Transverse Heat and Mass Transfer Correlations

Despite the small gap size, transverse temperature and concentration gradients may exist in microchannels. These gradients depend on the rates of heat and mass transfer versus the rate of reaction. Da, as discussed above, provides an estimate of transport effects but cannot be used for quantitative modeling. The Nusselt number (Nu) and Sherwood number (Sh) provide the heat and mass transfer coefficients, respectively, between a surface and the bulk gas, in the direction perpendicular to the flow. They are defined as ratios of heat or mass flux at the boundary to the net flux between the bulk and the surface  

Nu and Sh correlations developed for laminar flow [19] are often used to obtain transverse transport in both micro- and macroscale reactors [20]. Since the older correlations were developed using simplifying assumptions, they are not applicable for highly exothermic reacting flows and new correlations have been developed since [21, 22]. 

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