Enhancement of subcooled FC-72 pool boiling heat transfer by movable copper particles on a horizontal plate
|關鍵字:||池沸騰;熱傳增強;pool boiling;heat transfer;enhancement|
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.