ANALYSIS OF HEAT TRANSFER ENHANCEMENT IN TUBES WITH CAPSULE DIMPLED SURFACES AND AL2O3-WATER NANOFLUID

Öz

This study aims to numerically investigate and evaluate the enhancement of heat transfer by new capsule dimples on tube surfaces for flow of water and Al2O3-water nanofluid with different concentrations, under uniform surface heat flux. The originality of this work lies in combining two passive heat transfer enhancement methods such as geometrical improvements and nanofluids together. Capsule dimples with different depths were considered. Al2O3-water nanofluid was modeled as a single-phase flow based on the mixture properties. The effects of dimple depth and nanoparticle concentrations on Nusselt number, friction factor and performance evaluation criteria (PEC) were studied. Numerical computations were performed using ANSYS Fluent commercial software for 2000 14000 Reynolds number range. It was found that when laminar, transient and fully developed turbulent flow cases are considered, increase in the dimple depth increases the Nusselt number and friction factor for both pure water and Al2O3-water nanofluids cases. Also, the friction factor increases as dimple depth increases. Results show that increase in PEC is more pronounced in the laminar region than in the transition region, it starts to decrease for turbulent flows. For nanofluid, PEC values are considerably higher than pure water cases. The variation of PEC for capsule dimpled tubes are dependent on flow regimes and dimple depths. Increasing the nano particle volume concentration and dimple depth in laminar flows increase the PEC significantly.

Anahtar Kelimeler

• Heat transfer enhancement
• Nanofluid
• capsule dimples
• Computational analysis

BORU YÜZEYİNDEKİ KAPSÜL TİPİ KABARTMANIN VE AL2O3 SU NANOAKIŞKANIN ISI TRANSFERİNE ETKİSİNİN SAYISAL ANALİZİ

Öz

Bu çalışmanın amacı, duvar yüzeyinden düzenli ısı akısı uygulanan boru içi akışlarda geometrik modifikasyon yapılarak elde edilecek ısı transferi iyileştirmesinin numerik olarak incelenmesidir. Geometrik modifikasyon olarak kapsül tipi kabartmalar kullanılmış, akışkan olarak ise su ve Al2O3-su nano-akışkan kullanılmıştır. Isı transferi iyileştirmesi için hem geometrik modifikasyon yapılmış olması hem de bununla birlikte farklı yüzdelerde nano-akışkan kullanılmış olması çalışmayı benzerlerinden farklı bir noktaya taşıyabilmektedir. Kapsül tipi kabartmalar borunun iç yüzeyine farklı derinliklerde uygulanmıştır. Al2O3-su nano-akışkan 1%, 2% ve 3% konsantrasyonlarında tek fazlı akış olarak modellenmiş ve uygulanmıştır. Kabartmaların derinliğinin ve nono-akışkanın farklı konsantrasyonlarda uygulamalarının Nusselt sayısı, Sürtünme katsayısı ve Performans Değerlendirme Kriteri (PEC) üzerindeki etkileri çalışılmıştır. Sayısal analizler ANSYS Fluent kullanılarak 2000-14000 Reynolds Sayısı aralığında gerçekleştirilmiştir. Sonuçlar incelendiğinde, tüm akışkanlar için, laminer akış, geçiş akışı ve tamamen gelişmiş türbülanslı akış durumunda kabartma derinliği arttıkça Nusselt Sayısı ve aynı zamanda da sürtünme katsayısının arttığı görülmüştür. Laminer rejimde PEC daki artış etkisi türbülans rejimine göre oldukça fazladır. Performans Değerlendirme Kriterinin değişimi akış rejimine ve kabartma derinliğine oldukça bağlıdır. Genel olarak, laminer akışta nano-akışkan konsantrasyonu ve kabartma derinliği arttıkça Performans Değerlendirme Kriterinin önemli ölçüde arttığı görülmüştür.

Anahtar Kelimeler

• Isı transferi iyileştirmesi
• nano-akışkan
• kapsül tipi kabartma
• sayısal analiz

Kaynakça

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