## Materials and Methods
### Test Section  
The test section consists of a pipe flow and an atomiser, which is mounted on the centre axis. This setup is illustrated in Figure 1. The simple geometrical configuration is chosen in order to minimize secondary flow phenomena and to impose well-defined boundary conditions for numerical predictions. The hot gas mass flow of up to 1.5 kg s\({}^{-1}\) is provided by multiple rotary-screw compressors. By means of a natural gas fired heat exchanger, non-vitiated, preheated air at a constant elevated pressure is delivered to the test section.  
The operating condition of the hot gas flow is defined by temperature \(T_{\mathrm{hg}}\), pressure \(p_{\mathrm{hg}}\) and mass flow. The latter is measured by two orifice plates located upstream of the test section and is controlled to adjust a certain bulk flow velocity. Temperature and pressure are controlled at a definite distance upstream of the atomiser nozzle. It has to be mentioned that the mechanical design of the test section limits the operating range to a temperature of 873 K and an absolute pressure of 0.6 MPa. Still, it can be concluded that the exhaust gas conditions of diesel engines can be reflected over a wide range with this experimental setup. The operating conditions of the initial results presented in this paper will be elucidated in Section 3.  
A twin-fluid atomiser with an internal mixing chamber (CALDYN Apparatebau GmbH, Ettlingen, Germany) was mounted at the centre, coaxial to the hot air flow. The most important advantage in comparison to a simple pressure atomiser is a sharp spray angle and at the same time a very high quality of atomisation. The resulting spray is defined by a very narrow drop size distribution and  
Figure 1: Test section with instrumentation and measurement positions.  
a Sauter Mean Diameter (SMD) of less than 20 um. A snapshot of the spray in the near nozzle region is shown in Figure 2 in order to get a first impression. It should be noted that no flow around the atomiser was present. Hence, the illustrated spray represents the characteristics of the atomiser operated at quiescent ambient conditions. In particular, the narrow spray angle of approximately 15\({}^{\circ}\) is a very important feature, since the wall contact of droplets can be excluded over a long distance downstream of the atomiser. In addition, secondary atomisation can be assumed to be completed at a very short distance downstream of the atomiser nozzle. Hence, measurements close to the atomiser are feasible for deriving evaporation characteristics.  
The piping of the test rig consists of tube elements and elements equipped with windows providing optical access. These modules are interchangeable in order to investigate the spray characteristics at different distances \(x\) downstream of the atomiser. Possible measurement positions are depicted in Figure 1. The operating condition of the atomiser is defined by the mass flow of cold atomising air and the volume flow of UWS, which are mixed directly upstream of the atomiser exit. The temperature of the cold air \(T_{\text{cg}}\) is measured directly upstream of the mixing chamber. The final temperature of the mixture is determined in order to estimate the initial temperature of droplets \(T_{\text{d0}}\) at the atomiser exit.