Hydrodynamics and heat transfer of a falling particle curtain in air
2017-02-08T03:56:51Z (GMT) by
The behaviour of a particle curtain falling in a horizontally-flowing gas stream and in quiescent air was investigated in the study. An existing experimental apparatus was modified to generate a particle curtain that spanned the entire width of a duct, and a uniform gas stream that flowed horizontally through the duct. In order to maximise the gas-solids interaction, the existing solids feeder with a rectangular cross-section was also modified to generate a uniform particle curtain with relatively high voidage. Experimental studies were conducted to investigate the behaviour of the falling particle curtain under various conditions. In order to obtain a better understanding of the behaviour of a falling particle curtain, models based on single particle behaviour were developed, and an existing CFD model was also employed. The CFD model was developed by Dr. Andrew Lee and Dr. Madoc Sheehan from James Cook University (JCU) to predict the solids transport behaviour within a flighted rotary dryer. Through collaboration with the present author, they modified this CFD model to predict the behaviour of the falling particle curtain in a horizontally-flowing gas stream and in quiescent air using the same conditions as the experiments. The behaviour of a falling particle curtain in a horizontal gas flow was investigated for a range of inlet curtain thicknesses. The experimental results show that for an inlet thickness of 2cm, the curtain is found to diverge as it falls. Increasing the inlet thickness to 4cm causes the curtain to become slightly converging, and further increase of the inlet thickness to 6cm, 8cm, and 10cm causes the curtain to become increasingly converging. The CFD model predictions show a reasonable agreement with the measured trajectories of the particle curtain; however, the agreement between the predictions of the single particle model and the measured trajectories is found to be less satisfactory due to the non-uniform gas flow found in practice. The measurements of the gas velocity profiles immediately downstream of the particle curtain show very little gas flows through the upper section of the particle curtain, and most of the gas flows through the lower section where the voidage is higher.In order to study the gas-solids heat transfer of a falling particle curtain in a horizontal gas flow, a stream of particles at ambient temperature was fed through the rectangular feeder. Warm air with a uniform velocity profile flowed horizontally through the duct. The solids temperature inside the curtain and the temperature of air leaving the curtain were measured as a function of distance from the top of the duct. The experimental results show that for a falling particle curtain with small inlet thickness, the solids temperature is found to reach equilibrium with the gas temperature after falling only a short distance from the top of the duct. As the inlet thickness increases, the gas and solids temperature take longer to reach equilibrium. The heat transfer model based on single particle behaviour is able to predict the solids temperatures of the particle curtain with large inlet thicknesses (6cm, 8cm, and 10cm) reasonably well; however, the agreement with the measured gas temperatures is found to be less satisfactory, particularly in the upper section of the curtain. This is due to the fact that, in practice, the gas flow at the downstream of the curtain is not uniform as assumed by the model. In the study of a falling particle curtain in quiescent air, the air supply system was disconnected from the duct, leaving both end of the duct open to atmospheric pressure. The shape of the falling curtain was investigated for a range of inlet curtain thicknesses. The experimental results indicate that increasing the inlet curtain thickness from 2cm to 10cm causes the shape of the particle curtain to change from diverging to converging. A similar trend is also observed when the solids mass flowrate is increased from 0.015kg/s to 0.100kg/s. The CFD model predictions show a reasonable agreement with the experimental results. It is proposed that the structure of the particle curtain is determined by the balance between the particle pressure generated by the collisions amongst particles within the curtain and the compressive pressure caused by air pressure reduction due to the accelerating particles. When the particle pressure within the particle curtain is greater than the compressive pressure, the particle curtain is found to diverge as it falls. When the particle pressure is weaker that the compressive pressure, the particle curtain is found to converge.