Effect of CO₂/N₂ Dilution on Characteristics of Liquid Fuel Combustion in Flameless Combustion Mode

ABSTRACT

This paper presents the effect of CO₂ and N₂ dilution in the oxidizer stream on combustion and emission characteristics in a flameless combustor operating with kerosene and biodiesel as a fuel. The thermal input is varied from 30 kW (7.5 MW/m³) – 53 kW (13.3 MW/m³) and inlet air temperature are varied from 300 K to 800 K. CO₂/N₂ are added to dilute the oxidizer stream in a proportion of 15% of total air volume to reduce the O₂ concentration in the air stream. Reacting and non-reacting flow simulations show that the flow field improves significantly as both the flow velocity and temperature distributions become uniform. Distribution of the reactant dilution ratio becomes more uniform and a maximum value of R_dil = 4.89 is achieved for 30 kW conditions. The reduced O₂ concentration in the oxidizer results in enhanced dilution of fresh air-fuel mixture thereby enables uniform temperature and flow field. For the air preheating case, CO₂ dilution does not affect the velocity and thermal fields significantly. The measured CO emissions are marginally increased due to CO₂ dilution; however, NO_x emissions are significantly reduced. N₂ dilution results in increased levels of NO_x, contrary to the perception of the inert role of N₂ addition even though the combustor operating temperatures are lower than 1800 K. The measurements of acoustic pressure emissions show a sharp decrease during flameless combustion mode.

KEYWORDS:

• Flameless/MILD combustion
• Co₂/N₂ dilution
• reactant dilution ratio
• NO_x emissions
• spray combustion