Start-up in microgravity and local thermodynamic states of a hybrid loop thermosyphon/pulsating heat pipe

A wickless passive two phase closed loop heat transfer device especially designed for a future implementation on the heat transfer host module of the International Space Station is tested in relevant environment on board a parabolic flight. The tube internal diameter (3 mm) is larger than the static capillary threshold evaluated in normal gravity for this working fluid (FC-72), leading the device to work as a loop thermosyphon on ground and in hyper-gravity conditions, and as a Pulsating Heat Pipe when micro-gravity occurs. Novel start up tests, where the heat load has been provided after the occurrence of microgravity, show that the 20 s microgravity period is enough for the device activation and, most important, that the device activation is purely thermally induced and not affected by the previous acceleration field. Two miniaturized pressure transducers and direct fluid temperature measurement via two micro-thermocouples, allow to provide a detailed insight on the fluid local thermodynamics states both in the evaporator and in the condenser zone during microgravity. It is shown that the two-phase fluid close to the evaporator and the condenser is subjected to several degrees (up to 5 K) of superheating or subcooling. The level of subcooling seems to increase with the heat input level both in terms of temperature difference and in terms of percentage time with respect to the whole microgravity period

Start-Up and Operation of a 3D Hybrid Pulsating Heat Pipe on Board a Sounding Rocket

A large tube may still behave, to a certain extent, as a capillary in a micro-gravity environment. This very basic concept is here applied to a two-phase passive heat transfer device to obtain a new family of hybrid wickless heat pipes. Indeed, a Loop Thermosyphon, which usually consists of a large tube, closed end to end in a loop, evacuated and partially filled with a working fluid and intrinsically gravity assisted, may become a capillary tube in space condition and turn its thermo-fluidic behavior into a Pulsating Heat Pipe. This work presents the results obtained on such a hybrid device heated at 200 W both on board a sounding rocket (ESA REXUS 22, microgravity period ~120 s), and on ground in vertical and anti-gravity orientation. Since no steady state occurred in microgravity conditions, the comparison between flight and ground data focuses on the startup phenomenon, whereas the thorough ground test campaign describes the limits and performances of the device working in thermosyphon mode. The expected thermal behavior in microgravity conditions is between that of a purely conductive tube in anti-gravity conditions on ground and that of a gravity assisted thermosyphon. Since a microgravity period of approximately 120 s is not enough to reach a pseudo steady state regime, further investigation on a longer-term weightless condition is mandatory

Heat Transfer Host on the International Space Station: Design of Preliminary Experiments in Microgravity Conditions

Heat management has always been fundamental in spacecraft design since the dawn of space ight. A mission can fail, even catastrophically, for a poor heat management on a single piece of hardware. More so if it is a fundamental part that fails, like an engine or a solar panel. In the last few years, the progressive miniaturization of electronics and space missions more and more demanding have accentuated the attention on the research of new solutions to properly address the problem. Right now, two-phase heat transfer devices are becoming the predominant solution for heat management, in particular sintered wick Heat Pipes (HP) and Loop Heat Pipes (LHP). This is due to their reliability, lightness and, most of all, their capability to operate without the assistance of any acceleration field. The latter is obtained thanks to a wick, which is also the most complex and expensive element inside the system, enhancing capillarity. A Pulsating Heat Pipe (PHP) is a special kind of heat pipe that does not rely on a wick for capillarity but on the pipe itself and thus it results cheaper and more apt to a space environment. The work contained in this thesis is focused on the design, realization and the subsequent micro-gravity tests on a specific design of Pulsating Heat Pipe that will be hosted in the Heat Transfer Host on the International Space Station in 2020. In particular, it deals with the design, realization and integration of a Rack for testing, set in the 67th ESA Parabolic Flight Campaign in November 2017 and on a first analysis of the outcomes

