## 3 Results
### Drop Size Distribution Modelling  
Table 3 reports the shape and scale parameters together with the 95% confidence interval for their median values and the results of the Anderson-Darling goodness of fit test in function of spray pressure and for each nozzle type.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Pressure,**} & \multicolumn{3}{c}{**Shape Parameter and**} & \multicolumn{3}{c}{**Scale Parameter and**} \\  & **95\%** & **Confidence Interval** & \multicolumn{3}{c}{**95\%** & **Confidence Interval**} & **AD Test** \\  & **of the Median** & & & **of the Median** & & **of the Median** & & \\ \hline \hline  & \(\alpha\) & 2.5\% & 97.5\% & \(\beta\) & 2.5\% & 97.5\% & \(p\)-value \\ \hline \hline  & \multicolumn{6}{c}{ATR 80 orange} \\ \hline
0.3 & 2.3319 & 2.3101 & 2.3609 & 34.3492 & 33.9444 & 34.7725 & 0.2060 \\
0.5 & 2.5450 & 2.5248 & 2.5720 & 27.4295 & 27.1150 & 27.7032 & 0.2043 \\
1.0 & 2.4367 & 2.4205 & 2.4486 & 23.1820 & 23.0522 & 23.3008 & 0.0006 \\
1.5 & 2.4511 & 2.4394 & 2.4626 & 22.1915 & 22.0771 & 22.3346 & 0.0006 \\ \hline  & \multicolumn{6}{c}{TVI 8002} \\ \hline
0.3 & 1.1703 & 1.1344 & 1.2068 & 166.3876 & 160.1959 & 173.6039 & 0.0506 \\
0.5 & 1.4243 & 1.3825 & 1.4638 & 116.5647 & 112.6313 & 120.4293 & 0.2788 \\
1.0 & 1.5321 & 1.4890 & 1.5704 & 96.5119 & 93.5747 & 99.5777 & 0.4038 \\
1.5 & 1.5101 & 1.4829 & 1.5482 & 86.3048 & 84.2055 & 87.8912 & 0.2655 \\ \hline  & \multicolumn{6}{c}{AVI 11003} \\ \hline
0.3 & 1.3864 & 1.3524 & 1.4267 & 104.8953 & 101.6836 & 108.2892 & 0.2418 \\
0.5 & 1.4305 & 1.4029 & 1.4475 & 70.3632 & 69.0494 & 71.9837 & 0.0114 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Shape and scale parameters of gamma distribution.  
Figure 6: Cumulative volumetric droplet size distribution as affected by nozzle type and working pressure (data of all repetitions).  
For each nozzle type, the shape parameter was almost constant in function of the spray pressure. The 95% confidence interval for the median ranged from 2.310 to 2.572 for the ATR 80 orange nozzle, from 1.134 to 1.570 for the TVI 8002, and from 1.352 to 1.447 for the AVI 11003. On the contrary, the scale parameter was clearly decreasing when the pressure increased: from 34.349 (0.3 MPa) to 22.077 (1.5 MPa) for the ATR 80 orange nozzle, from 166.388 (0.3 MPa) to 86.305 (1.5 MPa) for the TVI 8002, and from 104.895 (0.3 MPa) to 70.363 (0.5 MPa) for the AVI 11003. The decreasing trend of the scale parameter with the increase in the pressure is an index of the higher drop pulverization (more drops of a lower diameter, Figures 4 and 5).  
The comparison between the three nozzles in terms of probability density functions and cumulative distribution functions when varying the spray pressure is shown in Figures 7-9. According to the results of the AD test (Table 3), the best fitting was obtained for the nozzle TVI 8002 at every pressure, for the ATR 80 up to 0.5 MPa, and for the AVI 11003 at 0.3 MPa. As a general result, the gamma distribution tends to underestimate the number of drops smaller than 100 \(\upmu\)m.  
Figure 8: Comparison between measured and theoretical distribution for the TVI 8002 nozzle. **(a)** Histograms and theoretical densities; **(b)** Empirical and theoretical cumulative distribution functions.