# Optimised Twin Fluid Atomiser Design for High-Viscosity, Shear-Thinning Fluids
## 3 Results  
### Nozzle Inserts  
First, atomisation investigations were performed with the nozzle configuration (NC) No. 4, consisting of six holes with a diameter of \(6\times 10^{-3}\) m each. The apex angle was chosen as 90\({}^{\circ}\). Compared to the literature, an apex angle of 90\({}^{\circ}\) is unusual, as most studies have experimented with external atomisers using apex angles up to 65\({}^{\circ}\)[10; 14; 33; 35; 37; 38].  
Figure 5 shows four different nozzle assemblies, each distinguishable by different nozzle inserts that are illustrated in black or by the absence of a nozzle insert. The first  
Figure 4: Image of (**a**) silicone oil atomisation and (**b**) corresponding, refined binary image.  
configuration tested, indexed by the number 1 in Figure 5, resulted in the production of fibre-like structures. In order to exclude doubts on the nozzle insert's length to diameter ratio of the second configuration, a third configuration with a ratio of 2.5 was investigated (3). The second and third configuration did not result in any improvements towards achieving atomisation.  
The formation of recirculation zones and vortices in twin fluid atomisation nozzles were researched at length [33; 39]. The formation of recirculation zones as well as vortices interacted with the free-falling liquid and deflected the downward-pouring liquid stream. It was notable that inhibiting recirculation zones and vortices must be taken into even greater consideration, as highly viscous liquids seemed to be more prone to insufficient atomisation when deflected. Those deflected structures were then not disintegrated further, as they appeared too far away from the geometrical impingement point of the gas jets and thus, the relative velocities between the gas jets and the disintegrated liquid columns were too low to achieve further breakup. Consequently, single-staged, free-fall atomisers were deemed unsuitable for the atomisation of the investigated high-viscosity surrogate liquid.  
It is known that highly viscous liquids require maximised relative velocities to achieve fine sprays [35]. The impingement distance between the liquid cluster agglomerating along the liquid nozzle tip and the gas jets (IDL) must be therefore minimised to ensure sufficient disintegration. This was achieved by the use of a nozzle insert shown in Figure 5, marked by the number 4. The variation in length of the nozzle insert is indicated in yellow.  
Preliminary investigations showed that both overly long and overly short nozzle inserts resulted in insufficient atomisation performance. As the radial distance from the core of the gas jet increases, the axial velocity of the jet diminishes. This reduction in velocity results from the momentum transfer between the jet and the surrounding air due to shear forces, leading to the dissipation of kinetic energy [40]. Consequently, it can be inferred that IDL should be minimised to maximise the shear rate. Following this reasoning, one might consider directing the gas jet to impact the nozzle insert such that the core velocity is fully utilised for atomisation. However, this would cause a loss of kinetic energy within the gas jet through the reduction of the effective gas volume flow rate available for atomisation. In this study, IDL was held constant at \(1.5\times 10^{-3}\,\mathrm{m}\).