Resistance fluid dynamics

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Kosterin, S. I., 1949, Study of Influence of Tube Diameter and Position upon Hydraulic Resistance and Flow Structure of Gas-Liquid Mixtures, Izvestiza Akademii Nauk SSSR, Otdelema Tekhni-cheskikh No. 12, 1824, Translation 3085, Henry Brutcher Tech. Translation, Altadena, CA. (3) Kottowski, H., and G. Grass, 1970, Influence on Superheating by Suppression of Nucleation Cavities and Effect of Surface Microstructure on Nucleation Sites, Proc. Symp. LIM Heat Transfer and Fluid Dynamics, p. 108, ASME, New York. (2)... 

While it should be self-evident that a rational reactor design demands a knowledge of both the fluid dynamic environment and the detailed process kinetics, the latter are rarely available. In many instances this leads to the severe limitation of many important reactions by an inadequate fluid dynamic intensity. Some of these are known to be fast, e.g., liquid-phase nitrations, while others are (incorrectly) assumed to be slow, e.g., most polymerizations. In these circumstances the pragmatic approach is to use a high-intensity reactor for each system and then to assess the impact upon the space-time productivity. Obviously, an intrinsically slow system is resistant to further acceleration and this will rapidly become evident. One significant qualification of this contention involves the very... 

In previous studies, Sooter (2) and Satchell (3) observed that reducing the particle size of the Nalcomo 474 catalyst from 8-10 mesh to 40-48 mesh did not have any significant effect on the desulfurization and denitrogenation of Raw Anthracene Oil under similar experimental conditions as employed in this study. This suggests that the fluid distribution and hence the fluid dynamic effects, were not important in the trickle bed reactors as operated for this work. If these effects were important then the reduction in particle size should increase conversion of the HDS and HDN by improving fluid distribution and reducing the intraparticle diffusion resistances. 

In the presence of significant thermal effects, one of the above-mentioned fluid dynamic scale-up criteria must be considered, together with the criterion US/V = constant, which can be made more realistic by considering that heat exchange surface S can be different from SL and by introducing a proper functional relationship for U. If the internal resistance to heat transfer prevails, U may be intended as the internal heat transfer coefficient h, so that the relationship... 

A boundary layer is formed between the two phases (fluid and solid). This is a stagnant film that represents a layer of less movement of the fluid and hence builds up a zone with resistance to mass transfer. The mass transfer coefficient and generally the mass transfer rate depend on the fluid dynamics of the system. Higher fluid velocities significantly reduce the thickness of the film. 

Reactors to obtain accurate intrinsic kinetic rate data which are needed for design, scaleup, and optimization purposes. In these reactors the fluid dynamics and various heat and mass-transfer resistances are either known or amenable to rigorous calculations. 

For enhancing the removal, it is important to reduce those resistances as much as possible by acting on the fluid dynamic (for kg and ki) and on the membrane properties (for km)- Increments of the liquid and gas flow rates, as a module design that promotes mrbulent flow, lead to a reduction of gas and liquid boundary layers, with consequent improvements of the mass transport. 

To find a rate, one must generally identify the driving force and the resistauice against flow. We elaborated this for a number of examples in sec. 1.6.4. All these examples involved macroscopic amounts of fluid, moving under the influence of external forces and having a resistance of a viscous nature. Under such conditions solution of the Navier-Stokes equation [1.6.1.15] or variants thereof, suffices to describe the fluid dynamics. For a droplet, spreading on a (Fresnel) surface, the situation is more complicated. Flow in the bulk of the drop obeys Navier-Stokes... 

Biofilm development is affected by both physical and chemical factors. The abundance and condition of bacteria in the water column plays a major role in initial rate of settlement on a surface 181]. Surface factors such as wettability [82] and critical surface tension [83], surface hydrophobicity [84], fluid dynamic forces [85], shear stress [86], electrolyte concentration [87] and metabolic inhibitors [88] can all affect microbial attachment, adhesion or growth. The low surface energy of a gorgonian octocoral has been implicated as a passive fouling resistance mechanism used in conjunction with other antifouling defences [82]. 

