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Research Papers: Heat Exchangers

Heat Exchanger Improvement Via Curved Microfluidic Channels: Impacts of Cross-Sectional Geometry and Dean Vortex Strength

[+] Author and Article Information
Samuel D. Marshall

Department of Mechanical Engineering,
National University of Singapore,
117608, Singapore
e-mail: mpesdm@nus.edu.sg

Bing Li, Rerngchai Arayanarakool, Poh Seng Lee, Lakshmi Balasubramaniam, Peter C. Y. Chen

Department of Mechanical Engineering,
National University of Singapore,
117608, Singapore

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 30, 2016; final manuscript received July 13, 2017; published online August 16, 2017. Assoc. Editor: Zhuomin Zhang.

J. Heat Transfer 140(1), 011801 (Aug 16, 2017) (9 pages) Paper No: HT-16-1320; doi: 10.1115/1.4037339 History: Received May 30, 2016; Revised July 13, 2017

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilize the enhanced fluid mixing characteristics of Dean vortices and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimizing the heat exchanger design based on the parameters of maximizing heat transfer while minimizing pressure drop and unit cost. A range of cross-sectional geometries for the curved channels was compared, showing significantly higher Nusselt numbers than equivalent straight channels throughout and finding superior performance factors for square, circular, and symmetrical trapezoidal profiles. Due to the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. The variation of Nusselt number over the length of the channel for a range of different curvatures (and hence Dean numbers) is also examined, showing significantly higher heat transfer occurring in strongly curved channels, especially in areas where the generated Dean vortices are strongest. The variation in Nusselt number was found to form the shape of an “arc.” In this way, a relationship between the Dean number and the Nusselt number is characterized and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

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Figures

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Fig. 1

Mesh independency study, showing average Nusselt number against Reynolds number for a sample square channel for four different element sizes (numbered element sizes correspond to columns from left to right)

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Fig. 2

Cross-sectional profiles for investigation: (a) square, (b) circle, (c) half-circle, (d) trapezoid, (e) reverse trapezoid (inside edge of curve larger than outside edge), (f) rectangular wide, (g) rectangular tall, and (h) symmetrical trapezoid

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Fig. 3

Full model geometries for investigation: (a) straight and (b) curved

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Fig. 4

Heat transfer against Reynolds number for all the geometries under constant wall temperature conditions (numbered geometries correspond to columns from left to right): (a) average Nusselt number of curved channel and (b) average Nusselt number enhancement over equivalent straight channel

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Fig. 5

Tangential velocity profiles at midpoint of channel for all the curved channels Re = 674: (a) square, (b) circle, (c) half-circle, (d) trapezoid, (e) reverse trapezoid (inside edge of curve larger than outside edge), (f) rectangular wide, (g) rectangular tall, and (h) symmetrical trapezoid

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Fig. 6

Flow impedance against Reynolds number for all the geometries under constant wall temperature conditions (numbered geometries correspond to columns from left to right): (a) friction factor of curved channel and (b) friction factor enhancement over equivalent straight channel

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Fig. 7

Thermal performance against Reynolds number for all the geometries under constant wall temperature conditions (numbered geometries correspond to columns from left to right): (a) TPF of curved channel and (b) TPFe over equivalent straight channel

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Fig. 8

Thermal performance against Reynolds number for selected geometries under constant heat flux conditions (numbered geometries correspond to columns from left to right): (a) TPF of curved channel and (b) TPFe over equivalent straight channel

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Fig. 9

Local Nusselt number against channel length at Re = 674 for a range of curved channels with different radii of curvature and an equivalent straight channel

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Fig. 10

Tangential velocity profiles at various lengths along a curved channel with 3 mm radius of curvature at Re = 674 (De = 301): (a) 50 mm from inlet and (b) 75 mm from inlet, 100 mm from inlet

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Fig. 11

Local Nusselt number and average swirling strength variation across channel length for a curved channel with 3 mm radius of curvature at Re = 674 (De = 301)

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