1,721,299 research outputs found
Heat transfer and pressure drops in micro-tubes for ground and space applications
Full text linkThe aim of the research is to compare and understand the bubble dynamics, heat transfer mechanism and pressure drops in subcooled flow boiling, at both, microgravity and normal gravity conditions. Nowadays, there are few studies on this topic, nevertheless the possible applications are many, from the spatial usage to microelectronics cooling. Micro-exchangers are the next generation of cooling systems useful for both terrestrial and space applications where weight and dimensions are important. Flow boiling is the best way to reach high heat fluxes. Micro-tubes, thanks to their dimension, can be used in a wide range of microgravity systems i.e. satellites for communications, thermal management for the International Space Station, cooling of hi-power electronic devices, nuclear space reactors, etc. To develop and design thermal systems for small applications, it is necessary to achieve a detailed understanding of all flow boiling aspects, also under low gravity conditions, at low pressure and at high thermal fluxes. A deepen analysis of available correlations in the literature has been performed and a comprehensive comparison with experimental data provided by two ENEA facilities (MICROBO and BOEMIA) has been carried out. The analysis considers 17 micro-channel correlations, 5 macro-channels and 2 models to calculate heat transfer. Instead, 5 correlations were chosen to calculate saturated boiling pressure drops and 3 for subcooled boiling. As most of data are in subcooled boiling, its effect on prediction is discussed and a model is proposed to calculate vapor quality in the channel. Moreover, a new methodology to calculate pressure drops has been developed and discussed
Flow boiling heat transfer and two-phase flow in microgravity
No full textFlow boiling heat transfer can provide high heat transfer rates due to latent heat transportation. Its possible use is therefore potentially important to reduce size and weight of cooling systems in space platforms and satellites. A comprehensive knowledge is also important for the safe operation of existing single-phase systems in case of accidental increase of heat generation rate. For space applications and appropriate design of components accommodating flow boiling heat transfer, it is important to understand the influence of microgravity conditions on forced convective boiling heat transfer. The number of existing researches on flow boiling in reduced gravity is very small due to large heat loads required and reduced available room in a 0-g apparatus for experiments, as well as complexity of the experimental facility for microgravity environment. This lecture will summarize the results of the few research carried out on flow boiling heat transfer in microgravity, discussing flow pattern, heat transfer coefficient, critical heat flux, with a particular emphasis to the recent experiments carried out at ENEA. The experiments were carried out at low gravity during the ESA (European Space Agency) parabolic flights campaign of November 2013. The paper will show the analysis of differences between flow patterns and vapour bubble parameters at normal and at zero gravity
Visual analysis of flow boiling at different gravity levels in 4.0 mm tube
No full textThe aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed during the parabolic flight campaign of October-November 2013. The paper will show the analysis of differences between the heat transfer coefficients and vapour bubble parameters at normal and at zero gravity. The results of 4.0 mm tube are presented and discussed. With respect to terrestrial gravity, heat transfer is systematically lower at microgravity in the range of the experimental conditions. Heat transfer differences for the two gravity conditions are related to the different bubble size in each of them. The size of a bubble in flow boiling is affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heated wall. Vapour bubble parameters (bubble diameter, bubble length, width, and nose velocity) have been measured
Saturated flow boiling of FC-72 in 1 mm diameter tube
No full textDue to the important role of microscale heat transfer, an analysis of the heat transfer coefficient for a 1 mm inner diameter tube, with FC-72 as working fluid, has been performed. The study is aimed to identify the best correlation or model to predict the available experimental database. A comparison of several models and correlations, available in the literature for micro- and macroscale, has been performed, focusing the present preliminary analysis to the saturated boiling conditions. Experimental data have been provided by ENEA through the facility BO.E.MI.A. (BOiling Experiments in MIcrochannel Apparatus), in the pressure range from 3 to 5 bar, with a mass flux from 800 to 1200 kg/m2s and thermal fluxes from 1.6 to 181 KW/m2. The best results, in this preliminary analysis of the saturated boiling points, were obtained for microscale correlations of Li and Wu, with more than 91% of data within ±30% error and a mean absolute percent error (MAPE) of 13.4%. Among the macroscale correlations only the Chen correlation provides good results, but with a lower agreement with the experimental data. Theoretical models are very promising but need further works to find the appropriate parameters valid for the specific fluid. A more detailed analysis including subcooling conditions will be performed in a future work. © 2016
Experimental study of thermal crisis in connection with Tokamak reactor high heat flux components
No full textThe results of an experimental research on high heat flux thermal crisis in forced convective subcooled water flow, under operative conditions of interest to the thermal-hydraulic design of TOKAMAK fusion reactors, are here reported. These experiments, carried out in the framework of a collaboration between the Nuclear Engineering Department of Palermo University and the National Institute of Thermal-Fluid Dynamics of the ENEA-Casaccia (Rome), were performed on the STAF (Scambio Termico Alti Flussi) water loop and consisted, essentially, in a high speed photographic study which enabled focusing several information on bubble characteristics and flow patterns taking place during the burnout phenomenology. © 2000 American Institute of Physics
Immunohistochemistry and multispectral analisys
No full textThe use of methods of image analysis has allowed us to make more reliable and reproducible results obtained by immunohistochemistry (IHC). Wider use and simplification of software allowing a colorimetric study has meant that these methods are available to everyone and made it possible to standardize the technique by reliable systems score. Moreover, the introduction in recent years of multispectral image acquisition systems methods has further refined these techniques allowing to avoid artifacts and release the evaluation of the data by the observer
Convective boiling heat transfer under microgravity and hypergravity conditions
No full textThe present paper concerns an experimental investigation of the gravitational effects on subcooled and saturated flow boiling heat transfer in a vertical 3.75 mm diameter aluminum tube, using perfluorohexane as working fluid. Experimental results obtained during parabolic flights are compared with data gathered in the same test facility for normal gravity. Results were obtained for mass velocities of 133, 252, 377 and 428 kg/m2s, heat fluxes up to 106 kW/m2 and vapor qualities up to 0.28. The assessments of prediction methods from literature for flow boiling heat transfer proposed for normal gravitational conditions were evaluated through comparisons of calculated and experimental results. In general, for either a mass velocity of 428 kg/m2s or a vapor quality lower than -0.2, the gravitational effects on the heat transfer coefficient were found as negligible. For lower mass velocities and saturated flow boiling, higher heat transfer coefficients were observed under hypergravity conditions. This behavior was associated to the fact that higher gravitational forces favor bubbles detachment. The method of Kanizawa et al. [1] provided reasonable predictions of the saturated flow boiling results under microgravity and hyper gravity conditions, predicting 100% of the data within an error band of ±30%
- …
