Hi Nikhil,
I'm confused by your description of the nitrogen as being stagnant. That implies a fixed volume of nitrogen. If the nitrogen is a fixed volume, then in a Steady State Thermal solution the nitrogen will attain the same temperature as the pipe inner wall so there would be no convective BC on the pipe inner wall, so I agree with Dave.
If the model has a convective BC of nitrogen on the inner wall, that implies an infinite supply of nitrogen at the ambient temperature. You can have that if you say that nitrogen is flowing in the pipe.
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