Transport Time-scales

For a passive scalar, the turbulent flow will be unaffected by the presence of the scalar. This implies that for wavenumbers above the scalar dissipation range, the characteristic time scale for scalar spectral transport should be equal to that for velocity spectral transport tst defined by (2.67), p. 42. Thus, by equating the scalar and velocity spectral transport time scales, we have23 t)... 

Krt < k scalar spectral transport time scale defined in terms of the velocity spectrum (e.g., rst). 

For the design of micro heat exchangers, it has to be considered that both heat and mass transport time-scales are strongly correlated with the characteristic dimensions of the exchanger according to diffusion theory [8,9] ... 

Finally, we demonstrate in discussions on weathering rates in the field, on the kinetics of colloid chemical processes, and on the role of surficial transport processes in geochemical and biogeochemical processes that spatial and temporal heterogeneities and chemical versus transport time scales need to be assessed in order to treat the dynamics of real systems. 

Mass and heat transport Time scale (s) influenced... 

Results liquid water increases time for current density to reach steady state temperature does not have significant effect on current density increasing porosity decreases mass transport time scale increasing GDL thickness delays influence of liquid water Cathode, anode, and membrane potentials during start-up and shut-down... 

Gradients depend on the ratio of time scales of various transport and chemical processes. Diffusion (conduction) time scales can easily be estimated from the square of the corresponding length scale divided by the diffusivity (thermal diffusivity). Temperature usually has a fairly small effect on transport time scales (an exception is surface diffusion that is often activated). On the other hand, the time scale of reaction depends very strongly on the chemistry (process) itself and the temperature (via Arrhenius kinetics) and secondary on species concentrations and pressure. Discontinuity at the walls (e.g. slip, lack of thermal accommodation) may also be encountered, but since these phenomena depend on transverse gradients, which are smaller than in large devices, are by-and-large less important in microdevices (I). 

Figure 10.1 Typical transport time scales in between 0.1 and 1 cm s (the latter is more microchemical systems. The chemistry time typical under operating conditions). The reactor...

For the axial convection inside the microreactor, the characteristic time (g X UUju (not accounting for flow acceleration of the gas due to combustion) was estimated between 6 and 33 ms. Diflhisive transport time scales of the gas in... 

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