• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.11a
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• pp.76-79
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• 2007

From the previous researches about flow characteristic of micro-nozzle， we found that viscosity and back pressure induced heavy losses in micro nozzle. To overcome thess losses, we began to study new conceptual micro propulsion system that is thermal transpiration based micro propulsion system. It has no moving parts and can pump the gaseous propellant by temperature gradient only (cold to hot). Most of previous research on thermal transpiration is in its early stage and mainly studied for application to small vacuum facility or gas chromatography in ambient condition using nanoporous material like aerogel. In this study, we focus on basic research of propulsion system based on thermal transpiration using polyimide material in vacuum conditions.

• Journal of computational fluids engineering
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• v.19 no.2
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• pp.40-48
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• 2014

For rarefied gas flow regimes, physical phenomena such as velocity slip and temperature jump occur on the solid body surface. To predict these phenomena accurately, either the Navier-Stokes solver with a slip boundary condition or the direct simulation Monte Carlo method should be used. In the present study, flow simulations of a wedge were conducted in Mach-10 flow of argon gas for several different flow regimes using a two-dimensional Navier-Stokes solver with the Maxwell slip boundary condition. The results of the simulations were compared with those of the direct simulation Monte Carlo method to assess the present method. It was found that the values of the velocity slip and the temperature jump predicted increase as the Knudsen number increases. Also, the results are comparatively reasonable up to the Knudsen number of 0.05.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.509-517
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• 1997

Results of flat plate compressible boundary layer calculation, based on discrete formulation of DSMC method, are presented in low Mach number and low Knudsen number range. The free stream is a uniform flow of pure nitrogen at various Mach numbers in low pressures (i.e. rarefied gas). Complete thermal accommodation and diffuse molecular reflections are used as the wall boundary condition, replacing unreal no-slip condition used in continuum calculations. In the discrete formulation of DSMC method, there is no need to use ad hoc assumptions on transport properties like viscosity and thermal conductivity, instead viscosity is calculated from values of other field variables (velocity and shear stress). Also the results are compared with existing self-similar continuum solutions. In all Mach number cases computed, velocity slip is most pronounced in regions near the leading edge where continuum formulation renders the solution singular. As the boundary layer develops further downstream, velocity slips asymptote to values that are between 10 to 20% of the magnitude of free stream velocity. When the free stream number density is reduced, so the gas more rarefied, the velocity slip increases as expected.

• Korean Chemical Engineering Research
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• v.58 no.2
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• pp.307-312
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• 2020

Fluidized bed reactors operated in subatmospheric pressure has been focused because several industrial applications such as vacuum drying and plasma cvd requires reduced pressure fludization. However, the hydrodynamics of fluidized beds in subatmospheric pressure has not been extensively investigated. The pressure drop in the fluidized bed has been measured with variation of downstream pressures from 1.33 to 101.3 kPa in the shallow and deep fluidized beds under the sub-atmospheric pressures. The obtained minimum fluidization velocity of powders is a function of pressure due to the changes of gas density and mean free path. We can experimentally determine the critical Knudsen number and the critical pressure to define the slip regime significantly to influence the hydrodynamics of fluidized beds.

• Journal of the Korean Society of Visualization
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• v.10 no.1
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• pp.40-46
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• 2012

Recently micro shock tube is extensively being used in many diverse fields of engineering applications but the detailed flow physics involved in it is hardly known due to high Knudsen number and strong compressibility effects. Unlike the macro shock tube, the surface area to volume ratio for a micro shock tube is very large. This unique effect brings many complexities into the flow physics that makes the micro shock tube different compared with the macro shock tube. In micro shock tube, the inter- molecular forces of working gas can play an important role in specifying the flow characteristics of the unsteady shock wave flow which is essentially generated in all kinds of shock tubes. In the present study, a CFD method was used to predict and visualize the unsteady shock wave flows using the unsteady compressible Navier-Stokes equations, furnished with the no-slip and slip wall boundary conditions. Maxwell's slip equations were used to mathematically model the shock movement at high Knudsen number. The present CFD results show that the propagation speed of the shock wave is directly proportional to the initial pressure and diameter of micro shock tube.

