# Optimised Twin Fluid Atomiser Design for High-Viscosity, Shear-Thinning Fluids
## 3 Results
### Atomisation Quality  
The atomisation quality of NC No. 4 was evaluated by the determination of the Sauter mean diameter and cumulative droplet size distribution. Figure 10 illustrates the cumulative droplet size distribution at the measurement positions MP 2 and MP 4 and the corresponding process parameters and Sauter mean diameters are obtained from Table 7. Based on the data in Table 7, secondary atomisation had not fully progressed at \(AX=62.5\times d_{L}\), as reflected by a further reduction in SMD at \(AX=131.25\times d_{L}\). \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**NC** & \(\hat{V}_{\text{G}}\times 10^{3}\) & \(T_{A}\) & \(Ma\) & \(p\times 10^{-5}\) \\  & **std m\({}^{3}\)/s** & **K** & **-** & **Pa** \\ \hline No. 4 & 52.78 & 298 & 0.9 & 1 \\ No. 4 & 105.56 & 288 & 1.0 & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Calculated gas flow properties at gas nozzle exit for measured \(\hat{V}_{\text{G}}\) and \(T_{A}\). Figure 9: ONC: High-speed image recordings at 75,000 FPS at measurement points MP 1 (**upper** images) and MP 4 (**lower** images). Blurry droplets caused by out-of-focus and velocity effects. Table 8 lists the operating conditions for the liquid mass flow rate and viscosity variations and the corresponding droplet size distributions are illustrated in Figure 11. Liquid mass flow was increased from Dataset 2 to 4. At constant viscosity and AX, decreasing the ALR resulted in higher SMD values and coarser droplet size distributions. The results in Tables 7 and 8, along with Figures 10 and 11, highlighted the influence of key operating parameters--namely, axial distance AX, air-to-liquid ratio ALR, liquid viscosity \(\eta_{L}^{0}\), and mass flow rate \(\dot{m}_{L}\) on atomisation performance, as quantified by the Sauter mean diameter \(d_{32}\). A qualitative sensitivity analysis suggests that ALR exerts the strongest influence on droplet size. For example, comparing Datasets 3 and 4 in Table 8, a reduction of ALR from 11,376 to 8532, by increasing \(\dot{m}_{L}\) from 9.00 to 12.00 g/s at constant \(\dot{V}_{G}\), lead to an increase in \(d_{32}\) from \(372\times 10^{-6}\) m to \(570\times 10^{-6}\) m, indicating a strong sensitivity of droplet size to the air-to-liquid ratio. A further observation concerns the broad droplet size distribution produced by the ONC, potentially limiting its applicability in processes that demand narrow spray spectra, as shown in Figure 11.  
In contrast, changes in viscosity at relatively high ALR (cf. Datasets 1 and 3, Table 8) showed a less pronounced effect: an increased \(\eta_{L}^{0}\) from 12.3 Pa s to 15.9 Pa s resulted in a rise in \(d_{32}\) from \(259\times 10^{-6}\) m to \(373\times 10^{-6}\) m. While high viscosity inhibits atomisation by dampening fluid instability waves, the overall effect appeared less pronounced than expected. As shown in a study of PDMS AK 1,000,000 [41], viscosity is strongly dependent on both temperature and shear rate. Under high shear, the viscosity differences between differently tempered silicone oils, however, decrease, resulting in reduced impact on the atomisation outcome. Comparable data for Wacker AK 10,000 are, to the best of the authors' knowledge, not available; however, a similar qualitative behaviour may be expected due to the analogous polymeric structure of the fluid. Figure 10: (**a**) Histogram of droplet size distribution and (**b**) corresponding (smoothed) cumulative distribution of the ONC at varying axial displacement AX. Process parameters according to Table 7.