# Effect of Solid Particles on Droplet Size Applying the Time-Shift Method for Spray Investigation
## 3 Experimental Setup and Measurement Techniques  
The investigations were conducted using the ATMOspheric spray test rig (ATMO), described in detail in [PERSON] et al. [14] and shown in Figure 1. For spray characterization, a time-shift and a laser diffraction system, as well as a high-speed camera, were used. A twin-fluid lance was supplied by liquid or suspension fluids through an eccentric screw pump (mass flow range: \(\dot{M}_{sus}=5\)-\(40\) kg\(\cdot\)h\({}^{-1}\)) controlled with a Coriolis mass flow and density meter. A stirred liquid storage tank was used to avoid sedimentation. A hot wire anemometer with a valve controlled the atomizing air mass flow in the range of: \(\dot{M}_{gas}=1\)-\(20\) kg\(\cdot\)h\({}^{-1}\). To prevent the recirculation of small droplets through the suction of the exhaust air, a honeycomb structure was placed at the inlet of the collection tank to serve as flow straighttener. The twin-fluid atomizer has a central tube with \(d_{liq}=2\) mm and a surrounding gas slit of \(s_{gas}=2.88\) mm. The influence of the tube separating the gas and liquid at the nozzle orifice was minimized by reducing the wall thickness to \(b=0.1\) mm. According to [PERSON] et al. [32], this configuration results in an undisturbed gas flow at the exit of the nozzle. The nozzle has parallel flow channels to enable the comparability of the experimental results with other investigations, for example those of [PERSON] and [PERSON] [10] and [PERSON] et al. [20]. For the characterization of the applied liquid and suspension fluids in terms of flow behavior and media properties, the following systems were used: The liquid viscosity was quantified by a Physica MCR 101 rheometer [33] equipped with a Searle-type measuring system [34]. The surface tension of the liquids was measured with an EasyDyne tensiometer [35] that employed the [PERSON] ring method [36]. The liquid surface tension was determined in accordance with the approach of [PERSON] and [PERSON] [37]. The density was measured using the weighing method. The complex refractive index \(n=n_{real}+iA\) was measured with an Abbe-refractometer for \(n_{real}\) and with visible spectroscopy for the attenuation \(A\). A high-speed camera was applied for the investigation of the primary breakup close to the nozzle orifice. This device was able to produce images at a frame rate of \(f=3600\) Hz at a resolution of 1 megapixel and up to \(f=500\) kHz at reduced resolution of 128 \(\times\) 16 pixels. The images were captured by applying backlight illumination of the region of interest with a special lighting setup. An array of 9 high-power light-emitting diodes (LED) with a total luminous flux of 9 \(\times\) 4500 lm was used. The position of each individual LED within the array was optimized for best light spread. Due to the high intensity and homogeneous distribution of the light, very short exposure times (\(t_{exp}\sim 7\) us) could be used. This light setup enabled a sharp representation of the droplets, even in case of fast flow conditions [38]. For the local measurement of droplet size and velocity, a so-called SpraySpy(r) from AOM-Systems was mounted on a 2-D traverse system at z = 150 mm downstream from the nozzle orifice. This measuring position was chosen for two reasons: (i) the secondary breakup is completed at even higher viscosities than those of the investigated liquids, as seen in previous experiments at \(\eta_{liq}=200\) mPa\(\cdot\)s [14]; (ii) the droplets are spherical, as revealed by high-speed camera images and which is also necessary for applicability of the SpraySpy(r) system. [PERSON] et al. [39] described the effect of non-spherical droplets on size measurement using the time-shift technique. The measurement system uses the time of flight method for calculation of droplet velocity and the time-shift method for detection of local droplet diameter [40]. The system was operated in backscatter mode with a working distance of \(\Delta x\) = 250 mm.