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Abstract

Spray combustion is crucial in numerous energy conversion systems, characterized by complex interactions among fluid dynamics, heat transfer, chemical kinetics, and phase transitions. Understanding these interactions is essential for enhancing combustion efficiency and minimizing pollutant emissions. Numerical studies on spray flames are important for improving our knowledge of these systems. The flamelet approach is advantageous for its efficient and precise separation of chemical processes from turbulence dynamics. Utilizing laminar spray flame structures in counterflow configurations is instrumental in establishing spray flamelet libraries that incorporate detailed chemical reaction mechanisms.

Research on numerical solutions for multiple flame structures in spray combustion, particularly under fuel-rich conditions, is limited but essential for developing comprehensive flamelet libraries. Additionally, the interaction and dynamics of heating and evaporation in multicomponent droplets within spray flames remain inadequately explored. Detailed investigations are essential for understanding systems like hydrous ethanol droplet sprays and titanium(IV) isopropoxide (TTIP)/p-xylene precursor solutions used in flame spray pyrolysis.

The research begins with an analysis of monocomponent ethanol spray flames under local fuel-rich conditions. In-depth results discussed include the observation of double and triple structures in ethanol spray flames under various conditions, employing regime diagrams to map their existence relative to initial gas strain rates, equivalence ratios, and droplet sizes. Double flame structures are observed for initial droplet radii larger than 30 um, while triple flame structures emerge exclusively under conditions of initial droplet radii ranging from 10 um to 30 um. Particularly, a flame structure with distinct evaporation and combustion zones is identified. A comprehensive analysis of these flame structures is presented, exploring the stability and transition mechanisms under differing operational conditions. Notably, stable spray flame structures with two chemical reaction zones are observed across all tested scenarios. The transition mechanisms among various spray flame structures underscore the critical interactions between energy-consuming vaporization processes, the positioning of droplets within the counterflow setup, and the exothermic nature of the chemical reactions involved.

A comprehensive numerical analysis of hydrous ethanol laminar spray flames is then conducted, comparing it with anhydrous counterparts. This analysis includes a validation of the extended multicomponent droplet spray flame model and an investigation of hydrous ethanol spray flame characteristics. Additionally, the study explores the multiple flame structures under identical conditions for hydrous ethanol sprays, highlighting the minor role of water content in the droplets on flame structure transitions.

Finally, a detailed investigation on TTIP/p-xylene spray flames explores the multicomponent heating, evaporation, and motion of the droplets, alongside thermal decomposition and the involved chemical reactions. A comparative analysis of pure p-xylene and TTIP/p-xylene spray flames indicates that p-xylene evaporates preferentially due to its higher volatility, thereby providing the necessary gaseous fuel for sustained combustion. In contrast, the thermal decomposition of TTIP which consumes energy essential for its phase change occurs in a relatively cooler zone, leading to a distinct separation of the reaction zones. The decomposition products of gaseous TTIP, notably C3H6 and CH3, are primarily observed in the gas-sided flame zone. Multiple flame structures are identified during the simulations. Among these structures, the flame structure with a single reaction zone on the spray side exhibits the greatest stability as strain rates increase.