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Experiments were conducted to investigate roughness effects on flow characteristics and heat transfer of air and CO2flow in four circular micr-tubes of approximately 1 mm inner diameter. The tubes were made by electrodepositing nickel on an aluminum sacrificial substrate. The desired roughness structures were machined or etched on the substrate before depositing nickel to generate a replica of the aluminum substrate on the internal surface of the nickel tubes. Four different surface roughness features were generated: (i) uniform roughness of 3.8 m, (ii) uniform roughness of 1.8 m, (iii) internal grooves 45 m deep and 218 m wide, and (iv) helical grooves 96 m deep with 1.9 mm pitch. Friction factor and Nusselt number data for the smooth tube are in good agreement with the conventional correlations in both the laminar and the turbulent flow regimes. In the rough tubes, the friction factors are significantly higher than that of the smooth tube. Heat transfer coefficients in the laminar flow regime for both smooth and rough tube agree well with the conventional correlation. However, in the turbulent flow regime, heat transfer was enhanced by the roughness features and the enhancement increased with increasing Reynolds number. [ABSTRACT FROM AUTHOR]

It has been well established that there are no differences between microscale and macroscale flows of incompressible liquids. However, surface roughness has been known to impact the transport phenomena. This work aims to systematically quantify the effect of structured roughness geometries on friction factor in the laminar and turbulent flows as a precursor to the detailed heat transfer studies on these geometries. Experiments were conducted by varying the pitch (150–400 μm) and height (36–131 μm) of transverse rib roughness structures in rectangular channels such that the pitch-to-height ratio ranged from 2 to 8. The channel width was fixed at 12.70 mm and the length at 152.4 mm, while the channel gap was varied (230–937 μm). Tests were conducted over a Reynolds number range of 5–3400. The results are compared to the existing models, which do not account for specific roughness features such as pitch and height. A theoretical model is developed to predict the effect of roughness pitch and height on pressure drop along the channel length. Validation of the proposed theory is carried out by comparing the predictions with the experimental results. The model and the experimental results provide an understanding of the effect of two-dimensional structured roughness on the frictional losses in fully developed laminar flow. [ABSTRACT FROM PUBLISHER]

New discoveries presented in the last decade for enhancing boiling performance utilizing nanoscale structures on surfaces are critically examined in this paper. Since the mechanism for such a phenomenon is not fully understood, this review mainly focuses on the experimental studies reported in the literature on the boiling phenomena on nanostructures, and implementation of nanostructures on various substrates. The paper also focuses on the interpretation of underlying phenomena for enhancing the boiling performance. The main influencing parameter in controlling is seen as the change in the surface energy of the boiling surface, which is characterized by the contact angle of the liquid and vapor phase interface at the heating surface. The nanostructures are seen to alter the contact angle. Design consideration and theoretical developments are also discussed, followed by practical aspects of nanostructure manufacturing. The issues related to performance, ease of fabrication, and durability (whenever available) are reviewed and recommendations are made for future research in this emerging area. [ABSTRACT FROM AUTHOR]

Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected, as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This article explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of these phenomena will help in overcoming these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels. [ABSTRACT FROM AUTHOR]

Recently, a set of new roughness parameters was proposed by Kandlikar et al. and Taylor et al. for reporting surface roughness as related to fluid flow. The average roughness Ra parameter is often used in microfluidic applications, but this parameter alone is insufficient for describing surface roughness; a specimen with deep grooves and sharp obstructions can share the same average roughness value as a relatively smooth surface with low uniform surface roughness. Because the average roughness parameter is broad, it is difficult to assess the surface topography features that result from different machining processes or etches. A profilometer and a digital microscope are used to examine the surface roughness profiles of various materials submitted to different machining techniques. The materials studied will be similar to those used for microchannels including aluminum, stainless steel, copper, and silicon. Depending on the material, these samples are submitted to several machining processes, including milling, grinding, fly cutting, and microfabrication techniques. These machining processes and microfabrication techniques are of practical interest in microfluidics applications. After studying the surface roughness patterns exhibited in these samples, the roughness parameters employed in some of the recent roughness models are evaluated. This study is expected to provide more understanding of assorted surface roughness. [ABSTRACT FROM AUTHOR]

