流体力学与传热课件Principles of Heat Flow in Fluids
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(流体力学与传热英文课件)Pressure drop and loss due to friction
The equation(1.4-10) is the equation usually used to calculate skin friction loss in straight pipe.
For laminar flow only, combining Eqs. (1.4-20 ) and (1.4-10) .gives
f 16 Re
64
Re
(1.4-22 )
It is not possible to predict theoretically the Fanning friction factor f for turbulent flow as was done for laminar flow.
1.4.3 Turbulent Flow in Pipes and Channels
Although the problem has not been completely solved, useful relationships are available.
• For turbulent flow the friction factor must be determined empirically, and it not only depends upon the Reynolds number but also on surface roughness of the pipe.
L R
Rearranging equation (1.4-2 ) gives
w
Rp 2L
Substituting from equation above into equation (1.4-7) gives
fV 2 w /2R V p 2//2 2 L4D L pf V 2 2
流体力学与传热学ppt课件
动（泵、风机等）
2) 流动状态
h紊流 h层流
层流运动：流体微团沿着主流方向做有规 则的分层运动
湍流运动：流体质点做复杂无规则的运动
3) 流体有无相变
h相变 h单相
单相换热：流体显热的变化实现对流换热中的热量
变换
相变换热：在有相变的换热过程中，流体相变热
（潜热）的释放或吸收常常其主要作用
4) 换热表面的几何因素
换热表面的形状，大小，换热表面与流体运动方向的相对位置以及换热表 面的状态（光滑或粗糙）
5) 流体的物理性质 流体的热物理性质对换热的影响很大： 热导率λ ；密度ρ；比热容c ； 动力粘度η ；运动粘度ν ；体胀系数β 综上所述，表面传热系数是众多因素的函数
h f (v, tw , t f , , cp , , ,, l)
在稳定的状态下 壁面与流体之间的对流传热量就等于贴壁处静止流体层的导热量
hx
tw
t
t y
w
,
x
对流传热过程微分方程式
hx取决于流体热导率、温度差和贴壁的温度梯度
要求解一个对流换热问题，获得该问题的对流传热系数或交换的热流量
获得流场的温度分布，即温度场
确定壁面上的温度梯度
计算出在参考温差下的对流传热系数
温度梯度或温度场取决于流体热物性、流动状态（层流或湍流）、流速的大 小及其分布、表面粗糙度等。
温度场取决于流场
§8.2 对流传热问题的数学描写
1、假设条件
为简化分析,对于影响常见对流换热问题的主要因素,做如下假设:
1) 流动是二维的; 2) 流体为不可压缩的牛顿型流体; 3) 流体物性为常数,无内热源;
流体力学与传热课件Heat Transfer to Fluids with Phase Change
The fine drops, in turn, coalesce into rivulets, which flow down the tube under the action of gravity, sweep away condensate, and clear the surface for more droplets.
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.
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.
流体力学与传热课件Principles of Heat Flow in Fluids
4.3 Principles of Heat Flow in Fluids
• Heat transfer from a warmer fluid to a cooler fluid, usually through a solid wall separating the two fluids, is common in chemical engineering practice.
mh c ph (T t ) mcc pc (t2 t1 ) (4.3-7)
4.3.3 Heat Flux and HeatTransfer Coefficients
Heat flux
In many types of heat-transfer equipment the transfer surfaces are constructed from tubes.
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case shown in figure.
Temp of condensing vapor T
Δt2 Δt Δt1
Length of tube L
Double-tube heat exchanger
It is assembled of standard metal pipe and standarized return bends and return heads. shown in figure.
• Heat transfer from a warmer fluid to a cooler fluid, usually through a solid wall separating the two fluids, is common in chemical engineering practice.
mh c ph (T t ) mcc pc (t2 t1 ) (4.3-7)
4.3.3 Heat Flux and HeatTransfer Coefficients
Heat flux
In many types of heat-transfer equipment the transfer surfaces are constructed from tubes.
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case shown in figure.
