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 both flow regimes. It is also found that Talbot's formula for the thermophoretic coefficient is accurate while Waldmann's free molecular formula is only applicable when Kn is greater than about 3.0.     

Nanograin magnetoresistive manganite coatings for EMI shielding against directed energy pulses

Two magnetoresistive manganites, La_0.83Sr_0.17MnO₃ and La_0.7Sr_0.3MnO₃, are synthesized by the environmentally friendly “deposition by aqueous acetate solution (DAAS)” technique. The manganite film has a grain size of 100 nm, and can be processed as thinly as 0.03 μm per layer, while the powder form has a crystallite size of 40 nm. These magnetoresistive materials are shown to be effective and inexpensive electromagnetic interference (EMI) shield for the extremely low frequency (ELF) EM fields. The electrical resistance of manganites is very sensitive to external influences, such as temperature and electromagnetic fields. Both permeability (μ) and conductivity (σ) of manganites tend to increase with increasing applied magnetic field. The manganites have been shown to “react” to field increases in a way that is particularly useful for shielding EMI field fluctuations (e.g., due to current or voltage spikes).

The manganite properties, e.g., crystal structure, film morphology, radiation absorption and reflection, electrical resistivity, and magnetization, etc., have been measured. The ceramic manganites have a metal–insulator transition at 300 K or higher, and are suitable for a room temperature operation. A thin film (approx. 0.1 μm) of La_0.83Sr_0.17MnO₃ was fabricated on a quartz tube or refractory ceramic fiber blanket. Using this thin manganite film, the EMI shielding effectiveness for the measured E-field attenuation is similar to that of a 25 μm thickness of copper tube, aluminum foil, and silver–nickel particle-dispersed paper. The silver–nickel impregnated paper has an EMI shielding effectiveness of 35 dB at 10 kHz, and 15 dB at 60 Hz (or frequency above 1 MHz). The ceramic manganites are chemically inert, thermally stable, and mechanically flexible. They provide low cost EMI shielding against directed energy pulses and may serve as a “signature reduction” barrier.     

PREDICTION AND RATIONAL CORRELATION OF THERMOPHORETICALLY REDUCED PARTICLE MASS TRANSFER TO HOT SURFACES ACROSS LAMINAR OR TURBULENT FORCED-CONVECTION GAS BOUNDARY LAYERS†

An approach originally developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is shown here to account extremely well for the thermophoretically reduced particle mass transfer rate to “overheated” surfaces experiencing either a forced boundary layer (BL)-flow of laminar or turbulent dusty gas. This laminar BL/hot wall situation occurs, e.g., in hot surface/cold envelope chemical reactors used for growing epitaxial silicon layers from mainstreams containing, say, silane vapor and inadvertent submicron dust particles. “Thermo-phoretic blowing” is shown to produce effects on particle concentration BL-structure and wall mass transfer rates identical to those produced by real blowing (transpiration) through a porous wall. Indeed, a “blowing parameter additivity” relationship is proposed to account for the simultaneous effects of both phenomena should they be acting in concert (or in opposition). Exact numerical BL calculations covering the parameter ranges: l≤T_w/T_e6, (particle thermophoretic-/gas thermal- diffusivity ratios between )0·1 and 0·8 and particle Schmidt numbers between 10⁰ and 2 × 10³ are used to establish the validity of the basic forced convection mass transfer correlations for self-similar laminar BLs and law-of-the-wall turbulent BLs. This includes parametric combinations of immediate engineering interest for which the deposition rate is thermophoretically reduced by no less than 10-decades! The applicability of our correlations to developing BL-situations is then illustrated using a numerical example relevant to wet-steam turbine technology.     

Effect of thermophoresis on sub-micron particle deposition from a laminar forced convection boundary layer flow onto an isothermal cylinder

Sub-micron deposition from a laminar forced convection boundary layer developing on a heated isothermal vertical cylinder has been investigated. Pseudo-similarity solutions, as proposed by Sparrow et al. (1970, AIAA J. 8, 1936), have been used to calculate the local cumulative particle deposition theoretically; with the thermophoretic velocity being that proposed by Talbot et al. (1980, J. Fluid Mech. 101, 737). It was demonstrated that the effect of thermophoresis can decrease particle transfer by about two orders of magnitude and that the effect increases with particle size and distance along the cylinder. Experiments were also conducted using mono-disperse fluorescent uranin particles with mean diameters of 0.05–0.25 μm. A calibrated fluorimeter was used to measure the mass of the particles deposited at different axial locations along the cylinder. Results showed similar trends to the theoretical model, but absolute values of the deposits were greater than theoretical predictions. On the other hand, good agreement was obtained for the percentage change in deposition relative to that obtained under isothermal conditions.     

