Two-Degree-of-Freedom Heater Control of a Loop Heat Pipe Based on Stationary Modeling

Loop Heat Pipes (LHP) are widely used for thermal control in satellites. They separate heat supply and rejection spatially by establishing a capillary-driven cooling cycle featuring an evaporator (supply) and a condenser (rejection). By this, it is possible to transport heat over longer distances than with common heat pipes. Mass shifts in a LHP between the evaporator and the condenser depending on the heat load and the sink temperature are counterbalanced by a so-called compensation chamber (CC). By heating the CC it is possible to move the natural operating temperature of the LHP to a desired operating temperature while obtaining the needed heat conductance. It is common practice to improve the design of LHP components with stationary models. In this paper, a stationary model is developed to calculate the CC heater power gap between the natural stationary operating temperature curve and the desired operating temperature level. The quantified stationary CC heater power is then used in a two-degree-of-freedom PI control. It achieves a stable temperature control and an improved reaction to disturbances than commonly used PI controllers by separating the response to setpoint changes from the response to disturbances. The stationary model relies on energy balances of the principal LHP components and on the relevant heat transfer kinetics. The model parameters are reliably derived from the experimental characterization of the LHP. Furthermore, a more detailed analysis of the condenser is needed for the thermal model to be able to map the underlying physics of the system. The operability of the model is confirmed by comparing results to corresponding experimental ones throughout the whole range of operating parameters considered. The designed control strategy is validated on a test bench with a LHP.