Reaction rate distribution

Impregnating these layers with PFSA ionomer for enhanced proton conduction or hydrophobizing agents like Teflon for sufficient gas porosity is optional. However, ionomer impregnation is indispensable in CLs with thicknesses of > 1 ftm. Ultrathin CLs with - 100-200 nm, on the other hand, can operate well without these additional components, based on sufficiently high rates of transport of dissolved reactant molecules and protons in liquid water, which could ensure uniform reaction rate distributions over the entire thickness of the layer. 

Overall, the rib effects are important when examining the water and local current distributions in a fuel cell. They also clearly show that diffusion media are necessary from a transport perspective. The effect of flooding of the gas-diffusion layer and water transport is more dominant than the oxygen and electron transport. These effects all result in non-uniform reaction-rate distributions with higher current densities across from the channels. Such analysis can lead to optimized flow fields as well as... 

Three dimensional electrode structures are used in several applications, where high current densities are required at relatively low electrode and cell polarisations, e g. water electrolysis and fuel cells. In these applications it is desirable to fully utilize all of the available electrode area in supporting high current densities at low polarisation. However conductivity limitations of three-dimensional electrodes generally cause current and overpotential to be non-uniform in the structure. In addition the reaction rate distribution may also be non-uniform due to the influence of mass transfer.1... 

Figure 18.15 shows the reaction rate distribution across the reactor bed. As observed during the initial few seconds of the reaction, the flux follows a nonuniform profile similar to that in... 

Determining the current distribution in the reactor is a central problem in electrochemical engineering [263, 264], The problem is to find the reaction rate distribution along a macroscopic surface (electrode) or along the walls of a microscopic feature... 

Fedotov, S. Nonumfoim reaction rate distribution for the generalized Fisher equation ignition ahead of the reaction front. Phys. Rev. E 60(4), 4958-4961 (1999). http //dx.doi. org/10.1103/PhysRevE.60.4958... 

In the case a = 1, Equation reduces to the original Tafel law with to the uniform reaction rate distribution. 

The nonuniform distribution of protons and potential in water-filled agglomerates and ultrathin catalyst layers is predominantly an electrostatic effect. It is determined by the Debye length. Ad- Resulting reaction rate distributions and effectiveness factors depend on the characteristic sizes of agglomerates (i a) or ultrathin CCLs ( L) and on the transfer coefficient a. 

In the oxygen depletion regime, jo I, only a thin sublayer with thickness L, adjaeent to the GDL, is active. The remaining sublayer with thickness L — 6eff) L, adjaeent to the membrane, is not used for reactions, due to the starvation in oxygen. For this situation with rather nonuniform reaction rate distribution, catalyst is used very ineffeetively. The inactive part causes overpotential losses due to proton transport in the polymer electrolyte, which could cause limiting current behavior, if the proton conductivity is low. 

With the new ERANOS calculation scheme and data, all of these effects are treated explicitly and, provided that the cross sections are sufficiently accurate and that there are no compensating errors (the power map distribution and the reaction rate distributions of a large core have not been used in the adjustment), the comparison should be satisfactory. 

As a conclusion, JUPITER experiment and analysis was found to possess sufficient consistency on the whole, especially for the prediction of criticality, space-dependency of C/Es in core region, and sodium void reactivity, which were persistent problems in the past JUPITER evaluations. It was also recognized that there is, however, some room for further improvements about the C28/F49 ratio, reaction rate distribution in blanket region, and Doppler reactivity. Efforts are now being conducted from various viewpoints such as re-evaluation of experimental and analytical errors, application of new most-detailed analytical tools, comparisons with other experimental cores, and refrnement of statistical tests for physical consistency. 

After the criticality test, the reactor physics test was performed and core reactivity worth, core reaction rate distribution, core flow rate distribution, etc. were measured. 

In the center of the original core and then in the core with neptunium there were measured by several methods, the ratios of average fission cross-sections for 16 isotopes, including minor actinides, and of capture cross-sections in aurum, neptunium, uranium-238, the central reactivity coefficients with the use of samples, the sodium void effect of reactivity, the efficiency of a mock-up of the central control rod with enriched and natural boron carbide, ad well as the fission reaction rate distributions with height. 

This section describes the calculational methods and modelling assumptions used for the neutron transport calculations which established the neutron flux and reaction rate distributions along the boiler duct penetrations througji the bio-shield. 

Electrochemical reactions occur only at those Pt particles where the three phases meet. Major constraints of this design are (1) statistical limitations of the Pt utilization due to the random three-phase morphology and (2) highly non-uniform reaction rate distributions that arise when the thickness of the layer ( Z, l )... 

At this point, it is important to realize that the ultimate optimization target of electrode design is not Pt utilization, which is a static statistical property of a catalyst layer, but more importantly the effectiveness factor, which includes as well the effects of non-uniform reaction rate distributions due to mass transport phenomena at finite current densities in the operating fuel cell. In simple ID... 

In the past, studies of the macrohomogeneous model have explored the effeets of thickness and composition on performance and catalyst utilization. At the outset, it should be noted that these works neglected the effects of liquid water accumulation in pores on performance. The specific effects due to the complex coupling between porous morphology, liquid water formation, oxygen transport, and reaction rate distributions will be discussed in Section 8.5.5. The results presented in this section are only valid at sufficiently small current densities, for which liquid water accumulation in secondary pores is not critical. 

The traditional way to calculate the physical characteristics of a fast reactor is to carry out the following steps (1) preparation of the effective cross sections for regions of the reactor (2) a three-dimensional calculation to obtain k-eff, and real and adjoint fluxes (3) edit the results of the previous steps to estimate the power and reaction rate distributions, neutron kinetics parameters, control rod effectiveness, etc., and (4) a bumup analysis, calculating the variation of the isotopic composition with time, and then recalculating the results obtained in the previous steps for particular bumup states. This scheme has been implemented, for example, in the TRIGEX code [4.49]. This code calculates k-eff, few group real and adjoint fluxes, power spatial distribution, dose factor and reaction rates distributions, breeding parameters, bumup effects, and kinetics parameters (effective delayed neutron Auction, etc.). 

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