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2.2 Air 133 A suffix often characterizes the masses moving: a for air; da for dry air; v for water vapour, m for moisture and w for water. Solving a mass transfer problem means determining two fields: that of the potentials (Po), which cause the transfer and that of the mass fluxes (g). The unknown quantities thus are Po(x,y,z,t) and g(x,y,z,t), the one scalar (Po), the other vectorial (g); computing requires a scalar and a vector equation. The scalar equation follows from mass conservation, stating that the mass flow exchanged between a system and its environment, plus the mass produced in or removed from the system per unit of time, called a source or a sink, equals the changing quantity of mass in the system per unit of time: div…gx † Sx ˆ @wx @t (2.8) with Sx the source or sink in kg/(m3.s). Flows can be diffusive and convective, the first depending on the gradient and the second on the driving force itself. The relationships between fluxes and driving forces offer the vector equations needed. 2.2 Air 2.2.1 Overview Dry air is a mixture of 21% by volume of oxygen (O2), 78% nitrogen (N2) and traces of other gases (CO2, SO2, Ar, Xe). Generally, dry air is assumed to behave as an ideal gas with equation of state: pdaV ˆ mdaRdaT (2.9) where pda is the (partial) dry air pressure in Pa, T the temperature in K, mda the mass of (dry) air in kg filling the volume V (m3), and Rda the gas constant for dry air, 287.055 J/(kg.K). In reality, dry air is non-ideal, obeying following equation of state: pdaV ndaRoT ˆ 1 ‡ Baa V=nl … † ‡ Caaa V=nl … †2 with nda the number of moles filling the volume V (m3), Ro the general gas constant, 8314.41 J/(mol.K), and Baa and Caaa viral coefficients, which at ambient temperature are almost zero: Baa ˆ 0:349568 104 0:668772 102 T 2:10141 T2 ‡ 92:4746 T3 m3=mol Caaa ˆ 0:125975 102 0:190905 T T2 m6=mol2 ‡ 63:2467 For the dry air concentration the ideal gas law gives: ρda ˆ mda V ˆ pda=…RdaT† (2.10)


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