Begell House Inc.
Multiphase Science and Technology
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21
4
2009
POOL BOILING IN MICROGRAVITY AND IN THE PRESENCE OF ELECTRIC FIELD: EVALUATION OF THE VOID FRACTION IN THE ARIEL EXPERIMENT
267-277
10.1615/MultScienTechn.v21.i4.10
Paolo
Di Marco
Department of Energy, Systems, Constructions and Territory Engineering, University of Pisa, largo Lucio Lazzarino 1, Pisa 56122, Italy
Walter
Grassi
Lo.Th.A.R. (Low gravity and Thermal Advanced Research Laboratory), DESTEC (Department of Energy, Systems, Territory and Constructions Engineering), University of Pisa−Largo Lucio Lazzarino, 56122 Pisa, Italy
The ARIEL experiment was flown aboard a Foton-M2 satellite and hosted in a FLUIDPAC facility, sharing its optical diagnostics and power systems. Its aim was to investigate boiling heat transfer in microgravity on a surface of industrial relevance, at high heat rates. The effect of an externally applied electrostatic field on boiling performance was also tested. The void fraction in microgravity was much larger than in normal gravity condition: the application of electric field was very effective in reducing it. In the absence of electric field, bubbles coalesced giving origin to a large mass of vapor residing near the surface. The application of reduced bubble coalescence and suppressed boiling heat transfer oscillations thus enhanced boiling performance in microgravity. In this paper, a quantitative evaluation of overall and near-wall void fraction was performed, based on the digital processing of the video images. The results showed that the cyclic oscillation of the near-wall void fraction was well correlated with temperature oscillations of the heated wall. Besides, the electric field was very effective in reducing overall void fraction even at low values of the applied voltage.
TWO-PHASE FLOW AND HEAT TRANSFER IN MICROTUBES UNDER NORMAL AND MICROGRAVITY CONDITIONS
279-295
10.1615/MultScienTechn.v21.i4.20
C.
Narayanan
ASCOMP GmbH, Technoparkstrasse 1, CH-8005 Zurich, Switzerland
D.
Lakehal
ASCOMP GmbH, Technoparkstrasse 1, CH-8005 Zurich, Switzerland
Detailed numerical simulations have been performed to study the effect of gravity on two-phase flow heat transfer (without phase change) in small-diameter pipes. Effect of the flow orientation with respect to gravity is investigated. Overall, the heat removal rate in two-phase flow was shown to be significantly higher than in single-phase flow. The downstream flow can be characterized as a periodic flow around each bubble/slug rather, where the shapes of the inclusions and the flow around them reach a repeatable state. The flow regime, viz. bubbly, slug, and slug-train, is found to have a strong influence on the heat transfer and pressure drop. The wall thermal layer is affected by the blockage effect of the inclusions, which manifests itself in a circulating liquid flow pattern superimposed on the equivalent single-phase fully developed flow. The Nusselt number distribution shows that the bubbly, slug, and slug-train regimes transport as much as three to four times more heat from the tube wall to the bulk flow than pure water flow. For upflow, the breakup into bubbles/slugs occurs earlier and at a larger frequency. Overall, the average Nusselt numbers are not significantly affected by the flow orientation with respect to gravity. A mechanistic heat transfer model is proposed, based on frequency and length scale of inclusions. The results found here suggest that a microgravity experimental campaign dealing with air water systems can be performed by European Space Agency on its space Fluid-Lab facility.
BOILING HEAT TRANSFER IN A VERTICAL MICROCHANNEL: LOCAL ESTIMATION DURING FLOW BOILING WITH A NON INTRUSIVE METHOD
297-328
10.1615/MultScienTechn.v21.i4.30
S.
Luciani
Polytech' Marseille Laboratoire IUSTI-UMR/CNRS 6595, 5, Rue Enrico Fermi, Technopole de Chateau-Gombert, 13453 Marseille cedex 13, France
David
Brutin
Aix Marseille University, CNRS, IUSTI UMR 7343, 13013, Marseille, France; Institut Universitaire de France, 75231 Paris, France
Christophe
Le Niliot
Aix-Marseille Université,IUSTI UMR 7343 CNRS
Lounes
Tadrist
Aix-Marseille Universite, CNRS, Laboratoire IUSTI, UMR 7343, Marseille 13453, France
Omar
Rahli
Laboratoire LTPMP, Fac GMGP, USTHB, 16111, Bab Ezzouar, Algiers, Algeria
This paper summarizes the results of experimental and umerical studies concerning boiling heat transfer inside vertical minichannels. The objective here is to provide basic knowledge on the systems of biphasic cooling in minichannels for several gravity levels (μ;g, 1g, 2g). To fully understand the high heat transfer potential of boiling flows in microscale's geometry, it is vital to quantify these transfers. To achieve this goal, an experimental device has been set up and, in order to study the influence of gravity, the device has been embarked on board A300 Zero-G to make Parabolic Flights experiments. Analysis is made up by using an inverse method in order to estimate the local heat coefficient while boiling occurs inside a minichannel. The results concern two-phase flow during convective boiling, and this investigation leads to solving an inverse heat conduction problem (IHCP). The estimation consists of inversing experimental data measurements located in the heating rod of our experiment to obtain the wall temperature and the heat flux on the minichannel surface. Images and video sequences have been performed with a high-speed camera. The experiments are conducted with HFE-7100 because this fluid has a low boiling temperature at the cabin pressure of the A300. This choice was governed by the capillary length, which is 4.3 mm in microgravity (0.05 m s-2). The experimental loop allows control of the heat flux and the liquid flow rate for three hydraulic diameters (DH): 0.49, 0.84, and 1.18 mm. The influence of three different parameters on the heat transfer coefficient are carried out: the gravity level, the Reynolds number, and the vapor quality. First results show that whatever the gravity level, the local heat transfer decreases sharply from the inlet to the outlet channel. In the annular slug regime, the average heat transfer is found around 6000 W m-2 K-1 and we observe that the heat transfer coefficient is higher in microgravity (30%) compared to normal and hypergravity conditions.
EXPERIMENTAL STUDY OF BUBBLE BEHAVIOR AND LOCAL HEAT FLUX IN POOL BOILING UNDER VARIABLE GRAVITATIONAL CONDITIONS
329-350
10.1615/MultScienTechn.v21.i4.40
N.
Schweizer
Chair of Technical Thermodynamics, Technische Universitat Darmstadt
Peter
Stephan
Institute for Technical Thermodynamics, Technische Universität Darmstadt,
Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany
This paper presents the results of a nucleate boiling experiment performed in the framework of the 42nd European Space Agency parabolic flight campaign. Nucleate boiling of FC-72 was established at a single artificial cavity on a thin stainless steel heating foil. The bubble shape and the temperature distribution of the heating foil were measured via high-speed imaging and infrared thermography at different gravity levels and during transition phases between these levels. The influence of gravity on bubble frequency and departure diameter was evaluated. The transient heat flux distribution was calculated by applying an energy balance at each pixel of the infrared temperature image. This heat flux distribution is presented for a complete bubble cycle (growing, detachment and rise), bubble coalescence, and satellite bubble merger.