The design of a thermal management system for an airborne, infrared, optical telescope system is described. This system provides transient thermal management for the optical elements of the system beginning at a high-temperature soak condition of 71°C (159.8°F) on the ground to a low-temperature operating condition of −30°C (−22°F) to −40°C (−40° F) within 45 min after aircraft takeoff. An active cooling system is employed to enable this rapid cooldown. In addition to the low-temperature requirement, the mirrors and lenses must be cooled so that temperature gradients across the optical elements are on the order of 1°C (33.8 °F) to 2°C (35.6 °F). The ambient air available for ground cooling is specified by the military environment to be 55°C (131.0 °F). As the aircraft takes off and climbs to an altitude of 11,582.4 m (38 kft), the ambient air temperature decreases to a low-temperature of −22°C (−7.6 °F) for steady, level flight at at Mach 0.9, this ambient air temperature results in a ram air inlet temperature on the order of 13.5°C (56.3 °F), after the air is captured and diffused to Mach 0.2 prior to entry into a ram air heat exchanger. This ram air heat sink is used to provide a chilled liquid for cooling of optical elements and the turret housing the system. The low temperatures required for this system, which are on the order of −30°C (−22 °F) to −40°C (−40 °F), make the use of forced-convection, liquid-cooling problematic because of the tendancy of liquids to become quite viscous as they approach these low temperature levels. Furthermore, the use of a single-phase heat transfer process will result in temperature gradients within the system. For these reasons, cooling concepts employing single-phase cooling using chilled-liquids have been eliminated from consideration. A low-temperature, low-pressure refrigerant, R-404a, is used as the working fluid. The themal management system uses the optical elements as the evaporator of a two-phase cooling system. The liquid refrigerant is introduced into the optical elements at the saturation temperature and saturation pressure of the liquid. The flow rate of the refrigerant will be controlled in such a manner that all of the heat transfer takes place in the liquid-vapor mixture region of the thermodynamic diagram for R-404a with the refrigerant exiting the elements at an arbitraily determned quality of approximately 0.8. This will assure that all of the heat transfer will be by boiling heat transfer and will take place at a constant temperature and essentially a constant pressure. Since the heat transfer coefficients are large and the process takes place at essentially a constant temperature, the optical elements will be controlled at the saturation temperature of the refrigerant and will be essentially a constant temperature across the expanse of the optical surface. The thermal management system is comprised of an array of TECs configured as a condenser HX. This TEC HX uses ram air as the eventual heat sink and will provide chilled-liquid produced by a liquid-to-ram air HX as the heat sink for the hot side of the TEC array. This system utilizes the system mass as the evaporator and a TEC HX as the condenser in a two-phase heat transfer process to provide rapid cooldown of the system mass to low temperatures in a short period of time and maintain that mass at proper operating temperatures with essentially zero temperature gradients throughout the system.
Discussion: “Free Convection, Forced Convection, and Acoustic Vibrations in a Constant Temperature Vertical Tube” (Jackson, T. W., Harrison, W. B., and Boteler, W. C., 1959, ASME J. Heat Transfer, 81, pp. 68–74)
