A nonlinear lumped circuit model for gunn diodes

A nonlinear lumped circuit model for Gunn diodes which includes the effects due to domain extinction and nucleation phenomena is presented. The model is based upon physical principles and allows an arbitrary nonlinear drift velocity curve v(E) and a nonlinear diffusion curve D(E) to be specified by the user. It is valid for simulating arbitrary Gunn-diode circuits operating in any matured high-field domain mode, or in the LSA mode. Under additional assumptions, the model simplifies to other existing models. Several computer-simulated examples of Gunn-diode circuits operating under both steady-state and transient regimes are presented. Finally, a rigorous definition of a ‘DC’ I-V curve for Gunn diodes is offered and shown to be rather useful for predicting the qualitative behaviour of Gunn-diode circuits during all time intervals where a matured domain exists.     

On The Drying of Food Products in a Tunnel Dryer

ABSTRACT

We present a dryer model for simulating the drying of hygroscopic-porous food products in a tunnel dryer. The model employs an improved receding-front formulation by taking into consideration the material volumetric shrinkage and the variation of the heat and mass transfer coefficient during drying. Predicted results show close agreement when compared with experimental data. We report a parametric analysis using the dryer model to study the drying transient and the need to cascade the drying process so as to maximise the drying potential of the air stream.     

CONVECTIVE DRYING OF BANANA, GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20^°C to 40^°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.     

Microwave-assisted synthesis of graphene-ZnO nanocomposite for electrochemical supercapacitors

Graphene-ZnO nanocomposite was successfully synthesized via microwave-assisted reduction of zinc ions in aqueous solution with graphite oxide dispersion using a microwave synthesis system. The electrochemical performance of the nanocomposite was analyzed through cyclic voltammetry and chronopotentiometry tests. The results showed that as compared with pure graphene, graphene-ZnO composite exhibited an improved electrochemical capacitance of 146 F/g with good reversible charge/discharge behavior.     

Application of rapid prototyping and computational fluid dynamics in the development of water flow regulating valves

Water flow regulating valves are widely used in instant water heaters to vary the rate of water flow. The water flow regulation from the valve has to be linear so that the temperature at the outlet is uniform. The shapes of valve trim affect the flow rate and have to be confirmed before releasing the designs for manufacture. In this project, the flow rate regulating characteristics were improved for a flow regulating valve with the application of rapid prototyping (RP) and computational fluid dynamics (CFD). In the development of flow regulating valves, precise, functional prototypes were required. Design concepts of three valve trim designs were evaluated applying different RP techniques and CFD analysis. Prototypes from different RP systems were used as functional models. Over the product development cycle, RP and CFD analysis stages played an important role particularly in the design selection stage. Finally, comparisons of RP samples from various RP processes and the results of CFD analysis were made on the flow rate and flow regulation. Based on these comparisons, the best design was chosen for manufacture of the valves.     

A new approach to overcome the overflow problem in computer-aided analysis of nonlinear resistive circuits

This paper is addressed to the so-called overflow problem commonly encountered in the computer simulation of nonlinear resistive circuits containing rapidly varying nonlinearities—such as exponentials found in the models of diodes and transistors. A novel approach which makes use of the arc-lengths of the nonlinear characteristic curves as the variables of iteration is proposed. It is proved, under rather mild conditions, that the arc-length approach not only overcomes the overflow problem, but also leads to a more rapid rate of convergence. Moreover, it is proved that for most practical diode-transistor circuits, the region of convergence associated with the arc-length approach is larger and the convergence of the Newton-Raphson algorithm is not sensitive to the initial guess. Since it is more difficult to make good initial guesses when the size of the network is large, in so far as choosing the initial guess is concerned, the advantage for using the arc-length approach over the conventional approach increases with the size of the network. Extensive numerical experiments confirm the superior convergence property of this approach even for circuits which violate the sufficient conditions invoked by the rigorous mathematical proofs. Although the approach is applicable to a much wider class of nonlinear networks, particular emphasis is focused on diode-transistor networks in this paper.     

