Search

The paper discusses the origins and course of the current crisis in Mali. The author outlines the reasons for the structural weakness of Mali, resulting from the shape of the borderline drawn at the time of decolonization, the confrontation between the Tuareg and the central government, the impact of the collapse of Gaddafi’s dictatorship in Libya at the onset of a new stage in the confrontation, the role of Islamist groups in northern Mali, and the course of French intervention there. The case of Mali as an almost classic ‘fallen state’ is treated separately., W artykule omówiono genezę i przebieg bieżącego kryzysu politycznego w Mali. Autor nakreślił przyczyny strukturalnej słabości Mali wynikające z dokonanego w czasie dekolonizacji wytyczenia granic, przebieg konfrontacji między Tuaregami a rządem centralnym, wpływ upadku dyktatury Kadafiego w Libii na otwarcie nowej fazy konfrontacji, rolę ugrupowań islamistycznych na północy Mali oraz przebieg francuskiej interwencji w tym państwie. W sposób odrębny potraktowany został przypadek Mali jako klasycznego niemal „państwa upadłego”.

The aim of this work was to characterize the microstructure of the as-cast Haynes® 282® alloy. Observations and analyses were carried out using techniques such as X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray spectroscopy (EDS), wave length dispersive X-ray spectroscopy (WDS), auger electron spectroscopy (AES) and electron energy-loss spectrometry (EELS). The phases identified in the as-cast alloy include: γ (gamma matrix), γʹ (matrix strengthening phase), (TiMoCr)C (primary carbide), TiN (primary nitride), σ (sigma-TCP phase), (TiMo)2SC (carbosulphide) and a lamellar constituent consisting of molybdenum and chromium rich secondary carbide phase together with γ phase. Within the dendrites the γʹ appears mostly in the form of spherical, nanometric precipitates (74 nm), while coarser (113 nm) cubic γʹ precipitates are present in the interdendritic areas. Volume fraction content of the γʹ precipitates in the dendrites and interdendritic areas are 9.6% and 8.5%, respectively. Primary nitrides metallic nitrides (MN), are homogeneously dispersed in the as-cast microstructure, while primary carbides metallic carbides (MC), preferentially precipitate in interdendritic areas. Such preference is also observed in the case of globular σ phase. Lamellar constituents characterized as secondary carbides/γ phases were together with (TiMo)2SC phase always observed adjacent to σ phase precipitates. Crystallographic relations were established in-between the MC, σ, secondary carbides and γ/γʹ matrix.

New simulation and experimental results have been obtained and are presented for a multi-tube fin heat exchanger unit, from which semi-analytical correlations for the Fanning friction and Colburn factors were developed. The multi-tube and fin heat exchanger represents the main component of the Fan Coil Unit, an essential component of HVAC systems used for domestic and commercial heating and cooling. Improving the efficiency of the heat exchanger typically comes at the expense of higher pressure drops or costlier materials and production costs. Here, an experimental setup was designed and constructed to evaluate the thermal performance of such a heat exchanger. Geometrical modifications were explored for thermal performance enhancement. Furthermore, full three-dimensional CFD case studies of the heat exchanger were investigated to examine the effect of the geometrical features on the air side of the heat exchanger to study the effect of fin spacing, transverse and longitudinal pitches. The CFD model developed was first globally validated against experimental results. The model results were used to predict the Fanning and Colburn factors and the local fin efficiency based on the carefully selected geometric parameters. The data obtained was utilised to develop two new semi-analytical models for the Fanning and Colburn friction factors which were well within ±10% error bands and showed strong correlation coefficients of more than 98 and 97% respectively.

Multi-tube multi-fin heat exchangers are extensively used in various industries. In the current work, detailed experimental investigations were carried out to establish the flow/heat transfer characteristics in three distinct heat exchanger geometries. A novel perforated plain fin design was developed, and its performance was evaluated against standard plain and louvred fins designs. Experimental setups were designed, and the tests were carefully carried out which enabled quantification of the heat transfer and pressure drop characteristics. In the experiments the average velocity of air was varied in the range of 0.7 m/s to 4 m/s corresponding to Reynolds numbers of 600 to 2650. The water side flow rates in the tubes were kept at 0.12, 0.18, 0.24, 0.3, and 0.36 m3/h corresponding to Reynolds numbers between 6000 and 30,000. It was found that the louvred fins produced the highest heat transfer rate due to the availability of higher surface area, but it also produced the highest pressure drops. Conversely, while the new perforated design produced a slightly higher pressure drop than the plain fin design, it gave a higher value of heat transfer rate than the plain fin especially at the lower liquid flow rates. Specifically, the louvred fin gave consistently high pressure drops, up to 3 to 4 times more than the plain and perforated models at 4 m/s air flow, however, the heat transfer enhancement was only about 11% and 13% over the perforated and plain fin models, respectively. The mean heat transfer rate and pressure drops were used to calculate the Colburn and Fanning friction factors. Two novel semiempirical relationships were derived for the heat exchanger’s Fanning and Colburn factors as functions of the non-dimensional fin surface area and the Reynolds number. It was demonstrated that the Colburn and Fanning factors were predicted by the new correlations to within ±15% of the experiments.