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    Effects of sinusoidal turbulator in cylindrical channel on heat transfer and flow characteristics
    (Maejo Univ, 2020) Altun, Aziz Hakan; Gurdal, Mehmet; Berber, Adnan
    Effects of heat transfer and flow characteristics on Reynolds numbers of different sinusoidal turbulators placed separately from a pipe in the turbulent flow were investigated experimentally. The experiments were carried out under forced flow and constant heat flow conditions. The elements, which drew a sine curve used as a turbulator, were positioned axially to the channel centre. The experiments were repeated for 3 width values of sinusoidal turbulators that correspond to 3D/4, D/2 and D/4 (D = inner diameter of test pipe). The experiments showed that although there were increases in the rate of heat transfer with the turbulator, they caused considerable pressure drop. As a result, as the width of the turbulator increased, the heat transfer ratio (the Nusselt number) and coefficient of friction increased. Increase in Nusselt number was about 39-86% at D/4 width, 70-119% at D/2 width and 72-170% at 3D/4 width. It was observed that the coefficient of friction was significantly affected by the width of the sinusoidal turbulator, being about 0.02 in the case of straight pipe, 0.184 - 0.242 at D/4 width, 0.345 - 0.415 at D/2 width, and 0.416 - 0.451 at 3D/4 width. In addition, increasing coefficient of friction increased pressure drop and power required for pumping.
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    Effects of trapezoidal and twisted trapezoidal tapes on turbulent heat transfer in tubes
    (Pergamon-Elsevier Science Ltd, 2022) Altun, Aziz Hakan; Nacak, Haluk; Canli, Eyub
    Trapezoidal and twisted trapezoidal tape elements which are used for heat transfer enhancement in tubes are experimentally investigated. The effects of the tapes on heat transfer, pressure drop, and overall performance factor are analyzed. Three different thread heights are used in the experiments, which are obtained by dividing the tape width by 4, 6, and 10. Pitch length is kept constant for the trapezoidal tapes, while the constant twisting ratio is used for the twisted trapezoidal ones. Tapes were held in the tube with no wall contact by stretching mechanically. Experiments were conducted under constant heat flux boundary condition and for Reynolds number interval of 5000-25,000, targeting turbulent flow at which overall performance factor values reported in literature are relatively small comparing to laminar flow. The experimental results show that trapezoidal tape elements enhance heat transfer and increase pressure drop with changing rates depending on thread heights. Overall performance factor in the tested interval is detected over unity for two twisted trapezoidal inserts, making them significant alternatives comparing to the literature. The best thermal performance of 1.164 is obtained with the twisted trapezoidal tape element with the maximum thread height, while Nu is 90.52 and friction factor is 0.12. A maximum Nu of 118.46 is achieved with the proposed twisted trapezoidal tape, which is 1.9 times higher than the plain tube case and 1.57 times higher than the conventional twisted tape case. Since the twisted trapezoidal tapes perform better than the plain trapezoidal ones, it is concluded that the combination of the twisted and trapezoidal forms may be favorable in terms of heat transfer enhancement and overall performance factor regarding combination of swirl flow effect and periodical flow disturbance effect.
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    Experimental investigation of the effects of horizontally oriented vertical sinusoidal wavy fins on heat transfer performance in case of natural convection
    (Pergamon-Elsevier Science Ltd, 2019) Altun, Aziz Hakan; Ziylan, Orkun
    In this work, natural convection heat transfer in horizontally oriented, vertical sinusoidal wavy fins, involving radiation heat transfer were investigated experimentally. Experiments were done with wavy fins having three different amplitudes, namely H/30, H/15 and H/10, with different heater power inputs. Results were compared to the results of a rectangular finned plate and also a reference horizontal base plate. Power of the heater was changed from 1.02 W to a maximum of 32.06 W and totally 69 experiments were done for 5 test sets. Results indicate that, heat transfer enhancement is better with wavy fins than with the rectangular fins. On the other hand, after a certain value of the wave amplitude the increase in the amplitude negatively affecting the natural convection due to the blockage of fluid motion. The results also indicate that, an important part of total heat transfer is by radiation and should be considered in natural convection studies of finned surface heat sinks. (C) 2019 Elsevier Ltd. All rights reserved.