Experimental Study and Analysis on Deposition Characteristics of Aerosols in Capillary Tubes

Abstract

Taking the release of radioactive aerosols in containment to the environment through cracks during nuclear power plant accidents as the research background, an experimental facility was established to study the deposition characteristics of aerosols in capillary tubes and the experiments were then conducted using the facility. The capillary tubes used in the experiments are PEEKsil tube. The gas flow inside the capillary was driven by pressure, with a maximum pressure difference of up to 500 kPa on both sides. The aerosol particles used in the experiments were titanium dioxide, with a mass median diameter of 1 mm. Organic filter membrane was used to collect the leaked aerosol, and then inductively coupled plasma-atomic emission spectrometry (ICP-AES) was adopted to obtain the titanium mass, which will be used to obtain the aerosol mass concentration at the capillary inlet and outlet. A methodology to calculate the aerosol deposition velocity based on the experimental data was developed in this paper, and then the relationship between dimensionless deposition velocity and relaxation time was built. It shows that the variation of dimensionless deposition velocity and relaxation time of aerosols in capillaries can be divided into rising zone and plateau zone, and the dimensionless relaxation time boundary between two regions is approximately between 70 and 100. In the rising zone, the dimensionless deposition velocity increases with the dimensionless relaxation time. In the platform zone, the dimensionless deposition velocity remains basically unchanged and is less than 0.03. Compared with the experimental results of aerosol deposition characteristics in conventional pipelines, the deposition velocity in this paper is relatively small. To analyze the deposition characteristics of aerosols in pipelines, numerical analysis has been conducted using the Lagrange method and the Eulerian method, among which the widely used Eulerian method is the three-layer deposition model. The model assumes that: 1) The particle flux across the boundary layer is steady and unidimensional, perpendicular to the surface; 2) The fluid is well mixed so that the particle concentration gradient exists only very close to the deposition surface; 3) There are no sources or sinks of particles within the boundary layer; and 4) The surface is a perfect sink for particles. To investigate the reasons for the smaller particle deposition velocity in the capillary tubes, this paper first restores the three-layer deposition model, which agrees well with the previous experimental results, and then adds Saffman force to the three-layer deposition model. In the modified three-layer deposition model, this paper introduces an average lag coefficient that reflects the difference in axial velocity between particles and fluid, and then evaluates the impact of lag coefficient on particle deposition velocity. The results show that when the lag coefficient increases to a certain value, the effect of Saffman force is significantly enhanced, and the dimensionless deposition velocity sharply decreases. In the experimental study on the deposition characteristics of aerosols in capillary tubes conducted in this paper, a compressible flow driven by high pressure difference is present in the capillaries. The airflow continuously accelerates in the flow direction, resulting in the acceleration of aerosol particles. The Saffman force always exists during the aerosol transportation in the capillary tubes. When the difference between particle velocity and gas flow rate reaches the certain value, the Saffman force greatly weakens the wall deposition of particles.

Keywords

• capillary
• pressure driven
• aerosol
• deposition velocity
• saffman force