Values Of Accommodation Coefficient Research Articles

Modeling liquid–vapor phase change experiments: Cryogenic hydrogen and methane

Mass accommodation coefficients are essential inputs to kinetic models of liquid–vapor phase change, yet after nearly 100 years there remains significant discrepancy in reported values. These discrepancies have been attributed to a wall material or geometric dependency resulting in the need for an empirical correction factors. The lack of experimental results for cryogenic fluids poses a serious impediment to modeling/predicting propellant behavior for long term space missions. Using a combination of neutron imaging experiments and multi-scale modeling, mass accommodation coefficients for liquid hydrogen and methane are determined. When the local variation in thermophysical properties are accounted for, the experimentally derived accommodation coefficients for hydrogen are invariant to container size, material and evaporation rate. The discrepancy in prior measurements of the accommodation coefficient for other fluids can be alleviated by a multi-scale analysis that incorporates local variation in thermophysical properties. The values of accommodation coefficients for hydrogen and methane are consistent with generalized transition state theory. This suggests that a mass accommodation coefficient is a solely a function of the liquid–vapor density ratio, making it a fluid-independent property easily determined without the need for empirical correction factors as reported in previous investigations.

Results from neutron imaging phase change experiments with LH2 and LCH4

Predicting evaporation and cryo-storage behavior of liquid hydrogen poses a challenge for both terrestrial energy infrastructure and long term space missions. The current understanding of cryogenic phase change and subsequent boil-off is limited, in part, because the values of accommodation coefficients (inputs to phase change models) are still lacking and experimental data to compute them are severely limited. In order to determine the accommodation coefficients, a new cryo/neutron imaging experiment was developed. Tests were conducted in the BT-2 Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) by introducing propellant vapor into cylindrical aluminum (Al 6061) and stainless steel (SS 316) cells placed inside a 70 mm cryostat. Saturation points between 80 kPa - 230 kPa were tested for liquid H2 and CH4. Phase change was induced through precise control of pressure and/or temperature. Neutron imaging was used to visualize the liquid and evaporation/condensation rates were determined through image processing. Both hydrogen and methane are perfectly wetting fluids as evidenced by the formation of micro-scale thin film at the onset of condensation and varies in thickness with rate of phase change. The rates of phase change are a function of both the size of the container and the degree of offset from saturation. The unique experimental data, made available through a data repository, have the potential to serve as a bench mark for future studies or serve as a dataset for model validation.

Thermophoretic force on a sphere of arbitrary thermal conductivity in a rarefied gas

The thermophoretic force on a sphere with arbitrary thermal conductivity immersed in a rarefied gas subject to a uniform temperature gradient is calculated in the framework of kinetic theory of gases in a range of the Knudsen number which covers the free molecular, transition and continuum regimes of the gas flow around the sphere. The condition of continuity of the radial heat flux at the gas–solid interface is used to calculate the thermophoretic force as a superposition of the thermophoretic force on a sphere with an uniform temperature and the radiometric force on a sphere due to its thermal polarization. Both solutions were previously obtained from the Shakhov model to the linearized Boltzmann equation and the Cercignani–Lampis model of gas–surface interaction. To analyze the influence of the gas and particle thermal conductivities on the thermophoretic force, spheres of PolyStyrene Latex (PSL), glass and nickel in helium and argon gases are considered. The force is calculated for several values of tangential momentum accommodation coefficient and normal energy accommodation coefficient as introduced in the Cercignani–Lampis scattering kernel. A comparison with experimental data available in the literature allowed to extract the values of the accommodation coefficients for helium interaction with the particle surface.

Effect of internal degrees of freedom in rarefied gas problems: Plane Couette flow

Present paper studies the influence of internal degrees of freedom (IDOF) on rarefied gas flow in a classic plane Couette problem. The numerical study is performed using Direct Simulation Monte Carlo method in a wide range of gas rarefaction, plates relative velocities and for different values of accommodation coefficients (ACs) on the walls. It is shown that: (1) in presence of IDOF a noticeable thermal non-equilibrium between translational and internal energy modes is observed, which increases with gas rarefaction and with plates relative velocity; (2) presence of IDOF can noticeably decrease shear stress and heat flux to the walls, which is mainly explained by the drain of energy from over-excited translational mode into internal ones, leading to lower values of translational temperature and, hence, viscosity; (3) the magnitude of IDOF influence is a non-monotonic function of gas rarefaction, taking maximal values in transitional regimes Kn≈1; (4) values of ACs can significantly change the influence of IDOF, both qualitatively and quantitatively; (5) the rate of energy exchange between translational and internal modes has a noticeable influence on the results, but the energy exchange model itself is not principal as long as this rate is reproduced correctly; (6) increasing number of IDOF leads to more noticeable influence on shear stress and heat flux. Therefore the influence of IDOF will be more significant for polyatomic gases.

