| Summary: | 博士 === 國立交通大學 === 環境工程系所 === 92 === This study investigates thermophoretic particle deposition efficiency in a tube flow. The highest temperature gradient near the wall occurs at the entrance of a tube when both flow and temperature are developing, thermophoretic deposition in the entrance region may be enhanced. Therefore, the effect of entrance flow on the thermophoretic deposition efficiency in laminar tube flow was first investigated numerically. In the previous study of Romay et al. (1998), the experimental data don’t agree well with theoretical results. In the present study, the thermophoretic particle deposition efficiency in tube flow was studied experimentally and compared with the theoretical predictions. To prevent particle deposition on tube wall, a common practice is to heat up the tube wall in industry. But the required wall temperature to effectively suppress particle deposition in tube wall is unknown. Thus the effect of tube wall temperature on particle deposition efficiency under laminar flow condition was investigated experimentally and numerically.
In the study of developing flow effect in a circular tube on thermophoretic particle deposition efficiency, the critical trajectory method was investigated numerically. The results show that when the flow is fully developed and temperature is developing, it is found that only near the thermal entrance region (or temperature jump region) of the tube the deposition efficiency is slightly higher than the combined fully developed case (flow and temperature), while the deposition efficiency remains the same for Z>5. When both flow and temperature are developing (or combined developing), the deposition efficiency is about twice of the combined fully developed case for Z>5 and is much higher near the entrance of the tube. Non-dimensional equations are developed empirically to predict the thermophoretic deposition efficiency in combined developing and combined fully developed cases under laminar flow condition.
In the experimental study of thermophoretic deposition of aerosols particles in laminar and turbulent tube flow. Thermophoretic deposition of aerosols particles (particle diameter ranges from 0.038 to 0.498 mm) was measured in a tube (1.18 m long, 0.43 cm inner diameter, stainless-steel tube) using monodisperse NaCl test particles under laminar and turbulent flow conditions. In the previous study by Romay et al. (1998), theoretical thermophretic deposition efficiencies in turbulent flow regime do not agree well with the experimental data. In this study, particle deposition efficiencies due to other deposition mechanisms such as electrostatic deposition for particles in Boltzmann charge equilibrium, and turbulent diffusion and inertial deposition were carefully assessed so that the deposition due to thermophoresis alone could be measured accurately.
In the aspect of suppression of particle deposition by thermophoretic force, flow through a tube with circular cross section was investigated numerically and experimentally for the case when the wall temperature exceeds that of the gas. Particle transport equations for convection, diffusion and thermophoresis were solved numerically to obtain particle concentration profiles and deposition efficiencies. The numerical results were validated by particle deposition efficiency measurements with monodisperse particles (particle sizes were 0.01, 0.02 and 0.04 mm). For all particle sizes, the particle deposition efficiency was found to decrease with increasing tube wall temperature and gas flow rate. An empirical expression has been developed to predict the dimensionless temperature difference needed for zero deposition efficiency in a laminar tube flow for a given dimensionless deposition parameter.
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