Net Zero: Flame Instability of Ammonia Aerosol Combustion

In the search for renewable and carbon-free fuels, the use of ammonia is considered an attractive solution for engine and gas turbine applications. When compared to other carbon-free fuels, such as hydrogen, it has significant advantages. Not only is it easy to produce from renewable sources of nitrogen and hydrogen, it is safer to store and transport and has a higher energy content. Furthermore, it can be produced, transported and distributed without changing the infrastructure already deployed by industries. However, for the successful application of ammonia as a fuel, one main challenge related to its combustion needs to be overcome: its low reactivity requires a high ignition energy, a narrow flammability range and low burning velocity. This complicates the stabilisation of the combustion flame and thus inevitably causes unreliable ignition and unstable combustion.

The combustion of clouds of fuel droplets (or aerosol clouds) is of practical importance in gas turbines, diesel and spark ignition engines, furnaces and hazardous environments. There is experimental evidence that, contrary to expectations, flame propagation in aerosol clouds, under certain circumstances, is higher than that in a fully vaporised homogeneous mixture (possibly by up to a factor of 3). Also, the presence of fuel droplets is shown to enhance the generation of flame wrinkling instabilities. With richer mixtures and larger droplets, it is possible for droplets to enter the reaction zone and further enhance existing gaseous phase instabilities through the creation of yet further flame wrinkling. Therefore, the flame experiences periodic deceleration and acceleration with these oscillations lasting for several cycles within 100ms. Surely, the burning velocity enhancement may be advantageous in giving more rapid burning when burning ammonia in a gas turbine. As ammonia aerosol combustion has not been extensively studied yet, it is necessary to make clear to what extent ammonia aerosol flames inherit this oscillating behaviour as this oscillation of the flame will couple with thermo-acoustic oscillations and damage the turbine blades.

While some theoretical research has studied flame propagation in aerosol clouds, the processes governing flame oscillations are still unclear, especially for ammonia. We will use the numerical techniques and hydrodynamics codes developed at the University of Leeds for STFC-funded astrophysical research to increase our comprehension of this phenomenon and advance ammonia as a carbon-free fuel.