NEW DESIGN PRINCIPLES OF FLASHING BLACK LIQUOR GUNS: MODELLING AND EXPERIMENTS

The first objective of this paper is to provide a brief presentation and experimental validation of a new two-phase flow simulation model for a flashing black liquor gun. The model is based on the one-dimensional mass, momentum, and energy balances in the viscous multiphase nozzle flow. A detailed growth model of a single vapour bubble in the super-heated black liquor was developed and implemented here as a vapour-generation source term for the overall multiphase flow model. The vapour bubble source model considers the extremely transient heat transfer to the bubble surface and the modified Rayleigh-Plesset equation for the conservation of radial kinetic energy, including also non-Newtonian effects on viscosity.

The second objective of this paper is to present some examples of how the nozzle flow model can be used to solve practical design problems. As an example, results on how to control flashing with certain small geometric changes will be presented. The effect of nozzle exit area reduction is one of the examples studied, showing how flashing can be effectively reduced by controlling exit velocity and droplet size.

The long-run objective is to provide an engineering simulation tool that will give reliable initial spray velocity data for CFD furnace combustion simulations of a recovery boiler. The new flow model can also be used effectively in the industrial design of better black liquor nozzles, especially under flashing conditions.