Numerical study on the multiphase flow in the CAP cyclone

Abstract

Air pollution is a serious problem nowadays. Traditional gas cyclones are widely used to purify industrial waste gases but are not effective in collecting fine and ultrafine particles. An innovative air purifying technology, cloud-air-purifying (CAP) technology, was recently developed to utilize a gas cyclone filled up with the supersaturated vapour. In this thesis, a numerical model is developed to study the multiphase flow in such a cyclone based on ANSYS/FLUENT. The complex multiphase flow of air, vapour and particles is simulated by the computational fluid dynamics (CFD). The air flow is modelled by solving the Navier-Stokes equation. Reynold stress model (RSM) is applied to simulate strong turbulence of the air flow in the cyclone. The supersaturated vapour in the cyclone is assumed constant firstly and then modelled by coupling the mixture species transport (MST) model with Eulerian wall film (EWF) model. Particle motion in the gas cyclone is traced by the Lagrangian particle tracking (LPT) method, with the condensation growth considered modelled via a user-defined-function (UDF). The model has been validated by good agreement with experiments in terms of the collection efficiency and vapour condensation rate. The effect of the amount of the supersaturated vapour is studied. The results show that in the supersaturated vapour, particles will grow in size due to the condensational growth and thus can be more effectively collected by the centrifugal flow field of the cyclone. In addition, the microscopic analyses are performed on different phases, which provide more clear microscale pictures for the collection of particles than experiments, For the particles, it is revealed that the same sized particles can grow into a narrow size distribution through the process, yet the average growth ratio is increased with the increase of the supersaturation ratio in the cyclone and the decrease of the initial particle size, resulting in a significant improvement of the collection efficiency for fine particles. Such improvement is further depicted through the analyses of the escaping particle flow patterns. For the vapour, the supersaturation ratio decreases from the top to the bottom of the cyclone which decreases the condensational growth in the lower part of the cyclone. To overcome this, an optimization method is proposed by injecting extra vapour in the middle of the cyclone to improve the supersaturation ratio at the lower part of the cyclone, which can lead to the second growth of the particles. Finally, based on the numerical data, a mathematical model is proposed to predict the collection efficiency with the existence of the supersaturated vapour. The numerical data and models are helpful to the understanding and optimisation of the CAP technology.

Date of Award	2019
Original language	English