Three-dimensional PDF simulation of a piloted turbulent non-premixed jet flame

Turbulent combustion is an important process in many technical applications, e.g. industrial gas furnaces used for heating water, production of steam and combustion in jet and diesel engines. To control the turbulent combustion, simulation tools with predictive power are required. The basic equations for turbulent combustion are well known, but their application to the complex flows appearing in most technical applications is not possible with the speed and storage capacity of present day computers. To simulate complex turbulent flows, averaged equations together with turbulence models for the unclosed terms are used. In turbulent reactive flows, the averaged equations can be applied only for simplified chemistry. To describe turbulent combustion accurately, the inclusion of realistic chemical kinetics requires a stochastic model that describes the statistics of the complex turbulence-chemistry interaction. To apply a stochastic model successfully in a turbulent combustion simulation, three problems have to be solved. Although theoretically arbitrary complex reaction mechanisms could be applied, they are numerically intractable and an appropriate, simplified chemistry scheme has to be found. The second problem of stochastic modelling is that the molecular species mixing appears in unclosed form. Different models are available, but their performance depends on the particular flow and the most appropriate one has to be selected. The third problem is that the solution of the stochastic model is computationally expensive and to reduce the computational time the simulations in most published research on turbulent combustion to date are two-dimensional. Since most of the flows in technical combustion devices are three-dimensional, the performance of the stochastic model for three-dimensional flows is an open question of research. In this work, the three problems have been addressed and applied to the Delft III flame. The Delft III flame is a piloted, non-premixed, turbulent jet flame. The turbulent flow field can be reasonably well simulated with prescribed accuracy. Therefore, the results of different mixing models and chemistry schemes can be studied. The pilot flames used to stabilize the flame introduce three-dimensional effects; the structure of this flame is very similar to that of flames used in industrial combustion devices. The simulations have been performed in three-dimensions using the commercial CFD-package FLUENT. Developing the three-dimensional PDF model