Improvements to heritage gas turbine engines will be needed in the coming years as the demand made on these systems increase. While industry continually presses for higher performance of both military and civilian aero engines, the government simultaneously raises the bar for emissions standards in the commercial sector to support public health. The next generation of aerospace gas turbine engines will be defined by their ability to operate at high power conditions while maintaining efficiency. This challenge is compounded by airlines' proposition of a return to supersonic flight- an operating regime characterized by higher total temperatures, and thus more NOx production.
Lean Direct Injection (LDI) is a combustion scheme that was proposed by NASA, and inherently addresses the needs of both the private sector and the military. LDI is a liquid fueled combustor that promotes rapid mixing of fuel and air at the entrance of the combustor. Despite the benefits of LDI, it has never been implemented, nor has any other lean burning scheme been implemented in an aircraft due to the system level complications of such technology. This dissertation focuses on the dynamics of thermoacoustic instability and lean blowout (LBO), two of the major complications that industry will face when they attempt to incorporate LDI in a production engine.
The present dissertation investigates these dynamics from a fundamental and applications standpoint. Fundamental insights on thermoacoustic instabilities are developed by investigating droplet dynamics in a self-excited flow field, and significant oscillations in droplet diameters are discerned. PDPA measurement will be taken to identify coupling of the fuel spray with the instability, and a phase locking algorithm will be used to develop a new spray parameter than is more indicative of combustion heat release that the standard Sauter mean diameter.
Next, while varying the swirl number and the venturi geometry of the combustor, the evolution of the flow field will be characterized. An in-house innovation called the Direct Rotation Swirler (DRS) is built for this purpose. The DRS uses an active geometry to provide continuously variable swirl number modulation. The effects of these changes on lean blow out, pressure drop and NOx emissions will then be experimentally determined. Venturis were rapidly manufactured using a ii casting procedure that was developed to make venturi geometries from a commercially available ceramic at very low cost. / Doctor of Philosophy / Improvements to heritage gas turbine engines will be needed in the coming years as the demand made on these systems increase. While industry continually presses for higher performance of both military and civilian aero engines, the government simultaneously raises the bar for emissions standards in the commercial sector to support public health. The next generation of aerospace gas turbine engines will be defined by their ability to operate at high power conditions while maintaining efficiency. This challenge in compounded by airlines' proposition of a return to supersonic flight- an operating regime characterized by higher total temperatures, and thus more NOx production.
Lean Direct Injection (LDI) is a combustion scheme that was proposed by NASA, and inherently addresses the needs of both the private sector and the military. LDI is a liquid fueled combustor that promotes rapid mixing of fuel and air at the entrance of the combustor. Rapid mixing yields a clean, even flame and eliminates the fuel-rich primary zone which is the heart of NOx production.
Despite the benefits of LDI, it has never been implemented, nor has any other lean burning scheme been implemented in an aircraft due to the system level complications of such technology. This dissertation focuses on two of the major complications that industry will face when they attempt to incorporate LDI in a production engine. Drastically reducing the local hot spots in the primary zone is fundamentally necessary to improve pattern factor and emissions, but this change is directly at odds with two dynamic phenomenon that plague combustors. These effects are thermoacoustic instabilities, and lean blow out.
Thermoacoustic instabilities are a major concern in any type of combustor and instabilities are more common and more intense in lean engines which is a significant safety risk to aircraft. A thermoacoustic instability occurs when pressure waves in an engine grow to high amplitudes.
Small pressure waves are normal in any combustion process, but when the acoustic waves couple with the heat release, the waves can grow uncontrollably. The amplitudes can reach magnitudes capable of damaging the combustor or significantly reducing its cyclic life. Due to the high iv standard of safety in the aerospace industry, lean combustion will not be implemented until engines can be designed to avoid instabilities throughout the entire flight envelope.
Lean blow out occurs when the fuel to air ratio of the engine becomes too low to sustain a flame.
Lean blow out is a transient phenomenon that is dependent on local flame speeds, local chemical time scales and turbulence parameters. Typically, lean blow out is combated by designing a rich flame anchoring region that burns with plenty of excess fuel so that even if the fuel flow rate is reduced, a core region is still within its flammability regions.
The present dissertation investigates these dynamics from a fundamental and applications standpoint. Fundamental insights on thermoacoustic instabilities are developed by investigating droplet dynamics in a self-excited flow field, and significant oscillations in droplet diameters are discerned. PDPA measurement will be taken to identify coupling of the fuel spray with the instability, and a phase locking algorithm will be used to develop a new spray parameter than is more indicative of combustion heat release that the standard Sauter mean diameter.
Next, while varying the swirl number and the venturi geometry of the combustor, the evolution of the flow field will be characterized. An in-house innovation called the Direct Rotation Swirler (DRS) is built for this purpose. The DRS uses an active geometry to provide continuously variable swirl number modulation. The effects of these changes on lean blow out, pressure drop and NOx emissions will then be experimentally determined. Venturis were rapidly manufactured using a casting procedure that was developed to make venturi geometries from a commercially available ceramic at very low cost.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116650 |
| Date | 10 November 2023 |
| Creators | Aradhey, Yogesh Sachin |
| Contributors | Mechanical Engineering, Meadows, Joseph, Schetz, Joseph A., Burdisso, Ricardo A., Lowe, Kevin T. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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