Experimantal investigation on a closed loop pulsating heat pipe in hyper-gravity conditions (original) (raw)
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Experimental Investigation on a Closed Loop Pulsating Heat Pipe in Hyper-Gravity Conditions
Proceedings of the 15th International Heat Transfer Conference, 2014
A Closed Loop Pulsating Heat Pipe made of a copper tube (1.1mm/2mm I.D./O.D.) and filled with FC-72 has been tested on the Large Diameter Centrifuge (LDC) of the European Space Agency in Noordwijk at different hypergravity levels up to 20g, different heat input levels and different orientations with respect to the gravity direction (vertical bottom heated and horizontal position). Results shows that both in the horizontal and vertical orientation the device operation depends on the combined effect of gravity and heat input level. For the horizontal orientation, fluid stratification and the consequent thermal crisis occur at different acceleration levels depending also on the heat input power level and on the heating/gravity history. During the vertical operation the PHP thermal performance is slightly enhanced by the lower hyper-gravity levels (up to 3g at 50W, up to 4g at 70W and up to 6g at 100W) while two different local thermal crisis affect the PHP thermal behavior with the higher acceleration levels.
Thermal response of a closed loop pulsating heat pipe under a varying gravity force
International Journal of Thermal Sciences, 2014
A Closed Loop Pulsating Heat Pipe made of a copper tube bent into a 2-D serpentine of 32 parallel channels and filled with FC-72, has been tested on ground and in micro/hyper gravity conditions during the 58th ESA Parabolic Flight Campaign. The device has been investigated both in horizontal and in vertical positions and at different heat loads (from 40 to 100 W). Beyond the standard thermal characterization, dynamic investigations have been performed on ground by changing the device orientation at constant heat input levels. Results show that in the vertical position the PHP thermal behavior is strongly affected by the variation of gravity field both on ground and on flight tests. In particular, during a parabolic flight, the first hypergravity period slightly assists the flow motion, while, during microgravity, the device undergoes a sudden temperature increase in the evaporator zone; the following hypergravity phase is then able to bring the PHP back to the previous thermal regime. The PHP in the horizontal position does not show any sensitive thermal variation during the parabola. A further analysis with a tilting bench in the ground lab proves that microgravity thermal behavior is comparable to the horizontal operation on ground: therefore for capillary closed loop pulsating heat pipes microgravity tests are not strictly necessary for the space application assessment.
A pulsating heat pipe for space applications: Ground and microgravity experiments
International Journal of Thermal Sciences, 2015
A novel concept of a hybrid Thermosyphon/Pulsating Heat Pipe with a diameter bigger than the capillary limit is tested both on ground and in hyper/micro gravity conditions during the 61 st ESA Parabolic Flight Campaign. The device is filled with FC-72 and it is made of an aluminum tube (I.D. 3 mm) bent into a planar serpentine with five curves at the evaporator zone, while a transparent section closes the loop, allowing fluid flow visualizations in the condenser zone. Five heaters, mounted alternatively in the branches just above the curves at the evaporator zone, provide an asymmetrical heating thus promoting the fluid flow circulation in a preferential direction. The device has been tested at different positions (vertical and horizontal) and at different heat power input levels (from 10 W to 160 W). Ground tests show that effectively the device works as a thermosyphon when gravity assisted: in vertical position the device can reach an equivalent thermal resistance of 0.1 K/W with heat fluxes up to 17 W/cm 2. In horizontal position the fluid motion is absent, thus the device works as a pure thermal conductive medium. The parabolic flight tests point out a PHP working mode: during the micro-gravity period, the sudden absence of buoyancy force activates an oscillating slug/plug flow regime, typical of the PHP operation, allowing the device to work also in the horizontal position. In some cases the hyper-gravity period is able to eliminate partial dry-outs restoring the correct operation until the occurrence of the next microgravity period.
2018
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. ...