Radiometric force on a sphere in a rarefied gas based on the Cercignani–Lampis model of gas–surface interaction

The radiometric force on a sphere due to its thermal polarization in a rarefied gas flow being in equilibrium is investigated on the basis of a kinetic model to the linearized Boltzmann equation. The scattering kernel proposed by Cercignani and Lampis to model the gas–surface interaction using two accommodation coefficients, namely, the tangential momentum accommodation coefficient and the normal energy accommodation coefficient, is employed as the boundary condition. The radiometric force on the sphere, as well as the flow field of the gas around it, is calculated in a wide range of the gas rarefaction, defined as the ratio of the sphere radius to an equivalent free path of gaseous particles, covering the free molecular, transition, and continuum regimes. The discrete velocity method is employed to solve the kinetic equation numerically. The calculations are carried out for values of accommodation coefficients considering most situations encountered in practice. To confirm the reliability of the calculations, the reciprocity relation between the cross phenomena is verified numerically within a numerical error of 0.1%. The temperature drop between two diametrically opposite points of the spherical surface in the direction of the gas flow stream, which characterizes the thermal polarization effect, is compared to experimental data for a spherical particle of Pyrex glass immersed in helium and argon gases.

Driving mechanism of thermal transpiration pump with porous material

The behavior of the rarefied gas in the thermal transpiration pump with the porous material is investigated numerically by the direct simulation Monte Carlo method. The mass flux achieved by the pump is analyzed for a wide range of Knudsen numbers and ratios of the pore length to the pore diameter. The results show that the thermal edge flows around the ends of the pore play an essential role in determining the maximum performance. The effect of the thermal edge flow leads to a qualitative difference in the driving mechanism from a similar thermal transpiration pump by Knudsen. The mass flow takes a maximum value at a considerably large Knudsen number when the pore length is much larger than the pore diameter. The numerical tests show that a larger mass flux is possible when the edge flow is suppressed. The mass flux is investigated for several values of accommodation coefficient and complex pore geometries. The present results show that only the latter leads to the reduction in the mass flux. The compression ratio, including the performance curve of the pump, is also analyzed for several cases. The results show that the small accommodation coefficient decreases the compression ratio of the pump.

The influence of particle composition upon the evolution of urban ultrafine diesel particles on the neighbourhood scale

Abstract. A recent study demonstrated that diesel particles in urban air undergo evaporative shrinkage when advected to a cleaner atmosphere (Harrison et al., 2016). We explore, in a structured and systematic way, the sensitivity of nucleation-mode diesel particles (diameter < 30 nm) to changes in particle composition, saturation vapour pressure, and the mass accommodation coefficient. We use a multicomponent aerosol microphysics model based on surrogate molecule (C16−C32 n-alkane) volatilities. For standard atmospheric conditions (298 K, 1013.25 hPa), and over timescales (ca. 100 s) relevant for dispersion on the neighbourhood scale (up to 1 km), the choice of a particular vapour pressure dataset changes the range of compounds that are appreciably volatile by two to six carbon numbers. The nucleation-mode peak diameter, after 100 s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between C22H46 and C24H50. The vapour pressure range, derived from published data, is between 9.23 × 10−3 and 8.94 × 10−6 Pa for C22H46 and between 2.26 × 10−3 and 2.46 × 10−7 Pa for C24H50. Therefore, the vapour pressures of components in this range are critical for the modelling of nucleation-mode aerosol dynamics on the neighbourhood scale and need to be better constrained. Laboratory studies have shown this carbon number fraction to derive predominantly from engine lubricating oil. The accuracy of vapour pressure data for other (more and less volatile) components from laboratory experiments is less critical. The influence of a core of non-volatile material is also considered; non-volatile core fractions of more than 5 % are inconsistent with the field measurements that we test the model against. We consider mass accommodation coefficient values less than unity and find that model runs with more volatile vapour pressure parameterisations and lower accommodation coefficients are similar to runs with less volatile vapour pressure parameterisations and higher accommodation coefficients. The new findings of this study may also be used to identify semi-volatile organic compound (SVOC) compositions that play dominating roles in the evaporative shrinkage of the nucleation mode observed in field measurements (Dall'Osto et al., 2011).

