The 1-D steady-state reactor models require a starting estimate of the solution from which to begin its iteration. The general form of this estimate is shown in Figure 16.1: The general form of the starting estimate . For this estimate, we presume that there is a reaction zone in which reactants are specified at one end of the reaction zone and the reactants change from their unreacted values to the products that are found on the far-end of the flow domain. The user can provide estimates for the location and thickness of this reaction zone. The user may also provide estimates of the product species fractions, or the species composition that is expected at the far-end of the domain.
For Premixed Laminar 1-D Flame, Flame-speed Calculator, Stagnation-flow, and Rotating-Disk models, if no product species estimates are given, Ansys Chemkin will calculate an equilibrium composition to be used as the product estimate. When equilibrium is used to estimate the product concentrations, the user can also provide a minimum value to be applied to all species when creating the product estimates. For Opposed-flow Flames, the "product" and "reactant" are represented by the two inlet streams compositions (for example, fuel and oxidizer compositions for diffusion flames) that provide the boundary conditions for the flow domain.
Within the reaction zone straight lines are used between the initial and final values for both the reactants and products. On the reactant side of the reaction zone the reactant species profiles are flat at the reactant values. On the product side, the product species are flat at the estimated product values. Note that any given species can be both a reactant and a product species. For example, the nitrogen in a hydrocarbon-air flame will be both a reactant and a product. The excess fuel in a rich flame will also be both a reactant and a product. Species can also be identified as "intermediates." Intermediates, such as short-lived radical species, are assumed to have a Gaussian profile that peaks in the center of the reaction zone. The peak height is specified in the input to the model, and the Gaussian width is such that the profile is at 1/10 of its peak value at the edges of the reaction zone.
The shape of the assumed species profiles is not too important. Smoother functions, such as cubic polynomials and hyperbolic tangents for the reactant and product species, have no apparent effect on the convergence properties of the method. Since the starting profiles are typically evaluated on a very coarse mesh, it makes little difference whether the underlying function is smooth or not. Therefore, simple linear starting profiles are used.