ACCURACY ANALYSIS OF DIRECT INFRARED TEMPERATURE MEASUREMENTS OF TWO-PHASE CONFINED FLOWS

The characterization and modeling of confined two-phase flows is still one of the most challenging and interesting objectives of the scientific community since it both helps the understanding of the thermo-fluid dynamic phenomena and the development of reliable design tools for the industries. The visualization techniques exploited so far in the literature, allowed to accurately describe the two phase hydrodynamic principles but very little information about evolution of the fluid temperature distribution can be found. The present work is devoted to the accuracy analysis of direct infrared temperature measurements of two-phase confined flows by means of a high resolution and fast infrared camera. A test rig is built to calibrate the camera by varying the fluid (n-perfluorhexane) and the ambient temperatures. A lumped parameter radiation model is developed to quantify the effect of the involved parameters (ambient temperature, back screen temperature and its emissivity, tube transmissivity, fluid transmissivity etc.), in case the experimental conditions differ from the calibration. The IR measurements are performed on a real two-phase passive heat transfer device. Considering the uncertainty related to the calibration procedure and the difference between the calibration and the actual experimental conditions, the maximum error of the IR temperature measurements is ±2°C. The IR technique also allowed to detect temperature gradients within the fluid and temporal temperature distributions of relatively fast thermo-fluid dynamic events

Beta-endorphin response to oral glucose tolerance test in obese and non-obese pre- and postmenopausal women.

Beta-endorphin (beta-EP) is a neuropeptide involved in several brain functions, regulating the reproductive axis and behavioral changes. Estrogens play a modulatory role on circulating levels of beta-EP in women. Previous clinical studies have demonstrated high plasma beta-EP levels in obese subjects and increased beta-EP release after an oral glucose tolerance test (OGTT) in normal or obese women. The aim of the present study was to evaluate plasma beta-endorphin levels in response to an OGTT in pre- and postmenopausal obese and non-obese women, in order to investigate if the decrease in gonadal steroid levels at menopause could modify in a different manner the control of beta-endorphin release in response to glucose administration. A group of 24 normal women (age range 45-55 years) were included in the study. The patients were subdivided in four groups of six subjects each: group A, premenopausal women with body mass index (BMI) 25 (obese); group C, post-menopausal women with BMI 25 (obese). All women were studied between 8.30 and 9.00 am, after overnight fasting, and underwent an OGTT. In obese premenopausal women, basal plasma beta-EP levels were significantly higher than in non-obese women (p < 0.01). In postmenopausal women, regardless of body weight, low basal plasma beta-EP levels were found. A significant increase in plasma beta-EP levels, at 30 and 60 minutes after oral glucose ingestion, was shown in control premenopausal women. No significant modifications to OGTT were shown in plasma beta-EP levels in the other three groups of women. In conclusion, while in premenopausal women the response of plasma beta-EP levels to OGTT is maintained, in postmenopause there is a lack of response to OGTT. This suggests that beta-EP release is dependent upon gonadal steroids, while it is only in part influenced by body weight

Large Diameter Pulsating Heat Pipe for Future Experiments on the International Space Station: Ground and Microgravity Thermal Response

This work describes the thermal characterization on ground and under a varying gravity field (parabolic flights) of a large diameter Pulsating Heat Pipe (PHP) especially designed for its future implementation on the heat transfer host module of the International Space Station (ISS) for long term microgravity experiments. A multi-turn compact closed loop PHP is made of aluminum and partially filled with FC-72 (50% vol.). The 3mm tube internal diameter is larger than the static capillary limit evaluated on ground conditions for the above working fluid, with the objective of dissipating larger heat power inputs compared to smaller diameter channels, under microgravity conditions, allowing the typical slug flow pattern of PHPs to occur. To provide a detailed insight on the thermo-hydraulics phenomena during the device start-up under the occurrence of microgravity, the PHP is equipped with a transparent sapphire tube insert, two miniature pressure transducers and two microthermocouples. The flow pattern and the liquid bulk temperature distribution are detected by a fast VIS camera and a medium wave IR camera respectively. The data recorded on the 67th ESA-NOVESPACE parabolic flight campaign are analyzed in the light of a future implementation on the ISS in 2020 and for the validation of actual numerical models. The device is continuously active during the whole microgravity periods without any stopover. The start-up tests (the heat power is provided after the 0-g occurrence) proved that the PHP operation is not primed by inertial effects. Finally, the thermal energy due to the sensible heat of the liquid phase is estimated showing a lower level than existing theoretical values