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The excess pressure losses due to end effects (mainly entrance) in the capillary flow of several types of polyethylenes were studied both experimentally and numerically under slip and no-slip conditions. These losses were first measured as a function of the contraction angle ranging from 158 to 908. It was found that the excess pressure loss attains a local minimum at a contraction angle of about 308 for all types of polyethylenes examined. This was found to be independent of the apparent shear rate. This minimum becomes more dominant under slip conditions that were imposed by adding a significant amount of fluoropolymer into the polymer. Numerical simulations using a multimode K-BKZ viscoelastic model have shown that the entrance pressure drops can be predicted fairly well for all cases either under slip or no-slip boundary conditions. The clear experimental minimum at about 308 can only slightly be seen in numerical simulations, and at this point its origin is unknown. Further simulations with a viscous (Cross) model have shown that they severely under-predict the entrance pressure by an order of magnitude for the more elastic melts. Thus, the viscoelastic spectrum together with the extensional viscosity play a significant role in predicting the pressure drop in contraction flows, as no viscous model could. The larger the average relaxation time and the extensional viscosity are, the higher the differences in the predictions between the K-KBZ and Cross models are.

In the present paper, a three-dimensional numerical investigation of pulverized dry lignite was undertaken, integrating the combustion of four different scenarios adopted experimentally in a 100-kW Chalmers laboratory-scale furnace. A hybrid unstructured grid computational fluid dynamics (CFD) code was used to model and analyze: an air-fired, oxy-fuel OF25 (25 vol % O2 concentration), oxy-fuel OF27 (27 vol % O2 concentration), and oxy-fuel OF29 (29 vol % O2 concentration). The appropriate mathematical models with the related kinetics parameters were implemented to calculate the temperature distributions, species concentrations (O2, CO2, CO, H2O, and H2), NO x emission concentrations, and the radiation heat transfer. The multistep chemical reaction mechanisms were conducted on the gas phase and solid phase of coal reaction in one-, two-, and three-step reaction schemes. The predicted results showed reasonably good agreement against the measured data for all combustion cases; however, in the three-step scheme, the results were highly improved, particularly in the flame envelope zone. For the NO x calculations, the obvious differences between the air-fired and oxy-fuel (OF27 and OF29) cases were evident. In the OF27 and OF29 cases, the expected increase in the flame temperatures and CO2 and H2O concentrations led to a slight increase in the radiative heat fluxes on the furnace wall, with respect to the air-fired case. As a continuation of improvement to the oxy-fuel combustion model, this numerical investigation might probably provide important information toward future modeling of a 550-MW, large-scale, brown coal oxyfuel tangentially fired furnace.

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