## 2 Experimental Setup
### Tdlas  
Tunable diode laser absorption spectroscopy (TDLAS) is a calibration-free, in-situ and integral measurement technique based on linear absorbance neglecting scattering and described by the Beer-Lambert law. A detailed overview of TDLAS in combustion technology, its fundamentals, different forms (e.g., direct absorption (DA) and wavelength modulation spectroscopy (WMS)) and fields of application, was given by, [PERSON] et al. [17] and [PERSON] et al. [18]. In this study, TDLAS in the form of direct absorption was used to determine temperature profiles and the concentration profiles of H\({}_{2}\)O and C\({}_{2}\)H\({}_{2}\). For temperature and H\({}_{2}\)O concentration, a multiline thermometry approach was used, based on a Boltzmann plot analysis and using the information from five different absorption lines. Two DFB diode lasers (NanoPlus GmbH) with central wavelengths \(\lambda_{cw}=1344.5\) nm and \(\lambda_{cw}=1392.3\) nm were used for this purpose. The experimental setup, selection of suitable absorption lines, characterization of the diode laser, data analysis and determination of experimental uncertainties for the determination of temperature and water concentration are described in detail in [19]. [PERSON] et al. [20; 21] presented the first TDLAS setup for C\({}_{2}\)H\({}_{2}\) in the near-infrared band around \(\lambda_{cw}=1535\) nm using the P17 absorption line of acetylene in flames. It has been shown that this absorption line is suitable for measurements in combustion processes. To determine C\({}_{2}\)H\({}_{2}\) concentration profiles, a third diode laser with central wavelength \(\lambda_{cw}=1535.4\) nm was integrated into the optical setup. A butterfly Mounted Distributed Feedback (DFB)-type diode laser (NEL Laser Diodes) with integrated Peltier cooling and a maximum output power of 20 mW was used. As with the two diode lasers used for temperature and H\({}_{2}\)O concentration, the tuning behavior (current and temperature) of the C\({}_{2}\)H\({}_{2}\) diode was studied in a preliminary investigation similar to that of [PERSON] et al. [22]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Method** & **Result** \\ \hline Olefin content & ASTM D 1319 & \(<\)4 vol.-\% \\ Aromatic content & & 16.5 vol.-\% \\ \hline Density (15 \({}^{\circ}\)C) & ASTM D 4052 & 800.8 kg/m\({}^{3}\) \\ \hline Sulfur Content & ASTM D 5453 (IP 336) & 600 mg/kg \\ \hline Naphthalene Content & ASTM D 1840 & 1.5 vol.-\% \\ \hline Lower heating value & ASTM D 240 mod. & 43.24 MJ/kg \\ \hline Carbon content & ASTM D 5291 & 85.0 m.-\% \\ Hydrogen content & & 13.1 m.-\% \\ \hline MAH & & 19.8 m.-\% \\ BAH & ASTM D 6379 & 2.1 m.-\% \\ Total AH & & 21.9 m.-\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of the used kerosene. A spectrum analyzer (771 Laser Spectrum Analyzer, Bristol Instruments) was used to determine the diode specific maximum wavenumber range and temperature tuning coefficients \(\delta\bar{\
u}/\delta T\) for a temperature tuning response. The wavelength tuning behavior due to current tuning was determined using a Fabry-Perot interferometer (SA200-12B, Thorlabs) with a free spectral range of 1.5 GHz. The optical components selected for the temperature and water concentration measurements (fiber optics, collimator and detector) have an operating wavelength range of 1050-1620 nm. For this reason, no additional components had to be installed for the use of the third diode laser. The beam parameters of all three lasers were determined experimentally and are shown in Table 3.  
The measured absorption spectrum is a combination of water, which is formed as a combustion product and parasitic water (humidity of the ambient air). Both were considered for the determination of temperature and water concentration. as described in detail in [19].