## 2 Materials and Methods
### Non-Intrusive and Traditional Diagnostics  
TDLAS was used for the measurements of H\({}_{2}\)O and T at port P1, see Figure 1. The UV absorption measurements of T and NO were made at port P2. The distance between P1 and P2 is 200 cm. The path length of the measurements is 65 and 78.5 cm at P1 at P2, respectively, and the inner diameter of the ECF is gradually increases between the ports. A detailed description of the TDLAS system at 1398 nm and the method for the evaluation of temperature and H\({}_{2}\)O concentration can be found in our previous studies [10; 11]. In brief, the laser wavelength from a fiber-coupled distributed feedback laser (Nanoplus  
Figure 1: Schematic of the pilot-scale experimental system. Description of the apparatus/techniques used in Ports 1–4 is given in Section 2.2. The origin of the x-axis characterizes the location from which the distances to the optical ports are calculated. GmbH) was controlled by a laser diode/TEC controller (ITC4001, Thorlabs). The laser was tuned over a range of up to \(\sim\)3 cm\({}^{-1}\) via the 100 Hz triangular modulation of the driving current produced by a data acquisition system from National Instruments with a NI PXIe-6356 card. The modulation was made at a constant diode temperature of 41 \({}^{\circ}\)C. After passing through the ECF, the light was focused on a photodetector (PDA10, Thorlabs). Temperatures and H2O concentrations were determined by least-squares fitting of the measured absorption spectra using spectral line parameters from the HITRAN 2012 [13] and HITEMP [14] databases reported in [10]. Information on the temperature-dependent self-broadening coefficient n\({}_{\text{self}}\) was taken from [15]. For the experimental conditions studied, the uncertainty of TDL H2O measurements is about \(\pm\)10% (relative) and TDL T is about \(\pm\)50-60 \({}^{\circ}\)C for H2O volume fraction above 1.5 vol-%, at lower H2O concentration the uncertainty might increase. The general description of the NO UV absorption measurements and the procedure of the data treatment is given in [9]. In short, fiber-coupled radiation from a high-brightness deuterium lamp of 9 W (Hamamatsu L10290) was used as a broadband UV light source. The UV radiation from the 600-micrometer diameter fiber was collimated using a short focus convex lens to produce a beam with an initial diameter of about 1 cm. The beam passed through ECF at P2, the beam diameter at the P2 outlet was about 2 cm. The beam was focused onto an optical fiber coupled to a UV-VIS range highspeed spectrometer (Ocean Optics HR2000 + GC) with a diffraction grating (Ocean optics, HC-1 Grating, 600 g/mm, blazed at 300 nm). The absorption spectra were recorded with a rate of 10 spectra per second, 100 spectra were typically averaged. The background signals were recorded during the fuel feeding stoppages/interruptions. The fitting of the experimental spectra was made using the well-known constants for (0, 0), (1, 1), and (2, 2) NO vibrational transitions of the \(A^{2}\Sigma^{+}-X^{2}\Pi^{2}\) electronic system. To reduce the computational time, a library of the molecular spectra calculated from 1300 to 1600 K with a step of 20 K was created. The uncertainty of the UV broadband absorption NO and T measurements determined for T \(<\) 1350 K is about \(\pm\)15% (relative) and \(\pm\)50 \({}^{\circ}\)C, respectively. A suction pyrometer used for in-flame temperature measurement is installed in port P3, located 447 cm from the beginning of the ECF. The pyrometer consisted of a type B thermocouple (rated to 1750 \({}^{\circ}\)C) inside a ceramic sheath, with a ceramic radiation shield around the thermocouple junction. Limas 11 analyzer (ABB) was used for online measurements of NO (0-1000 ppm UV and 0-10000 ppm infra-red modules) and NO\({}_{2}\) (0-1000 ppm UV module) in port P4, located 370 cm from the beginning of the ECF.