Investigation of the Factors That Effect the Performance of a Heat Pipe Heat Recovery System

Isı boruları ilk olarak 1944 yılında kullanılmaya başlanmış ve aradan geçen yıllar boyunca kullanım alanı uzay araştırmalarından nükleer enerji santrallerine, elektronik devrelerin soğutulmasına, enjeksiyon kalıplarının soğutulmasına ve ısı geri kazanım ünitelerine kadar geniş bir yelpazeye yayılmıştır. İklimlendirme sistemlerinde iç hava koşullarının sağlanmasında sağladığı enerji tasarrufu ile alışveriş merkezleri, hastaneler ve konutlarda geniş bir kullanım alanı bulmuştur. Bu çalışmada ısı borulu ısı geri kazanım sisteminin (IBIGKS) performansı farklı çalışma koşulları için FRİTERM AR-GE Merkezi tarafından geliştirilen “Heat Pipe” yazılımı kullanılarak araştırılmıştır. Buharlaştırıcı ve yoğuşturucu bölümleri için üç farklı ısı değiştiricisi modeli kullanılmıştır. Isı değiştiricileri alüminyum malzemeli dalgalı kanatlı ve bakır malzemeli yivli borudur. Farklı çalışma koşulları için 2, 3 ve 4 olmak üzere üç farklı boru sıra sayısı durumunda etkenlik değerleri hesaplanmıştır. IBIGKS etkenliği üzerine en büyük etkiye sahip parametrenin hava giriş hızı olduğu tespit edilmiştir.The heat pipes were first used in 1944 and over the years, the field of application has been spread over a wide range from space surveys to nuclear power plants, the cooling of electronic circuits, cooling of injection molds and heat recovery units. It has found a wide usage area in shopping centers, hospitals and residences with the energy saving in air conditioning systems. In this study, the performance of the heat pipe heat recovery system for the different operating conditions was investigated by using “Heat Pipe” software that is developed by FRİTERM R&D Center. Three different heat exchanger models were used for the evaporator and condenser sections. Heat exchangers have grooved pipes of copper material and wavy fins of aluminum material. For different operating conditions, the efficiency values were calculated in the case of three different number of tube rows of 2, 3 and 4. It has been determined that the parameter having the dominant effect on heat pipe heat recovery system effectiveness is air inlet velocity

Experimental investigation of effect of different tube row-numbers, fin pitches and operating conditions on thermal and hydraulic performances of louvered and wavy finned heat exchangers

Olcay, Ali Bahadir/0000-0003-0995-9173;Heat exchangers are used in air conditioners, heat pumps, marine, land and air vehicles, refrigeration systems, thermal and nuclear power plants, etc. Increasing heat transfer capacity of a heat exchanger means that the volume of the heat exchanger and the material used will be reduced. Besides, effects of some geometric parameters on heat transfer and pressure drop are more complex depending on the fin structure in fin and tube heat exchangers. In this study, three of the most dominant parameters affecting the thermal-hydraulic performance of a finned and tube heat exchanger were experimentally investigated. These are fin-type (louvered and wavy fins), fin pitch and number of tube-rows. The intermittent geometric structures of louvered fins break growing of the boundary layer and reduce its thickness yielding heat transfer enhancement. On the other hand, wavy fins cause an increase in the heat transfer area due to its large flow length and create instabilities in the flow due to flow separations increasing the heat transfer coefficient. In the present study, specifically five louvered firmed and three wavy finned and round tube heat exchanger prototypes were manufactured. Heat transfer and pressure drop experiments of these heat exchangers were performed at a wind tunnel in a conditioned room. Heat transfer and pressure drop characteristics were presented as heat transfer coefficient h(o), Stanton number S-t, Nusselt number Nu, dimensionless pressure drop coefficient C-p, Colbum-j factor, Fanning friction factor f, j(louver)/j(wavy), f(louver)/f(wavy), j/f(1/3) ratios and JF factor. The results were examined from the point of heat transfer and pressure drop mechanisms of louvered and wavy fins for the different number of tube-rows, fin pitches and air inlet velocities. It is found that Colburn-j factors and Fanning friction f factors of the LFRTHXs are higher than those of the WFRTHXs for all the studied cases. Colbum-j factors of the LFRTHXs are higher by 16.4-6.9%, 28.5-18.3% and 25-11.7% than those of the WFRTHXs for the cases of two tube-rows, three tube-rows and four tube-rows, respectively. On the other hand, pressure drops of the LFRTHXs are significantly higher than those of WFRTHXs. However, the thermal-hydraulic performances of the LFRTHXs are still higher than that of WFRTHXs. The thermal-hydraulic performance criteria j/f(1/3) ratios of the LFRTHXs are higher by 9.6-4.1%, 22.1-16% and 16.8-7.4% than those of the WFRTHXs for the cases of two tube-rows, three tube-rows and four tube-rows, respectively.Republic of Turkey Ministry of Industry and Technology; FRITERM TERMIK CIHAZLAR SAN. ve TIC A.S; [0649.STZ.2014]This study is a part of the Industrial Thesis Project with a contract number of 0649.STZ.2014. The fundings of the Republic of Turkey Ministry of Industry and Technology and FRITERM TERMIK CIHAZLAR SAN. ve TIC A.S are greatly appreciated. Authors are greatly appreciated with the help received from the R&D Department of FRITERM. Special thanks to M. Harun Sokucu, Mete Ozsen and Dr. Huseyin Onbasioglu

