Browsing by Author "Chatzi, Panagiota"

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    Conductivity grading in epoxy graphene oxide composites using thermal reduction.
    (Cranfield University, 2014-02) Chatzi, Panagiota; Skordos, Alexandros A.
    The aim of this project is the development of a method for the production of conductivity graded polymeric materials through the in-situ reduction of Graphene Oxide (GO). These types of composites can be used for the reduction of electrical field concentrations observed in several applications, such as cables, cable joints, microelectronic components and bushings. The evolution of electrical conductivity of GO was monitored during thermal reduction under isothermal and dynamic heating thermal conditions and compared with the kinetics of the reduction reaction as observed using calorimetry. These results were replicated in epoxy/GO composite to prove the feasibility of in-situ reduction and capabilities for controlling this process. In addition, reduction was achieved in epoxy/GO composites using infrared continuous and pulsed laser. It was shown that thermal reduction is possible in the cured epoxy composite at relatively low temperatures up to 26ᴼC. The kinetics of reduction follows a double mechanism which can be represented using a double n-th order process. Electrical measurements showed that conductivity increased significantly up to 0.12 S/m for dry particles and 0.09 S/m for the composite. Heat transfer by conduction and irradiation on specific areas resulted in selective reduction, and a conductive graded material.
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    Metal foam recuperators on micro gas turbines: Multi-objective optimisation of efficiency, power and weight
    (Elsevier, 2024-01-19) Chatzi, Panagiota; Efstathiadis, Theofilos; Skordos, Alexandros A.; Kalfas, Anestis I.
    Small size and high efficiency of micro gas turbines require a higher surface-to-volume ratio of recuperators. Conventional recuperators can achieve a range of 250–3600 m2/m3. Advances in materials and manufacturing, such as metal foams, can increase significantly the exchange surface and improve compactness ranging approximately from 500 to over 10,000 m2/m3, due to their exceptional micro geometry. The main advantage is that the increase of surface area does not impact the cost of the heat exchanger as much as conventional recuperators due to their easy manufacturing. This work addresses the optimisation of the recuperator using multiple objectives satisfying efficiency, power output and weight criteria, offering a holistic approach that takes into account the entire system rather than individual components or channels. A model is developed to represent the performance of a compact heat exchanger in micro gas turbines. The recuperator is an annular heat exchanger with involute profile filled with porous media in a counterflow arrangement on the hot and cold sides. The model allows the evaluation of the effect of the recuperator geometry features on the electrical efficiency, power output and weight savings in a micro gas turbine. Existing models for the global heat transfer coefficient, effective thermal conductivity, surface area and pressure drop of porous media are selected and implemented. The design variables of multi-objective are the pore density, porosity and number of channels, whilst the objectives are the overall electrical efficiency, power output and recuperator weight. The problem is solved using the Non-Dominated Sorting Genetic Algorithm (NSGA-II) to determine an approximation of the Pareto front, whilst the accuracy of the approximation is assessed against the solution obtained by an exhaustive search. The comparison shows that NSGA-II outperforms an exhaustive search by at least 90 % in terms of computational efficiency. These results allow the quantification of the impact of metal foam technology on performance metrics of the recuperator as well as the entire system. This quantitative analysis provides valuable insights into the behaviour of metal foam recuperators in micro gas turbines. An optimal design with 30 % efficiency and 28 kW power output appears in pore densities of approximately 10 and 20 pores per inch (PPI) for the air and gas side respectively, and a porosity of 85 %, which leads to a state-of-the-art recuperator weight of 48 kg. The efficiency improvement over the industry standard is 15 %, with only a 2.5 % reduction in power output.