Triem T. Hoang

Development of a Two‐Phase Capillary Pumped Heat Transport for Spacecraft Central Thermal Bus

Thermal requirements of future spacecraft and satellites will certainly outgrow the capability of conventional heat pipes in terms of heat transport, heat density, and temperature control. Emerging passive heat transport technologies such as Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP) have demonstrated in both ground testing and micro‐gravity flight experiments that they have the potential to replace heat pipes as primary heat transport devices in next generation thermal control technology. Like heat pipes, CPLs and LHPs are completely passive systems which have no mechanical moving part to wear out or to introduce unwanted vibration to the spacecraft. However, the heat transport capabilities of CPLs and LHPs are at least one order of magnitude higher than those of heat pipes. Despite sharing many operational characteristics. CPLs and LHPs do have differences. CPLs require a lengthy and tedious start‐up procedure to prime the wicks before heat is applied to the evaporator plate. Even with the sta...

Start -Up Behavior of an Ammonia Loop Heat Pipe

Loop Heat Pipes (LHP) are becoming important h eat transport devices for space -based thermal control systems simply because the thermal requirements of future spacecraft and satellites outgro w the capabilities of conventional heat pipes. Like heat pipes, LHPs are capillary -pumped two -phase fluid circu lation systems/devices containing no mechanical moving part to wear out or to introduce unwanted vibration to the host system. They are capable of transporting thousands of watts of waste heat over long distance s for heat rejection at remote locations. LHPs are, m ore importantly, maintenance -free . In spite of the increasing usage in recent years, some spacecraft engineers are still hesitant to utilize LHPs for what they believe to be the system start -up difficulties. In particular , the start -up process is sometimes problematic , especially when the evaporator is attached to a large thermal mass. Nevertheless the problem can be easily overcome by employing an electrical heater and/or a thermoelectric cooler to kick -start the loop as demonstrated in a numb er of LHP systems. The U.S. Naval Research Laboratory (NRL) recently carried out a test program of an ammonia LHP to investigate the feasibility of the LHP technology for the Navys future applications. Focus of the test program was placed on the star t-up process and the procedure to kick -start the LHP if it failed to start on its own. Like everything else, understanding the LHP start -up problem was the prerequisite to solving it. Results of the current test program provided valuable information regardi ng the LHP behavior during the start -up process. Nomenclature W p )

Performance of COMMx Loop Heat Pipe on TacSat 4 Spacecraft

The Central Thermal Bus design of the TacSat 4 spacecraft thermal control system (TCS) utilized an ammonia Loop Heat Pipe (LHP) to acquire/transport waste heat from the onboard electronics and reject it to space via eight radiator panels. The TCS underwent flight qualification at the U.S. Naval Research Laboratory (NRL) including the LHP. The thermal performance tests in vacuum demonstrated that the TCS was capable of maintaining the electronics boxes between 0 o C and +40 o C with maximum heat dissipation of 650W under anticipated orbital conditions. TacSat 4 was successfully launched on September 27, 2011 but, soon thereafter, it was apparent that the TCS did not perform as well as designed when the payload was activated for the first time in space. Nevertheless, the TCS was still able to keep the electronics within temperature limits. In early November of 2011, TacSat 4 entered the first eclipse season and then, in the middle of the month, the LHP heat leak seemed to get worse. The payload encountered an “overheat” whenever the heat dissipation was greater than 300W for more than 2 hours. Investigations of the TCS performance anomaly are being conducted independently by the manufacturer and NRL. Hence, not to influence the on-going investigations one way or the other, the authors will refrain from making comments regarding possible causes of the TCS deficiency in this paper.

Evaluation of a Magnetically-Driven Bearingless Pump for Spacecraft Thermal Management

