# Effect of Solid Particles on Droplet Size Applying the Time-Shift Method for Spray Investigation
## 4 Results and Discussion
### Conclusions  
This study investigated the influence of solid particles on spray formation for different process parameters. The dynamic viscosity was constant. To achieve this, two sets of Newtonian fluids (each consisting of one pure liquid and one suspension at the same respective viscosities) were used. The spray was recorded by a high-speed camera and a SpraySpy(r) system based on the time-shift effect, while a commonly used Spraytec(r) laser diffraction analyzer was employed for validation. In contrast to laser diffraction measurements, the SpraySpy(r) system allows for the detection of local spray characteristics. An external-mixing twin-fluid atomizer was operated at different gas velocity and corresponding GLR at constant liquid mass flow. The results of the experiments can be summarized as follows:  
* Newtonian flow behavior of the suspensions was achieved by suspending round glass beads of a specific size but different mass fraction in Newtonian liquids. For each suspension, a pure liquid with same viscosity was additionally used as reference fluid, which allows for the separation of the effect of the glass mass fraction and dynamic viscosity on atomization.
* A new measurement technique that applies the time-shift method for local measurement of droplet size and velocity in opaque suspension sprays was successfully used. The following effects were observed:
* With increasing gas velocity, the radial Sautter mean diameter distribution decreased.
* With increasing dynamic viscosity, the radial Sautter mean diameter distribution increased.  
Figure 9: Radial distribution of the Sautter mean diameter at \(z=150\) mm below the nozzle orifice as a function of the glass mass fraction for \(v_{gas}=80\) m\(\cdot\)s\({}^{-1}\) (dots: SpraySpy®, values: LDS) (left); High-speed camera images of the primary breakup for \(v_{gas}=80\) m\(\cdot\)s\({}^{-1}\) at \((\textbf{a})\,\phi_{P}=0\) wt-% and (**b**) \(\phi_{P}=50\) wt-% (right).  
With increasing solid mass fraction, an increasing droplet diameter was detected. This effect can be explained by the theory of tensile strength and a force balance.
* The primary breakup analysis of the viscous liquids and suspensions was performed with a high-speed camera. The primary breakup instability varied as a function of the dynamic viscosity, whereas a change in the solid mass fraction led to the same instability mode.
* The dependencies of droplet size on gas velocity, dynamic viscosity and solid mass fraction revealed by the SpraySpy\({}^{\text{\textregistered}}\) system were confirmed by a commonly applied laser diffraction system.  
Conceptualization, S.W. and T.J.; investigation, S.W.; data curation, S.W.; writing-original draft preparation, S.W.; writing-review and editing, T.J. and T.K.; visualization, S.W.; supervision, T.K. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the financial support of the Helmholtz Association of German Research Centers (HGF) in the context of the research program, Energy Efficiency, Materials and Resources (EMR). The present work contributes to the Helmholtz Virtual Institute for Gasification Technology-HVIGasTech (VH-VI-429) [[http://www.hvigastech.org/](http://www.hvigastech.org/)]([http://www.hvigastech.org/](http://www.hvigastech.org/)).  
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} ATMO & Atmospheric Spray Test Rig \\ CWS & Coal-water Slurry \\ EFG & Entrained Flow Gasification \\ GLR & Gas-to-Liquid Ratio \\ LDS & Laser Diffraction System \\ LED & Light-emitting Diode \\ \end{tabular}