The Oscillating Heat Pipe (OHP) is a novel, wickless heat pipe that relies on the phase change induced motion of a contained working fluid to transport heat between the evaporator and condenser. This paper investigates the effects of non-condensable gas (NCG) on OHP operation. A brief synopsis of the existing literature is presented before discussing the experimental setup, method, results, and conclusion. This paper clearly shows that: 1) NCG injection into an OHP produces an overall rise in the steady-state operating temperature, pressure, and thermal resistance; 2) Like LHPs OHPs are more tolerant of NCG than conventional heat pipes.
The oscillating heat pipe is a novel, simply formed, wickless heat pipe that relies on the phase-change-induced motion of a contained working fluid to transport heat between the evaporator (hot end) and condenser (cold end). The improved heat transfer capability, simplicity, and reduced mass of oscillating heat pipes have led to great interest in the oscillating heat pipe. This paper details the terrestrial and microgravity validation of an ultrasonic consolidation manufactured structurally embedded oscillating heat pipe. It is shown that 1) for conditions in which an oscillating heat pipe is terrestrially orientation-independent, it is also likely to be gravity-independent, and 2) for conditions in which an oscillating heat pipe is not terrestrially orientation-independent, it is likely to perform better in microgravity than in a terrestrial environment. Additionally, this test campaign provides evidence that the "knee" found in most oscillating heat pipe performance versus input heat curves roughly corresponds to the point at which 1) oscillating heat pipe performance variation drops off, 2) oscillating heat pipe performance becomes orientation-independent, and 3) the oscillating heat pipe performance becomes gravity-independent. These results can be used to better predict oscillating heat pipe performance using terrestrial test data.
On September 7th, 2017 the U.S. Air Force Research Laboratory launched the second Advanced Structurally Embedded Thermal Spreader (ASETS-II) flight experiment to space on Orbital Test Vehicle 5. The ASETS-II experiment is made of three low-mass, low-cost oscillating heat pipes (OHPs) and an electronics/experiment control box. The three primary science objectives of the experiment are to measure the initial on-orbit thermal performance, to measure long duration thermal performance, and to assess any lifetime degradation. The three OHPs on ASETS-II are of varying configuration (center heating with single-and double-sided cooling) and working fluids (butane and R-134a) in order to isolate specific performance parameters of interest. OHP #3 was specifically designed in order to explore the operating limits on OHP operation in microgravity without requiring excessive operating temperature or pressure. Data collected during the first 6 months of on-orbit operations are presented in this paper. It is shown that each OHP performed as expected, where on-orbit data for OHPs #1 and #2 mirrored ground-truth performance, and the OHP #3 on-orbit maximum operating evaporator temperature increased from ground-truth. The OHPs experienced no significant hysteresis effects and OHP #1 performed successfully in six-week long continuous operation.
The ASETS-II experiment consists of three oscillating heat pipes (OHPs), an electronics box, and mounting structures that control boundary conditions. Each OHP consists of 34 channels in a typical single-layer closed loop design. Butane was selected as the working fluid for OHP #1 and #2 for its performance stability. R-134a was selected for OHP #3 in order to explore the Bond number limit’s influence on OHP operation in microgravity.
The ASETS-II Flight and Flight Spare hardware were subjected to a comprehensive set of ground testing to baseline performance prior to flight testing. For most test conditions, the Flight and Flight Spare test results for OHP #1 and OHP #2 are within the margin of uncertainty in the measurements. OHP #3 on the Flight hardware performs similarly to OHP #3 on the Flight Spare hardware; however, the difference in performance is outside the margin of uncertainty in the measurements. This variation in performance may be attributable to the fact that OHP #3 is being pushed to operate near its Bond number limit.
Graphite Storable Tubular Extendable Masts (GSTEMs) are being investigated and developed for use as an extendable spacecraft boom that can be deployed and retracted on orbit. The advantages of GSTEMs over traditional metallic deployable booms include reduced thermal deformations due to reduced coefficient of thermal expansion and stronger/stiffer booms due to higher strain limit allowing thicker materials to be used. Despite the advantages of GSTEMs, much work is needed to understand and characterize their performance. The work presented here investigates thermal characteristics of GSTEMs. Challenges include accurately capturing anisotropic thermal properties and inherent overlapping construction. Consequently, experimental work was necessary to correlate thermal modeling efforts. Work presented here has illustrated the effectiveness of thermal models for predicting overlapped GSTEM temperatures in a simulated space thermal environment. These models were verified against thermal vacuum testing and can be used to extrapolate models to predict on-orbit GSTEM temperature distributions. These correlated models will provide a useful tool for predicting on-orbit thermal performance which can then be used to predict other performance aspects including thermal deformations.
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