A Theory for Temperature Oscillations in Loop Heat Pipes

Abstract

As the Loop Heat Pipe (LHP) technology becomes widely accepted for spacecraft thermal control systems, engineers are pushing the LHP operational envelope to design regimes that were not attempted before. As a result, a few so-called “anomalous phenomena” have been discovered, most of which occurred during ground testing. Temperature oscillation, notably on the liquid line, was reported as far back as 1999. At the time, the oscillation was generally deemed benign since it caused no ill effect on the LHP performance. Recently, however, results of LHP testing at the U.S. Naval Research Laboratory revealed that a pump deprime could happen following the appearance of the temperature oscillation, even when the measured loop pressure drop was far below the capillary limit of the primary wick. Within a certain operating regime, the temperature oscillation persisted with a fairly regular frequency and amplitude despite external conditions being kept constant. In an attempt to offer a sensible explanation for the aforementioned LHP oscillatory behaviors, a theory based upon the concept of stability of a nonlinear dynamical system is proposed in this paper. Below a critical value of the characteristic parameter μ, the LHP is able to reach and remain steady state. Above μ, its dynamical state undergoes a bifurcation to morph into a periodic but stable state. The underlying mechanism for the transformation is the mutual modulation of five synergistic processes taking place simultaneously in the loop: phase-change heat/mass transfer (i) in the capillary pump body, (ii) in the pump core, (iii) in the reservoir, (vi) in the condenser, and (v) fluid dynamics in the transport lines. The phenomenon of temperature oscillation is therefore dependent on the right combination of system components and the operating conditions.