The evolution of compact heat exchangers was driven by the need for reducing their size and weight and enhancing individual components and overall system performance. These needs are also becoming relevant in stationary refrigeration and air conditioning applications. The reduction in volume and footprint of the systems is foremost in the minds of urban architects, while refrigerant charge reduction and performance enhancement remain important safety and operating considerations. Minichannels provide a viable solution to address these concerns and are expected to be used widely in large as well as small commercial, industrial, and residential systems. This paper provides a roadmap for their implementation, with a critical review of the current status of our understanding in this field, recommendations for obtaining fundamental performance data and correlations, and an emphasis on innovative designs of minichannel heat exchangers. [ABSTRACT FROM AUTHOR]

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m2 (100 W/cm2) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5-10 MW/m2 (500-1000 W/cm2). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks. [ABSTRACT FROM AUTHOR]

The effect of flow instabilities on flow boiling heat transfer in microchannels is investigated using water as the working fluid. The experimental test section has six parallel rectangular microchannels, each having a cross-sectional area of 1054 × 197 microns. Flow restrictors are introduced at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. The mass flow rate, inlet temperature of water, and the electric current supplied to the resistive cartridge heater are controlled to provide quantitative heat transfer information. The results are compared with the unrestricted flow configuration. [ABSTRACT FROM AUTHOR]

An experimental facility has been developed to investigate single-phase liquid heat transfer and pressure drop in a variety of microchannel geometries. The facility is capable of accurately measuring the fluid temperatures, heater surface temperatures, heat transfer rates, and differential pressure in a test section. A microchannel test section with a silicon substrate is used to demonstrate the capability of the experimental facility. A copper resistor is fabricated on the backside of the silicon to provide heat input. Several other small copper resistors are used with a four-point measurement technique to acquire the heater temperature and calculate surface temperatures. A transparent pyrex cover is bonded to the chip to form the microchannel flow passages. The details of the experimental facility are presented here. The experimental facility is intended to support the collection of fundamental data in microchannel flows. It has the capability of optical visualization using a traditional microscope to see dyes and particles. It is also capable of performing micro-particle image velocimetry in the microchannels to detect the flow field occurring in the microchannel geometries. The experimental uncertainties have been carefully evaluated in selecting the equipment used in the experimental facility. The thermohydraulic performance of microchannels will be studied as a function of channel geometry, heat flux, and liquid flow rate. Some preliminary results for a test section with a channel width of 100 micrometers, a depth of 200 micrometers, and a fin thickness of 40 micrometers are presented. [ABSTRACT FROM AUTHOR]

Heat transfer during flow boiling in minichannels and microchannels is intimately linked to the liquid-vapor phase distribution in the channels. The vapor phase exists as nucleating bubbles, dispersed bubbles, elongated bubbles, an annular core, or all vapor flow completely filling the channel. Similarly, the liquid can exist as bulk liquid, slugs, thin film on the heated wall, or dispersed droplets. A detailed analysis of various structures, supported by high-speed video images, is carried out, and the effects on the associated heat transfer rates are discussed. This information provides insight into the flow boiling mechanisms and guidance for future research in this area. [ABSTRACT FROM AUTHOR]

