EXPERIMENTAL INVESTIGATION OF POOL BOILING HEAT TRANSFER IN NANOFLUIDS AROUND SPHERICAL SURFACES

Öz

In this study, the pool boiling heat transfer around spherical surfaces with high temperature quenched by nanofluids at saturated conditions and under atmospheric pressure was experimentally investigated. In the experiments, pure water-based silica, alumina, titania and copper oxide nanofluids with three different volumetric particle concentrations (0.01, 0.05 and 0.1%) were used. After the spherical test specimen made up off brass material was heated at high temperatures, it was suddenly plunged into the nanofluid suspensions at saturated conditions. Using the temperature-time data of the specimen, the cooling curves were drawn and the boiling curves were obtained. The experimental results showed that the cooling performance of test specimen depended on the type of nanofluids and nanoparticle concentration. In the first quenching tests, although the cooling trend were nearly identical to that in pure water, it was observed that the cooling time was considerably shortened with the repetition tests in nanofluids. This effect enhanced with the nanoparticle concentrations. For silica nanofluids, especially, the film boiling region vanished during the repetition tests and the critical heat flux dramatically increased. The experimental results also showed that a considerable change in nucleate pool boiling heat transfer was not observed for all nanofluids. Consequently, it was determined that the nanoparticles deposited on the test surface after the quenching tests by nanofluids increased the wettability and thus caused the increment in critical heat flux. 

Anahtar Kelimeler

• Heat transfer
• pool boiling
• nanofluids
• spherical surfaces

KÜRESEL YÜZEYLER ETRAFINDA NANO AKIŞKANLARDA HAVUZ KAYNAMA ISI TRANSFERİNİN DENEYSEL İNCELENMESİ

Öz

Bu çalışmada, atmosfer basıncı altında ve doyma şartlarında nano akışkanlarla su verilerek soğutulan yüksek sıcaklıktaki küresel yüzeyler etrafında havuz kaynama ısı transferi deneysel olarak incelenmiştir. Deneylerde %0,01, %0,05 ve %0,1 olmak üzere üç farklı hacimsel konsantrasyonda saf su esaslı silika, alümina, titania ve bakır oksit nano akışkanları kullanılmıştır. Bronz malzemeden yapılmış küresel test numunesi yüksek sıcaklıklara kadar ısıtıldıktan sonra, doymuş şartlardaki nano akışkan süspansiyonlarına aniden daldırmıştır. Numuneye ait sıcaklık-zaman verileri ile soğuma eğrileri çizilmiş ve kaynama eğrileri elde edilmiştir. Deneysel sonuçlar, test numunesinin soğuma performansının nano akışkan tipine ve konsantrasyonuna bağlı olarak değiştiğini göstermiştir. İlk su verme testlerinde saf suya yakın değerler elde edilmesine rağmen, nano akışkanlarda tekrarlı testler ile soğuma süresinin önemli ölçüde kısaldığı görülmüştür. Bu etki nano partikül konsantrasyonuyla artmıştır. Özellikle silika nano akışkanı için tekrarlı su verme testleri ile film kaynama kaybolmuş ve kritik ısı akısı önemli derecede artmıştır. Deneysel sonuçlar, ayrıca, çekirdek havuz kaynama ısı transferinde tüm nano akışkanlar için önemli bir değişimin olmadığını göstermiştir. Sonuç olarak, nano akışkanlar ile su verme testleri sonrasında, test yüzeyi üzerinde biriken nano partiküllerin yüzeyin ıslatabilirliğini artırdığı ve böylece kritik ısı akısında artışa neden olduğu belirlenmiştir.

Anahtar Kelimeler

• Isı transferi
• havuz kaynama
• nano akışkanlar
• küresel yüzeyler

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