Lean blowout sensitivities of complex liquid fuels

Abstract

Lean blowout is a process whereby a previously stable flame is either extinguished or convected out of its combustor. In aviation applications, blowout is a direct threat to passenger safety and it therefore sets operational limits on a combustor. Understanding the blowout problem is a key prerequisite to the deployment of alternative aviation fuels, as these fuels are expected to have comparable flame stability characteristics as traditional jet fuels. The objective of this work is to identify the fuel properties that govern lean blowout and to characterize their effect on the physics involved in the blowout process. The blowout performance of 18 different liquid fuels were experimentally compared in an aircraft relevant combustor. These experiments were repeated at 3 different air inlet temperatures, 300 K, 450 K, and 550 K, in order to vary the effect of fuel physical properties. Custom fuels were introduced that were specifically designed to decouple interrelated fuel properties and to accentuate the significance of preferential vaporization on lean blowout. The methodology that was used clearly demonstrated differences in the equivalence ratio at blowout between fuels, and a multiple linear regression analysis was performed to determine the relative contributions of each of the fuel properties. This work also characterizes the effect of fuel composition on the processes that precede blowout of the flame, thereby providing an explanation for why certain fuel properties govern lean blowout boundaries. By quantifying the time variation of flame luminosity and extinction “events” as a function of blowout proximity, it was demonstrated that local extinction processes are operative in and lead to blowout in spray flames. In addition, high speed imaging was used to analyze the space-time evolution of the most upstream point of the flame near blowout. Fast motion of these points upstream relative to the flow velocity was interpreted as flame re-ignition. These re-ignition processes become manifest when the stability of the flame is severely threatened by local extinction and often allow for recoveries that extend flame burning. Fuel composition was shown to have a clear effect on a flame’s propensity for extinction and re-ignition.