Sinan Yapici

Heat-transfer enhancement in a channel flow with perforated rectangular blocks

The present paper reports heat-transfer enhancement and the corresponding pressure drop over a flat surface in a channel flow due to perforated rectangular cross-sectional blocks attached on its surface. The channel had a cross-sectional area of 80×160mm2 with blocks 10×25mm2. The experiments covered the following range: Reynolds number (Re) 6670–40 000, the hole inclination angle (θ) 0–45°, the perforation open-area ratio (φ) 0.05–0.15, the diameter of the perforations (D) 2.5–8.0 mm, and the number of the blocks (Nb) 2–7 (giving the ratio of the distance between the blocks to the channel hydraulic diameter (Sx/De) 1.407–0.309). The blocks were transverse to the main flow. It was observed that the heat-transfer enhancement increased with increasing θ,φ and D and decreasing Sx/De and Re. The pressure drop was not affected by θ while it decreased with increasing D, Re, Sx/De, and φ. Correlation equations were developed for the average Nusselt number (Nu) and the friction factor (f). Performance analysis indicated that the solid blocks could lead to energy loses up to 20% despite significantly enhanced heat-transfer due to the increased surface area. The energy lost was recovered by perforations opened in the blocks by which means it was possible to achieve energy gains up to 40%.

Heat transfer and friction characteristics in decaying swirl flow generated by different radial guide vane swirl generators

In radial guide vane swirl generators, the flow direction must change from the radial direction to the axial direction. This can be achieved either abruptly or by means of a fairing section, and each technique can be used in conjunction with an inserted centre body (deflecting element). This research was conducted to study the effect of the geometry of the deflecting element in the radial guide vane swirl generator on the heat transfer and fluid friction characteristics in decaying swirl flow. The radial guide vane swirl generators used in this investigation had three different configurations related to the deflecting element: the swirl generator with conical deflecting element, with spherical deflecting element and with no deflecting element. These swirl generators were compared with each other by taking into account their heat transfer and friction characteristics. An augmentation up to 150% in Nusselt number relative to that of the fully developed axial flow was obtained with a constant heat flux boundary condition, depending upon the vane angles, Reynolds numbers and type of the swirl generators. It was observed that the swirl generator with no deflecting element gave the highest Nusselt numbers and also gave the highest pressure drop in both the swirl generator and the test pipe. Evaluating the effectiveness of the swirl generators for enhancing heat transfer, it was found that using the swirl generator with no deflecting element may be advantageous in terms of heat transfer enhancement and energy saving in comparison with swirl generators with a deflecting element.

Enhancement of heat transfer by turbulent decaying swirl flow

The heat transfer and friction characteristics of a decaying swirl flow were investigated experimentally. The swirling motion of the air was produced by a radial guide vane swirl generator. The vanes of the swirl generator were designed to be adjustable to obtain different swirl intensities. Different guide vane angles (15, 30, 45, 60 and 75°) were used for the swirling flow experiments. The results were correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and the vane angle as Nu=0.133Re0.65Pr0.4(1+tan θ)0.406. An augmentation of up to 98% in Nusselt number was obtained in the decaying swirl flow depending upon the Reynolds number and the vane angle. It was found that the performance of swirling flow is higher than corresponding axial flow with high vane angles and relatively low Reynolds numbers.

Second law analysis of rectangular channels with square pin-fins

Abstract This paper presents heat transfer and friction characteristics, and the second law analysis of the convective heat transfer through a rectangular channel with square cross-sectional pin-fins attached over a flat surface. Various clearance ratios (C/H) and interfin distance ratios (Sx/D) were used, and optimum pin-fin arrays that minimize entropy generation were determined. It was found that Nu number based on the projected area decreased with increasing Sx/D and C/H, whereas Nu based on the total heat transfer area increased with increasing Sx/D and with decreasing C/H. The friction factor was found to decrease with increasing Sx/D and C/H. Smaller entropy generation numbers were obtained at lower Reynolds number, higher clearance ratio, and higher interfin spacing ratio.

Thermal performance analysis for solid and perforated blocks attached on a flat surface in duct flow

The thermal performances of solid and perforated rectangular blocks attached on a flat surface in a rectangular duct were determined with respect to the heat transfer from the same plate without blocks. The data used in the performance analyses were obtained experimentally for varying flow and geometrical conditions. It was found that the solid blocks generated a net energy loss despite significantly enhanced heat transfer due to the increased heat transfer surface area. When the blocks were perforated, the loss in the net energy was recovered and depending on the geometrical and flow conditions, a net gain in energy, up to 20%, was achieved. For both the solid and the perforated blocks, increases in Reynolds number led to decreases in the performance.

