Water liquid saturation

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

Ohmura, R. Shimada, W. Uchida, T. Mori, Y.H. Takeya, S. Nagao, S. Minagawa, H. Ebinuma, T. Narita, H. (2004). Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance variations in crystal morphology. 

Ohmura, R. Matsuda, S. Itoh, S. Ebinuma, T. Narita, H. (2005d). Clathrate Hydrate Crystal Growth in Liquid Water Saturated with a Guest Substance Observations in a Methane + Water System. Crystal Growth Design, 5(3), 953-957. 

The primary drainage and steady-state flow simulations, described in the earlier section and typically devised in the petroleum/reservoir engineering applications,53 54 58,60 were deployed to evaluate the capillary pressure and relative permeability relations as functions of liquid water saturation, respectively. 

The capillary pressure response, a direct manifestation of the underlying pore morphology, can be evaluated from the two-phase LB drainage simulation and the corresponding transport relation as function of liquid water saturation can be devised as shown in Fig. 20 for the reconstructed CL micro structure.21 The overall shape of the capillary pressure curve agrees well with those reported in the literature for synthetic porous medium.55 The capillary... 

Figure 20. Capillary pressure — liquid water saturation relation for die reconstructed CL microstructure.

Figure 21. Capillary pressure - liquid water saturation relation for a non-woven GDL structure using the FM model.

With the liquid water distribution available from the two-phase LB simulation corresponding to a saturation level, the reduction in electrochemically active interfacial area (ECA) owing to liquid water coverage can be estimated from the 2-D saturation maps and subsequently a correlation between the effective ECA and the liquid water saturation can be established as the following 27,62... 

Using the 2-D saturation maps from the two-phase LB simulation, shown in Fig. 14, the effective ECA can be evaluated and correlated according to Eq. (26). Based on several liquid water saturation levels, the catalytic surface coverage factor for the CL microstructure is estimated and the following correlation can be constructed, which can be used as valuable input to macroscopic two-phase fuel cell models.27,62... 

Figure 23 shows the variation of the effective ECA with liquid water saturation from the evaluated correlation given in Eq. (27) along with the typical correlations with ad-hoc fitting of the coverage parameter otherwise used in the macroscopic fuel cell models.2 It is to be noted that the effect of liquid water is manifested via a reduction of the active area available for electrochemical... 

Figure 23. Catalytic site coverage relation as function of liquid water saturation for the CL structure.

This estimate could prove to be valuable input for more accurate representation of the pore blockage effect in the macroscopic two-phase fuel cell models. Figure 24 shows the variation of the effective oxygen diffusivity with liquid water saturation from the correlation in Eq. (29) along with the typical macrohomogeneous correlation with m = 1.5 and b = 1.5 otherwise used arbitrarily in the macroscopic fuel cell modeling literature. 

Figure 25 displays the anisotropic effective oxygen diffusivity variations with liquid water saturation in the GDL based on the evaluated pore blockage correlations. Furthermore, the impact of GDL compression on the pore blockage effect was also investigated.67... 

With the evaluated site coverage and pore blockage correlations for the effective ECA and oxygen diffusivity, respectively, and the intrinsic active area available from the reconstructed CL microstructure, the electrochemistry coupled species and charge transport equations can be solved with different liquid water saturation levels within the 1-D macrohomogeneous modeling framework,25,27 and the cathode overpotential, q can be estimated. 

The phase rule (eq. 3.1) implies there cannot be water ice in addition, but would replace one other (or, given the rules, in fact, two other) phase(s) (namely there would then, at lower temperatures, exist neither liquid water-saturated cyclohexanone with dissolved NaSCN nor solid NaSCN)... 

For many years, compilations of physical properties of liquid water, saturated steam, and superheated steam issued in steam tables have been standard references for mechanical and chemical engineers involved with steam cycles for electrical power generation. Steam tables are contained in Tables B.5-B.7 of this text. We recommend that you look at these tables as we describe what you can find in them. 

The role of the porous structure and partial liquid-water saturation in the catalyst layer in performance and fuel cell water balance has been studied in Ref. 241. As demonstrated, the cathode catalyst layer fulfills key functions in vaporizing liquid water and in directing liquid-water fluxes in the cell toward the membrane and cathode outlet. At relevant current densities, the accumulation of water in the cathode catalyst layer could lead to the failure of the complete cell. The porous structure controls these functions. 

A comprehensive water transport model was developed to account for effect of liquid water on the performance of the cell. This involved solution of an additional transport equation for liquid water saturation. Effects of convection, surface tension, electro-osmotic drag, gravity and surface tension are taken into account in this model. 

The liquid water saturation is obtained by integration over the distribution funetion... 

Figure 2.2. Model pore size distributions that represent bimodal porous structures in CCLs, calculated with Eq. (2.2) [25]. The three distributions posses the same total porosity but varying volume portions of primary and secondary pores. The equilibrium capillary radius, is shown for the specified operating conditions. Values of the corresponding liquid water saturations (areas under the distribution functions within the grey box) are given.

Figure 2.7. Catalyst utilization at macroscopic scale, i.e. the catalyst utilization factor /(Xptc, Xei) (>Sr)/Xptc in the exchange current density (top panel, cf. Eqs. (2.65) and (2.66)), and oxygen diffusion coefficient (bottom panel, cf. Eq. (2.63)) as functions of the liquid water saturation, plotted for the three different pore size distributions in Figure 2.2 [35]. The plots reveal the effect of porous structure on the basic competition between activity (top) and mass transport (bottom) in the CCL. The structure with a large fraction of primary pores is beneficial for catalyst utilization and detrimental for gas diffusion, and vice versa.

It relates the local distributions of liquid and gas pressures to the local capillary radius, re, which further determines the local liquid water saturation, 5 r, via Eq. (1.4). Thereby, all transport coefficients are determined and a closed system of equations is provided. Due to complex relations... 

When both phases exist, liquid chlorine saturated with water is in equilibrium with liquid water saturated with chlorine. The activity of water is by definition of equilibrium the same in both phases. Considering the composition of the water-rich liquid, we see that this activity is very nearly equal to that of pure water. Setting the activity of pure water at unity, we have for the water in the chlorine-rich phase 5 = 1, or... 

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