Probability density function modeling of turbulent premixed combustion and pulverized coal combustion.

摘要

The use of probability density function (PDF) methods for turbulent combustion simulations is very attractive because arbitrary finite-rate chemistry can be exactly taken into account. PDF methods are well developed for non-premixed turbulent combustion. However, many real flames involve a variety of mixing regimes (non-premixed, partially-premixed and premixed turbulent combustion), and the development of PDF methods for partially-premixed and premixed turbulent combustion has turned out to be a challenging task. This thesis demonstrates a promising way to overcome this problem by extending existing PDF methods to cover a variety of mixing regimes. This extension of PDF methods is done by a generalization of the standard scalar mixing time scale model to account for the fast chemical reactions that are present in premixed combustion. The suitability of the new mixing time scale model is shown by applications to several premixed turbulent Bunsen flames that cover various regimes ranging from flamelet to distributed combustion. Moreover, the combined performance of several mixing models and time scale models is investigated for a turbulent premixed flame.;Motivated by the success of the PDF method in turbulent gaseous combustion, the method is extended to describe dilute dispersed turbulent gas-solid reacting flows. Such flows are found in pulverized coal combustion and entrained flow coal gasification. The solid particles are modeled by a stochastic Lagrangian method which is combined with a joint velocity-composition PDF method for the gas phase. Coal specific devolatilization and char reaction models are included in the Lagrangian particle method and models for the mass, momentum and heat exchange between the phases are proposed. To account for the radiative heat transfer, a computationally efficient radiation model is coupled to the gas and particle phase equations. The proposed approach is applied in simulations of pulverized coal combustion in a semi-industrial scale furnace and validated by comparison of simulation results with available measurements.