negative velocity between Solid zone and Fluid zone
 

Hi,
I'm trying to calculate heating using fluent. My calculation domain has a solid zone and a fluid zone and these zones are connected. the boundary between solid zone and fluid zone is a temperature wall, and I set UDF on it. And also I set a mass source, which is changed depending on the temperature that is defined by UDF at the boundary, at a fluid cell zone that is adjacent to this boundary. the mass source value is always positive and it means that the mass flows to fluid zone, but the axial and radial velocity around the boundary are sometimes negative especially when the calculation reaches steady.
Why the axial and radial velocity are negative between solid zone and fluid zone although mass source are positive?

Negative Velocity
 

A mass source should never be alone. It should always be accompanied by sources for momentum, turbulence, thermal energy, etc. since mass never comes alone. If you add only mass, then Fluent has to redistribute the available momentum or thermal energy over new mass, which could lead to negative source generation, hence, flow reversals or negative velocity.