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    A diffusion based model for intermittent drying of rough rice
    (Springer, 2008) Cihan, Ahmet; Kahveci, Kamil; Hacihafizoglu, Oktay; de Lima, Antonio G. B.
    In this study, intermittent drying behavior of single layer rough rice with a moisture content of between 22 and 24% on the dry basis was simulated by means of a liquid diffusion model based on a prolate spheroid geometry. For this purpose, solution of the liquid diffusion equation was fitted to the experimental data for the drying air temperature 40 degrees C, drying velocity 1.5 ms(-1) and tempering periods ranging from 0 to 1 h. In order to make a comparison, solution of the liquid diffusion equation for a finite cylindrical geometry was also fitted to the experimental data. The results show that the liquid diffusion model based on a prolate spheroid geometry explains the drying behavior of rough rice more accurately. The results also show that greater variations occur in diffusion coefficient with increasing tempering time for prolate spheroid geometry which is more realistic geometry for a rough rice grain.
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    Energy-exergy analysis of gas turbine cycle in a combined cycle power plant
    (Sila Science, 2011) Hacihafizoglu, Oktay
    In this study, the performance characteristics of a gas turbine cycle in a combined cycle power plant (CCPP) were examined for a set of design and operational parameters. In this context, the effect of inlet air cooling (IAC), gas turbine inlet temperature (TIT) and steam injection gas turbine (STIG) on gas turbine power, thermal and exergy efficiency were taken into consideration. The analysis was carried using theoretical calculations by taking the experimentally measured data of a GT 13 D2 type gas turbine in operation conditions as a starting point. The results show that cooling the compressor inlet air temperature from 30 degrees C to 0 degrees C leads to 13.6% and 4% increase in power output and thermal efficiency, respectively. The results also indicate that changing the turbine inlet temperature from 990 degrees C to 1750 degrees C leads to an increase in thermal efficiency by 11% and exergy efficiency by 39% as well as an increase in turbine power output, as 130%. STIG method provides almost the same amount of power increase like IAC method and it is seen that the amount of increase in efficiency in this method is in a higher level compared to IAC method. Moreover, the amount of 5% compressor inlet air steam injection to combustion chamber leads to an increase in thermal efficiency by 10.6% and exergy efficiency by 37% and as well as an increase in turbine power output, as 11%.
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    Experimental and numerical investigation of heat transfer in a channel with multiple phase change materials (PCMs)
    (Elsevier, 2022) Akyol, Erhan; Hacihafizoglu, Oktay; Susantez, Cigdem; Kahveci, Kamil; Akyol, Ugur
    In this study, the thermal performance of an air-phase change material (PCM) unit used in free cooling applications was examined experimentally and numerically by Comsol finite element simulation software. Effects of the air inlet temperature, velocity and also PCM configuration on the efficiency of the air-PCM unit were investigated. Experiments were performed on PCM unit, which is a channel including PCM plates inside and exposed to medium air entrance from one side. The air in contact with the plates inside this unit is cooled and contributes the room temperature to decrease. The experimental work was carried out at two different ambient temperatures in a well-insulated room. Three different configurations of paraffin-based pure RT22HC and RT25HC were studied. Effects of air inlet temperature and velocity on the heat transfer, channel outlet temperature, performance of the air-PCM heat exchanger, cooling power of the system and also total amount of heat absorbed by the PCM were investigated. The initial temperature of the PCMs is 16 degrees C and experiments were carried out for two different average inlet temperature values (27 and 29 degrees C) and two different average velocities (1.3 and 1.8 m/s) of the air at the channel entrance. According to the results, it was concluded that the air inlet temperature and air inlet velocity have a substantial effect on the thermal performance of the air-PCM unit. It was observed that the melting time decreased and the channel outlet temperature increased with the increase of air inlet temperature and velocity. 2 degrees C increase of in the inlet temperature increased the total amount of heat absorbed by PCMs by 31%, while an increase of the inlet velocity by 0.5 m/s increased it by 16%. The results revealed that the increase of the inlet velocity and with the inlet temperature increased the thermal energy absorbed by 44%. In addition, it was observed that the average efficiency increased by 37% with 2 degrees C increase of inlet temperature and 0.5 m/s decrease in the inlet velocity.
