Microchannel aspect ratio

K(oo), and/Refd are quoted for this geometry as a function of the microchannel aspect ratio fi. It is... 

In Table 5 the values assumed by Vmax. e, K oo) and /Refd are quoted for this geometry as a function of the microchannel aspect ratio p. It is evident that, owing to the symmetry of the cross-section, for microchannels having P greater than 1, one can use l/p in order to calculate the integral parameters from Table 5. 

Flow is typically laminar in microchannel devices, although not always rigorously so. Correlations for fully developed laminar flow in perfectly rectangular microchannels have been validated in the literature [33-35]. Transition and turbulent flows in a microchannel have no such consistent treatise, and are highly dependent upon channel shape, aspect ratio, and surface characteristics [36, 37]. 

Taking into account typical numbers for a and D, this underlines that the channel width should be considerably smaller than 1 mm (1000 pm) in order to achieve short residence times. Actually, heat exchangers of such small dimensions are not completely new, because liquid cooled microchannel heat sinks for electronic applications allowing heat fluxes of 790 watts/cm2 were already known in 1981 [46]. About 9 years later a 1 cm3 cross flow heat exchanger with a high aspect ratio and channel widths between 80 and 100 pm was fabricated by KFK [10, 47]. The overall heat transport for this system was reported to be 20 kW. This concept of multiple, parallel channels of short length to obtain small pressure drops has also been realized by other workers, e.g. by PNNL and IMM. IMM has reported a counter-current flow heat exchanger with heat transfer coefficients of up to 2.4 kW/m2 K [45] (see Fig. 3). 

Dutta, D., Leighton, D.T., Jr., Dispersion in large aspect ratio microchannels for open-channel liquid chromatography. Anal. Chem. 2003, 75, 57-70. 

Samalam [43] modeled the convective heat transfer in water flowing through microchannels etched in the back of silicon wafers. The problem was reduced to a quasi-two dimensional non-linear differential equation under certain reasonably simplified and physically justifiable conditions, and was solved exactly. The optimum channel dimensions (width and spacing) were obtained analytically for a low thermal resistance. The calculations show that optimizing the channel dimensions for low aspect ratio channels is much more important than for large aspect ratios. However, a crucial approximation that the fluid thermophysical properties are independent of temperature was made, which could be a source of considerable error, especially in microchannels with heat transfer. 

Convective heat transfer in microchannels is significantly enhanced, depending on the values of the Knudsen, the Prandtl and the Brinkman numbers and the aspect ratio. Heat transfer characteristics can be significantly different from conventionally sized channels. 

The geometric parameters of individual rectangular microchannels, namely the hydraulic diameter and the aspect ratio, and the geometry of the microchannel plate have significant influence on the single-phase convective heat transfer characteristics. 

With a rectangular microchannel not only the hydraulic diameter but also the channel shape will influence the velocity profile. This is because of the impact of the channel geometry on the EDL. Figure 5 shows the relationship between the aspect ratio (H/IV) and the volumetric flow rate for a fixed hydraulic diameter. As the ratio of H W approaches 1 1 (for a square channel), the flow rate decreases. This is because of the larger role that comer effects have on the development of the EDL and the velocity profile in square channels. 

A special-purpose photo-structured glass called FOTURAN (available from Mik-roglas Chemtech GmbH - www.mikroglas.com), which is based on lithium aluminum silicate and is especially useful for creating microchannels and related structures with high aspect ratios. 

Often, microchannels are etched with finite (and sometimes high) width-to-depth aspect ratios, w/h, with typical values ranging from 2 to 20. For such channels, the time scale for diffusion across... 

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