Thermophoretic Particle Deposition Research Articles

Thermophoretic deposition of aerosol particles in laminar tube flow with mixed convection

A model is developed to investigate the thermophoretic deposition of aerosol particles on the wall of a relatively cool cylindrical tube. A key aspect of the flow is the presence of mixed convection. Continuity, momentum, and energy equations are solved to determine the velocity and temperature profiles of the system. Additionally, an aerosol population balance that incorporates thermophoresis and Brownian diffusion is solved. The results indicate that more particles deposit at shorter axial distances in downward flow through a vertical pipe than in upward flow. This behavior is traced to the larger residence time associated with downward flow, despite the presence of a reduced radial temperature gradient relative to the upward configuration. Model predictions also indicate that a bulk Richardson number ( Ri) of at least unity is necessary, but not sufficient for free convection effects to be important. The additional evaluation of a local Ri provides a more reliable indicator.

Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows

Thermophoretic deposition of aerosol particles (particle diameter ranges from 0.038 to 0.498 μm) 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., theoretical thermophoretic 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 laminar and turbulent diffusions were carefully assessed so that the deposition due to thermophoresis alone could be measured accurately. As a result, the semiempirical equation developed by Lin and Tsai in laminar flow regime and the theoretical equation of Romay et al. in turbulent flow regime are found to fit the experimental data of thermophoretic deposition efficiency very well with the differences of less than 1.0% in b...

Thermophoretic deposition of soot aerosol particles under experimental conditions relevant for modern diesel engine exhaust gas systems

A plate-to-plate thermal precipitation system has been used to investigate the deposition of agglomerate submicrometer soot aerosol particles under flow and temperature conditions relevant for modern heavy-duty diesel engine exhaust gas systems. Over a flow channel length of 45 cm , thermophoretic particle deposition efficiencies of 4–95% have been observed as a function of plate distance (0.45– 1.45 mm ), temperature gradient (10 4– 10 5 K m −1) , and flow velocity (2– 11 m s −1 ). Throughout the investigated particle size range of 34– 300 nm (electrical mobility diameter), the observed deposition efficiencies are nearly independent of particle size and Knudsen number. The experimental results are in good agreement with theoretical calculations assuming a constant thermophoretic coefficient K th=0.55 which had been derived by Waldmann and Schmitt (in C. N. Davies (Ed.), Aerosol Science, London, 1966) for the free molecular regime ( Kn⪢1). Our results demonstrate that K th≈0.55 is also applicable for agglomerate soot particles in the transition regime ( Kn≈1), which is supported by a recent theoretical study by Rosner and Khalil (J. Aerosol Sci. 31 (2000) 273) on the effect of reduced thermal conductivity on the thermophoretic coefficient of agglomerate particles. To our knowledge, the measurement data presented in this paper provide the first direct experimental validation for the theory of Rosner and Khalil under conditions relevant for diesel engine exhaust gas treatment. Moreover, they confirm the applicability of a simple formula of approximation for the thermophoretic deposition efficiency derived by Tsai and Lu (Aerosol Sci. Technol. 22 (1995) 172).

Thermophoretic deposition efficiency in a cylindrical tube taking into account developing flow at the entrance region

This study investigates the effect of developing flow in a circular tube on the thermophoretic particle deposition efficiency using the critical trajectory method numerically. When both flow and temperature are fully developed (combined fully developed), present results agree with previous theories for the long tube where the flow temperature approaches that of the wall. 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 the long tube. When both flow and temperature are developing (or combined developing), the deposition efficiency is about twice that of the combined fully developed case for the long tube 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 conditions.

Correlation for thermophoretic deposition of aerosol particles onto cold plates

A rational correlation for evaluating the effect of thermophoresis on aerosol particle deposition from laminar flow systems was developed. The flows considered were flat-plate flow and stagnation in plane and axisymmetric flows with variable properties for cooled plate surfaces. This procedure is based on the concept of high-mass transfer with a thermophoretic factor. This factor is treated with a product of two functions that are a suction-like convection term and a sink-like term. The particle flux to the plate surface, expressed in terms of the particle Stanton number, was obtained from the product of the particle Stanton number in the limiting case ( V T→0) and the thermophoretic factor. The results were correlated from the numerical solutions for self-similar boundary flows and provided a maximum possible error less than 3% over the range of 0.3⩽ κ⩽1.0 and 0.2⩽ T w / T ∞⩽1.0. To examine the effect of flow properties for stagnation flow upon the deposition rates between the variable and constant flow properties, the error increases with an increase in the temperature difference between the plate and free stream and becomes about 50% as T w / T ∞=0.2.

Sub-micron particle deposition on an isothermal horizontal rod in a turbulent flow system

The available theoretical models for particulate fouling were reviewed and used to explain some of limited fouling data which have been reported. An experimental measurement of the thermophoretic deposition of sub-micron particles on the smooth wall of a circular rod and a simple approximation of a two-dimensional method for predicting the thermophoretic effects on sub-micron particles are introduced. The particles were moving under fully developed turbulent flow conditions, and the investigation focusses on diffusive deposition of sub-micron particles ofdp=0.05 μm anddp=0.25 μm in diameter.

