Thermophoretic Deposition Of Aerosol Particles Research Articles

Thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders

The thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders was studied numerically. The objective of this research was to study the effect of free convection and the distance between cylinders, on deposition of particles. Continuity, momentum and energy equations were solved to determine the velocity and temperature profiles in the channel. The particle trajectories were evaluated by solving the Lagrangian equation of motion that included the drag, Brownian diffusion and thermophoresis forces. It was found that the temperature gradient near the channel wall, in mixed flow regime, is higher than the temperature gradient in forced convection regime. Increasing the temperature gradient increased the effect of thermophoresis on deposition of particles. It was observed that the deposition was increased with the Richardson number. The distance between cylinders is a parameter that influences the deposition of particles. Temperature gradient decreases with increasing the cylinders’ distance; on the other hand, the length of the high temperature gradient zone, which is located in the region between the cylinders where the most deposition occurs, will be increased. These two opposite phenomena cause the fact that at a distance which is four times longer than the cylinders’ length, a maximum cumulative deposition fraction occurs. It was eventually concluded that the thermophoresis and the inertial impaction are dominant deposition mechanisms of particles on the channel wall.

An Experimental Study of Thermophoretic Deposition of Aerosol Particles in Laminar Tube Flow with Mixed Convection

The thermophoretic deposition of aerosol particles in mixed-convection, laminar tube flow with a cooled wall is considered. The presence of free convection alters the velocity and temperature profiles for laminar tube flow relative to the constant-properties case. Previous modeling results (Walsh, Weimer, and Hrenya, in press), show that the modified flow fields lead to changes in the cumulative deposition-efficiency profiles (i.e., deposition as a function of axial distance). In particular, deposition in vertical, downward flow is higher at short axial distances as compared to the upward counterpart, while overall deposition efficiencies (i.e., total deposition in the tube) for both cases are closer in magnitude. In this study, experimental data is obtained in an attempt to qualitatively and quantitatively validate the model predictions for the effects of free convection on cumulative deposition efficiencies. To explore the effects of free convection, two cases are considered: (1) comparison of systems w...

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...

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.

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.

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 deposition of aerosol particles on transport tubes

Thermophoretic deposition of aerosol particles on transport tubes

Thermophoretic deposition in crossflow over a cylinder

tions.4 The pressure gradient to be used in the momentum equation is found either from the potential flow solution over the cylinder or from the best fit for the experimental data.4 The temperature gradient established in the thermal boundary layer drives the particles either toward or away from the cylinder surface. The velocity acquired by the small parti- cles relative to the gas velocity is known as the thermophoretic velocity vt. Following the standard assumptions,5 the conser- vation law for particle concentration, with the help of the continuity equation, reduces to HERMOPHORESIS causes small particles to be driven away from a hot surface and toward a cold one. This phenomenon has many practical applications. It affects the removal of small particles from gas streams, determines ex- haust gas particle trajectories from combustion devices, and helps in studying the particulate material deposition on turbine blades. It is also of importance in the manufacture of fumed silica, carbon black, and titania particles for the paint industry. All studies of thermophoreti c deposition in external flow (except Homsy et al.1 and Alam and Mehrotra2) are either for a zero pressure gradient in the boundary layer or for cases for which similarity solution is possible. Practical applications of thermophoresis in external flow, however, involve a nonzero pressure gradient for which no similarity solution is possible. Therefore, we study the thermophoretic deposition of aerosol particles in crossflow over a circular cylinder. Both theoretical and experimental pressure distributions in the hydrodynamic boundary layer over the cylinder are considered, unlike those in Homsy et al. 1 and Alam and Mehrotra,2 where only the former is studied. A finite-differe nce method is used for the solution. The working fluid is taken to be air.

The Measurements of Thermophoretic Deposition of Aerosol Particles in the Continuum and Slip Flow Regions

The thermophoretic deposition of limestone particles onto a cold plate above which a temperature gradient was established has been measured. The experimental setup and methodology are described. The particle size distribution was measured by means of an Andersen impactor. The concentration of particles of different sizes was also measured. The Knudsen numbers (Kn) covered both continuum and slip flow regimes (Kn = 1 × 10-2 to 0.93). The theoretical expressions for thermal velocity of particles in the two regimes were calculated using the theories of Derjaguin and Yalamow and Brock, respectively. The measured concentration of particles of each size (0.4 - 9 /μm) along with theoretical values of thermal velocity were used to predict the total particle flux. The comparison between the directly measured particle flux and the predicted values showed good agreement. This result validates the thermophoresis theoretical expressions which have been proposed for both flow regimes involved in the present investigation.

The influence of buoyancy on thermophoretic deposition of aerosol particles in a horizontal tube

Abstract The transport and deposition of aerosol particles in a horizontal tube with a linear wall temperature profile are analyzed. Assuming fully developed velocity and temperature fields, and using trajectory calculations, the deposition efficiency is calculated as a function of system parameters. It is found that buoyant flows tend to increase the rate of deposition of particles on the tube wall. Also, such flows can lead to azimuthal nonuniformity in the deposition process.

Analysis of thermophoretic deposition of aerosol particles from laminar-flow gas streams

Thermophoretic deposition of aerosol particles from fully-developed laminar flow was considered to investigate the applicability of the assumption of constant transverse temperature gradient in flow channels. From the numerical calculations, this assumption was found to hold only for particles depositing far from the entrance of the collecting channel. In particular, it is shown that the variation of gas viscosity with temperature can have significant effects on the deposition pattern of large particles.

Thermophoretic Deposition of Aerosol Particles in a Heat-Exchanger Pipe

Thermophoretic Deposition of Aerosol Particles in a Heat-Exchanger Pipe