TOWARD A NUMERICAL SIMULATION OF A CLOSED LOOP PULSATING HEAT PIPE IN DIFFERENT GRAVITY ENVIRONMENTS
A Closed Loop Pulsating Heat Pipe (CLPHP) is a new concept of wickless heat pipe, which represents a promising and a flexible solution for moderately high heat flux applications. Numerous are the attempts to simulate the complex behaviour of a CLPHP and in particular one model was already successfully implemented by two of the authors (Mameli and Marengo). However, none of the existing models is able to well represent the effects of various gravity levels, and therefore a novel lumped parameter numerical model for the transient thermo-hydraulic simulation of a CLPHP has been developed. It consists of a two-phase separated flow model applicable to a confined operating regime, meaning that capillary slug flow is assumed a priori. A complete set of balance ordinary differential equations (mass, momentum and energy) accounts for thermal and fluid-dynamic phenomena. The suppression of the common assumption of saturated vapor plugs and the consequent implantation of heterogeneous phase changes represent the principal novelties with respect to the previous codes. The numerical tool is here used to simulate the thermal-hydraulic behavior of a planar CLPHP made of a copper tube (I.D./O.D. 1.1mm/2.0mm) and partially filled with FC-72 in modified gravity conditions (1g, 2g and 10g). This simulated configuration has already been tested experimentally both on ground (normal gravity) and on the ESA-ESTEC Large Diameter Centrifuge which allows reaching accelerations up to 20g. First comparisons with these operating conditions are provided.
Applied Thermal Engineering, 2017
A hybrid Loop Thermosyphon/Pulsating Heat Pipe named Space Pulsating Heat Pipe (SPHP) is tested both on ground and in hyper/micro-gravity conditions during the 63 rd ESA Parabolic Flight Campaign. The device, partially filled up with FC-72 (50% Filling Ratio), is made of an aluminum tube (Inner/Outer Diameter 3 mm/5 mm) bent into a planar serpentine with five curves at the evaporator zone. A transparent section closes the loop in the condenser zone, permitting the fluid flow visualization. Each of the five heaters, mounted alternatively on the branches, just above the curves, is controlled independently, in order to test different heating distributions. The device is tested at different total heat inputs (50 W, 70 W and 90 W), both on ground and in hyper/micro gravity conditions. Data are collected by recording the heat power provided to each heater, the tube wall temperatures, the fluid pressure and fast speed images up to 200 fps are also recorded in the transparent section. An image processing software is developed in order to calculate the bubble flow velocity. On ground, where the device acts like a thermosyphon, the non-uniform heating promotes the fluid net circulation in a preferential direction, stabilizing the operation of the device and thus increasing the thermal performance with respect to the homogeneous heating. The parabolic flight tests point out a working mode in microgravity for such SPHP: the sudden absence of buoyancy force, activating an oscillating slug/plug flow regime, allows the device to work also without gravity assistance. It was found that particular heating distributions can shorten the stopover periods observed when the device is uniformly heated up, stabilizing a pulsating twophase flow motion when the gravity field is absent.
Microgravity Science and Technology, 2012
The Closed Loop Pulsating Heat Pipe (CLPHP) is a very promising passive two-phase heat transfer device for relatively high heat fluxes (up to 30 W/cm 2 ) patented by . Although the CLPHP has a simple structure, its working principles are very complex compared to the standard heat pipe with a porous wick. One of the most debated issues deals on how the thermal performance is affected by the inclination and by the action of different gravity fields (terrestrial, lunar, martian and microgravity). Even if the internal tube diameter satisfies the conventional slug flow regime requirement on the Bond number, gravity force still plays an important role on the PHP behaviour. Heat input and the number of turns are two of the most important indirect parameters linked to the gravity issue. A complete numerical campaign has been performed by means of a FORTRAN code at different inclination angles and gravity levels on various PHP. The numerical model is able to estimate both the hydrodynamic and the thermal performance of a CLPHP with different working fluids. The analysis shows that the effect of local pressure losses due to bends is important and must be taken into account, in particular in the horizontal operation which is the reference point for space applications. Numerical results are matched with the experimental data quoted in literature and both good qualitative and quantitative agreement have been found.
