Gas layer

There have been a few other experimental set-ups developed for the IR characterization of surfaces. Photoacoustic (PAS), or, more generally, photothemial IR spectroscopy relies on temperature fluctuations caused by irradiating the sample with a modulated monocliromatic beam the acoustic pressure wave created in the gas layer adjacent to the solid by the adsorption of light is measured as a fiinction of photon wavelength... 

Pt/Ru Catalyst Polymer Pt Catalyst Porous Gas Layer Electrolyte Layer Diffusion Membrane Electrode... 

F. J. Miller, J. W. Easton, A. J. Marchese, and H. D. Ross, Gravitational effects on flame spread through non-homogeneous gas layers, Proc. Combust. Inst. 29(2) 2561-2567, 2002. 

The air gas-diffusion electrode developed in this laboratory [5] is a double-layer tablet (thickness ca.1.5 mm), which separates the electrolyte in the cell from the surrounding air. The electrode comprises two layers a porous, from highly hydrophobic, electrically conductive gas layer (from the side of the air) and a catalytic layer (from the side of the electrolyte). The gas layer consists of a carbon-based hydrophobic material produced from acetylene black and PTFE by a special technology [6], The high porosity of the gas layer ensures effective oxygen supply into the reaction zone of the electrode simultaneously the leakage of the electrolyte through the electrode... 

The hydrophobic gas layer of the air electrode [4] possesses high porosity (ca. 0,9 cm2/g), such that an effective oxygen supply through this layer is obtained. From the experimental porogrames measured by both mercury and 7 N KOH-porometiy the contact angle 0en of the hydrophobic material with water electrolytes is obtained (0eff =116° 118°). Because of... 

The investigation of the pore size distribution (Fig. 1) shows that nano-size pores (radius ca. 20 nm) predominate in the gas layer from this hydrophobic material. 

It must be noted that the effective diffusion coefficient (Di)eff is obtained by electrochemical measurements of air gas-diffusion electrodes with sufficiently thick gas layer so that the limiting process is the gas... 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

Experimental AE - I curves can be used for comparison of the air electrodes with respect to the transport hindrances. In order to illustrate this possibility in Fig. 9 are presented the AE - I curves for air electrodes with identical catalytic layers and gas layers differing in their thickness only. Apparently the hindrances in the transport of molecular oxygen will be higher in the electrodes with thicker gas layers. 

Figure 9. AE -1 curves of air electrodes with identical catalytic layer and gas layer with different thickness.

From Figure 9 it is seen that the value of AE increases more rapidly with the current density for the electrodes with thicker gas layer which is due to the higher transport hindrances. 

In Figure 10, we presented the value of AE at current density 200 mA/cm2 as a function of the thickness of the gas layer of the electrodes from Figure 10. 

It is seen that the transport hindrances remain practically constant at thickness of the gas layer up to 1 mm. The further increase of the gas layer thickness results in significant increase of the transport hindrances. 

One long side of the compartment wall was split into a large number of thin, horizontal strips and the heat flux from the gas layer to the center of each strip calculated using the well known expression... 

In scenario B, however, both the horizontal concurrent flame spread and the downward flame spread, in and below the hot gas layer, are directly linked to the rate of heat release. 

Convective heat transfer coefficient for the configuration = 15 W/m2 K Specific heat of gas layer = 1 kJ/kg K... 

Although mass transfer across the water-air interface is difficult in terms of its application in a sewer system, it is important to understand the concept theoretically. The resistance to the transport of mass is mainly expected to reside in the thin water and gas layers located at the interface, i.e., the two films where the gradients are indicated (Figure 4.3). The resistance to the mass transfer in the interface itself is assumed to be negligible. From a theoretical point of view, equilibrium conditions exist at the interface. Because of this conceptual understanding of the transport across the air-water boundary, the theory for the mass transport is often referred to as the two-film theory (Lewis and Whitman, 1924). 

Corn (C7), however, points out that adsorbed films or gas layers may exist between the particles and may alter the nature of the adhesive force. Corn also indicates that various investigators have derived a value of the order of 4na for Kw when adsorbed liquid films are involved. Bradley (Bll) has derived an identical expression where <7 is defined as the surface energy of the solid. All of these forms yield values of Kw of the same general magnitude. There are, however, other reports (Fuchs, F4, pp. 363, 373) that indicate adhesive forces between particles as much as two orders of magnitude smaller than these. 

The hottest fires may be associated with those cases where the fire is big enough to give flames to fill at least half the structure volume, cases where it is stoichiometric or just under ventilated, and cases where the hot gas layer is 10 ft (3 m) or more deep. Heavier fuels would be less likely to give the hottest fires, asthey may not receive enough heat feedback to vaporize the liquid and therefore they may be self limiting in terms of the burn rate. Where these conditions may be encountered, heat fluxes of 1320-1584 BTU/ft (250 to 300 kW/m ) may be experienced. In certain circumstances, (which are not yet fully understood) highly efficient combustion can occur with fluxes of 1848-2112 BTU/ft (350-400 kW/m2) and temperatures of 2,500°F (1,400°C). 

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