Characteristics of oscillating flow in a micro pulsating heat pipe: Fundamental-mode oscillation

Theoretical and experimental analyses are performed for oscillating flow in a micro pulsating heat pipe (MPHP). A meandering rectangular micro-channel with a hydraulic diameter of 667 μm is engraved on a silicon wafer to form a five-turn closed-loop. Flow visualization through a glass top using a high-speed camera is conducted together with temperature measurement for thermal characterization of the MPHP. The MPHP is observed to have a harmonic oscillating motion: each liquid slug in the MPHP is observed to oscillate at frequency ranging from 40 to 50 Hz with a phase difference of 2π/5 between adjacent slugs. A closed-form expression for the oscillating motion of the slugs is suggested from a vapor spring-liquid mass model. To quantitatively explain the oscillating mechanism by the vapor spring, a link between the spring motion of the vapor plug and heat transfer to the vapor plug is found: Expansion and contraction of the vapor plug are shown to result from continuous evaporation and condensation at the liquid film enveloping the vapor plug. The evaporation heat transfer and the condensation heat transfer are shown to be out of phase and in turn result in a nonzero net heat transfer rate. To mathematically express the relationship between the net heat transfer rate and the spring motion of the vapor plug, a semi-analytic model is proposed. A semi-analytic expression for the vapor spring constant is developed and validated with experimental results. This study on the fundamental-mode oscillation in MPHP is expected to be used as a building block for investigating more complex oscillating motion in PHPs.