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Predicting the permeability and tensile behavior of high density polyethylene/tie/polyamide 6 three-layer films
(2004-12-01)
Huang, CH
;
Wu, JS
;
Huang, CC
;
應用化學系
;
Department of Applied Chemistry
Multilayer films with multiple properties have been widely applied in the packaging industry. In order to know the two most important properties, ie permeability and tensile property, of the multilayer films, we examined model predictions and the properties of the individual component layers to provide an efficient method to design desired multilayer structures. The series model was employed for predicting permeability; there is good agreement between experimental data and this model for predicting both gas and water vapor permeabilities of three-layer films with various thickness fractions of polyamide 6 (PA-6). We can also predict the parameters of the constitutive equation, which described the plastic deformation of the three-layer films, from those of the individual component layers, with particularly high accuracy by using the additive rule. However, there was a larger discrepancy between the model and experimental data at high crosshead speeds due to the generation of heat during deformation. (C) 2004 Society of Chemical Industry.
Analysis of micro-scale gas flows with pressure boundaries using direct simulation Monte Carlo method
(2001-07-01)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
The development of a two-dimensional direct simulation Monte Carlo program for pressure boundaries using unstructured cells and its applications to typical micro-scale gas flows are described. For the molecular collision kinetics, variable hard sphere molecular model and no time counter collision sampling scheme are used, while the cell-by-cell particle tracing technique is implemented for particle movement. The program has been verified by comparison of simulated equilibrium collision frequency with theoretical value and by comparison of simulated non-equilibrium profiles of one-dimensional normal shock with previous reported work. Applications to micro-scale gas flows includes micro-manifold, micro-nozzle and slider air bearing. The aim is to further test the treatment of pressure boundaries, developed previously by the first author, by particle flux conservation for gas flows involving many exits, complicated geometries and moving boundaries. For micro-manifold gas flows, excellent mass flow conservation between the inlet and two exits is obtained at low subsonic flows. For micro-nozzle gas flows, with fixed inlet pressure, the mass flow rate increases with decreasing pressure ratio (exit to inlet), but remains essentially the same at pressure ratios much lower than that obtained by continuum inviscid analysis. For higher specified pressure ratios, the locations of maximum Mach number moves further downstream as the pressure ratio decreases; while, for lower specified pressure ratios, the Mach number increases all the way through the nozzle to the exit. Eventually, supersonic speed is observed at the exit for pressure ratios equal to or less than 0.143. Finally, for slider air bearing gas flows of the computer hard drive, the simulated gas pressures, at different rotating speeds, agree very well with previous studies. However, there exists strong translational nonequilibrium in the gas flows at the high rotating speeds. The applicability of the treatment of pressure boundaries using the equilibrium Maxwell-Boltzmann distribution function is discussed in terms of the magnitude of the local Knudsen number at the pressure boundary for micro-nozzles and slider air bearing applications. (C) 2001 Elsevier Science Ltd. All rights reserved.
Pressure boundary treatment in micromechanical devices using the direct simulation Monte Carlo method
(2001-08-01)
Wu, JS
;
Lee, F
;
Wong, SC
;
機械工程學系
;
Department of Mechanical Engineering
Two numerical procedures in the Direct Simulation Monte Carlo (DSMC) method, applying particle flux conservation at inflow/outflow pressure boundaries, have been developed to treat the two most important boundary conditions encountered in micromechanical devices involving gaseous flows. The first one is for both specified pressures at inlet and exit; while the second one is for specified mass flow rate and exit pressure. Both numerical procedures have been tested on short and long microchannels in the slip and transitional regimes. Excellent agreement has been found between the current results and the previous reported numerical results as well as the experimental data for the first type of boundary conditions. Finally, the developed numerical procedures have been applied to backward-facing micro-step gaseous flows to demonstrate its general applicability in more complicated flows.
