Browsing by Author "Lim, C. -W."

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CVD Flow Field Modeling Using the Quiet Direct Simulation (QDS) Method
(2009)
Cave, H. M.
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Lim, C. -W.
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Jermy, M. C.
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Wu, J. -S.
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Smith, M. R.
In this paper, the Quiet Direct Simulation (QDS) method is used to model the unsteady jet development in a Pulsed Pressure Chemical Vapour Deposition (PP-CVD) reactor. QDS is a novel method of gas flow simulation which is able to compute true-direction fluxes of mass, momentum and energy in a computationally efficient and accurate manner. The scheme is ideal for the simulation of novel CVD processes like PP-CVD which include highly unsteady flow structures and which has previously proved extremely difficult to simulate. Here, the axisymmetric QDS solver is outlined and the injection phase of a PP-CVD reactor is simulated.
Multi-Species Fluxes for the Parallel Quiet Direct Simulation (QDS) Method
(2011-01-01)
Cave, H. M.
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Lim, C. -W.
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Jermy, M. C.
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Krumdieck, S. P.
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Smith, M. R.
Fluxes of multiple species are implemented in the Quiet Direct Simulation (QDS) scheme for gas flows. Each molecular species streams independently. All species are brought to local equilibrium at the end of each time step. The multi species scheme is compared to DSMC simulation, on a test case of a Mach 20 flow of a xenon/helium mixture over a forward facing step. Depletion of the heavier species in the bow shock and the near-wall layer are seen. The multi-species QDS code is then used to model the flow in a pulsed-pressure chemical vapour deposition reactor set up for carbon film deposition. The injected gas is a mixture of methane and hydrogen. The temporal development of the spatial distribution of methane over the substrate is tracked.
Validity and Inherent Viscosity of the Quiet Direct Simulation Method
(2011-01-01)
Jermy, M. C.
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Lim, C. -W.
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Cave, H. M.
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機械工程學系
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Department of Mechanical Engineering
The Quiet Direct Simulation (QDS) scheme is a numerical method for modelling gas flows, based on kinetic theory, with some similarities to the Lattice Boltzmann Method (LBM). It differs from LBM notably in that the discrete molecular velocities are not constant but are reset each timestep according to local values of bulk velocity and temperature. For this reason it performs well in highly compressible flows. Two features of the scheme limit its accuracy in low Mach number flows. QDS assumes a Maxwell distribution of molecular velocities. The validity of this assumption may be tested by calculating the gradient Knudsen number and average number of collisions per timestep. The separation of collision and streaming leads to excessive diffusion of momentum, leading to a very high effective viscosity of the modelled gas when the grid spacing is larger than the mean free path. This numerical dissipation is different in character from the dissipation due to the finite order of the spatial reconstruction, common to all finite volume methods, which is also present. The effective viscosity is quantified for simple shear flows and tested in models of a 2D channel flow. A crude model of intermolecular collision during streaming is implemented and shown to reduce the effective viscosity.