Temp of condensing vapor T
Δt2 Δt Δt1
Length of tube L
Double-tube heat exchanger
It is assembled of standard metal pipe and standarized return bends and return heads. shown in figure.
第二章 传热过程PPT课件
27
1-3 圆筒壁的稳定热传导
化工生产上最常采用圆筒形设备传热。
如热交换器里的管壁就是最常遇到的圆筒
壁。圆筒壁的传热面积随着圆筒半径的增
加而增加（平面壁的传热面积是不变的）。
设在图2-3 单层圆筒壁上取一厚度为dr的薄
层，此薄层距轴线的距离为r，圆筒的长度
为L，则 A2rL，故
q2rL dt
dr
28
q
δ1 λ1 A
q/
δ1 λ1
422 0.225 1.4
67.9℃
t2 t1 Δt1 930 67.9 862.1℃
Δt2
q
δ2 λ2 A
q/
δ2 λ2
422 0.250 0.15
703.5℃
t3 t2 Δt2 862.1 703.5 158.6 ℃
Δt3 t3 t4 158.6 40 118.6 ℃
6
化工生产中，间壁式传热设备用得最多。这类设 备通常称作热交换器或换热器。在所有化工厂设备中 换热器约占设置重量的40％左右，因此必须对传热机 理、传热过程的影响因素、传热过程的强化或抑止、 换热设备的传热面积计算，以及主要几种热交换器的 基本结构和性能有所了解。
7
§1 传导传热
1-1 传导传热
1 (3 4 7 1 33 )7 325
1
1.34104 [W]
从上述计算结果可以看出，其 r2 1.26 2 ，用平面
体的导热系数变化较小。大多数液体的导热系数随温 度升高而减小（水和甘油除外）。
气体的导热系数随温度的升高而加大。在相当大 的压力范围内，气体的导热系数和压力的关系不是很 大，只有在压力大于2000 [大气压]或是小于20 [毫米 汞柱]时，导热系数才随着压力的增加而加大。
1-3 圆筒壁的稳定热传导
化工生产上最常采用圆筒形设备传热。
如热交换器里的管壁就是最常遇到的圆筒
壁。圆筒壁的传热面积随着圆筒半径的增
加而增加（平面壁的传热面积是不变的）。
设在图2-3 单层圆筒壁上取一厚度为dr的薄
层，此薄层距轴线的距离为r，圆筒的长度
为L，则 A2rL，故
q2rL dt
dr
28
q
δ1 λ1 A
q/
δ1 λ1
422 0.225 1.4
67.9℃
t2 t1 Δt1 930 67.9 862.1℃
Δt2
q
δ2 λ2 A
q/
δ2 λ2
422 0.250 0.15
703.5℃
t3 t2 Δt2 862.1 703.5 158.6 ℃
Δt3 t3 t4 158.6 40 118.6 ℃
6
化工生产中，间壁式传热设备用得最多。这类设 备通常称作热交换器或换热器。在所有化工厂设备中 换热器约占设置重量的40％左右，因此必须对传热机 理、传热过程的影响因素、传热过程的强化或抑止、 换热设备的传热面积计算，以及主要几种热交换器的 基本结构和性能有所了解。
7
§1 传导传热
1-1 传导传热
1 (3 4 7 1 33 )7 325
1
1.34104 [W]
从上述计算结果可以看出，其 r2 1.26 2 ，用平面
体的导热系数变化较小。大多数液体的导热系数随温 度升高而减小（水和甘油除外）。
气体的导热系数随温度的升高而加大。在相当大 的压力范围内，气体的导热系数和压力的关系不是很 大，只有在压力大于2000 [大气压]或是小于20 [毫米 汞柱]时，导热系数才随着压力的增加而加大。
传热学对流传热的理论基础课件
特征数方程中的 几位人物
传热学对流传热的理论基础课件
（4） 与 t 之间的关系及 Pr
对于外掠平板的层流流动: uco,n st
动量方u程 u x： v u y y 2u 2
d d
p 0 x
此时动量方程与能量方程的形式完全一致:
u
t x
v
t y
a
2t y2
表明：此情况下动量传递与热量传递规律相似
上述理论解与实验值吻合。
普朗特边界层理论在流体力学发展史上具有划时代的意义！
传热学对流传热的理论基础课件
5.3 流体外掠等温平板传热的理论分析
当壁面与流体间有温差时，会产生温度梯度很大的温度 边界层（热边界层， thermal boundary layer ）
厚度t 范围 — 热边界层或温度边界层
预期解的形式
传热学对流传热的理论基础课件
4. 如何指导实验
• 同名的已定特征数相等 • 单值性条件相似：初始条件、边界条件、几何条件、
物理条件
实验中只需测量各特征数所包含的物理量,避免了测量的盲 目性——解决了实验中测量哪些物理量的问题 按特征数之间的函数关系整理实验数据，得到实用关联式 ——解决了实验中实验数据如何整理的问题 可以在相似原理的指导下采用模化试验 —— 解决了实物 试验很困难或太昂贵的情况下，如何进行试验的问题
Nu — 待定特征数 （含有待求的 h）
Re，Pr，Gr — 已定特征数
特征关联式的具体函数形式、定性温度、特征长度等的确 定需要通过理论分析，同时又具有一定的经验性。
传热学对流传热的理论基础课件
关联式中的待定参数需由实验数据确定，通常由图解法 和最小二乘法确定。如通过相似原理或理论分析，预期
化工原理英文教材传热原理Principles of heat flow in fluids
化工原理 Principles of Chemical Industry
Principles of heat flow in fluids
Typical heat-exchange equipment
Single-pass shell-and-tube condenser
Expansion joint
It is clear from Fig.11-4 that Δt can vary considerably from point to point along the tube, and, therefore, the flux also varies with tube length.
