Two-phase structure above hot surfaces in jet impingement boiling |
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Authors: | L Bogdanic H Auracher and F Ziegler |
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Institution: | (1) Technische Universit?t Berlin, Marchstra?e 18, 10587 Berlin, Germany |
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Abstract: | Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several
studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters
such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models
are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study
the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically
with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and
is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments
in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the
distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies
range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to
condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor
layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found. |
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Keywords: | Jet impingement boiling Optical probe Void fraction Contact frequencies |
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