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Abstract

Spray combustion under turbulent conditions occurs in many technical devices. Therefore, the proper prediction of the characteristics of turbulent spray flames is of vital importance for the design of new combustion technologies in view of efficiency and pollutant reduction, where the latter requires consideration of detailed chemical reaction mechanisms. Unfortunately, a direct inclusion of detailed chemical reactions dramatically increases the computational cost of the numerical simulations of technical combustion processes, and it is prohibitive in practical situations.

Models based on the assumption that turbulent ames can be seen as an ensemble of laminar stretched flame structures, the so-called flamelet models, represent a very promising approach for the cost effective inclusion of detailed chemical reaction mechanisms in the simulation of turbulent spray flames.

Several flamelet models are currently available in the literature for the simulation of pure non-premixed and pure premixed gas flames. Additionally, some two-regime flamelet formulations have been proposed in the last years for situations where nonpremixed and premixed gas combustion coexist and interact. These models, however, are not adequate for the simulation of turbulent spray combustion, since they do not take into account spray evaporation, which strongly affects the flame structure. Although a spray flamelet model has been proposed for the simulation of flames where non-premixed and evaporation-dominated combustion regimes coexist, most studies of turbulent spray flames use gas flamelet models, neglecting the effects of evaporation on the flame structure. In the present thesis, a common framework is developed in which the several single and two-regime flamelet models existing in the literature can be described and combined in order to advance the development of a comprehensive multi-regime spray flamelet model for turbulent spray flames. For this purpose, a set of multi-regime spray flamelet equations in terms of the mixture fraction and a reaction progress variable is derived, which describes all combustion regimes appearing in spray flames. The flamelet equations available in the literature for single and two-regime flames are retrieved from these multi-regime spray flamelet equations as special cases. Additionally, exact transport equations of the mixture fraction and its scalar dissipation rate are derived, which are then used to evaluate the validity of several assumptions commonly made in the literature during their derivation, such as the use of unity Lewis number and the negligence of spatial variations of the mean molecular weight of the mixture. These assumptions had not yet been tested for the calculation of the scalar dissipation rate of the mixture fraction in spray flames, and their validation is of vital importance for the formulation of any spray flamelet model.

Numerical simulations of axi-symmetric laminar mono-disperse ethanol/air counterflow spray flames are carried out to analyze the influence of spray evaporation on the flame structure. Parametric studies of the influence of the initial droplet radius and strain rate are presented, which clearly illustrate the major importance of evaporation in the determination of the flame structure. Additionally, the relative importance of non-premixed and premixed combustion regimes in the previously analyzed counterflow spray flames is studied by means of the derived multi-regime spray flamelet equations. The results show that premixed effects can be neglected in this kind of flame with all fuel injected in liquid phase. Moreover, the derived transport equations of mixture fraction and its scalar dissipation rate are solved for the counterflow spray flames considered in this work considering and without considering the assumptions of unity Lewis number and spatially uniform mean molecular weight of the mixture. The results are compared, and it is found that the assumption of unity Lewis number may lead to non-physical values of the scalar dissipation rate of the mixture fraction, whereas the use of a mass-averaged diffusion coefficient of the mixture is an acceptable approximation. Effects associated with the spatial variation of the mean molecular weight of the mixture are found to be small at low strain rate and negligible at high strain rates. These results confirm the validity of the use of Fick's diffusion law in highly strained flames. Finally, a set of non-premixed spray amelet equation is obtained by neglecting premixed effects in the previously derived multi-regime spray flamelet equations. This set of equations, which is valid in situations where non-premixed and evaporation-dominated combustion regime coexist, is similar to the classical non-premixed gas flamelet equations, but it contains two additional terms for the description of evaporation effects. These equations are then used to evaluate the relative importance of the effects attributable to evaporation. The results show that they are always relevant and they should be always considered.