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Journal of Vibration Testing and System Dynamics 6(3) (2022) 247--271 | DOI:10.5890/JVTSD.2022.09.001

Carmen Chicone$^1$, Z.C. Feng$^{2}$, Stephen J. Lombardo$^{2,3}$, David G. Retzloff$^{3}$

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Abstract

Dynamic evaporation and condensation processes contribute to successful operating regimes in oscillating heat pipes (OHPs), but they can also be responsible for evolution to a stagnant state where all liquid slugs are in the cold zone of the OHP system. In fact, accumulation of liquid slugs in the cold zone is considered to be the main cause for the slow startup of OHP systems. We formulate a model for the dynamics of liquid slugs and vapor plugs, which takes into account the effects of evaporation and vapor compressibility, and use it to examine mechanisms that drive the system from stagnation to oscillation. For example, modeled states where all menisci are located in the boundary between the hot and cold zones are equilibria in the presence of an imposed temperature gradient. Linear stability analysis at such equilibrium states corresponding to the stagnant case, i.e., all liquid slugs in the cold zone, is used to show that stagnant states bifurcate to self-excited oscillations when the system is subjected to sufficiently high temperature gradients. Our results extend those for an earlier model for one slug in a pipe closed at one end to a multi-slug model with pipe geometry characteristic of operational OHPs.

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