Computational Modeling of Non-Newtonian Fluid Flow in Simplex Atomizer

This thesis presents a computational analysis of flow of time-independent, purely-viscous,power-law fluids in simplex atomizers using the Volume-of-Fluid (VOF) method and a linear stability analysis of a liquid jet emanating from an atomizer. Effects of atomizer geometry and the power-law index on the flow and performance of the atomizer have been investigated. Flow of shear-thinning (0.4 < n <1), viscous Newtonian (n = 1) and shear thickening fluids (1 < n < 1.4) has been considered. Three geometry parameters have been considered, viz., the atomizer constant which is the ratio of inlet area to the product of swirl chamber diameter and the exit diameter, the ratio of swirl chamber diameter to exit orifice diameter, and the ratio of length to diameter of the exit orifice. The dimensionless film thickness at exit, spray cone angle, and the discharge coefficient for different values of the three geometry parameters as well as those with varying power-law index are reported. The pressure drop across the atomizer has been kept constant in all simulations. With fixed atomizer geometry, the shear thinning liquids tend to produce thinner liquid sheet, larger spray cone angle, and higher discharge coefficient compared to shear thickening fluids. A change in the power law index has significant effect on the flow of shear thickening liquids in the atomizer. However, flow of shear thinning liquids exhibits relatively smaller change with varying n. Changes in the atomizer geometry have a significant impact on the flow. The film thickness and the discharge coefficient increase and the spray cone angle decreases with increasing atomizer constant. With increasing Ds/do, the dimensionless film thickness at the exit first decreases up to about 3, and then increases. However, the dimensional film thickness at the exit decreases monotonically. The discharge coefficient and spray cone angle decrease with increasing D_s/d_o. With increasing the ratio of length to diameter of the exit orifice, a minimum in the dimensionless film thickness is reached at about l_o/d_o = 0.75. However, both discharge coefficient and spray cone half angle decrease monotonically with increasing l_o/d_o. A significant finding is that the variations of film thickness, spray cone angle, and discharge coefficient with a change in atomizer geometry are similar for different values of fluid power law index. This has important implications for atomizer design and industrial applications as most atomizers are designed and characterized for Newtonian fluids.

As the liquid sheet or jet exits from an atomizer, it becomes unstable and breaks up into ligaments and droplets. We consider a liquid jet emanating from an atomizer. Investigation of the temporal instability of a non-Newtonian liquid jet moving in an inviscid gaseous surrounding has been carried out considering axisymmetric disturbances. Solving a set of linearized Navier-Stokes equations, the dispersion relation between wave growth rate and wave number is derived. The liquid jet is considered visco-elastic and its behavior is represented by the Jeffrey’s model. The effects of variations in axial Weber number and in liquid swirl Weber number on the disturbance growth rate were studied. The stability characteristics of an inviscid, a Newtonian and a non-Newtonian jet have been discussed.