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    Control of the flow past a sphere near a flat wall using passive jet
    (Pergamon-Elsevier Science Ltd, 2019) Okbaz, Abdulkerim; Ozgoren, Muammer; Dogan, Sercan; Canpolat, Cetin; Akilli, Huseyin; Sahin, Besir
    In the present investigation, interactions between the vortices shedding from a single sphere and the boundary layer flow over a flat plate with various gap ratios have been investigated at Re = 5000 using the techniques of dye visualization and Particle Image Velocimetry. A passive jet has been created to control the flow past the sphere by a hole drilled into the sphere from the forward stagnation point to the rear of the sphere with various diameters. The results show that the complex flow downstream of the sphere, which is occurred from the combination of 'Carman vortex street and the boundary layer over the flat plate can be controlled by a passive jet. Because the jet flow interrupts periodicity of the vortices shedding from the sphere by supplying fluid flow into the wake with relatively high velocity. Presence of the hole significantly attenuates the magnitudes of the turbulence characteristics. Furthermore, the magnitudes of the turbulence characteristics decrease with increasing the hole diameter depending on the location of the sphere in the boundary layer. However, a larger hole diameter can result in higher turbulence levels in the region of the passive jet. Hence, the effectiveness of the each case (the diameter of the hole) changes with the location of the sphere over the flat plate.
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    Numerical comparison of thermal and hydraulic performances for heat exchangers having circular and elliptic cross-section
    (Pergamon-Elsevier Science Ltd, 2019) Dogan, Sercan; Darici, Selcuk; Ozgoren, Muammer
    The plain fin and circular tubes are generally used in heat exchangers of cooling systems. Numerous studies aiming at enhancing the thermal and hydraulic performances of this type of heat exchangers by changing the fin geometries are present in literature. In the present study, thermal and hydraulic performances of the heat exchangers with circular and elliptical tube have been compared using computational fluid dynamic method. Initially, the validation and comparison were done for the obtained data of the finned and circular tube design with respect to the performance results of the finned and circular tube design in the literature. The ellipticity ratios for the staggered arrangements of heat exchangers were changed within the range of b/a = 0.307-1.0. Numerical studies were performed with ANSYS-Fluent software within the range of Re = 750-2850 based on the fin spacing. As a result of the numerical validation studies, all analyses were solved by utilizing the SST k-omega turbulence model yielding closer values to the available studies similar to literature. Colburn factor j, friction factor f, JF values being the efficiency index and normalized j*, f* and JF* values were calculated and presented graphically. Moreover, temperature contours and streamline patterns were visually presented to evaluate the flow structures around the six different arrangements and to explain the physical reasons of pressure drop and heat transfer phenomena. The largest values of the Colburn factor j and the friction factor f known respectively as an indicator of heat transfer and a measure of pressure drop were obtained for the circular-tube-design ratio of b/a = 1.0 when compared to the other arrangements. According to the JF values, which is the efficiency index and the integrated outcome of heat transfer and pressure drop, the best result was obtained for the model with the diameter ratio of b/a = 0.307. Variations of the JF values for the arrangements of b/a = 0.333 and b/a = 0.307 depending on Reynolds numbers were found to be significantly close to each other. Finally, it is demonstrated that elliptical cross-section tubes with a better design in terms of lower pressure drop and less separated flow structures can be preferred in heat exchanger applications in order to provide energy saving. (C) 2019 Elsevier Ltd. All rights reserved.