Abstract
Spectral line shape caused by the collision between probing molecules and surrounding species is an important parameter that affects the accuracy of combustion diagnostics. Here, we report an in-situ calibration method of the pressure broadening coefficients using spherically expanding flames over continuously increasing flame pressure. A desirable isothermal condition is produced in the spherical expanding flame with unity Lewis number, in which the thermal and molecular diffusions are balanced during flame propagation. Laminar burning velocities and Markstein length of methane-air and ethylene-air flames are measured to understand their thermodynamic and transport properties. Mid-infrared absorption spectra of carbon dioxide at 4.2 µm are recorded from 0.3 atm to 5 atm in a single run of the experiment. Pressure broadening parameters are extracted using an empirical fitting procedure. After extending to wide ranges of flame conditions, the flame temperatures of the methane-air and ethylene-air flames are correctly determined using the in-situ-determined line shape parameters. The flame temperature deviation from adiabatic flame temperature is imperceptible in the weakly stretched quasi-steady spherically expanding flames. Such unity Lewis number spherical expanding flames are proven to be near-ideal reactors for spectroscopic calibration in combustion diagnostics.