Heat fluxes in IC chips and other electronics equipment have reached the current limits of air-cooling technology. Some of the applications require heat fluxes well beyond the limit of 100 W/cm 2 , thus demanding advanced cooling solutions. Liquid cooling technology has also received attention as the advances in single-phase liquid cooling in microchannels have shown considerable promise. The extension of compact heat exchanger technology to microscale applications offers many new possibilities. The liquid cooling technology is expected to reach heat dissipation rates as high as 10 MW/m 2 (1 kW/cm 2 ) with enhanced microchannels and a junction-to-air temperature difference of 50°C. The challenges facing flow boiling systems are also evaluated. This paper reviews the fundamental technological developments in liquid cooling as well as flow boiling and presents possible solutions in integrating the cooling system with a building's HVAC unit in a large server-type application. The opportunities and challenges are described in an attempt to provide the roadmap of cooling technology for cooling high flux devices in the next decade. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer in parallel minichannels and microchannels is one of the solutions proposed for cooling high heat flux .systems. The associated increase in the pressure drop poses a problem that needs to be studied in detail before the small diameter channels can he implemented in practical systems. The pressure drop fluctuations and the flow instability in a network of parallel channels connected by a common header also need to be addressed for the stable operation of flow boiling systems. The current work focuses on studying the flow patterns, pressure drop fluctuations, and flow instabilities in a set of six parallel rectangular minichannels, each with 333 μm in hydraulic diameter. Deionized and degassed water was used for all the experiments. The pressure fluctuations are recorded and signal analysis is performed to find the dominant frequencies and their amplitudes. These pressure fluctuations are then mapped to their corresponding flow patterns observed using a high speed camera. The results help us to relate pressure fluctuations to different flow characteristics and their effect on flow instability. [ABSTRACT FROM AUTHOR]

The increased circuit density on today's computer chips is reaching the heat dissipation limits for air-cooling technology. The direct liquid cooling of chips is being considered as a viable alternative. This paper reviews liquid cooling with internal flow channels in terms of technological options and challenges. The possibilities presented herein indicate a four- to ten-fold increase in heat flux over the air-cooled systems. The roadmap for single-phase cooling technology is presented to identify research opportunities in meeting the cooling demands of future IC chips. The use of three-dimensional microchannels that incorporate either microstructures in the channel or grooves in the channel surfaces may lead to significant enhancements in single-phase cooling. A simplified and well-established fabrication process is described to fabricate both classes of three-dimensional microchannels. Proof-of-concept microchannels are presented to demonstrate the efficacy of the fabrication process in fabricating complex microstructures within a microchannel. [ABSTRACT FROM AUTHOR]

Flow boiling in mini- and microchannels offer very high heat transfer capabilities and find applications in many emerging technologies, such as electronics cooling and fuel cells. The low flow rate employed in such geometries, coupled with the small flow channels, often results in a laminar flow with all flow as liquid. Since the single-phase flow with all liquid is in the laminar range, the flow boiling correlations developed for conventional tubes with an inner diameter larger than 3 mm and turbulent flow need to be carefully reviewed. In the present work, flow boiling correlation for large diameter tubes developed by Kandlikar [1, 2] is modified for flow boiling in minichannels by using the laminar single-phase heat transfer coefficient for all liquid flow. The correlation is also extended for flow boiling in microchannels using the nucleate boiling as the dominant part of the original correlation. The trends in heat transfer coefficient versus quality are compared in the laminar and deep laminar regions in minichannels and microchannels. Excellent agreement is obtained between predicted values and experimental data. [ABSTRACT FROM AUTHOR]

The article discusses various research articles published within the issue, including one by Steinke, Kandlikar, Magerlein, Colgan and Raisanen on the design and fabrication of a microfluidics bench and another by English and Kandlikar on the modeling of air-water adiabatic flow under conditions found in fuel cell applications.

Information about several papers and topics discussed at the Fifth International Conference on Nanochannels, Microchannels and Minichannels in Puebla, Mexico on June 18-20, 2007 is presented. Topics include the establishment of new ideas among researchers working in various fields such as traditional heat, mass transfer, sensors, biomedical applications and micromixers. The papers discuss various issues in the field of heat transfer, fluid flow and flow boiling in microchannels and minichannels.

The Fourth International Conference on Nanochannels, Microchannels and Minichannels (ICNMM) was held at the University of Limerick in Ireland during June 19-21, 2006, under the sponsorship of American Society of Mechanical Engineers. A total of 189 papers were presented and over 250 participants attended the conference. The participants came from 25 countries and presented papers in 22 different areas, including microfluidics, biomedical, heat transfer, single- and two-phase flows, electronics cooling, boiling and condensation, mixing, heat exchangers, and a number of related areas. The participants cited the intense interactions among researchers from various discipline as a major benefit of attending this conference. [ABSTRACT FROM AUTHOR]