It is often preferable from a physical point of view to model a system with a known temperature profile. Doing so eliminates the need to model heat losses in the energy equation, but it does require that the user be able to provide the temperature profile. There are also computational advantages to using a known temperature profile, rather than solving an energy equation. The most severe nonlinearities in chemical kinetics come from the exponential dependence of the reaction rates on temperature. Thus, eliminating temperature changes from the iteration may make the problem considerably easier to solve.

Even if the energy equation is being solved for the temperature distribution, the iteration often converges more reliably if the species profiles are first computed using a fixed temperature profile. Ansys Chemkin, therefore, provides for a two-stage or three-stage procedure to solve the fully coupled reacting-flow and energy equations, depending on the Reactor Model. For Premixed and Opposed-flow Flames, a two-stage procedure is used, where the first solution uses the initial temperature profile as a fixed-temperature constraint and only solves for the species composition and velocity profile. For Stagnation-flow and Rotating-disk CVD reactors, a three-stage procedure is used, where the first stage solves for velocity and temperature profiles, the second stage solves for species and velocity using a fixed temperature profile, and the third stage solves the fully coupled problem. In all cases, the preliminary stages can be skipped by using appropriate Solver Parameters (for example, Skip Non-reacting Problem, Skip Intermediate Fixed-Temperature Solution) if desired. This multi-stage continuation procedure, however, is usually more efficient than attempting to solve the fully coupled problem directly.

The user input for the temperature profiles is in the form of position-temperature pairs, which can be created using the Ansys Chemkin Profile tool in the User Interface. In general, the mesh used by does not have to correspond to the one on which the given temperature estimate is defined. A linear interpolation of the given temperatures onto the current mesh is used. In the case where the energy equation is not included, it is therefore important for the user to provide enough temperature points that the profile is well resolved and relatively smooth.