LEACHING KINETICS OF MALACHITE IN AMMONIA SOLUTIONS

Abstract In this study, the leaching kinetics of malachite in ammonia solutions was investigated in a magnetically stirred reactor, taking particle size, ammonia concentration, solid/liquid ratio and reaction temperature as parameters. It was observed that the leaching rate increases with increasing ammonia concentration, reaction temperature, decreasing solid/liquid ratio and particle size. The reaction order to the solid concentration is determined to be 1.0. It was also determined that the leaching process was controlled by the diffussion through the ash layer, which is the insoluble inert gangue material consisted of mainly SiO 2 , around the shrinking unreacted core. The activation energy for the leaching process was found to be 22.338 kJ mol −1 , and the Arrhenius constant was calculated to be 2.188 s −1 .

Dissolution kinetics of ulexite in ammonia solutions saturated with CO2

Abstract The dissolution kinetics of ulexite in ammonia solutions saturated with carbon dioxide were investigated. Experiments investigating the following parameters: concentration of ammonia; particle size; stirring speed; solid to liquid ratio; reaction temperature; and carbon dioxide flow rate; showed that ulexite can be dissolved completely in solutions of carbonic acid at atmospheric conditions. It was found that the dissolution rate increased with increasing ammonia concentration, reaction temperature and calcination temperature, and with decreasing particle size and solid to liquid ratio. It was observed that the flow rate of carbon dioxide and stirring speed have almost no effect on the dissolution rate. It was determined that the dissolution rate of ulexite can be described by a first-order pseudo-homogeneous reaction model and an empirical equation for the process was developed: −1 n (1−X)=1.0×10 5 (C A ) 0.22 (d) −0.92 ( S L ) −0.54 exp ( 6600 T −t) where CA is the ammonia concentration, d the initial particle size and ( S L ) the solid to liquid ratio. The activation energy for the process was found to be approximately 55 kJ mol−1.

An environmentally friendly process; Adsorption of radionuclide Tl-201 on fibrous waste tea

This work presents an investigation of the adsorption of the radionuclide of Tl-201 from waste water on the fibrous tea factory waste. The experimental parameters were chosen as temperature, pH, stirring speed, adsorbent dose and nominal particle size in the ranges of 10.0-40.0 degrees C, 2.0-10.0, 300-720 rpm, 1.0-15.0 g/L and 0.15-0.71 mm, respectively. The most effective parameter on the adsorption yield was found to be pH of the solution. Fourier transforms infrared and electron paramagnetic resonance spectroscopy studies were performed for the characterisation of the adsorption on tea waste. The experimental data were found to be in good agreement with the isotherm models of Freundlich, Halsey, Handerson and Dubinin-Radushkevich. Thermodynamic analysis showed that the values of Delta G and Delta H are negative. It was obtained that the adsorption rate can be represented very well by second-order pseudo homogeneous kinetic model. All the results proved that fibrous tea plant waste makes an excellent adsorbent for Tl-201 radionuclide.

Dissolution mechanism of ulexite in H2SO4 solution

Abstract This work comprises an investigation of the dissolution mechanism of ulexite in H 2 SO 4 solution and the effect of SO 4 2− ions and acid concentration on the dissolution process, using H 2 SO 4 , HCL+H 2 SO 4 and H 2 SO 4 +Na 2 SO 4 solutions. From the experimental observations, it can be suggested that the overall dissolution reaction can be written as Na 2 O·2CaO·5B 2 O 3 ·16H 2 O (s) +3H 2 SO 4(aq) →2Na + (aq) +2CaSO 4 ++10H 3 BO 3(aq) +SO 2− 4(aq) +4H 2 O (l) (6) or Na 2 O·2CaO·5B 2 O 3 ·16H 2 O (s) +3H 2 SO 4(aq) →2Na + (aq) +2CaSO 4 ·2H 2 O (s) +10H 3 BO 3(aq) +SO 2− 4(aq) depending upon the experimental conditions. It was observed that, increasing H 3 O + acid concentration increased the dissolution rate, but increasing SO 4 2− concentration reduced dissolution rate due to the precipitation of a solid film of CaSO 4 and/or CaSO 4 ·2H 2 O, determined by X-ray and SEM measurement.

The optimisation of the dissolution of phosphate rock with Cl2SO2 gas mixtures in aqueous medium

Abstract In the present study, aimed at the production of phosphate compounds, the optimum process conditions were sought for the dissolution of phosphate rock from the Mardin-Mazidaǧi concentrate. The dissolution of this phosphate rock in aqueous solution saturated with a mixture of Cl2SO2 gases was optimized by passing Cl2 gas through the final solution. Reaction temperature, solid-liquid ratio, Cl2 gas flow rate, reaction period and stirring speed were employed as parameters. Using the Taguchi Fractional Design Method, it was found that the optimum process conditions, at which 93.35% P2O5 conversion was reached, were as follows: Reaction temperature: 20°C Solid-liquid ratio: 1 7 (w/v) Cl2 gas flow rate: 120 cm3 min−1 Reaction period:20 min Stirring speed: 600 min−1