IMPROVED TECHNIQUE TO MEASURE TIMEAND SPACE-RESOLVED HEAT TRANSFER UNDER SINGLE BUBBLES DURING SATURATED POOL BOILING OF FC-72

Abstract

The objective of this work is to measure space - and time - resolved heat transfer variations during nucleate pool boiling of FC-72 using a microscale heater array in conjunction with a high-speed CCD. The feedback loops used in this work are vast improvements over those used in previous work, and are described here in detail. The heater array is constructed using VLSI techniques, and consists of 96 serpentine platinum resistance heaters on a quartz substrate. Electronic feedback loops are used to keep the temperature of each heater in the array at a specified value, and the variation in heater power required to do this is measured. Data are obtained with the bulk liquid subcooled by 2 degrees C at a system pressure of 0.8 atm. Isolated bubbles are obtained at a wall superheat of 29 degrees C. One nucleation site occurred in the middle of a 2 x 2 array of heaters and at least three heater lengths away from other bubbles. The heat transfer variation versus time from the four heaters directly around this nucleation site is plotted and correlated with images of the bubble obtained using the high-speed CCD. It is revealed that there are other major heat transfer mechanisms in addition to the microlayer evaporation, which had been thought to be the dominant heat transfer mechanism in saturated pool boiling. The purpose of this article is to provide a description of the experimental technique and demonstrate the technology.