A swirling jet comprises axial momentum superimposed on angular momentum. It is a complicated flow whose characteristics and dynamics are not fully understood, although it is widely used in a variety of combusting situations. It is known, for example, that swirling jets have a larger spreading rate and higher mass entrainment than non-swirling jets, however the mechanisms responsible for these characteristics are not known. In combusting flows, swirl is often a desirable commodity as it enhances mixing, stabilises the high intensity combustion process and promotes efficient and clean combustion in a variety of applications, including petrol and diesel engines, gas turbines and industrial furnaces. In determining the rate of combustion, a first approximation is to assume equal diffusivity of all reactants, then the rate of mixing is the controlling factor determining the rate of combustion. In reality the turbulence will preferentially move a scalar with high molecular diffusivity relative to one with a lower molecular diffusivity. This process, called differential diffusion, is a potential complication of the mixing process and will influence the speed and efficiency of combustion.
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