High-performance capillary heat transport devices such as Loop Heat Pipes (LHPs) and Capillary Pumped Loops (CPLs) are becoming important heat transport devices for spacebased thermal control systems (TCS) simply because the thermal requirements of future spacecraft and satellites outgrow the capabilities of conventional heat pipes. Like heat pipes, LHPs and CPLs contain no mechanical moving part to wear out or to introduce unwanted vibration to the host system. Each of the LHP and CPL technologies has many advantages in its own right. However, a complete TCS may require many specialized thermal control functions that neither LHP nor CPL alone can provide. Moreover, the need for smaller and lightweight TCS demands that the pumping capability of the heat transport loop be at least one-order-of-magnitude higher than those of LHPs and CPLs. The U.S. Naval Research Laboratory proposed the concept of hybrid two-phase capillary/mechanical pumped loop for next-generation space TCS. In the hybrid loop, a mechanical pump augments the capillary pumping head of a multiple-evaporator LHP or CPL. The capillary pumps provide a nearperfect liquid-vapor separation at the evaporators and, therefore, retain the effectiveness of the two-phase (evaporative) heat transfer of the LHP/CPL. A test program was carried out at NRL to assess the capability and reliability/durability of a magnetically-driven bearingless pump manufactured by Advanced Bionics Incorporated for use in the hybrid loop. Results of the test program are presented in this paper to demonstrate the feasibility of the hybrid loop concept for future TCS design.

Steerable Radiator Concept for Optimal Performance of Spacecraft Thermal Control System

Advances in commercial technology have pushed for spacecraft to include more functions requiring increased power dissipation in electronic boxes and active thermal components such as cryocoolers and thermal electric coolers. These combine to increase the thermal demand on conventional radiators increasing the temperature beyond the designed capability. In order to increase the radiator capability, NRL has proposed and demonstrated deployable radiators in which both sides of the radiator can be used, therefore doubling the surface area with only a small cost in mass to account for the motors. However, during an orbit, the view factor of the deployable radiator still changes and situations occur where the view factor of Earth or the Sun drive the radiator too warm or the view factor to deep space combined with low power dissipation (such as in a non-operational situation) drive the radiator too cold and freeze prevention heaters are required. The next step in radiator evolution is to have the ability to steer these deployable radiators, to optimize their view factor to both minimize heater power during a non-operational state and to increase cooling capabilities during fully operational stages.

Three-Dimensional Analysis of Mass and Heat Transfer in a Loop Heat Pipe Capillary Pump/Reservoir

Loop Heat Pipes (LHPs) have replaced the conventional heat pipes as the primary heat transports for the spacecraft thermal control systems (TCS). For the most part, the LHP operational capability and versatility provide the engineers many design options to meet even the most challenging thermal requirements of today’s spacecraft. Like heat pipes, LHPs are also passive capillary devices having no mechanical moving part to wear out or break down. Hence they are reliable, durable, and more importantly maintenance-free for space applications. Utilizing the capillary action for fluid pumping, however, has certain drawbacks regarding the LHP performance verification for micro-gravity operation with ground tests. Specifically, the liquid transport capacity of the LHP secondary wick can be greatly affected by the liquid level in the reservoir. If the liquid level is above the secondary wick, the liquid hydrostatic pressure creates favorable conditions for the fluid flow from the reservoir to the primary wick. In other words, a “successful” 1-g LHP test program may not fully verify the functionality of the secondary wick, perhaps, masking potential problems in space. To counter the positive effects induced by the reservoir liquid level in 1-g tests, the capillary pump/reservoir assembly is usually tilted with the reservoir-end down. In the tilt configurations, the buoyancy force acting on the vapor bubbles may impede the delivery of liquid to the primary wick that would not occur otherwise in space. The main objective of this research is to determine whether there exists a gravity-neutral tilt level for a particular LHP design at which both favorable and adverse effects of gravity cancel themselves out. If it does exist, the LHP ground tests should be carried out in this tilt configuration to assess more accurately the liquid transport capability of the secondary wick.

Loop Heat Pipe Utilization for Temperature Control of Electronics Deck

Loop Heat Pipes (LHPs) have become the mainstay of the spacecraft thermal control systems (TCS) over the last 10 years. Usage of LHPs enables the TCS to carry out many thermal management functions that could not be performed in the past, particularly tight temperature control of the on-board electronics. It had been demonstrated by various LHP vendors and researchers, in 1-g testing, that the LHP saturation temperature could be maintained within ± ± ± ±1 o C about a desired set point, regardless of the variability of the system operating conditions. More importantly, the temperature control was accomplished with the simple method of heating/cooling of the LHP reservoir. Nevertheless, even with the ability to control the LHP saturation temperature tightly, the TCS may not prevent the electronics from temperature swing of 10-15oC in some cases. For example, the spacecraft payloads can be either ON at full power or OFF in accordance with a certain duty cycle. Coupled with the large thermal mass of the payloads, the ±1 o C stability requirement for operational reliability is very difficult to meet. A thermal vacuum (TV) test program was carried out at the U.S. Naval Research Laboratory to determine how well a LHP-based system maintained the temperature of a typical electronics deck. In this paper, the essence of the LHP temperature control will be discussed. The pre-test model predictions and the TV test results will also be presented.

