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Research Papers

Scaling of Convective Heat Transfer Enhancement Due to Flow Pulsation in an Axisymmetric Impinging Jet

[+] Author and Article Information
Tim Persoons

e-mail: tim.persoons@tcd.ie

Darina B. Murray

Department of Mechanical and
Manufacturing Engineering,
University of Dublin, Trinity College,
Parsons Building,
Dublin 2, Ireland

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received June 21, 2012; final manuscript received December 18, 2012; published online September 23, 2013. Assoc. Editor: Sujoy Kumar Saha.

J. Heat Transfer 135(11), 111012 (Sep 23, 2013) (10 pages) Paper No: HT-12-1301; doi: 10.1115/1.4024620 History: Received June 21, 2012; Revised December 18, 2012

Impinging jets are widely used to achieve a high local convective heat flux, with applications in high power density electronics and various other industrial fields. The heat transfer to steady impinging jets has been extensively researched, yet the understanding of pulsating impinging jets remains incomplete. Although some studies have shown a significant enhancement compared to steady jets, others have shown reductions in heat transfer rate, without consensus on the heat transfer mechanisms that determine this behavior. This study investigates the local convective heat transfer to a pulsating air jet from a long straight circular pipe nozzle impinging onto a smooth planar surface (nozzle-to-surface spacing 1 ≤ H/D ≤ 6, Reynolds numbers 6000 ≤ Re ≤ 14,000, pulsation frequency 9 Hz ≤ f ≤ 55Hz, Strouhal number 0.007 ≤ Sr = fD/Um ≤ 0.1). A different behavior is observed for the heat transfer enhancement in (i) the stagnation zone, (ii) the wall jet region and overall area average. Two different modified Strouhal numbers have been identified to scale the heat transfer enhancement in both regions: (i) Sr(H/D) and (ii) SrRe0.5. The average heat transfer rate increases by up to 75–85% for SrRe0.5 ≅ 8 (Sr = 0.1, Re = 6000), independent of nozzle-to-surface spacing. The stagnation heat transfer rate increases with nozzle-to-surface distance H/D. For H/D = 1 and low pulsation frequency (Sr < 0.025), a reduction in stagnation point heat transfer rate by 13% is observed, increasing to positive enhancements for Sr(H/D) > 0.1 up to a maximum enhancement of 48% at Sr(H/D) = 0.6.

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References

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Figures

Grahic Jump Location
Fig. 1

Schematic of the impinging pulsating jet test facility. (a) pipe nozzle, (b) dummy pipe nozzle, (c) pneumatic valve, (d) square wave generator and amplifier, (e) mass flow controller, (f) pressure regulator and buffer vessel (not shown), (g) isothermally heated instrumented surface on traversing stage, (h) flush-mounted heat flux and embedded surface temperature sensors, and (i) computer with data acquisition system.

Grahic Jump Location
Fig. 2

Theoretical nozzle velocity for (○) f = 9 Hz (St = 156), (□) 27 Hz (St = 468), (△) 40 Hz (St = 693), and (♢) 55 Hz (St = 953). (a) Time-resolved mean velocity Um(t) and (b) velocity profiles at the start of the ejection phase U(r, f·t = 0) (···) and the end U(r, f·t = ½) (—).

Grahic Jump Location
Fig. 3

Heat transfer coefficient at the stagnation point of a steady impinging jet plotted as Nu0/(Re0.5Pr0.4) as a function of nozzle-to-surface spacing H/D

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Fig. 4

Local Nusselt number profiles for a steady impinging jet at (a) H/D = 1 and (b) H/D = 6

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Fig. 5

Local Nusselt number profiles for (a) H/D = 1 and Re = 6000, (b) H/D = 1 and Re = 10,000, (c) H/D = 6 and Re = 6000, and (d) H/D = 6 and Re = 10,000. The solid line represents steady flow. Markers represent pulsating flow at f = 9 Hz (○), 27 Hz (△), 36 Hz (♢), 40 Hz (▹), and 55 Hz (□).

Grahic Jump Location
Fig. 6

Local time-averaged (—) and fluctuating (−−) Nusselt number profiles for H/D = 2 and (a) Re = 6000 and (b) Re = 10,000. The solid and dashed lines represent steady flow. Markers represent pulsating flow at f = 9 Hz (○) and 55 Hz (□).

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Fig. 7

Frequency dependence of the area-averaged (—) and stagnation point heat transfer rate (···) plotted as (a)–(c) Frössling number Fr = Nu/(Re0.5Pr0.4(H/D)0.0436) and (d)–(f) heat transfer enhancement δNu, for H/D = 1 (a), 2 (b), and 6 (c), and Re = 6000 (○), 10,000 (□), and 14,000 (△).

Grahic Jump Location
Fig. 8

Scaling of (a) stagnation point and (b) area-averaged heat transfer enhancement δNu as a function of Sr(H/D) and Sr(Re)0.5. Markers represent the experimental data, with solid and hollow markers for H/D = 1–2 and 3–6, respectively.

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