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

Abstract

This paper deals with the analysis of the main features of forced microconvection of liquid and gas flows through microchannels. A critical oveiyiew 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 speciflc 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 flttings, 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.

The Challenge to Measure Single-phase Convective Heat Transfer Coefficients in Microchannels

“…Convective pressure-driven single phase fluid flows in micro-channels can be studied theoretically by determining the fluid velocity and temperature distribution using the mass, momentum and energy balance equations. If the continuum hypothesis is still valid for the working fluid within the microdevice, the classical formulation of the balance equations can be generally used [4,5].…”

“…In this paper, it will be demonstrated that the analysis of forced convection in micro-channels usually needs a combined experimental/numerical approach, as evidenced by Morini and Yang [5] and, more recently, by Wang et al [6]. This is partially due to the problems linked to the problems which afflict the measurement of the local fluid properties within the micro-systems.…”

Experiments on Measurement of Heat Transfer Rate of Gas Flow Through Microchannel with Constant Wall Temperature

“…Experimental investigations on heat transfer characteristics of gas flow in microchannels have been performed by a number of researchers using most common thermal boundary conditions, namely constant heat flux (CHF) and constant wall temperature (CWT) with emphasis on CHF [9][10][11][12]. The literature was well surveyed and critically reviewed by Lin et al, [10] and Morini et al, [11].…”

“…Experimental investigations on heat transfer characteristics of gas flow in microchannels have been performed by a number of researchers using most common thermal boundary conditions, namely constant heat flux (CHF) and constant wall temperature (CWT) with emphasis on CHF [9][10][11][12]. The literature was well surveyed and critically reviewed by Lin et al, [10] and Morini et al, [11]. It is reported that the thermal behavior at microscale of gas and liquid flows through microchannel 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 testing, the real thermal boundary conditions, the presence of fittings and position and type of the sensors, and so on [11].…”

“…The literature was well surveyed and critically reviewed by Lin et al, [10] and Morini et al, [11]. It is reported that the thermal behavior at microscale of gas and liquid flows through microchannel 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 testing, the real thermal boundary conditions, the presence of fittings and position and type of the sensors, and so on [11]. All these aspects have to be taken into account during the data post processing in order to obtain the correct evaluation of heat transfer characteristics.…”