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    Effect of new design of elongated jet hole on thermal efficiency of solar air heater
    (Elsevier, 2022) Pazarlioglu, Hayati Kadir; Tepe, Ahmet Umit; Tekir, Mutlu; Arslan, Kamil
    Effect of new design of elongated jet hole on the thermal efficiency (eta th) of a solar air heater (SAH) has been numerically studied in this paper. Jet impingement technique, which is one of the important active techniques, has been implemented on the absorber plate of the SAH surface to increase the convective heat transfer per-formance. Several designs of elongated jet hole (EJH) technique (gradually increasing, gradually decreasing along the crossflow direction, and equally extended), which is highly effective application to get rid of cross-flow, has been inserted to the jet plate. The main interest of this paper is to investigate the effect of different EJH configurations on convective heat transfer performance and eta th of the SAH. 15 x 7 array of jets have been mounted on the confinement plate. Results have been compared with the conventional SAH, which has the jet impingement Case1. Numerical computations have been conducted using RNG k -epsilon turbulence model. Average Nusselt number (Nu) on the absorber plate surface, eta th of the SAH, and performance evaluation criteria (PEC) have been quantitatively investigated comprehensively. As a result of calculations, the Nu and eta th of SAH can be enhanced by 31.80 % and 2.46 %, respectively, using Case4 instead of Case1 at Re = 25000. Besides, the pressure drop of SAH dramatically rises by 10.26 %, when Case4 and Case1 compared with each other at Re = 25000. Despite increment in pressure drop, the PEC value achieves to 1.28. Finally, it is concluded that the equally extended EJH presents well performance compared to gradually increasing and decreasing EJH.
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    Effect of slot-shaped pins on heat transfer performance in the extended jet impingement cooling
    (Elsevier France-Editions Scientifiques Medicales Elsevier, 2022) Yalcinkaya, Orhan; Durmaz, Ufuk; Tepe, Ahmet Umit; Uysal, Unal; Ozel, Mehmet Berkant
    In this study, effects of heat transfer performance on the roughened target surface with slot-shaped pins (SSP) were numerically investigated using an in-line array of extended jet impingement cooling models. The influence of pin heights on convective heat transfer for variable nozzle lengths, different pin arrangements, and Reynolds numbers (Re) in a rectangular channel flow was examined. The pins were located in a staggered and circular pattern. The numerical solution procedure has been verified by comparing numerical results with the available experimental data in the literature. The simulations were conducted with shear stress transport k-omega with low-Re correction turbulence model considering boundary conditions. The numerical model reasonably predicted heat transfer and pressure drop for flat plates and roughened plates with pins. Average Nusselt numbers on the surface were determined numerically using the model validated with experimental data for different dimensionless pin heights H-p/D-j (0.167, 0.417, and 0.667), various pin arrangements (R-1, R-2, R-3), dimensionless nozzle lengths G(j)/D-j (1.0, 2.0, and 6.0), and Re numbers (16250, 27100, and 32500). Numerical results were compared with the orifice plate; in other words, the conventional jet impinging configuration (G(j)/D-j = 6.0 and H-p/D-j = 0). When the system's flow characteristics and performance evaluation criterion were considered, optimum geometric parameters for all Re were determined as H-p/D-j = 0.167 and R(1_)G(j)/D-j = 1.0. Obviously, pin-roughened surfaces strongly affect the mean convective heat transfer and the homogeneity of the local heat transfer in extended jet impinging cooling systems. In this way, the negative effect of thermal stresses on the interested surface can be reduced, and the life of the material can be extended. Since the development of additive manufacturing technologies makes it possible to use these geometries in the internal cooling channels of gas turbine blades, a significant contribution will be made to the literature as a result of this study.
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    Enhancement of film cooling effectiveness using upstream vortex generator
    (Springer, 2021) Tepe, Ahmet Umit
    A numerical study was carried out to explore the influence of triangular with concave edge shaped (TCES) upstream ramp on the film cooling effectiveness (FCE) and flow characteristics. The main objective of this study is to obtain the performance of the newly proposed upstream ramp on FCE and to establish that it provides better thermal protection by increasing the lateral diffusion of coolant on the surface compared to the baseline case and continuous triangular-shaped (TS) upstream ramp. Four different cases were investigated, including baseline case, TCES with s/d = 1, TCES with s/d = 1.5 and triangular-shaped (TS) upstream ramp. Coolant streams were injected through a circular jet hole on the target surface with an inclination angle of alpha(c) = 35. Computations were carried out with different blowing ratio (M), including, 0.40, 0.85, 1.00, and 1.25. Results showed that TCES has a significant potential of improving coolant dispersion towards the lateral direction on the surface by means of generating a pair of strong anti-counter rotating vortex (anti-CRV) in the jet flow region compared to the baseline case. Correspondingly TCES with s/d = 1 upstream ramp is the best design especially considering thermal protection. Moreover, area-averaged FCE enhances 441.35% by using TCES with s/d = 1 upstream ramp on the surface under inspection compared with the baseline case for the blowing ratio of M = 1.25.