2019
An experimental study of a flat plate pulsating heat pipe has been performed under various gravity conditions during ESA 69 parabolic flight campaign. A molybdenum plate, with 14 milled rectangular channels with cross-section of 3×3 mm, was covered with a sapphire window, and tested in vertical position with ethanol as working fluid. If operation in normal and hyper-gravity conditions were characterized by nucleate boiling regime, FP-PHP working like in looped thermosyphon mode, transition into microgravity was accompanied by a flow pattern change into slug/plug regime with thin film evaporation, due to absence of buoyancy forces. Hydrodynamic instabilities, accompanied with short-term periods of emergence of nucleate boiling under microgravity, were observed during several parabolas. Formations of long vapor slugs in the channel lead to thin film evaporation. Combined optical and infrared visualizations showed velocity and amplitude increase of the menisci motions.
International Journal of Heat and Mass Transfer, 2022
A multi-parametric transient numerical simulation of the start-up of a large diameter Pulsating Heat Pipe (PHP) specially designed for future experiments on the International Space Station (ISS) are compared to the results obtained during a parabolic flight campaign supported by the European Space Agency. Since the channel diameter is larger than the capillary limit in normal gravity, such a device behaves as a loop thermosyphon on ground and as a PHP in weightless conditions; therefore, the microgravity environment is mandatory for pulsating mode. Because of a short duration of microgravity during a parabolic flight, the data concerns only the transient start-up behavior of the device. One of the most comprehensive models in the literature, namely the in-house 1-D transient code CASCO (French acronym for Code Avancé de Simulation du Caloduc Oscillant: Advanced PHP Simulation Code in English), has been configured in terms of geometry, topology, material properties and thermal boundary conditions to model the experimental device. The comparison between numerical and experimental results is performed simultaneously on the temporal evolution of multiple parameters: tube wall temperature, pressure and, wherever possible, velocity of liquid plugs, their length and temperature distribution within them. The simulation results agree with the experiment for different input powers. Temperatures are predicted with a maximum deviation of 7%. Pressure variation trend is qualitatively captured as well as the liquid plug velocity, length and temperature distribution. The model also shows the ability of capturing the instant when the fluid pressure begins to oscillate after the heat load is supplied, which is a fundamental information for the correct design of the engineering model that will be tested on the ISS. We also reveal the existence of strong liquid temperature gradients near the ends of liquid plugs both experimentally and by simulation. Finally, a theoretical prediction of the stable functioning of a large diameter PHP in microgravity is given. Results show that the system provided with an input power of 185 W should be able to reach the steady state after 1 min and maintain a stable operation from then on.
International Heat Transfer Conference 16, 2018
The thermo-fluid dynamic behaviour of a Single Loop Pulsating Heat Pipe (SLPHP) has been characterized during the 66 th ESA Parabolic. The SLPHP, with a 2 mm inner diameter, has been tested in bottom heated mode, varying the working fluid (FC-72 and ethanol), the heat power input (from 1W to 24W) and the gravity level (0.01g, 1g and 1.8g). Two transparent tubes connect the evaporator and the condenser, allowing local fluid flow visualization. A set of three-dimensional maps, derived from semi-empirical correlations usually adopted in literature to estimate the critical diameter at different gravity levels are drawn for the different fluids tested, liquid velocities and fluid temperatures. These maps are used to compare the flow velocity observed experimentally with the critical diameter value calculated. Additionally, an enhanced Volume of Fluid (VOF) model is utilised to simulate an imposed slug flow within a straight 2 mm inner diameter channel, replicating the same experimental conditions, with the primary aim to study the effect of the vapour bubble length and the liquid film thickness on the generated elongated bubble dynamics, in microgravity conditions.