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Phase-changing convection (相变对流传热) Chapter 7 Phase-changing convection (相变对流传热)

Condensation heat transfer Primary Contents Condensation heat transfer 凝结传热 phenomena & characteristics film condensation 膜状凝结 analytical solutions engineering calculation influencing factors enhancement

Primary Contents Boiling heat transfer the heat pipe 沸腾传热 modes & characteristics empirical relations of pool boiling 大容器沸腾传热 influencing factors and enhancement the heat pipe 热管

7-1 Modes of condensation heat transfer (凝结传热的模式)

dropwise condensation film condensation 膜状凝结 dropwise condensation 珠状凝结

Phenomenon & Characteristics liquid wets the surface g film condensation 膜状凝结 the surface is blanketed by the film; temperature gradient exists in the film; the film represents a thermal resistance.

liquid does not wet the surface g dropwise condensation 珠状凝结 no film barrier; higher heat transfer rates – 10 times higher than in film condensation difficult to maintain dropwise condensation

湖南大学2006年考研试题 判断题：蒸汽在低于饱和温度的壁面上出现 的膜状凝结形式或珠状凝结形式主要取决于 接触壁面表面的润湿能力。 浙江大学2006年考研试题 为什么电厂凝汽器中水蒸气与管壁之间的换 热可以不考虑辐射换热，而锅炉的炉膛内烟 气与水冷壁之间的换热必须考虑辐射换热？

小结 凝结换热产生的条件： 凝结换热有两种形式:膜状凝结、珠状凝结 膜状凝结 (1)凝结液体能很好地润湿壁面，并在壁面上均匀铺展成膜； (2)液膜层是膜状凝结的主要热阻。

小结 珠状凝结 凝结换热也属于对流换热，故牛顿冷却公式 仍然适用 (1)凝结液体不能很好地润湿壁面，在壁面上形成一个个小液珠; (2)其它条件相同时，珠状凝结的h远高于膜状凝结，但珠状凝结不能长久保持。 凝结换热也属于对流换热，故牛顿冷却公式 仍然适用

7-2 Analytical solutions and empirical relations of film condensation (膜状凝结分析解及计算关联式)

saturated 饱和的 subcooled 过冷的 layer Reynolds number 膜层雷诺数

Film condensation Analytic solution 饱和蒸汽 (clean saturated vapor, uniform wall temperature, vertical surface, laminar) Assumption the physical parameters are constant; the viscous shear of the vapor on the liquid-vapor interface is negligible at y=δ; the inertia force of the fluid is negligible ; 惯性力

tδ＝ts ; only conduction occurs in the film ; subcooled liquid is negligible ; 过冷 ρv＜＜ ρl ; no wave zone on the surface of the film.

Differential equations g t(y) u(y) Thermal boundary layers Velocity boundary layers 微元控制体 y x

Boundary conditions: y x g t(y) u(y) Thermal boundary layers Velocity boundary layers 微元控制体 y x Boundary conditions:

Results for vertical surface the film thickness the local heat transfer coefficient the average heat transfer coefficient qualitative temperature: qualitative temperature for r：ts for inclined wall：g=g·sinφ

Compared with horizontal tube the average heat transfer coefficient （7-4）

Compared with experimental solutions (vertical surface)

layer Reynolds number 膜层雷诺数 de - hydraulic diameter Ac – cross area P – wetted perimeter ul – average velocity in flow

l

Example A vertical square plate, 30 by 30cm, is exposed to steam at atmospheric pressure. The plate temperature is 98℃. Calculate the heat transfer and the mass of steam condensed per hour.