Study of Slip Flow in Unconventional Shale Rocks Using Lattice Boltzmann Method: Effects of Boundary Conditions and TMAC

Development and production from unconventional resources require understanding of flow mechanisms and rock/fluid interactions, which are different from those in conventional reservoirs. Several flow regimes including slip and transition can be dominated in these reservoirs due to the presence of micro- and nano-pores. Recently the lattice Boltzmann method has been considered as a well-accepted method for simulation of slip flow in shale rocks. In this paper, the two-relaxation-time-based Lattice Boltzmann approach was adopted to simulate the slip flow in a single channel and a simplified 2D and 3D porous media. Different boundary conditions including diffusive reflective and bounce back-specular reflection were used to capture the gas slippage at the wall surface. The simulation results were compared with the experimental data measured on three shale rock samples. The results showed that the gas slippage in the shale samples under study cannot be modelled using the literature tangential momentum accommodation coefficient values. It was also shown that the permeability enhancement was overestimated for Kn > 0.1 when the gas flow was simulated in a single channel. Furthermore, from the fitted line to the simulation results (up to Kn = 0.3), the first- and second-order slip coefficients of the N–S boundary conditions were found as 2 and 1.54, respectively.

Influence of the gas-surface interaction model on time-dependent rarefied gas simulations

In this paper, the influence of the gas-surface interaction on the time-dependent rarefied gas flow through a short tube into vacuum is investigated. Due to a significant scale separation, the flow is simulated using a hybrid scheme proposed by Vargas et al. [1, 2], in which the pressure change in the upstream chamber is coupled to the flow rate obtained by the Direct Simulation Monte Carlo (DSMC) computations. First, the influence of gas-surface interaction is demonstrated through comparison of DSMC simulation results obtained for different values of accommodation coefficients. The obtained dataset is then used to show how the boundary conditions can affect the time-dependent gas flow, even when applied on the small surface area of the device. It is argued that the domain of possible solutions is bounded by the cases of specular and diffuse scattering. Furthermore, it is investigated how this domain can be affected by measurement uncertainties present in the experimental setup. Next, the analysis of the gas-surface interaction is extended to binary mixtures. In the end, it is discussed how the obtained results can be used in order to improve the efficiency of gas separation mechanisms.

Modeling wall effects in a micro-scale shock tube using hybrid MD–DSMC algorithm

Wall effects in a micro-scale shock tube are investigated using the Direct Simulation Monte Carlo method as well as a hybrid Molecular Dynamics–Direct Simulation Monte Carlo algorithm. In the Direct Simulation Monte Carlo simulations, the Cercignani–Lampis–Lord model of gas–surface interactions is employed to incorporate the wall effects, and it is shown that the shock attenuation is significantly affected by the choice of the values of tangential momentum accommodation coefficient. A loosely coupled Molecular Dynamics–Direct Simulation Monte Carlo approach is then employed to demonstrate incomplete accommodation in micro-scale shock tube flows. This approach uses fixed values of the accommodation coefficients in the gas–surface interaction model, with their values determined from a separate dynamically similar Molecular Dynamics simulation. Finally, a completely coupled Molecular Dynamics–Direct Simulation Monte Carlo algorithm is used, wherein the bulk of the flow is modeled using Direct Simulation Monte Carlo, while the interaction of gas molecules with the shock tube walls is modeled using Molecular Dynamics. The two regions are separate and coupled both ways using buffer zones and a bootstrap coupling algorithm that accounts for the mismatch of the number of molecules in both regions. It is shown that the hybrid method captures the effect of local properties that cannot be captured using a single value of accommodation coefficient for the entire domain.

Numerical investigation of the effect of boundary conditions for a highly rarefied gas flow using the GPU accelerated Boltzmann solver

The effect of the gas–surface interaction model on the rarefied gas flow between parallel plates is investigated on the basis of the Boltzmann kinetic equation. The Cercignani–Lampis model for diffuse scattering with incomplete energy accommodation is provided as the boundary condition on plates. The numerical analysis of the heat transfer problem between parallel plates with uniform and sinusoidal temperature distributions is carried out. The computational algorithm is adapted for solving the Boltzmann equation onto Graphics Processing Units (GPUs). The speedup of the GPU-accelerated computation is up to 50 times as compared to the CPU one. It was found that a non-uniform temperature distribution on plates induced a steady flow. In addition, the flow field strongly depends on the value of accommodation coefficients imposed in the Cercignani–Lampis model and this effect is more visible for high Knudsen numbers. Presented results are in good agreement with open literature ones obtained by means of the direct simulation Monte Carlo method.

