Start-Up and Transient Behavior of an Oscillating Heat Pipe

Microelectronics devices require relatively low and uniform temperatures for maximum reliability and oscillating heat pipes (OHPs) may prove to be extremely well suited to this application. However, in some operating conditions, temperature overshoot has been observed at startup, which could lead to premature electronics failure. In this paper, the effect of various OHP operating parameters such as working fluid (deionized water, methanol, and acetone), inclination angle (vertical, 45°, and horizontal), fill ratio (50%, 60%, 70%, and 80%), and heat load (10W, 20W, and 30W) on start-up performance of a 20-turn OHP were investigated. This information could be used by a designer to optimize an OHP system to reduce the risk of start-up evaporator temperature overshoot before the system begins cooling. All working fluids performed best at high power. Water, for most cases, did not exhibit overshoot, but it performed well only in the vertical orientation with high fill ratios. However, methanol and acetone performed best at low fill ratios irrespective of inclination angle. Low power (10 W) was not sufficient to initiate oscillations when the fill ratio was high for these fluids, and overshoot occurred for both methanol and acetone at different conditions. Methanol, in general, did not overshoot either at low power and low fill ratio or high power and high fill ratio. However, acetone showed overshoot only at high heat load regardless of fill ratio and inclination angle. Start up behavior was somewhat unpredictable, with repeated test runs showing different results despite identical operating parameters.