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Thin layer drying of red pepper

The thin layer drying behaviour of red pepper slices is experimentally investigated in a convective dryer and the mathematical modelling by using thin layer drying models in literature is performed. Drying experiments were conducted at inlet temperatures of drying air of 55, 60 and 70 °C and at a drying air velocity of 1.5 m/s. Eleven different thin layer mathematical drying models were compared according to their coefficient of correlation to estimate drying curves. The effects of drying air temperature on the model constants and coefficients were predicted by regression models. According to the results, an approximation of the diffusion model could satisfactorily describe the drying curve of red peppers with a correlation coefficient (r) of 0.9987. The constants and coefficients of this model could be explained by the effect of drying air temperature.

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 TRANSFER ENHANCEMENTS IN A CONCENTRIC DOUBLE PIPE EXCHANGER EQUIPPED WITH SWIRL ELEMENTS

The effect of heat transfer rates of swirl generators with holes for the entrance of fluid were investigated by placing them at the entrance section of inner pipe of heat exchanger. Various swirl generators having different arrangements of boles were used. Hot air and cold water were passed through the inner pipe and annulus respectively. Experiments were carried out for both parallel and countercurrent flow models of the fluids at Reynolds numbers between 8500-17500

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.

Utilization of fly ash and polypropylene wastes in the production of a new porous composite material

This work was undertaken on two different wastes to produce a new construction material which may have good insulation and mechanical strength properties. Fly ash, a waste produced at power stations, and polypropylene, a waste generated mostly from packaging, were used to produce smooth-surfaced test blocks. Compressive and tensile strength, abrasion, water absorption, thermal conductivity and specific heat capacity tests were done on these blocks. It was concluded that these two wastes could be utilized to produce useful insulation and other building materials and at the same time eliminate environmental problems that may be caused by them if disposed off.

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.

The efficiency of heat exchangers with rotating inner pipes

Heat transfer correlations are given in terms of Nusselt, Reynolds and Rossby numbers, the flow mode (parallel or countercurrent) and the inner rotating-pipe surface area. The effects of these parameters on the pressure drop have also been evaluated. The results are compared with values for conventional systems. Rotation of the helically threaded inner pipe leads to Nu numbers which may be several times higher than those found for a smooth stationary pipe. Although the rotation of the inner pipe caused some increase in pressure drop, this effect was unimportant compared with the improvement in heat-transfer capacity.