Simplified numerical model for defining Ledinegg flow instability margins in MTR research reactor

Establishment of safety margins and the corresponding operating condition limits will ensure achievement of a safe operation of nuclear installations. For this purpose, several critical phenomena have been analyzed theoretically and experimentally and a great number of models and correlations are made available. Among these critical issues the well-known flow instability has been intensively investigated by several authors especially for nuclear power plants' (NPPs) operating conditions. However, limited published work is available for research reactor operation conditions. In general, the Whittle and Forgan correlation is widely used to define the margin to static flow instabilities in narrow parallel heated channels for research reactors.In the framework of verification and assessment of the capabilities of the RELAP5/Mod 3 system code to determine the onset of flow instability in research reactor conditions, a simple model based on steady-state equations adjusted with drift-flux correlations has been developed. The program is used to draw the pressure drop characteristic curves and to establish the conditions of the Ledinegg instability in a uniformly heated channel subject to constant outlet pressure. The model is assessed by using experimental data from a thermal hydraulic test loop by Siman-Tov and numerical results from RELAP5/Mod 3. The model presents acceptable estimation of the target mass flow that would induce flow instability and the latter could be then used to establish a conservative margin to the Ledinegg instability.     

Design of widely tunable semiconductor lasers and the concept ofbinary superimposed gratings (BSG's)

We propose a new concept of binary superimposed gratings (BSG's) as multiwavelength grating reflectors and show it as an effective structural improvement over the existing designs, to both the fabrication process and device performance. We also present a study of key design issues for widely tunable lasers based on grating mirrors with a comb-like reflection spectrum and summarize simple design rules for the grating part of the laser, based on analytical and numerical analysis. The binary supergrating consists of elements of equal size whose refractive index is allowed to be one of two possible values, which are sequenced according to a binary optics formalism to effect a spatial superposition of multiple sets of single-frequency gratings, and its implementation is well within the standard e-beam lithography limits. The calculations of lasing frequency, mode, and side-mode suppression ratio of the tunable laser are formulated and presented along with numerical examples     

An optimal inspection strategy for randomly failing equipment

This paper addresses the problem of generating optimal inspection strategies for randomly failing equipment where imminent failure is not obvious and can only be detected through inspection. Inspections are carried out following a condition-based procedure. The equipment is replaced if it has failed or if it shows imminent signs of failure. The latter state is indicated by measuring certain predetermined control parameters during inspection. Costs are associated with inspection, idle time and preventive or corrective actions. An optimal inspection strategy is defined as the inspection sequence minimizing the expected total cost per time unit over an infinite span. A mathematical model and a numerical algorithm are developed to generate an optimal inspection sequence. As a practical example, the model is applied to provide a machine tool operator with a time sequence for inspecting the cutting tool. The tool life time distribution and the trend of one control parameter defining its actual condition are supposed to be known.     

Ni supported on La2O3+ZrO2 for dry reforming of methane: The impact of surface adsorbed oxygen species

This research investigates the effect of doping Ni/La₂O₃+ZrO₂ with nitrates of Ca, Cr, Ga, and Gd on the conversion of the feed and H₂/CO molar ratio. The development of these catalysts is intended to tackle inefficiency of dry reforming of CH₄ which stems from deactivation and sintering of active metal. All promoted catalysts perform better than the un-promoted catalyst. Cr promoted sample gives the best performance with CH₄ and CO₂ conversion averaging 83 and 88% respectively. Also, it maintains good stability. The relative ease of reducing the catalyst plus its porous nature are responsible for its performance. Ca promoted sample has the same area as that of the support indicating the movement of Ni–Ca out of the pores of the support during calcination. According to thermogravimetric analysis, the un-promoted catalyst records the highest amount of carbon while the Ca promoted sample has about 18%.     

From mixed finite elements to finite volumes for elliptic PDEs in two and three dimensions

The link between Mixed Finite Element (MFE) and Finite Volume (FV) methods applied to elliptic partial differential equations has been investigated by many authors. Recently, a FV formulation of the mixed approach has been developed. This approach was restricted to 2D problems with a scalar for the parameter used to calculate fluxes from the state variable gradient. This new approach is extended to 2D problems with a full parameter tensor and to 3D problems. The objective of this new formulation is to reduce the total number of unknowns while keeping the same accuracy. This is achieved by defining one new variable per element. For the 2D case with full parameter tensor, this new formulation exists for any kind of triangulation. It allows the reduction of the number of unknowns to the number of elements instead of the number of edges. No additional assumptions are required concerning the averaging of the parameter in hetero‐ geneous domains. For 3D problems, we demonstrate that the new formulation cannot exist for a general 3D tetrahedral discretization, unlike in the 2D problem. However, it does exist when the tetrahedrons are regular, or deduced from rectangular parallelepipeds, and allows reduction of the number of unknowns. Numerical experiments and comparisons between both formulations in 2D show the efficiency of the new formulation. Copyright © 2003 John Wiley & Sons, Ltd.     

Production of a transformer oil from paraffinic basestocks

The work described in this paper is concerned with the production of a high quality transformer oil from local Egyptian paraffinic basestocks. Analysis showed that a neutral base oil 70N was most suitable for producing transformer oils. This neutral base oil was chemically treated with a sulphonation process. Optimum sulphonation conditions were achieved by using oleum or SO₃ at a concentration of 6 wt. %. The sulphonation reaction was carried out in three stages, each with an equal portion of the sulphonating agent. The acidic oil was then neutralised with an alkaline solution. Oil colour improvement was effected using activated clay as an absorbent. Laboratory results and field trials indicated that such a treated neutral paraffinic oil meets the IEC 296 international specifications for transformer oils, with the exception of pour point (−12°C). However, such a pour point is not relevant in certain hot climatic conditions. Comparative evaluation of the treated oil with commercial transformer oils has shown its suitability.     

Synthesis of some additives from upgraded petroleum residues of lubricant oil refineries

Aromatic extract residues occur as by‐products in petroleum refineries, through the process of refining lubricant oil. These residues are considered as having low economic value, and their disposal may even cause problems for the refineries. The present work looks at upgrading these residues by using them to synthesise different petroleum additives, e.g., antioxidants, ashless dispersants, pour‐point depressants, and flow improvers for lubricating crankcase oils and for fuels. The antioxidants were synthesised by reacting mono‐aromatics with phosphorus pentasulphide (P₂S₅) in the presence of zinc oxide (ZnO). Ashless dispersant additives were synthesised by reacting alkylated mono‐aromatics (using chlorinated paraffin wax as an alkylating agent) with formaldehyde solution (37%) in the presence of polyisobutylene succinimide. Fuel pour‐point depressants and flow improvers can be synthesised by acylating di‐aromatics via a Friedel‐Crafts reaction. Comparative evaluation of the synthesised products with commercial additives showed them to have good comparative performance properties.