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Öğe Effects of Extended Jet Holes to Heat Transfer and Flow Characteristics of the Jet Impingement Cooling(Asme, 2019) Tepe, Ahmet Umit; Arslan, Kamil; Yetisken, Yasar; Uysal, UnalIn this study, effects of extended jet holes to heat transfer and flow characteristics of jet impingement cooling were numerically investigated. Cross-flow in the impinging jet cooling adversely affects the heat transfer on the target surface. The main purpose of this study is to reduce the negative effect of cross-flow on heat transfer by extending jet holes toward the target surface with nozzles. This study has been conducted under turbulent flow condition (15,000 <= Re <= 45,000). The surface of the turbine blade, which is the target surface, has been modeled as a flat plate. The effect of the ribs, placed on the target surface, on the heat transfer has been also investigated, and the results were compared with the flat surface. The parameters such as average and local Nusselt numbers on the target surface, flow characteristics, and compressor power have been examined in detail. It was obtained from the numerical results that the average Nusselt number increases with decreasing the gap between the target surface and the nozzle. In addition, the higher average Nusselt number was obtained on the flat surface than the ribbed surface. The lowest compressor power was achieved in the 5Dj nozzle gap for the flat surface and in the 4Dj nozzle gap for the ribbed surface.Öğe Experimental and numerical investigation of jet impingement cooling using extended jet holes(Pergamon-Elsevier Science Ltd, 2020) Tepe, Ahmet Umit; Yetisken, Yasar; Uysal, Unal; Arslan, KamilIn this study, jet impingement cooling on flat surface was investigated experimentally. The aim of this study is to elucidate the effect of extended jet holes on the heat transfer performance of the in-line array jet impingement configuration. The studies were performed under fully turbulent flow condition (16250 <= Re <= 32500). Local Nusselt number (Nu) distribution on the surface of interest was obtained experimentally by using Transient Liquid Crystals (TLC) method. Numerical investigations were conducted as the same configuration with the experimental method to explore the flow and heat transfer characteristics. SST k-omega with low-Re correction turbulence model was used for solving turbulence equations. Experimental and numerical studies were conducted on 1 x 6 (in-line array) jet impingement cooling configuration. Dimensionless jet to jet spacing (X-n/D-j), dimensionless jet plate to target plate spacing (Z/D-j) and dimensionless target plate width (Y/D-j) were taken as 5.0, 6.0 and 6.0, respectively. Five different G(j)/D-j= (1.0, 2.0, 3.0, 4.0 and 5.0) were investigated and the results were compared with orifice plate jet impingement configuration (Z/D-j=G(j)/D-j=6.0). Average and local Nu number distributions, pressure drop of the system, flow characteristics and Performance Evaluation Criterion (PEC) were examined in detail. Numerical results were compared with the experimental data and it was obtained that SST k-omega turbulence model was able to accurately predict the average and local Nu number distributions on the surface of interest. The maximum average and local Nu numbers were obtained on the condition of G(j)/D-j=2.0. Furthermore, PEC shows that the most feasible dimensionless nozzle to target plate gap was G(j)/D-j=2.0 at all Re numbers. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Improvement of Performance by Using MgO+CuO/Water and MgOAl2O3/Water Nanofluids in Sequential Heat Pipe Heat Exchangers: A Comparative Experimental Study(Gazi Univ, 2021) Filiz, Cagdas; Yetisken, YasarSystems used in pre-heating the required clean air; waste heat recovery units used in industry and many waste heat plants. In this study, it is aimed to improve the thermal performance of the heat pipe heat recovery system by using pure water and nanofluids in air to air heat recovery systems. The fact that nanofluids, which have recently found a lot of application area, have been used in the experimental setup has added originality value to the study. As working fluid, MgO+CuO/Water hybrid nanofluid and MgOAl2O3/Water nanofluid were used in the heat pipe heat exchanger. With the use of these nanofluids, the improvement rates of thermal performance of the heat recovery system relative to pure water have been determined and attempted to comment. In the condenser region, which is the lower part of the heat pipes, when the cold air velocity was 0.751 m/s and Re=12300, it was measured that there was 77 % improvement for MgO+CuO/Water hybrid nanofluid and 91 % for MgOAl2O3/Water nanofluid.