TRANSFER: BASICS OF MASS AND HEAT TRANSFER - Biomass as a Sustainable Energy Source for the Future - page 124

(blowingeffect). This is a similar effect as describedby the difference between theRanz -

Marshall correlation Equation (4.26) and the correlation for the evaporating droplet
Equation (4.31). R SA is the ratio of the real surface area to the ideal surface area (sphere,
cylinder, or flat plat) and hence takes into account the effect of surface roughness. T b is
the bulk gas temperature, andT W is the temperature of the surroundings relevant for radi-
ative heat transfer. Note that in the radiative term for simplicity, the same emissivity ε e is

used for the particle surface and the surrounding wall. For each shape, an appropriate
heat transfer coefficient has to be used. In the case of devolatilization of a particle cloud,
if available, a particle size distribution and effects of random particle orientation can be
taken into account. In the absence of such information, the average particle dimension
can be used as the characteristic length. For more details, see Lu et al. (2010).

CHAPTER SUMMARY AND STUDY GUIDE

Transfer of mass is described by the combination of general transport equations for
species mass fractions and specific models for transport by diffusion (Fick ' s law as

a particular case, Stefan Maxwell equations are more generally applicable). Transfer

of energy is described by the combination of a general transport equation for an energy
variable (internal energy, total energy, enthalpy, temperature), a specific model for
transport by diffusion, and a source term due to radiation. Radiative energy transport
in general is described by the radiative heat transfer equation. In many cases, it is suf-
ficient to only consider a balance between radiative energies leaving different sur-
faces. The complexity of heat and mass transfer processes between a fluid and a
solid is described effectively using the concept of transfer coefficients. In processes
involving a phase change, such as evaporation of an ethanol droplet, the energy
needed for this phase change has to be taken into account. In many processes, mul-
tiphase systems appear, with solid, liquid, and/or gas present, and additional concepts
such as the porosity of a biomass particle are needed.

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KEY CONCEPTS

Diffusion
Convection
Radiation
Mass flux
Heat flux
Transfer coefficient
Nusselt number
Sherwood number
Evaporation
Devolatilization
Combustion
Reacting flow