Writing udf for Transient Heat conduction boundary value problem
 

I am solving a transient heat conduction boundary value problem and I am stuck at defining the udf for these boundary conditions.
Refer to page number 89 of the following paper for the boundary conditions.

http://eprints.nmlindia.org/5588/1/81-98.PDF

Is it possible to define a udf for this? If yes, how?

Just a tip: you're more likely to get an answer if you explain what it is you want to do and what have you done so far, rather then expect others to study your problem.
Cheers.

Regarding problem in boundary condition
 

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.

If you're ignoring fluid flow, what you have is a pretty straight heat conduction problem with phase change. You can solve that in Fluent without the need of UDFs by using the 3D solver with the solidification model.

The BC you mentioned will be handled automatically by Fluent in this case.