The flow regimes in a free falling particle stream and the dust emission rate are experimentally explored in this paper. The results show that the particle stream could be divided successively into the stable regime, the transitive regime and the dispersive regime. The length of each regime is quantitatively identified. In our testing range, the dust emission rate (ηr) increases monotonically with Z and the gratitude gradually increases when the particle stream impacts onto the rigid surface in the transitive regime. The dust generation rate shows linear to the dimensionless drop height. However, the rate decreases slightly monotonically with the drop height (Z) when the particle stream impacts onto the surface in the dispersive regime. The dust generation rate presents an exponent relation with the dimensionless drop height. The empirical equation about ηr is derived. It is found that the second fugitive dust still emits under the condition that the particle stream impacts into the water surface when the hopper outlet diameter is from 1 to 8mm. Reducing the expanding level and the impaction intensity of the particle flow is considerably important when the contact surface is in the transitive regime and the dispersive regime, respectively.

The generation of dust during the handling of high-temperature materials, which fall or impact contact surfaces, can be a significant source of danger for the health of operators. A greater knowledge on the effective control of these falling and impact processes must be illustrated. This paper presents recent experimental research on the characteristics and dust generation rates of materials of different temperatures during the free falling process. The results show that when experimental conditions are the same, the high-temperature material has a larger dispersion radius and a smaller impact velocity compared to the ambient-temperature material. In all cases, the first fugitive dust rate (ηw) which is produced by falling process increased as the material temperature (T) increased. The second fugitive dust rate (ηs) which is emitted by impacting process and the total dust rate (ηr) which consists of ηw and ηs, all decreased sharply at first with increasing T, after which the ηs–T curves has a gentle sloping section but the ηr–T curves increases slightly. In addition, a comprehensive analysis of the factors contributing to dust generation is also included in this paper. It was found that the effect of temperature on dust generation was more obvious with increasing percentage of the first fugitive dust.

利用DPM-CFD模型对非均一粒径颗粒流自由下落过程中气流速度的分布规律、颗粒的扩散和运动特性进行了研究,模拟结果与实验及理论吻合良好.研究发现,空气在轴心处形成射流,其周围有漩涡产生;气流速度沿径向先减小后增加,之后再次呈现下降趋势,在轴心处速度最大,且沿轴向增加;颗粒物浓度沿径向减小,扩散半径随粒径减小而增加;随着颗粒流的下落,颗粒速度的增幅由快到慢,逐渐趋于平缓;在小颗粒中混入大颗粒可使小颗粒沉降速度减小.