流体力学与传热 ：Chapter 4 Heat Transfer
第6章传热heattransfer共11页
q dQ dA
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ቤተ መጻሕፍቲ ባይዱ第6章 传热
6
q dQ dA
单位时间、通过单位传热面积所传递的热量。
某段时间内的累积传热量
6.1.3 定态与非定态
定态： t 0
非定态t：0
A
Q dA qd
0
0
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第6章 传热
7
热载体及其选择
载热体（加热剂、冷却剂）
握） • 6.2.3 通过单层圆筒壁和多层圆筒壁的定态导热过程计算（重
点掌握） 1、通过单层圆筒壁的定态导热过程计算
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第6章 传热
3
6.1 概述
传热过程在化工生产中的应用非常广泛,几乎所有的化工生产过程（反应过程、 单元操作等）都伴有传热过程。传热的目的:1)加热或冷却；2)回收热量；3)保 温或保冷（减少热损失或冷量损失）。常见的是冷热两种流体之间的热量交换。
用饱和水蒸汽冷凝放热来加热物料是最常用的加热方法，其优点是饱和水 蒸汽的压强和温度一一对应，调节其压强就可以控制加热温度，使用方便。 其缺点是饱和水蒸汽冷凝传热能达到的温度受压强的限制。
（2）冷却剂
工业中常用的有水（20～30℃）、空气、冷冻盐水、液氨（-33.4℃）等等。
水又可分为河水、海水、井水等，水的传热效果好，应用最为普遍。在水 资源较缺乏的地区，宜采用空气冷却，但空气传热系数小，传热速率慢。
热载体：为了将冷流体加热或热流体冷却，必须用另一种流体供给或取走 热量，此流体称为热载体。起加热作用的热载体称为加热剂；而起冷却作 用的热载体称为冷却剂。
（1）加热剂
工业中常用的有热水（40～100℃）、饱和水蒸气（100～180℃）、矿物油 或联苯或二苯醚混合物等低熔混合物 （180～540℃）、烟道气（500～ 1000℃）等；除此外还可用电来加热。
流体力学与传热课件Heat Transfer to Fluids with Phase Change
The condensation of mixed vapors is complicated and beyond the scope this text.
Dropwise and film-type condensation
A vapor may condense on a cold surface in one of two ways, which are well described by the terms dropwise and film-type.
• A phase change involves the addition or subtraction of considerable quantities of heat at constant or nearly constant temperature.
Friction losses in a condenser are normally small, so that condensation is essentially a constant-pressure process.
Because of this the heat-transfer coefficient at these areas is very high; the average coefficient for dropwise condensation may be 5 to 8 times that for film-type condensation.
流体力学与传热英文习题(64学时)
Problems of Fluid Flow and Heat Transfer for Unit Operations of Chemical EngineeringZHONG Li(College of Chemistry and Chemical Engineering, South China University of Technology)1. Water is pumped at a constant velocity 1m/s from large reservoir resting on the floor to the open top of an absorption tower. The point of discharge is 4 meter above the floor, and the friction losses from the reservoir to the tower amount to 30 J/kg. At what height in the reservoir must the water level be kept if the pump can develop only60 J/kg?2. The fluid (density 1200 kg/m3 ) is pumped at a constant rate 20 m3 /h from the large reservoir to the evaporator. The pressure above the reservoir maintains atmosphere pressure and the pressure of the evaporator keeps 200 mmHg (vacuum). The distance between the level of liquid in the reservoir and the exit of evaporator is 15 meter and frictional loss in the pipe is 120 J/kg not including the exit of evaporator, what is the pump effective work and power if the diameter of pipe is 60 mm?3. Water comes out of the pipe (Φ108x4 mm), as shown in Fig. The friction loss of the pipeline which does not cover the loss at the exit of pipe can be calculated by the following equation:h f =6.5U2where U is the velocity in the pipe, finda. water velocity at section A-A'.b. water flow rate, in m3 /h.4. Water passes through the variable pipe. The velocity in the small pipe is 2.5 m/s. The vertical glass tubes are inserted respectively at the section A and B to measure the pressure (see fig.) If the friction loss between two section is 15 J/kg, what is the water column difference between two glass tubes? By the way, draw the relative