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    Heat transfer enhancement of fin-tube heat exchangers using punched triangular ramp vortex generator on the fin surface
    (Pergamon-Elsevier Science Ltd, 2021) Tepe, Ahmet Umit
    Existence of the vortices in a flow field strongly affects the heat transfer. In this study, effect of newly proposed punched triangular ramp vortex generator (PRVG) on heat transfer performance for a fin-tube heat exchanger was investigated numerically. Normalized ramp height (H/d=0.196 and 0.161) and ramp angle (alpha(r)=20 degrees and 35 degrees) were examined as the geometric parameter. Numerical computations were carried out under turbulent flow conditions (5000 <= Re <= 20000). RANS equations were solved using ANSYS Fluent by using SST k-omega turbulence model. Nusselt (Nu) number, friction factor and Performance Evaluation Criterion (PEC) were comprehensively examined quantitatively. Flow characteristics were also investigated for elucidating the underlying physics of enhancement heat transfer by the PRVG winglet. Results were compared with the flat smooth fin surface (baseline case). Results showed that overall heat transfer on the entire channel wall increases up to 43.66% for Re=5000 by H/d=0.196 and alpha(r) =35 degrees. The influence of ramp angle on thermo-hydraulic performance was more significant compared to ramp height. Furthermore, PEC results showed that the most feasible geometric design of PRVG winglet for the fin-tube heat exchanger is H/d=0.161 and alpha(r) =20 degrees. Furthermore, decreasing flow speed in the channel increases the effectiveness of the vortex generator according to PEC results. (C) 2021 Elsevier Ltd. All rights reserved.
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    Numerical optimization of triangular concave edge shaped ramp in film cooling
    (Gazi Univ, Fac Engineering Architecture, 2022) Tepe, Ahmet Umit
    In this study, the effect of triangular concave ramp (UIBR) on film cooling efficiency (FSE) and flow characteristics in film cooling was numerically investigated. In order to protect the jet stream from the effect of the main stream, the UIBR is placed on the surface where the jet hole meets the surface and the main stream comes from. In order to determine the most suitable design parameters of the UIBR, three different inclination angles (ar) of 10 degrees, 25 degrees and 45 degrees, three different dimensionless ramp heights (h/d) as 0.15, 0.30 and 0.50 examined. However, calculations were performed at blowing ratios (M) of 0.30, 0.60, 0.85 and 1.25. In order to accurately determine the physical properties of the film cooling in accordance with the real operating conditions, air was used for the main flow and CO2 was used for the fluid injected to the surface in the calculations. Accordingly, the jet stream to main stream density ratio (YO) is 1.50. CO2 was injected into the surface through a circular jet hole with an angle of aj=35 degrees with the surface and a diameter of d=4 mm. Numerical studies were carried out using Ansys FLUENT developed for Computational Fluid Dynamics (CFD) and the Transition k-kl-omega turbulence model. The results are compared to a conventional flat surface without ramp. According to the numerical results, increasing the ramp height and decreasing the ramp angle significantly increased the FSE. As a result, the highest increase in area-averaged FSE compared to conventional film cooling was obtained as 305.44% in the ramp design with h/d=0.50 and ar=10 degrees.
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    Thermal-hydraulic performance of the circular-slice-shaped-winglet for tube bank heat exchanger
    (Elsevier France-Editions Scientifiques Medicales Elsevier, 2022) Tepe, Ahmet Umit; Yilmaz, Harun
    In this study, heat transfer performance of circular-slice-shaped-winglet (CSSW) for tube bank heat exchanger (TBHE) was numerically investigated. Circular-slice angle (alpha = 5 degrees, 10 degrees and 20 degrees), layer gap (h/d = 0.08, 0.14, and 0.20) and winglet diameter (D/d = 1.60, 1.76 and 1.92) were analyzed as geometric parameters to obtain optimal design. Comprehensive numerical simulations were conducted under both laminar and turbulent flow (1100 <= Re <= 8500) conditions. Navier Stokes Equations were solved by using RNG k-epsilon turbulence model (with enhanced wall treatment). Colburn-j factor (j), friction factor (f), modified London area goodness factor (j/f(1/3)) and performance evaluation criterion (PEC) were investigated quantitatively. Flow characteristics were also analyzed to elucidate the underlying physics of the effects of CSSW on thermal-hydraulic performance for TBHE. Results were compared with the data related to bare tube heat exchanger, and showed that the highest modified London area goodness factor is 26.34% with alpha = 10 degrees, h/d = 0.20 and D/d = 1.92 CSSW winglet design. Additionally, numerical calculations revealed that PEC increased to 1.356 with these CSSW design parameters, which corresponds to reduction of the required total heat transfer area by 36.60% compared to the bare tube heat exchanger for the same heat duty at a fixed pumping power.