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[en] This paper presents results for FC72 and FC87 saturated and subcooled nucleate pool boiling, at atmospheric pressure, on a downward facing surface and in confined spaces between a heated copper disk (diameter 12 mm and thickness 2 mm) and an unheated surface, for distances varying between 0.2 and 13 mm. At saturated boiling and low heat flux (≤45 kW/m2) a decrease in the distance between the plates causes an enhancement of the boiling. At subcooled boiling the results show that the heat transfer coefficient decreases with a reduction in the distance between the plates. A visualization of the subcooled boiling shows the effect of confinement and heat flux on the liquid-vapor configuration
[en] Single phase natural circulation is of interest in various energy systems, including solar heaters, nuclear reactors, geothermal power production, engine and computer cooling. The present paper deals with an experimental study on the influence of pressure drops on the behavior of a single-phase natural circulation loop. In a simple rectangular loop (MTT-1) located at DITEC, three series of tests have been carried out, with localized pressure drops (orifices of 6 mm, 10 mm and 14 mm diameter in the vertical legs) and different power levels. The experimental data are analyzed and compared with previous results obtained with smooth pipes, demonstrating the stabilizing effect of the pressure drops on the overall behavior of the loop
[en] In this paper the natural circulation phenomenon of a differentially heated single-phase natural circulation loop has been numerically investigated. The conservation equations of mass, momentum and energy are solved using a two-dimensional finite difference method. The preliminary results will be presented. The transient and steady-state behavior of the circuit has been investigated. The dependency of the temperature and velocity field on the power input and the wall thermal capacity has been analyzed, as well as the relationship between the most important non-dimensional numbers (Nusselt, Reynolds and the friction factor versus the modified Grashof number). A comparison of the numerical results is made with conventional one-dimensional analysis and with the experimental data. Generally, a good agreement is achieved in the stable region. (orig.)
[en] Single-phase natural circulation is of interest in various energy systems, including Nuclear Power Plants. Problems may arise from the occurrence of instabilities that, in extreme situations, may cause reversal of core flow. A simple loop has been constructed and operated and a series of experiments has been carried out over the power range 100 - 900 W; the Relap5 code has been extensively used having both purposes of code assessment and interpretation of experimental data. In this paper the calculations performed with Relap5/Mod3.2 are presented and analyzed in comparison with the experimental data. (author)
[en] Experiments were performed to examine the pool boiling heat transfer and critical heat flux on a smooth copper circular surface, confined by a face-to-face parallel unheated surface, by changing the gap between the surfaces and the unheated surface diameter. Pool boiling data at atmospheric pressure were obtained for saturated HFE-7100. The gap values investigated, between the boiling surface and the adiabatic one, were s 0.5, 1.0, 2.0, 3.5 mm. To confine the boiling surface, two different Plexiglas plates were used: the former characterised by a diameter D = 60 mm, large as the overall test section support, the latter characterised by a diameter D = 30 mm, large to cover only copper boiling surface (d = 30 mm). For each configuration, boiling curves were obtained up to the thermal crisis. For both different types of confinement, it was observed that the boiling curves match at low wall superheat, except for s = 0.5 mm, 1 mm. However, at high wall superheat, a drastic reduction in heat transfer as well as CHF appears decreasing the channel width s; for all gap sizes, this reduction is less pronounced for the smaller confinement wall (D = 30 mm). Instead, at low wall superheat for gap of 0.5 and 1.0 mm, the heat transfer coefficient is higher for diameter disc of 60 mm. CHF data were also compared with a literature correlation (Misale and al., 2009). (author)
[en] During the past years, several authors performed numerical and experimental studies on single-phase natural circulation loops (thermosyphons-NCL). However the problem of the loop stability, which is of interest in many industrial applications, cannot be considered solved. This paper is focused on the evaluation of the instantaneous mass and heat flow rates in the main loop sections. The available experimental apparatus is provided with a heat sink which can be feed with cooling fluid at controlled temperature. In this way the NCL mean temperature and heat flow rate can be varied independently. A set of CFD simulations with various heat sink temperatures, at fixed heat flow rate, has been carried out in order to improve the comprehension of the data collected during the experimental campaigns. (authors)
[en] Natural circulation (NC) is a heat transfer mechanism which occurs in case of density gradients inside a fluid. Even if the heat transfer coefficients are lower than in case of forced convection, NC guarantees a good reliability and low costs of maintenance as it does not need any mechanical part. Therefore the main industrial applications of NC systems are in the field of nuclear power plants, solar heaters and passive cooling systems of engines, turbines and electronic components. Most researchers focused their attention on large size systems, with particular care to performance optimization and stability analysis, while there are few studies about natural circulation inside small size devices. In this paper an experimental study focused on the macroscopic effects on the thermal performance of a mini-loop is presented. In particular, two working fluids were used during the tests: distilled water and a nanofluid (distilled water and Al2O3) characterized by two different concentrations (0.5% and 3.0% by volume). The analised parameters were: power transferred to the fluid, mini-loop inclination and temperature at the cooler. Experimental results of the two fluids were compared to the Vijayan’s correlation, developed for large scale natural circulation loop, showing good agreement.
[en] Natural circulation of distilled water and FC43 has been experimentally investigated in a rectangular loop characterized by internal diameter of 30 mm and total length of 4.1 m. The aim of the present study is to analyse the influence of thermal boundary conditions on the flow regimes inside the pipes and on the stability of the system. The new aspect of the present research is the possibility of tuning the heat sink temperature in a range between -20 oC and +30 oC by means of a cryostat. This kind of analysis could be useful for the design of systems characterized by a wide range of environment temperatures, as for example for aerospatial applications. The other parameters investigated were the heat flux transferred to the fluid, which varied between 0.1 kW and 2.5 kW, and the thermo-physical properties of the working fluid. The system showed both stable and unstable behaviour. In particular, in the case of FC43 the loop was more unstable and it was characterized by higher velocities and frequencies compared to the case of distilled water. It was found that the stability threshold could be crossed by varying only the heat sink temperature, demonstrating the importance of this boundary condition on the dynamics of the system. Different flow regimes and fluid velocities were observed. In the case of steady-state flow, Vijayan's correlation (Vijayan et al., 2000) was tested and found to give good agreement with experimental data. Linear stability analysis was made following the Vijayan's model. In particular, the effect of heat sink temperature was considered in the dimensionless Stanton number based on the overall heat transfer coefficient at the heat sink. Finally, Ultrasound Pulsed Doppler Velocimeter (UPDV) was used on a natural circulation loop for the first time, and gave a preliminary validation of the traditional fluid velocity measurement method based on the frequency analysis.