spray cooling
The general aim of the study is to better understand the hydrodynamics and heat transfer associated with spray impact onto a heated target and to improve the modeling of spray cooling. The spray impacting onto a rigid substrate creates on it a thin fluctuating liquid film. The flow in this film is influenced by the waves, craters, uprising sheets and jets, generated by drop impacts (Fig. 1). The heat transfer in this region is mainly determined by the convection in the liquid film, evaporation and conduction in the rigid target.
Following our preliminary theoretical estimations, gravity influences the splash of a single drop (particularly the height of the uprising sheet and the duration of its propagation). It influences also the average thickness of the film and its stability, thus affecting the heat transport associated with the spray impact.
The specific goals of the experiment are the investigation of the stability of the wall film created by spray impact, the measurement of the film thickness and the measurement of the temperature distribution in the target at various gravity conditions (Fig. 2).
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| (a) | (b) | (c) |
| Fig. 2: Experiment configurations:(a) 2 - 20g, (b) -2 - -20g, (c) transversal position | ||
The most relevant applications of the proposed study include spray cooling in the production of metals, cooling of electronic modules and cryogenic cooling of human tissues. Spray cooling is also a part of spacecraft technology.
experimental apparatus
The experimental setup integrated in the centrifuge consists of a test cell, a liquid supply system, a gas supply system, an extraction system, a measurement and control system and an optical visualization system. The water spray is generated by an atomizer installed in the test cell. The droplets of the spray impact onto a convex heated target. The walls of the cell are transparent which allows observation of the spray impact leading to the formation of the liquid wall-film and generation of the secondary spray.
The water for the spray is provided by the liquid supply system. The volume flux can be varied by changing the injection pressure in the water supply line. The test cell is ventilated by a gas co-flow provided by a gas supply system. The deposited liquid and the gas are exhausted from the cell using the extraction system. The pressure in the test cell, the volume flux of water and gas are controlled using the measurement and control system.
The temperature in the target is measured at different locations at various distances from the surface. This allows us to estimate the temperature distribution in the target and the heat flux.
The surface of the target is convex allowing us to observe the film motion and the profile of the liquid film at the target's generatrix. A high-speed camera system is installed that allows us to measure the film thickness at various time instants and to determine spray parameters with the help of image processing analysis
results
A large diversity of phenomena is associated with the flow in the liquid film layer initiated by single drop impacts onto a heated target and their interactions. Due to the complexity of the problem, the hydrodynamics of the films created by sprays is not entirely understood and no reliable model describing exclusively the behavior of the liquid film or depicting spray cooling is present until now.
By focusing on the dynamics of the thin liquid film produced by spray impact and the corresponding heat transfer in the wall region it is expected to provide a sound basis for a reliable modeling of spray cooling. Both phenomena are investigated under terrestrial conditions and compared with observations gained under zero gravity and hypergravity conditions.
In the centrifuge experiment the setup focuses on the influence of various gravity conditions on the average film thickness, the film fluctuations and the film patterns. Additionally, the influence of the spray and film characteristics on the heat transport associated with spray impact will be investigated.
On the basis of the edge detection technique to delineate the liquid film contour of the wetted target and on the basis of a robust method to investigate the liquid film dynamics it has been shown in preparatory works that there exists a dependency of several liquid film characteristics not only on the parameters of the impacting spray but also on the gravity level. The spray cooling efficiency also alters with various gravity levels.
publikations
Kyriopoulos O., Roisman I. V., Gambaryan-Roisman T., Stephan P., Tropea C. (2008) Dynamics of a liquid film produced by spray impact onto a heated target. Jap. Soc. Micro. Appl. Journal, Nara, Japan, 25, 231ff, No.3
Celata G. P., Colin C., Colinet P., Dimarco P., Gambaryan-Roisman T., Kabov O., Kyriopoulos O., Stephan P., Tadrist L. and Tropea C. (2008) Bubbles, drops, films: transferring heat in space. Europhysics News, 39, 23-25, number 4, DOI: 10.1051/epn 2008401
Gambaryan-Roisman T., Kyriopoulos O., Roisman I. V., Stephan P., Tropea C. (2007) Gravity Effect on Spray Impact and Spray Cooling. Microgravity Sci. Technol., Springer Netherlands, Kyoto, Japan, 19, 151ff, Issue 3-4, ISSN: 0938-0108 (Print), 1875-0494 (Online), DOI: 1007/BF 02915782
Roisman I. V., Gambaryan-Roisman T., Kyriopoulos O., Stephan P., Tropea C . (2007) Breakup and atomization of a stretching crown. Phys. Rev. E, 76, 026302
conferences, talks and reports
Kyriopoulos O., Roisman I. V. , Gambaryan-Roisman T., Stephan P. Tropea C. (2008) Sounding Rocket: Liwuid Film Dynamics and spray cooling, 3rd International Topical Team Workshop on Two-Phase Systems for Ground and Space Aoolications, Brussels, Belgium, awarded Best Poster
Kyriopoulos O., Roisman I. V. , Gambaryan-Roisman T., Stephan P. Tropea C. (2008) Dynamics of a liquid film produced by spray impact onto a heated target, . Proc. 22nd European Conf. on Liquid Atom. And Spray Systems ILASS-Europe, Como Lake, Italy
Roisman I.V., van Hinsberg N. P., Kyriopoulos O., Tropea C., Gambaryan-Roisman T., Stephan P. (2007) Wall flow produced by spray impact EUROMECH 493, Interface dynamcis, Stability and Fragmentation, Grenoble, France
Kyriopoulos O., Roisman I. V., Gambaryan-Roisman T., Stephan P., Tropea C. (2008) Spray impact: hydrodynamics and heat transfer under microgravity conditions. 3rd Int. Symp. on Physical Science in Space, Nara, Japan
Gambaryan-Roisman T., Kyriopoulos O., Roisman I. V., Stephan P., Tropea C. (2007) Gravity Effect on Spray Impact and Spray Cooling. 2nd International Topical Team Workshop on Two-Phase Systems for Ground and Space Applications, Kyoto, Japan
Kyriopoulos O. (2006) Experimentelle Untersuchungen zur Halbmodellmesstechnik. Deutscher Luft- und Raumfahrtkongress, Luft- und Raumfahrt: Lehre, Forschung, ndustrie - gemeinsam innovativ, Braunschweig, Germany, Claudius Dornier Jr. Stiftungspreis
Kyriopoulos O. (2008) 22 Sekunden - und viele Monate: Ein langer Weg bis zum FOrschen unter Schwerelosigkeit. DLR Portal
Kyriopoulos O. (2007) Völlig losgelöst. TUD-hoch³: Schwerpunkt Schwerelos, 1/Jahrg 3
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