Effect of two-component liquid fuel mixtures on the dynamics of a swirl-stabilized spray flames array subjected to a forced transverse acoustic mode

Reducing the environmental impact of aircraft engines could be achieved in particular through the use of drop-in sustainable aviation fuels. However, changing the fuel composition could give rise to operating issues, among which are transverse/azimuthal thermoacoustic combustion instabilities. In this context, the paper aims at characterizing the effect of two-component liquid fuel mixtures of n-heptane and dodecane on the dynamics of swirl-stabilized spray flames in a forced transverse acoustic mode of the combustion chamber. This is done by using a linear array of three flames, TACC-Spray, in which the central flame is placed at a pressure antinode. Analyses of the flame dynamics through the downstream-pressure-based Flame Describing Function, and of the thermoacoustic coupling qualified by a Rayleigh index show that increasing the proportion of n-heptane favors the propensity to instabilities. Flame dynamics is typified by the propagation of longitudinal flame intensity waves, whose amplitude significantly increases with the proportion of n-heptane. The fuel spray dynamics participates in the flame dynamics through the presence of droplet number waves and the oscillations of the Sauter mean diameter. The propagation of droplet number waves correlates well with that of flame intensity waves, indicating that the former waves are a fundamental mechanism at the basis of the swirl-stabilized spray flames dynamics. The decrease of the evaporation rate as the proportion of dodecane is increased appears as an essential feature which induces a weaker flame dynamics.