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Highlights • Heat and mass transfer of an oscillatory rarefied gas flow in a square cavity is investigated. • Effects of frequency and speed of the oscillating lid are studied. • The hot to cold heat transfer could be dominant for a highly rarefied oscillatory flow. • The convective heat transfer could be dramatically enhanced by lid oscillation. • The average Nusselt number on the lid is reported in all the flow regimes. Abstract Although effective cooling of micro-electro-mechanical systems (MEMS) with oscillatory components is essential for reliable device operation, the role of oscillation on heat transfer remains poorly understood. In this work, heat and mass transfer of the oscillatory gas flow inside a square cavity is computationally studied by solving the Boltzmann model equation, i.e. the Shakhov model. The oscillation frequency of the lid and rarefaction and nonlinearity of the flow field are systematically investigated. Our results show that, when the oscillation frequency of the lid increases, the usual cold-to-hot heat transfer pattern for highly rarefied flow changes to hot-to-cold, which contradicts the well-known anti-Fourier (i.e. cold-to-hot) heat transfer in a non-oscillatory lid-driven cavity. In addition, the thermal convection will be dramatically enhanced by lid oscillation, which may play a dominant role in the heat transfer. Meanwhile, the average Nusselt number varies non-monotonically with the oscillation frequency, with the maximum occurring at the anti-resonance frequency. Finally, the average Nusselt number on the lid at various oscillation frequencies is found to reduce when the gas becomes more rarefied. These findings may be useful for the thermal design of MEMS. [ABSTRACT FROM AUTHOR]

Approaches to predict flowfields that display rarefaction effects incur a cost in computational time and memory that is considerably higher than methods commonly employed for continuum flows. For this reason, to simulate flowfields where continuum and rarefied regimes coexist, hybrid techniques have been introduced. In the present work, analytically defined gas-kinetic schemes based on the Shakhov and Rykov models, for monatomic and diatomic gas flows, respectively, are proposed and evaluated, with the aim of being used in the context of hybrid simulations. This should reduce the region where more expensive methods are needed by extending the validity of the continuum formulation. Moreover, because, for rarefied gas flows at high velocities, it is necessary to take into account the nonequilibrium among the internal degrees of freedom, the extension of the approach to employ diatomic gas models with rotational relaxation is a mandatory first step toward realistic simulations. Compared to the previous works the presented scheme is defined directly on the basis of kinetic models that involve a Prandtl number correction. Moreover, the methods are defined fully analytically instead of making use of Taylor expansion for the evaluation of the required derivatives. The scheme has been tested for various test cases and Mach numbers proving to produce reliable predictions in agreement with other approaches for near-continuum flows. Finally, the performance of the scheme, in terms of memory and computational time, compared to discrete velocity methods makes it a compelling alternative in place of more complex methods for hybrid simulations of weakly rarefied flows. [ABSTRACT FROM AUTHOR]

A numerical analysis of the rarefied gas flow caused by a temperature gradient in the direction tangential to a wall through a planar channel of finite length is carried out based on the nonlinear Shakhov model of the Boltzmann kinetic equation. The Maxwell diffuse and Cercignani–Lampis gas–surface interaction models are provided as a boundary condition at solid walls. An implicit scheme for solution of the S-model kinetic equation was applied and the algorithm has been optimized for the use of massive parallelization in both physical and velocities spaces. The competition between rarefaction, gas–surface scattering model and temperature gradient effects on a gas flow is discussed in terms of both mass flow rate and streamwise heat flux. A comparison with available literature results has been carried out and showed a good agreement. It was found that the temperature gradient as well as the gas–surface scattering dynamics play a significant role for the highly rarefied gas flow, beginning from the transition flow regime. At the same time, under certain gas–surface scattering conditions a change of the value of temperature gradient does not affect the heat flux. For the weakly rarefied flow the temperature gradient effect on the mass flow rate is small for any gas–surface scattering condition, while the heat flux does not depend neither on value of temperature gradient nor choice and details of gas–surface scattering dynamics. [ABSTRACT FROM AUTHOR]

