Observations on braided thin wire nucleate boiling in microgravity

A microgravity experiment was conducted on the Space Shuttle Endeavor (STS-108) to observe sustained nucleate boiling of water. Subcooled water was boiled with a single strand and a braid of three 0.16 mm diameter and 80 mm long Nichrome resistive wires. A CCD video camera recorded the experiment while six thermistors recorded the temperature of the fluid at various distances from the heating element. This paper reports experimental results in observations, measurements, and data analysis. Bubble explosions were found to take place shortly after the onset of boiling for both the single and braid of wires. The explosion may produce a high heat transfer rate, as it generates a cloud of microbubbles. The number, size, and departure rate of the bubbles from the heater wire were measured and compared with theoretical models as a function of time. The temperature measurements revealed a complex temperature distribution in the fluid chamber due to bubbles ejected from the wire that carried thermal energy close to the temperature sensors. Drag forces on departing bubbles were calculated based on bubble movement and used to predict bubble propagation. Results from this experiment provided further understanding of nucleate boiling dynamics in microgravity for the eventual design and implementation of two-phase heat transfer systems in space applications.