Experimental and numerical investigation on soot formation characteristics in n-decane diffusion flames at elevated pressures

The effect of pressure on soot formation is investigated in nitrogen-diluted laminar n-decane diffusion flames at pressures from 2 to 7 bar. The flame temperature, soot volume fraction, and soot particle size are evaluated from color-ratio pyrometry, laser-induced incandescence, and time-resolved laser-induced incandescence measurements, respectively. Numerical simulations are performed to further reveal the influence mechanism of pressure on soot formation based on a two-dimensional co-flow flame model. The mass flow rates of n-decane are kept constant at all pressures to obtain tractable measurements. The rates of soot nucleation, surface growth, and oxidation increase with increasing pressure inferred from the evolution of the soot volume fraction along the flame centerline. An overall weakening trend for the facilitating role of pressure in soot generation can be observed within the tested pressure range. There is an approximately linear relationship between soot particle size and pressure in n-decane flames. The simulation predicts the maximum soot volume fraction accurately and captures the distribution of the soot. The simulated rates of all soot generation processes also rise as pressure increases, and the promoting effect of pressure indeed decreases. The simulation demonstrates that compared with HACA surface growth and soot nucleation, PAH condensation presents the greatest sensitivity to pressure above 5 bar. The primary factor for the enhancement in PAH condensation rate is the increase of soot volume fraction.