The default transport model built into PHOENICS may be used with the thermodynamic and transport properties calculated in the CHEMKIN and TRANLIB routines. However, for laminar reaction- diffusion processes it is frequently the case that a more accurate model is required.
As an alternative to the default model, the"Curtiss-Hirschfelder" model of mass transport, including thermal
diffusion (the Soret effect) has been supplied as an option.
However, the present implementation of the "Curtiss-Hirschfelder"
model is under investigation at CHAM, and so it is recommended
that Ficks law be employed, as done for example in CHEMKIN
Library cases C204 and C205.
The default model calculates the diffusive mass-flux, J(k), from the formula
There is a difficulty with the thermal diffusion term which is that the term is proportional, through the dependence of kT on Y(k), to the mass-fraction itself rather than to the spatial derivative.
The thermal diffusion term behaves more as a convective flux than as a
diffusive flux, and so the same difficulties that are associated
with the convective terms can be expected to be encountered.
Theimplication is that the thermal-diffusion flux should be upwinded.
For the enthalpy equation there is a further potential complication. In addition to the conductive heat flux, there is also an enthalpy flux associated with the mass-diffusion. The additional term is
The user is advised that the present implementation of the transport model
The enhanced transport algorithm involves coding sequences in group 8 of EARTH. The finite difference form for the species gradient term is;
The finite difference form for the thermal diffusion term is
Su(k;+) = Su(k;+) + J''(k).Y(k)
Ap(k;+) = Ap(k;+) + J''(k)
Su(k) = Su(k) - J''(k).Y(k;+)
Ap(k) = Ap(k) - J''(k)
In addition, the modifications to the diffusion coefficient, ie. the division by mean molecular weight, necessitate an entry to GXCHKI to multiply the laminar wall function coefficient for a mass-fraction by the mean molecular weight.
The enhanced transport model is activated by setting 2 variables in the Q1 file:
If the modified diffusion law is in use, any laminar wall- functions for mass-fractions should be setup using a PATCH-name starting with CKWAL and GRND9 in the COefficient slot of the COVAL. For example,
PATCH(CKWALIN,SWALL,1,NX,1,NY,1,NZ,1,LSTEP) COVAL(CKWALIN,H2,GRND9,0.025)
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