The role of reaction directions for thermochemistry impacts on chemical kinetic model predictions

Thermochemical species properties can have a significant effect on kinetic model predictions due to their impacts on the chemical equilibria of elementary reactions, as demonstrated in several recent studies by means of sensitivity analysis and uncertainty quantification methods. Since the reverse rate coefficients are commonly obtained from the forward rate coefficients and the equilibrium constants (which depend on the thermochemistry data of the involved species), the sensitivities of quantities of interest on the thermochemical properties are expected to depend significantly upon the directions for which the forward rate coefficients are provided in a model. Nevertheless, this dependence has not yet been well quantified in the literature. Deeper insight is thus of high interest. The present work systematically assesses the extent to which the sensitivities on thermochemical properties as well as the resulting uncertainties in model predictions depend upon the choice of forward reaction directions. The mechanisms of two exemplary fuel components, i.e., diethyl ether and $n$-heptane, are assessed for ignition conditions. When all reactions are defined in their respective main directions of net reaction flux at the conditions of interest, the impacts of thermochemistry data on model predictions are shown to become relatively low. In particular, only the thermochemistry data of reactants and products of the low-temperature isomerization reactions are found to be moderately sensitive at intermediate temperatures in that case. Conversely, the prediction uncertainties due to thermochemistry data can be more than an order of magnitude higher when all reactions are defined contrary to their main directions of net reaction flux. These results highlight the relevance of the selection of forward reaction directions in terms of minimization of model prediction uncertainties caused by the thermochemistry data, especially if relatively accurate kinetic rate data are available. Further practical implications are finally discussed.

Novelty and significance statement This work evaluates quantitatively the impacts of the choice of forward reaction directions on the sensitivities of prediction targets on thermochemical properties and the resulting uncertainties in model predictions for the first time. It is found that the model prediction uncertainties can be minimized, if all reactions are defined in their respective directions of net reaction flux. The results of this work highlight the importance of the selection of reaction directions in the model development for the minimization of model prediction uncertainties caused by the thermochemistry data.