INVESTIGATION OF THE EFFECT OF SWIRL ON MIXING WITHIN A KRAFT RECOVERY BOILER

摘要

An experimental study is performed to investigate the mixing characteristics of the flow within a Kraft recovery boiler. The focus is on a comparison of the degree of mixing between a boiler operating under conditions of balanced flow which exhibit grossly unsteady flow and one operating with turbulent flows which exhibit a more definite flow pattern, as when modest swirl is introduced into the flow. Such a study is pertinent for actual boiler operations where the air ports can clog, leading to changes in the boiler flow. The determination of operating conditions which can increase mixing can lead to increased operational efficiency for the recovery boiler. The experiments are performed on a 1/28 geometrical scale water model using laser Doppler velocimetry to measure the flow field and particle tracking to investigate mixing. In the latter case, neutrally buoyant tracer particles are injected through specific ports in the boiler models. These particles are illuminated by means of a laser sheet shone horizontally across a boiler cross section. The activity of the particles in the laser plane are recorded with a S-VHS camera and the pictures are digitized for analysis. The results show that the flow patterns in the boiler are significantly different for the two cases investigated; namely the balanced unsteady flow and the swirling flow. The rms-velocity values for the balanced unsteady flow are greater than those for the swirling flow. Mixing, as quantified by a parameter involving the flux of particles, is more complete for the swirling flow. This contrasts with the usual assumption that greater unsteadiness provides greater mixing. Comparisons of experimental results with computational results show good agreement in values and trends. Such agreement provides confidence for future computational investigation of mixing.