BubblesHT_web - ENU 4134 Boiling(with Bubbles D Schubring...

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ENU 4134 – Boiling (with Bubbles) D. Schubring October 23, 2009
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Two-Phase Heat Transfer We’ll spent the next several classes on two-phase heat transfer, covering the following topics: I Boiling with bubbles (introduction, nucleation, incipience, bubble departure, saturated/subcooled boiling) I Boiling without bubbles (film boiling and CHF) Note: Due to the density of projects and exams in this part of the course, there are no required HW problems on two-phase heat transfer. Your project and HW #6.5 cover these issues.
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Boiling with Bubbles (and Intro) I Introduction to pool and flow boiling I Nucleate superheat I Boiling incipience and bubble departure
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Pool Boiling (Fig. 12-2) Four regions (values for water near atmospheric pressure): I Single-phase NC (Δ T < 5 C , q 00 10 4 W m - 2 ) I Nucleate boiling (5 C < Δ T < 30 C , q 00 10 4 - 10 6 W m - 2 ) I Transition boiling (30 C < Δ T < 50 C , q 00 10 6 - 10 5 W m - 2 ) I Film boiling (50 C < Δ T , q 00 10 5 W m - 2 and up) Compare: fix Δ T , measure q 00 to fix q 00 , measure Δ T . Critical Heat Flux : 1.5 MW m - 2 at one atmosphere, increases to about 5 MW m - 2 at 6 MPa.
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Flow Boiling (Fig. 12-4) In flow boiling, the “regimes” of boiling behavior are: single-phase convection, subcooled boiling ( T wall > T sat > T bulk ), saturated nucleate boiling, convection through liquid film (annular flow), convection to vapor (with drops), convection to superheated vapor.
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