Modeling of transport phenomena in gases based on quantum scattering
Introduction
The direct simulation Monte Carlo (DSMC) method [1] used to calculate rarefied gas flows consists of decoupling of the free motion of gaseous molecules from collisions between them. The second stage requires a physical intermolecular potential in order to obtain reliable results. Recently, a procedure to implement any potential into the DSMC method was proposed in our previous paper [2] using the phenomenological Lennard-Jones potential as an example. In contrast to phenomenological models, ab initio (AI) potentials are free from any adjustable parameter usually extracted from experimental data. Nowadays, such potentials practically for all noble gases and their mixtures are available in the open literature, see e.g. [[3], [4], [5], [6], [7], [8], [9], [10]]. Thus, the DSMC method based on AI potential [11] also becomes free from such adjustable parameters. The idea of the procedure to implement any potential into the DSMC is to generate look up tables of the deflection angle depending on the relative velocity of interacting particles and their impact parameter. The method was used to study the influence of the interatomic potential on various phenomena in rarefied gases [[12], [13], [14], [15], [16]] considering the intermolecular interaction based on the classical mechanics, that is justified at high temperatures for heavy gases. However, the quantum effects in intermolecular interactions is not negligible for light gases, e.g. helium, hydrogen, tritium, especially at moderately low temperatures [[17], [18], [19], [20], [21]]. It can be important to model helium, hydrogen and tritium flows in many technological fields such as cryogenic pumps [[22], [23]], cryogenic systems used in the huge fusion reactor ITER [[24], [25]], monochromatic beams of helium [[26], [27]], helium microscope [[28], [29]], acoustic thermometry at a low temperature [[30], [31]], experimental set-up to measure the neutrino mass [[32], [33]], etc. In spite of the high practical interest to model gases at low temperatures, the quantum scattering has not been implemented yet in the DSMC method.
The aim of the present paper is to propose a new technique to implement the quantum scattering into the DSMC method using any potential and to show the influence of quantum effects on transport phenomena in rarefied gases. For this purpose, a procedure of generation of deflection angle matrix based on quantum scattering is elaborated and a couple of classical problems of fluid mechanics is solved to evaluate the influence of quantum effects. A temperature range where the classical approach fails and the quantum theory becomes an unique alternative to simulate the transport phenomena in rarefied gases will be pointed out. It will be also shown that even at a high temperature when the classical approach works, the quantum approach reduces computational effort that makes it preferable for the whole range of the temperature. It should be emphasized that we are interested in quantum effects only in interatomic iterations. Other effects, like high densities at low temperatures when the interatomic distance is comparable to the de Broglie wavelength, are not considered here.
Section snippets
Numerical method
The DSMC method consists of a decoupling the free-motion of molecules from intermolecular collisions during each time steps . Here, the free-motion of particles is considered to be classical that is valid under the condition [[34], [35]] where is the gas number density, is the Planck constant, is the atomic mass of the gas, is the Boltzmann constant and is the gas temperature. This condition is well satisfied at the atmospheric pressure and at any temperature above
Differential cross section
According to the quantum theory of scattering [[17], [18], [19], [20], [21]], the DCS of undistinguishable particles with a spin consists of two terms and reads for boson and fermions, respectively. Both and are expressed via the speed and deflection angle as
Matrix of deflection angle
The vector and the matrix were calculated for helium-3 and helium-4 having the atomic masses [39] 3.01605 u and 4.00260 u, respectively. The AI potential for these two species is the same and can be found in Refs. [[3], [4], [6]]. For our purpose, the potential proposed in the work [3] was chosen as the most complete and exact at the moment. The authors of the paper [40] used this potential to calculated the viscosity and thermal conductivity of both helium-3 and helium-4. The
Examples
In order to illustrate the technique and to estimate the influence of the quantum effects, two classical problems of fluid mechanics related to transport phenomena through helium were solved.
The first problem is a heat transfer between two parallel plates fixed at . The plate at is kept at a temperature , while the other plate has a lower temperature . We are interested in the heat flux as a function of the gas rarefaction and of the equilibrium temperature .
Conclusions
An interatomic interaction based on quantum scattering was implemented into the direct simulation Monte Carlo method applied to transport phenomena through rarefied gases. Such an implementation allows us to model flows of light gases like helium over the whole temperature range beginning from 1 K up any temperature when no ionization happens. As an example, two helium isotopes He and He have been considered in two classical problems of fluid mechanics, namely, heat transfer between two
Acknowledgment
The author acknowledges the Brazilian Agency CNPq, Brazil for the support of his research, grant 303697/2014-8.
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