# Mass Flow Scaling of Gas-Assisted Coaxial Atomizers
## 3 Experimental Setup  
As the experiments were carried out over a wide range of liquid mass flows (\(\dot{M}_{liq}\) = 20-500 kg\(\cdot\)h\({}^{-1}\)), two different spray test rigs were employed. The ATMOspheric spray test rig (ATMO), which is described in detail in [PERSON] et al. [38], was utilized for liquid mass flows at the lab-scale between \(\dot{M}_{liq}\) = 20-100 kg\(\cdot\)h\({}^{-1}\). The burner test rig (BTR), which is shown in Figure 2, was applied for the investigation of the nozzles featuring liquid mass flows on the industrial scale of \(\dot{M}_{liq}\) = 500 kg\(\cdot\)h\({}^{-1}\). The nozzle was mounted on a twin-fluider dance, that was supplied with liquids from a storage tank. The liquid mass flow was adjusted by a Coriolis mass flow and density meter and pumped through an eccentric screw pump with a mass flow range of \(\dot{M}_{liq}\) = 400-1300 kg\(\cdot\)h\({}^{-1}\). Pressurized gas was provided by a screw compressor with a 5 m\({}^{3}\) pressure vessel at \(p_{sys}\) = 11 bar. The measurement and regulation of the gas mass flow (\(\dot{M}_{gas}\) = 50-400 kg\(\cdot\)h\({}^{-1}\)) was performed by a hot wire anemometer with a coupled valve. The nozzle dimensions are given in Table 1. As the central tube thickness between liquid and gas phase \(b\) has a significant influence on the resulting spray [26], \(b\) was reduced to a minimum size of \(b\) = 0.1 mm. According to [PERSON] et al. [29], \(b\) has to be minimized to avoid disturbances at the exit of the nozzle. To enable a comparison with earlier studies, all applied nozzles feature parallel flow channels of the gas and liquid phase. Water was used at \(T\) = 20 \({}^{\circ}\)C and \(p_{sys}\) = 1 bar, with \(\eta_{liq}\) = 1 mPa\(\cdot\)s, \(\sigma\) = 0.0719 N\(\cdot\)m\({}^{-1}\) and \(\rho_{liq}\) = 998 kg\(\cdot\)m\({}^{-3}\). For the detection of primary jet breakup and for validation of the droplet size results, a high-speed camera was used in all experiments. An appropriate illumination of the images was achieved by a 9 \(\times\) 4500 lm light-emitting diodes (LED) array in a backlight configuration. For every operating condition, a set of 2000 images was recorded near the nozzle orifice, as well as in the measuring plane of the phase Doppler anemometer to guarantee a high-quality data base. The camera enabled images with 1 megapixel at a 3600 Hz frame rate. A more detailed description of the setup is given in [22]. At the industrial scale, the high-speed camera was also used to investigate the droplet size distribution. For each operating condition, 2000 images in the measurement plane were recorded, whereas every 20 th image was applied for the droplet size calculation to avoid the double determination of droplets. The calculation was performed by means of an algorithm with a global threshold method by [PERSON] [39]. Out of 100 images, at least 29,000 droplets were analyzed per operating condition, achieving a reliable data base [40]. The lowest detectable droplet size was \(d_{drop}~{}=~{}225~{}\upmu\)m, which equaled three pixels in the high-speed camera sensor. As the droplet size at the industrial scale was expected to be significantly above the measurement limitation, the resolution was considered to be sufficient. For spray characterization at the lab-scale, a fiber phase Doppler anemometer (PDA) with a SprayExplorer was utilized in a forward scattering arrangement (first-order refraction) to investigate the droplet diameter locally. The settings of the setup were optimized in accordance with [41], which led to a measuring range of droplet diameter from 2-1357 \(\upmu\)m for water, respectively [42]. The settings, evaluated by means of a sensitivity analysis adapted from [PERSON] et al. [43], are presented in Table 2.