Dursun Pehlivan

Modelling of drying kinetics of single apricot

Abstract In this work, the drying behaviour of single apricots, hanging in the flow direction of hot air in a drying chamber, was investigated. In the experiments, the air was passed through the chamber at a variety of flow rates (0.2, 0.5, 1.0 and 1.5 m/s) and temperatures (50, 60, 70 and 80 °C). In order to obtain drying data, the changes in the mass and inner temperatures of the apricots were recorded as well as the drying air properties. The variations of drying rates with time and moisture content were used to test fourteen different one-layer drying models given in the literature and a new model was developed. Among all the models, the logarithmic model was found the best for explaining the drying behaviour of apricots. The effects of drying air flow rates and temperatures on the constants and coefficients of the models were also studied by multiple regression analysis. It was found that the new model could represent single apricot drying kinetics within 99.9% accuracy. The moisture transfer from the apricots occurring during the falling-rate period of drying was characterised by determining experimentally the apparent diffusion coefficient into the air. It was found, by multiple regression analysis, that the diffusion coefficients could be expressed within 97.3% accuracy, depending on the air flow rates and inside temperatures of the single apricot. In studying the consistency of all the models, some statistical tests, such as reduced χ 2 , mean bias error and root mean square error were also used as well as correlation coefficients. The results of these tests have also confirmed the consistency of the logarithmic model.

Mathematical modelling of solar drying of apricots in thin layers

Abstract Solar drying experiments in thin layers of apricots grown in Elazig, Turkey, were conducted. An indirect forced convection solar dryer consisting of a solar air heater with conical concentrator and a drying cabinet were used in the experiments. Air was forced into the solar air heater by a blower and hot air obtained there was passed over the apricots. The changes in the mass of apricots and principal drying parameters were recorded continuously from morning to evening in each test day. Drying curves obtained from the data were fitted to a number of mathematical models and the effects of drying air temperature, velocity and relative humidity on the model constants and coefficients were evaluated by the multiple regression and compared to previously given models. The logarithmic drying model was found to satisfactorily describe the solar drying curve of apricots with a correlation coefficient ( r ) of 0.994. The constants and coefficients of this model could be explained by the effect of drying air temperature, velocity and relative humidity with a correlation coefficient ( r ) of 1.000.

Drying of apricots in a rotary column cylindrical dryer (RCCD) supported with solar energy

Rapid deterioration of fruits and vegetables necessitates their consumption in a limited area. The solutions to this problem may be provided by technological precautions, which not only increase economic efficiency but also motivates agricultural developments. Among the technological operations applied to increase the market life of the foods, drying operation is of prime importance which is applicable to almost all the agricultural products. In this work, drying of apricots in a newly developed rotary column cylindrical dryer (RCCD) equipped with a specially designed air solar collector (ASC) was investigated to find optimum drying air rate and rotation speed of dryer, to maintain uniform and hygienic drying conditions and to reduce drying times. For this purpose, the changes in the moisture contents of the Sugarpiece (Şekerpare) foods were studied. Drying is an operation of prime importance used for almost all the agricultural products. Type apricots dried in the present dryer were compared to those of the same type apricots dried open on the ground. It was shown that co-operation of RCCD and ASC increased drying rate, reduced drying times and rotation of drying chamber provided gains in both time and labour.

Heat transfer and pressure drop in a heat exchanger with a helical pipe containing inside springs

In this study, a heat exchanger which is constructed by placing spring-shaped wires with varying pitch within a helical pipe was considered. The pressure drop and the overall heat transfer rates were measured for the case of air flow at various Reynolds numbers inside and constant water flow outside. The results show that the Nusselt number increases with decreasing pitch/wire diameter ratio, as much as five times with respect to an empty pipe for the same Dean number, and for this relationship, a tentative empirical formula is suggested. Although a rise up to 10 times in the inlet/outlet pressure drop values with respect to the conventional empty helical case is observed, the increase in Nusselt number, naturally, reflects an increase of about 30% in the effectiveness of the helical heat exchanger.

