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Ventura A.,University of Colorado at Colorado Springs | Ketsdever A.,University of Colorado at Colorado Springs | Webb R.,University of Colorado at Colorado Springs | Alexeenko A.,Purdue University | And 2 more authors.
AIP Conference Proceedings | Year: 2012

Thermophoretic force on a sphere in rarefied gas is studied experimentally and computationally for Knudsen numbers on the order of 0.1. Both experiment and numerical modeling have shown the force maximum for a Knudsen number of about 0.3 based on the sphere diameter. The measured and computed maxima agree within the error bars of the experiment and simulation. The phenomenon of negative thermophoresis, where the force on the sphere acts in the direction of temperature gradient (cold to hot) is established numerically and, for the first time, experimentally. © 2012 American Institute of Physics.


Gimelshein N.,Gimel Inc. | Gimelshein S.,University of Southern California | Pradzynski C.C.,University of Gottingen | Zeuch T.,University of Gottingen | Buck U.,Max Planck Institute for Dynamics and Self-Organization
Journal of Chemical Physics | Year: 2015

A hybrid Lagrangian-Eulerian approach is used to examine the properties of water clusters formed in neon-water vapor mixtures expanding through microscale conical nozzles. Experimental size distributions were reliably determined by the sodium doping technique in a molecular beam machine. The comparison of computed size distributions and experimental data shows satisfactory agreement, especially for (H2O)n clusters with n larger than 50. Thus validated simulations provide size selected cluster temperature profiles in and outside the nozzle. This information is used for an in-depth analysis of the crystallization and water cluster aggregation dynamics of recently reported supersonic jet expansion experiments. © 2015 AIP Publishing LLC.


Gimelshein N.,Gimel Inc. | Gimelshein S.,University of Southern California | Lilly T.,University of Colorado at Colorado Springs | Moskovets E.,Masstech, Inc.
Journal of the American Society for Mass Spectrometry | Year: 2014

Gas and ion transport in the capillary-skimmer subatmospheric interface of a mass spectrometer, which is typically utilized to separate unevaporated micro-droplets from ions, was studied numerically using a two-step approach spanning multiple gas dynamic regimes. The gas flow in the heated capillary and in the interface was determined by solving numerically the Navier-Stokes equation. The capillary-to-skimmer gas/ion flow was modeled through the solution of the full Boltzmann equation with a force term. The force term, together with calculated aerodynamic drag, determined the ion motion in the gap between the capillary and skimmer. Three-dimensional modeling of the impact of the voltage applied to the Einzel lens on the transmission of doubly charged peptide ions through the skimmer orifice was compared with experimental data obtained in the companion study. Good agreement between measured and computed signals was observed. The numerical results indicate that as many as 75% of the ions that exit from the capillary are lost on the conical surface of the skimmer or capillary outer surface because of the electrostatic force and plume divergence. [Figure not available: see fulltext.] © 2014 American Society for Mass Spectrometry.


Ketsdever A.,University of Colorado at Colorado Springs | Gimelshein N.,Gimel Inc. | Gimelshein S.,University of Southern California | Selden N.,Tanner Research, Inc.
Vacuum | Year: 2012

Many of the major scientists of the late 19th and early 20th century have weighed in on the mechanisms that influence the forces produced in the Crookes radiometer. However, even today, there is still some uncertainty, and in fact confusion, about the order of magnitudes of forces produced by these mechanisms and the dominant mechanisms in various operational regimes and configurations. The development of a better understanding of the radiometric force, aimed at resolving that confusion, has been helped by modern computational techniques. In the transitional flow regime, where the radiometric force is observed, kinetic approaches are necessary for valid results. These modern kinetic approaches, coupled with significant increases in computational speed, have qualitatively improved the level of understanding of radiometric flows. For the first time, flowfields near the vanes of a radiometer can be fully investigated in an attempt to quantify the importance of various mechanisms involved in the production of the radiometric force. In many cases, the computational techniques have been validated by highly accurate experiments. Various operational regimes and geometric configurations have been studied in an attempt to maximize the force. This review looks at the historic context of three types of radiometers, most notably the Crookes radiometer, develops a modern understanding of the mechanisms behind the radiometric force, and documents recent applications. © 2012 Elsevier Ltd. All rights reserved.


Selden N.,Tanner Research, Inc. | Gimelshein N.,Gimel Inc. | Gimelshein S.,Gimel Inc. | Ketsdever A.,University of Colorado at Colorado Springs
AIP Conference Proceedings | Year: 2012

The force that acts on a thin vane immersed in rarefied gas when a temperature gradient is imposed along or across the vane has historically been known as the Radiometric force. First observed by Fresnel in 1825, the radiometric force has regained its former popularity in recent decades due to the advent of micro-machines, where a transitional flow regime can occur at atmospheric pressures. Whether used for its force potential or simply viewed as a nuisance, this force cannot be ignored in micro-devices where thermal gradients exist. Potential applications of radiometric force now span from atomic force microscopy to astrophysics to high altitude flight. This paper describes an application of these forces to a conceptual micro-scale energy harvester, where two possible geometries of operation are described. It is shown that one configuration is significantly simpler to fabricate while the other geometry is more efficient at producing larger forces. The effect of pressure, feature separation, and feature-to-ring gap are analyzed. For consistency and the accurate treatment of the relevant flow conditions, an implementation of the SMOKE code that solves the ES BGK equation was used in all computations. © 2012 American Institute of Physics.


Bosworth R.W.,University of Colorado at Colorado Springs | Ventura A.L.,University of Colorado at Colorado Springs | Ketsdever A.D.,University of Colorado at Colorado Springs | Gimelshein S.F.,Gimel Inc.
Journal of Fluid Mechanics | Year: 2016

The rarefied gas flow phenomenon of thermophoresis is studied experimentally on a macroscopic spherical particle with a diameter of 5.1 cm for pressures ranging from 0.01 to 10 Pa (Knudsen numbers Kn from 10 to 0.01, respectively). Size scaling with matching Knudsen numbers makes the results applicable to microscale particles such as aerosol droplets at atmospheric pressure. Two sets of measurements are presented. The first set, complemented by numerical modelling based on the solution of the ellipsoidal statistical Bhatnagar-Gross-Krook kinetic equation, is focused on a spherical particle of high thermal conductivity in close proximity to a heated wall. The second set is conducted for the same particle placed in a linear thermal gradient established between two parallel walls. Results show the first reproducible measurements of negative thermophoretic force acting on a spherical particle in the direction from cold to hot, with values of the order of 5% of the maximum hot to cold force production. © 2016 Cambridge University Press.


Ventura A.,University of Colorado at Colorado Springs | Gimelshein N.,Gimel Inc. | Gimelshein S.,University of Colorado at Colorado Springs | Ketsdever A.,University of Colorado at Colorado Springs
Journal of Fluid Mechanics | Year: 2013

A numerical and experimental study of radiometric forces on vanes of different thickness is presented for the flow regime where the radiometric force is near its maximum. For single-and multi-vane geometries, it is shown that radiometric force decreases by only ∼10-15 % when the vane thickness-to-height ratio increases fourfold from 0.5 to 2. For a single-vane geometry, the shear force on the lateral side of the vane is attributed to a vortex flow generated by the interaction of cold chamber walls and heated walls of the vane. In that case, it always acts to reduce the total radiometric force governed by the pressure difference between the hot and the cold sides of the vane. For a multi-vane geometry, represented by a perforated vane, the shear force becomes positive for larger thickness-to-height ratios and lower pressures, primarily due to strong vane-driven transpiration flow through the gaps. © 2013 Cambridge University Press.

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