Macro-channel

Steam-liquid flow. Two-phase flow maps and heat transfer prediction methods which exist for vaporization in macro-channels and are inapplicable in micro-channels. Due to the predominance of surface tension over the gravity forces, the orientation of micro-channel has a negligible influence on the flow pattern. The models of convection boiling should correlate the frequencies, length and velocities of the bubbles and the coalescence processes, which control the flow pattern transitions, with the heat flux and the mass flux. The vapor bubble size distribution must be taken into account. 

For the most part of the experiments one can conclude that transition from laminar to turbulent flow in smooth and rough circular micro-tubes occurs at Reynolds numbers about RCcr = 2,000, corresponding to those in macro-channels. Note that other results were also reported. According to Yang et al. (2003) RCcr derived from the dependence of pressure drop on Reynolds number varied from RCcr = 1,200 to RCcr = 3,800. The lower value was obtained for the flow in a tube 4.01 mm in diameter, whereas the higher one was obtained for flow in a tube of 0.502mm diameter. These results look highly questionable since they contradict the data related to the flow in tubes of diameter d> mm. Actually, the 4.01 mm tube may be considered... 

In the study by Qu et al. (2004), experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel. The bubbly, stratified and churn flow patterns commonly encountered in macro-channels were never observed in the study. No water droplets were observed in the nitrogen bubble, nor were any nitrogen bubbles present in the water slugs. 

In large tubes, as well as in tubes of a few millimeters in diameter, two-phase flow patterns are dominated in general by gravity with minor surface tension effects. In micro-channels with the diameter on the order of a few microns to a few hundred microns, two-phase flow is influenced mainly by surface tension, viscosity and inertia forces. The stratified flow patterns commonly encountered in single macro-channels were not observed in single micro-channels. 

Several popular macro-channel correlations and recently recommended small-channel correlations were examined by Lee and Mudawar (2005b). Predictions were adjusted for the three-sided wall heating and rectangular geometry using the following relation ... 

Calculate the transport-limited current for the one-electron oxidation of a 1 mM aqueous solution of ferrocyanide, Fe(CN)g , at a macro-channel electrode of size 4.0 mm X 4.0 mm in a flow cell of cross-section dimensions 6 mm x 0.4 mm at flow rates (Vf) of 10 and 10 cm s . Assume a value of 6x10 cm s for the diffusion coefficient of ferrocyanide. 

For a macro-channel electrode, axial diffusion maybe ignored so that the steady-state mass transport equations for species A, B and C are ... 

Leonard, 2004, A novel and template-free method for the spontaneous formation of aluminosilicate macro-channels with mesoporous walls, Chem. Commun., 1674. 

The behavior of the flow in micro-channels, at least down to 50 pm in diameter, shows no difference with macro-scale flow. For smooth and rough micro-channels with relative roughness 0.32% turbulent flow occurs between 1,800 < Recr < 2,200, in full agreement with flow visualization and flow resistance data. In the articles used for the present study there was no evidence of transition below these results. 

Annular flow pattern is characterized by a thin water film, which flows along the channel wall with the nitrogen comprising the central core. Unlike annular macro-... 

The placement of catalysts/carriers in micro channels can be done by various means. In a conventionally oriented variant, catalyst powders or small grains are inserted as mini fixed beds [7]. However, more specific catalyst arrangements are also known, originally designed for novel ways of processing at the macro scale, such as catalyst filaments [8], wires [9] and membranes (Figure 3.2) [10, 11]. 

The falling film micro reactor (Figure 5.1) transfers this well-known macro-scale concept to yield films of a few tens of micrometers thickness [1-3]. For this reason, the streams are guided through micro channels. To obtain a reasonable throughput, many micro channels are operated in parallel. 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

The parameters D and Dk > whether for macro (denoted by subscript m) or for micro (denoted by subscript ju) regions, are normal bulk and Knudsen diffusion coefficients, respectively, and can be estimated from kinetic theory, provided the mean radii of the diffusion channels are known. Mean radii, of course, are obtainable from pore volume and surface area measurements, as pointed out in Sect. 3.1. For a bidisperse system, two peaks (corresponding to macro and micro) would be expected in a differential pore size distribution curve and this therefore provides the necessary information. Macro and micro voidages can also be determined experimentally. 

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