Effects of modified Clay on the morphology and thermal stability of PMMA/clay nanocomposites

The potential to improve the mechanical, thermal, and optical properties of poly(methyl methacrylate) (PMMA)/clay nanocomposites prepared with clay containing an organic modifier was investigated. Pristine sodium montmorillonite clay was modified using cocoamphodipropionate, which absorbs UVB in the 280–320 nm range, via ion exchange to enhance the compatibility between the clay platelets and the methyl methacrylate polymer matrix. PMMA/clay nanocomposites were synthesized via in situ free-radical polymerization. Three types of clay with various cation-exchange capacities (CEC) were used as inorganic layered materials in these organic–inorganic hybrid nanocomposites: CL42, CL120, and CL88 with CEC values of 116, 168, and 200 meq/100 g of clay, respectively. We characterized the effects of the organoclay dispersion on UV resistance, effectiveness as an O₂ gas barrier, thermal stability, and mechanical properties of PMMA/clay nanocomposites. Gas permeability analysis demonstrated the excellent gas barrier properties of the nanocomposites, consistent with the intercalated or exfoliated morphologies observed. The optical properties were assessed using UV–Visible spectroscopy, which revealed that these materials have good optical clarity, UV resistance, and scratch resistance. The effect of the dispersion capability of organoclay on the thermal properties of PMMA/clay nanocomposites was investigated by thermogravimetric analysis and differential scanning calorimetry; these analyses revealed excellent thermal stability of some of the modified clay nanocomposites.     

An Investigation of the Thermal Stresses Induced in a Thin-Film Thermoelectric Cooler

This work aims at investigating the thermal stresses induced within a four-layered thin-film thermoelectric cooler. The one-dimensional (1D) temperature and thermal-stress distributions are firstly analyzed under the consideration of Joule heating, the conduction heat transfer as well as Thomson heating. Followed are two-dimensional (2D) calculations of the thermal stresses with the commercial software ANSYS. The validity of the 1D analytical model is then examined by a comparison of its predicted thermal stresses with the numerical ones obtained from the 2D model. In the 2D model, the thermoelectric element becomes curved due to the shrinkage and the fixed boundary conditions. The latter also causes huge values and rapid changes of thermal stresses near the ends. In the middle portion of the thermoelectric element where the thermal effect dominates, the thermal stresses predicted by the 1D model are not much different from those computed from the 2D model. Quantitative differences arise from the fact that the 1D model does not count the stresses induced by the non-zero Poisson's ratios. In addition, the normal-stress distributions are pretty uniform across the layer thickness (the variation is less than 1MPa within each layer in the worst case). These results verify the possibility of using the 1D model for a preliminary estimate of the thermal stresses induced within the layered thin-film thermoelectric element. The 1D model nonetheless fails to capture the behaviors near the ends of the thermoelectric element.     

COMBINED RADIATION AND LAMINAR MIXED CONVECTION IN THE THERMAL ENTRANCE REGION OF HORIZONTAL ISOTHERMAL RECTANGULAR CHANNELS

The interaction of thermal radiation with laminar mixed convection for a gray fluid in the thermal entrance region of a horizontal isothermally heated rectangular channel is numerically investigated. The vorticity-velocity formulation of the Navier-Stokes equation and the integral formulation for radiation solved by finite-element nodal approximation are employed. The effects of radiation and convection on local Nusselt number, the development of bulk temperature, and the friction factor are examined. Secondary flow induced by the buoyancy effects leads to a significant enhancement in heat transfer in the entrance region. The result shows that the existence of secondary flow causes fluctuations in local Nusselt number and this phenomenon is reduced by the effect of thermal radiation and a large aspect ratio.     

Development of a One-Dimensional Model to Predict the Flame Temperature in Cylindrical Micro Combustors

Measurement of the flame temperature in a micro combustor is essentially difficult due to the size constraint. A one-dimensional (1D) flame model coupled with the heat conduction in the solid wall is employed to analyze the heat transfer occurring in a cylindrical micro combustor. The flame temperature is given explicitly by taking into account the effects of the heat loss (from the flame to the wall) in the reaction zone and heat recirculation through the solid wall. With the data obtained from the simulation results of the 1D adiabatic freely propagating CH₄–air laminar flames, the flame temperature in a cylindrical micro combustor can be solved iteratively. In order to validate the 1D model, the two-dimensional (2D) numerical simulations of premixed combustion of the CH₄–air mixtures are carried out in a 0.5 mm radius cylindrical micro combustor. The comparisons of the flame temperature and heat recirculation between the 1D model and 2D numerical simulation indicate that despite the simplifications and assumptions made in the present study, the 1D theoretical model is able to predict the flame temperature to a reasonable accuracy.     

