The numerical prediction of the acoustic response of liquid-fuelled, swirl-stabilised flames

2019-05-29T15:14:51Z (GMT) by Nick Treleaven
This thesis sets out and tests several different approaches to predicting and understanding the acoustic response of an industrially representative liquid fuelled, swirl-stabilised, lean-burn fuel injector using numerical simulations. This work is important as it contributes to the design of fuel injectors with a low susceptibility for thermoacoustic instabilities or ‘rumble’.
The flame transfer function (FTF), a transfer function relating the mass flow rate through the fuel injector and heat release rate of the combustor, has been chosen as the best way to describe the flame response as it can be used in conjunction with a acoustic field solver to predict the stability of a combustion system. The FTF of a chosen injector geometry has been predicted using conventional compressible methods and a novel incompressible method which has been shown to be consistent with the compressible method at two frequencies of forcing. This is in contrast with mass flow forced incompressible simulations that fail to reproduce the correct downstream flow field fluctuations. The single phase flow field and acoustic response of the injector has also been predicted and compared to experiments with good agreement.
The injector hydrodynamic response has also been investigated along with how hydrodynamics, acoustics, the fuel spray and heat release are related. Acoustic forcing can be seen to actively alter the strength of large scale fluid structures, the mean pressure field and the mass flow rates through the different injector passages. The fuel spray may also couple with these structures causing additional local changes to the mixture fraction field and heat release rates. The effects of fuel spray SMD (Sauter Mean Diamter) and fuel spray injection velocity have on the flame have also been tested showing that the fuel spray atomisation, which can also be affected by acoustic forcing, may play a significant role in combustion instabilities.
Several novel numerical methods have been developed and are also discussed in detail includ- ing methods relating to the reproduction of acoustic forcing in incompressible simulations and the reproduction of turbulent fields at inlets. Several innovative post-processing techniques have been employed to identify the relationship between large scale flow structures, the fuel spray and combustion.
Modifications of the original injector geometry have been proposed to reduce the sensitivity of the injector to instabilities. These include better atomisation and mixing, better placement of swirl vanes, better aerodynamic design and improved hydrodynamic stability.