NUMERICAL INVESTIGATION OF THERMAL AND HYDRAULIC PERFORMANCE A TWISTED TUBE MODIFIED BY USING “CUO/WATER” NANOFLUID

In this study, several numerical simulations were constructed in order to enhance theheat transfer performance by exploiting smooth and twisted tubes with three different pitchlengths was (50mm, 100mm, 250mm). Water was selected as base fluid for analysis, andthe (CuO-Water) nano-fluids effect was tested on the heat transfer through a circular tubein order to improve the thermal and hydraulic performances. The nano-fluid used was asingle-phase of four different volume fractions: 1, 2, 3, and 4%. The analysiswas experimented for Reynolds number values between 6000-40000. Constant uniformheat flux of 50,000 W/m2 was applied to the outer surface of the test tube. Besides; kepsilon standard turbulence model was adopted through the use of CFD (ANSYS)program. The inlet fluid temperature was taken as 293K and a constant hydraulic diameterof 26.9mm was considered. The optimum results (Nu = 443.76, h=12129.98 W/m2, f =0.1582, ∆p = 2782.85 Pa, PEC=2.39) obtained were at pitch length of 50 mm, Reynoldsnumber of 35613 and volume fraction of 4% (CuO-Water) in base fluid.Finally, The results in terms of thermal and hydraulic performances showed decreasingNusselt number, heat transfer coefficient, friction factor and pressure as the number ofpitches length in twisted tube increased. In addition, the thermal and hydrauliccharacteristics increased too when increasing the value of CuO-water nano-fluid fraction.</p

NANOAKIŞKAN KULLANARAK VE İÇ ELEMAN YERLEŞTİRİLMİŞ DÜZ YATAY BORUDA ISI TRANSFERİNİN TAGUCHI YÖNTEMİ YARDIMIYLA ARAŞTIRILMASI

Bu tez çalışması sabit ısı akısı ile ısıtılan ve türbülanslı akış şartlarında bir ısı değiştirici borusu üzerinde deneysel olarak ısı transferi iyileştirme çalışmalarını kapsamaktadır. Baz akışkan olarak su ve su-etilen glikol karışımı kullanılmıştır. Deneyler, Reynolds sayısı 6000-23000 arasında gerçekleştirilmiştir. Isı transferi iyileştirme çalışmaları boru içerisine 7 farklı tasarımda bükülmüş levhalar (BL) yerleştirilerek ve baz&nbsp; akışkanlara %0.25, %0.50, %0.75 ve %1.00 olmak üzere 4 farklı ağırlıksal oranlarda (wt) TiO2 nano partikülü katılmasıyla gerçekleştirilmiştir. Bükülmüş levhalar, tipik bükülmüş levha (TBL), delinmiş bükülmüş levha (DBL) ve gamzeli bükülmüş levha (GBL) olmak üzere 3 farklı ana tasarımdadır. BL’lar üzerindeki delik ve gamzeler arası mesafenin büküm uzunluğuna oranı (P/y) 1.00, 0.50 ve 0.25 şeklinde tasarlanmıştır. En yüksek termohidrolik performans değeri, içerisine GBL (P/y=0.25) yerleştirilmiş boru çalışmasında, nano partikül ağırlıkça oranı %1.00 olduğu Su-TiO2 nanoakışkanı kullanımında ve Reynolds sayısının 6319 olduğu deneyde elde edilmiştir. Deney sonuçlarının en yüksek ısı transferi katsayısı ve en düşük basınç düşümüne göre optimizasyonu yapılmıştır. Su baz akışkanı için en optimum deney, 0.25 m3/h hacimsel debi, ağırlıkça wt=%1.0 nano partikül kullanımında ve boru içerisine GBL (P/y=0.25) yerleştirildiği konfigürasyonudur. Su-etilen glikol karışımı baz akışkanı için en optimum deney, 1.04 m3/h hacimsel debi, ağırlıkça wt=%1.0 nano partikül kullanımında ve boş boru konfigürasyonudur.Thesis study includes experimental heat transfer augmentation studies on a heat exchanger tube that is heated under a constant heat flux and turbulent flow conditions. Water and water-ethylene glycol mixture were chosen as a base fluid. The experiments are conducted for Reynolds number between 6000 and 23000. Heat transfer augmentation studies are performed by placing twisted tapes with 7 different designs in the tube and 4 different weight ratios (wt) of TiO2 nanoparticles into the base fluids as 0.25%, 0.50%, 0.75% and 1.00%. Twisted tapes are in three main designs which are typically twisted tape, perforated twisted tapes and dimpled twisted tapes. The ratio of the distance between the holes and dimples on the twisted tapes to the twist length (P/y) is designed as 1.00, 0.50 and 0.25. While the amount of nanoparticle increases in the base fluid in the same Reynolds number increases, the Nusselt number increases and the friction factor does not change. Inserting of twisted tape in the tube causes to increase both the Nusselt number and the friction factor. The highest THP value was obtained by using of dimpled twisted tapes (P/y = 0.25) in the tube, the nanoparticle ratio was 1.00% by weight in the use of water-TiO2 and Reynolds number of 6319. The optimum test configuration for the water base fluid is obtained as volumetric flow of 0.25 m3/h, wt=%1.0 and DTT (P/y=0.25) in the pipe. The optimum test configuration for the water-ethylene glycol mixture base fluid is volumetric flow of 1.04 m3/h, wt=%1.0 and smooth pipe.</p

