# Temperature Fields of the Droplets and Gases Mixture
## 2 Experimental Set-Up and Procedure
### LIP Measurement Technique
For optical calibration of the measurement system (correcting and comparing images from two cameras), a flat calibration target was used with an in-line array of markers, 2 mm in diameter located 5 mm away from each other. We used the DaVis software (LaVision Inc., Gottingen, Germany) for  
Figure 3: The emission (left) spectrum [26] and calibration (right) curve (**a**); video frame of BAM:Eu particles before being processed (**b**); typical airflow temperature field without droplets (**c**), air flow (**d**) and combustion products flow (**e**) after droplets pass-through obtained by Laser-Induced Phosphorescence (LIP) technique (for \(T_{\mathrm{a}}\approx\) 200 \({}^{\circ}\)C). experimental image processing and temperature field recovery. Image processing comprised a series of consecutive operations according to the conclusions made in [24; 25]. A background was subtracted from each initial image obtained. The image was then calibrated using the calibration target image. A series of software filters were applied to remove noise, to mask areas with no tracer particles, and to smooth the luminous intensity of tracers. The fluorescence gradient in the image was adjusted with White Image Correction, and a temperature field was plotted based on the analysis of the emission intensity from two optical filters. The LIP technique experiments were conducted in two stages: The first one is the temperature calibration of the system in the absence of sprayed flow in the observation area. For each standalone experiment, the LIP temperature field could only be recorded while BAM:Eu particles were in the observation area (1.5-2 s), which is not enough time to record the whole blending process. That is why the second stage included a series of experiments, starting with the initial droplet flow injection (recording initiated at 0 s), and then with 2 s increment until the temperature recovery relative to the undisturbed gas flow. When conducting their experiments, the authors focused on such matters as the power fluctuations of the Nd:YAG laser and the concentration of the thermographic indicator in the measurement area [26; 27]. The experimental results have shown that, when the laser power ranges from 70 to 130 mJ, the BAM:Eu emission changes proportionally. However, the emission intensity ratio of BAM:Eu (420/450 nm) remains the same and does not exceed the systematic error of the technique. The result obtained is in good agreement with the results from the work in [27], where the authors explored the impact of laser power on the emission intensity and heating of thermographic phosphor particles. On the other hand, for reliable measurements, it is important to maintain the optimal concentration of BAM:Eu particles in the measurement area. The relative volume concentration (\(\gamma_{\rm p}\)) of BAM:Eu particles in a frame (Figure 3b) was derived indirectly from the intensity of the laser radiation emitted by the particles. As a criterion to estimate \(\gamma_{\rm p}\), we used the ratio of the total area of BAM:Eu particles in a frame (\(S^{\star}\)), derived from the binarization threshold, to the observation area size (\(S_{0}=x\,y\)): \(\gamma_{\rm p}\)=\(S^{\star}\)/\(S_{0}\). Moreover, experiments have established that the average fluorescence of droplets did not exceed 25-30 intensity counts when they were on screen. At the same time, the fluorescence of BAM:Eu particles was 10-12 times higher and reached 100-170 intensity counts for the left side of the emission spectrum (420 nm) and 160-220 intensity counts for the right side (450 nm) of the same spectrum. Thus, by specifying the binarization threshold above the luminous intensity of droplets, we discarded droplets from analysis so that they did not affect the determination of \(\gamma_{\rm p}\). When constructing temperature fields, we only considered the experimental frames where the relative concentration of BAM:Eu particles \(\gamma_{\rm p}\) was within 0.05-0.1.