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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]

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]

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]

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]

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.