# Optimised Twin Fluid Atomiser Design for High-Viscosity, Shear-Thinning Fluids
## 3 Results
### Atomisation Quality
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**DS** & **NC** & **AX** & \(\eta_{L}^{0}\) & **ALR** & \(\dot{m}_{L}\times\mathbf{10^{3}}\) & \(d_{32}\times\mathbf{10^{6}}\) \\  & & **m** & **Pa s** & **-** & **kgs\({}^{-1}\)** & **m** \\ \hline
0 & 4 & 62.5 \(\times\)\(d_{L}\) & 12.3 & 12,286 & 8.33 & 367 \\
1 & 4 & 131.25 \(\times\)\(d_{L}\) & 12.3 & 12,286 & 8.33 & 259 \\
2 & 4 & 131.25 \(\times\)\(d_{L}\) & 16.7 & 20,477 & 5.00 & 343 \\
3 & 4 & 131.25 \(\times\)\(d_{L}\) & 15.9 & 11,376 & 9.00 & 372 \\
4 & 4 & 131.25 \(\times\)\(d_{L}\) & 15.9 & 8532 & 12.00 & 570 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Sauter mean diameter and corresponding operating conditions. In addition, the axial measurement position influenced the observed SMD. As shown in Figure 10, a downstream shift from \(AX=62.5\times d_{L}\) to \(AX=131.25\times d_{L}\) resulted in a notable reduction in droplet size, indicating continued secondary atomisation. An extended breakup distance was expected for high-viscosity liquids, as an elevated Ohnesorge number has been shown to delay the onset of liquid column deformation and ligament breakup [42]. High-viscosity liquids were typically characterised by Oh > 1 [21]. The surrogate fluid used in this study, Wacker AK 10,000, exhibits an Oh number exceeding 19 under an estimated local shear rate of 50,000 s\({}^{-1}\) in the near-nozzle region (see Table 1). Further downstream measurements could have revealed a continued decrease in SMD; however, such investigations were not feasible due to vertical space limitations in the test facility. Interestingly, a downstream distance of 0.35 m was selected as the measurement zone in a previous study [14], corresponding to the ignition zone of CWS with viscosities of 0.8 Pa s to 0.9 Pa s. The observation implies for CWS with viscosities in the range of 0.8 Pa s to 0.9 Pa s that secondary atomisation had sufficiently progressed by that point. In contrast, the delayed droplet break up observed in the present study is consistent with the higher viscosity and Oh number of the fluid. Secondary atomisation was found to progress notably between the measurement positions at 0.5 m and 1.05 m downstream. It is worth noting that stable atomisation was consistently achieved across ambient temperatures, ranging from approximately 0 \({}^{\circ}\)C (winter conditions) to 25 \({}^{\circ}\)C (summer conditions), suggesting a certain degree of operational redundancy with respect to external environmental factors. The robustness was further confirmed by the fact that repeatable atomisation could be achieved in all trials using the ONC. Several aspects, however, remain unresolved and merit further investigation. The influence of varying characteristic atomiser distances, in particular the liquid impingement distance (IDL) and the gas impingement distance (IDG), on droplet size distribution has not yet been evaluated. IDL and IDG are expected to impact droplet size distribution due to altered velocity gradients. Moreover, the present study was limited to sonic gas  
Figure 11: Smoothed cumulative distribution of droplet sizes based on the data presented in Table 8.  
velocities. It remains an open question how supersonic gas flow conditions would alter the atomisation performance. Overall, the ONC configuration demonstrates promising atomisation characteristics. Broader validation, however, is required to enable its practical implementation in resource recovery applications. One of the primary limitations lies in the scalability and adaptability of the nozzle to real high-viscosity sludges, given their stronger fouling tendencies, compositional heterogeneity, and higher solids content. Ensuring long-term operational stability, resistance to clogging, and easy maintenance under industrial conditions represents a key technical challenge that needs to be addressed in future studies.