Ph.D. Thesis Proposal by

Nicholas Rock

 “Lean Blowoff Sensitivities of Alternative Aviation Fuels”

Tuesday, June 5, 2018 at 2:00 p.m.

West Village Ensemble Room

Abstract:

            Lean blowoff is a process whereby a previously stable flame is convected out of its combustor. In aviation applications, blowoff is a direct threat to passenger safety and it therefore sets operational limits on a combustor. Understanding the blowoff 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 effect that different fuel physical and chemical properties have on the stability of spray flames remains a scientific unknown. This knowledge gap hinders the ability of chemists to develop optimized fuels and raises the risk that a prospective fuel will fail the certification process, thereby discouraging potential investors. The objective of this work is to identify the fuel properties that govern blowoff and to characterize the physics of the blowoff process.

            The blowoff performance of 18 different liquid fuels was experimentally compared in an aircraft relevant combustor. The methodology that was used clearly demonstrated differences in the fuel-air ratio at blowoff between fuels. Custom fuels were used that were specifically designed to decouple interrelated fuel properties and to accentuate the significance of preferential vaporization on blowoff. Additionally, the experiments were repeated at 3 different air inlet temperatures, 300 K, 450 K, and 550 K. These different temperatures are intended to vary the effect of fuel physical properties.

            This work additionally seeks to understand the physics associated with the blowoff process, i.e., what does blowoff look like? Premixed gaseous studies have determined that blowoff is more of a process than an event. These studies have shown that the flame experiences partial or complete extinction over an extended duration prior to blowing out. Specifically, holes were observed to form in the flame that increase in size and frequency as blowoff is approached. Preliminary work has provided evidence that these same mechanisms are at least somewhat operative in spray flames, as a fuel’s autoignition delay time typically correlates with its blowoff propensity. High speed imaging will be used to determine whether these autoignition differences manifest themselves as the extinction-autoignition processes that are seen in premixed flames. By tracking the leading luminosity point throughout successive CH* chemiluminescence images, differences in autoignition behavior can be compared between fuels. Flame edge tracking and machine learning techniques will also be applied to these images to identify features that precede the blowoff process. Lastly, photomultiplier tube measurements will be used to monitor changes in flame intensity as blowoff is approached over a longer time interval than would be possible using cameras alone.

Committee:

Professor Tim C. Lieuwen (Advisor)

Professor Suresh Menon

Professor Jerry Seitzman