Influence of the ultrasounds on the heat transfer in single phase free convection and in saturated pool boiling

2012 ◽  
Vol 36 ◽  
pp. 12-21 ◽  
Author(s):  
Federica Baffigi ◽  
Carlo Bartoli
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Single Phase ◽  
Pool Boiling

Heat Transfer Enhancement Using Ultrasonic Waves in Presence of Liquid: A Basic Research for Cooling Electronic

2013 ◽  
Vol 464 ◽  
pp. 163-170 ◽  
Author(s):  
F. Baffigi ◽  
C. Bartoli
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Single Phase ◽  
Cooling System ◽  
Basic Research ◽  
Horizontal Cylinder ◽  
Ultrasonic Waves ◽  
Department Of Energy ◽  
Subcooled Boiling ◽  
Joule Effect

This work collects the experimental results obtained in the Thermal Fluid Dynamics Lab at the Department of Energy, Systems, Land and Constructions Engineering at the University of Pisa, concerning a basic physics research on the influence of ultrasounds in single phase free convection and in subcooled boiling, at atmospheric pressure. The ultrasounds are applied at the set frequency of 40 kHz, with a transducer output changing from 300 to 500W, on a circular horizontal cylinder heated by Joule effect, immersed in distilled water. The tests in single phase free convection, without ultrasonic waves, are validated by means of the classical correlations reported in literature, but they do not produce distinctive augmentation of the heat transfer. The enhancement of the heat transfer coefficient is maximum in subcooled boiling conditions (about 57%). In this regime a detailed investigation was performed to optimize the variables involved, such as the ultrasound generator power, the position of the cylinder and, especially, the subcooling degree. This paper, makes clear systematically the effects of ultrasounds on the heat transfer and shows as they could be very useful as cooling system for the last generation electronic components.

Free Convection of Water-Al2O3 Nanofluid from Horizontal Porous Coated Tube

2013 ◽  
Vol 597 ◽  
pp. 15-20
Author(s):  
Janusz T. Cieśliński ◽  
Katarzyna Krygier
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Boiling Heat Transfer ◽  
Single Phase ◽  
Horizontal Tube ◽  
Porous Coating ◽  
Enhance Heat Transfer ◽  
Thermal Systems ◽  
Heating Systems ◽  
Theoretical Considerations

Nanofluids are expected to enhance heat transfer in many thermal systems. Present efforts are concentrated on boiling heat transfer of nanofluids, although single phase convection is of key importance in various appliances such as car radiators or heating systems. This paper presents experimental results of heat transfer during free convection of water-Al3O3 nanofluid from horizontal tube covered with metallic porous coating. Contrary to theoretical considerations deterioration of heat transfer was observed.

Optimization of Free Convection Heat Transfer From Vertical Plates Using Nanofluids

2012 ◽  
Vol 134 (4) ◽  
Author(s):  
Massimo Corcione ◽  
Marta Cianfrini ◽  
Emanuele Habib ◽  
Alessandro Quintino
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Single Phase ◽  
Effective Properties ◽  
Convection Heat Transfer ◽  
Operating Conditions ◽  
Fundamental Result ◽  
Convection Heat ◽  
Free Convection Heat ◽  
Base Liquid

Free convection heat transfer in nanofluids from vertical flat plates at uniform temperature is investigated theoretically. The main idea upon which the present work is based is that nanofluids behave more like single-phase fluids rather than like conventional solid–liquid mixtures. This assumption implies that all the convective heat transfer correlations available in the literature for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this connection, two empirical equations, based on a wide variety of experimental data reported in the literature, are used for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity. Conversely, the other effective properties are computed by the traditional mixing theory. The heat transfer enhancement that derives from the dispersion of nanosized solid particles into the base liquid is calculated for different operating conditions, nanoparticle diameters, and combinations of solid and liquid phases. The fundamental result obtained is the existence of an optimal particle loading for maximum heat transfer. In particular, for any assigned combination of suspended nanoparticles and base liquid, it is found that the optimal volume fraction increases as the nanofluid average temperature increases, the nanoparticle size decreases, and the Rayleigh number of the base fluid decreases.

