Numerical Investigation on Combustion Regime and Conditional Statistics of Turbulent Spray Combustion

Abstract

Turbulent spray combustion is common in practical combustors like internal combustion engine, gas turbine, rockets and burners. The characteristics of combustion are significantly dependent on the characteristics of liquid sprays. The better understanding of the turbulent spray combustion is required. In this thesis, numerical simulations are performed in order to understand the characteristics and conditional statistics of turbulent spray combustion. 3-D DNS and RANS simulations are performed under the conditions resembling diesel engine combustion. The DNS studies are performed for distributed droplet combustion in a slab layer and transient spray jet flame respectively. The spray droplets are assumed to be point sources without any consideration of interfacial boundary or liquid volume fraction under resolution limit. Spray combustion regimes are identified according the ratio of chemical and transport time scales, which are group and collective combustion regimes. The chemical time scale is defined as the ignition delay time of the most reactive mixture fraction, while the transport time scale is defined as the mixing time of evaporated fuel from a single droplet. The conditional statistics show dependence on the spray combustion regimes. Reasonable agreements are shown with scaled AMC and simple linear evaporation rate model in the ranges of meaningful probability. CMC model was originally derived for gas phase flow. In this work, a balance equation of conditional sensible enthalpy for two phase flow is derived and checked the budgets of the component terms, which involves evaporation source terms. From DNS studies, although evaporation source terms are dominant during the initial period of evaporation, reaction term has become important since ignition occurs in both combustion regimes. Scalar dissipation rate term tends to be more important in collective combustion regime than in group one. In the DNS of the transient spray jet flame, the conditional statistics are obtained for multiple Lagrangian groups of sequentially evaporating fuel vapor. It is important to consider different residence times and histories of injected sprays, especially for a long injection duration or multiple injection. Hence multiple flame structures are required for accurate predictions of the ignition and combustion phenomena. For accurate description of ignition and combustion, it is not only important how many flame groups are defined, but it's also important when new flame groups are defined in the domain. Conditional mean temperatures show that sufficient number of flame groups should be defined at proper times according to reasonable strategy of defining new flame group. It is also shown that there's possibility of flame propagation with interaction among neighboring flame groups from DNS. An OpenFOAM solver, involving the Lagrangian CMC for two-phase flow and conditional submodels such as AMC and linear evaporation rate model, is applied to ECN experiment. The ECN has provided the measured data of reacting n-heptane sprays under diesel-like conditions in a closed vessel. All ECN test cases fall into the group combustion regime, so that the flame configuration is similar to that of DNS for transient spray jet in group combustion regime. The budgets are checked for component terms of the conditional sensible enthalpy equation for two phase flow, whose qualitative trends are similar to the results of DNS studies. In the OpenFOAM simulation, multiple flame groups are defined to have equal fuel mass according to the evaporation sequence of the fuel spray. It is shown that flame group interaction is required for Lagrangian CMC with multiple flame groups. The absence of the interaction among flame groups leads some unrealistic phenomena like too long ignition delay and rapid pressure rise. A new Lagrangian CMC with flame group interaction is proposed and validated. The flame group interaction is considered as the flame propagation along iso-mixture path in the EBU form between flame groups in burned and unburned state. Better agreements are obtained by the new Lagrangian CMC with measurements lying between predictions by single flame group and multiple flame groups without flame group interaction.