Enhanced performance of triboelectric nanogenerator based on polyamide-silver antimony sulfide nanofibers for energy harvesting

Triboelectric nanogenerators (TENGs) are new renewable energy harvesting devices that convert smallscale mechanical movements into electrical energy. Nowadays, the dielectric materials with high tribopotential are being investigated significantly to improve the energy conversion efficiency of TENGs. Nanofibers are widely used as dielectric materials in TENGs due to their high surface area and flexibility. In this study, polyacrylonitrile nanofibers and AgSbS2 doped Nylon 6.6 nanofibers were tested as dielectric layers in spring assisted TENGs. Decorating Nylon 6.6 with AgSbS2 both enhanced the output voltage and markedly advanced the power density of the TENGs, and thus improved triboelectric performance of the TENGs. According to the results, tribopotential of Nylon 6.6 was enhanced as AgSbS2 additive amount increased. Compared to PAN/Nylon 6.6 nanofibers based TENG, PAN/10 wt% AgSbS2@Nylon 6.6 nanofibers based TENG exhibited 2.95 and 1.68 fold enhancement in power density and output voltage, respectively. The peak power density of PAN/10 wt% AgSbS2@Nylon 6.6 nanofibers based TENG reached 6.81 W/m(2) under a load resistance of 10 MU. From the perspective of the choices of materials and design, the results demonstrate that grafting AgSbS2 nanocrystal materials into Nylon 6.6 nanofibers is an effective way to make better the triboelectric performance of nanofibers mat based TENG. Therefore, the study not only shows a high triboelectric performance of nanofibers based TENG, but also shed on light new glance into the material selection, fabrication, and design for contact separation mode TENGs. (C) 2021 Elsevier Ltd. All rights reserved.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [217M212, 121M608]We gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the financial support provided under project numbers 217M212 and 121M608

High-performance triboelectric nanogenerator with optimized Al or Ti-embedded silicone tribomaterial

In this study, we have enhanced the electricity generation capacity of a silicone and glass fiber-based vertical contact separation mode triboelectric nanogenerator (TENG) by optimizing the thickness of the silicone material and by tuning the triboelectrification characteristics embedding Al or Ti conductive materials. Considering electrical outputs, the optimum thickness of the silicone triboelectric material layer decorated with 2.5 wt% Al or 2.5 wt% Ti is found to be 0.85 mm. The silicone and glass fiber-based TENG delivers the highest performance with 2.5 wt% Al or 5 wt% Ti embedding. The peak power density of 2.5 wt% Al or 5 wt% Ti-embedded silicone glass fiber TENGs are 22.6 W/m(2) and 21.3 W/m(2), respectively. Embedding 2.5 wt% Al or 5 wt% Ti enhances output power density by 2.19 folds and 2.06 folds, respectively compared to the pure silicone tribonegative layer. We have demonstrated that using conductive fillers significantly increases the TENGs' powers and charging capacities.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [121M608]We gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the financial support pro-vided under project number 121M608

Flow optimization in a microchannel with vortex generators using genetic algorithm

