Two-equation turbulence model

Hello,
I tried to add two-equation turbulence model (k-epsilon turbulence model) for simulating air flow behaviour in an air cooled heat exchanger. I referred to the heat sink example for problem setting because the basic physics is similar. However, I always encountered a problem “loss went to Nans” during training. My SimNet version is 21.06.
Would you please tell me what’s wrong it is and share some experience to improve the convergence? What’s more, I am wondering if you could kindly send me an example (two-equation turbulence model) for reference.

Thanks,
Zhiyong Wu