The measurement of flame surface evolution in both space and time is necessary for the advancement of knowledge concerning the physical processes contributing to lifted jet flame stabilisation. Previous studies either reproduce the flame front accurately in three-dimensional space or in time. In this study a measurement system capable of both is presented. Based on the Mie-scattering of oil droplets added to the jet flow, the system reconstructs the volumetric surface at the base of a lifted jet flame from a series of two-dimensional slices. The slices are created using a pulsed high-speed laser and a polygonal laser scanner unit which serves to sweep the laser beam through the measurement volume. A single high-speed camera is used for recording the subsequent measurement slices. The achieved temporal and spatial resolution as well as the accuracy and precision of the sheet placement are discussed in respect to the flames’ scales. The first results of the reconstruction of the lifted jet flame at its stabilisation point show the potential of such measurements to avoid the ambiguities in interpreting conventional 2D-data.
This paper presents an experimental study of flame propagation through a partially-premixed flow following ignition. A combination of simultaneous high-speed acetone planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) was utilized for time-resolved measurements of mixture fraction, flow field and flame position. This provides access to the major quantities needed to characterize non-premixed flames. High quality mixture fraction measurements with signal-to-noise ratios up to 120 for unity mixture fraction were made feasible using a combination of a conventional high-speed laser at 10 kHz for LIF excitation and a wavelet based de-noising algorithm to reject camera noise. It was observed that flame propagation in the far-field of a partially-premixed jet takes place in a premixed mode, with the flame propagating through highly stratified mixtures until it approaches locations containing mixtures outside the flammability limits. In these areas the flame recedes and further propagation is controlled by mixing processes of air and fuel. Even though flame propagation is then mixing-controlled, the flame is not observed to switch into a non-premixed mode. Instead, mixing ahead of the flame takes place until locally premixed flammable mixtures are recovered for subsequent flame propagation.
Circumferential upstream propagation of a premixed flame in a region confined between two concentric tubes is considered. The cold flow in this configuration features rotational motion and the flame is modelled as an interface separating the burned and unburned gases. Through an analytical solution of the integral form of the governing equations, it is shown that the static pressure increases across the flame. Hence, the circumferential propagation of the flame is associated with the generation of an adverse pressure gradient. The theoretical prediction of the pressure increase is, further, supported by the experimental observations and discussed in the context of the theory of flame back pressure. The results extend the recent findings on the generation of adverse pressure gradient during the axial propagation of swirling flames, to the circumferential direction. It is argued that the demonstrated pressure gain across the flame can significantly facilitate flame flashback.
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