This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in microelectronics and other high heat-flux cooling applications. Microchannels are defined as flow passages that have hydraulic diameters in the range of 10 to 200 micrometers. The impetus for microchannel research was provided by the pioneering work of Tuckerman and Pease [1] at Stanford University in the early eighties. Since that time, this technology has received considerable attention in microelectronics and other major application areas, such as fuel cell systems and advanced heat sink designs. After reviewing the advancement in heat transfer technology from a historical perspective, the advantages of using microchannels in high heat flux cooling applications is discussed, and research done on various aspects of microchannel heat exchanger performance is reviewed. Single-phase performance for liquids is still expected to be describable by conventional equations; however, the gas flow may...

Evolution of Microchannel Flow Passages-Thermohydraulic Performance and Fabrication Technology

Single-phase Convective Heat Transfer in Microchannels: a Review of Experimental Results

The extremely high rates of heat transfer obtained by employing microchannels makes them an attractive alternative to conventional methods of heat dissipation, especially in applications related to the cooling of microelectronics. A compilation and analysis of the results from investigations on fluid flow and heat transfer in micro- and mini-channels and microtubes in the literature is presented in this review, with a special emphasis on quantitative experimental results and theoretical predictions. Anomalies and deviations from the behavior expected for conventional channels, both in terms of the frictional and heat transfer characteristics, are discussed.

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A Comparative Analysis of Studies on Heat Transfer and Fluid Flow in Microchannels

() A nonmechanical pumping mechanism, thermocapillary pumping TCP , is de- scribed for moing nanoliter- and picoliter-sized drops of liquid within microfabricated flow channels. In TCP, one end of a single drop is heated to create a surface tension difference between the ends of the drop. The induced surface tension difference causes a capillary pressure difference between the two drop ends and results in drop motion. TCP ¤elocities of up to 20 mmrmin were measured for seeral liquids at temperature differ- ences between 10 and 708C. An expression deeloped for TCPelocity yields predictions that agree with experimentalelocities within corresponding uncertainty limits. Seeral techniques for assisting TCP are also suggested when contact angle hysteresis, the major factor limiting TCPelocities, is too large. These techniques include using surface treat- ments to reduce the contact angle hysteresis, conerging channels to offset hysteresis, or an applied pressure to assist in moement.

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Thermocapillary Pumping of Discrete Drops in Microfabricated Analysis Devices

We present a centrifugal process for the extraction of plasma from sediment by a decanting structure, terminating with metered plasma which is readily available for subsequent on-disk processing. Our technique supplies 2 ml plasma from 5 ml of whole blood at moderate spinning frequencies of 40 Hz within 20 s, only. The residual cell concentration in the purified plasma amounts to less than 0.11%, independent of the frequency of rotation. A capillary duct connects the extracted plasma to subsequent on-disk processing units.

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Centrifugal extraction of plasma from whole blood on a rotating disk

To investigate the performance of microengineered fluid channels in liquid dosing applications, flow-rate measurements have been performed with various channel geometries in a range from 0.01 to 1000 μl min -1 . An optical flow-measurement technique has been developed to enhance the measurement range into the desired low flow range (10 -3 to 1 μl min -1 ), and is compared to a standard gravimetric method, which is preferably used for flow rates above 1 μl min -1 . In addition, influences of the temperture-dependent viscosity and effects arising from fluidmechanical characteristics are studied. These influences are also calculated from laminar flow theory and semi-emprical models to obtain a theoretical model. It is found that the theoretical model is able to describe the measurement results well in the whole flow range. The model is implemented on a PC-based system, which measures the pressure drop across the microchannel and the fluid temperature and calculates the flow. In a temperature range from 20 to 50°C excellent agreement is found.

D. Weiβ

Papers

Microchannels for applications in liquid dosing and flow-rate measurement