Aspects of ethanol

Abstract

The aspects of ethanol combustion presented in this dissertation provide insights in the modelling techniques used to simulate reactions of complex ethanol blends and its burning characteristics in off-design transient regimes. The tools developed will assist in the design of gas turbine combustors fired on blends of ethanol and other species. The present PhD thesis focuses on exploring a variety of combustion aspects of one of the most attractive liquid bio-fuels: ethanol. The two areas studied are the combustion characteristics of the ethanol as prevaporised gas fuel in turbulent flames, and the forced ethanol spray flame response to fluctuations of the gas velocity. The first part of the dissertation is concerned with the combustion of prevaporised ethanol and the combustion is treated with a tabulated chemistry approach based on an optimized choice of the reaction progress variable. A Computational Singular Perturbation algorithm is used, along with a sensitivity analysis of the chemical time scales performed in a Perfectly Stirred Reactor, to determine the optimal combination of the species mass fractions defining the reaction progress variable. Blends of ethanol/water/iso-octane were used to test the capability of the method and it is found that the adopted framework simulation can be successfully extended to complex fuel mixtures. In such simulation framework, the database is implemented in the commercial software Ansys CFX, where Reynolds averaged Navier-Stokes equations are solved in steady state regime. In the second part of the thesis, numerical simulations of a piloted turbulent ethanol spray flame are presented. Both steady and transient simulations are performed in the Eulerian-Lagrangian framework. The reference test-case is the Sidney spray flame, in particular the EtF6 and the EtF7. The spray is modelled under the assumption of the dilute spray regime. Finally, the forced spray flame response is studied with URANS simulations and presented in the last chapter. Two frequencies signals are chosen as upstream perturbation of the gas flow. The spray flame response is also compared to the response of prevaporised ethanol flames, at constant global equivalence ratio. The analysis is made both in the time and frequency domain, and a comparison of the discrete flame transfer function is performed between spray and gaseous flames.