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Research Papers

# Heat Transfer From Novel Target Surface Structures to a $3×3$ Array of Normally Impinging Water Jets

[+] Author and Article Information
Nicholas M. R. Jeffers

Stokes Institute, Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Irelandnick.jeffers@ul.ie

Jeff Punch

CTVR, Stokes Institute, Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Irelandjeff.punch@ul.ie

Edmond J. Walsh, Marc McLean

Stokes Institute, Mechanical and Aeronautical Engineering, University of Limerick, Limerick, Ireland

1

Corresponding author.

J. Thermal Sci. Eng. Appl 2(4), 041004 (Jan 28, 2011) (11 pages) doi:10.1115/1.4003220 History: Received April 14, 2010; Revised October 21, 2010; Published January 28, 2011; Online January 28, 2011

## Abstract

Impinging jet arrays provide a means to achieve high heat transfer coefficients and are used in a wide variety of engineering applications such as electronics cooling. The objective of this paper is to characterize the heat transfer from an array of $3×3$ submerged and confined impinging water jets to a range of target surface structures. The target surfaces consisted of a flat surface, nine 90 deg swirl generators, a $6×6$ pin fin array, and nine pedestals with turn-down dishes that turned the flow to create an additional annular impingement. In order to make comparisons with a previous single jet study by the authors, each impinging jet within the array was geometrically constrained to a round, 8.5 mm diameter, square-edged nozzle at a jet exit-to-target surface spacing, of $H/D=0.5$. A custom measurement facility was designed and commissioned in order to measure the heat transfer coefficient and the pressure loss coefficient of each of the target surface augmentations. The heat transfer results are presented in terms of $Nu/Pr0.4$, and the pressure results are presented in terms of pressure loss coefficient. Comparing the array of jets to a single jet showed a decrease in heat transfer. Full field velocity magnitude images showed that this decrease in heat transfer was caused by neighboring jet interference cross-flow coupled with a greater back pressure effect. The analysis of the different target surface augmentations showed that the performance of the pedestal with the turn-down dish was the least compromised by the addition of the surrounding jets. It showed both the highest fin efficiency of 95.1% and fin effectiveness of 2.27. However, it showed the highest overall pressure loss coefficient compared with the other target surfaces, and therefore the nine 90 deg swirl generators performed the best in terms of both pressure loss coefficient and thermal performance. The findings of this paper are of practical relevance to the design of primary heat exchangers for high-flux thermal management applications, where the boundaries of cooling requirements continue to be tested.

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## Figures

Figure 1

Confined and submerged multiple jet impingements

Figure 2

Four jet impingement target surfaces: (a) nine 90 deg swirl generators, (b) nine pedestals, (c) 6×6 pin fin array, and (d) flat surface

Figure 3

Turn-up and turn-down dish configurations

Figure 4

Jet array bulk heat transfer test facility

Figure 5

The different heater block positions in the jet array test facility

Figure 6

PIV setup

Figure 7

Nusselt–Reynolds plot for nine normally impinging jets onto a flat target surface

Figure 8

Flow visualization for the array of jets at Re=9500: plot of the velocity magnitude for dissections across (a) the center three jets and (b) the front three jets; plot of the turbulence intensity across the center three jets for (c) the vertical direction (v) and (d) the horizontal (u)

Figure 9

Velocity distribution along two separate horizontal slices: at H/D=0.42 near the jet exit and H/D=0.08 near the target surface

Figure 10

Flow resistance and the resultant back pressure effect caused by the wall jet development of the neighboring jets

Figure 11

Nusselt–Reynolds plot for the correlations extracted from the literature and compared with the fully integrated array of jets impinging on a flat target presented in this investigation

Figure 12

Nusselt–Reynolds plot for the individual jets within a 3×3 array impinging onto a target surface consisting of nine pedestals with turn-down dishes to create an additional annular impingement

Figure 13

Nusselt–Reynolds plot for the individual jets within a 3×3 array impinging onto a target surface consisting of a 6×6 pin fin geometry

Figure 14

Different fluidic effects causing enhanced and diminished heat transfers

Figure 15

Nusselt–Reynolds plot for the individual jets within a 3×3 array impinging onto a target surface consisting of nine 90 deg swirl generators (SGs)

Figure 16

Nusselt–Reynolds plot for a fully integrated target surface heat transfer performance (Nu¯eff/Pr0.4) under the 3×3 array of jets

Figure 17

Pressure drop as a function of Re for the large scale array of jet impinging onto the different target surfaces tested

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