liquid column height of two tubes in the Fig.5. A centrifugal pump takes brine (density 1180 kg/m3 , viscosity 1.2 cp) from the bottom of a supply tank and delivers it into another tank. The line between the tanks is 300 m of 25 mm diameter pipe (inner diameter). The flow rate is 2 m3 /h. In this line, there are two gate valves, four elbows (90o ) and one return bend, what is the friction loss if the roughness of pipe is 0.025 mm?6. The orifice meter (diameter of orifice 0.0001 m) is installed for measuring the flow rate. The indicating liquid of orifice is mercury if U shape pressure gauge reading is 0.6 meter and orifice coefficient can be taken as 0.61, what is the flow rate of water?7. Water flows through a pipe with a diameter di 100 mm as shown in figure.a. when the valve is closed, R is 600 mm and h equals 1500 mm. While the valve opens partially, R=400 mm and h=1400 mm, f=0.00625 (Finning factor) and k c =0.5 (contraction coefficient), what is the flow rate of water, in m3 /h?b. If the valve opens fully, what is the pressure of section 2-2', in N/m2 ? The equivalent length of the valve is1.5 m and the Fanning factor f keeps the same?(ρH2O=1000kg/m3, ρHg=13600kg/m3)8. The rotameter is installed to measure the water flow rate, as shown in figure. If the total length includingequivalent length of pipeline A is 10 m and the reading of rotameter is 2.72 m3 /h, what is the flow rate for pipeline B? (f A =0.0075, f B =0.0045)9. Water is transported by a pump (efficiency of pump 65%) from reactor to higher tank, as shown in Figure. The total equivalent length of pipe is 200 m including all local frictional loss. The pipeline is φ89⨯4.5 mm , theorifice coefficient C o and orifice diameter d o are 0.61 and 20 mm, respectively. Frictional coefficient λis 0.025 and the readings of vacuum gauge in reactor and pressure gauge in tank are 200 mm Hg and 49000N/m2 , respectively.Find:(1) Water mass flow rate, in kg/s when the reading R of U pressure gauge in orifice meter is 600 mm Hg? (ρH2O =1000 kg/m3, ρHg =13600 kg/m3)(2)Effective work of pump, in J/kg?C (density of air 1.205 kg/m3 , viscosity 1x10-5 Pa.s). Calculate the maximum diameter of particle if the settle obeys the Stoke s’ Law?11. A filter press(A=0.1 m2 ) is used for filtering slurry. The vacuum inside the filter is 500 mm Hg. One liter filtrate can be got after filtering of 5 min and 0.6 more liter filtrate is obtained after 5 more min. How much filtrate will be got after filtering of 5 more min?12. The following data are obtained for a filter press (A=0.0093 m2) in a lab.------------------------------------------------------------------------------------------------pressure difference (kg f /cm2 ) filtering time (s) filtrate volume (m3 )1.05 502.27⨯10-3660 9.10⨯10-33.50 17.1 2.27⨯10-3233 9.10⨯10-3Find1) filtering constant K, q e , t e at pressure difference 1.05 kg f /cm2 ?2) if the frame of filter is filled with the cake at 660 s, what is the end filtering rate (dV/dt)E at P 1.05 kg f /cm2 ?3) compressible constant of cake s?13. A slurry is filtered by a 0.1 m2 filter press at constant pressure if the cake is incompressible. The filter basic equation is as follows:(q+10)2 = 250(t+ 0.4)where q---l/m2 t----minfind (1) how much filtrate is got after 249.6 min?