Spray formation in non-Newtonian uids

Lead Research Organisation: Imperial College London
Department Name: Chemical Engineering

Abstract

While both several experimental and numerical studies for Newtonian sprays have been conducted, the exploration of the non-Newtonian ows has received comparatively little attention. Achieving fundamental understanding of the physical phenomena governing spray formation of this type of ow remains a challenge. The present project aims to set the basis for the numerical examination of non-Newtonian atomisation and spray systems. To achieve this, a Direct Numerical Simulations (DNS) approach is followed where all the temporal and spatial scales are completely resolved. We begin with the simulation of the filament thinning of an Oldroyd-B viscoelastic two-dimensional/axisymmetric framework, using the volume-of-fluid technique to track the interface and the log-conformation transformation for the solution of the viscoelastic constitutive equation. This permits the rapid exploration of parameter space, capturing the effect of the elastic, viscous and inertia forces (i.e. Deborah and Ohnesorge numbers), which characterise the internal relaxation and macroscopic time scales that viscoelasticity presents. This serves as a departure point for the development for the very first time of two-dimensional numerical simulations of an Oldroyd-B impulsive, as well as released with constant velocity jets into a stagnant gas phase for exploring the effect of viscoelasticity on the ejected droplet size. These simulations constitute the basis for further work involving three-dimensional simulations of atomisation processes of viscoelastic jets. The numerical simulations of the spray formation of a non-Newtonian fluid still offers substantial challenges, but it is reflective of industrial applications (e.g. spray-drying) and can lead to the optimisation of spray processes, containing fluids of a very complex behaviour.

Publications

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Description Sprays are the result of atomisation processes, following a number of different types of instabilities, which occur when a jet of liquid discharges into a gaseous phase. Spray formation in a non-Newtonian fluid is central to numerous industrial applications such as spray-drying and atomisation involving large interfacial deformations and complex dynamics between the fluid droplets and interconnecting ligaments. The aim of the present work is to establish a robust numerical basis for systematic examination of viscoelastic spray systems, i.e. dissolved polymers. To achieve this, we begin with two-dimensional axisymmetric numerical simulations of an impulsively started jet entering into a stagnant gaseous phase. We have achieved to simulate for the first time the jetting process of a viscoelastic fluid, including the nozzle, which seem to affect the breakup dynamics.

To be more specific, we expanded the thoroughly examined case of the breakup of a free viscoelastic filament, adding the effect of the momentum flux that comes in the system due to the injection. Regarding the flow inside the flow where a pressure gradient is imposed in the inlet, we successfully captured the phase lag as it is expected in the case of a pulsating poiseuille flow, while we also broadened this study to incorporate the elasticity effects. We validated the numerical simulations against the predictions of linear stability analysis. Subsequently, we explored the role of the nozzle in the thinning dynamics and the breakup process of the viscoelastic jet, as we analysed the evolution of the polymeric stresses in time and in space, moving downstream. We still examine the role of the finite extensibility of the polymeric chains in the breakup dynamics. The main objective of this research is to elucidate the open question of what is the valid rate of thinning that needs to be considered for a viscoelastic jet, where the role of the momentum flux into the system should be thoroughly captured. Hence, the validated numerical setup and study will permit the efficient exploration of material parameter space, capturing the competing effects of the elastic, viscous, and inertial forces on the ejected droplet size.

It is worth noting that this project is being carried out in collaboration with Prof.Gareth McKinley at "Hatsopoulos Microfluids Laboratory" at MIT, one of the most prominent laboratories in this area of research. A visit at MIT is also planned for carrying out spray-specific experiments using special equipment which is not available at Imperial College, which is currently suspended until the pandemic would permit for normal travel to resume.
Exploitation Route The objective of this research is to establish the numerical framework for the successful study of non-Newtonian jetting processes, which find significant industrial applications, i.e. injekt printing, as well as playing a central role in the transmission of infectious diseases. This work will be the departure point for the accurate prediction of the ejected droplet size, the extensional properties of viscoelastic fluids, and the thorough examination of the elasticity on the spray behavior, particularly when three-dimensional numerical simulations will be developed, as a next step.

This work also aims to set the basis for incorporating the non-isothermal behavior on the polymeric filament and its thinning dynamics, capturing effectively the corresponding Marangoni stresses. In particular, this will be carried out in collaboration with Johnson Matthey, which is interested in processes where the temperature effects on the jetting processes are important, i.e. in battery production. Johnson Matthey is providing funding through an iCase.
Sectors Chemicals,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology