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This paper describes the performance benefits and current technology progress of an active heat pump loop (HPL) thermal control bus for spacecraft and planetary thermal control applications. Having initiated this research more than 14 years ago, this paper also briefly highlights the technical developments and obstacles overcome during this 14-year development. This paper discusses the unique features of the HPL approach that make it an attractive design choice for future manned thermal control applications: the use of an heat pump to reject heat to space at a temperature above the heat acquisition temperature, the use of non-toxic thermally stable working fluids, and the use of high-performance lubrication-free (gravity independent) refrigeration compressors. The HPL approach has the performance benefits of a traditional two-phase pumped loop thermal bus coupled with the simplicity of a single-phase pumped loop.

This paper investigates the use of several zero-ozone depleting potential (zero-ODP) HFC refrigerants, including HFC-134a, HFC-227ca, HFC-227ea, HFC-236ea, HFC-236cb, HFC-236fa, HFC-245cb, and HFC-254cb, for centrifugal chiller applications. We took into account the thermodynamic properties of the refrigerant and aerodynamic characteristics of the impeller compression process in this evaluation.. For a given operating temperature lift, there are significant differences in the pressure ratio required by each refrigerant and this variation in pressure ratio directly affects compressor size, efficiency, and performance. A comparison of the HFC refrigerant candidates with CFC-114 shows that HFC-236ea, HFC-227ca and HFC-227ea are viable alternatives for centrifugal water chillers. HFC-236ea has properties closest to CFC-114, and will result in comparible performance, but will require a slightly larger impeller and a purge system.

This paper will describe the screening and development of absorbents for HFC-134a in the chemical/mechanical heat pump. The absorbents must have low volatility, low melting point, high solubility for HFC-134a vapor, high heat of mixing with HFC-134a, suitable vapor pressure/temperature concentration characteristics when mixed with HFC-134a, low toxicity, low flammability, and thermal stability. A screening procedure was used to select approximately 15 absorbents for experimental evaluation. Measurement of the key physical and thermodynamic properties of the absorbent/HFC-134a mixtures, such as vapor pressure/temperature/concentration properties, materials compatibility, and thermal stability, is described. From these measurements, activity coefficients, enthalpy of mixing, and entropy of mixing of the liquid solution were determined.

This paper describes the transfer of Russian fine pore sintered powder metal wick structure fabrication technology to the United States for use in the construction of U.S. made loop heat pipes (LHPs), capillary pumped loops (CPLs) and heat pipes. Sintered powder metal wick structures have been used in U.S. made heat pipes for over twenty-five years. The typical pore radii for these wick structures range from 10 to 100 microns. Use of a wick material with a pore radius less than 10 microns was limited due to the high pressure drop encountered when used in a standard heat pipe. Conversely, the Russian loop heat pipe is able to get around this high pressure drop constraint due to its unique evaporator design. Prior to the work presented in this paper, the U.S. concentrated on the development of wick structure materials above 10 microns which created a technology void with the advent of the LHP.

The development of chemically vapor deposited (CVD) diamond promises to greatly impact numerous technologies, and in particular the field of thermal management. Despite its current high cost, the physical properties of CVD diamond are so attractive, compared to currently implemented materials, that its use is justified in a few demanding or previously impossible applications. Unfortunately, at $50 or more per carat, the cost/performance ratio is well beyond the limits that would make it useful for many more widely spread applications. This paper describes an affordable variant of CVD diamond that is under development for thermal management in electronics. This material, designated “diamond/carbon/carbon composite,” consists of a carbon/carbon composite that is partially infiltrated with CVD diamond in the surface region. The performance advantages of DCC relative to current thermal management materials are analyzed, and the cost advantages relative to pure CVD diamond are discussed.

This paper describes a program to develop a lightweight centrifugal compressor for aircraft air cycle environmental control systems (ECS). This compressor will use magnetic bearings for lubrication-free operation and high performance three dimensional impellers. The use of 3-D impellers and magnetic bearings will result in centrifugal compressors superior to current aircraft compressors with improved pressure ratios, flow ranges, and efficiency, as well as reductions in size and weight. The use of the magnetic bearings and three dimensional impeller design extends the centrifugal compressor flow range, reducing the minimum flow rate limit significantly. This oil-free design has applications for aircraft Brayton open cycle cooling, as an alternative to engine bleed air.

In this paper, we describe our accomplishments in infiltrating a carbon/carbon composite material with polycrystalline diamond. This development results in an ultra high thermal conductivity electronics baseplate featuring an electrically insulating coating with high thermal conductivity. This novel composite material exhibits thermal conductivity higher than all existing coldplate materials. We have prepared diamond-infiltrated composites, and characterized the resulting material including the thermal conductivity measurement of the base carbon/carbon, SEM and Raman spectroscopy, and DC electrical resistivity measurements of the integrated diamond/carbon/carbon composites. We have achieved metallization of the diamond surface, demonstrating in part the feasibility of surface mount technology applied to the dielectric heat sink composite. We discuss our technical accomplishments to date, our objectives for the ongoing technical program and future applications of this material.

This paper describes an effort to demonstrate the benefits of an innovative, lightweight, lubrication free centrifugal compressor that allows the use of environmentally safe alternate refrigerants with improved system efficiencies over current State-of-the-Art technology. This effort couples the recently developed 3-D high efficiency centrifugal compressor and fabrication technologies with magnetic bearing technology and will then prove the performance, life and reliability of the compressor.

The basic feasibility and anticipated benefits to using heat pipe technology to cool the turbine vanes of gas turbine engines are presented in this paper. This concept involves fitting out the vane interior as a heat pipe, extending the vane into an adjacent heat sink and then transferring the vane incident heat through the vane to the heat sink. An advanced military fighter engine is used as a baseline and the bypass air is the chosen heat sink. The results of this study show a 7.2% increase in engine thrust, a 0.2% decrease in specific fuel consumption with overall engine weight increased by less than 1% by using this technology.

The Capillary Pumped Loop (CPL) is a two phase aerospace thermal transport system with many advantageous performance characteristics. While retaining the passive nature of the heat pipe, it has demonstrated an order of magnitude greater thermal transport capacity over high performance arterial heat pipes. In this survey paper, the CPL is described and its brief history discussed. A postulated analytical design model based on thermodynamic principles is presented. Both demonstrated and potential performance advantages are given. Finally, opportunities for future research are suggested.