The study of liquid sprays is fundamental in understanding their applications in a variety of atomization devices, such as IC Engines, Agriculture, Inkjet printers etc. In this present study, we are specifically interested in high-speed atomization, pertaining to both rectangular liquid sheets and cylindrical liquid jets. The first part of the thesis aims to provide insights into the breakup of liquid sheets and jets using theoretical analysis such as linear stability analysis, and highly resolved computational fluid dynamics simulations. The second part of the thesis discusses an application of liquid sprays in the engine exhaust involving injection of urea-water solution sprays to reduce harmful NOx from the engine exhaust.
The physics governing the instability of high-speed liquid sheets being injected into a quiescent gas environment is studied using a combination of mean-flow spatial linear stability and two-dimensional Volume-of-Fluid (VoF) simulations, where it is found that the gas shear layer thickness, δG, plays an influential role. From the mean-flow spatial linear stability analysis, it is observed that at high gas shear layer thicknesses (δG), a new sinuous mode emerges, characterized by large wavelengths. To further investigate this phenomenon, perturbations corresponding to the most dominant sinuous and varicose modes are applied at the inlet of a Volume of FLuid (VoF) simulations. The results reveal that large-scale sinuous modes play a critical role in the catastrophic breakup of the liquid sheet. An energy analysis is conducted for both large-scale sinuous and small-scale varicose modes reveal that, for large-scale sinuous modes, the energy contributions from pressure fluctuations in the gas phase significantly enhance the catastrophic breakup of the sheet. In contrast, for small-scale varicose modes, these pressure fluctuations inhibit the growth of instabilities, opposing their development.
An obvious extension of the work on liquid sheets is to understand the physics of high-speed cylindrical jets injected into a quiescent gas environment. Volume of Fluid (VoF) simulations of the 3D cylindrical jets show that the complete fragmentation of the jet is caused by a large wavelength mode, which is helical in nature. Upon careful observation, these modes are observed when the gas shear layer thickness (δG) spreads significantly. Plotting the perturbation kinetic energy (PKE) shows that near the region of the development of large-scale asymmetric modes, the PKE in gas spreads and drives the growth of these modes eventually leading to the jet breakup. This phenomenon observed in the VoF simulations is explained through a mean-flow linear stability analysis (MFLSA). From MFLSA, when δG is increased for the mean flow, the asymmetric (helical) modes shift to a higher wavelength compared to the axisymmetric modes. Energy analysis for axisymmetric modes shows that the PKE in liquid is always higher compared to the PKE in gas for all values of gas shear layer thickness (δG). Whereas in the case of helical modes, the PKE in liquid is higher for low values of δG and then shifts to a higher PKE in gas as the δG is increased. This observation from the MFLSA directly correlates to the spread of PKE in gas observed in the VoF simulations when large-scale modes develop at higher gas shear layer thickness.
The last study presented in this thesis studies one of the applications of the sprays in Selective Catalytic Reduction (SCR) system in an engine exhaust system, used to reduce the nitrogen oxides (NOx) into nitrogen (N2) and water (H2O). The prevalent method this is done is by injecting urea-water solution (UWS) sprays, producing ammonia (NH3) which reacts with the NOx in the SCR system. In this study, we perform a Lagrangian-Eulerian method to simulate the UWS sprays in the engine exhaust system, using Converge-CFD software. The concentration of NH3 at the inlet of SCR system is analyzed by simulating the engine exhaust system for various conditions of engine exhaust such as mass flow rate, temperature and composition. In addition, the simulations are also conducted for various UWS injection conditions. The data from the simulations are validated with experiments and could be potentially used to create reduced order models assisting in predicting the NH3 available for reduction of NOx.