Experimental investigation of enhanced convective boiling in small channels.

摘要

Performance characteristics are experimentally determined for enhanced convective boiling of FC-87 in horizontal, small-cross-sectional-area, single-channel heaters. One method of enhancement investigated is decreasing channel size. Flow boiling studies have shown that heat transfer coefficients are enhanced with decreases in channel size, as long as channel dimensions are not made too small. Three different channels are tested which have square cross sections with side lengths of 2, 1, and 0.5 mm, respectively. Each of the three channels are 8 cm in length.; Another method of enhancement investigated is the application of a recently developed, microporous surface coating. Although few studies have been performed to quantify the effects of internal porous coatings on flow boiling, their application in this area is promising. This boiling enhancement paint provides enhancements over the entire nucleate boiling curve by dramatically increasing the number of active nucleation sites. The heated surface for two of the three channels is tested both with, and without, the microporous coating. Tests are conducted over wide ranges of velocity, subcooling and heat flux. Use of the microporous coating in a confined channel combined with subcooled inlet conditions enables dissipation of high heat loads. Two-phase pressure drop and local and average heat transfer coefficients are determined.; A nucleate boiling heat transfer mechanism study is performed to provide insight into the mechanisms of boiling enhancement. A unique photographic method is used to quantify volumetric flow rate of vapor departing from a heated wire immersed in a saturated liquid. The vapor flow rate is determined by measuring the volume of bubbies after departure from the boiling surface in consecutive frames of high-speed video. This consecutive-photo method requires relatively few video images to be analyzed to obtain steady-state vapor volume flow rates. The volumetric flow rate data are used to calculate the latent heat transfer and, indirectly, the sensible heat transfer which comprises the nucleate boiling heat flux. Studies are performed to investigate boiling enhancement due to surface tension reduction and length scale reduction.