Flow Characterization of a Pulsating Heat Pipe through the Wavelet Analysis of Pressure Signals

Pulsating Heat Pipes are two phase passive heat transfer devices characterized by a thermally induced two phase oscillating flow. The correct detection of the dominant frequencies of such oscillations is fundamental to fully characterize the device thermofluidic operation but the studies available in the literature are very heterogenous and results are often discordant. In this work, the concept of dominant frequency in Pulsating Heat Pipes is thoroughly discussed and defined analytically. The wavelet transform is used to characterize the fluid pressure signal in the frequency domain varying the heat power input at the evaporator and in the condenser zone of a full-scale Pulsating Heat Pipe tested in microgravity conditions. During the slug-plug flow regime, the dominant frequencies falls in the range 0.6–0.9 Hz, showing an increasing trend with the heat load input. The Cross-Correlation reveals that the two signals at the evaporator and at the condenser are very similar. Finally, the instantaneous angle of phase is calculated and lies between 310 and 360 deg. This value can be physically interpreted as a repeatable time shift between the two signals that can be used to evaluate the flow local mean velocity (0.09–0.13 m/s) constituting a valuable alternative to the visualization techniques

Synthesis of ZnO/PMMA nanocomposite by low-temperature atomic layer deposition for possible photocatalysis applications

Zinc oxide is one of the most widely used semiconductors, thanks to its shallow band-gap of 3.3 eV, low cost, inertness, and abundance in nature. On the other hand, poly (methyl methacrylate) (PMMA) is a common thermoplastic material used in many applications namely because of its transparency, environmental stability, and low cost. The realization of novel inorganic/polymeric hybrid nanomaterials is appealing, being beneficial in a variety of applications including photocatalysis, sensing, energy harvesting and storage, and optoelectronics, but also challenging. In this work, ZnO and PMMA were combined using the atomic layer deposition (ALD) technique. The morphology of the samples was evaluated by scanning electron microscopy (SEM), while the crystallinity has been investigated using X-ray diffraction (XRD) analyses. In order to give a proof of concept of a possible application of the materials synthetized, the photocatalytic activity of the nanocomposites has been tested by the degradation of two organic pollutants in water: methylene blue (MB) dye and sodium lauryl sulfate (SDS), an anionic surfactant. The results have shown that all samples are active in the removal of both pollutants (i.e., MB and SDS), opening the route for the application of the proposed nanocomposites in water treatment.peer-reviewe

Suitability of different titanium dioxide nanotube morphologies for photocatalytic water treatment

Photocatalysis has long been touted as one of the most promising technologies for environmental remediation. The ability of photocatalysts to degrade a host of different pollutants, especially recalcitrant molecules, is certainly appealing. Titanium dioxide (TiO2) has been used extensively for this purpose. Anodic oxidation allows for the synthesis of a highly ordered nanotubular structure with a high degree of tunability. In this study, a series of TiO2 arrays were synthesised using different electrolytes and different potentials. Mixed anatase-rutile photocatalysts with excellent wettability were achieved with all the experimental iterations. Under UVA light, all the materials showed significant photoactivity towards different organic pollutants. The nanotubes synthesised in the ethylene glycol-based electrolyte exhibited the best performance, with near complete degradation of all the pollutants. The antibacterial activity of this same material was similarly high, with extremely low bacterial survival rates. Increasing the voltage resulted in wider and longer nanotubes, characteristics which increase the level of photocatalytic activity. The ease of synthesis coupled with the excellent activity makes this a viable material that can be used in flat-plate reactors and that is suitable for photocatalytic water treatment.peer-reviewe

Ag/ZnO/PMMA nanocomposites for efficient water reuse

This work attempts to produce photocatalytic surfaces for large-scale applications by depositing nanostructured coatings on polymeric substrates. ZnO/poly(methyl methacrylate) (PMMA) composites were prepared by low-temperature atomic layer deposition (ALD) of ZnO on PMMA substrates. In addition, to increase the photocatalytic and antibacterial activities of ZnO films, Ag nanoparticles were added on ZnO surfaces using plasma-enhanced ALD. The morphology, crystallinity, and chemical composition of the specimens were meticulously examined by scanning and transmission electron microscopies, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. The noteworthy photocatalytic activity of the nanocomposites was proved by the degradation of the following organic pollutants in aqueous solution: methylene blue, paracetamol, and sodium lauryl sulfate. The antibacterial properties of the samples were tested using Escherichia coli as a model organism. Moreover, the possible toxic effects of the specimens were checked by biological tests. The present results unambiguously indicate the Ag/ZnO/PMMA nanocomposite as a powerful tool for an advanced wastewater treatment technology.peer-reviewe

Multi-parametric experimental validation of a numerical model for the Pulsating Heat Pipe transient simulation