Effect of the laryngeal jet on particle deposition in the human trachea and upper bronchial airways

Preferential sites of particle deposition within the human respiratory system are known to correlate with primary cancer sites, and are therefore important in the etiology of neoplastic respiratory diseases. In this study, we characterized the intrabronchial and intratracheal patterns of deposition in a hollow cast of a human larynx-tracheobronchial tree, and examined the effects of airflow and turbulence on particle deposition by performing airflow measurements in the hollow cast and an “ideal” airway bifurcation model.

Experimental results revealed a deposition “hot spot” for particles greater than 2 μm in mass median aerodynamic diameter in the trachea at 2 cm below the larynx. The enhancement of deposition in the trachea was studied by making comparative airflow and detailed morphometry measurements in a hollow lung cast and in an “ideal” model. The larynx had a significant effect on the local flow field in the trachea of the hollow cast and this effect extended to and beyond the tracheal bifurcation. This accounted for some of the difference in flow field at the bifurcation between the cast and an “ideal” model. Additional differences were related to the different shapes of the transitional regions near the bifurcation.     

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.     

Effects of air temperature and humidity on particle deposition

Particle deposition in a fully developed turbulent duct flow was studied. The random walk model of Lagrangian approach was used to predict the trajectories of 3000 particles with a density of 900 kg/m³. The effects of thermophoretic force and air humidity were also considered. The results were compared with the previous studies with a particle size range of 0.01–50 μm and air flow velocity of 5 m/s. The profile of dimensionless deposition velocity with relaxation time presents a V-shaped curve and the results are in good agreement with the previous studies.The effects of air temperature and humidity on particle deposition with a particle size of 1 μm were also investigated. The results show that thermophoretic force accelerates particle deposition onto the duct walls with increasing temperature difference between air flow and the duct wall surface. Meanwhile, it was found that particle deposition velocity increases with air humidity.     

Annular flow configuration with high deposition efficiency for the experimental determination of thermophoretic diffusion coefficients

An experimental study was carried out to produce reliable data for the determination of the thermophoretic diffusion coefficient K_th of suspended oil particles in air, in the transition regime. An original device was used for the thermophoretic deposition efficiency measurement, involving a turbulent flow through a concentric tube annulus, with the inner tube cooled (5 °C) and the outer heated. Experimental parameters varied in particle diameter (0.039–5.13 μm), flow rate (150, 200, and 250 Nl min⁻¹, corresponding to Reynolds number in the range 5000–10 000) and hot wall temperature (65–125 °C). This configuration, based on three controlled temperatures (gas inlet, cold wall, hot wall), the so-called “3T”, permits an overall deposition efficiency enhancement compared to conventional “2T” penetration devices (hot gas flow in a cooled tube). In the 3T configuration, significant thermophoretic deposition efficiencies have been obtained (up to 27%), together with limited gas temperature axial variations, thus permitting a reliable determination of the thermophoretic diffusion coefficient K_th.An analytical model was developed for the prediction of the thermophoretic deposition efficiency, for a given value of the thermophoretic diffusion coefficient K_th. This model has been used, together with our measurement results, to derive the K_th experimental values, for a Knudsen number ranging from 0.01 to 3. These K_th values were compared with evaluations based on various models available in the literature. Although widely used, Talbot's model always provides K_th values higher than our experimental results in the transition regime. The most relevant model appears to be the one proposed by Beresnev and Chernyak, particularly for an energy accommodation slightly lower than one.     

THE CONVECTIVE VELOCITY OF HEAVY PARTICLES IN TURBULENT AIR FLOWS

This paper examines the influence of the “crossing trajectories” effect on the convective velocity for heavy particles suspended in a turbulent air flow. The fluid energy spectrum “experienced” by a falling particle is deduced from experimental data and is used to evaluate the turbulent component of the convective velocity. The results indicate a significant increase in the rms value of the turbulent component due to the “crossing trajectories” effect.     

