1,721,045 research outputs found
Experimental analysis of the melting process in a pcm/aluminum foam composite material in hypergravity conditions
No full textPhase change materials [(PCMs), e.g., paraffin waxes, fused silica salts, and polyethylene glycol] can be successfully used for thermal management and heat storage in ground and space applications. Open-cell metal foams embedded in the PCM material increase the overall thermal conductivity and accelerate the melting process. The literature shows that the pore size and relative density strongly affect the melting process performance. Most studies have shown that the high thermal conductivity of the open-cell metal foam dominates the melting process. The natural convection effect usually is attenuated; however, it can be relevant if it occurs. An experimental activity has been designed and carried out under the framework of the European Space Agency student program Spin Your Thesis 2017 to analyze the effect of different hypergravity levels and configurations on the melting performance of a composite aluminum foam (10 pores per inch)/paraffin wax material at two different heat fluxes. The gravity level ranges from 5g up to 20g using a large diameter centrifuge facility. The effect of gravity on the melting process has been investigated by measuring the melting time and the dynamic evolution of the melted area. The experiments show that the hypergravity condition accelerates the melting process: it is 12% faster ranging from 5g to 10 g. Infrared visualization allowed us to define the melting front dynamic evolution. A natural convection regime was observed in all of the experiments. The natural convection incipience accelerates the melting process. A critical analysis of the scaling criterion in the literature has been qualitatively done and a modified Rayleigh number is proposed to characterize the melting process
A pulsating heat pipe embedded radiator: Thermal-vacuum characterisation in the pre-cryogenic temperature range for space applications
No full textThe use of Pulsating Heat Pipes (PHPs) in the space field is still an open issue because of the lack of data obtained during actual operation in relevant environment. A considerable amount of data is available in the literature on the thermal response of PHPs to a variable gravity condition and on the operation in the cryogenic field but in both cases the heat load at the condenser is rejected by convection to a constant temperature sink. On the other hand, barely any work in the PHP's literature deals with thermal radiation to a low temperature sink as only heat transfer mode. The present work attempts to fill the gap by testing a PHP radiator (16 turns, 1.1 mm inner diameter, 50% filled with FC-72 at 293 K) in thermo-vacuum conditions in horizontal orientation at different heat loads and different environment (chamber) temperature. Fluid pressure measurements coupled with the frequency analysis characterised the effect of the cold source temperature on the device operational limits and efficiency. Results show that the device thermal performance in the radiative configuration is mostly affected by the lower operating temperatures needed to obtain a sensible heat rejection, rather than the heat transfer mode itself. The decrease of the environment temperature shortens the operational heat load range: the start-up occurs at higher heat input levels while the thermal crisis occurs at lower heat loads. The frequency analysis reveals that the equivalent thermal resistance is positively affected by higher values of the dominant frequency for all the cases
Heat transfer delay method for the fluid velocity evaluation in a multi-turn pulsating heat pipe
No full textMetal foam/PCM melting evolution analysis: Orientation and morphology effects
No full textPhase Change Materials (PCM) are promising materials for thermal energy storage systems. Since they present a relatively low thermal conductivity, they are often embedded in an open cell a metallic foam to enhance the overall thermal conductivity. In this paper, both experimental and numerical results on PCMs coupled with aluminum foams under different heat fluxes, porosities, number of Pores Per Inch (PPIs) and orientation are presented. The test cell is equipped with a Zincum Selenide window that allows to capture the whole temperature distribution by means of a IR camera. The melting front position in time is tracked by means of a MATLAB® algorithm based on IR camera images that are useful for a more robust tracking of melting front. Numerical simulations are performed with references to the porous media volume-averaged approach, under the assumption of local thermal non-equilibrium between the two phases. The most updated correlations for the porous media closing coefficients are taken from the literature. All the experiments are compared with numerical simulations, showing a very good agreement. After showing the effects of the different input parameters on melting front evolution, an analysis in terms of different convective heat losses to the environment and melting temperature range is presented to appreciate how these two variables affect the melting front position. Finally, total melting front evolution has been compared between experiments and simulations, showing a good agreement. This has been evaluated for different conditions, showing that a decrease in the porosity drastically reduces the melting time, while PPI has no relevant effect and small effects can be observed from orientation
