Atomization of viscous and non-newtonian liquids by a coaxial, high-speed gas jet. Experiments and droplet size modeling

Abstract

This paper describes a collaborative theoretical and experimental research effort to investigate both the atomization dynamics of non-Newtonian liquids as well as the performance of coaxial atomizers utilized in pharmaceutical tablet coating. In pharmaceutically relevant applications, the coating solutions being atomized are typically complex, non-Newtonian fluids which may contain polymers, surfactants and large concentrations of insoluble solids in suspension. The goal of this investigation was to improve the understanding of the physical mechanism that leads to atomization of viscous and non-Newtonian fluids and to produce a validated theoretical model capable of making quantitative predictions of atomizer performance in pharmaceutical tablet coaters. The Rayleigh-Taylor model developed by Varga et al. has been extended to viscous and non-Newtonian fluids starting with the general dispersion relation obtained by Joseph et al. The theoretical model is validated using droplet diameter data collected with a Phase Doppler Particle Analyzer for six fluids of increasing rheological complexity. The primary output from the model is the Sauter Mean Diameter of the atomized droplet distribution, which is shown to compare favorably with experimental data. Critical model parameters and plans for additional research are also identified.

Studying the effects of elongational properties on atomization of weakly viscoelastic solutions using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER)

“…This has been primarily due to the fact that the proposed models can not capture one other key feature, which is the saturation of the e↵ect at large values of the relaxation time. Previous analyses have argued that the droplet average size should be proportional to the wavelength of the final instability process during the atomization event, and a combination of linear stability analyses for the di↵erent instability modes in the air spray should predict the measured values for the mean droplet diameter < d > [2,89]. Although this scaling may work for inviscid jets, recent work by Marmottant and Villermaux [3] for viscous Newtonian fluids shows that the e↵ects of viscosity on the lifetime of the ligaments that form in the final stages of detachment from the core liquid jet play a dominant role in setting the average droplet size in the spray.…”

“…This limit can be circumvented by studying the dynamics of capillary breakup using the ROJER device because of the much smaller length and time This study helps us understand the subtle e↵ect of extensional rheology on the breakup and the atomization of weakly viscoelastic liquids which are widely used in many biological and industrial applications. For Newtonian liquids, two di↵erent scalings for ligament sizes are suggested based on linear stability analysis [89,91]. The di↵erence between the two scalings arises from the Rayleigh Taylor stability analysis, in which the direction of air acceleration relative to the liquid/air interface can change at di↵erent air/liquid flow rate ratios.…”

“…The di↵erence between the two scalings arises from the Rayleigh Taylor stability analysis, in which the direction of air acceleration relative to the liquid/air interface can change at di↵erent air/liquid flow rate ratios. The competing e↵ect of viscosity and surface tension on the average droplet size in an air atomizer can be understood by a simplified version of the original scaling from Aliseda et al [89]:…”

A New Spray Approach to Produce Uniform Ultrafine Coatings

“…ese problems are particularly important for ultrafine coating due to its strict requirements. In order to solve these problems, the common practices are optimization design of the operating parameters such as adjustment of pressure, nozzle orifice, and gas-to-liquid flow ratio [8][9][10][11]. ese approaches can alleviate the uneven spray pattern, whereas the effect is limited and not essential to solve the above problems.…”

Numerical Models for Viscoelastic Liquid Atomization Spray

“…Dumouchel [10] has reviewed the experimental investigate dedicated to the primary atomization step for Newtonian liquids. Aliseda et al [11] extracted the images of liquid jet break-up which is closed to the nozzle orifice from high-speed visualizations for several Weber and Reynolds numbers for viscous and non-Newtonian liquids. They observed that at lower Weber numbers the Kelvin-Helmholtz instability grew slowly and several intact wavelengths were observed prior to break-up, In contrast at larger weber number, the Rayleigh-Taylor (R-T) instability was arresting.…”