| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 101 === An experiment is carried out here to investigate how the saturated pool boiling heat transfer of liquid FC-72 over a horizontal heated copper plate of 3×3 〖cm〗^2 in surface area is affected by placing metallic particles above the surface, intending to explore the possible pool boiling heat transfer enhancement by the moving particles. Both copper and stainless steel particles are tested. The particles are freely placed above the heated plate with a rectangular fence surrounding the plate so that the particles can be moved by the force induced by the boiling flow without being blown away. In the experiment, the imposed heat flux is varied from 0.1 to 6 W/cm2 for the diameter of the 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 100 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 imposed heat flux level and the size, material and number of the particles.
The data obtained from the present study for the saturated pool boiling indicate that placing the movable particles can significantly increase the pool boiling heat transfer coefficient of FC-72 at low and medium heat fluxes (wall superheats). For the copper particles the enhancement can be up to 430% over that for a bare surface for a certain combination of the experimental parameters. The best enhancement can be as high as 530% for the stainless steel particles. Even when more than one layer of particles are placed on the plate relatively significant boiling heat transfer enhancement can still be obtained. However, the boiling heat transfer enhancement varies nonmonotonically with the particle diameter, number and material and the heat flux applied, reflecting the complex mutual influences of the movable particles and bubble motion near the heated surface. An optimal boiling heat transfer enhancement could be procured by a suitable choice of the experimental parameters. Besides, the wall superheat for the incipient boiling can be substantially reduced by the moving metallic particles. However, at high heat flux (wall superheat) placing the particles on the plate can greatly 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 results in two opposite effects of enhancing and retarding the boiling heat transfer. At high heat flux the retarding effect is strong.
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