Enhancement of FC-72 Pool Boiling Heat Transfer by Ceramic Particles on a Horizontal Plate
|關鍵字:||沸騰;熱傳;顆粒;熱傳係數;冷煤;boiling;heat transfer;particle;heat transfer coefficient;coolant|
|摘要:||本論文針對放置可移動陶瓷粒子於加熱銅板上對FC-72池沸騰熱傳增強實驗研究。可移動顆粒放置於加熱銅板表面上並且由一個四方型的壓克力圍牆圍住，來防止可移動顆粒不會因為液體的流動，導致可移動顆粒被沖離了加熱銅板表面。在實驗中探討顆粒種類、顆粒直徑和顆粒的數量。在實驗參數範圍上，熱通量q從0.1到6 W/cm2，顆粒種類為氧化鋯和氮化矽，氧化鋯顆粒直徑有1.0和1.6 mm，顆粒的數量從100到1800(對於直徑為1.0 mm的顆粒)和100到700(對於直徑為1.6 mm的顆粒)，而氮化矽顆粒直徑有1.6和2.0 mm，顆粒的數量從100到700(對於直徑為1.6 mm的顆粒)和100到500(對於直徑為2.0 mm的顆粒)。
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 fine ceramic particles above the surface, intending to explore the possible pool boiling heat transfer enhancement by the boiling flow driven moving particles. Both zirconia and silicon nitride 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 with the diameter of the particles fixed at 1.0 and 1.6 mm for the zirconia particles and at 1.6 and 2.0 mm for the silicon nitride particles. Besides, the total particle number placed on the plate ranges from 100 to 1800 for the small zirconia particles, from 100 to 700 for the particles at d_p=1.6 mm, and from 100 to 500 for the large silicon nitride particles at d_p=2.0 mm. The measured data are presented in terms of boiling curves and boiling heat transfer coefficients for the case 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 ceramic particles on the heated plate can significantly increase the pool boiling heat transfer coefficient of FC-72 at low and medium heat fluxes (wall superheats). For the zirconia particles the enhancement can be up to 550% over that for a bare surface for a certain combination of the experimental parameters. The best enhancement can be as high as 560% for the lighter silicon nitride particles. Even when more than one layer of particles is 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 size, 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 ceramic particles. However, for cases at high heat flux (wall superheat) placing the large zirconia particles on the plate can noticeably reduce the boiling heat transfer. Moreover, the boiling heat transfer enhancement by the lighter ceramic particles is found to be somewhat better than that by the heavier metallic particles. Furthermore, the boiling heat transfer retardation at high heat flux by the ceramic particles in less severe. The results from the visualization of the bubble and particle movement in the boiling flow over the copper plate reveals the complicate interactions between the particles and boiling flow, which is useful in identifying the mechanisms of enhancing and retarding the boiling heat transfer by the particles at different levels of the wall superheat. At high heat flux the retarding effect by the particles can be strong.