Phase-Averaged Temperature Characterization in Swirl Burners

Abstract

Methods have been developed to represent the rotating temperature fields associated with a three-dimensional, time-dependent coherent structure, the precessing vortex core (PVC), formed in the exit of swirl burners. In conjunction with laser sheet visualization of the phenomenon and phase-averaged rotating velocity measurements a description of the phase-averaged rotating temperatures and associated flow fields has been made for the flow in a 100 kW swirl burner (modelled on a 2 MW system for the efficient combustion of low calorific value gases), fired on premixed natural gas and air with some 10 per cent of the fuel used being injected on the central axis as a pilot. The resulting calibrated temperature maps show comparable patterns to the laser Doppler anemometry velocity measurements and some basic structures associated with the PVC can be identified. Past the burner exit the PVC is shown to precess about the central axis and create a crescent-shaped region of high-velocity flow close to the exit wall and extending over about 180°. This structure extends to about one exit diameter downstream. Two flame regions are formed in this system. An inner yellow diffusion flame forms on the boundary of the PVC due to its coherence as the 10 per cent of the gas introduced on the axis passes directly into this structure. A much larger diameter annular blue premixed flame forms, which encloses the inner flame, restricts the oxygen supply and gives good flow visualization. The inner flame surrounding the PVC only breaks up when it hits the side of the reversed flow zone (RFZ) and bursts with the PVC, then coalescing with the outer flame. As a result of the PVC, uneven burning results on the outside of the flame with hot and cold regions corresponding to low- and high-velocity regions respectively. The PVC was found to be helical in shape with the helix angle (140° from x/De = 0.13 to x/De = 039 above the burner exit) acting in the opposite sense to that of the flame. The work has relevance to many areas where swirl burners or swirl-assisted combustion devices are used. In particular, high levels of swirl are probably best used in primary stages of air-staged low NOx burners where high levels of flame stability are sought under fuel-rich conditions. Subsequent air stages at lower swirl levels will tend to even out the non-uniform burning processes. In terms of acoustic resonance care must be taken to mismatch the PVC frequency with any of the natural frequencies of the system, as otherwise large-scale pressure fluctuations can result.