Experimental analysis and transient numerical simulation of a large diameter pulsating heat pipe in microgravity conditions
Full text linkA 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
Heat transfer delay method for the fluid velocity evaluation in a multi-turn pulsating heat pipe
Full text linkA multi-turn closed loop pulsating heat pipe made of aluminium is tested in vertical bottom heated mode and
different condenser temperatures with the aim of providing quantitative information regarding its flow dynamics
through a novel post-processing technique on the local wall-to-fluid heat flux, evaluated within the adiabatic
section. The studied device is made of an annealed aluminium tube (inner/outer diameter: 3/5 mm), folded in 14
turns and partially filled with methanol (volumetric filling ratio: 50%). The aluminium channels are coated with
a high-emissivity opaque paint, thus allowing thermographic measurements on the outer wall by means of a
high-resolution medium wave infrared camera. The proposed method, named Heat Transfer Delay Method, is
validated by means of a dedicated experimental approach. Then, the acquired time-space temperature maps are
used as input data for the inverse heat conduction problem resolution approach to estimate the local convective
heat flux locally exchanged at the inner wall-fluid interface. The resulting wall-to-fluid heat fluxes are then post-
processed by applying the Heat Transfer Delay Method to the oscillatory and circulatory flow modes. The average
fluid velocity is assessed at varying working conditions during the circulatory flow, finding values up to 0.77 m/s
and 0.3 m/s for condenser temperature equal to 20 ◦C and 10 ◦ C, respectivel
Heat and mass transfer for a small diameter thermosyphon with low fill ratio
No full textThermosyphons of smaller dimensions are more commonly sought after as electronics cooling devices. The interactions of the tube wall and working fluid become more significant as the dimension of a thermosyphon is reduced, particularly for high surface tension fluids such as water. This paper aims to experimentally investigate a water-charged, small diameter (8 mm) thermosyphon as it operates with a low (25%) filling ratio for a relatively long evaporator length of 200 mm. High speed videography provides in-situ flow pattern visualization at different heat input power. The boiling regimes for each level of heat flux are determined by analyzing the flow patterns from the high-speed video footage. The interdependence of the flow regimes and the heat and mass transfer mechanisms is evaluated using the measured wall temperature variations and derived thermosyphon performance metrics, such as the average heat transfer coefficients and thermal resistances. It was observed that the heat and mass transport was dominated by Geyser-type boiling at lower heat fluxes with associated low heat transfer coefficients in the evaporator and condenser. With increasing thermal power, less liquid was observed to return to the evaporator resulting in more aggressive boiling events which improved the heat transfer coefficients in both the evaporator and condenser. For all power levels tested, the dominant thermal resistance was found to be that associated with the condenser. The ultimate failure of the thermosyphon was a result of liquid hold-up in the condenser section and subsequent falling liquid film and evaporator dryout
Simulations of paraffine melting inside metal foams at different gravity levels with preliminary experimental validation
Full text linkIn this work, the results of a numerical code based on the porous media Local Thermal Non-Equilibrium (LTNE) and the apparent heat capacity methods, are compared with experiments aiming at a preliminary validation. The test cell consists in a 50 mm aluminum foam cube filled with a paraffin wax, heated and cooled on the same face. The heat flux is measured by two miniaturized sensors, while the temperature is measured in three different locations along the cube edge. Finally, one side is equipped with a Zinc Selenide window which is transparent to the long wave InfraRed. This system allows to track the paraffin melting front evolution together with the temporal trend of the whole temperature distribution simplifying the comparison with the numerical outputs at different time steps. The numerical model is then set with the same boundary conditions (heat flux) to predict the experimental temperature fields, considering both conduction in the solid domain and natural convection in the liquid domain. The preliminary validation shows that the numerical results match the experimental data with good agreement. Results are also presented for different gravity levels. This study can be a starting point for all those applications where gravity has a major role
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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