Effects of ambient turbulence and fuel properties on the evaporation rate of single droplets
(2001-12-01)
Wu, JS
;
Liu, YJ
;
Sheen, HJ
;
機械工程學系
;
Department of Mechanical Engineering
Results of the evaporation of a single liquid fuel droplet in various free-stream turbulence intensities and scales are reported. Experiments are carried out at room temperature by using n-heptane and n-decane, fuels at Re-d = 100. A low-speed vertical wind tunnel with different turbulence intensities and scales, controlled by using different sizes of disk, is constructed. The free-stream turbulence intensities are varied in the range from 1% to 60% and the. integral length scales are from 2.5 to 20 times of the initial droplet diameter. Results show that the time history of droplet diameter follows the d(2)-law in turbulent environments with generally higher evaporation rates as compared with those in quasi-laminar cases. Combined effects of liquid fuel properties and ambient turbulence properties on the evaporation rate can be reasonably well explained by the correlation of normalized evaporation rate with the effective vaporization Damkohler number, Da(v). (C) 2001 Elsevier Science Ltd. All rights reserved.
The direct simulation Monte Carlo method using unstructured adaptive mesh and its application
(2002-02-10)
Wu, JS
;
Tseng, KC
;
Kuo, CH
;
機械工程學系
;
Department of Mechanical Engineering
The implementation of an adaptive mesh-embedding (h-refinement) scheme using unstructured grid in two-dimensional direct simulation Monte Carlo (DSMC) method is reported. In this technique, local isotropic refinement is used to introduce new mesh where the local cell Knudsen number is less than Some Preset Value. This simple scheme, however, has several severe consequences affecting the performance of the DSMC method. Thus, we have applied a technique to remove the hanging node, by introducing the an-isotropic refinement in the interfacial cells between refined and non-refined cells. Not only does this remedy increase a negligible amount of work, but it also removes all the difficulties presented in the originals scheme. We have tested the proposed scheme for argon gas in a high-speed driven cavity flow, The results show an improved flow resolution as compared with that of un-adaptive mesh. Finally, we have used triangular adaptive mesh to compute a near-continuum gas flow, a hypersonic flow over a cylinder, The results show fairly good agreement with previous studies. In summary, the proposed simple mesh adaptation is very useful in computing rarefied gas flows, which involve both complicated geometry and highly non-uniform density variations throughout the flow field. Copyright (C) 2002 John Wiley Sons, Ltd.
Parallel particle simulation of the near-continuum hypersonic flows over compression ramps
(2003-01-01)
Wu, JS
;
Tseng, KC
;
機械工程學系
;
Department of Mechanical Engineering
This paper describes the analysis of the near-continuum hypersonic flow over a compression ramp using the two-dimensional parallel direct simulation Monte Carlo (DSMC) method. Unstructured and triangular solution-based adaptive mesh depending on the local mean free path is used to improve the resolution of solution for the flow field with highly varying properties. In addition, a freestream parameter is defined to help reduced the cell numbers in the freestream area, resulting in appreciable decrease of the computational time (20-30%) without sacrificing the accuracy of the solution. The two-step multilevel graph partition technique is used for physical domain decomposition, employing estimated particle number distribution in each cell as the graph vertex weight. 32 IBM-SP2 processors are used throughout the study unless otherwise specified. The Effect of the outflow vacuum boundary condition, compression ramp angle, freestream condition, and length of the ramp to the flow field are investigated. Computational results are compared with previous numerical results whenever available.
Adhesion, permeability, and mechanical properties of multilayered blown films using maleated low-density polyethylene blends as adhesion-promoting layers
(2003)
Huang, CH
;
Wu, JS
;
Huang, CC
;
Lin, LS
;
應用化學系
We have fabricated three-layer films, comprising a varying content of ethylene-vinyl alcohol copolymer (EVOH) as the internal layer and blends of low-density polyethylene (LDPE) and low-density ethylene grafted with maleic anhydride (LDPE-g-MAH) as the external layers, by a coextrusion blown-film process. We chose to use blends to promote the adhesion between EVOH and LDPE and to reduce the number of layers in the coextrusion system. The peel strength increased sharply when the amount of LDPE-g-MAH was greater than 12.5 wt% and we associate it with the promotion of adhesion between layers brought about by specific interactions between EVOH and LDPE-g-MAH. FT-IR spectroscopic analysis showed an increase in the intensity of the absorbance of the ester band upon increasing the content of LDPE-g-MAH, which indicates that a chemical reaction occurs that promotes adhesion at the interface. The tensile strength did not change significantly with increasing LDPE-g-MAH content, which had a small effect on elongation and modulus in both the machine direction (MD) and transverse direction (TD). Tear strength decreased continuously with increasing LDPE-g-MAH content, in both the MD and TD, as a result of the greater ease of crack propagation in the EVOH layer. The oxygen permeability of the three-layer films remained almost constant upon varying the amount of LDPE-g-MAH. These three-layer films all followed the theoretical prediction made by the inverse additivity rule. The water vapor permeabilities of the three-layer films, however, were affected by the degree of hydrogen bonding, which was analyzed by FT-IR spectroscopy. The effects of this hydrogen bonding resulted in a discrepancy between the experimental findings and the theoretical predictions, especially when the EVOH content was increased.
