Hi, okey I'll try to give a deep explanation.

Purpouse: Study the heat transfer during the flight of a ballon untill 32 Km of altitude. The ballon is supposed to move with a +4 m/s in the Y axis.

Geometry: - One fluid volume for the air and one fluid volume for the Helium inside the balloon.

                  - The air volume is a cube whose edge are 20R lenght.

                  - The balloon it's a sphere of R m Radius.

                  - Top cube face will be the inlet and bottom face will be the outlet.

FLUENT: - Time set to transient, since I'm going to simulate the temperature and pressure change with UDFs (code is shown in previous post).

                - Air for the cube, helium for the sphere(cell zone conditions) and film porperties for the ballon skin (applied in wall menu).

                - Velocity inlet (top face): Set velocity to 4 m/s normal to boudary and temperature defined by UDF.

                - Presure Outlet (bottm face): Gauge Pressure defined by UDF(see previous post).

                - Skin-wall: Set as a wall and asigned a thickness, shell conduction considered.

                - Operating Conditions: Set gravity as -9.81 in the Y axis, Operating pressure as 0 and Operating Density as 0.

                - Viscous Model k-epsilon and energy enabled (later on will try to implement radiation if able).

                - Material properties for helium and air: desnity set by the ideal-gas model.

Problems: - When using Pressure Solver, not able to get any results.

                 -  When using Desnity based Solver, anormally high temperature shown in results such as 500 K. When the max temperature                          expected is 288.15 K (could be more with radiation).

                 - In reality the ballon change its size during the fligh.