(2) if the pressure difference is double and the resistance of cake is constant, how much filtrate can be obtained after 249.6 min? (cake is incompressible)14. A flat furnace wall is constructed of 120 mm layer of sil-o-cel brick, with a thermal conductivity 0.08 w/(m o C), backed by a 150 mm of common brick, of conductivity 0.8 w/(m o C), the temperature of inner face of the wall is 1400 o , and that of the outer face is 200o C.a. What is the heat loss through the wall in w per square meter.b. To reduce the heat loss to 600 w/m2 by adding a layer of cork with k 0.2 w/(m o C) on the outside of common brick, how many meters of cork are required?15. The vapor pipe (d o=426 mm) is covered by a 426 mm insulating layer (k=0.615 w/m o C). If the temperature of outer surface of pipe is 177 o C and the temperature outside the insulating layer is 38 o C, what are the heat loss per meter pipe and the temperature profile within the insulating layer?16. A steel spherical shell has inside radius r i and outside radius r o. The temperatures inside and outside walls are t i and t o, respectively and the conductivity is k. Derive the equation for heat transfer by conduction.17. Air at the normal pressure passes through the pipe (d i 20 mm) and is heated from 20o C to 100o C. What is the film heat transfer coefficient between the air and pipe wall if the average velocity of air is 10 m/s? The properties of air at 60 o C are as follows:density 1.06 kg/m3 , viscosity 0.02 cp, conductivity 0.0289 w/(m o C), and heat capacity 1 kJ/kg-K18. A hot fluid with a mass flow rate 2250 kg/h passes through a ∅25⨯2.5(outer diameter of pipe⨯ thickness of pipe wall) mm tube. The physical properties of fluid are as follows:k=0.5 w/(m o C), C p =4 kJ/kg-K, viscosity 10-3 N-s/m2 , density 1000 kg/m3 Find:a. Heat transfer film coefficient h i , in w/(m2 -K).b. If the flow rate decreases to 1125 kg/h and other conditions are the same, what is the h i ?c. If the diameter of tube (inside diameter) is decreased to 10 mm, and the velocity u keeps the same as that ofcase a, calculate h i .d. When the average temperature of fluid and quantity of heat flow per meter of tube are 40o C and 400 w/m, respectively, what is the average temperature of pipe wall for case a?e. From this problem, in order to increase the heat transfer film coefficient and enhance heat transfer, what kinds of methods can you use and which is better, explain why?Hint: for laminar flow, Nu=1.86[Re Pr]1/3for turbulent flow Nu=0.023Re0.8 Pr1/319. In a double pipe exchange (Φ23⨯2 mm), the cold fluid (Cp=1 kJ/kg, flow rate 500 kg/h) passes through the pipe and the hot fluid goes through the outside. The inlet and outlet temperatures of cold fluid are 20 and 80 o , and the inlet and outlet temperatures of hot fluid are 150 and 90o , respectively. The h i (film coefficient inside pipe) is 700 w/(m2 o C)and overall heat transfer coefficient U o (based on the outside surface of pipe) is 300w/(m2 o C), respectively. If the heat loss is ignored and the conductivity of pipe wall (steel) is taken as 45 w/(m o C), find:(1) heat transfer film coefficient outside the pipe h o?(2) the pipe length required for counter flow, in m?(3) what is the pipe length required if the heating medium changes to saturated vapor(140 o C) and it condenses to saturated liquid and other conditions keep unchanged?