I am studying a case with natural convection using Fluent. Gravity is activated. And I want to set up the thermal expansion coefficient in the materials. However there is no option to specify thermal expansion coefficient. The interesting thing is that: in the tutoral "Modeling Radiation and Natural Convection" provided by Fluent, you can set up thermal expansion coefficient. I am confused. What happened?

Are you sure you cannot? Define/Materials Set the density to 'boussinesq' from the drop-down list. Then you can set the 'beta' here, to an appropriate value.