  Two-dimensional direct numerical simulations are conducted for convection
sustained by uniform internal heating in a horizontal fluid layer. Top and
bottom boundary temperatures are fixed and equal. Prandtl numbers range from
0.01 to 100, and Rayleigh numbers (R) are up to 5x10^5 times the critical R at
the onset of convection. The asymmetry between upward and downward heat fluxes
is non-monotonic in R. In a broad high-R regime, dimensionless mean temperature
scales as R^{-1/5}. We discuss the scaling of mean temperature and
heat-flux-asymmetry, which we argue are better diagnostic quantities than the
conventionally used top and bottom Nusselt numbers.