An Aerosol CVD Method Using Internal Jet for Optical Fiber Synthesis

The present study has proposed a novel aerosol CVD utilizing an internal jet in the conventional MCVD reactor for the purpose of enhancing the deposition efficiency(and rate) and the uniformity of deposited film. The use of impingement of high temperature jet through a thin inner tube ensures the reduction of non-uniform particle deposition zone as well as higher thermophoretic particle deposition. It is shown that significant improvements have been achieved for both aspects of deposition efficiency and uniformity. As jet temperatures increase, the tapered length is reduced and deposition efficiency is significantly increased.

Thermophoretic transport and deposition of particles in vertical tube flow with variable wall temperature and thermal radiation

A particulate two phase flow with variable wall temperature has been studied for examining the deposition of particles in the thermal radiation and mixed convection flow associated with the manufacture of optical fiber preforms. The two-dimensional governing equations of continuity, momentum and energy have been solved numerically including the effects of thermal radiation and buoyancy (upward or downward flow) in the vertical tube flow. A particle trajectory model has been adopted to predict the particle transport, and P-1 approximation has been used to evaluate the radiation heat transfer. In the upward flow case, a high deposition efficiency is obtained and the deposition zone of the downward flow is broader than that of the upward flow. Thermal radiation makes the deposition zone broader and the deposition efficiency smaller.

Thermophoretic deposition of small particles in a direct numerical simulation of turbulent channel flow

This paper presents results for the deposition rate of small particles on the walls of a turbulent channel flow. The results were obtained by direct numerical simulation of a horizontal turbulent channel flow. A temperature profile typical of ceramic oxide aerosol reactors was imposed across the channel. Thermophoresis played an important role in the deposition of particles for the range of conditions that were studied. An interaction between turbophoresis and thermophoresis was found to play an important role in the deposition process.

Thermophoretic deposition of aerosol particles in turbulent pipe flow

Thermophoretic deposition of aerosol particles was measured in a 0.965 m long, 0.49 cm ID stainless-steel pipe under laminar and turbulent flow conditions. The experiment consisted of heating an aerosol to a specified temperature, and then passing it through an annular water cooled (20°C) heat exchanger to induce thermophoretic deposition onto the pipe walls. Experimental parameters varied were particle diameter (0.1–0.7 μm), flow rate (5, 20 and 35 l min -1), and the gas temperature at the entrance to the cooling section (300–425 K). The flow rates, 5, 20 and 35 l min -1 corresponded to Reynolds numbers of 1379, 5517 and 9656, respectively. Particle materials used were sodium chloride and polystyrene latex spheres. Experimental results showed a complex coupling of the turbulent and thermophoretic deposition mechanisms with turbulent deposition dominating over thermophoresis for the larger particle sizes at 35 l min -1. The thermophoretic deposition component was extracted from the total deposition and was compared with theoretical predictions. A theoretical expression was derived and compared with other expressions from the literature. Experimental deposition efficiencies were found to be between 1.1 and 2.0 times greater than the theoretical predictions. The discrepancy between theory and experiment increased as Reynolds number and particle size were increased. This discrepancy was attributed to factors not included in the theory which may involve deposition enhancement due to turbulent eddy transport or non-uniform particle concentration gradients at the entrance to the pipe cooling section.

Three-dimensional analysis of heat transfer and thermophoretic particle deposition in OVD process

Heat transfer and particle deposition in the OVD process are simply modeled as a buoyant jet flow impinging on a circular target cylinder and numerically analyzed using a finite volume method on the flow, heat transfer and thermophoretic particle deposition over the cylinder. Effects of the three-dimensionality, conjugate heat transfer and longitudinal traversing of the jet on the particle deposition are investigated. The surface temperature, which is the most important value in the whole OVD process, is estimated considering the conjugate heat transfer. As the conductivity of the cylinder decreases, Nusselt number, particle deposition rate and deposition efficiency greatly decrease due to the reduced temperature gradient. Since the increase of the jet traversing speed keeps the surface temperature relatively low, Nusselt number and the particle deposition increase with the traversing speed.

Analysis of thermophoretic deposition of particles from laminar-flow gas streams with considerable transversal temperature drops

A theoretical model was developed describing the thermophoretic deposition of aerosol particles in fully-developed laminar flow through channels with their walls at different fixed temperatures. Precipitation of particles occurs under the considerable transversal temperature drops. Gas flow takes place when the gas density, the dynamic viscosity and the thermal conductivity depend on the temperature of the gas. The nonlinear system of equations of gas dynamics describing the distribution of the temperature and the longitudinal component of the mass velocity was solved analytically. The expressions for the distributions of the temperature and the longitudinal component of the velocity in the gas flow enabled us to obtain formulae for direct estimation of the channel length at which the complete thermophoretic precipitation of particles occurs. Numerical evaluation has shown that the increase of the upper plate temperature causes a decrease in the length of complete thermophoretic entrapment of particles.