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    Finite Element Simulation of Drying of Rough Rice
    (Trans Tech Publications Ltd, 2011) Hacihafizoglu, Oktay; Cihan, Ahmet; Kahveci, Kamil
    Rough rice has high moisture content at harvest, and if their moisture content is not reduced to about 12% on the dry basis they decay due to infestation and mold growth. Drying process is generally performed by heating ambient air and then sending it by forced convection over the rough rice to be dried. The aim in heating air to a certain temperature is to reduce the relative humidity of the air, which has a positive effect on the drying potential. In this study, drying behavior of single layer rough rice for different drying air temperature was simulated by means of a liquid diffusion model numerically by finite element modeling and simulation software. The results show that temperature is an effective factor on the drying rate. The results also show that as drying proceeds, a moisture gradient develops within the grain. This slows down the drying rate considerably. Therefore, it can be concluded that performing drying with an intermittent period instead of continuous drying will cause a considerable energy-saving.
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    Investigation of models of the yarn-bobbin drying process by determination of their parameters using genetic algorithm
    (Sage Publications Ltd, 2017) Susantez, Cigdem; Hacihafizoglu, Oktay; Kahveci, Kamil
    In the first part of this study, the drying behavior of wool-acrylic yarn bobbins was investigated by a theoretical model and genetic algorithm method. Each candidate solution for D-o, D-1 and D-2 was presented on a single chromosome. The values of D-o, D-1 and D-2 yielding the best fit between the experimental and predicted moisture contents were obtained using the genetic algorithm. In the second part of this study, the suitability of various empirical and semiempirical models in the modeling of the drying process was investigated by the genetic algorithm. The population number was taken as 30 and the tournament selection method was used. The calculations were performed until the 20th generation for the theoretical model and 100th generation for the empirical and semiempirical models. The results show that the genetic algorithm can be successfully used in the modeling of the drying process of yarn bobbins. The results also show that the Verma etal. and Diffusion Approach models yield the best fit with experimental data.
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    A liquid diffusion model for thin-layer drying of rough rice
    (Springer, 2008) Hacihafizoglu, Oktay; Cihan, Ahmet; Kahveci, Kamil; de Lima, Antonio G. B.
    In this study, the drying behavior of single-layer rough rice with a moisture content of between 22 and 24% on the dry basis was simulated by means of a liquid diffusion model, based on a prolate spheroid geometry. For this purpose, the solution of liquid diffusion equation was fitted to the experimental moisture ratios for drying air temperatures between 40 and 60 degrees C and velocity 1.5 m s(-1). In order to make a comparison, the predictions of liquid diffusion equations for a spherical and finite cylindrical geometry were also fitted to the experimental results. Modeling was performed by selecting the diffusion coefficients in diffusion equations in such a manner as to minimize the sum of the squared differences between the experimental results and the theoretical predictions. It was found that the liquid diffusion model, based on a prolate spheroid geometry, explains single-layer drying behavior of rough rice well. It was also found that the model, based on a prolate spheroid geometry, has better agreement with the experimental results than the other geometries.
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    Modelling of intermittent drying of thin layer rough rice
    (Elsevier Sci Ltd, 2007) Cihan, Ahmet; Kahveci, Kamil; Hacihafizoglu, Oktay
    Various mathematical models in describing the intermittent drying characteristics of thin layer rough rice were investigated. Experimental values for drying temperature 40 degrees C, drying velocity 1.5 ms(-1) and tempering period ranging from 0 h to 1 h were fitted to the theoretical models to relate the parameters of the drying models with the drying conditions. Suitability of fitting of the experimental data by models was specified as comparing the correlation coefficient, standard error and mean square deviation. The Midilli model was found to be the most adequate model in describing the intermittent drying of thin layer rough rice. The coefficient a and b, drying coefficient k and exponent n in the Midilli model can. be expressed as a polynomial function of tempering time. (c) 2006 Elsevier Ltd. All rights reserved.
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    NUMERICAL INVESTIGATION OF INTERMITTENT DRYING OF A CORN FOR DIFFERENT DRYING CONDITIONS
    (Vinca Inst Nuclear Sci, 2019) Hacihafizoglu, Oktay; Susantez, Cigdem; Kahveci, Kamil; Yilmaz, Sercan
    In this study, intermittent drying process of corn was studied numerically for various intermittent periods and drying air temperatures. An Arrhenius type dffusi-coefficient D = e((-b.T)). 10(-9) m(2)/s was proposed for the moisture diffusion inside the corn. Numerical simulations were performed by choosing the suitable value for drying constant, b, that yields the best agreement with experimental drying rates. The experimental results were obtained via an experimental setup for intermittent periods of 30 minute and 60 minute, and drying air temperatures of 40 degrees C, 50 degrees C, 60 degrees C, and 70 degrees C. The results show that overall agreement between the experimental and theoretical prediction is good. On the other hand, the theoretical results overestimate the moisture ratio at the initial stage and underestimate it at the later stage of drying.