Measurements of thermophoretic velocities of aerosol particles in microgravity conditions in different carrier gases

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Abstract

Measurements of the thermophoretic velocities of aerosol particles (paraffin) in different carrier gases (helium, nitrogen, argon, xenon) were performed in microgravity conditions (the drop tower facility, in Bremen). The experiments permitted the study of thermophoresis in conditions which minimize the impact of gravity.

Monodisperse aerosol particles were observed through a digital holographic velocimeter, a device allowing the determination of 3-D coordinates of particles in the viewing volume. Particle trajectories, and consequently particle velocities, were reconstructed by analysing the sequence of particle positions. We successfully observed thermophoretic velocities in low-gravity conditions. The experiments show that the thermophoretic velocity decreases from helium (He) to nitrogen (N2), argon (Ar), and xenon (Xe).

Talbot et al. [1980. Thermophoresis of particles in a heated boundary layer. Journal of Fluid Mechanics 101, 737–758] predict thermophoretic velocities that nearly equal the observed values in Xenon, but are larger than observed values in N2 and Ar and smaller than the observed values in He.

Yamamoto and Ishihara [1988. Thermophoresis of a spherical particle in a rarefied gas of a transition regime. Physics of Fluids 31, 3618–3624] predict thermophoretic velocities that are smaller than observed values and also predict negative values in N2, Ar and Xe.

Beresnev's theory [1995. Thermophoresis of a spherical particle in a rarefied gas: Numerical analysis based on the model kinetic equations. Physics of Fluids 7, 1743–1756] fits the experimental data well when the coefficient of tangential momentum accommodation is set to one and the coefficient of energy accommodation is set to a value between 0.4 and 0.9, depending upon the gas.

Introduction

Thermophoresis describes a phenomenon in which particles suspended in a fluid with nonuniform temperature are subject to a force, named thermophoretic force, which is counteracted by the fluid drag on the particle. In steady state, particles move at a constant velocity due to equal thermophoretic and drag forces, known as thermophoretic velocity.

In addition to intrinsic theoretical interest, thermophoresis plays a role in the scavenging of aerosol particles in cloud, and is of considerable importance in many industrial processes: scale formation on the surfaces of heat exchangers, with the consequent reduction of heat transfer efficiency; micro-contamination control in the semiconductor industry; chemical vapour deposition processes for the manufacture of high-quality optical fibres; the performance of thermal precipitators in removing small particles from gas streams; the increase in the deposition of diesel or gasoline car exhaust particles on the surface of the oxidation catalyst.

Experiments to measure the thermophoretic force or velocity have mainly been performed in normal gravity, with monoatomic (Ar, He) and polyatomic gases (Air, N2, CO2), employing a variety of experimental methods.

In normal gravity it is not possible to study the phoretic effect alone, as particles move due to gravity and due to natural convection resulting from temperature gradients established to study thermophoresis.

Such difficulties can be overcome by utilizing a microgravity environment, e.g. parabolic flights or drop tower facilities.

Section snippets

Theoretical background and previous experiments in microgravity

Theoretical investigations suggest that thermophoretic velocity depends on many factors, i.e. the temperature gradient, thermal conductivities of gases and particles, Knudsen number (defined as the ratio λ/r, where λ is the molecular mean free path of the fluid surrounding an aerosol particle, and r is the radius of the particle), and momentum and energy accommodation coefficients of gas molecules on the particle surface.

Such parameters influence the thermal and viscous slip flow and the flow

Experimental set-up

The experimental procedures, similar to those described in Prodi et al. (2006), are briefly mentioned here.

The microgravity experiments were carried in the drop tower facility, which provides a weightlessness stage of about 4.7 s under free fall conditions with residual acceleration of about 10-6g0.

The experimental apparatus is housed in a special pressurized capsule. The electronic system of the Bremen drop tower comprises a ground station computer system, a telemetry remote

Results

Fig. 2, Fig. 3 report the thermophoretic velocity (Eq. (4)), normalized to 315 K, vs. Knudsen number, obtained from microgravity experiments performed on different gases. It can be observed that the thermophoretic velocity decreases from He, N2, Ar, Xe.

In order to explain this trend, it must be borne in mind that one of the factors determining the movement of an aerosol particle in the presence of a thermal gradient is thermal creep. Utilizing gas kinetic theory, Maxwell (1879) predicted that

Conclusions

The aim of the experiments presented here is to evaluate the influence of the physical parameters of different gases on the thermophoretic velocity and thermophoretic force of aerosol particles.

Previous experiments in microgravity conditions (Prodi et al., 2006) did not evidence a clear dependence of Vth on the thermal conductivity of the aerosol. This outcome disagrees with Talbot and Yamamoto theories, but agrees with data obtained in 1g experiments (Keng and Orr, 1966, Li and Davis, 1995,

Acknowledgements

The authors wish to thank the ESA PRODEX Programme and the Belgian Federal Science Policy Office for their support. They also extend thanks to the Bremen Drop Tower Management and the Service Company ZARM FABmbH (Germany).

Special acknowledgement goes to Marcello Tercon and André Mullaert for the technical assistance provided. The contribution of C. Minetti and P. Queeckers in the equipment installation and software installation is greatly appreciated.

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