Cryogenic Loop Heat Pipe for Zero-Boil-Off Cryogen Storage

For long-duration space missions, cryogens in liquid phase such as Hydrogen, Oxygen or Helium may be needed as fuels or for other purposes. For terrestrial applications, liquid Hydrogen storage coupled with high efficiency Hydrogen fuel cells offer greater endurance for the unmanned air vehicles and other autonomous systems. Cryogen storage systems are thermally insulated from the warmer environment to minimize the heat leak. However, the heat leak does not completely go away. The liquid cryogen changes phase to vapor as it absorbs the heat leak from the environment. Pressure inside the tank increases as the vapor continues to build up in it. At some point, a relief valve must open to vent excess vapor to the environment to keep the tank under the pressure limit. The loss of the cryogen shortens the life of the storage of the mission. The Zero-Boil-Off (ZBO) storage concept calls for the heat leak to be removed by active cooling. Thus, no cryogen is lost due to the heat leak. Most of the previous ZBO research efforts focused on active cooling of an area on the tank outer wall. This created a local cold spot that could lead to a thermal instability condition. In the current research, a cryogenic Loop Heat Pipe (LHP) is utilized to remove an amount of the generated vapor out of the storage tank and transport it to a cryocooler where it is condensed back to liquid and then returned to the tank. Since the stored cryogen is used as the working fluid for the two-phase cooling loop, the tank pressure can be easily controlled and maintained at an almost constant level. In addition, the LHP is a capillary-pumped cooling transport having no mechanical moving parts to wear out or to generate unwanted vibration to the host system.

Non-Intrusive Fluid Flow Measurement Method for Loop Heat Pipes

The ability to accurately calculate the rate of fluid flow in a Loop Heat Pipe (LHP) is essential in the analytical modeling of the loop performance. The first step in the process of validating a LHP model is, therefore, to verify the predicted flow rate against the measured value in the actual loop operation. Since the LHP operational characteristics are sensitive to the system pressure drop, any mechanical measuring device utilized as part of the fluid loop will certainly affect the thermal performance, most likely, in a bad way. Accordingly, the U.S. Naval Research Laboratory (NRL) developed a non-intrusive method of determining the mass flow rate in a single-phase section of the LHP transport lines simply by measuring the time rate of change of its wall temperature. Hence, the LHP flow rate can be determined at any time during operation, whether under a steady state or transient condition. A proofof-concept test program was carried out to demonstrate the accuracy and responsiveness of the NRL flow measurement method. Different algorithms were employed to deduce the flow rate from the measured temperatures. Preliminary data assessment showed that the method produced excellent results at various flow settings for either gasor liquid-phase fluid flow.

Analytical Model for Transient Loop Heat Pipe Operation

Loop Heat Pipes (LHPs) have proven themselves as reliable and robust heat transport devices for spacecraft thermal control systems (TCS). As they gained increasing acceptance, LHP-based TCS have become more and more complex; more than one LHP may be used to carry the waste heat from a common heat source to multiple radiator locations for rejection. In Low Earth Orbit (LEO), orbital variation of the thermal environment does not allow the LHPs to reach a “steady state” as it does in Geosynchronous Earth Orbit (GEO). Hence, their heat transport requirements are dependent not just on the applied heat load but also on the orbit beta angle, thermal mass and the TCS operation-specific conditions. LHP performance is characterized in 1-g for steady-state operation and is a good starting point for selecting an initial design. But in order to demonstrate that the TCS can meet its requirements during all phases of the mission, many or all of which are not steady-state, an integrated transient thermal model of the LHPs and the spacecraft environment becomes necessary. The thermal model must be flexible enough to handle different TCS configurations in trade studies and yet accurate enough to predict the TCS performance for both 1-g and 0-g operation. It must also be run-time efficient. To this end, an LHP transient fluid/thermal model has been developed.