Convection-diffusion equation

$$\rho C ( \dot{T} + \beta_i \frac{\partial T}{\partial x_i} ... ...{x_2}^2} + \frac{\partial^2 T}{\partial {x_3}^2} ) - a T + f$$

The primary unknown is the condif_temperature $T$. Further notation: $\rho$ group_condif_density; $C$ group_condif_capacity; $x$ space coordinate; $\beta_i$ group_condif_flow in $i$-direction; $k$ group_condif_conductivity; $a$ group_condif_absorption; $f$ force_element_volume. Typical applications are heat conduction and heat conduction in a flow.