Experimental investigation of premixed ammonia combustion at high Karlovitz number conditions

Abstract: This thesis aims to acquire deep knowledge on turbulent premixed combustion at high Karlovitz (Ka) number conditions with the help of laser-based diagnostic measurements.Considering that ammonia (NH3), as a carbon-free energy carrier, is a promising candidate for replacing conventional fossil fuels in the future, all the investigations in this work were carried out on ammonia/air premixed flames. Different optical diagnostic techniques, including planar laser-induced fluorescence (PLIF), laser Doppler anemometry (LDA) and Rayleigh scattering thermometry, have been employed for the measurement of key species, flow velocity and temperature, respectively.Firstly, experimental research on ammonia/air premixed flames was conducted on the Lund University Pilot Jet burner (LUPJ). The flame structure was visualised through the simultaneous measurement of the temperature field together with NH radical distribution or with NO distribution. Five stoichiometric flames withKarlovitz (Ka) numbers ranging from 274 to 4720 were studied. The NH layer, used as a marker of fuel consumption layer, remains thin at the burner exit, but becomes progressively thickened along the flame height with increasing turbulent intensity (u'/SL) when the Ka is higher than 1900. This thickness increase is attributed to the penetration of the small eddies and the merging of flame branches. The NO pollutant, mainly generated in the reaction zone, was observed to exist in a wide region, across the whole flame, because of the turbulent diffusivity and the flow convection.Limited by the geometric scale, the turbulent Reynolds number (Ret) in the LUPJ flame is much smaller than the operational ranges in industrial applications. For a better understanding of highly turbulent premixed combustion, a DRZ (distributed reaction zone) burner was introduced. This burner has integral scales (l0) between 30 – 40 mm and wider working conditions with a maximum turbulent intensity (u'/SL) and Karlovitz (Ka) number up to 240 and 1008. OH-/NH-PLIF and LDA measurements have been carried out to expand fundamental understanding. The results show that NH layer thickness remains almost constant and independent of turbulent intensity (u'/SL), although all the cases are located in the distributed reaction zone regime (Ka > 100). The turbulent eddies can only wrinkle the flame surface instead of penetrating into the inner structures. The ratio of turbulent to laminar burning velocity (ST/SL) increases monotonously with the Karlovitz (Ka) number. All the signs indicate that the flamelet theory is still applicable for ammonia premixed combustion at high Karlovitz (Ka) number conditions, which is inconsistent with Peters’ assumption in the Borghi-Peters diagram.