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Collision Frequency (collision + frequency)

Distribution by Scientific Domains

Mathematics and Statistics	50%
Chemistry	50%

Selected Abstracts

On linear kinetic equations involving unbounded cross-sections

MATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 9 2004
Bertrand Lods
Abstract This paper deals with the spectral properties of singular neutron transport equations involving unbounded collision frequencies and unbounded collision operators. Generation results are given as well as estimates of the essential type. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Characterization of granular flow of wet solids in a bladed mixer

AICHE JOURNAL, Issue 8 2006
Azzeddine Lekhal
Abstract In this study, we measure instantaneous, average, and fluctuating velocity fields at exposed surfaces for dry and wet grains in a vertical cylindrical mixer, agitated by four pitched blades. When the material is dry, the free surface of the granular bed deforms, rising where the blades are present, and falling between blade passes. Although average velocities are predominantly azimuthal, instantaneous velocities tracked in time reveal three-dimensional particle circulations, including significant periods of particle motion in the opposite direction to that of the blades, indicative of bed penetration. When moisture is added to the solid particles, the flow dynamics change from a regime dominated by the motion of individual grains to a regime controlled by the motion of small clumps that form as a result of the cohesive forces. This transition is characterized by a reduced particle,particle collision frequency and exhibits a sharp decrease in the granular temperature at the free surface. This transition is also characterized by an increase in bed porosity, which is attributed to increased cohesiveness arising from liquid bridges. A Fourier transform analysis conducted on the tangential component of the velocities (dominant flow) shows that a group of high frequencies exceeding the blade rotation frequency become significant with added moisture. These are characteristics of the large number of wet agglomerates flowing between successive blade passes. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source]

A new linear ion trap mass spectrometer

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 6 2002
James W. Hager
Characteristics of mass selective axial ion ejection from a linear quadrupole ion trap in the presence of an auxiliary quadrupole field are described. Ion ejection is shown to occur through coupling of radial and axial motion in the exit fringing fields of the linear ion trap. The coupling is efficient and can result in extraction of as much as 20% of the trapped ions. This, together with the very high trapping efficiencies, can yield high sensitivity mass spectral responses. The experimental apparatus is based on the ion path of a triple quadrupole mass spectrometer allowing either the q2 collision cell or the final mass analysis quadrupole to be used as the linear trap. Space charge induced distortions of the mass resolved features while using the pressurized q2 linear ion trap occur at approximately the same ion density as reported for conventional three-dimensional ion traps. These distortions are, however, much reduced for the lower pressure linear trap possibly owing to the proposed axial ejection mechanism that leads to ion ejection only for ions of considerable radial amplitude. RF heating due to the high ejection q -value and the low collision frequency may also contribute. Two hybrid RF/DC quadrupole-linear ion trap instruments are described that provide high sensitivity product ion scanning while operated in the linear ion trap mode while also retaining all conventional triple quadrupole scan modes such as precursor ion and neutral loss scan modes. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Barium Sulphate Agglomeration in a Pipe , An Experimental Study and CFD Modeling

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2003
J. Ba
Abstract Agglomeration effects, observed during precipitation of barium sulphate in the unpremixed feed two-dimensional tubular precipitator, are studied experimentally and interpreted theoretically. Effects of process parameters on precipitation,agglomeration phenomena are predicted using a CFD based model that describes micromixing (the multiple-time-scale turbulent mixer model is used) and precipitation (including nucleation, growth and agglomeration of crystals). Agglomeration rate is defined as a product of the collision frequency and the probability of agglomeration. [source]

The Andersen thermostat in molecular dynamics

COMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 1 2008
Weinan E
We carry out a mathematical study of the Andersen thermostat [1], which is a frequently used tool in molecular dynamics. After reformulating the continuous- and discrete-time Andersen dynamics, we prove that in both cases the Andersen dynamics is uniformly ergodic. A detailed numerical analysis is presented, establishing the rate of convergence of most commonly used numerical algorithms for the Andersen thermostat. Transport properties such as the diffusion constant are also investigated. It is proved for the Lorentz gas model where there is intrinsic diffusion that the diffusion coefficient calculated using the Andersen thermostat converges to the true diffusion coefficient in the limit of vanishing collision frequency in the Andersen thermostat. © 2007 Wiley Periodicals, Inc. [source]

On singular mono-energetic transport equations in slab geometry

MATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 13 2002
Mohamed Chabi
In this paper we establish the well posedness of the Cauchy problem associated to transport equations with singular cross-sections (i.e. unbounded collisions frequencies and unbounded collision operators) in L1 spaces for specular reflecting boundary conditions. In addition, we discuss the weak compactness of the second-order remainder term of the Dyson,Phillips expansion. This allows us to estimate the essential type of the transport semigroup from which the asymptotic behaviour of the solution is derived. The case of singular transport equations with periodic boundary conditions is also discussed. The proofs make use of the Miyadera perturbation theory of positive semigroups on AL -spaces. Copyright © 2002 John Wiley & Sons, Ltd. [source]