Effects of radiative heat transfer on premixed spherically expanding NH3-air flames

This article numerically investigates the effects of radiation on the laminar burning velocity (LBV) extrapolated from microgravity spherical expanding NH 3 /air premixed flames, with equivalence ratios and pressures ranging from 0.7 to 1.3 and 1 to 50 atm, respectively. The simulations employ three levels of thermal radiation: i) an adiabatic model that neglects radiation, ii) the optically thin method (OTM) that neglects absorption, and iii) a detailed model combining an optimized statistical narrow-band correlated- k approach with the discrete ordinates method. Model results show that radiation affects the LBV through two competing mechanisms. The first involves radiative emission by H 2 O, which cools the burnt gases and subsequently lowers both the flame temperature and the LBV. The second mechanism involves radiation absorption by NH 3 , which preheats the unburnt gas, thereby enhancing both the flame temperature and the LBV. Regardless of the equivalence ratio, the first mechanism dominates at pressures below approximately 15 atm, while the second prevails at higher pressures. The OTM model overestimates radiative cooling in the burnt gas and ignores fuel re-absorption, leading to an underestimation of the LBV that significantly worsens with increasing pressure. Conversely, the adiabatic model remains on the whole more accurate due to compensation of errors, with errors typically within ± 5% for stoichiometric and rich mixtures. This suggests that adiabatic simulations can be used for kinetic mechanism validation under these specific conditions. However, for lean mixtures, the induced errors are noticeably higher due to the much lower laminar burning velocity. In such cases, interpreting experimental data with adiabatic simulations requires a corrective factor to derive the ”adiabatic” LBV from measurements. While previously proposed corrections were derived without accounting for fuel re-absorption and are found to be valid only up to 5 atm, this study proposes a new correlation specifically for lean NH 3 / air flames for the entire pressure range. Novelty and significance statement Radiation affects premixed NH 3 -air spherically expanding flames in a complex manner due to the capability of NH 3 to re-absorb radiative emission by hot water vapor. The main contribution of this article is to quantify the competing radiative mechanisms over wide ranges of equivalence ratio and pressure through a detailed analysis based on state-of-the-art radiative modeling and chemical kinetic mechanisms. The conclusions of this analysis are then used to provide insights on the interpretation of experiments in propagating spherical flame configuration. This has important implications for the validation of ammonia chemical kinetic mechanisms.