Evaluating active control strategies for airblast-injection

This research project focuses on unsteady two-phase flow behaviour at high pressure and temperature levels, and is ground-research oriented. The aimed field of application is the stability of airblast atomisation, which is the common injection device used in modern aeroengines. Collateral fields are for instance liquid-fueled rocket engine stability, internal combustion engine injection, drying spray process control (pharmaceutical + agri-food industry), and coating quality control (metallurgy). New designs for performant and sustainable gas turbine combustors include low-NOx injection technologies, such as lean-premixed-prevaporised (LPP) burners. These systems that operate at high pressure ratios and near the blow-out limit are known to be sensitive to self-induced combustion oscillations. If "heavy" combustion control systems are already applied with success on stationary gas turbines, their transfer to aeroengines is not trivial. There is still a lack of understanding on the physics of unsteady multiphase flow that prevents to master the steadiness of combustion. Actuating the injection is the most feasible solution to damp combustion instability. In this study, we want to rate different airblast actuation strategies required to stabilise the equivalence ratio of the mixture at the level of the flame, or to phase-control this mixture. We propose to act as follows: The approach of the study will be numerical, using an Euler-Lagrange scheme for the simulation of a two- phase flow on a 3D computational domain A set of basic experiments will be required to validate the numerical models, as well as measure finely the effect of a specific actuation, for precise boundary condition input Studied parameters will be the effect of pulsed air only, pulsed liquid only, and simultaneously pulsed air and liquid (phase-shifted) on the mixture in the far field Specific numerical development will concern the implementation of unsteady boundary conditions, introduction of the liquid phase, interaction air-particle, simultaneous particle transport and evaporation under unsteady conditions Specific experimental development regards the selection or development of actuators, their control, their testing, and the development of synchronised measurement techniques Works previously realised by the applicant at ONERA and DLR will serve as extra data banks for our parametric analysis (airblast atomisation and evaporation fully characterised for specific geometries, at high pressure and temperature conditions, under steady and unsteady conditions) At the end of this project, the defined guidelines for the realisation of an ad-hoc airblast atomiser (order of magnitude of the liquid/air flow actuation and phase-control issues) will be the input of a future project, where these results will incorporate combustion, be experimentally tested and validated This project requires two PhD`s positions over three years (simulation and experiments) plus one PhD over one and a half year (fast control technology). The study will take place at the Combustion Unit, Institute for Thermal Turbomachinery and Machine Dynamics, at the Graz University of Technology.

Over the last decades research on gas turbine combustors for both propulsion and power is strongly focussed on low-emission technologies and optimised system performance. Main drivers are the impact on the environment, the finiteness of fossil resources and the rising fuel burn cost. These optimisation efforts lead to combustors operated in the lean combustion domain in which combustion instabilities by means of pressure fluctuations and unsteady heat release may occur and could result in a severe damage of the system. Within this project the air-blast atomisation, as it is state-of-the-art in gas turbine combustion, has been investigated both numerical and experimental. In numerical studies countermeasures on combustion instabilities like forced air pulsations and pulsed liquid injection were analyzed in a wide operational domain. The influence of these air pulsations and pulsed liquid injection on the primary breakup of a liquid sheet into ligaments and further into droplets was analyzed in experiments using Laser Doppler Velocimetry, Phase Doppler Anemometry and imaging techniques.