Single-Phase Flow in Channels

At the catalyst-electrolyte surface we have gas-phase diffusion, and there can also be additional surface diffusion. In surface diffusion, gas molecules physically or chemically absorb onto a solid surface. If it is physical absorption, the species are highly mobUe. If it is chemisorption and the molecule is more strongly bonded to the specific site, species are not directly mobile but can move via a hopping mechanism. Surface diffusion rates can be measured by direct measurement of the flux of a nonreacting gas across the material surface. The difference between the measured diffusion and predicted Knudsen diffusion is calculated to be the surface diffusion component. Values of the surface diffusion coefficient (Ds) are 10 cm /s in solids and liquids, but these vary widely since surface interaction is involved. Also, Ds is a strong function of temperature and surface concentration. Surface diffusion adds to the overall diffusion but is typically less than one-half of the Knudsen component and so has been mostly neglected in fuel cell analysis. 

To help summarize the concepts covered on diffusion transport. Table 5.12 is provided. Tortuosity and porosity further modify the gas-gas diffusivities if flow is through a porous media. Pressure, temperature, and other parametric dependencies vary between the modes. 

It is the goal of reactant flow manifold and channel design in PEFCs to provide excellent access to the catalyst while suffering the lowest possible pressure drop. Since parasitic blowers or fans typically must power air and possibly fuel through the fuel ceil, the pressure loss is directly related to the power output of the fuel cell. For some higher pressure systems, the power consumed by pumps and blowers can reach 30% of the power generated by the fuel ceil. 

One of the most important aspects of fuel cell design is the reactant flow through the manifolds and flow field, because parasitic losses from driving the flow through the cell can 

Frictional Losses This is the most easily recognized source of pressure loss in the flow channel. In all channels, the viscous losses retard the flow so that a pressure drop per length of the channel is required to maintain a given flow rale. In the limit of high stoichiometry, this loss dominates pressure drop. 

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Calame JP, Myers RE, Binari SC, Wood FN, Garven M (2007) Experimental investigation of micro-channel coolers for the high heat flux thermal management of GaN-on-SiC semiconductor devices. Int J Heat Mass Transfer 50 4767-4779 Celata GP, Cumo M, Zummo G (2004) Thermal-hydraulic characteristics of single- phase flow in capillary pipes. Exp Thermal Fluid Sci 28 87-95 Celata GP (2004). Heat transfer and fluid flow in micro-channels. Begell House, N.Y. 

We attempt here to reveal the acmal reasons of disparity between the theoretical predictions and measurements obtained for single-phase flow in micro-channels. For this purpose, we consider the effect of different factors (roughness, energy dissipation, etc.) on flow characteristics. Some of these factors were also discussed by Sharp et al. (2001), and Sharp and Adrian (2004). 

The experimental results of single-phase flow in smooth micro-channels are summarized in Table 3.3. 

In experiments related to flow and heat transfer in micro-channels, some parameters, such as the flow rate and channel dimensions are difficult to measure accurately because they are very small. For a single-phase flow in micro-channels the uncertainty of ARe is (Guo and Li 2002,2003)... 

Bo = q/Gh] Q, where t is the period between successive events, U is the mean velocity of single-phase flow in the micro-channel, Jh is the hydraulic diameter of the channel, q is heat flux, m is mass flux, /zlg is the latent heat of vaporization). The dependence t on Bo can be approximated, with a standard deviation of 16%, by... 

Figure 6.35 shows dependence of the dimensionless initial liquid thickness of water and ethanol 5, on the boiling number Bo, where 5 = 5U/v, f/ is the mean velocity of single-phase flow in the micro-channel, and v is the kinematic viscosity of the... 

Figure 6 (a) Nusselt number and (b) pressure drop for single-phase flow in narrow channels. 

Figure 5.14 Estimated Nusselt numbers for segmented flow [19] and hydrodynamically developing single-phase flow In cylindrical pipes Pry = 7 dy,/L, = 0.02) [20]. (Experimental values taken from Ref. [20]) (squared channel, Pry = 7 dy /L, = 0.02).

The aim of this section is to understand the features of single-phase flow in the cathode channel of a PEFC or DMFC. The model below takes into account mass and momentum transfer through the channel/GDL interface. The model gives exact solutions and helps in clarifying how the electrochemical reactions and electro-osmotic effect affect the flow in the fuel cell channels (Kulikovsky, 2001). 

Mackowiak J. Extended channel model for prediction of the pressure drop in single-phase flow in... 

In Chap. 3 the problems of single-phase flow are considered. Detailed data on flows of incompressible fluid and gas in smooth and rough micro-channels are presented. The chapter focuses on the transition from laminar to turbulent flow, and the thermal effects that cause oscillatory regimes. 

One drawback of a micro-channel heat sink is a relatively high temperature rise along the micro-channel compared to that for the traditional heat sink designs. In the direction of the flow, the wall temperature rises in a single-phase flow even when the wall heat flux is uniform. In a micro-channel heat sink, the large amount... 

The problems of micro-hydrodynamics were considered in different contexts (1) drag in micro-channels with a hydraulic diameter from 10 m to 10 m at laminar, transient and turbulent single-phase flows, (2) heat transfer in liquid and gas flows in small channels, and (3) two-phase flow in adiabatic and heated microchannels. The smdies performed in these directions encompass a vast class of problems related to flow of incompressible and compressible fluids in regular and irregular micro-channels under adiabatic conditions, heat transfer, as well as phase change. 

The investigations of fluid flow in micro-channels may be divided in two groups (1) single-phase flow, and (2) evaporative two-phase flow. The first was intensively investigated beginning from the pioneer work by Tukermann and Pease (1981). Two-phase flow is much less understood. 

The subject of the book is fluid dynamics and heat transfer in micro-channels. This problem is important for understanding the complex phenomena associated with single- and two-phase flows in heated micro-channels. 

It should also be noted that in single-phase flow heat transfer, the effect of channel size is expressed by equivalent diameter. This concept, however, should be... 

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