This thesis describes the application of the first-order Conditional Moment Closure (CMC) to the autoignition of high-pressure fuel jets, and to piloted and lifted turbulent jet flames using classical and advanced CMC submodels. A Doubly-Conditional Moment Closure (DCMC) formulation is further proposed.

Conditional Source-term Estimation (CSE) is a closure technique for modelling turbulent combustion phenomena. CSE uses the Conditional Moment Closure (CMC) hypothesis for closing chemical source terms: conditionally averaged chemical source terms are closed by conditional averaged scalars, which are obtained by inverting an integral equation, instead of solving transport equations (as in CMC). Since CSE has been successfully applied to both premixed and non-premixed configurations, it represents an attractive method for dealing with the more general and complex case of partially premixed combustion. The objectives of the present study are to (i) consolidate the premixed formulation of CSE through numerical simulations of a turbulent bluff body premixed flame; (ii) formulate, implement and test the Doubly conditional CSE (DCSE) in the context of partially premixed combustion; (iii) compare the DCSE predictions with well documented turbulent partially premixed flames. The canonical example of partially premixed flames is represented by turbulent lifted flames. A series of lifted turbulent jet flames is investigated in RANS by using DCSE. The DCSE calculations are successful in predicting the lift-off heights at three different conditions and reproducing many aspects of the flame structure in agreement with the experimental observations. The current results show that important aspects of the stabilization mechanism can be reproduced by the DCSE combustion model. The applicability of DCSE is further evaluated by applying this approach to a series of turbulent V-shaped flames for which experimental data is available. Premixed and stratified conditions are investigated. Overall, the agreement between numerical results and experimental findings is good, demonstrating the capability of DCSE to deal with partially premixed combustion. Future work includes implementation of CSE in LES and investigation of different fuels such as propane and biofuels.