ID 341

A Closer look at Linear Stability Theory in Spray Modeling

 

Abstract:

Primary atomization in liquid sprays is widely modeled on the basis of linear stability theory, where the growth rates and associated liquid length scales are used in prescribing initial Lagrangian droplet characteristics. Under this conventional view of the breakup process, the most unstable mode (from linear stability) grows and develops to such an extent that it breaks up the jet. The present work is aimed at first examining the extent to which this linear stability and associated flow conditions hold in a realistic spray configuration using the ECN spray A geometry. Secondly, it is aimed at analyzing the degree to which the conventional view of spray breakup, adopted broadly in spray models, conforms to a much more detailed and realistic simulation of fuel injection and atomization via a Volume-of-Fluid formulation (both internal and external flow computation). The results show that within the first 4 diameters beyond the orifice, the non-linear components of the Navier-Stokes have grown to 10% of the corresponding linear part in both the liquid and the gas phase, and continue to grow exponentially. The non-axial and non-fully developed flow profiles are particularly significant even within one diameter, but do not develop as strongly as the non-linear components. Additionally, the interfacial disturbances are highly irregular and asymmetrical as opposed to the conventional sinusoidal disturbances adopted in linear theory. The disturbances of the liquid jet result in the atomization of jet surface within approximately 7 diameters downstream of the orifice. However, they are excessively small to cause complete fragmentation of the liquid jet, which happens much farther downstream at L/do = 37.8. This finding raises questions on the notion that disturbances stemming from linear stability are responsible for primary atomization and casts doubts on the conventional view of jet breakup.