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I am solving a heat conduction problem in whcich a spherical shell is formed on a solid sphere.
Model equation :
From 0<r<R0 and 0<t<ttotal
(1/r^2)*d(Kb*r^2dT/dr)/dr=rho*Cb*dT/dt
From R0<r<R and 0<t<tb
(1/r^2)*d(Kf*r^2dT/dr)/dr=rho*Cf*dT/dt

Initial conditions :
(i)t=0 and 0<r<R0
T=T0
(ii)t=0 and R0<r<R
T=Tbath

Boundary conditions :
(i) r=0 and 0<t<ttotal
dT/dr=0
(ii)r=R0 and 0<t<tb
Kb*dT/dr|b=Kf*dT/dr|f
(iii) r=R and 0<t<tb
T=Tmpbath
(iv) 0<t<tb
Kf*dT/dr|f=rhof*deltaH*dr/dt+h*(Tbath-Tmpshell)
tb<t<ttotal
Kb*dT/dr|b=rhof*deltaH*dr/dt+h*(Tbath-Tmpshell)

Where :
r=variable radius in sphere and shell
R0=Radius of sphere
R=Radius of spherical Shell
t=time
tb=time for base solid dissolve
ttotal=time when entire shell ans sphere dissolve
rho=density
deltaH=latent heat
Kb=thermal conductivity of base sphere
Kf=thermal conductivity of spherical shell
T=temperature
T0=room temperature
Tbath=1250 degree celsius
Tmpbath=1540 degree celsius
Tmpshell=melting point of shell


How to write boundary condition (ii) and (iv) in comsol expressions function.