Vaporization and combustion of metal slurry droplets

Analytical and numerical studies on metal slurry droplet combustion and metal slurry spray combustion are discussed. The slurry droplet consists of liquid hydrocarbon carrier (n-octane) and metal fuel (aluminum). Two limiting geometrical configurations for isolated droplets are studied: one with large metal particle enveloped by liquid hydrocarbon and another with many fine metal particles inside a liquid drop. The initial objective is to formulate vaporization models for isolated slurry droplets which can be used in spray combustion calculations. By studying transient gas-phase effects on the ignition characteristics of solid particles, it is found that in absence of forced convection unless the ambient air is preheated to large temperatures comparable to the aluminum oxide melting point, the flame does not possess sufficient energy to ignite the metal. Forced convection effects are studied by constructing a film theory based model. Ignition times for the metal particle are found to depend on the solid mass loading and can be several times larger than the liquid fuel burnout time. In the geometrical configuration consisting of many fine metal particles, which are initially uniformly distributed in a liquid fuel droplet, it is found that depending on the shell characteristics different modes of liquid vaporization from the droplet can occur. A general model is formulated to describe the various modes of vaporization. Calculations are made with metal particle size and initial volume fraction as parameters.; An analytical model is constructed to describe the combustion of aluminum particles in air. The particle transient heating is considered, and the phase-equilibrium conditions of the fuel, and vapor and condensed products are analyzed. Mass and energy interactions between the slurry droplets and gas flow are studied in an idealized configuration consisting of parallel droplet streams. Results show that, at different combustor locations, interacting and distinct premixed and diffusion type reaction zones are present. The heating and burning times of the metal agglomerate are found to be much larger in comparison to the liquid fuel vaporization times and they increase with increasing metal particle size and metal loading of the slurry droplets.