Research Papers: Heat Transfer Enhancement

Effect of Pin Tip Dual Clearance on Flow and Heat Transfer at Low Reynolds Numbers

[+] Author and Article Information
Michael L. Seibert

Command Power & Integration Directorate,
Aberdeen Proving Ground, MD 21005
e-mail: Michael.l.seibert.civ@mail.mil

Neal E. Blackwell

Fort Belvoir, VA 22060

Danesh K. Tafti

Mechanical Engineering Department,
Virginia Polytechnic and State University,
Blacksburg, VA 24061

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received July 31, 2013; final manuscript received March 7, 2014; published online April 10, 2014. Assoc. Editor: William P. Klinzing. This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Heat Transfer 136(7), 071901 (Apr 10, 2014) (9 pages) Paper No: HT-13-1385; doi: 10.1115/1.4027184 History: Received July 31, 2013; Revised March 07, 2014

This paper examines the augmentation of heat and mass transfer due to dual clearances on cylindrical pin fins, relative to a channel between parallel plates, in mini/microchannel reactors at low Reynolds numbers. In this work, diffusion limitations to heat and mass transfer in smooth-walled mini/microchannel reactors were minimized by the implementation of microcylinder pin fins with dual clearances that, (1) promote the production of instabilities in the wakes that enhance mixing and (2) reduce the viscosity dominated regions at pin-wall interfaces. A smooth catalyst coating is assumed on all exposed surfaces of the microchannel interior walls and pin fins. Due to the analogy of heat and mass transfer, augmentation of the Nusselt number is equivalent to the augmentation of the Sherwood number. Heat transfer augmentation is investigated in air (Pr = 0.705) at dual clearances ranging from 0 to 0.4 of the channel height and Reynolds numbers from 10 to 600. The pin fins and the clearance augmented the heat transfer coefficient by a factor of 4.0. The combination of the augmentation of the heat transfer coefficient and the increase in the surface area, by the clearances, results in an increase in the conductance over a plane channel, by a factor of 7.1. The results are extendable to overcoming laminar diffusion with laminar periodic wakes of fuel vapors such as methanol vapor in air where Scfuel ∼ Prair. For turbulent wakes impinging upon downstream pins, the results can be extended to fuel vapors with (Scfuel)turb ∼ (Prair)turb. A large eddy simulation (LES) approach was used in this study.

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Fig. 1

Definition of pin fin channel geometry and computational domain

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Fig. 2

Validation of numerical method

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Fig. 3

Friction factor as a function of ReD

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Fig. 4

Nusselt number as a function of ReD

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Fig. 5

Instantaneous local Nusselt number on the bottom wall (left) and vorticity (right) above and below the pin at ReD = 10 and tip clearances t = 0.1 H, 0.2 H, and 0.4 H

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Fig. 6

Snapshot of instantaneous local Nusselt number (left) and vorticity (right) at t = 0.1 H and ReD = 150, 300, and 600

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Fig. 7

Flow instability as seen in the variation of spatially averaged Nusselt number with respect to time with a tip clearance of t = 0.1 H

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Fig. 8

Streamlines around pins with tip clearance 0.1*H and ReD = 300

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Fig. 9

Streamlines around pins with tip clearance 0.1*H and ReD = 600

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Fig. 15

Nusselt number augmentation as a function of pin diameter based Reynolds number

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Fig. 14

Conductance augmentation as a function of pin diameter based Reynolds number

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Fig. 13

Conductance augmentation versus pumping power

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Fig. 12

Effect of pin fins on flow rate at constant pumping power as compared to a plane channel

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Fig. 11

Required pumping power through the fin array

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Fig. 10

Onset of instability at increasing Reynolds number is shown by the increase in the standard deviation of the spatially averaged Nusselt number in an arbitrary time period




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