This paper presents experimental and three-dimensional numerical study of early onset of separation with rarefied gas flow through a tube with single sharp 90° bend. Experiments are conducted for nitrogen gas flowing at low pressures in three conventional size tubes. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number range (0.0003 < Kn < 0.0385) covers part of the continuum and the slip flow regime while maintaining the Reynolds number between 0.27 and 418.5. The static pressures along the inner, outer and top walls are measured for different mass flow rates and analyzed to understand the flow behavior. The static pressure measurement indicates adverse pressure gradient near the bend along the inner and outer walls of the tube at much lower value of Reynolds number as compared to conventional flow. The numerical solution of the Navier–Stokes equations with the Maxwell’s slip boundary condition shows good agreement with experimental data and helps bring out the complex flow behavior near the bend. The adverse pressure gradient, velocity profile, flow streamlines and velocity vectors in the bend plane clearly indicates secondary flows near the bend at as low a Reynolds number as unity. The flow acceleration and the presence of secondary flows near the bend causes a larger pressure drop as compared with a straight tube. Empirical correlations for Poiseuille number and additional pressure drop coefficient are proposed as part of this work. It is noted that limited experimental data exists in the literature for such flows; these results should therefore help enhance the fundamental understanding of gas flow in microchannels with bend. [ABSTRACT FROM AUTHOR]

The NACA 0012 airfoil and the sharp 25–55-deg biconic cases are usually considered in evaluating the accuracy of new forms of velocity slip and temperature jump wall boundary conditions in the Navier–Stokes–Fourier equations. These cases have been investigated by many researchers in previous works with the first order Maxwell slip and Smoluchowski jump boundary conditions. Recently, we proposed a new form of the second order temperature jump for simulating rarefied gas flows and obtained accurate results for low-speed microflows. The current work is an extension of our previous CFD work where we applied both the second order velocity slip condition and the second order temperature jump boundary condition in new forms to simulate rarefied external gas flows for these cases. The second order boundary conditions are implemented into the rhoCentralFoam solver in the OpenFOAM software. The NACA 0012 airfoil case is investigated with angles-of-attack of 0 and 10 degrees and Mach numbers of 0.8 and 2. The sharp 25–55-deg biconic case is simulated at Mach of 15.6. The simulation results obtained for the NACA 0012 airfoil show that the combination of the second order slip/jump boundary conditions gives suitable agreements with DSMC for surface gas temperature and drag coefficients in comparison with those of the first order conditions. However, the results of the biconic case figure out that the second order conditions are not superior to the first order conditions. [ABSTRACT FROM AUTHOR]

In a rarefied gas the temperature field and the gas motion are closely related, and the temperature field can cause, in a confined flow geometry, a steady flow without the help of external forces. This is due to the creep of the fluid along the walls induced by a wall temperature gradient, as a consequence of the molecular transfer of momentum to the wall. Experiments performed in microgravity conditions in two small cells, with mono- and bi-atomic carrier gases (Ar, N2), and a thermal gradient between upper and bottom horizontal plates, allowed the measurement of the width of the thermal creep and the induced velocity of the gas near the vertical cell wall, due to thermal gradient. In addition, experiments demonstrated the existence of the thermal creep flow in the cells, even with Knudsen number as low as 10-5. The work evidences experimentally, and for the first time, the thermal creep flow in small cells, due to wall temperature gradient, with Knudsen number as low as 10-5. In view of these results, the no-slip boundary conditions of the Navier-Stokes law in the "continuum" regime can be inadequate in non-isothermal flow geometry and in microgravity conditions. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of an experimental study concerning the gas flow through a packed bed of spheres located between two concentric cylinders, in non isothermal conditions. This problem is prototypal for more complex geometries which can be found in Micro Electro-Mechanical Systems (MEMS) where the heat transfer is a crucial issue. In these applications the characteristic lengths are such that the gas flow regime could fall in almost rarefied regime where the features of heat and mass flows can be markedly different with respect to the continuum regime. We investigated a wide range of regimes, spanning from the compressible continuum flow to rarefied conditions with a variation of the peculiar Knudsen number of about three orders of magnitude, i.e. from Kn =10−3, which corresponds to almost continuum regime, to Kn ≈1 which falls in the transitional region. The heat transfer through the porous medium have been studied by the analysis of the temperature field and the heat flux as a function of the flow rate, the boundary conditions and the porous materials. [Copyright &y& Elsevier]

The direct simulation Monte Carlo method is used to study a plane-parallel supersonic gas flow through a grid formed by a series of parallel infinite cylinders. Characteristic features of the shock disturbance formation during the interaction of a supersonic flow with a permeable grid and the effect of this disturbance on the flow parameters behind the grid are revealed. The boundaries of the domain of supersonic flow breakup ahead of the grid and the laws of the total momentum loss on the grid are obtained. Kinetic and energetic characteristics of the flow behind the grid are determined. [ABSTRACT FROM AUTHOR]

The author opines on a research related to data on internal rarefied gas flows from hydrodynamic to free-molecular flow regime by F. Sharipov and V. Seleznev that was published in a 1998 issue of the periodical and mentions that entire system cannot be considered isolated.

A response from the authors of an research related to data on internal rarefied gas flows from hydrodynamic to free-molecular flow regime, that was published in a 1998 issue is presented.