Modeling of turbulent dilute spray combustion

Abstract

In a real turbulent spray flame, dispersion, continuous phase turbulence modification, dispersed phase inter-particle collisions, evaporation, mixing and combustion occur simultaneously. Dealing with all these complexities and their interactions poses a tremendous modeling task. Therefore, in order to advance current modeling capabilities, it seems reasonable to aim for progress in individual sub-areas like breakup, dispersion, mixing and combustion, which however cannot be viewed in complete isolation. Further, one has to consider advantages and disadvantages of the general modeling approaches, which are direct numerical simulation (DNS), large eddy simulation (LES), simulations based on Reynolds averaged equations and probability density function (PDF) methods. Not least one also has to distinguish between Eulerian and Lagrangian dispersed phase descriptions. The goal of this paper is to provide a review of computational model developments relevant for turbulent dilute spray combustion, i.e. the dense regime, including collisions as well as primary and secondary atomization, is not covered. Also not considered is breakup in dilute sprays, which can occur in the presence of sufficiently high local turbulence. It is intended to guide readers interested in theory, in the development and validation of predictive models, and in planning new experiments. In terms of physical phenomena, the current understanding regarding turbulence modification due to droplets, preferential droplet concentration, impact on evaporation and micro-mixing, and different spray combustion regimes is summarized. In terms of modeling, different sets of equations are discussed, i.e. the governing conservation laws without and with point droplet approximation as employed by DNS, the filtered equations considered in LES, the Reynolds averaged equations, and Lagrangian evolution equations. Further, small scale models required in the context of point droplet approximations are covered. In terms of computational studies and method developments, progress is categorized by the employed approaches, i.e. DNS, LES, simulations based on Reynolds averaged equations, and PDF methods. In terms of experiments, various canonical spray flame configurations are discussed. Moreover, some of the most important experiments in this field are presented in a structured way with the intention to provide a database for model validation and a guideline for future investigations.