Detailed Microphysical Modeling of the Formation of Organic and Sulfuric Acid Coatings on Aircraft Emitted Soot Particles in the Near Field

The development of a detailed microphysical model that describes the complex multicomponent interactions between organic vapors and soot particles emitted from aircraft gas turbine engines is presented. Our model formulation includes both soot surface activation by organic vapors and organic vapor condensation on the activated part of the soot surfaces. To enable this formulation, approaches to estimate chemical and physical properties of aerosols containing complex mixtures of sulfuric acid, water, and organic molecules were developed. Relevant distributions of a list of organic surrogates at the engine exit plane were used to represent complex organic emissions from aircraft engines. A parametric study was performed using this new formulation to understand the effects of ambient conditions, organic emissions levels, and mass accommodation coefficient values on the evolution of near field volatile particulate matter emissions from aircraft engines at ground level.Copyright 2014 American Association for Aerosol Research

Velocity distribution and velocity correlation of mixture of gases in a nanochannel

Molecular dynamics study of non-reacting and disparate mass binary gas mixture is conducted. To investigate the gas–surface interaction properties of the gas mixture, in non-continuum regime, the rarefied gas confined in a nanochannel is considered. While keeping the bulk number density a constant, the interaction properties of gas mixtures at different molar concentration of individual components are determined. The study discusses the calculation of accommodation coefficients and prediction of the reflected gas distribution using accommodation coefficients. The Maxwell-type model that use calculated values of accommodation coefficients accurately predicts the reflected tangential and normal distributions. The model is validated for different wall-fluid conditions. The velocity correlation of individual gases demonstrates that the individual components of gas mixture are accommodated in different proportion. The result from the study will be beneficial in the analysis and development of potential applications involving the flow of gas mixture in a nanochannel.

Computational modelling of fluid structure interaction at nano-scale boundaries with modified Maxwellian velocity distribution

The soft collisions among fluid–fluid and fluid-wall molecules are modeled from first principles. In particular, the assumption of Maxwellian distribution of velocities for thermalized molecules, in both parallel and perpendicular directions to the wall, has been re-evaluated with supporting experimental and/or numerical evidence.It is proposed that the normal component of molecular velocity post collision is conserved for all fluid molecules. The slip effect at the wall boundary, introduced by the surface roughness, is accounted by an accommodation coefficient f. A moving least square method is used to calculate macroscopic velocity values. The influence of molecular interaction on the macroscopic velocity distribution is investigated at 40MPa and 300K for slit pore, inclined and stepped wall configurations. The accommodation coefficient values f=0, 0.07, 0.257, 0.45, 0.681 and 1; and acceleration values ranging from zero to 1×1011m/s2 and 250×1011m/s2 are used for comparison.The distribution of macroscopic velocity parallel to the wall is studied to observe the effect of the slip behaviour. The detailed study of average of velocity values at various magnitudes of acceleration has shown an evidence of characteristic low and high speed of molecular flows that is considered as significant and a comparison is sought with an equivalent laminar and turbulent flow style behaviour. The two dimensional vector and contour plots of macroscopic velocity provide further insights in understanding Continuum velocity distributions resulting from molecular fluid-wall interaction at nanoscale. The research has highlighted the need to develop molecular dynamics simulation techniques for non-periodic boundary conditions.

New method of calculating adsorption and scattering for Xe-Pt(111) using Direct Simulation Monte Carlo techniques

A new scheme is presented for calculating adsorption and scattering for a gas–surface system in the thermal scattering regime using Direct Simulation Monte Carlo techniques. Results are compared with experimental data for the Xe-Pt(111) gas–surface system at surface temperatures of 95 K (adsorption) and 800 K (scattering). Fair agreement is found for the initial adsorption probability, while good agreement is found for the adsorption probability versus coverage. When implemented with the Cercignani–Lampis–Lord scattering kernel, the new scheme was found to improve scattering properties. Any scattering kernel or method amenable to Direct Simulation Monte Carlo techniques may be applied with the new scheme. Optimal values of accommodation coefficients were also found for the surface (αn = 0.74, αt = 0.60) and the adlayer (αn,ad = 0.89, αt,ad = 0.60).

Molecular-dynamics study of Poiseuille flow in a nanochannel and calculation of energy and momentum accommodation coefficients

We report a molecular-dynamics study of flow of Lennard-Jones fluid through a nanochannel where size effects predominate. The momentum and energy accommodation coefficients, which determine the amount of slip and temperature jumps, are calculated for a three-dimensional Poiseuille flow through a nano-sized channel. Accommodation coefficients are calculated by considering a " gravity"- (acceleration field) driven Poiseuille flow between two infinite parallel walls that are maintained at a fixed temperature. The Knudsen number (Kn) dependency of the accommodation coefficients, slip length, and velocity profiles is investigated. The system is also studied by varying the strength of gravity. The accommodation coefficients are found to approach a limiting value with an increase in gravity and Kn. For low values of Kn (<0.15), the slip length obtained from the velocity profiles is found to match closely the results obtained from the linear slip model. Using the calculated values of accommodation coefficients, the first- and second-order slip models are validated in the early transition regime. The study demonstrates the applicability of the Navier-Stokes equation with the second-order slip model in the early transition regime.

