Curvature and burning velocity of Bunsen Flame tips.

Abstract

The burning velocity of a premixed flame propagating in a given flammable mixture is known to depend on the difference between the curvature of the flame and the so-called curvature of the flow, which is the strain rate of the flow of fresh gas along the normal to the flame divided by the burning velocity of the planar flame. The difference between the local burning velocity and the burning velocity of a planar flame in a gas at rest is proportional to the difference of the flame and flow curvatures. The proportionality factor is the product of the burning velocity of the planar flame and the Markstein length, which is an intrinsic property of the flame that characterizes its dynamics. The Markstein length can be determined experimentally by simultaneously measuring the curvature of the flame and the strain rate of the flow. To achieve this goal, we have set up a laminar jet burner and used two PIV systems to measure the gas flow velocity in two perpendicular planes normal to the flame. Each PIV system is composed by two Q-switched Nd: YAG pulse lasers (New Wave, maximum 120 mJ/pulse at 532 nm wavelength), a double-shuttered cross-correlation camera (PCO, 1392 × 1040 pixels) and a pulse generator (ILA GmbH) to synchronize all the components as well as the two PIV systems. Oil droplets are used for tracking the flow and the flame. They are formed by condensation after oil evaporation in a seeding chamber placed in the air line. Seeded air and fuel gas (CO,H2, CH4) are mixed in a settling chamber upstream of the burner and burned in a stationary Bunsen flame. The oil droplets evaporate in the flame preheating region, thereby allowing a dual tomography of the front.