Spacecrafts frequently make use of a wide range of variable conductance devices for thermal management. These devices, also known as thermal switches, help to maintain the temperature of heat sources that operate under varying thermal environments and thermal loads within a spacecraft. Many such applications are already operating in Lunar and Mars landers and rovers, and in satellites. Being highly reliable, scientists may be using thermal switches in the future for human spacecraft transiting through space.
Two-phase thermal switches are low-mass, and they meet the above requirements very well. The temperature of the heat source passively triggers the switching mechanism. The operation of thermal switches is similar to the functioning of a heat pipe with flexible walls.
A two-phase thermal switch consists of a hermetic enclosure housing sealed metallic bellows. The bellows have one of its ends fixed to the enclosure, which, in turn, is in contact with the heat source. Within the bellows, there is a wick structure along with a small amount of saturated working fluid.
The heat from the source enters the enclosure and the bellows, heating the working fluid. The heat vaporizes the fluid, increasing the saturated vapor pressure inside the bellows. The increasing pressure causes the bellows to expand until it makes contact with the other end of the enclosure, which is in contact with a heat sink.
The temperature of the saturated vapor that causes the pressure at which the bellows makes contact with the heat sink end of the enclosure, is the setpoint temperature of the two-phase thermal switch. The design of the two-phase thermal switch determines its setpoint temperature. One of the components deciding the set point temperature is the gas pressure within the enclosure, as it opposes the expansion of the bellows. Users can remotely adjust the set point temperature of a two-phase thermal switch by changing this counter pressure. The switch maintains the heat source at its set-point temperature as the heat sink conducts heat away from the enclosure.
As the name suggests, a two-phase thermal switch operates in two phases. The first phase is similar to a conventional thermal switch. The device switches from a low conductance state to a high conductance state and back as the heat source supplies heat or removes it.
The second phase of the switch comes into effect during its high conductance state. In this condition, the device also operates as a variable conductance device for maintaining the heat source at its set-point temperature. The design of the device allows it to maintain the temperature of the heat source at the set point while the heat sink temperature varies wildly. The variable conductance is a result of the dynamic motion of the bellows as it oscillates and periodically connects with the heat sink.
Two-phase thermal switches are capable of dissipating a wide range of heat loads during widely ranging thermal environments. Their low mass, simple design, low cost, and higher on to off conductance ratios are positive factors in spacecraft applications. At high temperatures of the heat source, the bellows may not disconnect from the sink, essentially acting as a heat pipe.