Studies on condensational growth of particles through expansion in a capillary tube were performed both numerically and experimentally. The effects of various conditions on the capillary flow and the particle growth were examined. From one-dimensional analysis on the flow and the seed particle growth, we derived three major parameters - flow parameter, heating parameter, and growth parameter. Two-dimensional numerical analysis of the capillary flow and the formation and growth of particles was performed. This kind of particle growth system has been used for the generation of monodisperse particles, molecular cluster beams, and ceramic precursors. And it also can be used for the measurement of fine particles. We developed a new type of condensation nucleus counter (CNC) using a capillary and examined its performance.
Using the method of moments, the droplet size distribution formed from the spontaneous homogeneous nucleation and the subsequent growth was calculated in Lagrangian viewpoint. In two-dimensional calculation of the flow, the effect of the vapor depletion and the release of latent heat were considered using particle-source-in cell (PSI-CELL) method. The growth of seed particles was calculated with the rigorous expression of the growth rate in a rapid changing environment. The count efficiency of the prototype CNC at various conditions was compared with that of TSI-3010 Condensation Particle Counter.
Numerical results show that the flow and the saturation ratio in the capillary tube strongly depend on the flow parameter. The size and the total mass of droplets formed from the homogeneous nucleation increase with decreasing the flow parameter. The final size of the seed particle also depends on the flow parameter. As the flow parameter increases and approaches to 1, the final size decreases slowly. As the flow parameter exceeds 1, however, the size decreases rapidly. At high value of the flow parameter, the supersaturated region still exists at the center of capillary and seed particles still grow. As the heating parameter increases, the final size decreases due to the latent heat release at the surface of the particles. The final size of seed particle is proportional to the square root of the growth parameter. At low values of heating parameter and flow parameter, the size becomes approximately the square root of the growth parameter. We found that the proper control of these three major parameters makes it possible to control the seed particle growth and the formation of the droplet independently. The count efficiency of the prototype CNC was very low, because the small portion of particles passes the measuring volume located in the free jet expansion region behind the nozzle exit. As the seed particle size decreases, the count efficiency of the prototype decreases. However, the count efficiency does not decrease so much as that of TSI-3010 at 10 nm of seed particles. This indicates that the new type CNC has a capability to detect particles less than 10 nm. The increase of temperature at the saturator increases the count efficiency of the prototype and the increase of the length of capillary decreases the count efficiency. These trends are well agreed with our analysis and numerical results.