A method for reducing the deposition of small particles from turbulent fluid by creating a thermal gradient at the surface

The present paper suggests the use of thermophoretic phenomena to decrease the rate of particle deposition onto pipe walls from a turbulent flow. When a tube is externally heated; the particles will be subjected to thermal force within the laminar sublayer in a direction away from the surface preventing or reducing their deposition. A theory proposed by EI-Shobokshy and Ismail (1980) has been used for estimating the deposition velocity. The thermal velocity component was calculated and the effective velocity of particles approaching the wall surface computed. The results present the relationship between particle penetration and particle size at different values of pipe wall temperature and Re. The experimental results showed a good agreement with theoretical results for particle sizes 6 -10 μm diameter, Re = 6000 – 8000 and pipe wall temperatures 50 – 150°C.     

EVALUATION OF UV DOSE IN FLOW-THROUGH REACTORS FOR FRESH APPLE JUICE AND CIDER

High absorptivity and turbidity interfere with the UV disinfection of apple cider. Three different configurations of flow-through UV reactors were evaluated to overcome this interference. Two approaches were employed: use of an extremely thin film UV reactor and increasing the turbulence within a UV reactor. Multiple-lamp UV reactors including the thin-film laminar flow “CiderSure” (8 lamps) and turbulent flow “Aquionics” (12 lamps) and annular single-lamp “UltraDynamics” reactor were studied. UV disinfection performance in laminar and turbulent flow reactors was compared by evaluation of UV dose delivery. UV fluence rate (irradiance) distribution was calculated using the multiple point source summation method. E. coli K12 was used as a target bacterium in a bioassay, and the log reduction per one pass was determined for each UV reactor. Finally, the UV decimal reduction dose (D₁₀) was calculated by dividing the average UV fluence by log bacterial reduction per pass. Variations of the UV decimal dose were observed with various designs of UV systems. The least inactivation of E. coli K12 but the highest UV decimal reduction dose, ranging from 90 to 150 mJ/cm², was observed in the Aquionics UV reactor in apple cider with apparent absorption coefficient (a) of 5.7 mm^-1. The lower value of UV decimal reduction dose of 7.3-7.8 mJ/cm² was required for inactivation of E. coli K12 in malate buffer and apple juice in the annular single-lamp UltraDynamics reactor. However, the decimal reduction dose for E. coli K12 in apple cider was significantly higher, about 20.4 mJ/cm². Similar UV decimal reduction doses from 25.1 to 18.8 mJ/cm² for inactivation of E. coli K12 were observed in the thin-film 'CiderSure' UV reactor in apple cider with identical absorption coefficient. Mathematical modeling of UV irradiance can improve the evaluation of UV dose delivery and distribution within the reactors.     

Deposition and coagulation of polydisperse nanoparticles by Brownian motion and turbulence

Turbulent and Brownian coagulation rates as well as deposition coefficients of polydisperse nanoparticles were measured experimentally. The coagulation rates were obtained from the change in the total number concentration of polydisperse sodium chloride aerosols, with geometric mean diameters ranging from 30 to 120 nm, in a closed chamber at atmospheric pressure. The geometric standard deviations of the experiments were in the range of 1.55–1.65. The experimental coagulation rates took deposition rates into account, because coagulation and deposition occur simultaneously in a closed chamber. As a result of deposition, it was shown that the experimental deposition coefficients of polydisperse aerosols were agreed well with the theoretical data of Park et al. [(2001). Wall loss rate of polydispersed aerosols. Aerosol Science and Technology, 35, 710–717]. It was shown that the effect of the coagulation was much greater than that of the deposition in the high particle number concentration. In addition, the results represented that bigger turbulent coefficients, caused by higher fan rotation speeds, make the turbulent coagulation process become stronger. In the small particle size range, however, the coagulation rates tend to converge though turbulent coefficients are different. In conclusion, it was shown that experimental coagulation rates followed the values of Lee and Chen [(1984). Coagulation rate of polydisperse particles. Aerosol Science and Technology, 3, 327–334], which were calculated for polydispere aerosols in the gas-slip regime and free-molecule regime.     

Thermophoretic Deposition Efficiency for a Nonuniform Inlet Particle Concentration

The calculation of the cumulative thermophoretic efficiency of particles in tube flow with nonuniform inlet particle concentration is considered. Using previous results, a simple but exact expression is found for the cumulative deposition efficiency. An illustrative calculation is performed for a specific initial particle distribution with one parameter. The total deposition efficiency is computed as a function of this parameter and the results are compared to the uniform inlet concentration case. It is observed that the deposition efficiency is higher when the initial particle concentration is greater near the tube wall.     

