A modified boundary condition of velocity for continuity equation with non-uniform density distribution at outlet boundary plane

Boundary conditions in computational fluid dynamics significantly affect the prediction of flow field. However, the outlet boundary conditions for the continuity equation have been rarely investigated. In addition, the velocities at the outlet boundary might not be accurately predicted with the conventional outlet boundary conditions when a flow that has non-uniform density distribution on the outlet boundary is simulated. In the present study, we modified a boundary condition for the continuity equation in consideration of the non-uniform density distribution on the outlet boundary plane, comparing the numerical results of combustion between the conventional and modified boundary conditions. As a result, the proposed boundary condition can resist the generation of an unrealistic temperature field better than the conventional methods

Improvement of the prediction accuracy of NO emissions in counter-flow diffusion flames on using NO mass fraction as a progress variable

Computational fluid dynamics has been widely used to predict the production of nitrogen oxide (NO). Flamelet approach is commonly used as a modelling technique to perform turbulent combustion simulations. As the prediction of NO emissions with the flamelet approach is not reliable, when predicting the NO emission, the NO emissions are calculated with the conservation equation of NO mass fraction, and the NO production rate is predicted with the flamelet approach. In this study, we used the mixture fraction and NO mass fraction to predict the NO production rate in the conservation equation of the NO mass fraction, comparing the numerical results calculated with proposed method with those with the conventional methods and detailed chemistry model. Numerical simulations of counter-flow diffusion flames where NO was not supplied, that was supplied with fuel, and that was supplied with oxidizer indicated that the distribution of NO mole fraction calculated with the proposed method was in better agreement with that of the detailed chemistry model than that of the conventional methods

A Brief Review on How to Make a Database for Flamelet Approach

The simulation of turbulent combustion has been of more importance to develop practical combustors. The flamelet (or tabulated chemistry) approach is one of the techniques to estimate combustion reaction rate and to describe turbulent combustion field. This approach can reduce computational cost in comparison with the reduced reaction schemes because the combustion reaction rate is just looked up from the pre-calculated database. The difficulty of the flamelet approach comes from the variety of how to make this pre-calculated database. In the present study, we reviewed previous researches in terms of the flamelet approach and summarized the ideas to make the database for the flamelet approach. First, the fundamental combustion reaction models such as the detailed reaction model and skeletal model were introduced. And then, how to generate the dataset for the database in flamelet, flamelet progress variable, and flamelet generated manifolds models were mentioned. Moreover, how to process the dataset into the database for the turbulent combustion simulation was pointed out. Finally, apart from making the database, the treatment of the database in the computational fluid dynamics was described briefly

Turbulent combustion simulation based on flamelet model

Turbulent combustion simulation based on flamelet mode