David B. Sarraf

Intermediate Temperature Fluids Life Tests - Experiments

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 725 K (170 to 450°C), including space nuclear power system radiators, fuel cells, and high temperature electronics cooling. Historically, water has been used in heat pipes at temperatures up to about 425 K (150°C). Recent life tests, updated below, demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C), due to water’s favorable transport properties. At temperatures above roughly 570 K (300°C), water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. An electromotive force method was used to predict the compatibility of halide working fluids with envelope materials. This procedure was used to reject aluminum and aluminum alloys as envelope materials, due to their high decomposition potential. Titanium and three corrosion resistant superalloys were chosen as envelope materials. Life tests were conducted with these envelopes and six different working fluids: AlBr

Heat Pipe Cooling of Concentrating Photovoltaic (CPV) Systems

Concentrating photovoltaic systems (CPV) utilize low cost optical elements such as Fresnel lens or mini-reflecting mirrors to concentrate the solar intensity to 200 to 1000 suns. The concentrated solar energy is delivered to the solar cell at up to 20 to 100 W/cm. A portion of the energy is converted to electricity, while the remainder must be removed as waste heat. Solar cell cooling must be an integral part of the CPV design, since lower cell temperatures result in higher conversion efficiencies. A heat pipe cooling system was developed to passively remove the high heat flux waste heat at the CPV cell, and reject the heat to ambient through natural convection. With a heat flux of 40 W/cm, the heat pipe heat sink rejected the heat to the environment by natural convection, with a total cell-toambient temperature rise of only 40°C. In contrast, the ∆T between the cell and ambient would be over 110°C using natural convection from the backplate.

High Temperature Water-Titanium Heat Pipe Radiator

Space nuclear systems require large area radiators to reject the unconverted heat to space. System optimizations with Brayton cycles lead to radiators with radiator temperatures in the 400 to 550 K range. To date, nearly all space radiator systems have used aluminum/ammonia heat pipes but these components cannot function at the required temperatures. A Graphite Fiber Reinforced Composites (GFRC) radiator with high temperature titanium-water heat pipes is currently under development. Three candidate fin materials have been evaluated: K13D2U fibers with 5250-4, EX1551, and HPFE resin. Titanium was selected over Monel as the baseline envelope material, due to its lower mass and previous experience with bonding titanium into honeycomb panels. Graphite foam saddles are used to bond the heat pipes to the radiator fins. In addition to providing a heat transfer path between the round heat pipes and flat fins, the graphite saddle also provides micrometeroid protection, and reduces the effects of the coefficient of thermal expansion difference between the heat pipe and the fin. This paper also discusses mechanical and thermal tests of the laminate material, as well as a series of test panels.

High Temperature Water Heat Pipe Life Tests

NASA is interested in Brayton cycle converters for both nuclear powered spacecraft, and lunar/Mars missions. The radiator to dissipate the waste heat will operate at temperatures in the 400–550 K range. To date, nearly all space radiator systems have used aluminum heat pipes with ammonia working fluid, but these heat pipes are not suitable in the higher temperature range. A Graphite Fiber Reinforced Composites (GFRC) radiator with high temperature water heat pipes is currently under development. Previous short‐term life tests indicated that water would be compatible with titanium and Monel heat pipes. This paper presents the results to date of heat pipe life tests with commercially pure titanium, titanium alloys, as well as Monel K500 and Monel 400. To date, the life test pipes have operated successfully at 500 and 550 K. The Monel and CP‐Titanium pipes have operated for 11,800 hours, while the titanium alloy pipes have operated for 3,300 hours. The life tests are ongoing.

Intermediate Temperature Fluids Life Tests — Theory

There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, and high temperature electronics cooling. Potential working fluids include organic fluids, elements, and halides, with halides being the least understood, with only a few life tests conducted. Potential envelope materials for halide working fluids include pure aluminum, aluminum alloys, commercially pure (CP) titanium, titanium alloys, and corrosion resistant superalloys. Life tests were conducted with three halides (AlBr3, SbBr3, and TiCl4) and water in three different envelopes: two aluminum alloys (Al‐5052, Al‐6061) and CP‐2 titanium. The AlBr3 attacked the grain boundaries in the aluminum envelopes, and formed TiAl compounds in the titanium. The SbBr3 was incompatible with the only envelope material that it was tested with, Al‐6061. TiCl4 and water were both compatible with CP2‐titanium. A theoretical mod...

