Experimental Study of Flow Structures of Circular Pulsating Air Jet

Tan Tan

doi:10.3844/ajeassp.2009.171.175

Abstract

Problem Statement: Applications of impingement jets in industry for heating and cooling purposes requires a high convective heat transfer coefficient. Numerous studies have been conducted to improve the convective heat transfer coefficient for a steady impinging jet. A pulsating jet has a very high potential in replacing steady jet after it been found able to increase the heat transfer coefficients at certain pulsating frequencies. The objectives of this study were to; (i) determine the velocity profile of a circular pulsating air jet at different pulse frequencies and Reynolds Number using a rotating valve pulse jet system and (ii) to compare the normalized steady and pulsed jet velocity at highest Reynolds number of 32 000 and highest pulsating frequency of 80Hz. Approach: Pulsation of the air jet was produced by a rotating cylinder valve mechanism at frequencies between 10-80 Hz. Flow structures of the heated steady and pulse single circular axisymmetric air jet velocity were measured using a calibrated hot-wire anemometer and presented in non-dimensional form. The measurements were carried out at three different Reynolds numbers which was set at 16000, 23300 and 32000. The jet exit velocity profile for all the test frequency is determined by plotting the graph of radial distance against the non-dimensional jet exit velocity. Results: The corresponding Reynolds number in this test is based on time-averaged centerline velocity. The results of the velocity measurement were plotted side by side using non-dimensional parameters in order to make direct comparison of the velocity profile at different frequencies and Reynolds numbers. Stagnation point velocities are the same for steady and pulsating jet for all pulse frequencies. As the radial distance from the stagnation point increases, pulsating velocity increases between 20-30% from radial distance of 2-22 mm. Conclusion: Results of the flow structures plotted show a distinctive exit air jet profile which can affect the impingement heat transfer characteristics. This was the result of enhanced turbulence intensity due to pulsating jet produced by the rotating cylinder. From the jet exit velocity profile obtained, it is found that mass flow rate for different test frequencies are slightly different due to the difference in the local velocity measurement affected by the pulses. The jet exit velocity profile data will be used to form a correlation between the pulsating jet velocity and heat transfer data.

Highlights

High convective heat transfer coefficient is a very important factor that leads to numerous applications of impingement jets in industry for heating and cooling purposes
The purpose of the first part of this study is to investigate steady and pulsating single circular air jet flow structures
The stable flow structure created from the experiment is important in order to correctly measure the instantaneous heat transfer

Summary

Introduction

High convective heat transfer coefficient is a very important factor that leads to numerous applications of impingement jets in industry for heating and cooling purposes. Applications of impinging air jets include the cooling of electronic equipment, aircraft engine nacelle and blade, drying of textiles, annealing of metals and tempering of glass. Extensive research has been conducted on steady impinging jet to understand their heat and mass transfer characteristics. Numerous studies and reviews on the subject of steady jet heat transfer have been published over the last several decades[1,2,3]. Kataoka and Suguro[4] show that stagnation point heat transfer for axisymmetric submerged jets is enhanced by the impingement of large-scale structures Heat transfer in pulsating flows has been the subject of renewed interest in recent years since the present of flow pulsations has been found to increase the heat transfer coefficients.

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Discussion

Conclusion