10. Combustion and Reacting Flows

Combustion is a complex phenomenon involving many parameters, and a combustion model that represents physically the reaction is used to estimate the combustion phenomenon. The Arrhenius model is generally used as a combustion model. The Arrhenius model, however, is applied to each elementary reaction to determine each characteristic value related to combustion, which requires time-consuming calculations. The authors proposed the Hyperbolic Tangent Approximation (HTA) model, which considers the combustion reaction as a one-step overall reaction and approximates the reaction progress variable by the Hyperbolic Tangent function. This model is more useful than the Arrhenius model because the characteristic values of combustion can be calculated paradoxically from the reaction progress, and the computational cost can be reduced drastically. However, the HTA model does not consider the effect of endothermic reactions in the actual reaction process due to the assumption of a one-stage overall reaction, which results in differences from the actual phenomenon. In addition, this difference is expected to be larger for ammonia and hydrogen because of the large endothermic effect. In the present research, a new combustion model that considers endothermic reactions while reducing the computational cost was developed by using a simplified three-reaction model and introducing the HTA model for predicting each reaction process. The results of applying this model to methane combustion indicated that the mole fraction and temperature variations in the flame were in approximate agreement with experimental results, thus confirming the dramatic improvement from the results obtained with the conventional HTA model. We believe that this indicates the effectiveness of this model. In the present presentation, the calculation method of the proposed combustion model and the results of the validation of its usefulness will be explained in detail.