Interphase heat transfer correlations

Superheated steam has heat transfer properties superior to air at the same temperature. Since there is no resistance to diffusion of the evaporated water in its own vapor, the drying rate in the constant rate period is dependent only on the heat transfer rate. The convective heat transfer coefficient (h) between steam and the solid material surface can be estimated using standard correlations for interphase heat transfer. Neglecting sensible heat effects, heat losses, and other modes of heat transfer, the rate at which surface moisture evaporates into steam is given simply by... 

Our discussion of multiphase CFD models has thus far focused on describing the mass and momentum balances for each phase. In applications to chemical reactors, we will frequently need to include chemical species and enthalpy balances. As mentioned previously, the multifluid models do not resolve the interfaces between phases and models based on correlations will be needed to close the interphase mass- and heat-transfer terms. To keep the notation simple, we will consider only a two-phase gas-solid system with ag + as = 1. If we denote the mass fractions of Nsp chemical species in each phase by Yga and Ysa, respectively, we can write the species balance equations as... 

In the rate-based models, the mass and energy balances around each equilibrium stage are each replaced by separate balances for each phase around a stage, which can be a tray, a collection of trays, or a segment of a packed section. Rate-based models use the same m-value and enthalpy correlations as the equilibrium-based models. However, the m-values apply only at the equilibrium interphase between the vapor and liquid phases. The accuracy of enthalpies and, particularly, m-values is crucial to equilibrium-based models. For rate-based models, accurate predictions of heat-transfer rates and, particularly, mass-transfer rates are also required. These rates depend upon transport coefficients, interfacial area, and driving forces. It... 

For interphase limitations (boundary layer effects) the situation seems, at first glance, as simple as that for internal gradients, since most correlations for heat-and mass-transfer eoeffieients show a proportionality to the flow velocity of the surrounding fluid, u", where normally 0.6 < n < 1. At the lower velocities associated in particular with laboratory reactor operation, however, n tends to be closer to 0.6 than to 1, and the transport coefficients become insensitive to flow velocity and changing flow velocity is not an effective diagnostic. 

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