BubblesHT_web - ENU 4134 – Boiling (with Bubbles) D....

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Unformatted text preview: ENU 4134 – Boiling (with Bubbles) D. Schubring October 23, 2009 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. Boiling with Bubbles (and Intro) I Introduction to pool and flow boiling I Nucleate superheat I Boiling incipience and bubble departure 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. 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.convection to vapor (with drops), convection to superheated vapor....
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This note was uploaded on 07/14/2011 for the course ENU 4133 taught by Professor Schubring during the Spring '11 term at University of Florida.

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BubblesHT_web - ENU 4134 – Boiling (with Bubbles) D....

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