Thermophoretic Particle Deposition Around a Circular Cylinder in a Uniform Laminar Air Dlow

Thermophoretic particle deposition on a circular cylinder in a uniform laminar air flow was numerically investigated using a control volume method based on the generalized non-orthogonal coordinate system. Variation of air properties due to the change of temperature was taken into account. Effects of variable property on the distribution of heat transfer and deposition rates of particle were discussed. A new correlation of thermophoretic particle deposition on a circular cylinder was proposed in the present study.

A study of thermophoretic particle deposition in a particle laden stagnation flow including the effect of radiative heat transfer

A study of thermophoretic particle deposition has been carried out for a particle laden stagnation flow considering the effect of radiative heat transfer. Energy, concentration and radiative transfer equations are all coupled and have been solved iteratively assuming that absorption and scattering coefficients were proportional to the local concentration of particles. Radiative heat transfer was shown to strongly affect the profiles of temperature and particle concentration. e. g., radiation increases the thickness of thermal boundary layer and wall temperature gradients significantly. As the wall temperature gradients increase, the particle concentration at the wall decreases due to thermophoretic particle transport. The deposition rate that is thermophoretic velocity times particle concentration at the wall decreases as the effects of radiation increases. The effects of optical thickness, conduction to radiation parameter and wall emissivity have been determined. The effects of anisotropic scattering are shown as insignificant.

CONJUGATE HEAT TRANSFER AND PARTICLE TRANSPORT IN OUTSIDE VAPOR DEPOSITION PROCESS

A numerical study of conjugate heat transfer and particle transport has been carried out for the outside vapor deposition process. A buoyant jet flow impinging on a two-layered cylinder has been analyzed including heat conduction occurring through the two-layered cylinder, which consists of the original target rod and the deposited porous layers. Temperature and flow fields have been obtained by an iterative method, and thermophoretic particle deposition has been studied. Of particular interest are the effects of the thickness of deposited layers, the torch speed, the rotation speed of the cylinder, and the distance between the torch and the cylinder on the heat transfer and particle deposition. Effects of variable properties and tube rotation are also included.

Modelling of chemical vapour deposition for optical fibre manufacture

A study of numerical modelling has been carried out for chemical vapour deposition processes with applications to manufacture of optical fibres. Temperature distributions and thermophoretic particle deposition have been calculated for the modified chemical vapour deposition (MCVD) and the outside vapour deposition (OVD) processes. A two torch formulation and a heat flux boundary condition are used for MCVD and the present model is shown to be capable of predicting tube wall temperatures and deposition profiles correctly. The present results are in agreement with experimental data. For OVD modelling, nonorthogonal body-fitted coordinates have been utilized to solve a conjugate problem including the jet flow and heat conduction through a two-layered cylinder that consists of an original target and the deposited porous layers. Surface temperatures and efficiencies of particle deposition have been obtained.

Thermophoretic deposition of airborne particles in an annular flow: experiments and simulations

The thermophoretic deposition of a polydisperse glass aerosol influenced by a fixed temperature gradient in the flow direction was investigated in a prototype thermal cell. The cell was constructed as an annulus and the flow was between the cylinders. The inner cylinder is a high resistance wire electrically connected with a power supply. A fixed thermal gradient between the cylinders can be applied with the outer cylinder maintained at a low temperature and the inner cylinder electrically heated to a higher one. Two optical counters were connected on-line with the cell to perform the measurements. The results were consistent for repeated measurements when the aerosol source is stable and are presented here with an assessment of thermal instability in the horizontally aligned thermal cell.

Thermophoretic deposition of fine particles from longitudinal flow over a cylinder

Thermophoretic deposition of fine particles onto the surface of a cylinder in longitudinal non-self-similar flow was examined. The equations of motion, thermal and mass balance were solved using the integral method of the boundary layer theory. As a result the mass rate of deposition was predicted.

Experimental study of thermophoretic particle deposition in laminar tube flow

The deposition of aerosol particles due to the presence of a temperature gradient (called thermophoresis deposition) was studied experimentally in a laminar tube flow, for the special case of uniform distribution of particle concentration and gas temperature at the inlet of a deposition tube with cooled walls. The experimental results clearly indicated a decrease in thermophoretic velocity as particle diameter increased. They also demonstrated that the limit deposition efficiency for an infinitely long tube was independent of the flow rate. On the basis of these results two correlations were established which can be used to predict thermophoretic deposition of submicron or micronsized particles in a laminar tube flow.

Thermophoresis of highly absorbing, emitting particles in laminar tube flow

The effect of radiation on the thermophoresis phenomenon owing to the presence of highly absorbing, emitting particles (such as soot or pulverized coal) suspended in a two-phase flow system is investigated numerically for a laminar tube flow. The analysis of conservation equations for a gas-particle flow system is performed on the basis of a two-fluid model from a continuum Eulerian viewpoint. In addition, the P-1 approximation is used to evaluate radiation heat transfer. Under a strong radiative effect, the rate of thermophoretic particle deposition becomes considerably less. The effects of the thermal loading ratio and the Stokes number on thermophoresis phenomenon also are examined.