Example 一大气压下，一块30×30cm的竖壁放置在蒸汽中。壁面温度为98℃。请计算换热量，以及每小时的凝结量。

[Solution] From Appendix 10, we have From Appendix 9, we have

Assume laminar film condensation Checking the Reynolds number, we have

So that the laminar assumption was correct. The heat transfer rate is The total mass flow of condensate is

小结 凝结换热过程热平衡关系式： 膜状凝结的工程计算 (1)流态判别： 对竖壁,由于Re为待定准则,因而常需先假定流态，待求出h后再校核，计算过程需迭代。 对横管,计算Re时，需用πd代替l。 对横管,因为直径较小,一般都在层流范围。

小结 (2)竖壁： (3)水平单管和管束： 说明： (1)定性温度： (2)特征长度：

7-3 Influencing factors of the film condensation and its enhancement (膜状凝结影响因素及其传热强化)

Non-condensing gases Superheated vapor Subcooled liquid 不凝结气体 过热蒸汽 过冷液体

Influencing factors Non-condensing gases Rows of tubes 不凝结气体 Rows of tubes Condensation inside the tube

Influencing factors Velocity of the vapor Superheated vapor 过热蒸气 Subcooled liquid and nonlinear temperature

西安交通大学2003年考研试题 安装新空调的时候，在充灌制冷剂之前 往往要对系统抽真空，请从传热学的观 点阐述这样做的理由？

Example Steam at atmospheric pressure condenses on the outside of a horizontal copper tube. The tube wall temperature is maintained at 90℃. There are two cases: (a) a 10-cm-diameter tube; (b) ten 1-cm-diameter tubes. Which is better in order to obtain more amount of condensate fluid? Does the conclusion related to the steam pressure and the surface temperature of the tube?

Example 一大气压下的蒸气在水平铜管外凝结，管壁的温度保持在90℃。管子有两种布置方式: (a) 一根10cm直径的管子; (b) 10根1cm直径的管子。为了得到更多的凝结液，哪种布置更好？这个结论与蒸气压力及管壁温度有关吗？

[solution] so

Condensation enhancement Principle - thinning or damaged film thickness

Condensation enhancement low-fin tube(低肋管) saw-tooth tube(锯齿管)

7-17 在高为L的竖管外,等间距布置n个泄出罩,且加罩前与加罩后管壁温度及其它条件保持不变。 （1）导出加罩后全管平均表面传热系数与未加罩时平均表面传热系数间的关系式； （2）如希望把表面传热系数提高2倍，应加多少个罩？ （3）若L/d=100，为使竖管平均表面传热系数与水平管一样，需加多少个罩？

L L

[Solution] (1) When there is no drainage ring, we have Assume the number of drainage ring is n, the average heat transfer coefficient is

问题： 冷凝器为什么通常横管布置？ (2) when We have So 15 drainage rings need to be installed. (3) For a horizontal tube, we have So When , we have

小结 膜状凝结的影响因素 降低气液界面蒸气分压力； 增加了不凝结气体层。 对液膜表面产生粘滞力； 与流速大小、方向有关。 用焓差代替潜热。 蒸气流速 对液膜表面产生粘滞力； 与流速大小、方向有关。 蒸汽过热度 用焓差代替潜热。 液膜过冷度及温度分布的非线性 使表面传热系数h增大。 管子排数 凝结表面几何形状 若使液膜层减薄，则h增大。 扰动使h增大。 液膜波动 膜状凝结的强化措施

7-4 Modes of boiling heat transfer (沸腾传热的模式)

subcooled/local boiling pool boiling 大容器沸腾（池沸腾） in-tube boiling 管内沸腾 subcooled/local boiling 过冷沸腾 saturated/bulk boiling 饱和沸腾

nucleate boiling film boiling Critical Heat Flux nucleation site 核态沸腾 film boiling 膜态沸腾 Critical Heat Flux 临界热流密度 nucleation site 汽化核心 surface tension 表面张力

Pool boiling In-tube boiling 大容器沸腾（池沸腾） When the heated surface is submerged below a free surface of liquid. In-tube boiling 管内沸腾 Heated Surface Liquid flow Bubble flow Slug flow Annular flow Mist flow

Subcooled/local boiling 过冷沸腾 The temperature of the liquid is below the saturation temperature. Saturated/bulk boiling 饱和沸腾 The liquid is maintained at saturation temperature.

