FUNDAMENTAL CONSIDERATIONS FOR COAL SLURRY ATOMIZATION

Presented herein are the results of an investigation of theoretical models of atomization, which can be used for a phenomenological theory for coal slurry atomization. This investigation was conducted in several phases. The primary phase involved an intensive analysis of the rheological properties of coal slurry fuels. Analysis was performed including viscosity as a function of shear rate, the extensional viscosity, and the viscoelastic properties. During the second phase, atomization was studied over a sufficiently wide range of rheological properties. During this phase, simulated fluids as well as coal slurries were studied. The simulated fluids consisted of corn syrup solutions. In some blends, xanthan gum was used and both Newtonian and non-Newtonian simulated fluids were studied. The atomization properties of both coal slurries and simulated fluids were determined under a variety of spray conditions using a Malvern size analyzer. Three basic theoretical models were analyzed to determine the best approach to characterizing these complex fluids. The first model was a linearized Navier-Stokes equation for a cylindrical fluid stream breaking up into drops under the impact of a high-velocity air stream. The second model was a collisional model, by which the collision of the air stream and the fluid stream produced droplets. Energy and momentum conservation were used to derive relationships between the drop size and the relevant physical parameters. A third model studied was a statistical model using a Boltzmann-type transport equation for the propagation of drops under the interactions of a high-velocity airstream. The effects of drop coalescence and breakup are incorporated into this model. By comparing the various theoretical models with the atomization data and the rheological data, a phenomenological model was constructed that correctly predicted the trends of the Sauter mean diameter as a function of air/fuel ratio, rheological properties, and spray angle. An effective viscosity was defined that included the effects of viscous losses, extensional properties, and viscoelastic properties. In addition, the effects of yield point were incorporated and shown to be important in predicting atomization properties.