Ammonia doping and pressure effects on 2D temperature distribution in n-heptane laminar flames via two-line OH-PLIF

Accurately measuring the two-dimensional temperature distribution in n-heptane/ammonia co-flow diffusion flames is essential for understanding combustion dynamics, providing insights into reactive species distribution, and validating numerical models. This study investigates the combustion characteristics of pressurized n-heptane/ammonia co-flow diffusion flames using two-line OH-PLIF thermometry. The effects of ammonia addition and elevated pressures on flame structure, OH radical distribution and temperatures were systematically examined. As the ammonia mass fraction increased from 0 % to 50 %, OH-LIF signals dropped by approximately 22.69 % and 24.50 % for the R₂(12.5) and P₁(2.5) transitions, respectively. Despite these chemical changes, peak flame temperatures remained stable, varying by less than 5 %. Rising pressure from 1.0 bar to 3.0 bar intensified flame brightness due to increased soot formation and radiative heat transfer, resulting in about 70-fold increase in chemiluminescence intensity. Peak temperatures rose from 1850 K at 1.0 bar to 2030 K at 3.0 bar, while the temperature distribution narrowed. Numerical simulations showed a consistent temperature increase but slightly overpredicted peak temperatures due to idealized assumptions such as neglecting radiative heat loss and soot formation. These findings demonstrate ammonia's viability as a sustainable co-fuel, enabling stable combustion with reduced carbon emissions. The study underscores the importance of accurate chemical mechanisms, radiation models, and experimental validation for advancing high-pressure combustion research.