Air Flow in the Channel

Heat fluxes along the y- and z-axes are negligible the dominant role in heat removal is played by air flow in the channel (Figure 5.2). 

Let us consider one more physical phenomenon, which can influence upon PT sensitivity and efficiency. There is a process of liquid s penetration inside a capillary, physical nature of that is not obvious up to present time. Let us consider one-side-closed conical capillary immersed in a liquid. If a liquid wets capillary wall, it flows towards cannel s top due to capillary pressure pc. This process is very fast and capillary imbibition stage is going on until the liquid fills the channel up to the depth l , which corresponds the equality pcm = (Pc + Pa), where pa - atmospheric pressure and pcm - the pressure of compressed air blocked in the channel. 

Monolithic catalytic converters continue to receive attention in the literature because of their applications in air pollution control and clean energy production. They differ from packed-bed reactors in their configuration as there are many parallel channels coated with a layer of catalyst. The flow in the channels is typically laminar. Because of its large void fraction, it is expected that the temperature transients will exhibit a significant impact on the performance of the monolith, particularly with respect to thermal stability. 

CONDITIONS FOR FLUIDIZATION. Consider a vertical tube partly filled with a fine granular material such as catalytic cracking catalyst as shown schematically in Fig. 7.9. The tube is open at the top and has a porous plate at the bottom to support the bed of catalyst and to distribute the flow uniformly over the entire cross section. Air is admitted below the distributor plate at a low flow rate and passes upward through the bed without causing any particle motion. If the particles are quite small, flow in the channels between the particles will be laminar and the pressure drop across the bed will be proportional to the superficial velocity... 

So far, we have assumed that the oxygen concentration in the cathode channel is constant. In this section, we will relax this assumption. Suppose that the cell is equipped with the single straight air channel let the axis z be directed from the channel inlet to the outlet (Figure 23.9). We will assume that the flow in the channel is a plug flow, that is, a well-mixed flow with a constant velocity v. The validity of this approximation is discussed in [3]. The oxygen mass conservation equation in the channel then reads... 

The speed of sound in atmospheric pressure air at T = 350 K is about 3 10 cm s and it increases with temperature. Therefore, gaseous flow in the channel is always deeply subsonic and hence incompressible (Section 4.1). This means that the pressure in the channel varies mainly due to the viscous friction of the flow over the chaimel walls. 

Now we return to the stack element in Figure 5.2. The heat flux (W m ) transported by the air flow along the channel is... 

Heat exchange with the air flow in the cathode channel is negligible. The heat capacity of a liquid methanol-water mixture is much larger than the heat capacity of air and thus at moderate oxygen stoichiometries the contribution of air flow to the overall heat balance is small. 

Figure 3.16 Simulated domains for (A) the fuel-rich concept in Fig. 3.1 B and the (B) i-CST concept in Fig. 3.15a. In both concepts, the catalytic channel has a length L-75 mm and a half-height 6=0.6 mm. In (A), bypass air flows in adjacent channels with height H=2b —. 2 mm.

The three-dimensional simulation model for the fuel cell based on steady-state, single-phase and incompressible flow analysis is presented here. The other basic assumptions that have been made in formulating the model are as follows (1) gas flows in the channels are assumed to be incompressible, (2) water generation takes place only at the anode membrane interface, (3) there is no water generation and water transport in the electrolyte, (4) water exists only in the gas phase in the fuel cell, and (5) humidified hydrogen and air are assumed to be ideal gases. 

Fig. 10 Humidity distribution and oxygen concentration along the channel for a standard cell and a cell with reduced air flow in the beginning and sevenfold subsequent air injection during the first 35% of the length of the air channel. Calculated for a cell temperature of about 75 °C and an air stoichiometry of 1.8...

Flow inside a microchannel may operate in multiple flow regimes. Let us consider a long microchannel with the entrance pressure to be atmospheric and exit conditions to be near vacuum. As air flows down the channel, pressure drops to overcome viscous forces in the channel. For prevailing isothermal conditions, density also drops. The conservation of mass requires the flow to accelerate down the constant area tube. The fluid acceleration affects the pressure gradient, resulting in nonlinear pressure drop in the channel. The Mach number increases down the tube, limited only by the choked flow condition. Mean free path increases with the corresponding increase in Knudsen number. The simple duct flow manifests all possible complexity in flows. Note that similar situation may take place when the entrance pressure is high, that is, at 7 atm and exit pressure is atmospheric. 

Flooding. When a stable rathole forms in a bin and fresh material is added, or when material falls into the channel from above, a flood can occur if the bulk sohd is a fine powder. As the powder falls into the channel, it becomes entrained in the air in the channel and becomes fluidized (aerated). When this fluidized material reaches the outlet, it is likely to flood from the bin, because most feeders are designed to handle sohds, not fluids (see Eluidization). Fimited Discharge Kate. Bulk sohds, especially fine powders, sometimes flow at a rate lower than required for a process. This flow rate limitation is often a function of the material s air or gas permeabihty. Simply increasing the speed of the feeder does not solve the problem. There is a limit to how fast material... 

As more air was added to the channel, the slug flow became unstable, the slug bubble broke down, and eventually the churn flow occurred in the channel. As shown in Fig. 5.3d, the most significant feature of flow characteristics in the churn flow is that the pressure oscillated at a relatively high amplitude, since the gas plug and liquid bridge flowed through the test section alternatively. 

Two-phase flow in parallel pipes, fed from a common manifold, displays interesting phenomena, as two phases may split unevenly when entering the parallel piping. Ozawa et al. (1979, 1989) performed experimental smdies on two-phase flow systems in parallel pipes of 3.1 mm diameter. They simulated the flow in boiling channels by injection of air and water into the pipes. 

Only a few experimental investigations deal with heat transfer of gas-liquid flow in the conventional size channels. There is a significant discrepancy between experimental results on heat transfer presented for channels of dh = 1-100 mm. No data is available in the literature on gas-liquid heat transfer in miero-channels, except for the results on the study of heat transfer in the test section that contains 21 parallel triangular micro-channel of r/h = 130 pm reported in the present chapter. In the range of superficial velocities Uls = 0.015-0.244 m/s, Ugs = 0.50—28.6 m/s the heat transfer coefficient increases with increasing liquid velocity and decreases with increasing air velocity. 

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