Jeffrey M. Cohen

Active Control of Combustion Instability in a Liquid-Fueled Low-NOx Combustor

A practical active control system for t nitigation of combustion instability has been designed and demonstrated in a lean, premixed, single-nozzle combustor at realistic engine operating conditions. A full-scale engine fuel nozzle was modified to incorporate a simple fuel flow actuator Results indicate that the system was capable of reducing pressure fluctuations by 82 percent (15 dB or 5.6×) while maintaining or reducing NO x and CO emissions levels.

Experimental investigation of near-blowout instabilities in a lean, premixed step combustor

An experimental characterization of combustion instabilities in a lean, premixed backwardfacing step combustor was performed. Specifically, the instabilities of interest were those encountered as the equivalence ratio was reduced to levels approaching the combustors lean extinction limit. High-response measurements of static wall pressures, inlet velocity and local combustor heat release rate were acquired. In addition, the unstable flow field was visualized using high speed cinematography and a focused shadowgraph system. The results indicate that the magnitude of a longitudinal acoustic disturbance grows with decreasing equivalence ratio, until it eventually triggers a lower-frequency, high amplitude instability. This low frequency instability is visible as a flapping of the flame and increases in severity with decreasing equivalence ratio until it causes combustor blowout

Measurement of spray/acoustic coupling in gas turbine fuel injectors

A diagnostic to measure the acoustic coupling of air flow with a fuel injector spray has been developed and tested. The instrument measures the mass of fuel within a plane of the spray using planar laser-induced fluorescence. The signal is monitored continuously to measure mass flow fluctuations during acoustic excitation of the flow. A comparison with the acoustic signal provides a measure of the response/of the spray to acoustic excitation for a given nozzle design. This paper describes the approach to acquiring a planarintegrated time-dependent signal for response measurements. Results for several nozzle designs are also presented.

High Fidelity Simulation of the Spray Generated by a Realistic Swirling Flow Injector

Practical aero-engine fuel injection systems are highly complicated, combining complex fuel atomizer and air swirling elements to achieve good fuel-air mixing as well as long residence time in order to enhance both combustion efficiency and stability. While detailed understanding of the multiphase flow processes occurring in a realistic injector has been limited due to the complex geometries and the challenges in near-field measurements, high fidelity, first principles simulation offers, for the first time, the potential for a comprehensive physics-based understanding. In this work, such simulations have been performed to investigate the spray atomization and subsequent droplet transport in a swirling air stream generated by a complex multi-nozzle/swirler combination. A Coupled Level Set and Volume Of Fluid (CLSVOF) approach is used to directly capture the liquid-gas interface and an embedded boundary (EB) method is applied to flexibly handle the complex injector geometry. The ghost fluid (GF) method is also used to facilitate simulations at realistic fuel-air density ratio. Adaptive mesh refinement (AMR) and Lagrangian droplet models are used to efficiently resolve the multi-scale processes. To alleviate the global constraint on the time-step imposed by locally activated AMR near liquid jets, a separate AMR simulation focusing on jet atomization was performed for relatively short physical time and the resulting Lagrangian droplets are coupled into another simulation on a uniform grid at larger time-steps. The high cost simulations were performed at the U.S. Department of Defense high performance computing facilities using over 5000 processors. Experiments at the same flow conditions were conducted at UTRC. The simulation details of flow velocity and vorticity due to the interaction of the fuel jet and swirling air are presented. The velocity magnitude is compared with experimental measurement at two downstream planes. The two-phase spray spreading is compared with experimental images and the flow details are further analyzed to enhance understanding of the complex physics.Copyright

Characterization of a Superheated Fuel Jet in a Crossflow

An experiment was conducted to characterize a superheated fuel jet (Jet-A) injected into an unheated crossflow of air. The liquid phase of the fuel jet was characterized with high speed imaging and phase Doppler interferometry. The transition from a shear-atomized to a flash-atomized spray at a fuel temperature of 513 K (465°F) was observed at an ambient pressure of 1 atm, which is consistent with the bubble and dew point curves predicted for JP-8. The explosive breakup that was seen in the flash-atomized spray produced submicron droplets with a high radial momentum. This unique behavior makes superheated fuels an attractive design feature for fuel preparation devices that can employ flash boiling to enhance fuel atomization and mixing in a compact volume.

Active Control of Combustion Instability in a Liquid–Fueled Low–NOx Combustor

A practical active control system for the mitigation of combustion instability has been designed and demonstrated in a lean, premixed, single -nozzle combustor at realistic engine operating conditions. A full -scale engine fuel nozzle was modified to incorporate a simple fuel flow actuator. Results indicate that the system was capable of reducing pressure fluctuations by 82% (15 dB or 5.6X) while maintaining or reducing NOx and CO emissions levels.Copyright

Liquid Film Formation by an Impinging Jet in a High- Velocity Air Stream

The impingement of a liquid jet on a solid surface, and the development of a shear-driven liquid film is characterized in a planar experiment. Weber number and momentum-flux ratios were chosen to be representative of gas turbine fuel injector operations. High-speed digital imaging was used to visualize the formation of the liquid film from impinging droplets and the development of a continuous, wavy film. Film thickness measurements indicated growth of the film along the length of the impinging surface (streamwise direction), and reduction in the film thickness in the crossstream direction. In general, film thicknesses increased with increasing momentum flux ratio, as more liquid drops reached the filmer surface. Three different mechanisms of atomization from the liquid film were identified.namely droplet splashing, film surface atomization via aerodynamic instability and film breakup at channel exit.