The Thermal Control Subsystem (TCS) has always played a key role in spacecraft design. Its primary objective is to maintain all spacecraft, payload and subsystems within their required temperature constraint during each mission phase. Moreover, the continuous miniaturization of electronic components along with the complexity rising of mission re- quirements are pushing towards the research of new solutions. An emerging technology, the Pulsating or oscillating Heat Pipes (PHP), is one of the cheapest and most reliable thermal management systems. For these reasons is one of the most promising not only for space applications but also for civil ones. However, since the phenomena that govern its functioning are not well understood, its large scale adoption is still far from being reality. In the last two decades many significant efforts have been done to develop numerical models of such devices, but just few of them are able to perform a complete thermo-hydraulic simulation. One of the most sophisticated models present in literature is the one developed by V. Nikolayev, which reproduced many physical phe- nomena observed in PHPs. Nevertheless, it has not been validated yet due to the lack of experimental data. This thesis represents a first step towards a complete validation of Nikolayev’s model using experimental data provided by the University of Pisa. First, an experimental data post-processing has been performed to obtain all the relevant quantities for the valida- tion. Then, a model update was developed in order to represent as closely as possible the real device with all its features. Startup simulation results, carried out on multiple parameters, show a remarkable agreement from both qualitative and quantitative point of view. This could not be possible without a good description of all the interactions between dominant phenomena governing PHP operations

Experimental analysis and transient numerical simulation of a large diameter pulsating heat pipe in microgravity conditions

International audienceA 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 185W should be able to reach the steady state after 1min and maintain a stable operation from then on

Experimental validation of a Pulsating Heat Pipe transient model during the start-up in micro-gravity environment

International audienceA large diameter Pulsating Heat Pipe (PHP) is a device operating as a thermosyphon on ground and as a PHP under microgravity conditions. Such a prototype with a transparent (sapphire) tube section in the adiabatic region has been tested during the 67th ESA parabolic flight campaign. Infrared visualizations of the fluid in the sapphire section along with measurements of all the relevant quantities, which characterize the device state (pressures, temperatures), are acquired during the tests and exploited for the validation of CASCO code of PHP simulation. After accurate implementation of the PHP geometry and material properties, transient simulations have been carried out. A comparison with the experiment is possible only for the cases where the PHP can be initially assumed at equilibrium. The transients for tube wall temperatures, liquid plug velocities, lengths and temperatures show a good agreement with the experiments during the start-up phase in microgravity conditions reducing the gap towards the development of a fully validated PHP design tool

Development of Innovative modular experimental apparatus for the investigation of start-up and dryout processes in Pulsating Heat Pipe

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the start-up and the thermal crisis phases, that are crucial to define their operational limits. This is due to the complex phenomena involving the thin liquid film evaporation at the triple line (tube wall/liquid/vapor) during the start-up and dry-out process. The design of an innovative single loop modular experimental PHP, especially designed to address the above open issues, is described here. It consists in a square loop made of four borosilicate transparent glass tubes (2mm inner diameter; 5 external diameter) joined at corners by means of brass connectors. The external tube surface is coated by means of series of transparent Indium Tin Oxide (ITO) heaters (16 patches of 30 mm length). This solution allows to directly visualize and analyze the liquid film dynamics inside the tube by means of a high-speed grey-scale camera and consequently to characterize the start-up and dry-out processes. Cooling is provided by removable condensers consisting in two aluminum spreaders sandwiched on tube and cooled down by a peltier cells/cold-plates system. Each brass connector is equipped with a fluid thermocouple, a pressure transducer and an optic fiber phase detector to determine the local thermodynamic state. The modular design of evaporators and condensers allows to change their position and length to reproduce a large number of PHP configurations. A large test case matrix comprising all the varying parameters of interest (e.g. flow patterns, dominant frequencies, local heat transfer coefficients, overall thermal resistance) is proposed. The obtained data will be fundamental for the understanding of the PHP governing phenomena and for models validatio

Preliminary Multiparametric Validation of a Numerical Model for the Pulsating Heat Pipe Transient Simulation

Abstract. A relatively new and emerging technology, Pulsating or oscillating Heat Pipes (PHP), is one of the cheapest and most reliable thermal management systems. However, since the phenomena that govern its functioning are not well understood, its industrial application is still far. In the last two decades many significant efforts have been done to develop numerical models of such devices, but just few of them are capable to perform a complete thermo-hydraulic simulation. One of the most sophisticated models present in literature is the code CASCO (French abbreviation for Advanced PHP simulation code), which reproduced, at least qualitatively, many physical phenomena observed in PHPs. Its preliminary validation is presented in this work. Experimental data has been collected during the 67th European Space Agency parabolic flight campaign. A large diameter PHP prototype has been tested at different power levels (18-200 W). Temperature measurements at several location of the device as well as infrared images of the fluid (in a sapphire section of the tube) were recorded. After implementing topology, geometrical features and material properties of the prototype into CASCO, transient simulations have been carried out. Temporal evolution of temperatures, liquid plugs’ velocity length and temperature distribution have been closely predicted by the model

Experimental validation of a Pulsating Heat Pipe transient model during the start-up in micro-gravity environment

International audienceA large diameter Pulsating Heat Pipe (PHP) is a device operating as a thermosyphon on ground and as a PHP under microgravity conditions. Such a prototype with a transparent (sapphire) tube section in the adiabatic region has been tested during the 67th ESA parabolic flight campaign. Infrared visualizations of the fluid in the sapphire section along with measurements of all the relevant quantities, which characterize the device state (pressures, temperatures), are acquired during the tests and exploited for the validation of CASCO code of PHP simulation. After accurate implementation of the PHP geometry and material properties, transient simulations have been carried out. A comparison with the experiment is possible only for the cases where the PHP can be initially assumed at equilibrium. The transients for tube wall temperatures, liquid plug velocities, lengths and temperatures show a good agreement with the experiments during the start-up phase in microgravity conditions reducing the gap towards the development of a fully validated PHP design tool