Chemiluminescent OH^∗ and CH^∗ flame structure and aerodynamic scaling of weakly buoyant, nearly spherical diffusion flames

Normal-gravity experiments were conducted with inverse diffusion flames of small density difference with their surrounding ambient to study low Grashof number (Gr) flames that were several centimeters in diameter. The intensity of buoyancy was minimized by ejecting diluted air from a porous, spherical burner into a lower-density fuel atmosphere of hydrogen or hydrogen and methane at reduced pressures (<0.25 atm). The resulting weakly buoyant, almost spherical flames were imaged by a UV camera, with narrow-band-limited filters corresponding to electronically excited OH (OH^∗) and CH (CH^∗), and then deconvoluted to obtain intensity profiles corresponding to the chemiluminescent species. The experimental results were then compared with computations allowing for detailed chemistry and transport. For the hydrogen flames, the comparison was very satisfactory, hence substantiating the adequacy of the chemistry and the experimental approach. For hydrogen/methane flames, OH^∗ chemiluminescence exhibited two peaks, demonstrating the importance of the H + O + M ⇆ OH^∗ + M reaction in addition to the CH +O₂ ⇆ OH^∗+CO reaction. The hydrogen/methane flames also experienced a mild degree of buoyancy, which shifted the peak OH^∗ and CH^∗ locations from the calculated values. Through a separate experimental investigation, it was subsequently determined that the effects of weak buoyancy, based on the flame dimension, scale with Gr^1/2 and are therefore in accord with the low-Gr scaling for heat transfer phenomena. The associated correction satisfactorily explains the shifts in the locations of the experimental OH^∗ and CH^∗ peaks.