Loop Heat Pipe for TacSat‐4

The TacSat‐4 micro‐satellite uses an aluminum/ammonia loop heat pipe (LHP) to transport 700 W of heat from the electronics to two radiator sections. In addition to the thermal requirements, there were additional specifications for the primary and secondary wicks, and the flow balancer between the two LHP condensers. This paper discusses the experimental test rigs designed to verify the LHP performance against these requirements. The measured LHP performance at various operating conditions including start‐up, un‐balanced condenser heat removal, transient power, high power, and shut‐down is discussed.

Aluminum foil lined composite tubing

This paper describes the development of lightweight aluminum foil lined polymer matrix composite tubing for applications ranging from heat pipe construction to fluid transport tubing and tankage structure for future spacecraft. The metal lining is completely hermetic and endows the tubing with metal like characteristics without compromising its lightweight or strength advantages. It consists of one wrap of 0.076 mm thick aluminum foil that is rolled in a cylindrical shape and seam welded. Each end of the foil tube transitions to a short section of heavy wall aluminum tubing that is welded to the foil tube creating a leak tight lining. Composite fibers are braided over the lining and then resin transfer molded. The epoxy resin bonds to the fibers and to the lining, forming an integral tube. The demonstration tubing that was constructed was 25.4 mm in diameter, 4.57 m long and had an average mass per unit length of 0.131 kg/m. Extension of this technology to other metal lining materials for containment of v...

Pressure Controlled Heat Pipe for Precise Temperature Control

This paper discusses the design and test of a pressure controlled heat pipe (PCHP) for spacecraft thermal management. The PCHP combines a conventional grooved aluminum‐ammonia heat pipe with a variable‐volume non‐condensable gas reservoir to create a heat pipe whose conductance can be precisely controlled. Testing showed that a prototype PCHP was capable of maintaining a stable evaporator temperature within 0.1 K despite wide swings in heat load and heat sink temperature. A similarly‐sized variable‐conductance heat pipe (VCHP) yielded temperature swings of over 3.5 K for the same variation of heat load and sink temperature. Using a non‐optimized control system, the PCHP was capable of maintaining evaporator temperature within 0.05 K over time. The PCHP had a much faster transient response than other devices such as heated‐reservoir VCHPs, as well as providing a means for changing the set point temperature after assembly. The PCHP is a significant advance over other means of temperature control, even in it...

HEAT PIPES FOR HIGH TEMPERATURE THERMAL MANAGEMENT

Copper water heat pipes are a well-established solution for many conventional electronics cooling applications; however they have several problems when applied to high temperature electronics. The high vapor pressure of the working fluid combined with the decreasing strength of an already soft material leads to excessive wall thickness, high mass, and an inability to make thermally useful structures such as planar heat pipes (vapor chambers) or heat pipes with flat input surfaces. Titanium/water and Monel/water heat pipes can overcome the disadvantages of copper/water heat pipes and produce a viable thermal management solution for high temperature electronics. Water remains the fluid of choice at temperature up to about 280°C due to its favorable transport properties. Life tests have shown compatibility at high temperature. At temperatures above roughly 300°C, water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. Preliminary halide life test results are presented, giving fluids that can operate at temperatures as high as 425°C. At higher temperatures, alkali metal heat pipes are suitable. Water and the higher temperature working fluids can offer solutions for cooling high-temperature electronics, or those working at or above 150°C.Copyright

Pressure Controlled Heat Pipes

In a Variable Conductance Heat Pipe (VCHP), a Non-Condensable Gas (NCG) is added to the heat pipe to allow the conductance to vary. A Pressure Controlled Heat Pipe (PCHP) is a VCHP variant, where the heat pipe operation is controlled by varying either the gas quantity or the volume of the gas reservoir. This paper will discuss two applications for PCHPs: 1. Precise Temperature Control, and (2) Switching thermal power between multiple sinks. A prototype aluminum/ammonia PCHP was built and tested to demonstrate the capability of controlling the evaporator section of an aluminum/ammonia pressure controlled heat pipe to milli-Kelvin levels over an extended period of time. The external (simulated radiator or heat sink) temperature was varied and the heat input into the evaporator section was varied during those tests. Temperature set point changes were also demonstrated. PCHPs can also be used to switch power between multiple high temperature reactors. In a second program, a heat pipe solar receiver was designed to accept, isothermalize and transfer the solar thermal energy to reactors for oxygen production from lunar regolith. The receiver has two PCHPs and two CCHPs to supply heat to two reactors. During operation, one reactor is producing hydrogen at low solar power, while the other reactor is warming up a fresh batch of regolith. The PCHPs switch power between the two reactors as required.