| Abstract |
The first vortex combustors were forest •fire whirls', where the fire itself provided the energy to maintain the system and confining walls were absent. Commercial vortex incineration is a contained and controlled version of this natural phenomenon. An inwardly spiralling vortex air-flow is set up within a cylindrical combustion chamber, by the use of tangential air injected into the chamber, to burn off in a highly turbulent, swirling airstream. The correct exploitation of the unique properties of vortex air-flows can result in complete and efficient burn-out of dirty and/or toxic wastes, at heat release rates greater than 2MW/n\3/atm. The earliest successful combustor based on these principles was probably that of Hurley (1930). Like most early units, it was designed to burn pulverised coal. In 1940 combustion chamber design took a huge step forward when the Whittle jet utilised similar cyclonic fields, to promote the high intensity combustion of kerosine. Subsequent to this, all gas turbine combustion 'can' designs have utilised the twin concepts of swirl and recirculation to promote, stable, faster and cleaner combustion. This current research dates back to 1969, when the advice of one of the authors (FDM) was sought in connection with the malfunctioning of a commercial vortex incinerator. An inspection of this unit revealed that the combustion was extremely sensitive to design parameters and predicting these in advance was difficult. As a result a three part investigation was undertaken; a theoretical study of vortex combustion, together with, aerodynamic experiments on 1/12 scale water and air models of both specific and general incinerators. Vortex incineration is a special case of swirling combustion, in fact most of the literature fails even to mention incineration. The topic can be classified under two headings; swirl induced burners, and swirl combustion chambers. Swirl combustors can be further subclassified into slagging, and non-slagging types, depending on the behaviour of any ash present. Slagging combustors tend to possess a single tangential air inlet and are termed cyclone combustors, whilst the non-slagging types have at least two air inlets and are then termed vortex combustors. The early water model, of a specific incinerator, used polystyrene beads and nigrosine dye to visualise the air and fuel flows, and clearly illustrated the complex but stable nature of the double helix, vortex flow. In practical terms, this means that noxious residues can be completely burnt off within the chamber and that the unit is only about 20% of the size of a conventional incinerator, for the same duty. A later, and much more useful, air model test bench was then set up to produce velocity profiles using automated impact head probe devices. This study indicates that the best practical design of such an incinerator is a non-slagging, axially fired, vortex combustor with dual tangential inlets for the secondary air supply. |
