Critical heat flux for subcooled flow boiling in micro-channel heat sinks

Critical heat flux (CHF) was measured and examined with high-speed video for subcooled flow boiling in micro-channel heat sinks using HFE 7100 as working fluid. High subcooling was achieved by pre-cooling the working fluid using a secondary low-temperature refrigeration system. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid, which is consistent with the mechanism of Departure from Nucleate Boiling (DNB). CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink’s inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels. This technique is shown to be successful at correlating micro-channel heat sink data corresponding to different hydraulic diameters, mass velocities and inlet temperatures.     

Dynamic scheduling for heterogeneous Desktop Grids

Desktop Grids have emerged as an important methodology to harness the idle cycles of millions of participant desktop PCs over the Internet. However, to effectively utilize the resources of a Desktop Grid, it is necessary to use scheduling policies suitable for such systems. In this paper, we analyze the performance of a policy which is shown to perform well in highly heterogeneous Desktop Grids. The policy utilizes the solution to a linear programming (LP) problem which maximizes system capacity. We suggest robust modifications to address several limitations of the policy.     

Learners’ navigation behavior identification based on trace analysis

Identifying learners’ behaviors and learning preferences or styles in a Web-based learning environment is crucial for organizing the tracking and specifying how and when assistance is needed. Moreover, it helps online course designers to adapt the learning material in a way that guarantees individualized learning, and helps learners to acquire meta-cognitive knowledge. The goal of this research is to identify learners’ behaviors and learning styles automatically during training sessions, based on trace analysis. In this paper, we focus on the identification of learners’ behaviors through our system: Indicators for the Deduction of Learning Styles. We shall first present our trace analysis approach. Then, we shall propose a ‘navigation type’ indicator to analyze learners’ behaviors and we shall define a method for calculating it. To this end, we shall build a decision tree based on semantic assumptions and tests. To validate our approach, and improve the proposed calculation method, we shall present and discuss the results of two experiments that we conducted.     

Resource Allocation for Concrete Batch Plant Operation: Case Study

A concrete batch plant is an important element in any concrete construction process, whether it is working as a central mixing plant onsite or is offsite supplying ready mixed concrete to a project. This study tackles the problem of optimizing plant production to maximize profit and, if possible, revenue. A linear programming model has been designed to optimize the plant operation. The maximum production rate for each type of concrete can be obtained by solving the model under the given constraints. A sensitivity analysis is conducted to provide management with a flexible range of prices per cubic yard (cubic meter) and material storage limits. In addition, a model has been designed to determine the optimal number of transit mixers based upon the required quantity of concrete. A chart has been developed to determine the quantities of concrete ingredient materials required daily to organize the available storage space and to plan their delivery.     

Interval-based global optimization in engineering using model reformulation and constraint propagation

This paper deals with the preliminary design problem when the product is modeled as an analytic model. The analytic models based method aims to use mathematical equations to address both multi-physic and economic characteristics of a product. The proposed approach is to convert the preliminary design problem into a global constrained optimization problem. The objective is to develop powerful optimization methods enough to handle complex analytical models. We propose to adapt an approach to solve this problem based on interval analysis, constraint propagation and model reformulation. In order to understand the optimization algorithm used for engineering design problems, some basic definitions and properties of interval analysis are introduced. Then, the basic optimization algorithms for both unconstrained and constrained problems are introduced and illustrated. The next section introduces the reformulation technique as main accelerating device. An application of the reformulation device and its global optimization algorithm on the optimal design of electrical actuators is presented.     

Enhanced confined light transmission by single subwavelength apertures in metallic films

The diffraction of light that emerges from a metallic circular aperture is studied. Near- and far-field results are presented. Spectral angular transmitted intensities are performed versus the incident wavelength for four kinds of aperture. It is shown that, for a definite configuration, a large enhancement of transmission--compared with the basic case of a single hole--occurs combined with a spectacular angular confinement of light. Such effects are, for example, of great interest in optical near-field microscopy for which the probe is a nanosource.     

Photographic study of pool boiling CHF from a downward-facing convex surface

Heat transfer measurements and photographic studies are performed to capture the detailed evolution of the liquid–vapor interface near critical heat flux (CHF) for a 90-degree downward-facing convex surface. The test surface, with a width of 3.2 mm and a 102.6-mm radius, consists of a series of nine heaters that dissipate equal power. Instrumentation within each heater facilitates localized heat flux and temperature measurements along the convex surface, and transparent front and back windows enable optical access to a fairly two-dimensional liquid–vapor interface. Near CHF, vapor behavior along the convex surface is cyclical, repeatedly forming a stratified vapor layer at the bottom of the convex surface, which stretches as more vapor is generated, and then flows upwards along from the surface. Subsequently, heaters at the bottom of the convex surface, followed by the other heaters, are wetted with liquid before the nucleation/coalescence/stratification/release process is repeated. This study proves that despite the pronounced thickening of the vapor layer as it propagates upwards along the convex surface, CHF always commences on the bottom of the surface.     

Quantification of metallothionein-like proteins in the mussel Mytilus galloprovincialis using RP-HPLC fluorescence detection

A HPLC-fluorescence method, using the fluorophore SBD-F (ammonium-7-fluorobenz-2-oxa-1,3-diazole-4-sulfonate), was adapted for the quantification of metallothioneins and their isoforms from the Moroccan mussel Mytilus galloprovincialis. The method was first optimized using a rabbit liver metallothionein. The effects of EDTA, tris(2-carboxyethyl)phosphine, and SBD-F on the labeling efficiency were studied. The optimized method was then applied to evaluate the amount of metallothionein in the mussels either exposed to cadmium in the laboratory or collected from the Casablanca coast, Morocco. The concentrations of metallothioneins measured in the field samples describe the degree of contamination of the sites and are reflected by distinct isoform patterns.     

Photographic study of high-flux subcooled flow boiling and critical heat flux

This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.     

Exploring isolated lignin material from oil palm biomass waste in green composites

Lignin obtained from oil palm biomass empty fruit bunches (EFB) fibers, has been used as curing agent in green epoxy composites. Epoxy–lignin composites, with varying lignin content (15%, 20%, 25% and 30%), reinforced with EFB fiber were prepared. The effect of EFB-based lignin on the mechanical, thermal and morphology properties of the composites were investigated and compared with the composites cured with isophorone diamine curing agent. The improved thermal stability and the observed microstructure of the fractured surface of the composites were attributed to good fiber–matrix interaction, induced by the curing agent. The epoxy composites cured with 25% lignin content proved to be a better matrix and gave optimum value compared with other formulations which was confirmed by its mechanical, thermal and morphological properties.