放置可移動擾動銅粒子在一水平加熱銅板上對FC-72池沸騰熱傳增強研究

Abstract

本論文乃針對放置可移動的擾動銅粒子在一沉浸於次冷態FC-72中的加熱銅板之池沸騰熱傳增強研究。在本實驗中，我們探討了流體次冷度、擾動粒子的直徑、擾動粒子的數目，以及所施加之熱通量之間的關係。其中，次冷度範圍從5℃到20℃，輸入熱通量q從0.1到6W/cm²，銅粒子的直徑分別有1.0和1.5mm兩種，在小粒徑時數量從100至1800顆，而大粒徑時數量從100到800顆。 實驗結果分別以沸騰曲線和h-T圖表示，並且把加上擾動粒子的結果和光滑銅表面作相互比較。實驗結果發現，當我們在銅板上加上了擾動粒子，熱傳效果在低和中熱通量時，會有明顯的增強效果，而隨著放置於加熱銅板上的粒子量越多，熱傳效果會變得越來越好。在本實驗中，最好的一組熱傳效果大約產生相較於光滑表面之400%的提升，而熱傳效果也會因為參數的相互組合不同而有所變化。另外，在高熱通量的情形下，熱傳效果會有明顯的下降趨勢，這個效果主要歸因於擾動粒子對於氣泡的脫離產生了阻擋的負面效果。在本實驗中，熱傳降低的幅度最大約光滑表面的20%。而理想的熱傳效果，必須搭配選用適當的擾動粒子粒徑、數目、以及流體次冷度才能達到。

An experiment is carried out here to investigate how subcooled pool boiling heat transfer of liquid FC-72 over a horizontal heated copper plate is affected by placing a large number of copper particles above the plate surface, intending to explore the possible pool boiling heat transfer enhancement by the moving particles. During the experiment, the copper particles are freely placed above the heated plate with a rectangular acryl fence surrounding the plate so that the particles can be moved by the force induced by the boiling flow without been blown away from the heating plate. In the experiment the liquid subcooling ranges from 5℃ to 20℃ and the imposed heat flux is varied from 0.1 to 6 W/cm2 for the diameter of the moving metallic particles fixed at 1.0 and 1.5 mm. Besides, the total particle number placed on the plate ranges from 100 to 1800 and from 200 to 800 respectively for the small and large particles. The measured data are presented in terms of boiling curves and boiling heat transfer coefficients for the heating surface with the presence and absence of the particles. The experimental parameters include the liquid subcooling, imposed heat flux level, and the size and number of the particles. Results obtained from the present study for the subcooled pool boiling of FC-72 show that placing the movable copper particles can significantly increase the pool boiling heat transfer at low liquid subcooling for ∆T_sub≤10℃. For the small copper particles at liquid subcooling ∆T_sub=10℃ and N_p=1600, the enhancement can be up to 200% over that for a bare surface for a certain combination of the experimental parameters. The best enhancement in this study can be as high as 300% for the small copper particles at the liquid subcooling of 5℃ and N_p=1400 & 1800. Even when more than one layer of particles are placed on the plate, relatively significant boiling heat transfer enhancement can be obtained. However, the boiling heat transfer enhancement varies nonmonotonically with the liquid subcooling, particle size and number, and the heat flux applied, reflecting the complex mutual influences of the movable particles and bubble motion near the heated surface. We also note that at higher liquid subcooling, the copper particles are less effective in augmenting the boiling heat transfer. For the high ∆T_sub of 20℃ the boiling heat transfer is retarded by the copper particles especially when a large number of the particles are placed on the plate. Besides, the wall superheat for the incipient boiling can be substantially reduced by the moving metallic particles in some cases for N_p⁄N_pf ≥1.0 and ∆T_sub≤10℃. However, at high heat flux (wall superheat) placing the particles on the plate can substantially reduce the boiling heat transfer especially for the large particles. The results from the visualization of the boiling flow over the copper plate indicate that placing the movable particles above the plate produce two opposite effects of enhancing and retarding the boiling heat transfer. At high heat flux the retarding effect is strong and boiling heat transfer is impeded by the particles.