Effects of layer thickness and binder saturation level parameters on 3D printing process

Various parameters, such as binder properties, printing layer thickness, powder size, and binder saturation level, have effects on the strength and surface finish of the three-dimensional printing (3D printing) process. The objective of this research is to study the effects of two parameters of layer thickness and binder saturation level on mechanical strength, integrity, surface quality, and dimensional accuracy in the 3D printing process. Various specimens include tensile and flexural test specimens and individual network structure specimens are made by the 3D printing process under different layer thicknesses and binder saturation by use of ZCorp.'s ZP102 powder and Zb56 binder. Two printing layer thicknesses, 0.1 and 0.087 mm, are evaluated at 90% and 125% binder saturation levels. Experimental findings show that under the same layer thickness, increment of binder saturation level from 90% to 125% would result in an increase of tensile and flexural strengths of the specimens and decrease of dimensional accuracy and surface uniformity of specimens. On the other hand, under the same binder saturation conditions, increase in layer thickness from 0.087 to 0.1 mm would decrease tensile strength and increase flexural strength. Also, it gives better uniformity on the surface.     

Rapid Prototyping Applications in Medicine. Part 2: STL File Generation and Case Studies

Rapid prototyping (RP) technology has extended traditional manufacturing applications in areas other than product engineering. Using RP to fabricate custom implants and prosthesis for surgical planning and education is now an important area of research. Although, in theory, RP is capable of producing objects of any complexity, designing freeform shapes is difficult using current CAD systems. These CAD systems are geared toward the design of parts manufactured by traditional methods; they do not help designers exploit the expanded opportunities offered by RP technology. Medical data cannot be input into these CAD systems directly for further modification and manipulation. The purpose of this project is to explore a new approach for modelling and prototyping biomedical objects. The work extends from volume modelling to RP and medicine. In Part 1 of two papers, a new approach to modelling complex objects, NURBS-based volume modelling, is proposed. A NURBS representation of volumes is developed to represent not only the surface boundary but also the interior of a 3D object. NURBS-based volume modelling inherits advantages from both NURBS modelling and voxel-based modelling. The key idea of the NURBS-based volume modelling is to exploit the flexibility of NURBS modelling and use the voxelised NURBS volumes as components for constructing complex objects. This paper, Part 2, deals mainly with issues of interfacing volume models to RP systems. A new approach to generate STL files through volume modelling and iso-surface extraction is proposed. This approach guarantees the validity of the final STL file inherently. Software development and case studies are also given.     

Creating machinable textures for CAD/CAM systems

Texture is applied on three-dimensionally modelled surfaces in computer graphics to enhance visual effect. This research focuses on the development of three-dimensional textured surfaces which are suitable for manufacturing. Three approaches for creating the three-dimensional texture are presented.The first approach is to process a design from either an artist's sketch or an image from a two-dimensional scan. The second approach uses a three-dimensional scanned texture. Both these approaches depend on the quality of the scanned image and are more tedious than the third approach, which is to convert texture using parameters and is a more direct approach. In the user interface design, two custom-made forms are developed to cater for both regular and irregular textures.The case studies have shown that the textures created are not only good for a visual effect, but are also machinable. The development work is incorporated into the ArtCAM system which is a specialised CAD/CAM system that is capable of generating three-dimensional shapes from two-dimensional artwork.     

Study of Trapped Material in Rapid Prototyping Parts

Since the launch of the first commercial system in 1988, rapid prototyping (RP) technology is fast being accepted in the fields of product design and development, in the tool and die making industry and recently, for biomedical applications. Although, in theory, RP is able to produce an object of any complexity, problems such as having material trapped inside parts with hollow features still frequently occur during normal operation. This research project seeks to achieve a better understanding of the formation of trapped material inside RP parts, with the eventual aim of developing an algorithm to avoid this problem. In this paper, the causes and effects of trapped material will be discussed for stereolithography apparatus (SLA), solid ground curing (SGC), selective laser sintering (SLS), and laminated object manufacturing (LOM). Finally, current remedies will be presented.