Browsing by Author "Krumdieck, S. P."

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CVD Flow Field Modeling Using the Quiet Direct Simulation (QDS) Method
(2009)
Cave, H. M.
;
Lim, C. -W.
;
Jermy, M. C.
;
Wu, J. -S.
;
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.
Implementation of the Transient Adaptive Sub-Cell Module for the Parallel DSMC Code
(2009)
Tseng, K. C.
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Cave, H. M.
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Kuo, T. C.
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Jermy, M. C.
;
Krumdieck, S. P.
A method of transient adaptive sub-cells (TAS) suitable for unstructured grids in Direct Simulation Monte Carlo (DSMC) simulations is introduced. The TAS algorithm is implemented within the frame work of a parallelized DSMC code (PDSC). Benchmarking tests are conducted for driven cavity (low, hypersonic flow over a two-dimensional cylinder and the unsteady vortex shedding behind a two-dimensional cylinder. The use of TAS enabled a reduction in the computational expense of the simulation since larger sampling cells and less simulation particles could be employed. Furthermore, the collision quality of the simulation was maintained or improved and the preservation of property gradients and vorticity at the scale of the sub-cells enabled correct unsteady vortex shedding frequencies to be predicted. The use of TAS in a parallel DSMC code allows simulations of unsteady processes at a near continuum level to be carried out efficiently, accurately and with acceptable computational times.
Implementation of unsteady sampling procedures for the parallel direct simulation Monte Carlo method
(2008-06-01)
Cave, H. M.
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Tseng, K. -C.
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Wu, J. -S.
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Jermy, M. C.
;
Huang, J. -C.
An unsteady sampling routine for a general parallel direct simulation Monte Carlo method called PDSC is introduced, allowing the simulation of time-dependent flow problems in the near continuum range. A post-processing procedure called DSMC rapid ensemble averaging method (DREAM) is developed to improve the statistical scatter in the results while minimising both memory and simulation time. This method builds an ensemble average of repeated runs over small number of sampling intervals prior to the sampling point of interest by restarting the flow using either a Maxwellian distribution based on macroscopic properties for near equilibrium flows (DREAM-I) or output instantaneous particle data obtained by the original unsteady sampling of PDSC for strongly non-equilibrium flows (DREAM-II). The method is validated by simulating shock tube flow and the development of simple Couette flow. Unsteady PDSC is found to accurately predict the flow field in both cases with significantly reduced run-times over single processor code and DREAM greatly reduces the statistical scatter in the results while maintaining accurate particle velocity distributions. Simulations are then conducted of two applications involving the interaction of shocks over wedges. The results of these simulations are compared to experimental data and simulations from the literature where there these are available. In general, it was found that 10 ensembled runs of DREAM processing could reduce the statistical uncertainty in the raw PDSC data by 2.5-3.3 times, based on the limited number of cases in the present study. (c) 2008 Published by Elsevier Inc.
Multi-Species Fluxes for the Parallel Quiet Direct Simulation (QDS) Method
(2011-01-01)
Cave, H. M.
;
Lim, C. -W.
;
Jermy, M. C.
;
Krumdieck, S. P.
;
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.
The direction decoupled Quiet Direct Simulation method for rapid simulation of axisymmetric inviscid unsteady flow in pulsed pressure chemical vapour deposition
(2013-11-05)
Lim, C. W.
;
Smith, M. R.
;
Cave, H. M.
;
Jermy, M. C.
;
Wu, J. -S.
Pulsed Pressure-Chemical Vapour Deposition (PP-CVD) is a thin film deposition process which employs a highly unsteady flow with wide dynamic range of pressure. The large, time-varying density gradient during a PP-CVD process cycle produces a flow field in which the Knudsen number varies from the near-continuum to the rarefied regimes, making Direct Simulation Monte Carlo (DSMC) prohibitively expensive. The present directional decoupled Quiet Direct Simulation (DD-QDS) method is a novel kinetic-based flux scheme that computes fluxes of mass, momentum and energy at the interface of computational cells in a highly computationally efficient manner. The Maxwell-Boltzmann equilibrium distribution is enforced locally at each computational cell at each time step. In this paper, an axisymmetric second order directional decoupled QDS scheme is used to simulate highly unsteady flows encountered in PP-CVD reactor. Two simulations were conducted to investigate the PP-CVD reactor flow field at 1 Pa and 1 kPa reactor base pressures. The assumption of the local Maxwell-Boltzmann equilibrium distribution used in the QDS scheme is verified by examining the gradient length local Knudsen number based on the density, and by estimating the average number of molecular collisions within each computational cell in one computational time step. The validity of the local equilibrium assumption is found satisfactory at 1 kPa reactor based pressure but not at 1 Pa. The limitation of the QDS scheme in modelling PP-CVD flow was also investigated. The time required to establish the quasi-steady under-expanded jet is found to be similar to 4 ms, and the jet dissipates within 0.5 ms of the end of injection. This important information is required to set up PP-CVD operating conditions which give uniform film deposition. (C) 2013 Elsevier Ltd. All rights reserved.