(4) When the exchanger is used for a year, it is found that it cannot meet the need of production (the outlet temperature of cold fluid cannot reach 80o C), explain why?20. Water flows turbulently in the pipe of Φ25⨯2.5 mm shell tube exchanger. When the velocity of water u is 1 m/s, overall heat transfer coefficient Uo (based on the outer surface area of pipe) is 2115 w/(m2 o C). If the u becomes 1.5 m/s and other conditions keep unchanged, Uo is 2660 w/( m2 o C ). What is the film coefficient ho outside the pipe? (Heat resistances of pipe wall and scale are ignored)21. Water and oil pass parallelly through a exchanger which is 1 m long. The inlet and outlet temperatures of water are 15 and 40 o C, and those of oil are 150 and 100 o C, respectively. If the outlet temperature of oil decreases to 80 o C, and the flow rates and physical properties and inlet temperatures of water and oil maintain the same, what is the pipe length of new exchanger? (Heat loss and pipe wall resistance are neglected)22. Air which passes through the pipe in turbulent flow is heated from 20 to 80 o C. The saturated vapor at 116.3 o C condenses to saturated water outside the pipe. If air flow rate increases to 120% of the origin and inlet and outlet temperatures of air stay constant, what kind of method can you employ in order to do that? (Heat resistance of pipe wall and scale can be ignored)23. Water flows through the pipe of a Φ25⨯2.5 mm shell-tube exchanger from 20 to 50 o C. The hot fluid (C p 1.9 kJ/kg o C, flow rate 1.25 kg/s) goes along the shell and the temperatures change from 80 to 30 o C. Film coefficients of water and hot fluid are 0.85kw/(m2 o C) and 1.7 kw/(m2 o C). What is the overall heat transfer coefficient Uo and heat transfer area if the scale resistance can be ignored? (the conductivity of steel is 45w/(m o C).思考问答题1. If the inlet and outlet temperatures of fluids are given, the LMTD of countercurrent flow is always larger than that of parallel-current flow?2. For countercurrent flow, the outlet temperature of cold fluid can be higher than that of hot one.3. The value of overall heat transfer coeff. U is closed to that of larger heat transfer film coeff.4. Dirty overall heat transfer coef. is smaller than clean overall heat transfer coef.5. If h i and h o are 10 and 1000 w/(m2o C), respectively, we should try to increase h o in order to elevate overall heat transfer coefficient U.6. For no phase change, ΔT of 1-2 pass shell-tube exchanger is smaller than LMTD of countercurrent flow.7. Explain simply the advantages of countercurrent flow over parallel-current flow and in what situations, parallel flow should be used.8.Dimensional analysis can directly produce the numerical results without experimental data.9. Decrease of thermal boundary layer thickness can increase h and enhance heat transfer.10. Newton's cooling law says that heat transfer film coefficient is a constant.11. The tube length changes only affect the heat transfer area during the convection heat transfer.12. Increase of Reynolds Number can elevate the heat transfer film coeff. of free convection.13. Increase of Reynolds Number can raise the heat transfer film coeff. of forced convection.14. Heat transfer film coeff. of the return bend pipe is larger than that of the same diameter straight pipe.15. The smaller the heat transfer