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Show downstream from the main jet-entry and mixing region, the flow field in the smaller furnaces can be close to laminar. With these limitations it is no surprise that successes in predictive modelling have been limited, as noted by Smoot in a recent review [12]. In this context, the approach using DNS essentially requires complete reconstruction of the systems of equations. In addition, there remains the central problem that also accompanies existing techniques and that has never been satisfactorily resolved. This is the problem of being certain that the sub-model components are themselves adequately verified. Accordingly, the approach mapped out for these developments has been to introduce minimal changes to each step in the modelling, starting with the simplest conceptual system. This approach is also one that, potentially, can be tested experimentally, as intended in future developments. In this approach, we have taken experimental temperature and gas concentration fields reported in the literature [3] measured in pulverized coal flames in the IFRF furnace. Into these combined fields, using a single jet, we inject a gas stream containing single particles at initially different locations in the jet. The DNS solution procedure is then used to predict the particle location, point-by-point through the hot gas field, with prediction of particle motion governed by standard viscosity and drag relations for particles in a flow field with relative velocity. With each incremental calculation, the particle path is traced out through the gas field; and, knowing the location at each point, we also know the gas temperature and reactive gas concentrations from the experimental field data initially read into the program as local conditions. While the particle is pyrolyzing, the particle temperature is taken as equal to the gas temperature. After pyrolysis, the char particle is reacting, and its temperature is allowed to rise above the gas temperature using a standard energy balance based on the rate of reaction and rate of energy" loss by relevant heat transfer methods. The associated reduction of particle diameter (and density) is then incorporated in the drag relations for the particle motion in the next time interval calculation. This use of experimental fields to predict measurable parame- 2 |
