The transport and deposition of particles suspended in a non-isothermal particulate gas flows was studied experimentally and numerically for a wide range of particle sizes (having diameters from about 0.1 to 30 microns).
First, for two-dimensional stagnation point flows around flat ribbon, relative thermophoretic particle deposition rates were measured by SEM (scanning electron microscopy) techniques. Using an LDA (laser-Doppler anemometry) system, the velocities of the two-phase and the particle size were measured simultaneously. Particle sizes were measured using the maximum pedestal amplitude of Doppler signal and the visibility. Also, particle motion that considered the effects of thermophoresis and inertial impaction was computed by numerically solving Eulerian partial differential equations of momentum, energy and mass for the gas and particle phases (two-fluid model). The experimental data were compared to the numerical results for variation of Stokes numbers. Good agreement between experimental and numerical results was obtained for velocity, temperature and particle deposition rates. Also the importance of combined thermophoresis and inertial impaction was examined.
Next, for the cross flows around a circular cylinder, the local particle deposition rates for two Reynolds numbers (Re=36, 54) and three wall-to-gas temperature ratio parameters ($T_w/T_\infty$=0.731, 0.853, 0.897) were obtained by using SEM techniques. Mainstream particle number density and velocity fields were measured also by LDA system. Calibration for particle concentration was accomplished by the particle size and number density data obtained from inertial impaction and filtration method. The effects of particle size, target temperature ratio parameter, and flow Reynolds number on the particle deposition rates at each local angular position were estimated. The experimental particle deposition rates for small particles is compared to the theoretical thermophoresis correlation.
Also, particle deposition onto a cylinder surface in a mixed convection flows was studied numerically. Time-dependent Navier-Stokes and energy equations have been solved for the buoyancy assisting and opposing flows. The Boussinesq approximation was employed. A Lagrangian trajectory model implemented to the particle deposition in non-isothermal cross flow fields which was obtained above was presented. The equations include the Brownian diffusion, thermophoresis, Stokes' drag, gravity force, and shear-induced (Saffman) lift force. Monte Carlo simulations for Brownian dynamics were employed. The dependence of the deposition rate on the particle size, local angular position, target temperature ratio parameters, and flow Reynolds number was clarified. Further, the combined effects of the Brownian motion, thermophoresis, inertial impaction, gravity, and shear-induced lift force on the particle deposition on cylinder surface were estimated.
고체입자를 포함하는 비등온 기체유동에서 약 0.1$\sim$0.3$\mu$m 정도의 광범위한 입자에 대하여 입자의 전달 및 부착현상에 관한 실험 및 수치계산 연구가 수행되었다.
먼저, 평판리본 주위를 흐르는 2차원 정체유동에서 전자현미경법에 의한 입자부착율의 온도구배 효과를 측정하였다. 그리고 LDA (laser-Doppler anemometry) 시스템을 이용하여 입자의 크기와 속도를 동시에 측정하였다. 이러한 측정 데이타들은 열확산(thermophoresis)과 관성효과의 상호작용이 동시에 고려되는 오일러의 2유체 모델에 의한 수치계산 결과와 비교되어졌다. 그 결과, 입자 및 유체 유동장에 대하여 매우 잘 일치하였으며, 특히 비등온 입자 부착에서 관성효과의 영향이 입자 크기에 따라 정량적으로 관찰되었으며, 수치계산에 의하여 검증되었다.
그리고 실린더 주위 유동인 경우에 실린더 각 국부(local) 표면에서의 절대적인 무차원 입자부착율을 실험적으로 측정하였다. 즉, 실린더 주위 유동에서 입자의 크기, 비등온 온도구배 인자($T_w/T_g$=0.731, 0.853, 0.987), 레이놀드 수(Re=36,55) 및 그라숍 수 등의 변화에 대한 입자부착율을 구하였다.
그리고 실린더의 복합대류 유동장에서 입자부착율을 수치적으로 계산하였다. 먼저 유동장은 유선-와류함수 방법에 의하여 계산되었으며 부시네스크(Boussinesq) 근사가 적용되었다. 입자 방정식은 여러가지 인자 (브라운 운동, 열확산, 관성 등)를 동시에 고려할 수 있는 라그란지의 궤적(trajectory) 모델을 이용하였으며, 입자의 브라운 운동은 몬테카를로 (Monte-Carlo) 시뮬레이션을 적용하였다. 이상에서 비등온 유동장에서 열전달 및 입자부착을 지배하는 중요한 인자들, 즉, 레이놀드 수, 그라숍수, 온도구배 인자 및 입자크기 등의 효과가 실린더의 각 위치에서 비교 검토되었다.
