The mixing of gases at different temperature levels is a common task in industrial processes. In this paper the design of a mixing arrangement in the offgas treatment system of an electric steelmaking plant is discussed with the help of computational fluid dynamics (CFD). Dust-laden offgases originating from three sources in the plant (electric arc furnace primary suction system, ladle furnace suction system and canopy suction system) are mixed in a mixing chamber. The gas is then directed to a bag house where it is filtered through bag filters in order to prevent environmental pollution. Gas temperatures in the bag house must not exceed a certain temperature during any mode of operation and in any place of the filter in order to prevent damage to filter bags (consisting of polyester felt). For this reason high mixing efficiencies at low pressure drops are requested to minimize blower power consumption (typically in the range of 1-2 MW). The first section deals with the description of the mathematical models applied in the simulations. Then the mixing performance of two different mixing devices is compared: a swirl vane and a 'static mixer' located in the pipe connecting the mixing chamber to the bag house. The mixing performance of the swirl vane is found to be superior at equal pressure drop. Appropriate simulation models (RNG k-epsilon model, Reynolds stress model) must be used to resolve the main features of the flows and to obtain reliable results. The subsequent discussion of simulation results from the industrial process arrangement and experimental validations (online temperature measurements) shows excellent agreement. The last section of the paper discusses a recently developed and built design optimization of the industrial arrangement which leads to highly improved mixing performance for all modes of operation.
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