Hoerner, S.F. (1965,1993) Fluid-Dynamic Drag practical information on aerodynamic drag and hydrodynamic resistance, Hoerner Fluid Dynamics, Midland Park, N J. 

However, the definitions related to specific processes have to be kept in mind. In chromatography the plate height is a measure that lumps together the contribution of the fluid dynamic non-idealities (axial dispersion) and the mass transfer resistance... 

Resolution can be increased if one of the three terms in Eq. 4.14 is increased. As mentioned in Chapter 2.4.3 the first term describes the influence of selectivity. This term should be maximized by maximizing a in a selectivity screening with different adsorbents and mobile phases. The second term should be kept in a certain range and not be maximized, because the maximum value of 1 is reached for an infinite retention factor. At infinite retention the productivity would decrease due to the high cycle time. The last term of Eq. 4.14 describes the efficiency of the column in terms of the number of plates. Resolution can be increased by selecting efficient adsorbents with small particle size and appropriate narrow particle size distribution. For these adsorbents fluid dynamic and mass transfer resistances are minimized. Con-... 

Drop deformation occurs when fluid dynamical forces, often referred to as shear forces , in the surrounding fluid act on its surface. Surface and internal viscous forces resist it. Drop dispersion (breakage) occurs when the shear forces exceed the combined resistance force. 

The results of this section indicate that the models based on a purely diffusive mechanism of mass and heat transfer are incomplete because nonequilibrium surface concentration, interfacial mass transfer resistance, and the fluid dynamic regime around and within the droplets must be taken into account as described by Eqs. (42) to (49). 

Dynamic shear rheology involves measuring the resistance to dynamic oscillatory flows. Dynamic moduli such as the storage (or solid-like) modulus (G ), the loss (or fluid-like) modulus (G"), the loss tangent (tan 8 = G"IG ) and the complex viscosity ( / ) can all be used to characterize deformation resistance to dynamic oscillation of a sinusoidally imposed deformation with a characteristic frequency of oscillation (o). 

The maximum temperature of the specimens was observed at the top of the nose due to the fluid dynamic conditions. The target temperature was set at 1,800 K to maintain the margin at the melting point of Si02, the oxidation resistant layer produced from SiC. 

The distribution of most chemical species in the middle atmosphere results from the influences of both dynamical and chemical processes. In particular, when the rates of formation and destruction of a chemical compound are comparable to the rate at which it is affected by dynamical processes, then transport plays a major role in determining the constituent distribution. In an environment like the Earth s atmosphere, air motions, and hence transport of chemical species, are strongly constrained by density stratification (gravitational force) which resists vertical fluid displacements, and the Earth s rotation (Coriolis force) which is a barrier against meridional displacements. Geophysical fluid dynamics describes how atmospheric motions are produced within these constraints. 

The contribution of concentration polarization to overall dialytic mass transfer resistance suggests that dialyzera should be operated with flows in the lurbolenl flow region to minimize boundary-layer formation. While this is typically the case on the dialysate side of the membrane, diaiyzers usually are operated with leminar flow on the fsed side. In hemodialysis, this is partly to avoid undue mechenical stress on the blood cell9 which mey result in their destmction. However, a consideration of fluid dynamics dictates that feed-side flow be leminar in nearly all applications. 

If external resistance is rate controlling, a mass transfer coefficient, Ks, can be introduced. From the fluid dynamics of the reaction vessel, it is possible to estimate substrate concentration at enzyme catalytic sites according to the steady state relationship ... 

The elution profile of an ideal chromatogram depends only on the courses of the equilibrium functions characterizing the chromatographic system. To understand and predict real chromatograms additional mass transfer resistances and details regarding the fluid dynamics have to be taken into account. 

The plate height lumps together the contributions of fluid dynamic nonidealities (e.g., axial dispersion), mass transfer resistances, and finite adsorption and desorption rates, which all contribute to undesired band broadening. It can be defined as the rate of the local gradient of the width of a Gaussian peak (Equation 2.31) ... 

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