Extended Nonlinear Dynamical Modeling and State Estimation for the Temperature Control of Loop Heat Pipes

For the thermal control of aerospace systems, loop heat pipes (LHPs) are often utilized as passive, two-phase heat transport systems. Their high heat transfer coefficient is reached through evaporation and condensation of a working fluid, whose mass flow in the loop is established by capillary forces in a porous wick. However, the LHP operating temperature depends on the dissipated heat of the heat sources and the temperature of the heat sink. Thus, a heater on the compensation chamber of the LHP enables the temperature control under varying operating conditions. To realize the designed state-dependent nonlinear controller on actual LHPs, a nonlinear dynamical state estimation based on the square-root unscented Kalman filter (SR-UKF) is designed in this paper. This filter provides the controller with the required LHP states. Furthermore, the underlying nonlinear LHP state-space model with two temperature measurement is extended to four temperature measurements by modeling the overheated vapor in the LHP. It is shown in a numerical LHP simulation that the extended LHP state-space model improves the control performance achieving a higher robustness against the varying operating conditions.