Continuous Casting

Hi,
I am modeling solidification of casting process using the following model:
www.comsol.com/model/continuous-casting-382

The alloy I am using is different hence the parameters are different. The geometry is similar, except that I have a larger diameter.

I have estimated the heat transfer coefficients and they are 3-4 times that used in this problem. My solution does not converge for these higher values. However, using lower values of heat transfer coefficients, I obtained convergence.

What could be the source of this error? How do I tackle it?

Thanks in advance