Development of a Single-Point SV-LDI-1 System: A Benchmark Study for Future Experiments

Abstract

As the demand for high-efficiency, low-emissions gas turbine propulsion systems continues to rise, the aerospace industry faces mounting challenges in balancing performance, environmental impact, and noise generation. Commercial aviation is under increasing legislation to meet stricter emissions standards while improving fuel efficiency due to concerns of increased NOx production. These competing goals underscore the need for novel combustion technologies capable of operating under optimized conditions without compromising performance or emissions.

Lean Direct Injection (LDI) has emerged as a promising solution to these challenges. Originally proposed by NASA, LDI promotes rapid mixing of fuel and air at the combustor inlet, generating a more uniform flame front and significantly reducing the formation of fuel-rich zones, the primary source of NOx emissions in traditional combustors. Despite its potential, LDI has yet to be implemented in production engines due to two critical system-level complications: thermoacoustic instability and lean blowout (LBO). These phenomena present significant barriers to operational reliability, and understanding their behavior is essential for advancing LDI to practical use.

This thesis investigates the operability limits of a redesigned SV-LDI-1 combustion testing rig using Jet-A fuel, with a particular focus on defining LBO equivalence ratio (ER) limits, defining trends in SPL as a function of ER and swirl number (SN), and generating operability maps that describe testing conditions for future experiments with the rig.

A modular redesign of the rig, including upgrades to the fuel injector, swirler, and data acquisition systems, enabled synchronized flame imaging and dynamic pressure measurement. Systematic testing revealed a linear relationship between swirl number and minimum global ER during LBO events, and frame-synchronized data helped characterize flame structures and extinction patterns. Additionally, frequency-domain operability maps were created to visualize how sound pressure level (SPL) varied across a range of SN and ER conditions.

Building on these findings, the second phase of this research explored dynamic pressure behavior and instability coupling mechanisms in greater detail. Using optimized spectral domains, the study tracked peak SPL and frequency evolution across seventeen swirl numbers and a range of fuel-lean equivalence ratios. The 350 Hz mode was identified as the dominant instability contributor, and a detailed sweep of ER values at the baseline swirl number (SN = 0.49) revealed an optimal condition (ER = 0.60) where pressure fluctuations were reduced by over 90% from the maximum pressure amplitude measured near stoichiometric combustion at the same SN condition. These findings contribute to a growing understanding of how SN, fuel-air ratio, and combustion geometry interact to shape instability behavior. All work presented in this thesis was conducted to support future testing with SAF and to refine methodologies for evaluating combustor designs with the listed procedures readily amendable to explore further combustion behaviors and advanced combustion concepts.