Secondary Flow Effects in High Tip Speed Free Convection

1987 ◽  
Vol 109 (1) ◽  
pp. 97-103 ◽  
Author(s):  
P. W. Eckels ◽  
J. H. Parker ◽  
A. Patterson
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
High Speed ◽  
Single Phase ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Secondary Flows ◽  
Heated Wall ◽  
Rotating Frame ◽  
Two Phase

Experimental analyses of the effects of secondary flows on heat transfer in high tip speed rotating apparatus are not readily available. This paper provides data on the heat transfer within two different test modules which were rotated at high speed with the heat transfer surfaces perpendicular and parallel to the Coriolis acceleration. One module contained a heated wall and another a parallel plate free convection experiment. Uniform heat fluxes were maintained. Rayleigh numbers in excess of 1015 were achieved with liquid helium as the transfer medium. Some of the findings are that secondary flows can reduce heat transfer by as much as 60 percent in single-phase heat transfer, the transitions to fully turbulent flow are in agreement with existing prediction methods, the critical heat flux in two-phase flow boiling is significantly increased, forced convection correlations underpredict single-phase thermosyphon performance, and the usual nondimensional parameters of free convection establish similitude between various fluids and speeds. These results suggest that techniques used to enhance heat transfer in the rotating frame should be verified by tests in the rotating frame.

Pool Boiling of Nanofluids in Vertical Porous Media

2013 ◽  
Vol 388 ◽  
pp. 18-22 ◽  
Author(s):  
Ridho Irwansyah ◽  
Nandy Putra
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Media ◽  
Single Phase ◽  
Pool Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Sintered Copper ◽  
Order Of Magnitude ◽  
Screen Mesh

The development of electronic components such as microprocessor requires a better thermal management system to overcome the high heat flux produce by the component. The method to absorb the heat produce by the microprocessor is still use the conduction or either natural or free convection which still in a single phase heat transfer. One of heat transfer method that suitable for a high heat flux application is pool boiling which has a two order of magnitude higher than of a single phase heat transfer and does not require a pump to move the fluid. In this study has been conducted the pool boiling experiment with four different porous media surface which are sintered copper 300 µm and 400 µm, copper screen mesh and stainless steel screen mesh with four different fluid which are Al2O3-Water 1%, 3% and 5%. The sintered copper 400 µm has shown a better heat transfer performance compared to the other porous media. The Water, Al2O3-Water 5% has shown a performance no better than Al2O3-Water 1% and 3%.

Stability characteristics of a single-phase free convection loop

1975 ◽  
Vol 67 (1) ◽  
pp. 65-84 ◽  
Author(s):  
H. F. Creveling ◽  
J. F. De Paz ◽  
J. Y. Baladi ◽  
R. J. Schoenhals
Keyword(s):  
Heat Transfer ◽  
Free Convection ◽  
Single Phase ◽  
Vertical Plane ◽  
High Heat ◽  
Convection Flow ◽  
Transfer Rates ◽  
High Heat Transfer ◽  
Thermodynamic Critical Point ◽  
Fluid Properties

Stability characteristics of a single-phase free convection loop are presented. In the experiments, water was placed inside a toroidal glass loop oriented in a vertical plane. The lower half of the loop was heated and the upper half was cooled. At low heat-transfer rates and also a t high heat-transfer rates the free convection flow was observed to be steady. For the intermediate range, however, the flow was found to be highly oscillatory. Stability predictions are also developed. The comparison between theory and experiment yields favourable agreement.Observations of unstable behaviour have been reported previously for single-phase fluids in the vicinity of the thermodynamic critical point. In these situations it has been assumed that the unusual behaviour of the fluid properties in the near-critical region necessarily constitutes the underlying cause of such instabilities. In contrast t o this view, analyses by Keller (1966) and Welander (1967) indicate that instabilities can occur for ordinary fluids as well. Results of the present study confirm this contention, since instabilities were clearly observed for water at atmospheric pressure and moderate temperatures.

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