Mixing characterization of transient puffs in a rotary kiln incinerator.

Abstract

Studies on both full size and pilot scale rotary kiln incinerators have suggested that mixing between evaporating waste and ambient air may be an important factor in the destruction of hazardous wastes in such facilities. Research presented here has addressed the issue of gas phase mixing between one intermittent gas stream, representing a transient puff of evaporating waste, and a continuous stream, representing the primary flame flue gas in a rotary kiln incinerator. Experiments were performed in a plexiglas cold flow model which is a full scale replica of the rotary kiln simulator used by EPA's Combustion Research Branch. The intermittent gas stream was seeded with TiO₂ particles. Quantitative data were obtained through a combination of laser flow visualization and digital image analysis. Experiments were completed for different main air flow rates, representing Reynolds numbers from 1,700 to 17,000. The experimental results were compared to a computer simulation model, based on the linear eddy modeling technique. Model results agreed very well with experimental data, if a recirculation zone was explicitly accounted for. The model was used to predict mixing at low Schmidt number, with input conditions obtained from the existing evaporation model of Wendt and Linak (1988). Conclusions from this research include the following: (1) digital image analysis, based on flow visualization, can be used to quantify gas phase mixing processes; (2) deviations from Reynolds number similarity in these experiments can be explained from differences in relative (main air and puff) flowrates, and in Strouhal number; (3) recirculation zones have a significant effect on macro-mixing (average concentration profiles). However, the effect on micro-mixing is limited; (4) interpretation of high Schmidt number, low resolution measurements as if they where fully resolved low Schmidt number measurements is generally adequate. Specifically for the EPA kiln, it was concluded that: (5) three separate mixing mechanisms are present: jet mixing in the cavity, recirculation zones, and shear turbulence; (6) unmixedness is a likely explanation for failure modes in the EPA rotary kiln incinerator; (7) insufficient macro-mixing (controlled by transient evaporation and large scale motion) is more important than local unmixedness (governed by fine scale turbulence and molecular diffusion).