OCR Text |
Show 5.6.16 up to ten times that given by the steady state view, before reaching the wall. Indeed, particles spend a significant proportion of their total combustion time traversing the central fixed vortex region, a region devoid of particles in a steady state view, and then move out into the free vortex region to complete their combustion, never achieving a stabilised orbit. By choice of suitable design parameters any particles remaining after combustion can either continue on their spiral path outward to the wall to be collected by peripheral tap-off, or else, will reverse their radial movement and spiral inwards towards the flue, to be collected later on from the flue gases. In order to start the particle's spiral path to the centre of the chamber, peripheral injection was investigated. The required inward radial injection velocity for a particular particle was found to be not only very high, but for a particular sized particle also very sensitive to particle size. The result was that, either over or undershoot of the central position easily occurred, giving a much shorter time of flight for combustion. Similar conclusions were reached by Bailey in his study of spray drier kinetics. In view of the need to cope with a range of particle sizes, the only practical method is to inject the particles at the centre of the vortex chamber, and allow them to combust on their outward spiralling path. The outward path is opposite to that followed by the combustion gases and leads to excellent counter-current heat transfer to the fuel and with good flame stabilisation. To verify the numerical simulation studies and the mathematical model on which they were based, studies were carried out on small vortex combustors of one metre diameter. Glass spheres of a specific size range were injected into the cold operating combustor and their trajectory recorded photographically and also by means of samples taken at various points within the chamber. These results were used to make minor changes to some of the initial assumptions used in the model, in order to obtain a closer fit between calculated and experimental results. Subsequent to this a computer simulation package has been successfully used to design a number of vortex combustors burning oil and coal, as well, as a novel natural gas fired silica processing chamber, for the production of ballotini from crushed glass. |