In this study, delta winglet-type vortex generators, widely used in conventional macro channels and proven to be effective, are used in microchannels to increase their heat transfer capacities. The effects of vortex generators on heat transfer and pressure loss characteristics are studied numerically for different angles of attack, vortex generator arrangement type, the transverse and longitudinal distance between vortex generators, vortex generator length and height, and different Reynolds numbers. The thermal and hydraulic characteristics are presented as the Nusselt number, the friction factor, and the performance evaluation criteria number (PEC) that takes into account the heat transfer enhancement and the corresponding increase in pressure loss. The variation of Nu/Nu0, f/f0, and PEC are found to be in the range of 1.03-1.87, 1.04-1.8, and 0.92-1.62, respectively. A multi-objective optimization study are performed with the response surface methodology analysis to see how different parameters affect heat transfer and pressure loss and to determine the most optimum design. Besides, local sensitivity analysis study is carried out through the RSM, and water inlet velocity for heat transfer enhancement is found to be the most effective parameter. Among the geometric parameters, vortex generator height is determined as the most effective factor. Finally, practical Nusselt number and friction factor correlations taking many parameters into account are proposed to be able to compare the results of other researchers, and for engineers designing microchannel cooling systems

Computational investigation of heat transfer and pressure drop in a typical louver fin-and-tube heat exchanger for various louver angles and fin pitches

In this study 3-D numerical simulations on heat transfer and pressure drop characteristics for a typical louver fin-and- double-row tube heat exchanger were carried out. The heat transfer improvement and the corresponding pressure drop amounts were investigated depending on louver angles, fin pitch and Reynolds number, and reported in terms of Colburn j-factor and Fanning friction factor f. The heat transfer improvement and the corresponding pressure drop amounts were investigated depending on louver angles between 20° ≤Ө≤ 30°, louver pitch of Lp=3.8 mm and frontal velocities of U between 1.22 m/s - 3 m/s. In addition, flow visualization of detailed flow features results, such as velocity vectors, streamlines and temperature counters have been shown to understand heat transfer enhancement mechanism. The present results indicated that louver angle and fin pitch noticeably affected the thermal and hydraulic performance of heat exchanger. It has been seen that increasing louver angle, increases thermal performance while decreasing hydraulic performance associated to pressure drop for fin pitches of 3.2 mm and 2.5 mm. Fin pitch determines the flow behaviour that for fin pitch of 2 mm, increasing louver angle decreased heat transfer and pressure drop. Velocity vectors and streamlines give considerable information about the flow whether it is duct directed or louver directed. For all conditions the flow is louver directed

High-performance triboelectric nanogenerator based on carbon nanomaterials functionalized polyacrylonitrile nanofibers

Triboelectric nanogenerators (TENGs) are one of the most promising energy sources for self-powered electronic devices in the near future. Improving the dielectrics with high tribo-potential is a primary requirement to increase the output performance of TENGs. In this study, spring supported TENGs consisting of polyvinylpyrrolidone/ethyl-cellulose (PVP/EC) nanofibers and various carbon-doped polyacrylonitrile (PAN) nanofibers as positive and negative dielectric layers, respectively, were fabricated. According to the experimental results, reduced graphene oxide (rGO) and carbon nanotube (CNT) which were grafted to PAN matrix, both increased surface charge density and enhanced the output voltage of the TENGs. On the other hand, carbon black (CB) reduced the tribo-potential of PAN as a negative dielectric layer. As the best result, a 40 x 40 mm(2) TENG constructed of PVP/EC and 3 wt% CNT doped PAN nanofibers demonstrates high triboelectric characteristics with a charge capacity of 260 nC (under 0.022 mF capacitive load), a maximum peak output voltage of 960 V (under a 70 MU load resistance), and a maximum peak power density of 14.6 W/m(2) (under a 14.6 MU load resistance). In other words, the addition of 3 wt% CNT to PAN increased the charge amount by 136%, and the maximum peak power density by 125%. This work presents an effective way to take advantage of the coupling effect of carbon additive and nanofiber structure to significantly enhance the output performance of TENGs. (C) 2021 Elsevier Ltd. All rights reserved.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [121M608]We gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the financial support provided under project number 121M608

