Thermophoretic Behavior Research Articles

Thermophoretic motion behavior of submicron particles in boundary-layer-separation flow around a droplet

As a key mechanism of submicron particle capture in wet deposition and wet scrubbing processes, thermophoresis is influenced by the flow and temperature fields. Three-dimensional direct numerical simulations were conducted to quantify the characteristics of the flow and temperature fields around a droplet at three droplet Reynolds numbers (Re) that correspond to three typical boundary-layer-separation flows (steady axisymmetric, steady plane-symmetric, and unsteady plane-symmetric flows). The thermophoretic motion of submicron particles was simulated in these cases. Numerical results show that the motion of submicron particles around the droplet and the deposition distribution exhibit different characteristics under three typical flow forms. The motion patterns of particles are dependent on their initial positions in the upstream and flow forms. The patterns of particle motion and deposition are diversified as Re increases. The particle motion pattern, initial position of captured particles, and capture efficiency change periodically, especially during periodic vortex shedding. The key effects of flow forms on particle motion are the shape and stability of the wake behind the droplet. The drag force of fluid and the thermophoretic force in the wake contribute jointly to the deposition of submicron particles after the boundary-layer separation around a droplet.

Collective thermodiffusion of colloidal suspensions

The thermophoretic behavior of concentrated colloidal suspensions can be understood as the sum of single particle and collective effects. Here, we present a simulation model to investigate the particularities of the collective thermodiffusive effects in concentrated uncharged solutions, where the influence of different colloid-colloid interactions is analyzed. The concentration dependence found in our simulations qualitatively agrees with experimental results. Colloids with repulsive interactions are found to accumulate more effectively than the solvent in the warm areas, such that the corresponding Soret coefficients are negative and decrease with increasing concentration. The accumulation of colloids in the cold regions is facilitated by attraction, such that colloids with attractive interactions have larger values of the Soret coefficient. A thermodynamic argument that explains our results from equilibrium quantities is discussed as well.

Opposite ReD-dependencies of nanofluid (Al2O3) thermal conductivities between heating and cooling modes

The dynamic thermal conductivities of nanofluids (Al2O3) in heating or cooling under fully developed laminar flow conditions show opposite dependence on Reynolds numbers, i.e., the dynamic conductivities under the heating conditions increase with increasing ReD but under the cooling conditions, the dynamic conductivities decrease with increasing ReD. Furthermore, the dynamic conductivities for cooling are higher than those for heating, and the thermal conductivities of stationary nanofluids with uniform distributions fall between these two values, for the entire tested ReD range from 300 to 800. We believe that the main reason for this distinction is because of the drastically different cross-sectional nanoparticle concentration distributions that are in turn attributed to the opposite thermophoretic behavior near the tube wall between heating and cooling. The near-wall nanoparticle concentrations for cooling are substantially higher than those for heating; however, the stationary nanofluid with no thermo...

Thermophoresis of colloids by mesoscale simulations

The motion of a colloid induced by a temperature gradient is simulated by means of multiparticle collision dynamics, a mesoscale simulation technique. Two algorithms to quantify the thermophoretic behavior are employed and contrasted. The validity of the methods is verified as a function of the temperature gradient, system size, and algorithm parameters. The variation of the solvent–colloid interaction from attractive to purely repulsive interestingly results in the change of the colloid behavior from thermophobic to thermophilic.

Impact of Thermodiffusion on Carbon Nanotube Growth by Chemical Vapor Deposition

Thermal diffusion, the process by which a multicomponent mixture develops a concentration gradient when exposed to a temperature gradient, has been studied in order to understand if its inclusion is warranted in the modeling of single-wall carbon nanotubes (SWNTs) synthesis by thermal chemical vapor deposition (CVD). A fully coupled reactor-scale model employing conservation of mass, momentum, species, and energy equations with detailed gas phase and surface reaction mechanisms has been utilized to describe the evolution of hydrogen and hydrocarbon feed streams as they undergo transport, as well as homogeneous and heterogeneous chemical reaction within a CVD reactor. Steady state velocity, temperature, and concentration fields within the reactor volume are determined, as well as concentrations of adsorbed species and SWNT growth rates. The effect of thermodiffusion in differing reactor conditions has been investigated to understand the impact on SWNT growth. Thermal diffusion can have a significant impact on SWNT growth, and the first approximation of the thermal diffusion factor, based on the Chapman–Enskog molecular theory, is sufficient for modeling thermophoretic behavior within a CVD reactor. This effect can be facilitatory or inhibitory, based on the thermal and mass flux conditions. The results of this investigation are useful in order to optimize model and reactor designs to promote optimal SWNT deposition rates.

Effect of particle morphology on thermophoretic velocity of aggregated soot particles

A particle under a temperature gradient experiences a force toward the colder region if a particle is very small. This phenomenon is called thermophoresis, a mass transfer phenomenon induced by the temperature gradient. It is well known that the magnitude of the thermophoretic force depends on the size of particle and the temperature gradient. In addition, the magnitude of the thermophoretic force depends on various factors such as the ambient gas’s kinematic viscosity and thermal conductivity and the morphology of the particle. To understand thermophoresis in detail, the effects of these factors need to be evaluated. In this study, we accurately measured the thermophoretic velocity of aggregated particles in order to understand the effect of particle morphology. We used carbon black particles of well-defined aggregation parameters to systematically understand the effect of morphology. In addition, we introduced a new optical system to measure the velocity and the size of each particle simultaneously. Five different samples of carbon black particles with different aggregation parameters were used to systematically understand the effect of morphology. The measured thermophoretic velocities were almost proportional to ν∇ T/ T. The measured dimensionless thermophoretic velocities, U T /( ν∇ T/ T), were much larger than those expected based on the size of aggregates and rather close to those expected based on the primary particle size even when the size of aggregates are larger than 100 mm. This result infers that thermophoretic velocity of an aggregated particle is governed by the primary particle size. The dimensionless density, the ratio of the bulk density to the true density, which represents the overall packing degree of aggregate, is also found to have a significant effect on the thermophoretic behavior.