High Frequency Low Amplitude Temperature Oscillations in Loop Heat Pipe Operation 2003-01-2387

The operating temperature of a loop heat pipe (LHP) with a single evaporator is governed by the compensation chamber (CC) temperature, which in turn is a function of the evaporator power, condenser sink temperature, and ambient temperature. As the operating condition changes, the CC temperature will also change during the transient but eventually reach a new steady temperature. Under certain conditions, however, the LHP never really reaches a true steady state, but instead displays an oscillatory behavior. This paper presents a study on the oscillation of the loop operating temperature with amplitudes on the order of 1 Kelvin and periods on the order seconds to minutes. The source of the high frequency temperature oscillation is the fast movement of the vapor front in the condenser section, which usually occurs when the vapor front is near the condenser inlet or the condenser outlet. At these locations, the vapor front is unable to find a stable position for the given operating condition, and will move back and forth. The oscillation of the vapor front causes an oscillation of the liquid flow in the condenser and liquid line, leading to an oscillation of the CC and loop temperatures. Factors that affect the vapor front movement include evaporator power, condenser sink temperature, body forces and whether or not the CC temperature is actively controlled. As long as there are no large thermal masses attached to the evaporator, the loop can self adjust rather quickly and the vapor front will move rapidly around the condenser inlet or outlet, leading to high frequency temperature oscillations. The amplitude of temperature oscillation is usually the largest in the liquid line, up to 20 Kelvin in many cases, but diminishes to less than 1 Kelvin in the CC. Furthermore, the high frequency temperature oscillation can occur at any CC temperature with the "right" combinations of the evaporator power and condenser sink temperature.