NUMERICAL INVESTIGATION OF HEAT TRANSFER ENHANCEMENT ON THE EFFECT OF ADDING (AL2O3) NANOPARTICLES TO WATER FLOW IN A HORIZONTAL TUBE INSERTED PENTAGON RINGS

In this work, a numerical study has been presented to investigate heat transfercharacteristics of (Al2O3-Water) nanofluids flow in a circular horizontal tube that fittedpentagon rings by three different pitch lengths (d, 2d, and 3d). To provide fullydeveloped flow inside the test section, specified distances are performed on the entranceand the exit of the tube. The analyses are carried out with three different nanofluidsvolume fractions 1.5, 3, and 4.5 % in the range of Reynolds number from 5,000 to15,000. The numerical solution procedure includes the investigation of heat transfercoefficient, Nusselt number, friction factor, surface temperature and the pressure dropcharacteristics with using finite volume method by applying (k-w) Standard turbulencemodel to solve the equations of continuity, momentum, and energy in a threedimensional domain. A single-phase approach is employed to model nanofluids andproperties of the fluid are applied constant with temperature. The use of (Al2O3-Water)nanofluids with pentagon ring leads to increase in heat transfer and pressure drop overthe smooth tube. The heat transfer coefficient increases with the increment of thenanofluid concentrations and Reynolds number, and it shows descending trend with theincrement of pitch length or in another meaning increases with the increment of thenumber of pentagon rings into a tube. Finally, the results showed that the configurationof Al2O3-Water nanofluid with pentagon ring could be used effectively in heat transferprocesses</p

İçerisinde grafen katkılı nanoakışkan ve konik iç eleman kullanılan boruda termohidrolik performans ve entropi üretiminin incelenmesi