Influence of fluid rotation on the heat transfer and pressure drop in double-pipe heat exchangers

In this work the effect of propellers, which were able to rotate freely due to the force exerted by the fluid flowing through the inner pipe of the double-pipe heat exchangers, on the heat transfer and pressure was investigated expermentally. The experiments were undertaken for the Reynolds number range of 2500-15000 and for several propellers. In the system, heat-transfer rates were about 250% better than those found with the system without propellers. Thus, because of rotating the fluid, propellers and their number become influential upon heat transfer. The increase in pressure-drop changed between 500 and 1000% in comparison with that for the empty tube, depending on Reynolds number and number of propellers.

Effect of twisted strips on heat transfer and pressure drop in heat exchangers

Abstract Twisted narrow, thin metallic strips were placed in the inner pipe of a concentric double-pipe heat exchanger. Their effects on heat transfer and pressure drop were studied for parallel and countercurrent flow. These turbulators were prepared by twisting the strips through certain angles and designed to touch the inside wall at each step. In the system, hot air was passed through the inner pipe, while cold water was flowing through the annulus. Experiments performed with an empty inner tube confirmed the results with the Dittus-Boelter expression given for the Nusselt number. It was observed that the Nu number could increase up to 100% at a cost of about 130% increase in pressure drop for the tube with twisted strips inside.

Heat transfers and pressure drops in rotating helical pipes

Helical pipes are employed extensively in chemical reactors, reservoirs, nuclear generators, evaporators and condensation flow systems as well as energy saving applications. In this study, in order to increase the heat-transfer rates in helical pipes of various diameters, the rotation of the pipes was considered experimentally. An experimental set-up for determining the heat-transfer rates and pressure drops occurring, because of the centrifugal effects on the rotating helical pipes, was designed. The heat-transfer rates and pressure drops were measured for various air-flow rates and helical pipe rotational speeds. Although the rotation caused an increase in pressure drop, heat-transfer capacities of the helical pipes were observed to improve notably. The Nusselt numbers rose due to the increases in Reynolds numbers and heat inputs into the system. Beside these, further improvements in heat transfer were observed with the decreasing curvature diameter of the helical pipe.

The performance of a solar air heater with conical concentrator under forced convection

Abstract In this work, the efficiency of a new solar air heater with a conical concentrator, which can track the sun from sunrise to sunset, was investigated under forced convection conditions of air. The collector, whose absorber was arranged as a two-pass exchanger and mounted on the focal axis of the conical concentrator, was tested for a number of tilting angles and a variety of air mass flow rates. In addition, major operation parameters related to efficiencies of the collector and heat flow in the absorber were determined and correlated with each other and compared with flat-plate solar collectors. The highest efficiencies and maximum increase in the air temperatures were obtained at 28.4° tilting and the lowest mass flow rate, respectively. Although, the efficiencies were similar, the maximum outlet temperatures of the air were found to be approximately twice and heat transfer rates inside the absorber were higher than those reached by conventional flat-plate solar air heaters.

THERMAL EFFICIENCY OF AN AIR SOLAR COLLECTOR WITH EXTENDED ABSORPTION SURFACE AND INCREASED CONVECTION

A modified solar air heater, which incorporated aluminium wool on a perforated plate placed diagonally on the passageway of the air to serve as a front absorbing medium above the absorber plate was designed, constructed and tested. Its efficiency was compared with the same sized conventional solar collector. The efficiencies of these two collectors were evaluated in terms of the effects of temperatures and duration times for the same airflow rates. It was found that the heat transferred to the air was increased greatly in the case of solar air heater with front absorbing surface.

REACTIONS OF METHOXY-SUBSTITUTED AROMATIC COAL MOOELS IN SUPERCRITICAL SOLVENTS

Abstract Reactions of α- and β-methoxy naphthalene and anisole in supercritical benzene and toluene were studied at 420 °C and 3.9-4.4 MPa pressure. The decomposition of these lower rank coal related model compounds in both solvents was found to obey the first order rate law. While naphthols were present in the reaction mixture of methoxy naphthalenes, phenol and cresoles were present in that of anisole. In addition to these products, diphenyl or dibenzyl was also present in the reaction mixture of supercritical benzene or toluene, respectively. The reactions between models and solvents might be explained by free radical and intermolecular rearrangement reaction pathways.