## 3 Results and Discussion  
The discussion of the initial results is based on the data taken at four measurement positions and at one hot gas operating condition, which are defined in Figure 1 and Table 2, respectively. The operating condition of the hot gas is set in order to preserve the Reynolds number  
\[Re=\frac{D}{\upsilon_{\text{hg}}}=100,000 \tag{2}\]  
of the pipe flow. In Equation (2), \(D\) represents the diameter of the pipe of \(70.3\,\mathrm{mm}\), \(\bar{u}_{\text{hg}}\) the bulk velocity, and \(\upsilon_{\text{hg}}\) the kinematic viscosity of the hot gas. The main reason for preserving \(Re\) is to ensure well-defined boundary conditions for the turbulent dispersion of droplets.  
The subsequent discussion of the results is focused on the capabilities of the diagnostics. In this context, the first measurement position downstream of the atomiser (\(x=$50\,\mathrm{mm}$\)) is of great importance for the methodology, since the droplet data recorded at this position can be utilised as droplet starting conditions for numerical simulations of spray evaporation. Hence, it must be ensured that sufficient information about the spray is captured in order to avoid inconclusive results.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Measurement volume & \(\mathrm{mm}^{3}\) & \(1.2\times 0.9\times 0.4\) \\ Field of view & \(\mathrm{mm}^{2}\) & \(1.2\times 0.9\) \\ Resolution & \(\mathrm{\SIUnitSymbolMicro m}/\mathrm{pixel}\) & \(0.75\) \\ Detectable diameters & \(\mathrm{\SIUnitSymbolMicro m}\) & \(4\cdot 300\) \\ Numerical aperture & & \(\simeq 0.1\) \\ Exposure time & \(\mathrm{\SIUnitSymbolMicro s}\) & \(\simeq 0.01\) \\ Interframing time & \(\mathrm{\SIUnitSymbolMicro s}\) & \(0.5\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Major properties of the microscopic imaging diagnostics.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Hot gas temperature & \(T_{\text{hg}}\) & \(773\,\mathrm{K}\) \\ Hot gas pressure & \(p_{\text{hg}}\) & \(0.12\,\mathrm{MPa}\) \\ Hot gas bulk velocity & \(\bar{u}_{\text{hg}}\) & \(96\,\mathrm{m}\,\mathrm{s}^{-1}\) \\ UWS volume flow & \(\dot{V}_{\text{UWS}}\) & \(10\,\mathrm{L}\,\mathrm{h}^{-1}\) \\ Cold gas mass flow & \(\dot{m}_{\text{cg}}\) & \(5.84\,\mathrm{kg}\,\mathrm{h}^{-1}\) \\ Cold gas pressure & \(p_{\text{cg}}\) & \(0.49\,\mathrm{MPa}\) \\ Initial droplet temperature & \(T_{\text{d0}}\) & \(325.8\,\mathrm{K}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Operating condition of the hot gas and the twin-fluid atomiser for experimental investigation of spray evaporation of urea–water solution (UWS).  
Furthermore, the accuracy of measuring the gas velocity by using the smallest detectable droplets as seeding particles is assessed by a Stokes number analysis. The resulting gas velocity may yield important information for the analysis of spray systems, since the relative velocity of droplets with regard to the gas phase can be derived. Finally, it will be elucidated that the described methodology will provide experimental validation data, which can be used to study the evaporation characteristics of UWS under technical relevant flow conditions.