Mesh quality is as indicated in the very first image , i.e. orhto quality approx 0.15 ish.
Regarding reference temperature, I am not sure what the correct value should be. I know that my bulk inlet is at 70C and wall is at 28C, and have a mass flow rate for a certain reynolds.
I tried various reference temperature as follows
22.85 was randomly chosen, this is when the htc was negative
49 was chosen as a mean of bulk and wall temperature. Here the residual do converge, however, i am unsure if the htc is correct. (Exp result for this bc of 70 C bulk and 28C wall should yeild htc of 2592.03)
30 was chosen becuase I thought ref temperature could be the free stream temperature around the tube wall, so would be slightly higher than wall temperature of 28. This didn't converge
70 was assumed becuase, again, I thought, ref temperature for internal flow could be of bulk temperature, as that would be the free stream temperature as athe temperature converges. However, this too didn't converge.
( Not sure if it is important , but I also got turbulent viscosity greater than 1e5 in so and so cells in many of the meshes that I tested. I corrected this with initializing epsilon to 0.01 and k to 100 from another question on this forum. However, my guess is this should not have appeared as the flow is of internal tube with 13mm diameter tube, that too a periodic unit. MEsh was already at min ortho of 0.1, while points out in that question that scale is important and should be corrected with changing the limits.)

So, the question that I would need help with is, what do I keep my reference temperature, if wall is 28C and bulk is 70C, for a periodic tube, with mesh of min 0.1 orthogonal quality?