Mechanisms of aerosol deposition in a nasal model

Total and regional aerosol deposition were investigated in a model of a normal human nasal airway. Contributions of fluid turbulence and particle inertia were evaluated using monodisperse aerosols. At fixed turbulent flow conditions, deposition percentage increased with particle size greater than 1 μm, suggesting that turbulent inertial deposition is a primary mechanism.

With same size aerosol, deposition increased with increasing fluid turbulence but its contribution was less with larger size aerosol. Turbulent diffusion was the dominant transport mechanism for particles less than 1 μm, where deposition decreased with particle size. Two major deposition sites were visualized with radio-aerosol in the anterior region of the nasal airway. One is close to the ostium internum where turbulent eddies are well developed, and the other is the anterior region of the middle turbinate where direction of airflow changes from upward to horizontal.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Eulerian Model for Prediction of Particle Transport and Deposition in Turbulent Duct Flows with Thermophoresis

The Reynolds-averaged equations for turbulent particle population/transport in an Eulerian framework must be closed by specifying models for several terms: a turbophoretic force; a turbulent thermophoretic force; and a turbulent particle-diffusion term. In this article, new models are proposed for the turbophoretic term, as a particle-size dependent extrapolation of the corresponding turbulent fluid-velocity correlation, and for the turbulent thermophoretic term as an eddy-viscosity-scaled multiple of the corresponding mean thermophoretic term, appropriate for small low-inertia particles with τ⁺_p < 10. When the turbophoresis model is incorporated in a system of equations that describes particle motion within the surrounding fluid, it predicts particle deposition velocities that are in good agreement with experimental data over a range of particle sizes. When this equation system is included in a computational model to predict particle transport in turbulent pipe flows, the efficiency of particle deposition in pipes with upstream heating and downstream cooling is found to be in fair agreement with experimental measurements at two different Reynolds numbers, and over a range of particle sizes and temperature differences.

Copyright 2015 American Association for Aerosol Research     

Thermophoretic deposition of ultrafine particles in a turbulent pipe flow: Simulation of ultrafine particle behaviour in an automobile exhaust pipe

Thermophoresis of ultrafine particles in a turbulent pipe flow was studied using high-temperature and high-concentration polydisperse ultrafine particles. Experimental conditions were designed to simulate particle behaviour in an automobile exhaust pipe, with a particular focus on establishing similar particle residence time. From the experimental results, thermophoresis was found to be a dominant mechanism for ultrafine particle deposition in the turbulent pipe flow. A previous thermophoretic deposition model was found to be inadequate with respect to estimating the results of the experimental conditions. In this study, the experimental data and the computational analysis results reflect the necessity of establishing a new formula for thermophoretic deposition for high-concentration polydisperse ultrafine particles in a pipe flow.     

沿同心环面层流流动的气体中颗粒的传热沉积物的预测:发展中的流动和已充分发展的流动的比较

Thermophoresis is an important mechanism of micro-particle transport due to temperature gradients in the surrounding medium. It has numerous applications, especially in the field of aerosol technology. This study has numerically investigated the thermophoretic deposition efficiency of particles in a laminar gas flow in a concentric annulus using the critical trajectory method. The governing equations are the momentum and energy equations for the gas and the particle equations of motion. The effects of the annulus size, particle diameter, the ratio of inner to outer radius of tube and wall temperature on the deposition efficiency were studied for both developing and fully-developed flows. Simulation results suggest that thermophoretic deposition increases by increasing thermal gradient, deposition distance, and the ratio of inner to outer radius, but decreases with increasing particle size. It has been found that by taking into account the effect of developing flow at the entrance region, higher deposition efficiency was obtained, than fully developed flow.     

Using a new approach to analyze a depth profile of double bond conversion in model formulations

A new approach of analyzing the depth profile of double bond conversion as a function of film depth has been studied. By using a combination of statistical calculation and traditional FTIR, a new approach to analyze the depth profile of conversion “layer by layer” in the characterization of photopolymerization was explored. Utilizing a formula (X₁ + X₂ +  + X_n)/n = average conversion, n = 1, 2, 3, n is a number of layers (μm), an average conversion of any 5 μm depth could be calculated from the prior 5 μm conversion and the total average conversion. More detailed information of photopolymerization, such as the depth profile of conversion and a difference in conversion between the top 5 μm and the bottom 5 μm in a 25 μm film as a function of film depth, was obtained. This investigation was accomplished using a variation of film depth, non-photo bleaching photo initiator [PhI] as well as the concentration of PhIs in the presence of air and in the absence of air. Results of analyzing double bond conversion between traditional FTIR and the new approach (statistical calculation/FTIR) were compared.