Influence of water addition upstream diffuser inlet on lean blowout and emissions in the teless-II combustor

Water is likely to be ingested into aeroengines under conditions of rainfall or hail, increasing the risk of flame blowout and posing great influence on the combustion characteristics. The chemical and thermal effects of inlet water addition on lean blowout and emissions in the TeLESS-II centrally staged LPP (Lean Premixed Prevaporized) combustor are investigated in this paper. Experiments are carried out at various conditions relevant to gas turbine combustors: the inlet temperature range is 450 K – 533 K, the inlet pressure range is 0.3 Mpa – 0.4 Mpa. The range of water mass fraction investigated is 0% - 5%. Chemical kinetics calculations are conducted to analyze the chemical effect of water addition on combustion characteristics of aviation kerosene. Time-averaged CH chemiluminescence imaging is applied to provide information of flame front and heat release. Combustion emission concentrations (CO, UHC, and NO_x) are measured by a gas analyzer. Based on Lefebvre’s model, an updated semi-analytical correlation is proposed to predict lean blowout limits of centrally staged combustor considering water addition. Chemical calculation results show that the addition of water leads to lower laminar flame speed and lower adiabatic flame temperature. The lower flame speed and higher flow velocity due to water injection result in higher lean blowout limits. Higher combustor inlet temperature can extend the LBO (lean blowout) limits to lower values at the same water fraction. Flame observations indicate that during the flame blowout process with the decrease of fuel flow rate, the post flame region first extinguished, and then upstream to the flame root. Flame close to the lean blowout presents a V shape and stabilizes in the recirculation zone near the nozzle. The heat release decreases with the increasing water injection. Emission results show that the water injection can reduce NO_x emissions due to the thermal effects of flame temperature, while the CO, UHC emission increase on the contrary. The semi-analytical correlation to predict LBO limits shows fair agreement with experiments values at different water fractions. The prediction error compared to experiment data is within -11.3 % to + 12.8 % in the present centrally staged combustor configuration.