Multiple quasi-steady states in a closed loop pulsating heat pipe

The novel fact that, keeping all the operating and boundary conditions fixed, a single loop pulsating heat pipe exhibits multiple operational quasi-steady states is reported in this paper. For a specified heat power input level and volumetric filling ratio, continuous online measurements of static pressure and temperature at crucial locations, along with flow visualization, have been carried out for more than twelve hours per experimental run of device operation. Four distinct quasi-steady states have been observed in these experimental runs. Each quasi-steady state is characterized by a unique specific two-phase flow pattern and corresponding effective device conductance, revealing the strong thermo-hydrodynamic coupling guiding the thermal performance. The quasi-steady state corresponding to best thermal performance consists of continuous unidirectional flow circulations, while the state corresponding to poor thermal performance is characterized by the intermittent bidirectional flow reversals. A temporal scaling analysis is presented to estimate the order of magnitude of the equilibrium frequency of phase change and ensuing oscillations. These order-of-magnitude estimates closely match with the experimentally observed frequencies. The spectral contents of each quasi-steady state are analyzed and it is found that dominant frequencies of flow oscillations are in the range of 0.1 to 3.0 Hz with each quasi-steady state exhibiting a characteristic power spectrum. This provides the necessary velocity scaling estimates, primary information needed for any progress in design of pulsating heat pipes. (C) 2008 Elsevier Masson SAS. All rights reserved.