Fix density formula and momentum equation

[imitrichev]

Fix density formula rho=(PM)/(RT)

and momentum equation (A/V is already taken into account in \dot m_s)

Also, please provide a description for \dot w_k, I don't understand what is it and if it is really needed in conservation eq. for fluid species k.
I feel like \dot w_k is molar rate of species k formation/consumption, and it is NOT needed in eqs. (4) and (5), due to eqs. are rewritten on MASS basis and this term is simply the same as the one with \dot s_k_j.

[speth]

Thanks, these changes make sense to me.

$\dot \omega_k$ is the molar rate of production of species $k$ from gas phase reactions, which definitely does need to appear in the species and energy equations. $\dot s_k$ in contrast is the molar rate of production from surface reactions. These are the same definitions used in the other reactor formulations (though the definitions should probably be given again here).

[imitrichev]

Dear Ray,
thank you for approving this.

Also, thank for explaining about \dot w - it is about gas-phase reactions. Of course, if PFR can account for gas-phase chemistry, these terms should be present in the equations.