Bu tez çalışmasında ısı değiştiricilerinin performansını artırmada kullanılan tekniklerarasında bulunan iç eleman ve nanoakışkan kullanımının, bir ısı değiştiricisiborusunun termohidrolik performansına ve entropi üretimine olan etkileri deneyselve sayısal olarak incelenmiştir. Özgün tasarlanan helisel sarılmış konik iç elemanlar,iki farklı mesafe oranında (P/D=2 ve 3), ve Iraksak, Iraksak-Yakınsak, Yakınsakolmak üzere üç farklı boru içi yerleşime sahip olacak biçimde üretilmişlerdir. Üçfarklı su-etilen glikol karışım oranına sahip akışkanların oluşturduğu baz akışkanlariçerisine, diğer nanopartiküllere oranla yüksek ısıl iletkenlik ve yüksek stabilitedeğerleriyle öne çıkan Grafen nanopartikülü, yine üç farklı ağırlık oranında,%0.5, 0.75 ve 1, eklendikten sonra, oluşturulan nanoakışkan konsantrasyonlarınıntermofiziksel özellikleri tespit edilmiştir. Reynolds sayısının 4000-27,000 aralığındadeğiştiği deneysel ve sayısal çalışmaların birbiri ile uyumlu olduğu tespit edilmiştir.Nusselt sayısının hem iç eleman kullanımına hem de nanoakışkan kullanımına bağlıolarak artış gösterdiği, sürtünme katsayısının iç eleman kullanımına bağlı artışgösterdiği fakat nanoakışkan kullanımında ise radikal bir değişiklik göstermediğitespit edilmiştir. Her iki pasif tekniğin tekil performansları, birlikte kullanımperformanslarına göre düşük kalmış, en yüksek termohidrolik performans değerini%100Su-GnP nanoakışkan-Iraksak-P/D=2 konik iç eleman bileşik konfigürasyonu6283 Reynolds sayısında 1.72 olarak sağlarken, en düşük boyutsuz entropi üretimsayısı değeri yine aynı konfigürasyonda 25,451 Reynolds sayısında, 0.42 olarakgerçekleşmiştir.&nbsp;In this thesis, the effects of the use of internal ribs and nanofluids, which areamong the techniques used to increase the performance of heat exchangers, on thethermohydraulics performance and entropy production of a heat exchanger tubewere investigated experimentally and numerically. The specifically designed helicallycoiled conical internal ribs were produced in two different distance ratios (P/D = 2and 3), and have three different tube layouts: Divergent, Divergent-Convergent,Convergent. Graphene nanoparticle, which stands out with its high thermalconductivity and high stability values compared to other nanoparticles, withthree different weight ratios, % 0.5, 0.75 and 1 was added in base fluid thatcomprises three different water-ethylene glycol mixing ratios, after created nanofluidthermophysical properties of concentrations were examined. It was determined thatthe experimental and numerical studies were compatible with each other for theReynolds number varies in the range of 4000-27,000. It was acquired that the Nusseltnumber increased depending on both the use of internal ribs and the use of nanofluid,the friction coefficient increasesd depending on the use of internal ribs, but it didnot show a radical change in the use of nanofluids. The individual performancesof both passive techniques were low compared to their combined use performances,the highest thermohydraulics performance value was provided by %100 Su-GnPnanofluid-Divergent-P/D=2 conical internal rib compound configuration as 1.72 inthe 6283 Reynolds number, the dimensionless entropy production number value wasrealized as 0.42 for the 25,451 Reynolds number, in the same configuration.</p

NUMERICAL INVESTIGATION OF HEAT TRANSFER ENHANCEMENT ON ADDING (TiO2) NANOPARTICALES TO WATER FLOW IN NOZZLES ADDED TOA HORIZONTAL TUBE

This study presents a quantitative investigation regarding the thermal performance ofinserting two types of nozzle; (a) Non-drilled nozzles, (b) Drilled nozzle with threedifferent pitch lengths (126, 180 and 315 mm) through a water-based TiO2 nanofluidflowing into a horizontal tube. The considered nanofluid volume fractions are limitedfrom 0.2% to 2.0%. A uniform constant heat flux of 50 kW/m2was applied onto theouter surface of the tube. The k-ω standard turbulent model was chosen to simulateturbulent flow, and analyses were implemented for the Reynolds number ranging from4000 to 14,000. The nanofluid flow was modeled using the mixture model as it is moreaccurate than the single-phase model. The thermophysical properties of nanoparticlesand water are considered independent of temperature. The thermophysical properties ofthe nanofluid were calculated with equations and empirical correlations based on theliterature. Thus, as the volume fraction of TiO2 in both the smooth tube and drillednozzle inserted tube rise, the heat transfer coefficient increases. While increasing pitchlength of the nozzles means decreasing the number of nozzles in the tube, yet whenusing drilled nozzle, it’s higher than the non-drilled nozzle. As such, inserting morenozzles into the tube fosters the heat transfer, but increases the pressure drop penalty.The highest heat transfer coefficient for drilled nozzle is obtained as 72% which isbigger than the non-drilled nozzle, and the heat transfer of non-drilled nozzle is higherthan the smooth tube about 54.5% for nanofluid flow through the smooth tube insertednozzle with a pitch length of 126 and volume fraction of 2.0%, and friction factorincreases approximately 5 times more than the smooth tube and less than the non-drillednozzle approximately 7%.&nbsp;</p