Learning Forum

[Nikhil Bhat]

Hi

I have a query with regards to heating up a pipe which has gas (nitrogen) inside the pipe. The following are the boundary conditions

Heat is applied to the outer surface of the pipe

The pipe is covered by an insulation material with ambient air outside

Air and Nitrogen gas at ambient temperature

Air and Nitrogen fluids are represented through convective heat transfer coefficients in the Ansys Mechanical model

See Figure below

 For the boundary condition

Heat is applied to the pipe as a heat flux to the outside surface of pipe(see below)

Convection to air outside the insulation is defined through convection coefficient

My main concern is with the boundary condition I have applied to the inner surface of the pipe. To represent the heat transfer happening to nitrogen gas, I have given a convective heat transfer coefficient as shown below

However, I do not think I have defined it correctly. In theory the inside of the pipe is filled with stagnant nitrogen, so theoretically as the inside surface temperature of the pipe increases, the temperature of the nitrogen gas will also increase (from ambient) in proportion to the inside surface temperature of pipe. However, based on the boundary condition I have fixed it to 31 Deg C which I think will artificially increase the heat transfer from the pipe to the nitrogen gas inside

I am not sure how to define this boundary condition of heat transfer from pipe to nitrogen gas inside the pipe as the temperature of nitrogen gas will rise in proportion to the temperature of the inside surface of the pipe. Any help or example on this will be highly appreciated

Thanks

[Dave Looman]

Your assessment seems right that it's not correct to specify a convection boundary conditions between the pipe and the nitrogen.  If you feel the thermal mass of the nitrogen is significant you could model it as a solid body or (more likely) you could just ignore it and not have any convection on the inside pipe surface.

[peteroznewman]

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.