Investigation of interactions between a sphere wake and free surface

Bu çalışma, küre çapına göre tarif edilen Reynolds sayısının 2,5x103 ve 1x104 aralığında farklı daldırma yükseklikleri için açık bir su kanalında serbest su yüzeyi ve kürenin art izi bölgesi arasındaki akış yapısı etkileşimini sunmaktadır. Anlık ve zaman ortalaması alınmış hız alanı, akım çizgisi topolojisi, girdaplar ve hız konturları gibi akış modellerini göstermek için Parçacık Görüntülemeli Hız ölçme (PIV) ve boya görselleştirme teknikleri uygulanmıştır. Küre, serbest yüzeye kısmen daldırılma konumundan serbest yüzeyden iki küre çapı kadar aşağı doğru çeşitli daldırma konumlarına 0,25? h/D ?3,0 aralığında sabitlenmiştir. Daldırma oranı, küre alt kolunun serbest yüzey ile olan mesafesinin küre çapına oranı olarak tanımlanmıştır. Kürenin yerleştirildiği konumlara bağlı olarak, art izi bölgesinin özellikleri üç farklı akış olayı sergilemiştir. Bunlar; daldırma oranı 0,25? h/D ?1,0 aralığı için üniform akış bölgesi ve serbest yüzey arasındaki kısıtlı bir art izi bölgesi, daldırma oranı 1,25? h/D ?2,0 aralığı için serbest yüzey etkisinden dolayı simetrik olmayan akış modelleri ve daldırma oranı 2,5? h/D ?3,0 aralığı için simetrik akış yapıları olarak sınıflandırılmıştır. Akış fiziği açısından bahsedilen akış yapıları için daha detaylı sonuçlar karşılaştırılmalı olarak sunulmuş ve yorumlanmıştır.This paper presents flow-structure interaction between a sphere wake and a free water surface for different submerged elevations in an open water channel for Reynolds numbers in the range of 2.5x103 ?Re?1x104 based on the diameter of the sphere. Particle Image Velocimetry (PIV) and dye visualization techniques were performed to demonstrate flow patterns such as instantaneous and time-averaged velocity field, streamline topology, vortices, velocity contours. The sphere was fixed at various submerged locations from the piercing the free surface case to the two sphere diameters beneath the free surface in the range of 0.25? h/D ?3.0. Here, submerged ratio was defined as ratio of bottom shoulder of the sphere from the free surface to the sphere diameter. Depending on the sphere locations, characteristics of wake region exhibit three different flow phenomena. These are classified as; a restricted wake region between the free-surface and uniform flow stream for the submerged ratios of 0.25? h/D ?1.0, unsymmetrical flow patterns due to the free surface effect for the submerged ratios of 1.25? h/D ?2.0 and symmetrical flow structures for the submerged ratios of 2.5? h/D ?3.0. More detailed results for the aforementioned flow structures from the point of flow physics were comparatively presented and interpreted

Turbulent shear flow downstream of a sphere with and without an o-ring located over a plane boundary

Flow-structure interaction of separated shear flow from the sphere and a flat plate was investigated by using dye visualization and the particle image velocimetry technique. Later, a passive control method was applied with 2mm oring located on the sphere surface at 55° from front stagnation point. The experiments were carried out in open water channel for Reynolds number value of Re=5000. Flow characteristics have been examined in terms of the 2-D instantaneous and time-averaged velocity vectors, patterns of vorticity, streamlines, rms of velocity fluctuations and Reynolds stress variations and discussed from the point of flow physics, vortex formation, lengths of large-scale Karman Vortex Streets and Kelvin-Helmholtz vortices depending on the sphere locations over the flat plate. It is demonstrated that the gap flow occurring between the sphere bottom point and the flat plate surface has very high scouring effect until h/d=0.25 and then unsymmetrical flow structure of the wake region keeps up to h/D=1.0 for smooth sphere. For the sphere with o-ring, the wake flow structure becomes symmetrical at smaller gap ratios and reattachment point on the flat plate surface occurs earlier. Moreover, o-ring on the sphere diminishes peak magnitudes of the flow characteristics and thus it is expected that the flow-induced forces will be lessened both on the sphere and flat plate surface. Vortex formation lengths and maximum value occurring points become closer locations to the rear surface of the sphere with o-ring