Öğe Jet impingement cooling on a rib-roughened surface using extended jet holes(Pergamon-Elsevier Science Ltd, 2020) Tepe, Ahmet Umit; Uysal, Unal; Yetisken, Yasar; Arslan, KamilIn this study, jet impingement cooling on a rib-roughened surface has been investigated experimentally. The aim of this study is to investigate the effect of extended jet holes on the heat transfer performance and flow characteristics of the jet impingement cooling on a rib-roughened surface. The studies have been conducted under turbulent flow condition (16,250 <= Re-j <= 32,500). Transient Liquid Crystals (TLC) method has been employed to investigate the average and local Nusselt number (Nu) distributions on the surface of interest. Six inline arrays of jet impingement configuration have been examined as the jet impingement cooling system. Jet holes were extended towards the target surface with the nozzles. Various dimensionless nozzles to the target surface gaps (G(j)/D-j = 1.0, 2.0, 3.0, 4.0 and 5.0) have been investigated. Rectangular cross-sectional ribs were located on the surface of interest for the augmentation of heat transfer. Experimental studies were conducted on the dimensionless rib height as H-r/D-j = 0.42. In addition, numerical studies were carried out to investigate the flow and heat transfer characteristics. The effect of various dimensionless rib heights (H-r/D-j) on convective heat transfer performance has also been investigated numerically. SST k-omega with low-Re correction turbulence model was used for solving turbulence equations. Average and local Nu number distributions, flow characteristics and Performance Evaluation Criterion (PEC) were examined in detail. Results were compared with the orifice plate (G(j)/D-j = 6.0) jet impingement configuration. Results showed that SST k-omega turbulence model accurately reveals the experimental data. Application of extended jet holes is a feasible method for practical application of the jet impingement cooling, especially at relatively low dimensionless nozzle gap (G(j)/D-j <= 4.0). Furthermore, improperly designed rib height has been found to diminish heat transfer performance.Öğe Upgrading of the Performance of an Air-to-Air Heat Exchanger Using Graphene/Water Nanofluid(Springer/Plenum Publishers, 2021) Soezen, Adnan; Filiz, Cagdas; Aytac, Ipek; Martin, Kerim; Ali, Hafiz Muhammad; Boran, Kurtulus; Yetisken, YasarThe aim of this study is to improve the thermal performance of air-to-air heat recovery units, containing heat pipes by using graphene/water nanofluid as a working fluid. The experimental set up of this work consists of two air ducts. To study the effect of the airflow rate and the temperature on the performance of the heat recovery unit, different values of airflow rates and temperatures are used. The values of Re numbers are calculated for each air duct. These Re numbers referred to the turbulent flow type in all cases. To compare the results of the graphene/water nanofluid and the pure water working fluid, thermal efficiency and thermal resistance values are calculated for both of them. The results showed that the graphene/water nanofluid was more efficient than pure water in all different conditions. Re number in the cold air duct was 6800, and the Re number in the hot air duct was 9000. The maximum thermal efficiency values were 34.1 % and 20.1 % for graphene/water nanofluid and pure water, respectively. The maximum improvement rate in thermal efficiency was 87.7 % when the average Re number in cold and hot air ducts was equal to 11,150 and 11,650, respectively. By comparing the results of graphene/water nanofluid with those of the pure water, it can be seen that using graphene/water nanofluid decreased the thermal resistance of the heat pipes. Therefore, the heat transfer increased. The maximum decreasing value of the thermal resistance was 52.3 % when cold and hot air duct Re numbers were 11,700 and 11,000, respectively.
