Gravity Effects Observed in Partially Premixed Flames

INTRODUCTION Partially premixed flames (PPFs) contain a rich premixed fuel–air mixture in a pocket or stream, and, for complete combustion to occur, they require the transport of oxidizer from an appropriately oxidizer–rich (or fuel–lean) mixture that is present in another pocket or stream. Partial oxidation reactions occur in fuel–rich portions of the mixture and any remaining unburned fuel and/or intermediate species are consumed in the oxidizer–rich portions. Partial premixing, therefore, represents that condition when the equivalence ratio (φ) in one portion of the flowfield is greater than unity, and in another section its value is less than unity. In general, for combustion to occur efficiently, the global equivalence ratio is in the range fuel–lean to stoichiometric. These flames can be established by design by placing a fuel-rich mixture in contact with a fuel-lean mixture, but they also occur otherwise in many practical systems, which include nonpremixed lifted flames, turbulent nonpremixed combustion, spray flames, and unwanted fires. Other practical applications of PPFs are reported elsewhere [1, 2]. Partially premixed flames contain multiple reaction zones [3, 4, 5, 6] that have either a premixed-like structure or a transport-limited nonpremixed flame characteristics in which mixing and entrainment effects are significant. A rich premixed reaction zone is established on the fuel rich side, a lean premixed zone on the fuel lean side, with a nonpremixed zone in between these two in the case of triple flames. Double flames are established when a fuel rich mixture burns in air and have two reaction zones, rich premixed and nonpremixed. Heat release from a flame causes flow dilatation and accelerates the gas across the flamefront. In normal gravity, free convection accelerates the heated gas due to buoyancy, which leads to air entrainment into the flame it its ambient is air rich. For low Froude numbers (Fr=υ/(gl), where υ denotes the fuel stream velocity, l the characteristic length scale and g the gravitational acceleration) flames, gravity effects on a flame can dominate the effects of other physical phenomena, e.g., radiative heat transfer and diffusive transport. Although extensive experimental studies have been conducted on premixed and nonpremixed flames under microgravity [7, 8], there is a absence of previous experimental work on burner stabilized PPFs in this regard. Previous numerical studies by our group [9, 10] employing a detailed numerical model [11, 12, 13] showed gravity effects to be significant on the PPF structure. We report on the results of microgravity experiments conducted on two-dimensional (established on a Wolfhard-Parker slot burner) and axisymmetric flames (on a coannular burner) that were investigated in a self-contained multipurpose rig [9,14,15]. Thermocouple and radiometer data were also used to characterize the thermal transport in the flame.