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Research Papers

Guidelines for the Determination of Single-Phase Forced Convection Coefficients in Microchannels

[+] Author and Article Information
Gian Luca Morini

e-mail: gianluca.morini3@unibo.it

Yahui Yang

e-mail: yahui.yang2@unibo.it
DIN - Alma Mater Studiorum,
Università di Bologna,
Viale Risorgimento 2,
Bologna 40136, Italy

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received June 29, 2012; final manuscript received December 10, 2012; published online September 11, 2013. Assoc. Editor: Sushanta K. Mitra.

J. Heat Transfer 135(10), 101004 (Sep 11, 2013) (10 pages) Paper No: HT-12-1326; doi: 10.1115/1.4024499 History: Received June 29, 2012; Revised December 10, 2012

This paper deals with the analysis of the main features of forced microconvection of liquid and gas flows through microchannels. A critical overview of the main effects that tends to play an important role in the determination of Nusselt number in microchannels is presented. Some experimental data obtained at the Microfluidics Lab of the University of Bologna together with the main results which appeared recently in the open literature both for liquids and gases are used in order to highlight the peculiar characteristics of the convective heat transfer through microchannels and to suggest the guidelines for a physically based interpretation to the experimental results. By means of specific examples, it is shown that the thermal behavior at microscale of gas and liquid flows through microchannels in terms of convective heat transfer coefficients can be strongly affected by scaling and micro-effects but also by practical issues linked to the geometry of the test rig, the real thermal boundary conditions, the presence of fittings, position and type of the sensors, and so on. All these aspects have to be taken into account during the data post processing in order to obtain a correct evaluation of the Nusselt numbers. It is also highlighted how it is always useful to couple to the experimental approach a complete computational thermal fluid-dynamics analysis of the whole tested microsystem in order to be able to recognize “a priori” the main effects which can play an important role on the convective heat transfer analysis. It is demonstrated in this paper that this “a priori” analysis is crucial in order to: (i) individuate the main parameters which influence the convective heat transfer coefficients (this is important for the development of new correlations); (ii) compare in a right way the conventional correlations with the experimental results.

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Figures

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Fig. 1

Average Nusselt numbers as a function of the Reynolds number for water flow (#1 d = 440 μm; #2: d = 280 μm; #3: d = 146 μm) from Ref. [36] and comparison with the Gnielinski [33] and Sieder and Tate [35] correlations

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Fig. 2

Average Nusselt numbers as a function of the Reynolds number and comparison with Gnielinski [33], Sieder and Tate [35], and Choi et al. [8] correlations for nitrogen flow (#4: d = 172 μm; #5: d =750 μm) from Ref. [37]

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Fig. 3

Axial distribution of the wall and bulk temperature for microtube #4 at Re = 400 for: (a) nitrogen and (b) water

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Fig. 4

A sketch of the outlet section of the test rig used in Ref. [37] where the temperature sensor for the measurement of the gas outlet temperature is placed

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Fig. 5

(a) Axial temperature distribution along the outlet plenum centerline; (b) 2D temperature distribution within the outlet plenum at Re = 1400 (c) 2D temperature distribution within the outlet plenum at Re = 600 (only half of the outlet plenum displayed)

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Fig. 6

Average Nusselt numbers obtained by integrating experimental and numerical data and comparison with Gnielinski [33], Sieder and Tate [35], and Choi et al. [8] correlations for nitrogen flow

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