The local flux dq/dA is related to the local value of Δt by the equation
because, as inspection of Figs11-4a and b will show, it is
not possible with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow.
The temperatures plotted Fig11-4 are average stream temperatures.
The temperature so defined is called the average or mixing-cup stream temperature.
Principles of heat flow in fluids
Typical heat-exchange equipment
Single-pass shell-and-tube condenser
Expansion joint
It is clear from Fig.11-4 that Δt can vary considerably from point to point along the tube, and, therefore, the flux also varies with tube length.
The local flux dq/dA is related to the local value of Δt by the equation
because, as inspection of Figs11-4a and b will show, it is
not possible with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow.
The temperatures plotted Fig11-4 are average stream temperatures.
The temperature so defined is called the average or mixing-cup stream temperature.
流体力学与传热课件Centrifugal pump theory
• This assumption in turn is equivalent to assumption that there are an infinite number of vanes, of zero thickness, at an infinitesimal distance apart.
2
2
r2
2
g
ΔH is the ideal theoretical head developed
In practice, the total head across the pump is less than this due to energy dissipation in eddies and in friction.
The capacity Q is proportional to the diameter D, the head H is proportional to D2, and the brake horsepower W is proportional to D3.
The developed head of an actual pump is considerably less than that calculated from the ideal pump relation.
Pump performance also depends on β2.
β2 > 90o are forward facing and Head
The velocity V has radial and
tangential components Vr and Vt, respectively. The angular momentum of a mass m of
2
2
r2
2
g
ΔH is the ideal theoretical head developed
In practice, the total head across the pump is less than this due to energy dissipation in eddies and in friction.
The capacity Q is proportional to the diameter D, the head H is proportional to D2, and the brake horsepower W is proportional to D3.
The developed head of an actual pump is considerably less than that calculated from the ideal pump relation.
Pump performance also depends on β2.
β2 > 90o are forward facing and Head
The velocity V has radial and
tangential components Vr and Vt, respectively. The angular momentum of a mass m of
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The temperature of the fluid in the tubes increases continuously as the fluid flows through the tubes.