film coef. h, the less the convective heat transfer resistance.16. What happens to the viscosity of liquid or gas when the temperature increases?17. At steady-state heat transfer by conduction, the temperature at all points in a solid is equal.18. Direction of heat flow is opposite to that of the temperature gradient.19. The thicker the insulating layer, the smaller the heat loss.20. For heat transfer through a series of layers at steady-state, the smaller the temperature drop at a certain layer, the larger the heat resistance at the same layer.21. At steady-state heat transfer conduction through a pipe wall, q/A is a constant.22. For heat transfer through two layers of insulating materials having the same thickness but different conductivities at a flat wall. Temperatures of both sides of insulating materials keep constant. If two layers of insulating materials change their places how does heat loss change? Explain why?23. For the same conditions as the above question, but heat transfer through a cylinder, how does heat loss change? Explain why?24. Which can develop more total head H, one pump or two same pump which work in series?25. The dimension for viscosity in SI system is______, and what about the unit for it?26. What is the relationship between the gauge and absolute pressure, the vacuum and absolute pressure? If the reading at entrance of pump is 0.029 MPa(vacuum), what are vacuum in mmHg and gauge pressure in mmHg? If reading at the exit of pump is 0.67 Mpa (gauge), what is absolute pressure (atmosphere pressure 0.1MPa)?27. The total (developed) head of centrifugal pump H means______ and maximum suction lift implies________ and net positive suction head (NPSH) is_______.28. What is cavitation? At what situations, the cavitation will occur?29. If the temperature of fluids increases, what happen to viscosities of liquid and gas?30. The pressure or pressure difference of liquid can be measured by U-shape pressure gauge. If the reading of R becomes smaller, what kind of gauge can be used in order to keep the accurate measurement?31. What are going on flow rate, total head and brakepower of centrifugal pump if the fluid of density 1200 kg/m3 is transported in the same pipe line compared to? (Other properties of fluid are the same as those of water).32. Somebody says that the total mechanical energy entering section 1-1 equals that leaving section 2-2, what do you think about that? If you consider that it is wrong, what is the correct statement?33. When the pipe changes from horizontal to vertical position and velocity keeps the same, what happens to energy loss?34. The solid dust is removed from gases in a gravity-settling chamber. If settling is within the laminar region, compare the productivity at 20 and 200 o C and which is larger?35. For shell-tube exchangers, what is one-pass? When the flow rate is given, velocity of fluid will _________ and Reynolds Number will______and convective film coefficient will ______ if one-pass change to two-pass.36. For the same velocity entering cyclone, separation factor (efficiency) will _____ with the increase of diameter of cyclone.37. The filter basic equation is derived based on __________.38. What is equivalent diameter for 0.5m square?39. How do you adjust the flow rate of centrifugal and reciprocating pumps?。
