05-05-2011, 02:56 PM
Jet impingement heat transfer – Part I: Mean and root-mean-square heat transfer and velocity distributions
Abstract
Impinging jets provide a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The currentwork forms the first stage of a two part investigation of heat transfer distributions from a heated flat surface subject to an impinging airjet for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit spacing, H/D, from 0.5 to 8. In the present paper,the relative magnitudes of the local heat transfer coefficients are compared to the fluctuating components and to the mean and rootmean-square local velocity components. It has been shown that at low nozzle to surface spacings (<2 diameters) secondary peaks inthe radial heat transfer distributions are due to an abrupt increase in turbulence in the wall jet. In particular the velocity fluctuationsnormal to the impingement surface have a controlling influence on the enhancement in the wall jet._ 2007 Elsevier Ltd. All rights reserved.Keywords: Jet impingement; Heat transfer; Turbulence
1. Introduction
Impinging jets are known as a method of achieving particularlyhigh heat transfer coefficients and are thereforeemployed in many engineering applications. Impinging jetshave been used to transfer heat in diverse applications,which include the drying of paper and the cooling of turbineblades. Hollworth and Durbin [1] investigated theimpingement cooling of electronics, Roy et al. [2] investigatedthe jet impingement heat transfer on the inside of avehicle windscreen and Babic et al. [3] used jet impingementfor the cooling of a grinding process. In these, and in othercases, research has been conducted with a specific applicationin mind but there have also been many fundamentalinvestigations into the fluid flow and heat transfer characteristics.These have led to the identification of severalparameters which influence heat transfer on the impingementsurface. Thus, the main variables for jet impingementheat transfer are the angle of impingement, the jet Reynoldsnumber and the height of the nozzle above theimpingement surface. The current investigation is concernedwith heat transfer to a submerged normally impingingaxially symmetric air jet.Comprehensive studies of the mean fluid flow characteristicsof both a free and an axially symmetric impinging airjet have been presented by Donaldson and Snedeker [4],Beltaos [5] and Martin [6]. In many investigations, includingthat by Gardon and Akfirat [7], the heat transfer to animpinging jet has been correlated with what is often termedthe ‘‘arrival” flow condition. This is the flow condition atan equivalent location in a free jet.Jet flow characteristics are highly complex and can beinfluenced easily by varying the flow rate and nozzle geometry;the effects of nozzle geometry on the potential corelength were investigated by Ashforth-Frost and Jambunathan[8]. Four jet exit conditions were studied, namely flatand fully developed flow for unconfined and semi-confinedjets. For unconfined jets, it was shown that the potential corelength can be 7% longer for the fully developed flow case
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