# Optimised Twin Fluid Atomiser Design for High-Viscosity, Shear-Thinning Fluids
## 1 Introduction  
Sewage sludge incineration is being increasingly adopted in many countries due to its advantages in volume reduction, pathogen elimination, and potential for energy and material recovery. Alternative treatments, such as landfill disposal, are limited or prohibited by concerns related to heavy metals, microplastics, and pharmaceuticals. To support circular economy objectives and address phosphorus scarcity, Germany will begin phosphorus recovery from sewage sludge starting in 2029 [1]. One potential technological approach for phosphorus recovery could involve the following two-stage process: In the first stage, sewage sludge is atomised and dried in a spray drying tower, i.e., fluidised bed reactor. In the second stage, the dried sludge particles are combusted in a rotary kilin. The resulting ash contains concentrated phosphorus recoverable through chemical or thermo-chemical processes [2, 3]. A previous study successfully realised the above mentioned cost-effective two-stage process, in which an atomiser, with no specific information available on the atomiser design, was utilised inside a fluidised bed reactor for sewage sludge incineration [4]. According to the reference [4], the nozzle must be wear-resistant and generate droplet sizes between 30 and 500 microns. It is clear that robust atomisation is essential forthe effective implementation of this approach, highlighting the critical role of atomiser design. This study is therefore motivated by the need to design a suitable atomiser for the high-viscosity sewage sludge employed in a specific resource recovery application. The use of polydimethylsiloxane (silicone oil) in our study as a surrogate fluid allows for the simulation of atomisation behaviour under controlled conditions, as its density, surface tension, and shear-dependent viscosity are comparable to those of the target sludge. The odourless, homogeneous, and non-separating nature of silicone oil facilitates laboratory handling and optical analysis. The literature on coal-water slurry (CWS) atomisation was consulted, as CWS generally constitutes a shear-thinning fluid containing suspended solid particles [5; 6]--a characteristic shared with sewage sludge. Internal-mixing twin fluid atomisers offer enhanced atomisation efficiency in comparison to external designs [7]; however, their inherent tendency to suffer from erosion and clogging when handling particulate-laden fluids remains a major drawback [8]. Several studies on high-viscosity twin fluid atomisation employ nozzle throat diameters below 8 mm, despite the associated increase in clogging risk [5; 9; 10; 11; 12]. In our proposed external-mixing atomiser design, a nozzle diameter of 8 mm was selected to minimise the risk of clogging encountered with viscous, particle-laden fluids. By keeping gas and liquid streams separate prior to atomisation, the external geometry reduces abrasion and facilitates easier maintenance. External atomiser designs, however, result in higher gas consumption and larger droplet sizes compared to internal-mixing variants [13], a trade-off that was deliberately accepted in favour of operational robustness. Research on shear-thinning high-viscosity fluids, i.e., bio slurry, water-based solutions, and suspensions, typically examine a single nozzle geometry and lack systematic evaluation of critical nozzle design parameters, such as apex angle of the atomising agent and orifice arrangement [5; 9; 10; 11; 14; 15; 16]. Multiple nozzle designs, in fact, originate from atomising low-viscosity fluids [9], raising concerns regarding their atomisation efficiency, when applied to high-viscosity media. One area that remains unexplored in the available literature is the atomisation of PDMS at a zero-shear viscosity of 10 Pa s. This study addresses this shortcoming by systematically analysing how variations in atomiser geometry and process parameters affect the atomisation quality of the high-viscosity PDMS.