Buoyancy driven heat transfer of water-based nanofluids in a differentially heated, tilted enclosure is investigated in this study. The governing equations (obtained with the Boussinesq approximation) are solved using the polynomial differential quadrature method for an inclination angle ranging from 0 deg to 90 deg, two different ratios of the nanolayer thickness to the original particle radius (0.02 and 0.1), a solid volume fraction ranging from 0% to 20%, and a Rayleigh number varying from 10 4 to 10 6 . Five types of nanoparticles, Cu, Ag, CuO, Al 2 O 3 , and TiO 2 are taken into consideration. The results show that the average heat transfer rate from highest to lowest is for Ag, Cu, CuO, Al 2 O 3 , and TiO 2 . The results also show that for the particle radius generally used in practice (␤ ϭ 0.1 or ␤ ϭ 0.02), the average heat transfer rate increases to 44% for Raϭ 10 4 , to 53% for Raϭ 10 5 , and to 54% for Raϭ 10 6 if the special case of ϭ 90 deg, which also produces the minimum heat transfer rates, is not taken into consideration. As for ϭ 90 deg, the heat transfer enhancement reaches 21% for Raϭ 10 4 , 44% for Raϭ 10 5 , and 138% for Raϭ 10 6 . The average heat transfer rate shows an increasing trend with an increasing inclination angle, and a peak value is detected. Beyond the peak point, the foregoing trend reverses and the average heat transfer rate decreases with a further increase in the inclination angle. Maximum heat transfer takes place at ϭ 45 deg for Raϭ 10 4 and at ϭ 30 deg for Raϭ 10 5 and 10 6 .

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SUMMARYEnergy and exergy analysis were carried out for a combined-cycle power plant by using the data taken from its units in operation to analyse a complex energy system more thoroughly and to identify the potential for improving efficiency of the system. In this context, energy and exergy fluxes at the inlet and the exit of the devices in one of the power plant main units as well as the energy and exergy losses were determined. The results show that combustion chambers, gas turbines and heat recovery steam generators (HRSG) are the main sources of irreversibilities representing more than 85% of the overall exergy losses. Some constructive and thermal suggestions for these devices have been made to improve the efficiency of the system.

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AbstractÇelen S., Kahveci K. (2013): Microwave drying behaviour of tomato slices. Czech J. Food Sci., 31: 132-138.The microwave drying behaviour of tomato slices was investigated experimentally to determine the effects of microwave power on the drying rate, energy consumption, and dried product quality in terms of colour, and a theoretical model was proposed to define the drying curves of tomato slices. The experiments performed with the microwave power of 90, 180, 360, and 600 W indicate that the drying time and the energy consumption decreased considerably with an increase in microwave power. The experiments also revealed that the drying rate shows first an increase and then a decrease during drying, and that the colour quality of the product deteriorates significantly with the increase of the microwave power. A theoretical model was developed using the solution of energy equation considering the microwave power as an internal heat source. The electric field strength inside the material was assumed to be dependent on the moisture content and the constants emerging from this assumption were obtained by minimising the sum of squared differences between the theoretical results and experimental data obtained for various drying conditions. The results show that the values proposed for the constants provide a good agreement between the theoretical and experimental drying behaviour.

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