PDE and ODE implementation

Hi,

I would like to make a frost growth model where ODE and PDE equations are involved such as:

ys * d(rho,t) = d(P,T)
rho* d(ys,t) = m - d(P,T)

And energy balance equation
k*d^2(T,y) =(m - rho*d(ys,t))/ys*L

With a B.C.
k*d(T,y) = h(Ta-T)+m*L

My variables in equations presented above are ys, T, and rho, the others are known.

My guess is that the first 2 eq. can be implemented in global equations ODE.
Row 1: Name: T; f(u,ut,utt,t): rho rhot*ys- d(P,T)
Row 2: Name: T; f(u,ut,utt,t): ys yst*rho-m+d(P,T)

but i face problems. when i am trying to implement energy balance eq in the PDE with 2 variables ys and T
and also the BC in the flux/source node from PDE

Can I please have some hints how should I do?

Thank you