GÖZENEKLİ ORTAMDA DİKEY SALINIMLI HALKASAL BİR AKIŞKAN KOLONUNDAN ISI GEÇİŞİ: DENEY SONUÇLARININ TERMODİNAMİK ANALİZİ

Bu çalışmada gözenekli ortamda bulunan, salınımlı olarak dikey yönde hareket ettirilen, halkasal bir akışkan kolonunda tek fazlı veya iki fazlı habbecikli akışlı ısı geçişi sanki-sürekli durumda, teorik ve deneysel olarak incelenmiştir. Zorlanmış salınımlar suya DC motor ve piston-silindir aracılığıyla uygulanmıştır. Isı geçişi merkezdeki sabit elektrikli ısıtma elemanından salınımlı akışa olmaktadır. Tek fazlı veya iki fazlı akış rejimindeki salınımlı halkasal akıştan ısı geçişi paslanmaz çelik yün gözenekli ortam vasıtasıyla değiştirilmektedir. Tek fazlı akış bölgesinde oluşan hidrodinamik sınır tabakasının merkezdeki akışı takip edemediği için ısı geçişini iyileştirdiği tespit edilmiştir. Basitleştirilmiş bir termodinamik analiz kullanılarak salınımlı akışta habbecikli (kabarcıklı) akış kaynaması da teorik ve deneysel olarak incelenmiştir. Gözenekli ortamın sebep olduğu kanat etkileri; akıştaki sınır tabakalarının karışımı; efektif yüzey pürüzlülüğünün artışı ve habbelerin büyüklüklerinin gözenekli ortam hücre hacmiyle kısıtlı olması gibi kaynama döngüsü değişiklikleri sebebiyle kaynama başlangıcı sıcaklığının parlatılmış metal yüzey üzerindeki havuz kaynaması ve kaynamalı akışa nazaran kaydadeğer derecede düştüğü tespit edilmiştir. Çevrim ortalama Nusselt sayısı için bir korelasyon geliştirilmiş, ve korelasyonun deney sonuçları ile uyumlu olduğu görülmüştür. Kazanlar, kompakt ısı değiştiricileri, ısı boruları ve buhar jeneratörleri yapılan çalışmanın uygulama alanları içerisindedir.

HEAT TRANSFER FROM AN OSCILLATED VERTICAL ANNULAR FLUID COLUMN THROUGH A POROUS DOMAIN: A THERMODYNAMIC ANALYSIS OF THE EXPERIMENTAL RESULTS

Heat transfer in an oscillating vertical annular fluid column flowing through a porous domain in the singlephase or bubbly flow two-phase regime (sub-cooled or saturated nucleate flow boiling) are investigatedexperimentally and theoretically, in quasi-steady state conditions. Forced oscillations are applied to water via afrequency controlled dc motor and a piston-cylinder device. Heat transfer is from the stationary concentric tubularelectric heating element outer surface to the reciprocating flow. The heat transfer in an oscillating vertical annularfluid column flowing in the single phase or in the bubbly flow regime is altered by using stainless steel wool porousmedium. For the single phase region of flow, it is understood that, the effective heat transfer mechanism is enhancedand it is due to the hydrodynamic boundary layer which can not follow the core flow. Bubbly (nucleate) flow boilingin oscillating flow is also investigated experimentally and theoretically using a simplified thermodynamic analysis.The onset of boiling temperature is distinctly dropped compared to the pool and flow boiling experiments on polishedsurfaces due the finned surface effect of the steel porous domain, due to the enhanced mixing of the boundary layerflow and core flow; due to the improvement of apparent surface roughness and due to the alteration of ebullition cycle(bubbles are limited by the cell volume here). The developed correlation predicted cycle-space averaged Nusseltnumber is shown to be in good agreement with the experimental data. The present investigation has possibleapplications in moderate sized wicked heat pipes, boilers, compact heat exchangers and steam generators

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