Measurements of tangential momentum accommodation coefficient for various gases in plane microchannel

Mass flow rate measurements in a single silicon microchannel were carried out for various gases in isothermal steady flows. The results obtained from hydrodynamic to near free molecular regime by using a powerful experimental platform allowed us to deduce interesting information, notably about the reflection/accommodation process at the wall. In the 0–0.3 Knudsen range, a continuum analytic approach was derived from the NS equations, associated with first or second order slip boundary conditions. Identifying the experimental mass flow rate curves to the theoretical ones the tangential momentum accommodation coefficient (TMAC) of various gases was extracted. Over the full Knudsen range [0–30] the experimental results were compared with theoretical values calculated from the kinetic approaches: using variable accommodation coefficient values as fitting parameter, the theoretical curves were fitted to the experimental ones. Whatever the Knudsen range and whatever the theoretical approach, the TMAC values are found decreasing when the molecular weights of the gas increase (as long as the different gases are compared using the same approach). Moreover, the values of the various accommodation coefficients are rather close to one another but sufficiently smaller than unity indicating that the full accommodation modeling is not satisfactory to describe the gas/wall interaction.

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.

Transfer of gas-ion momentum and energy to an electrically conductive surface partially coated by a thin dielectric layer

Thedynamic interaction of bodies with a rarefied gas flow is characterized to a significant extent by exchange of momentum and energy, or the corresponding accommodation coefficients. The momentum and energy accommodation coefficients are used to determine aerodynamic characteristics and heat exchange of bodies in a rarefied medium and are an important element of the computation relationships no matter what pattern of gas-atom interaction with the surface is chosen. The present study will examine interaction of gas atoms with purely crystalline structures. There are available a significant numberof studies which have performed numerical simulations of atomic particle collisions with a solid surface and offered approximate analytical solutions characterizing the mechanism of gas-atom momentum and energy transfer to ideal crystalline surfaces [i, 2]. In practical targets with an ideal single-crystal structure are rarely found. In the majority of cases the surface flowed over a polycrystalline structure with individual crystallites randomly oriented. In numerical study of collision of atomic particles with an atomically smooth polycrystalline surface it is necessary to average the interaction characteristics, which significantly complicates the problem [3]. The situation becomes even more complicated for technological materials such as alloys with complex structure and surface relief. The literature provides an insufficient volume of data on calculated and experimental values of gas particle accommodation coefficients for the velocity range u~ ~ i0 km/sec which is of practical interest in aerodynamics. Experimental values of the accommodation coefficients are few in number, and refer to varied experimental conditions. Calculations of aerodynamic characteristics and heat exchange of bodies moving in a rarefied medium require knowledge of a complex of parameters characterizing the dynamic interaction of the body with the incident flow. In connection with this it becomes necessary to perform general studies to determine the gas-particle momentum and energy accommodation coefficients on the surface flowed over. The present study will experimentally investigate the effects of a number of factors characterizing interaction of a gas with a surface upon the values of the momentum and energy accommodation coefficients of gas ions with atomic masses from 4 to 131 on the surface of an aluminized polymer film with a conductive face surface coated by a glass screen (dielectric grid with transparency coefficient of %0.12), the outer surface of vacuumscreen thermal insulation [4]. i. The force action of a flow of partially ionized gas of low density on a surface being flowed over having a "floating" potential is determined by bombardment of electrons, ions, rapid and slow neutral particles produced by ion charge exchange with the residual gas, metastable particles, etc.:

Comparative study of accommodation coefficients of helium and neon on clean tungsten under transition, temperature jump, and free molecule conditions

Measurements of heat conduction by neon and helium between concentric cylinders under free molecule, transition, and temperature jump conditions have been made with particular care to insure that a well defined clean surface state of the inner (tungsten) cylindrical surface was maintained. Several kinetic theoretical treatments applicable to the various regimes have been utilized to calculate the thermal accommodation coefficients of these gases. Comparison of these accommodation coefficient values obtained by analysis of the data in the transition and temperature jump regimes by the Smoluchowski-Kennard temperature jump method, the Langmuir-Wachman mean free path method, and the Lees-Liu moment method with the accommodation coefficient values obtained on the same filament by the well established low pressure Knudsen method has been made, and the values are in remarkable agreement.