# Experimental investigation on heat transfer from square jets issuing from perforated nozzles

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## Abstract

This paper reports the results of an experimental investigation of fluid flow and heat transfer carried out with square jets issuing from perforated nozzles. This is accomplished by an impinging square jet on a uniformly heated plate of finite thickness (5 mm). The medium under consideration is air. Three different nozzle configurations are used in the study namely a single nozzle and perforated nozzles with four and nine holes, which are accommodated in the same available jet area 4.6 mm × 4.6 mm. This arrangement is akin to introducing a wire mesh at the nozzle exit plane. The effects of dimensionless jet-to-plate distance (2–9) and the mass flow rate of the jet fluid on the heat transfer rate are studied. Jet centerline mean velocity and turbulence intensity measurements are made with a hot-wire anemometer. The pressure drop across the orifice nozzle plate is measured and corresponding pumping power values are calculated. A comparison of the heat transfer performance and pumping power penalty of the three nozzle configurations is done.

## Keywords

Nusselt Number Mass Flow Rate Turbulence Intensity Heat Transfer Characteristic Average Nusselt Number## List of symbols

*a*,*b*Calibration constants of the hot-wire anemometer

*B*Width of the square hole on single orifice plate (mm)

*E*Voltage across the hot wire probe (V)

*I*Current (A)

- \(k_f\)
Thermal conductivity of air (W/mK)

*L*Width of the impingement plate (mm)

*m*Mass flow rate (kg/s)

*Nu*Average Nusselt number

*P*Pumping power \((\dot{\text {Q}}\, \Delta p)\) (W)

*q*Uniform heat flux supplied (W/\(\text {m}^{2}\))

- \({\dot{\hbox {Q}}}\)
Volume flow rate (l/min)

- \(Q_{supp}\)
Heat supplied (W)

- \(Q_{loss}\)
Heat loss (W)

- \(Q_{net}\)
Net heat supplied (W)

*Re*Jet Reynolds number, \(\frac{m}{\phi B\mu }\)

- TI
Turbulence intensity, \(\frac{\sqrt{\overline{u'^2}}}{U_e}\times 100\)%

- \(T_{ave}\)
Average impingement surface temperature (K)

- \(T_{j}\)
Jet temperature at the orifice exit (K)

- \(u'\)
Fluctuating component of the jet centerline velocity (m/s)

- \(U_{c}\)
Jet centerline velocity (m/s)

- \(U_{e}\)
Jet exit velocity (m/s)

- \(U_{c}/U_{e}\)
Normalized jet centerline velocity

*V*Voltage supplied to the heater (V)

*Z*Jet-to-plate distance (mm)

*Z*/*B*Dimensionless jet-to-plate distance

## Greek letters

- \(\mu \)
Dynamic viscosity of air (kg/m s)

- \(\rho \)
Density of air (kg/\(\text {m}^3\))

- \(\phi \)
Flow area ratio

- \(\Delta p\)
Pressure drop (Pa)

## Notes

### Acknowledgements

Authors acknowledge Dr. Arvind Pattamatta and Sangamesh C Godi, HTTP Laboratory, IIT Madras, for the help provided in hot-wire anemometer measurements. Also, the first author Pullarao Muvvala would like to thank Prof. K. Srinivasan, TDCE Laboratory, IIT Madras, for the lectures on course titled “Jet flow and Acoustics”, which gave foundation knowledge for the research problem.

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