# Multiparameter Determination of Thin Liquid Urea-Water Films
## 1 Introduction  
In many industrial applications, the formation of thin liquid films, whether it be desired or not, has a significant influence on the underlying processes. Examples range from printing or coating processes [1, 2] and lubrication purposes [3, 4] to the formation of oil films on piston heads, causing pool fires and leading to an increase in pollutants in exhaust gas [5, 6]. Another process where film formation has a high relevance is the selective catalytic reduction process (SCR), which is a state-of-the-art method to reduce nitrous oxides from the exhaust gas of diesel engines. A urea-water solution (UWS or AdBlue) is injected into the hot exhaust gas stream upstream of the SCR catalyst. Through thermolysis and hydrolysis processes, ammonia is formed, which acts as the reducing agent. However, depending on the operation conditions, a liquid UWS film can form on the surrounding walls, which can subsequently form solid deposits.  
In order to comply with the limits of nitrogen oxides in the future, the SCR process must be understood in detail. Film formation, in particular, is a crucial parameter that can lead to a reduction in the efficiency of the SCR [7]. The investigation of the formation of the liquid film and deposit formation on the overall SCR process plays a leading role. Thus, key film parameters, such as film thickness, temperature and concentration of urea in water, must be monitored [8].  
A wide range of measurement of non-intrusive methods have been proposed in the past for the determination of film parameters, which include laser-induced fluorescence [9, 10], ultrasonic measurements [11, 12] and Raman spectroscopy [13, 14]. However, among this selection of multiple methods, only laser absorption spectroscopy currently offers the ability to measure film thickness, concentration and temperature of a fluid simultaneously.  
In this work, we propose the extension of the film measurement laser absorption spectroscopy (FMLAS) method, which has previously been validated and implemented for film thickness measurements, for the determination of temperatures and concentrations of a thin UWS film [15, 16, 17]. The FMLAS method provides some significant advantages: it isnon-intrusive, allows for a high temporal resolution and has a cost-effective setup based on readily available near-infrared laser diodes. The research presented here prepared for the use of diode lasers by selecting the optimal emission wavelength.  
FMLAS is based on the Beer-Lambert-Law for induced absorption:  
\[I_{\
u}=I_{0,\
u}\;exp[-\kappa_{\
u}(c,T)\;\delta]=I_{0,\
u}\;\exp(-\alpha_{ \
u}). \tag{1}\]  
where the detected light intensity \(I_{\
u}\) is a function of the incident light \(I_{0,\
u}\) and the absorbance \(\alpha_{\
u}\) at the wavenumber \(\
u\). In turn, \(\alpha_{\
u}\) is a function of the absorption coefficient \(\kappa_{\
u}\) and the film parameters of film thickness \(\delta\), concentration \(c\) and temperature \(T\). With the FMLAS approach, \(\delta\) can be determined by intensity measurements at two suitable wavenumbers through quotient formation after calibration (see [16]).  
The aim of this work is to find optimized wavenumber combinations that allow for the determination of \(\delta\) as well as the additional film parameters of \(T\) and \(c\) (concentration %mass of urea in water). For this purpose, the broadband transmission spectra of a comprehensive calibration data set are recorded with a Fourier transform infrared (FTIR) spectrometer. In contrast to \(\delta\), the parameters of \(T\) and \(c\) exhibit a nonlinear dependency on the absorbance \(\alpha_{\
u}\), which is why a novel evaluation strategy for the spectral data is additionally required. We introduce an approach to spectroscopically determine the film parameters through an adapted calibration method, which is then used to evaluate a test set. Furthermore, an optimization strategy is presented for the wavenumber selection.