Boiling curve 核态沸腾 膜态沸腾 qmax q × 10-5 W/(m2) qmin 4 Temperature excess (Tw-Ts) （℃） q × 10-5 W/(m2) Free convection Nucleate boiling Transition boiling Film boiling qmax DNB CHF Solitary bubbles qmin 4

Boiling curve three important zones two important state points nucleate boiling 核态沸腾区 transition boiling 过渡沸腾区 film boiling 膜态沸腾区 two important state points CHF (critical heat flux) 临界热流密度 DNB (departure from nucleate boiling) 偏离核态沸腾点

Bubble dynamics 气泡动力学 nucleation site - create & grow 汽化核心 

existential conditions pl πR2(pv-pl) pv – vapor pressure pl – liquid pressur σ – surface tension pv R 2πRσ 表面张力 The force balance

小结 沸腾换热产生的条件： 沸腾换热的分类: 大容器沸腾、管内沸腾 过冷沸腾、饱和沸腾 沸腾换热也属于对流换热，故牛顿冷却公式 仍然适用：

小结 大容器饱和沸腾曲线 (1)四个区域： 自然对流区-单相，加热表面上无汽泡产生 核态沸腾区-h大,一般工业应用都在此范围 膜态沸腾区-汽膜层使h降低,tw增大,辐射强 (2)两种加热方式：控制热流、控制壁温 (3)临界热流密度

小结 汽化核心 (1)汽化核心是汽泡产生的源泉，壁面上的凹穴、裂缝容易残留气体，是最好的汽化核心； (2)汽化核心存在和长大的条件： ——汽泡外液体必须过热 ——

7-5 Experimental correlations of pool boiling (大容器沸腾传热的实验关联式)

Engineering Calculation Saturated nucleate pool boiling cpl – specific heat of saturated liquid Prl – Prandtl number of saturated liquid s = 1.0 for water and 1.7 for other liquids Cwl – constant, Table7-1

Critical Heat Flux (Saturated pool boiling )

小结 沸腾换热的工程计算 (1)大容器饱和核态沸腾： (2)临界热流密度计算公式： 说明： 所有物性均按饱和温度查取。 在使用公式时，要注意各物理量的单位。

Example 1 A heated brass tube of 20 mm diameter and 1 m long is submerged in a container of water, the surface temperature of the brass tube is 65℃, and the saturated temperature of the water is 55℃. Calculate the heat flow and vaporation rate.

Example 1 一加热用的黄铜管直径为20 mm 长为 1 m，把它浸入盛满水的容器中，黄铜管的表面温度维持在65℃, 水的饱和温度为55℃. 请计算热流量和蒸发率。

[Solution] From formula (7-17) , we have From Appendix 9 , we have

From Appendix 10 , we have From Table 7-1 , we have

The total heat flow is The evaporation rate is

Calculate the current flow that will burnout the nickel wire. Example 2 镍丝 A nickel wire of 1 mm diameter is submerged in a container of water at atmospheric pressure. The electric resistance of the nickel wire is 0.129Ω/m. Calculate the current flow that will burnout the nickel wire.

Example 2 一根直径为1mm的镍丝被浸入到1大气压下盛满水的容器中，镍丝的电阻值为0.129Ω/m. 请计算镍丝发生烧毁的电流是多少？

[Solution] From formula (7-20) , we have From Appendix 9, we have From Appendix 10 , we have

Example 3 常压下，直径为30cm的圆盘形、铜质沸腾换热面浸入水中，每小时产生4.5kg水蒸汽。问此时壁面温度应为多少？

[solution] From Appendix 9, we have From Appendix 10 , we have

Assume nucleate boiling So that the nucleate boiling assumption was correct.

7-6 Influencing factors of boiling and its enhancement (沸腾传热的影响因素及其强化)

Influencing factors Non-condensing gases enhanced Degree of subcooling Liquid lever Liquid level

Gravitational acceleration Structure of the boiling surface

小结 沸腾传热的影响因素 可以强化沸腾传热。 在核态沸腾的起始段，使换热增强。 只有低液位沸腾会强化换热。 不凝结气体 过冷度 在核态沸腾的起始段，使换热增强。 液位高度 只有低液位沸腾会强化换热。 重力加速度 在一定的重力场范围内会使换热强化。 沸腾表面结构 增加表面凹坑会使换热强化。 沸腾传热的强化措施

作业： 英文版：9-11 中文版：7-13,7-30