• Journal of Korea Technical Association of The Pulp and Paper Industry
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• v.30 no.3
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• pp.46-56
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• 1998

The objective of this study was to investigate the relationship between water vapor diffusion properties and the pore structure of paper. Gas-phase molecular diffusivity of water vapor through pores was determined based on the kinetic theory of gas. A mathematical model was derived to characterize the dimensional changes of the pore caused by the fiber-swelling mechanism. A modified-Fickean diffusion model was designed to simulate the water-vapor diffusion phenomena in porous paper web. Structural characterisocs of paper pores including the tortuosity and the shape factor was studied on a theoretical basis of Knudsen flow diffusion. Results are summarized as follows: 1. The theoretical water vapor diffusivity in gas-phase was 0.092$cm^2$ /min, 2. Porosity was inversely proportional to the degree of wet-swelling of paper, 3. Solid-phase water-diffusivity of fiber was 1.2 $ \times 10^{-5}cm^2/min$, 4. Modified diffusion model was fairly consistent to the experimental data (from part I), and 5. The Fickean pore tortuosity, ranging from 1,000 to 2,500, was in inverse proportion to the porosity of paper, and the Knudsen shape factor and length-angle factor for micro-pores in paper were 0.5~3.5 and about 340, respectively.

• Korean Chemical Engineering Research
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• v.43 no.1
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• pp.66-70
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• 2005

For the application in HI decomposition reaction of thermochemical water-splitting IS process, the carbonized membranes using the polymer material (polyimide) were prepared, and SiC membrane was also prepared by SiO treatment on those carbonized membranes. The weight change by the carbonation of polyimide was about 50%, and the weight decreased with an increase of carbonation temperature. The gas permeance ($H_2$ or $N_2$) of carbonized membrane decreased with an increase of carbonation temperature led to the pore closing. The gas permeance ($H_2$ or $N_2$) of SiC membrane increased with an increase of SiO treatment concentration, and the gas permeation mechanism was changed from the activiation energy flow to Knudsen flow.

• Journal of the Korean Institute of Gas
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• v.21 no.2
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• pp.50-57
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• 2017

Shale gas reservoir are composed of very fine grained particles, and their pores are very small, at the scale of nanometers. In this study, a parametric study was implemented to investigate the effect of knudsen diffusion, relative permeability and permeability reduction in shale gas reservoir. Shale gas reservoir model in Horn-River was developed to confirm the productivity for different design parameters such as diffusion, relative permeability, connate water saturation, and permeability reduction.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.10
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• pp.947-954
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• 2009

Starting from the Eu's GH(Generalized Hydrodynamic) theory, the multi-species GH numerical solver is developed in this research and its computatyional behaviors are examined for the hypersonic rarefied flow over an axisymmetric body. To improve the accuracy of the developed multi-species GH solver, various slip-wall boundary conditions are tested and the computed results are compared. Additionally, in order to validate the entropy characteristics of the GH equation, the entropy production and entropy generation rates of the GH equation are investigated in the 1-dimensional normal shock structure test at a high Knudsen number.

• Computers and Concrete
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• v.32 no.6
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• pp.567-576
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• 2023

In this study, a 3D microstructure-based model is established to simulate the effective thermal conductivity of cement paste, covering varying influencing factors associated with microstructure and thermal transfer mechanisms. The virtual cement paste divided into colloidal C-S-H and heterogeneous paste are reconstructed based on its structural attributes. Using the two-level hierarchical cement pastes as inputs, a lattice Boltzmann model for heat conduction is presented to predict the thermal conductivity. The results suggest that due to the Knudsen effect induced by the nanoscale pore, the thermal conductivity of air in C-S-H gel pore is significantly decreased, maximumly accounting for 3.3% thermal conductivity of air at the macroscale. In the cement paste, the thermal conductivities of dried and saturated cement pastes are stable at the curing age larger than 100 h. The high water-to-cement ratio can decrease the thermal conductivity of cement paste.