# Optimised Twin Fluid Atomiser Design for High-Viscosity, Shear-Thinning Fluids
## 4 Conclusions  
The twin fluid atomisation of silicone oil, mimicking a specific target sewage sludge, was studied. The investigated single-stage, free-fall atomiser configurations did not produce atomisation due to flow instabilities that deflected the high-viscosity liquid stream prior to reaching the designed impingement location. In such a design, a method to stabilise the free-falling liquid must be identified. Two-step atomisation, as shown in [36], may present a promising solution to this problem. Testing a two-stage free-falling nozzle design, however, was not feasible due to the geometrical constraints of the current nozzle design. This limitation arose from the excessive radial distance between the vertical gas streams (\(\theta\) = 0\({}^{\circ}\)) and the coaxially flowing liquid. A nozzle insert was introduced into the atomiser to mitigate the encountered issues. It turned out that relatively few, but large-diameter, discrete gas nozzle holes achieved the best atomisation results for the investigated atomiser design. This behaviour is attributed to larger-diameter free jets retaining their kinetic energy over extended distances (IDG) along the jet core. The gas velocity is therefore maintained at a maximum within the jet core. It was found that at least four discrete and equally distant gas jet orifices were necessary to enable atomisation. An atomiser design comprising six discrete and equally distant gas orifices, each \(6\times 10^{-3}\) m in diameter, was identified as optimal (ONC). Ambient temperature fluctuations caused variations in the viscosity of the surrogate liquid. A minimum feasible ALR of 8532 was determined for the ONC at a zero-shear viscosity of 15.9 Pa s, corresponding to a SMD of 570 \(\times\) 10\({}^{-6}\) m. ALR was found to depend on the viscosity: an increase from 15.9 Pa s to 16.7 Pa s led to an ALR rise from 11,376 to 20,477 in order to achieve comparable SMD values. This study aimed to investigate the feasibility of atomising high-viscosity, shear-thinning sewage sludge by employing a surrogate with comparable properties. Atomisation efficiency was found to improve through the optimisation of the orifice arrangement and apex angle. The findings contribute to the optimisation of atomiser designs for high-viscosity fluids exhibiting comparable characteristics to the surrogate, as encountered in a wide range of resource recovery applications. This is particularly relevant, since conventional nozzle designs are based on low-viscosity atomisation, which may limit their efficient applicability to high-viscosity fluids. The ONC demonstrated atomisation for silicone oil with a zero-shear viscosity of 10 Pa s at 25 \({}^{\circ}\)C, a result not previously reported in the literature, highlighting the potential of this design for challenging fluids. Nevertheless, several challenges remain before industrial application can be realised. These include scalability to real sludges with more complex composition and fouling tendencies, as well as ensuring long-term operational stability. Another challenge lies in the broad droplet size distribution observed with the ONC, limiting its applicability in scenarios demanding uniform spray characteristics. Future work should address the influence of characteristic atomiser dimensions (IDL, IDG) and explore atomisation under supersonic gas flows. Improved diagnostics such as Phase Doppler anemometry are additionally recommended to enhance droplet characterisation. Conceptualisation, M.D. ; methodology, M.D. ; software, M.D. ; validation, M.D. ; formal analysis, M.D. ; investigation, M.D. ; resources, C.H. ; data curation, M.D. ; writing--original draft preparation, M.D. ; writing--review and editing, M.D. and C.H. ; visualisation, M.D. ; supervision, C.H. ; project administration, M.D. and C.H. ; funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data Availability Statement: Data is contained within the article.