for the entire ranges of Re and Ra, it was found that increasing the cross flow strength decreases the effective cooling of the surface.

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Abbreviations

A :

Area of the hot target surface, m2

A j :

Area of j the enclosure side wall, m2

B :

Slot width, m

F :

View factor

G :

Irradiations, W/m2

Gr :

Grashof number

g :

Gravity acceleration, m/s2

H :

Separation distance, m

h :

Average heat transfer coefficient, W/m2 K

I :

Electric current, Amp.

J :

Radiosity, W/m2

k a :

Thermal conductivity of air, W/m K

k g :

Thermal conductivity of fiberglass, W/m K

k w :

Thermal conductivity of plexigalss enclosure wall, W/m K

Nu :

Average Nusselt number

Pr :

Prandtl number

q :

Convection heat transfer rate, W

q c :

Conduction heat transfer rate, W

q r :

Radiation heat transfer rate, W

Ra :

Rayleigh number

Re :

Air Reynolds number

Ri :

Richardson number

T H :

Hot surface temperature, K

T j :

Jet inlet temperature, K

T w :

Wall surface temperature, K

t g :

Thickness of fiberglass insulation, m

t w :

Thickness of plexiglass wall, m

V :

Voltage input, volt

Vj :

Jet exit velocity, m/s

α :

Thermal diffusivity, m2/s

β :

Coefficient of volumetric expansion, 1/K

ε :

Emissivity, dimensionless

v :

Kinematic viscosity, m2/s

ρ :

Density, kg/m3

σ :

Stefan–Boltzmann constant, W/m2 K4

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Authors and Affiliations

  1. Department of Mechanical Engineering Technology, High Institute of Technology, Benha University, Benha, Egypt

    S. A. Nada

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  1. S. A. Nada
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Nada, S.A. Buoyancy and cross flow effects on heat transfer of multiple impinging slot air jets cooling a flat plate at different orientations. Heat Mass Transfer 45, 1083–1097 (2009). https://doi.org/10.1007/s00231-009-0480-2

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  • DOI: https://doi.org/10.1007/s00231-009-0480-2

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