Verification of Loop Heat Pipe Stability Theory − Part I Low-Frequency/High-Amplitude Oscillations

The temperature oscillation phenomenon in Loop Heat Pipes (LHP) was reported as far back as 1990. At the time, the oscillations were generally deemed benign for it caused no ill effect on the LHP performance. Recently, a few deprimes were attributed to the oscillations, even when the measured loop pressure drop was far below the capillary limit of the primary wick. Within a certain operating regime, the temperature oscillations persisted with a fairly regular frequency and amplitude despite the external conditions being kept constant. A theory based upon the stability/instability concept of dynamical systems was formulated. It identified two primary mechanisms that cause the oscillations: (i) the direct condensation of vapor in the reservoir destabilizes the system dynamics and (ii) the thermal masses of the LHP components including the liquid mass help damping out the disturbances. The theory pointed out that the two temperature oscillation phenomena – high-frequency/low-amplitude and low-frequency/high-amplitude – were actually one of a kind even though the conditions that trigger the instability are quite different. The next step in the research effort is to validate the theory by verifying the model predictions against the available LHP data. The current research is a work in progress with this endeavor being the first in a series of the planned verification efforts. The theory/model prediction versus test data comparison for the low-frequency/high-amplitude oscillations is the focus of this paper.