Temperature ºC
The temperatures of the condensing vapor and of the liquid are plotted against the tube length. The horizontal line represents the temperature of the condensing vapor, and the curved line below it represents the rising temperature of the tube-side fluid.
流体力学与传热课件Principles of Heat Flow in Fluids
• Heat transfer from a warmer fluid to a cooler fluid, usually through a solid wall separating the two fluids, is common in chemical engineering practice.
Heat is transferred between warm and cool fluids by conduction and convection.
4.3.1 Typical Heat-Exchange Equipment
Typical heat-exchange equipment Single-pass shell-and-tube condenser
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case shown in figure.
Single-pass shell-and-tube condenser
If the vapor entering the condenser is not superheated and the condensate is not subcooled, the temperature throughout the shell-side of the condenser is constant.
connection G leads to a trap, which is a device that allows flow of liquid but holds back vapor.
The fluid to be heated is pumped through connection H into channel D2.
One fluid flows through the inside pipe and second fluid through the annular space between the outside and inside pipes.
Double-pipe eFra bibliotekchanger are useful when not more than 9 to 14 m2 of surface is required.
It consists essentially of a bundle of parallel tubes A, the ends of which are expanded into tube sheets B1 and B2.
The tube is inside a cylindrical shell C and is
Temp of condensing vapor T
Δt2 Δt Δt1
Length of tube L
Double-tube heat exchanger
It is assembled of standard metal pipe and standarized return bends and return heads. shown in figure.
• The heat transferred may be latent heat accompanying a phase change such as condensation or vaporization, or
it may be sensible heat from the rise or fall in the temperature of a fluid without any phase change.
For larger capacities , more elaborate shelland-tube exchangers, containing up to thousand of square meter of area, are used.
Countercurrent and parallel-current flows
provided with two channels D1 and D2, one at each end, and two channel covers E1 and E2.
Steam and other vapor is introduced through nozzle F into the shell-side space surrounding the tubes, condensate is withdrawn through connection G, and any noncondensable gas that might be enter with the inlet vapor is removed through vent K.
Temperature ºC
The temperatures of the condensing vapor and of the liquid are plotted against the tube length. The horizontal line represents the temperature of the condensing vapor, and the curved line below it represents the rising temperature of the tube-side fluid.
流体力学与传热课件Principles of Heat Flow in Fluids
• Heat transfer from a warmer fluid to a cooler fluid, usually through a solid wall separating the two fluids, is common in chemical engineering practice.
Heat is transferred between warm and cool fluids by conduction and convection.
4.3.1 Typical Heat-Exchange Equipment
Typical heat-exchange equipment Single-pass shell-and-tube condenser
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case shown in figure.
Single-pass shell-and-tube condenser
If the vapor entering the condenser is not superheated and the condensate is not subcooled, the temperature throughout the shell-side of the condenser is constant.
connection G leads to a trap, which is a device that allows flow of liquid but holds back vapor.
The fluid to be heated is pumped through connection H into channel D2.
One fluid flows through the inside pipe and second fluid through the annular space between the outside and inside pipes.
Double-pipe eFra bibliotekchanger are useful when not more than 9 to 14 m2 of surface is required.
It consists essentially of a bundle of parallel tubes A, the ends of which are expanded into tube sheets B1 and B2.
The tube is inside a cylindrical shell C and is
Temp of condensing vapor T
Δt2 Δt Δt1
Length of tube L
Double-tube heat exchanger
It is assembled of standard metal pipe and standarized return bends and return heads. shown in figure.
• The heat transferred may be latent heat accompanying a phase change such as condensation or vaporization, or
it may be sensible heat from the rise or fall in the temperature of a fluid without any phase change.
For larger capacities , more elaborate shelland-tube exchangers, containing up to thousand of square meter of area, are used.
Countercurrent and parallel-current flows
provided with two channels D1 and D2, one at each end, and two channel covers E1 and E2.
Steam and other vapor is introduced through nozzle F into the shell-side space surrounding the tubes, condensate is withdrawn through connection G, and any noncondensable gas that might be enter with the inlet vapor is removed through vent K.