NoBubblesHT_web - ENU 4134 – Boiling(without Bubbles D...

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Unformatted text preview: ENU 4134 – Boiling (without Bubbles) D. Schubring October 23, 2009 Two-Phase Heat Transfer (2) 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 without Bubbles I CHF mechanisms I Film boiling correlations I CHF correlations CHF Mechanism – Pool Boiling Fluidization of the pool (Kutateladze) – liquid is suspended by vapor streams at the wall. Vapor velocity: j g = C 1 σ ( ρ f- ρ g ) g ρ 2 g 1 / 4 (1) C 1 ≈ . 13- . 18. q 00 cr = ρ g j g h fg (2) Remark: critical heat flux does not usually have anything to do with critical flow. The repeated use of the “cr” subscript can be confusing. CHF Mechanism – Pool Boiling (2) Once CHF occurs, film boiling is present. A lower j g ( q 00 ) is necessary to sustain it: j g = C 2 σ ( ρ f- ρ g ) g ( ρ f + ρ g ) 2 1 / 4 (3) C 2 ≈ . 09 (4) q 00 = j g ρ g h fg (5) Mechanism: stability analysis – heavy liquid on top of lighter vapor. Enrichment: Rayleigh-Taylor instability (neat Wikipedia page as of 10-12-09) CHF Mechanisms – Flow Boiling Figure 12-21 in T&K shows a schematic. At low quality and high heat flux, CHF occurs by departure from nucleate boiling (DNB). A sufficiently large number of bubbles form to generate a continuous layer of vapor along the wall....
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NoBubblesHT_web - ENU 4134 – Boiling(without Bubbles D...

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