流体力学与传热电子教案--chapter10
t-Δt
t
t+Δt
Q
dA n
温度梯度是一个点的概念。
温度梯度是一个向量。 方向垂直于该点所在等温面，以温度增加的方向为正
Basic law of conduction
Fourier’s law (傅立叶定律)
dq = −k ∂T dA ∂n
(10-1)
negative --- gradient is opposite that of
Heat transfer by conduction Equal temperature surface 等温面
在同一时刻，温度场中所有温度相同的点组成的面。
★不同温度的等温面不相交
等温
t1
面
t2
Q
t1>t2
temperature gradient 温度梯度
r grad t
=
lim
Δt
=
∂t
Δn→0 Δn ∂n
Chapter10. Heat transfer by conduction Basic conception
¾Steady-state conduction 稳态导热
( ) Ttimhtee=,tceamnfpbexera,thtyuer,fezuanrcetiionn∂∂dθtoepf=epn0odsietniotnof,
对流--- 流体内部质点发生相对位移的热量传递过程
Newton’s law of cooling
q A
=
h(ts
−
t
f
)
h---heat transfer coefficient
ts--- surface temperature
传热学第4章对流换热(Convective Heat Transfer)
第一节：概述 工程应用背景
第四章 对流换热(Convective Heat Transfer)
第一节：概述
热对流 对流换热：
计算关系式
Φ hAtw tf
Φ hAtf tw
本章的主要任务：确定 h 的具体表达式
——请千万小心，步步都是富贵险中求。殊不知多少江湖英豪；名门侠女都 曾栽在这块看似山青湖静，实则风阴涛涌的领域！
第二节：对流换热问题的数学描写—对流换热微分方程组
二维、常物性、不可压、稳态
u v 0 x y
u
u x
v
u y
Fx
1
p x
2u x 2
2u y 2
u
v x
v
v y
Fy
1
p y
2v x 2
2v y 2
u
t x
v
t y
a
2t x 2
2t y 2
t
h tw t y w
第四章 对流换热(Convective Heat Transfer)
第一节：概述 求h主要有以下基本途径：
Φ h At w t f
h
第四章 对流换热(Convective Heat Transfer)
第一节：概述
影响对流换热的基本因素： 流动因素、几何因素和物性参数 流动因素 a 流动起因 自然对流(Natural Convection)—— 强迫对流(Forced Convection)—— b 流动状态 层流(Laminar Flow)—— 紊流(Turbulent Flow)—— c 流体有无相变(Phase Change) 凝结换热(Condensation Heat Transfer) 沸腾换热(Boiling Heat Transfer)
工程传热学双语 四川大学Chapter 4-convective heat transfer
4.1 Some Preliminary Concepts Regarding Convective Heat Transfer 对流传热概论 4.2 An Exact Laminar Boundary Layer Theory 边界层理论 4.3 Correlation of Heat Transfer Data 传热关联式 4.4 Heat Transfer Analysis in Pipe Flow 管内对流传热分析 4.5 Models of Convection in Laminar Flows 层流对流传热模型
流体流过壁面,而且与壁 面存在温度差时,流体与 壁面间的热量传递过程 称为对流传热。
u∞，T∞ Q
Ts
Ts < T∞ 总体要求 ： • 定性—对流换热基本概念，强化对流换热的机理 • 定量—表面传热系数计算，换热量计算
对流特点 • 流体的宏观运动 + 微观的导热 • 流动与换热密不可分 • 对流换热的机理与通过紧靠换热面的薄膜层的热传导有关
thermal diffusivity 热扩散率
α = k ρC p
δ ⎛v⎞ = fn ⎜ ⎟ = fn (Pr ) Pr = v δ' ⎝α ⎠
α
fn ( Pr ) = Pr1 3
j-factor for heat transfer 传热 j 因子
f Nu = Re Pr1 3 2
jH ≡
Nu RePr 1 3
−1
friction factor f 摩擦因子
f ≡
τ
1 ρU 2 2
= CD δ ≡
1 2
μU fρ U
第4章传热共224页文档
Q A dt
dx
式中：Q——导热速率，W A——导热面积，m2 λ——材料的导热系数，W/（m.℃）或W/（m.K）
dt/dx——沿x方向的温度梯度，℃/m或K/m。x方向为热 流方向，即温度降低的方向，故为负值。
物质的导热系数主要与物质的种类和温度有关。
纯金属>合金>非金属建筑材料>液体>绝缘材料>气体
1、 固体的导热系数
金属：金属是最好的导热体。
纯金属：熔融状态时λ变小。
合金：随纯度↑—λ↑。
随T↑—λ↓ 。
非金属建筑材料和绝热材料 λ与温度、组成和结构的紧密程度有关。 随T↑—λ↑ ， 随密度↑—λ↑ ，存在最佳密度，使λ最小。
三、平壁的稳态热传导
1、单层平壁热传导
由傅立叶定律：
Q A dt
dx
Q
分离变量后积分，可得导热速率方程为：
t1
Q b A(t1 t2)
t2
Q
(t1
t2) b
t R
传热推 热阻
动
力
b
A
上式也可写为：
Q
q
A
b(t1
t2)
式中q为单位面积的导热速率，称为热流密度，单位为W/m2。
五、传热速率和热流密度
传热速率Q（rate of heat transfer）：又称热流量（rate of heat flow），指单位时间内通过传热面的热量，单位W 。 整个换热器的传热速率表征了换热器的生产能力。
热流密度q：又称热通量（heat flux），指单位时间内通过 单位传热面积传递的热量，单位W/m2 。 在一定的传热速率下，q越大，所需的传热面积越小。因 此，热通量是反映传热强度的指标，又称为热流强度。
《传热学》教学大纲【可修改文字】
可编辑修改精选全文完整版《传热学》课程教学大纲一、课程名称：传热学/ Heat Transfer二、课程编号：0300302三、学分学时：3学分/48学时四、使用教材：《传热学》（第4版）杨世铭、陶文铨编，高等教育出版社，2014年12月五、课程属性：专业基础课/必修六、教学对象：新能源科学与工程专业七、开课单位：机械工程学院八、先修课程：高等数学、大学物理、流体力学九、教学目标：1、掌握传热学的基本概念、基本理论和基本计算方法，2、培养和建立学生的工程观点和理论联系实际解决工程实际问题的初步能力，并为学习后续的专业课程提供必要的理论基础支撑。
十、课程要求：通过本课程的学习，学生需掌握热量传递的三种基本方式及综合传热过程所遵循的基本规律，学会对传热过程进行分析处理和计算的基本方法，能运用这些规律提出增强传热、提高热经济性和削弱传热减少热损失的途径，具备分析工程传热问题的能力，并基本掌握换热设备的两种基本计算方法；结合热工实验课，使学生掌握一定的传热实验的技能。
主要以课堂讲授为主，充分采用多媒体教学。
