Direct Numerical Simulation Of Complex Multiphase Tubulent Combustion

Combustion is the main technology for energy production in our modern world. Meanwhile, it causes serious environmental problems. In order to improve the design of combustion devices with enhanced combustion efficiency and reduced pollutant emission, it requires a deep understanding of the complex multi-physics and multi-scale interactions coupled in multiphase turbulent combustion and an accurate predictive capability of these processes. Under this background, this dissertation has developed a direct numerical simulation (DNS) code for multiphase combustion and investigated some fundamental problems involved in complex multiphase turbulent combustion, including gas-phase turbulent combustion, sprays turbulent combustion and pulverized coal turbulent combustion. Various DNS databases of gas-phase and multiphase turbulent combustion are established. And they are employed to provide insights into complex turbulence, chemistry and particle interactions in multiphase turbulent combustion, as well as to validate and develop predictive models for multiphase turbulent combustion.Regarding gas-phase turbulent combustion, a laboratory scale H2/N2turbulent lifted flame is studies using DNS.A comprehensive comparison of the DNS results and the experimental measurements is carried out for various scalars. Good agreements are observed for both of the mean and fluctuation components, which validate the present approach and code. Spatially developing H2/air swirling premixed flames are analyzed. The emphasis is placed on the investigation of the interactions between turbulence and combustion. Preferential diffusion effects in swirling premixed flames are studies. It is found that preferential diffusion effects observed in planar flames are suppressed in swirling flames. DNS of freely propagating H2/air premixed flames in isotropic turbulence is conducted. Combustion characteristics and various models for the mean chemical source term are discussed for both of planar flames and spherical flame kernels.As for spray turbulent combustion, DNS of reacting sprays in isotropic turbulence is performed. It is shown that the mean normalized squared droplet diameter decreases linearly with time for the cases studies, featuring the d2law. Turbulence is proved to be of advantage to evaporation while droplet diameter greatly affects the evaporating rate and the evaporation lifetime. Similar to purely gaseous combustion, ignition is always found in low scalar dissipation rate regions. The conditional evaporating rate is found to increase linearly with the mixture fraction. The log-normal distribution for the scalar dissipation rate is examined, and reasonably good agreements are obtained between the DNS results and the model predictions.A parameter characterizing the amount of burning in premixed regime is analyzed.For pulverized coal turbulent combustion, a pulverized coal jet flame in hot co-flow is investigated using DNS. A comprehensive model for coal combustion is applied and the reaction mechanism of CH4with five species and two-step reactions is adopted for gas-phase combustion. The simulation is partially validated against the experiment, and qualitative agreements are obtained. The particle behavior is investigated. It is found that in the upstream region, the reaction rate is quite scattered and a single particle is found inside the burning flame to form an individual particle combustion mode. While in the downstream region, the reaction zone is more continuous with a large number of particles enclosed, which characterizes the group combustion mode.Systematic investigations of the combustion sub-models in the framework of conditional moment closure (CMC) are carried out for premixed, non-premixed and multiphase turbulent combustion. The presumed PDF for the mixture fraction/progress variable is examined. The conditional velocity and scalar dissipation rate are closed with the linear model and AMC model, respectively. The gradient diffusion approximation is used for the conditional turbulent flux. For spray combustion, the conditional evaporation rate is closed using a linear model. For pulverized coal combustion, conditional statistics for the temperature and heat release rate with respect to the mixture fraction are obtained to provide insights into coal combustion and the related models.