The system trades mass-optimized heat rejection for a human-rated, single-fluid pumped system of greater heat rejection range and passive control.
Fluid-pumped radiators are used to reject heat from structures to space. A fluid travels inside the structure to collect heat, and then travels external to the structure through radiators where the heat is rejected to space via radiation heat transfer. A radiator is essentially several tubes attached to a thermally conducting plate or face sheet. The fluid cools as it travels along the inside of the tubes, and then returns to the inside of the structure to repeat the heat rejection cycle. If the structure contains humans, the fluid in the structure must be nontoxic and nonflammable. Further, as space can be extremely cold (4 K), the fluid external to the structure may freeze, particularly during low-power operations where heat rejection needs are minimal. Freezing of the fluid renders the radiator inoperable, and unfreezing a radiator can be very difficult, power-intensive (i.e. heaters), and/or timely. For these reasons, two fluids may be used: one inside that is compatible with humans (e.g. water), and one outside that has a low freezing point (e.g. ammonia). The heat is then transferred from the inner loop to the external loop through a heat exchanger. This dual-loop system is more complex and heavier than a single-loop system. However, as the outer loop does not freeze as easily, the dual-loop radiator system can be operated at lower heat rejection loads, increasing its overall heat rejection range (or turn-down ratio) over that of the single-loop system.
Dual-loop fluid-pumped radiator systems that can operate over large ranges of heat rejection (i.e. accomplishing large turn-down ratios) are massive and complex due to multiple components. Handling of the external fluid is also nontrivial in terms of ensuring material compatibility and minimizing human hazards. The heat rejection range of single-fluid pump systems are limited due to the fluid freezing. Thawing techniques to ensure reliable performance, or extend the range, require increased power and/or mass, and still fall short of the reduction achievable with a dual-loop system.
This invention allows a spacecraft to have one single-fluid loop radiator system that uses nontoxic, nonflammable, noncorrosive dielectric fluid for variable heat rejection in variable environments. The invention enables radiator turn-down from high heat rejection in hot environments to low heat rejection in cold environments using a single-fluid loop. A single-fluid loop system is less complex with less risk (and typically less mass) than a dual-fluid loop system. The system can be passive or control can be added to actively increase the turn-down further. Finally, the invention specifically allows for quick recovery from a low heat rejection operation to a high heat rejection operation.
The innovation is trickle flow to enable large reduction in heat rejection without freezing the radiator, and enables quick stagnation recovery. As a fluid cools inside a tube, the fluid nearest the tube walls will cool faster than the fluid at the center of the tube. This causes a radial temperature gradient in the fluid across the tube. Many fluid properties vary with temperature, which can cause the cooler fluid to stagnate (cease flowing) while the warmer fluid in the center continues to flow. As the heat rejection is decreased, more fluid will stagnate and the warmer fluid flow in the center will reduce to a trickle. At the system level, the tube with the trickle flow and its corresponding radiator area will appear to not contribute significantly to heat rejection. Subsequently, the active radiator area is reduced passively, and allows the radiator to be turned down.
The system is designed such that some or all tubes develop some level of trickle flow and do not fully stagnate. Mass flow or bypass valve control can increase the turn-down ratio without having the entire radiator stagnate. The trickle flow is then available to essentially act as a passive in-line heater and aids in quick heat rejection recovery.
As heat rejection needs decrease, trickle flow will develop either in all tubes or a set of pre-designated tubes (through mechanical design). As heat rejection needs increase, the trickle flow will either increase in flow rate and/or increase in temperature, which will act to un-stagnate the surrounding fluid within the tube. Increased mass flow through the radiator is caused by a command to the pump and/or a bypass valve upstream of the radiator that diverts more fluid to the radiator. Increased fluid temperature would simply be a response in the system to the increased heat load should mass flow be held constant.
Trickle flow can be accomplished with fluid of thermal conductivity that decreases with temperature, has low or no freezing point, and/or with fluid of viscosity that increases inversely with temperature. As thermal conductivity lowers with temperature, the fluid near the tube wall is less able to transfer heat and thus, in essence, insulates the warmer inner fluid at the center of the tube. As viscosity increases with temperature, the higher drag on the tube walls due to friction causes the fluid to flow slower and cool faster. If it cools fast enough, it could stagnate and create a smaller-diameter tube through which the warmer fluid travels. Dynamic control of fluid mass flow rate through the radiator via either a bypass valve or variable speed pump could increase further the turn-down ratio.