十一、教学内容：本课程主要由以下内容组成（理论教学48学时）第一章绪论（2学时）知识要点：传热学的研究对象及其在工程技术中应用；热量传递的基本方式；导热、对流和辐射，传热过程及热阻重点难点：热量传递的三种基本方式，传热过程与传热系数教学方法：课堂讲授、讨论第二章稳态热传导（6学时）知识要点：温度场、等温面、等温线，温度梯度及傅立叶定律，导热系数，各向同性、具有内热源的导热微分方程及导热过程单值性条件的确定；通过单层、多层和复合平壁的稳态导热，通过单层和多层圆筒壁的稳态导热，通过肋壁的稳态导热，具有变导热系数的单层平壁导热问题的处理方法，肋效率、等截面直肋和环肋的工程计算，接触热阻及形状系数。
重点难点：傅立叶定律，导热微分方程及其单值性条件；能够依据直角坐标系下导热微分方程和导热过程单值性条件对常物性、无内热源、简单几何形状的物体的一维稳态导热问题进行分析计算教学方法：课堂讲授、讨论第三章非稳态导热（4学时）知识要点：非稳态导热过程特点，一维非稳态导热问题分析解及其讨论，诺模图，简单几何形状一维、二维和三维非稳态导热的计算，周期性变化边界条件和常热流通量边界条件下半无限大物体非稳态导热。
传热学第六章单相流体对流传热特征数关联式
传热学 Heat Transfer 3. 局部表面传热系数 hx 的变化
传热学 Heat Transfer
二、管槽内湍流换热实验关联式
1. 迪图斯-贝尔特（Dittus-Boelter）关联式：
Nuf 0.023Ref0.8Prfn ;
适用的参数范围：
0.4 n 0.3
(tw tf ) (tw tf )
传热学 Heat Transfer
二、横掠单管（柱）对流换热实验关联式
1. 流动的特征
流体横向绕流单 管时的流动除了具 有边界层的特征外， 还要发生绕流脱体， 而产生回流、漩涡 和涡束。
传热学 Heat Transfer 2. 换热的特征
边界层的成长和脱体决定 了外掠圆管换热的特征。
低雷诺数时，回升点反 映了绕流脱体的起点。
0.14
(Ref
Prf
d l
)1/
3
f w
2
传热学 Heat Transfer 2. 层流充分发展换热的 Nu 数
对于圆管：
Nuf 3.66 (tw const) Nuf 4.36 ( qw const)
传热学 Heat Transfer
例题：在一冷凝器中，冷却水以1m/s的流速流过内径为 10mm、长度为3m的铜管，冷却水的进、出口温度分别 为15℃和65℃，试计算管内的表面传热系数。
tf
tw
传热学 Heat Transfer
一、纵掠平壁换热实验关联式 （以层流为例）
局部对流传热系数关联式
Nux
0.332
Re
1/ x
2
Pr1/ 3
平均对流传热系数关联式
Nu 0.664 Re1/2 Pr1/3
适用范围
传热学
物理概念
物理概念
热管热传递速率曲线图传热学的作用是利用可以预测能量传递速率的一些定律去补充热力学分析，因后者只 讨论在平衡状态下的系统。这些附加的定律是以3种基本的传热方式为基础的，即导热、对流和辐射。传热学是研 究不同温度的物体或同一物体的不同部分之间热量传递规律的学科。
随着激光等新的实验技术的引入谢观看
应用领域
应用领域
传热不仅是常见的自然现象，而且广泛存在于工程技术领域。在能源动力、化工制药、材料冶金、机械制造、 电气电信、建筑工程、文通运输、航空抗天、纺织印染、农业林业、生物工程、环境保护和气象预报等部门中存 在大量的热量传递问题。而且常常还起着关健作用。例如，提高锅炉的蒸汽产量，防止燃气轮机燃烧室过热、减 小内燃机气缸和曲轴的热应力、确定换热器的传热面积和控制热加工时零件的变形等，都是典型的传热学问题。
传热方式
传热方式
传热的基本方式有热传导、热对流和热辐射三种。 1、热传导是指在不涉及物质转移的情况下，热量从物体中温度较高的部位传递给相邻的温度较低的部位， 或从高温物体传递给相接触的低温物体的过程，简称导热。 从微观角度来看。气体、液体、导电固体和非导电固体的导热机理是有所不同的。 （1）气体中，导热是气体分子不规则热运动时相互碰撞的结果。众所周知，气体的温度越高，其分子的运 动动能越大。不同能量水平的分子相互碰撞的结果，使热量从高温处传到低温处。 （2）导电固体中有相当多的自由电子，它们在晶格之间像气体分子那样运动。自由电子的运动在导电固体 的导热中起着主要作用。 在非导电同体中，导热是通过晶格结构的振动，即原子、分子在其平衡位置附近的振动来实现的。 （3）至于液体中的导热机理，还存在着不同的观点。有一种观点认为定性上类似于气体，只是情况更复杂， 因为液体分子间的距离比较近，分子间的作用力对碰撞过程的影响远比气体大。另一种观点则认为液体的导热机 理类似于非导电固体。 2、热对流是指不同温度的流体各部分由相对运动引起的热量交换。
传热与流体力学的相互作用研究与应用
传热与流体力学的相互作用研究与应用引言传热和流体力学是研究物质内部的能量传递和流动行为的两个重要学科。
它们在自然界和工程领域中都有广泛的应用。
传热与流体力学的相互作用是两个学科交叉的领域，具有重要的理论研究和应用价值。
本文将介绍传热与流体力学的基本概念及其相互作用的研究进展，并探讨其在不同领域的应用。
传热的基本概念传热是热量从高温物体传递到低温物体的过程。
热量的传递有三种方式：导热、对流和辐射。
导热是通过物质内部的分子热运动进行热量传递，它的主要方式是热传导。
对流是通过流体的运动进行热量传递，它的主要方式是强迫对流和自然对流。
辐射是通过电磁波辐射进行热量传递。
传热的研究内容包括传热机理、传热模型和传热传质的耦合。
流体力学的基本概念流体力学研究物质流动的规律和性质。
流体力学主要分为两个方面：流体静力学和流体动力学。
流体静力学研究静止的流体，并研究其受力平衡的问题。
流体动力学研究流体的运动，包括流体的速度、压力和密度等变化规律。
流体力学的研究内容包括流体的本构关系、能量方程和动量方程等。
传热与流体力学的相互作用传热与流体力学相互作用的研究是传热与流体力学学科交叉的领域。
它研究在流体中传热过程中热量传递与流体流动之间的相互影响。
传热对流体流动产生的影响主要包括：热源对流体流动的推动作用、热传导对流体流动的抑制作用和热辐射对流体流动的影响。
流体流动对传热的影响主要包括：流体流动对热传导的增强作用、流体流动对传热界面的清洗作用和流体流动对热辐射的干扰作用。
传热与流体力学的相互作用是一个复杂的问题，需要通过实验和数值模拟来研究。
传热与流体力学的应用传热与流体力学的相互作用在工程领域有着广泛的应用。
以下是几个应用的例子：1. 热交换器热交换器是传热与流体力学相互作用的典型应用。
热交换器通过流体的对流传热来实现热量的转移。
流体在热交换器内部流动时，会与热交换器壁面进行热传递，从而实现热量的平衡。
热交换器的设计和优化需要考虑传热与流体力学的相互作用，以提高传热效率和节约能源。
热工与流体力学基础 热工篇第4章
热力循环
可逆循环可以表示在状态参数坐标图 上，且为一条封闭的曲线。 根据热力循环所
产生的不同效果
一、正向循环和热效率
1. 正向循环
——将热能转变为机械能的循环称为正向循环,也称 为动力循环或热机循环 。
• 一切热力发动机都是按正向循环工作的。
热工与流体力学基础 热 工篇第4章
2020年4月28日星期二
学习导引
热力学第二定律揭示了能量传递与转换过程进行的 方 向、条件和限度。热力学第二定律与热力学第一定律是热 力 学的两个最基本定律，共同组成了热力学的理论基础。
本章主要讲述了热力学第二定律的实质和表述，阐述 了 热力循环、卡诺循环、卡诺定律、熵的基本概念及熵增原 理
➢此外，即使同为热能，当它们储存的热源温度 不同时，它们的品质也是不同的。储存于高温水 平热源的热能品质较高。当热由高温物体自动的 传向低温物体时，同样也使能的品质下降了。
➢热力学第二定律的实质是能量贬值原理，即在 能量的传递和转换过程中，能量的品质只能降低 不能增高。它是一个非守恒定律。
第三节 卡诺循环与卡诺定律
热力学第二定律
第二类永动机 :从单一热源取热并使之完全转变 为功的热机 .
