Mass convection internal flow

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

Ary given catalytic material can be abstracted based on the same underlying similar architecture — for ease of comparison, we describe the catalytic material as a porous network with the active centers responsible for the conversion of educts to products distributed on the internal surface of the pores and the external surface area. Generally, the conversion of any given educt by the aid of the catalytic material is divided into a number of consecutive steps. Figure 11.13 illustrates these different steps. The governing transport phenomenon outside the catalyst responsible for mass transport is the convective fluid flow. This changes dramatically close to the catalyst surface from a certain boundary onwards, named the hydrodynamic boundary layer, mass transport toward and from the catalyst surface only takes place... 

Flow In Round Tubes In addition to the Nusselt (NuD = hD/k) and Prandtl (Pr = v/a) numbers introduced above, the key dimensionless parameter for forced convection in round tubes of diameter D is the Reynolds number Re = (.7 ) u where G is the mass velocity G = m/Ac and Ac is the cross-sectional area Ac = kD2I4. For internal flow in a tube or duct, the heat-transfer coefficient is defined as... 

Peng, X. and Wang, B., (1993), Forced convection and flow boiling heat transfer in flat plates with rectangular microchannels. International Journal of Heat and Mass Transfer 36,14, pp. 3421-3427. 

Many formulations based upon these assumptions can be derived. One formulation can be converted into another using the definitions of density, internal energy and the ideal gas law. Though equivalent analytically, these formulations differ in their numerical properties. Each formulation can be expressed in terms of mass and enthalpy flow. These rates represent the exchange of mass and enthalpy between zones due to physical phenomena such as plumes, natural and forced ventilation, convective and radiative heat transfer, and so on. For example, a vent exchanges mass and enthalpy between zones in connected rooms, a fire plume typically adds heal to the upper layer and transfers entrained mass and enthalpy from the lower to the upper layer, and convection transfers enthalpy from the gas layers to the surrounding walls. 

The most important processes in monolith channel convection of exhaust gas, heat and mass transfer between the flowing gas and the washcoat, internal diffusion, catalytic reactions in the washcoat, heat and mass accumulation and heat conduction—are schematically depicted in Fig. 7. 

Cotta, R.M., Orlande, H.R.B., Mikhailov, M.D., and KakaQ, S. (2003), Experimental and Theoretical Analysis of Transient Convective Heat and Mass Trmsfer - Hybrid Approaches, Invited Keynote Lecture, ICHMT International Symposium on Transient Convective Heat And Mass Transfer in Single and Two-Phase Flows, Cesme, Turkey, August 17 - 22. 

ABSTRACT Internal temperature distribution and mass were measured during drying and pyrolysis of cylindrical samples of wood of 0, 14 and 44% moisture. A onedimensional model of drying and pyrolysis is modified to reflect the anisotropy of the wood. Inclusion of an instant axial convective mass flow is shown to reduce the time of conversion compared to simulations with no axial flow. This mass flow, contrary to a convective mass flow through a porous structure, is not in thermal equilibrium with the solid phase. The gas leaves at a lower temperature compared to the temperature of the solid phase and is thus neglected in the energy equation. 

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