Evolution of Microchannel Flow Passages: Thermohydraulic Performance and Fabrication Technology

Abstract

This paper provides a roadmap of development in the thermal and fabrication aspects of microchannels as applied in the 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, 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 expected to be still describable by the conventional equations; however the gas flow may be influenced by the rarefaction effects. Two-phase flow is another topic that is still under active research. The evolution of research into microchannel heat sinks has paralleled the advancements made in microfabrication technology. The earliest microchannels were built using anisotropic wet chemical etching techniques based on alkali solutions. While this method has been exploited successfully, it does impose certain restrictions on silicon wafer type and geometry. Recently, anisotropic dry etching processes have been developed that circumvent these restrictions. In addition, dry etching methods can be significantly faster and, from a manufacturing standpoint, create fewer contamination and waste treatment problems. Advances in fabrication technology will continue to fuel improvements in microchannel heat sink performance and cost for the foreseeable future. Some fabrication areas that may spur advances include new materials, high aspect ratio patterning techniques other than dry etching, active fluid flow elements, and micromolding.

Experimental investigation of fluid flow and heat transfer in a single-phase liquid flow micro-heat exchanger

“…A layout of channels at both sides of a plate has been used for efficiency and simplicity purposes. The plate should be thin (as Table 2 Kandlikar and Grande [2] classification scheme. possible but restricted to channels mechanization needs) and made of a material with high conductivity, apart of the necessary dimensional stability.…”

“…Tables 1 and 2 show the classifications proposed by Mehendale et al [1] and Kandlikar and Grande [2]. Both classifications are defined over the value of the smaller dimension of the channels.…”

Enhancement of flow boiling heat transfer in microchannels by nano- and micro-surface treatments

“…Kandlikar and Grande [2] suggested another classification based on the hydrodynamic diameter: conventional channels (D h > 3 mm), minichannels (200 µm < D h < 3 mm) and micro-channels (D h < 200 µm).…”

“…The test channel is rectangular with hydraulic diameter of 0.96 mm and length of 180 mm. The confinement number is equal to 2.6, and hence the test channel is considered as microchannel according to Kew and Cornwell theory [1], but as minichannel according to Kandlikar and Grande [2] classification. The local wall temperature and heat flux are directly measured.…”

Selected studies of flow maldistribution in a minichannel plate heat exchanger

“…Different classification was postulated in [4], i.e., channels can be defined as: conventional when D h > 3 mm, mini when 0.2 mm < D h < 3 mm and micro when 10 µm < D h < 200 µm. Mini-or microchannel heat exchangers are widely used in many industries, i.e., power industry (water cooled turbine blades, rocket nozzle cooling, fusion reactor blanket cooling, domestic micro-combined heat and power) [5][6][7][8][9], information technology industry (computer data centers) [10,11], avionics industry (avionics cooling) [12], space industry (cooling of satellite electronics) [13], solar industry (solar photovoltaic panels) [14], automotive industry (cooling of hybrid vehicle power electronics) [15], chemical and biological industry [16,17], refrigeration industry (microfin tubes in residential cooling) [4] or cryogenic industry (heat exchangers for hydrogen storage systems) [18]. Kandlikar [19] reported that microchannels may provide a heat flux dissipation ratio up to 10 3 W/cm 2 .…”