Browsing by Author "Izadi, Saeed"

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Open Access
Experimental investigation of a low-NOx swirl-assisted and jet-stabilized gas turbine combustor concept
(2025) Izadi, Saeed; Aigner, Manfred (Prof. Dr.-Ing.)
Today's aircraft engine emission standards regulate, among other aspects, the emissions of nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (UHC) at low altitudes, i.e. during the take-off and landing cycle. It is expected that international aviation regulatory bodies will extend the standards to include high-altitude emissions. This will reduce the global impact of these pollutants. In particular, NOx emissions will need to be reduced due to their role in the greenhouse effect as one of the major non-CO2 factors at higher altitudes. Therefore, in order to meet the upcoming stricter emission standards while maintaining optimal combustor reliability, affordability and efficiency, innovative combustor concepts are required. As a low-NOx combustion technology for future gas turbine engines, a low-swirl, lean premixed prevaporized concept can be an alternative to current conventional combustor systems. The concept is characterized by a lean-fuel and a high degree of mixing of the fuel with air prior to the reaction zone. This results in minimized hot spots and a significant reduction in thermal NOx levels. This work aims to investigate an innovative jet-stabilized concept. Initially, a single-nozzle jet-stabilized gas turbine combustor as a reference combustor was tested using both spray and superheated injection (flash atomization) of Jet A-1 at atmospheric pressure. Non-reactive tests using Mie scattering showed that as the fuel temperature increased, the fuel spray gradually vanished and was replaced by a rapidly evaporating fuel plume. The primary effect was a re-duction in the size of the fuel droplets, but also a rapid axial acceleration of the fuel vapor. As a result of the superheated injection, the Jet A-1's radial penetration was significantly reduced. This resulted in poorer mixing of the fuel with the air and led to shifting flame downstream of the flow. Additionally, the high temperatures caused carbon deposits to form within the fuel lines and the injector, which limited the operation of the combustor. These initial tests showed that fundamental changes to the combustor design are required to utilize superheated fuel injection with low emissions and a wide operating range in the jet-stabilized single-nozzle com-bustor. Due to the narrow operating range of the single-nozzle jet-stabilized combustor under spray conditions and the extremely unstable flame under superheated conditions, the combustor was iteratively developed to incorporate additional components. This was followed by a thorough study of how each component affected fuel vaporization and emissions. The results showed that, the additional components allowed for improved fuel-air mixing, fuel atomization, and evaporation prior to the reaction zone. The axial swirler slowed the rapidly expanding, high-velocity, superheated fuel by providing moderate swirling motion. The swirler hub proved to be an effective baffle, allowing the expanding and superheated fuel to mix better with the air. In addition, a prefilmer channel was installed around the axial swirler to increase the velocity through the swirler vanes, which allowed for improved secondary atomization of the fuel by means of an air-blast effect. As a result, a systematic variation of combustor operational and geometric design parameters was experimentally performed to study their effects on a newly developed swirl-assisted jet-stabilized combustor. The operational parameters included the adiabatic flame temperature, the thermal power, and the air and fuel temperatures, while the geometric parameters were the type of fuel injector, swirl number, the flame tube and the air nozzle diameters. In addition, to evaluate their behavior under sprayed and superheated injection regimes, four different liquid fuels with different thermochemical properties were tested. Finally, water vapor was added to the fuel-air mixture for evaluation of flame resistance to perturbations such as dilution and combustion inhibitors. For the characterization of the physical phenomena, established methods of combustion diag-nostics have been applied. Mie scattering was used in non-reactive and reactive tests for quali-tative analysis of fuel spray angle, penetration depth and degree of evaporation in the flame tube. Flame length (FL) and height above burner (HAB) of the heat release zone were deter-mined using OH* chemiluminescence. Furthermore, an emission analyzer was used to evaluate the pollutants emitted from the flames. These pollutants include NOx, CO, UHC and particu-late matter (PM). The mean residence time, bulk velocity, and recirculation rate and shape in the flame tube were primarily affected by variation of the flame tube diameter (DFT). This led to a change in reaction zone’s HAB and FL. The lowest NOx and CO levels were consistently observed with the smallest air nozzle diameter (DAN). This could be attributed to improved fuel-air mixing resulting from increased air dispersion at the nozzle exit, which led to increased turbulence at higher jet velocities. For both Jet A-1 and natural gas combustion, the injection of steam re-duced NOx emissions by lowering the adiabatic flame temperature. The characterized combustor concept features very low-emission combustion of a variety of liquid fuels over a wide operating range. The combustor concept is insensitive to spray quality so that injectors with poorer spray characteristics can be used. For the presented concept it was also shown that the injection of superheated fuel does not offer significant advantages due to the fuel preparation in the combustor.