Investigation of transfer processes in swirling flows in application to vortex furnaces for coal fuel

Abstract

In this work, the swirling flow structure was studied experimentally and numerically in a wide range of operating parameters at a laboratory model of the promising furnace of pulverized-coal boiler with a four-vortex combustion circuit. The basic aerodynamic features of construction, affecting the efficiency of fuel combustion, were investigated by the example of air under the atmospheric pressure (Re ~ 104) for the isothermal case. The data on distribution of averaged and pulsation velocity characteristics were experimentally obtained in various areas of the model using the non-contact PIV method (Particle Image Velocimetry). The results of measurements carried out in a wide range of initial velocities allow distinguishing three flow regimes (β is the ratio of flow velocities at the outlet of the side and frontal nozzles): the flow regime with regular structure (four symmetric vortices, β ≥ 1.8), the transitional regime (1.8>β > 0.9) and regimes with irregular vortex structure (β ≤ 0.9). It is shown that formation of irregular structures is associated with uneven organization of air supply to the model. In regimes with β < 1.8, the flow becomes sensitive to asymmetry in the input conditions. At the same time, at high β ≥ 1.8, a stable four-vortex flow structure is formed, even if the input conditions are asymmetric within 10% (by velocity). In the transitional regime, when β decreases from 1.8 to 0.9, the flow begins distorting, and this is presented by the flow asymmetry. With a further decrease in β ≤ 0.9, the development of asymmetry is accompanied by the breakdown of large vortices into smaller vortices. A stable four-vortex flow structure at β ≥ 1.8 is important for practical applications, since in this regime, effective “washing” of the heat-exchange surfaces of the boiler will be ensured, thus avoiding their slagging. The results of laboratory measurements obtained in the range of flow regimes, self-similar by the Reynolds number (104<Re < 106), allowed us to verify the mathematical model. Numerical calculations of aerodynamics were performed using the AnsysFluent CFD package. All considered turbulence models (URANS k-w SST, URANS RSM, DES) reproduce well the averaged flow structure in the combustion chamber. The results of numerical calculations are in good agreement with experimental data. The verified mathematical model and calculation method allow us a future transition to simulate the combustion of coal fuel, taking into account all the main processes of heat and mass transfer in the furnace.