Effects of Non-Darcian and Inlet Conditions on the Forced Convection Along a Vertical Plate With Film Evaporation

The present study analyzes theoretically the non-Darcian effects and inlet conditions of forced convection flow with liquid film evaporation in a porous medium. The physical scheme includes a liquid–air streams combined system; the liquid film falls down along the plate and is exposed to a cocurrent forced moist air stream. The axial momentum, energy, and concentration equations for the air and water flows are developed based on the steady two-dimensional (2-D) laminar boundary layer model. The non-Darcian convective, boundary, and inertia effects are considered to describe the momentum characteristics of a porous medium. The paper clearly describes the temperature and mass concentration variations at the liquid–air interface and provides the heat and mass transfer distributions along the heated plate. Then, the paper further evaluates the non-Darcian effects and inlet conditions on the heat transfer and evaporating rate of liquid film evaporation. The numerical results show that latent heat transfer plays the dominant heat transfer role. Carrying out a parametric analysis indicates that higher air Reynolds number, higher wetted wall temperature, and lower moist air relative humidity will produce a better evaporating rate and heat transfer rate. In addition, a non-Darcy model should be adopted in the present study. The maximum error for predictions of heat and mass transfer performance will be 21% when the Darcy model is used.     

On monitoring of multiple non-linear profiles

Most state-of-the-art profile monitoring methods involve studies of one profile. However, a process may contain several sensors or probes that generate multiple profiles over time. Quality characteristics presented in multiple profiles may be related multiple aspects of product or process quality. Existing charting methods for simultaneous monitoring of each multiple profile may result in high false alarm rates. Or worse, they cannot correctly detect potential relationship changes among profiles. In this study, we propose two approaches to detect process shifts in multiple non-linear profiles. A simulation study was conducted to evaluate the performance of the proposed approaches in terms of average run length under different process shift scenarios. Pros and cons of the proposed methods are discussed. A guideline for choosing the proposed methods is introduced. In addition, a hybrid method combining the salient points of both approaches is explored. Finally, a real-world data-set from a vulcanisation process is used to demonstrate the implementation of the proposed methods.     

Vendor selection in a modified re-buy situation using a strategy-aligned fuzzy approach

The supplier/vendor selection is a decision-making problem at the strategic management level that involves a semi-structured process. The majority of the existing approaches obtained their optimal solutions without considering their fits with the strategy of the firm. This study utilizes the supplier positioning matrix, modified from the product-process change matrix, to link the capability of suppliers with the requirements of the customers to identify the strategy-aligned criteria for vendor selection in a modified re-buy situation. A fuzzy factor rating system is then used to evaluate the potential vendors based on the type of components required by the customers. In addition, in the proposed model, the final decision-maker is given the authority to synthesize the scores of individual alternatives based on the risks of individual vendors to accommodate the differences among the vendors to the specific environment. An empirical case study is performed to demonstrate the efficacy of the proposed system and to identify the best potential vendor(s) for further negotiation and development. The results of the case study also provide interesting managerial implications.     

Optimum design of a cogeneration system in processes

Abstract

A systematic method is presented for the preliminary design of a cogeneration system with the desire of achieving minimum operating costs. The ratio of power to process heat is shown to be a critical factor in the preliminary selection of energy conversion systems. However, the final decision should be based on the operating costs and economical index, ROI. A fuel substitution factor is proposed to define theoretically the most economical power generation in plants. Two case studies are demonstrated to describe this approach.     

Numerical simulation and optimization of WC-SS420 induction brazing process

Tungsten carbide (WC) is a very hard, brittle, and relatively expensive cermet. It is often joined to more economical material, such as martensitic 420 stainless steel (SS420), to form a hybrid cutting tool stock. This article summarizes a study on the optimization of an induction brazing process joining these two materials using BAg-22 as filler metal. This study uses an integrated approach consisting of finite element analysis (FEA) modeling and experimental verifications. The thermal behaviors and the metallurgical transitions during the brazing cycles of WC-BAg22-SS420 joints are studied. This study investigated process and metallurgical issues due to mismatches in thermal expansion coefficient, thermal conductivity, and electromagnetic permeability. Using the optimum process parameters derived from simulation, joint strengths exceeding 370?±?40?MPa for shear were obtained. The hardness of WC was unaffected by the brazing heat. However, softening in the SS420 next to the fusion interface was constantly observed in the experiments. The FEA modeling results demonstrated that using a controlled heating and cooling process schedule was effective in mitigating this softening phenomenon. Experimental verification of this mitigation method is currently under study.     

Robust d‐stability analysis of linear uncertain discrete time‐delay systems

Abstract

In this work, under assumption that all the eigenvalues of the linear nominal discrete time‐delay system lie within a specified circular region, a sufficient condition is proposed to preserve the assumed property when the structured parameter perturbations are added into the linear nominal discrete time‐delay system. By mathematical analysis, the presented criterion is proved to be less conservative than the existing one reported in the literature.