Hi. Just for illustration purposes I am trying to model the temperature in the walls of a hollow sphere. The purpose is to (very crudly) approximate the heating effects of a combustion process (running engine). It is assumed that there is gas inside of the sphere. Assumptions: A. Inner and Outer radii are constants - Ri and Ro, respectively. B. The temperature outside of the sphere is constant - Tw (w stands for "water") C. The temperature of the gas (Tg) is assumed to be uniform and either a) Tg(t) = C (constant) or b) Tg(t) = C1 + C2*sin(w*t) where "t" naturally denotes time. I want to study mainly the long term solution, but for fun also the transients. So far so good. Now to the problematic part. How to model and solve this? I understand the following: I. Because of symmetry it can be modeled as a one-dimensional problem (T(r,t)) II. The Outer surface temperature of the sphere (T(Ro,t)) should be modeled as T(Ro,t)=Tw. Or maybe it shouldn't? After that I am lost! Is it a good idea to model the Inner surface temperature (T(Ri,t)) of the sphere as T(Ri,t)=Tg(t)? I don't think it corresponds very well to reality, or does it? Maybe it would be better to model the flux across the surface in some way, but how? Please help! I really would like to model and solve this in an appropriate manner.