热力学第二定律说明，用于热功转换的热机至 少要有高温、低温两个热源（即要有温度差） 。为此，热力学第二定律也可以表述为“第二 类永动机不可能实现”。
热力学第二定律
应注意：不能将热力学第二定律简单的理解为“功 完全可以转变为热，而热不能完全转变为功”。
二、热力学第二定律的实质和表述
热力学第二定律指出了能量在传递和转换过程中 有关传递方向、转化的条件和限度等问题。
流体力学与传热(英文版)Review
Velocity distribution of laminar flow
r ur 1 umax rw
Hale Waihona Puke 2Where umax=-pri2/4L
ur is a local velocity
Laminar Flow
If the flow is laminar, the following is satisfied. 1 parabolic profile of velcity
The
relationship between gauge pressure and absolute pressure as well as vacuum
Fluid-flow phenomena
Ideal
fluid
Boundary layer
Two
kinds of flow patterns
Laminar flow Turbulent flow Criterion of flow pattern:
Re d u
Newtonian fluid
The shear stress is proportional to the shear rate (Newtonian Law of viscosity)
Unit
Example:
what is the dimension and unit of viscosity, in SI system?
Unit
conversion: P=1atm=?mmHg=?mH2O=?N/m2
Fluid
Statics and Its Applications
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Basic Mechanisms of Heat Transfer
The mechanisms by which the heat flows are three:
❖Conduction
❖Convection
❖ Radiation
4.1.2 Conduction
If a temperature gradient exists in a continuous substance, heat can flow unaccompanied by any observable motion of matter.
Chapter 4 Heat Transfer and Its Applications
INTRODUCTION AND MECHANISMS OF HEAT TRANSFER
4.1.1 Nature of heat flow
When two objects at different temperature are brought into thermal contact , heat flow from the object at the higher temperature to that at the lower temperature.
The forces used to create convection currents in fluids are of two types:
natural convection and force convection
Natural convection
If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natourier’s law, the heat flux is proportional to the temperature gradient and opposite to it in sign. For one-dimensional heat flow
dq k dt dA dx
In gases, conduction occurs by the random motion of molecules.
4.1.3 Convection
When a current or macroscopic particle of fluid crosses a specific surface, such as the boundary of a control volume, it carries with it a definite quantity of enthalpy.
Thermal conduction for various materials
• In metals, thermal conduction results from the motion of free electrons.
In solids that are poor conductor of electricity and in most liquids, thermal conduction results from momentum transfer between adjacent vibrating molecules or atoms.
The net flow is always in the direction of the temperature decrease.
Steady-State Heat Transfer
The heat transfer occurs in the control volume where the rate of accumulation of heat is zero and the temperatures at various points in the system do not change with time it is called as steady-state heat transfer.
Such a flow of enthalpy is called a convective flow of heat.
The convection flux is usually proportional to the difference between the surface temperature and temperature of the fluid, as stated in Newton’s law of cooling
q A
h(tw
t
f
)
(4.1-2)
Note that the linear dependence on the temperature driving force tw-tf is the same as that for pure conduction in a solid of constant thermal conductivity.
The heat-transfer coefficient is not an intrinsic property of the fluid,
but depends on the flow patterns determined by fluid mechanics as well as on the thermal properties of the fluid.
forced convection
If the currents are set in motion by the action of a mechanical device such as a pump or agitator, the flow is independent of density gradients, and is called forced convection.