Blends of poly(propylene) and polyacetal compatibilized by ethylene vinyl alcohol copolymers
(2003-08-08)
Huang, JM
;
Cheng, HJ
;
Wu, JS
;
Chang, FC
;
應用化學系
Polymer blends of poly(propylene) (PP) and polyacetal (polyoxymethylene, POM) with ethylene vinyl alcohol (EVOH) copolymers were investigated by differential scanning calorimetry (DSC), rheological, tensile, and impact measurements, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The PP-POM-EVOH blends were extruded with a co-rotating twin-screw extruder. The ethylene group in the EVOH is partially miscible with PP, whereas the hydroxyl group in the EVOH can form hydrogen bonding with POM. The EVOH tends to reside along the interface, acting as a surfactant to reduce the interfacial tension and to increase the interfacial adhesion between the blends. Results from SEM and mechanical tests indicate that a small quantity of the EVOH copolymer or a smaller vinyl alcohol content in the EVOH copolymer results in a better compatibilized blend in terms of finer phase domains and better mechanical properties. (C) 2003 Wiley Periodicals, Inc.
Parallel implementation of DSMC using unstructured mesh
(2003-09-01)
Wu, JS
;
Tseng, KC
;
Yang, TJ
;
機械工程學系
;
Department of Mechanical Engineering
The parallel implementation of the Direct Simulation Monte Carlo (DSMC) method on memory-distributed machines using unstructured mesh is reported. Physical domain decomposition is used to distribute the workload among multiple processors. A high-speed driven cavity flow is used as the benchmark problem for the validation of the parallel implementation. Three static partitioning techniques including simple coordinate partitioning, two-step partitioning (JOSTLE) and multi-level partitioning (METIS) are used for static domain decomposition, respectively. A cell renumbering technique is used to improve the memory management efficiency. Results of parallel efficiency show that two-step partitioning using JOSTLE performs the best, with 63% up to 25 processors, due to better load balancing among the processors. The powerful computational capability of the parallel implementation is demonstrated by computing a 2-D, near-continuum, hypersonic flow over a cylinder as well as a 3-D hypersonic flow over a sphere, respectively, both using 25 processors. Results compare reasonably well with previous simulated and experimental studies.
Parallel three-dimensional direct simulation Monte Carlo method and its applications
(2003-09-01)
Wu, JS
;
Lian, YY
;
機械工程學系
;
Department of Mechanical Engineering
The development of a parallel three-dimensional direct simulation Monte Carlo (DSMC) method using unstructured cells is reported. Variable hard sphere molecular model and no time counter method are used for the molecular collision kinetics, while the cell-by-cell ray-tracing technique is implemented for particle movement. Developed serial code has been verified by comparing the results of a supersonic corner flow with those of Bird's three-dimensional structured DSMC code. In addition, a benchmark test is performed for an orifice expanding flow to verify the parallel implementation of DSMC method by comparing with available experimental data. Static physical domain decomposition is used to distribute the workload among multiple processors by considering the estimated particle weighting distribution. Two-step multi-level graph partitioning technique is used to perform the required domain decomposition. Completed code is then applied to compute a hypersonic flow over a sphere (external flow) and the flow field of a spiral drag pump (internal flow), respectively. Results of the former are in good agreement with previous numerical results using axisymmetric DSMC method and experimental data. Results of the latter also agree well with previous numerical results. (C) 2002 Elsevier Science Ltd. All rights reserved.

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