Fuel injectors play a crucial role in many combustion situations. They mix reactants, atomizing one if it is a liquid and set up conditions for flame attachment. Subtle changes in injector design often have large effects downstream. In our research we aim to understand the physical mechanisms behind these effects and develop practical rules for injector design.
The application of this knowledge is not limited to fuel injectors. This research can also be applied to industrial furnaces, to non-reacting injection and to centrifugal separators, such as the Dyson vacuum cleaner.
Good mixing is essential in a combustion chamber. In a rocket engine, for instance, mixing affects combustion efficiency, with a very strong influence on fuel consumption and maximum range. In an aeroplane engine, good mixing avoids hot spots in the chamber and thus reduces NOx emissions. In all cases, high turbulence is desired in order to achieve high flame surface density.
Much of our research focuses on confined shear injectors, which are commonly used in aerospace applications. We were the first to discover that confinement has a very strong effect on the local instability of shear flows, making them absolutely unstable rather than convectively unstable . This can lead to development of unstable global modes , which generate large scale motion in the flow, aiding mixing.
Our current work explores the behaviour of confined shear injectors. As well as developing the theory further, we are designing and constructing an experimental rig.
If one of the reactants is a liquid, it must evaporate before it can burn. The evaporation rate largely determines the heat release rate in the combustion chamber. Good atomization of the liquid jet is therefore essential, since it creates a large surface area for evaporation.
Conceptually, we differentiate between primary atomization, which is the generation of large initial droplets from the initial flow, and secondary atomization, which is the atomization of these large droplets to smaller droplets. Our group is mainly concerned with the effect of the injector's geometry on primary atomization.
In some situations, flames attach themselves to the lip of the fuel injector. This leads to a flame tip which is relatively insensitive to flow perturbations and which is therefore quite stable. However, if the flame detaches, it can become sensitive to oscillations or even blow out entirely. This is undesirable.
However, flames are not always attached to injector lips. Many are stabilized by a region of recirculating hot gas just downstream of the injector. This region is generated by swirling the flow and, again, is very dependent on the injector geometry and flow conditions.