Abstract
The study was carried out to determine the effect of pulsating frequencies on the local and average heat transfer characteristic of a heated circular air jet. The velocity profile of a heated circular pulsating air jet was measured in the first part of the study. The same set-up was used to measure the heat flux of the pulsating jet impinging on a wall. The heat flux of the heated air jet impinging on the plate was measured using a heat flux microsensor at different radial positions. Measurement of the heat flux was used to calculate the average and local Nusselt Number for different pulsating frequencies and at different flow Reynolds Number. The pulsating frequencies were between 10-80 Hz and the Reynolds number used were 16 000, 23 300 and 32 000. Results obtained show that the local Nusselt number calculated were higher at all radial position away from the stagnation point. The pulsed jet Nusselt number was higher than the average steady jet Nusselt number for all values of frequencies due to the higher localised heat transfer. The higher Nusselt number obtained at localized radial positions can be due to the higher instantaneous velocity as was shown from the velocity profile plotted in the first part of the experiment. Enhanced turbulence intensity found was due to the pulsed jet.
Highlights
Jet impingement has been used in many applications of heat transfer such as the cooling of electronic equipment, aircraft engine nacelle and blade, drying of textiles, annealing of metals and tempering of glass
To confirm the accuracy of the present work, the value of the steady jet Nusselt number versus Reynolds number was plotted and compared to the results of several previous researchers work as reported by Jambunathan et al.[10]
The stagnation point heat transfer does not show any enhancement for the three Reynolds numbers investigated
Summary
Jet impingement has been used in many applications of heat transfer such as the cooling of electronic equipment, aircraft engine nacelle and blade, drying of textiles, annealing of metals and tempering of glass. If enhanced heat transfer can be obtained by pulsing an air jet, a lot of savings can be made from efficient heating or cooling system which can lead to reduced costs. Earlier findings regarding the effect of pulsation on heat transfer have been conflicting due to many different factors. Test carried out by Nevins and Ball[5] on heat transfer between a flat plate and a pulsating jet showed that no significant heat transfer enhancement was obtained by using a pulsed air jet. Nevins and Ball[5] did not document the extent of secondary flow structures in their experiments and since the experiment was studied at a very low Strouhal number this might affect their ability to demonstrate pulsed flow heat transfer enhancement. The application of pulse air jet was left dormant for many years due to this earlier finding and the difficulty of accurately controlling many pulse air jet parameters
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