Jet impingement heat transfer – Part II: A temporal investigation of heat transfer and local fluid velocities Tadhg S. O’Donovan * , Darina B. Murray Department of Mechanical and Manufacturing Engineering, Trinity College Dublin, Ireland Received 1 August 2006; received in revised form 28 January 2007 Available online 5 April 2007 Abstract Impinging jets are a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The temporal nature of both the fluid flow and heat transfer has been investigated for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit distance, H/D, from 0.5 to 8. At the impingement surface simultaneous acquisition of both local heat flux and local velocity signal has facilitated a comprehensive analysis of the effect that fluid flow has on the heat transfer. Results are presented in the form of surface heat transfer and fluid velocity signal spectra, and coherence and phase difference between the corresponding velocity and heat flux signals. It has been shown that the evolution of vortices with distance from the jet exit has an influence on the magnitude of the heat transfer coefficient in the wall jet. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Jet impingement; Heat transfer; Vortices; Spectra; Coherence; Phase 1. Introduction Impinging air jets are employed in a wide range of appli- cations for enhanced cooling, as detailed in the first part of this two part investigation. The effect of local mean veloc- ities and turbulence intensities on the heat transfer has been outlined in part 1, whereas the objective of this part is to explore in more detail the influence of the turbulence char- acteristics of the flow on heat transfer. In particular, the effect of naturally occurring vortices on the mean heat transfer from the impingement surface is presented. In a jet flow, vortices initiate in the shear layer due to Kelvin Helmholtz instabilities. As the vortices move down- stream of the jet nozzle each vortex can be wrapped and develop into a three dimensional structure due to second- ary instabilities. These secondary instabilities can lead to the ‘‘cut and connect” process as described by Hui et al. [1] and Hussain [2] in which the toroidal vortices break down into smaller scale motions, generating high turbu- lence. Vortices, depending on their size and strength, affect the jet spread, the potential core length and the entrain- ment of ambient fluid. In certain cases jet vortices can pair, forming larger but weaker vortices. In general, vortices pass in the shear layer of the jet at the same frequency as that at which they roll up but in the vortex pairing case the passing frequency halves as the vortices pair off. Turbu- lent jets have a fundamental frequency at which the pairing process stabilises and this is determined by the turbulence level of the jet. With distance from the jet nozzle the vorti- ces break down into random small scale turbulence. It is clear that vortices influence the arrival velocity of the impinging jet flow and therefore influence the shape and magnitude of the heat transfer distribution. Artificial jet excitation can control the development of vortices in the jet flow and therefore is thought to have the potential to enhance heat transfer from the surface. Liu and Sullivan [3] excited an impinging air jet acousti- cally and reported on the resulting flow and heat transfer distributions. It was found that, depending on the fre- 0017-9310/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2007.01.047 * Corresponding author. Tel.: +353 1 896 3878; fax: +353 1 679 5554. E-mail address: tadhg.odonovan@tcd.ie (T.S. O’Donovan). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 50 (2007) 3302–3314