[saharesobh]

I am trying to solve the natural convection (mixed convection) in a Chemical vapor deposition reactor. The reactor has top and bottom plates and inlet and outlet tubes. The reactor is heated by top smaller rectangular plate embodying two heating rod. the top plate is at 250 oc and the bottom plate is at 150 oc. The temperature at top and bottom plate is controlled by the Omnic temperature controller and the heater built in. The temperature controller did a good job to remain constant temperatures of top and bottom plate.As for the tube walls ,the fluid temperature which enters the pipe( tube wall) increases from 26 C up to about 154 C while passing through the pipe. Considering the fact that the length of the pipe is only about 6 mm, this temperature difference seems really huge (128 C). when I got to Ansys Fluent, boundary condition, in tube wall zone,it shows me thermal condition ( heat flux, temperature, convection, radiation,mixed). So because the temperature is not constant at the tube wall and there is not radiation, and the heat flux is not constant, so I chose convection.
when i went to convection, it asked me to get heat transfer coefficient, free stream temperature and heat generation rate. My question is that I don't know how to get free stream temperature and what this temperature means? as for heat transfer coefficient, first I found a Nusslet number correlation for pipe and internal flow and through that, I got heat transfer coefficient. is my approach correct? would you please give me some advice?
Thank you much for your help in advance