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12. BoilingHeatTransfer_1_web

12. BoilingHeatTransfer_1_web - ENU 4134 Boiling Heat...

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ENU 4134 – Boiling Heat Transfer – Part 1/3 D. Schubring August 23, 2010
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Outline I Introduction to pool and flow boiling I Nucleation superheat I Boiling incipience and bubble departure I Correlations for nucleate boiling heat transfer coefficient I Critical heat flux (CHF) mechanisms I CHF correlations I Film boiling correlations (post-CHF heat transfer)
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Learning Objectives (1/2) I 2-a-i Identify and characterize the four regimes of pool boiling I 2-a-ii Reproduce the “pool boiling diagram” (heat flux or heat transfer coefficient vs. temperature difference) for water at atmospheric pressure I 2-b-i Identify physical mechanism for nucleation superheat I 2-b-ii Compute nucleation superheat and boiling incipience point I 2-c-i Identify and characterize the regimes of flow boiling and their relationships to flow regimes
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Learning Objectives (2/2) I 5-a Develop familiarity with terminology and mathematical symbols common to nuclear TH, including those symbols which do not map one-to-one to terminology I 5-b Use correlations and/or models to analyze problems in nuclear thermal hydraulics I 5-f Identify TH-related safety limits for light water reactor operation I 5-g Consider conservatism (or lack thereof) present in a model and evaluate implications of this for reactor safety analysis
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Pool Boiling Experiment 1: Enclose saturated liquid water in a reservoir open to pressure P at the top. Assume the reservoir is much large than bubble sizes, but not so large that the bottom of the reservoir is at an elevated pressure ( e.g. , large stovetop pot). Increase the heat flux at the bottom of the reservoir and observe the (increase) in temperature of the bottom surface of the reservoir. Experiment 2: Starting at the high heat flux, reduce the heat flux and measure the temperature. Experiment 3: Starting a low superheat temperature ( T wall - T sat ), increase the temperature and observe the changes in the heat flux.
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Pool Boiling Curve – Figure 12-2 in T&K Experiment 1 is curve A-B-C-C 0 -E. Experiment 2 is E-D-B-A. Experiment 3 is curve A-B-C-D-E. (Provided dq 00 / dt is small.) 5.8* Qx<G 3^¡ a}ar Ma¡ >¡ Lv B9v Ty ¡o¢fÓ Ls[´fÓ J[»Ã±[£Ó 8¤¡Æf`¼¥¤¦Ó JÃ`¢f[¼fÓ 5¤§¨§¦oÓ U°[¦´y¼¥¤¦Ó 5¤©¢©¦oÓ P9v T¼[]ªfÓ Ay«¬Ó 5¤¥ ¢¥¦oÓ $?;NG7[ 6!ͺ!ம y2ŏ>:$qԷŏ##5Է&#N5>'(Է:4;¡!Է0#;Է-$%!;Է4'3!;Է$%6#"ŏ?!;>:Էŏ;!""48!¢Է ǫ%ԅ )"-ԅ /!ԅ%!!-ԅ%'ɯќѝ 2-0ԅ72'5ԅ 9$2-'ԅ] < ԅ'5!ԅ4$&*"'2$-ԅ$4ԅ/6//+!%ԅ +!",%ԅ '$ԅ "ԅ *$&!ԅ !ƹ!)'2=!ԅ5!"'ԅ'&"-%4!&ԅ x %$ԅ'5!ԅ-6)+!"'!ԅ/$2+2-0ԅ5!"'ԅ v6öԅ)"-ԅ /!ԅ$-!ԅ '$ԅ'7$ԅ $&,!&%ԅ $4ԅ *"0-2'6,!ԅ5205!&ԅ'5"-ԅѵ5"'ԅ$4ԅ'5!ԅ%2-0+!U95"%!ԅ-"'6&"+ԅ)$-=!)'2$-ԅ5!"'ԅv6ö¡ԅt$7!=!&¢ԅ "/$=!ԅ"ԅ02=!-ԅ2-2'2"+ԅ5!"'ԅv6öԅ#9$2-'ԅc.ԅ'5!ԅ-6)+!"'2$-ԅ&"'!ԅ/!)$*!%ԅ5205ԅ!-$605ԅ'$ԅ +!",ԅ'$ԅ4$&*"'2$-ԅ$4ԅ"ԅ)$-'2-6$6%ԅ="9$&ԅ£+*ԅ"'ԅ'5!ԅ %6&4")!Nԅ>2+*ԅ/$2+2-0ԅ)"-ԅ "+%$ԅ/!ԅ !%'"/+2%5!,ԅ"'ԅ +$7!&ԅ5З"'ԅ v6ö!%ԅ 24ԅ'5!ԅ %6&4")!ԅ '!*9!&"'6&!ԅ 2%ԅ %64£)2!-'+(ԅ5205<ԅ "%ԅ 21ԅ The line C-C 0 is actually closer to 1.5 MW m - 2 for real pool boiling a 1 atmosphere. This heat flux is termed the critical heat flux , as it represents the edge of a sudden transition in behavior.
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Flow Boiling (Fig. 12-4)
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