第十章 对流换热教程文件

第十章对流换热

第十章 对流换热

英文习题

1. Finding convection coefficient from drag measurement

A 2 m×3 m flat plate is suspended in a room, and is subjected to air flow parallel to its

surfaces along its 3-m-long side. The free stream temperature and velocity of air are

20℃ and 7m/s. The total drag force acting on the plate is measured to be 0.86 N.

Determine the average convection heat transfer coefficient for the plate (Fig. 10-1).

2. Cooling of a hot block by forced air at high

elevation

The local atmospheric pressure in Denver, Colorado (elevation 1610

m), is 83.4 kPa. Air at this pressure and 20℃ flows with a velocity of 8

m/s over a 1.5 m×6 m flat plate whose temperature is 140℃ (Fig. 10-

2). Determine the rate of heat transfer from the plate if the air flows

parallel to the (a) 6-m-long side and (b) the 1.5-m side. 3. Cooling of a steel ball by forced air

A 25-cm-diameter stainless steel ball (ρ=8055 kg/m 3, and C p =480 J/kg.℃) is removed from the oven at a uniform temperature of 300℃. The ball is then subjected to the flow of air at 1 atm pressure and 25℃ with a velocity of 3 m/s. The surface temperature of the ball

eventually drops to 200℃. Determine the average convection heat

transfer coefficient during this cooling process and estimate how long

the process will take.

4. Flow of oil in a pipeline through the icy waters of a

lake

Consider the flow of oil at 20℃ in a 30-cm-diameter pipeline at an

average velocity of 2 m/s (Fig.10-3). A 200-m-long section of the

pipeline passes through icy waters of a lake at 0℃. Measurements

indicate that the surface temperature of the pipe is very nearly 0℃.

disregarding thermal resistance of the pipe material, determine (a) the

temperature of the oil when the pipe leaves the lake, (b) the rate of heat

transfer from the oil. 5. Heat loss through a double-pane window

The vertical 0.8-m-high, 2-m-wide double-pane window shown in

Fig.10-3, consists of two sheets of glass separated by a 2-cm air gap at

atmospheric pressure. If the glass surface temperatures across the air

gap are measured to be 12℃ and 2℃, determine the rate of heat

transfer through the window.

FIGURE 10-1

FIGURE 10-2

FIGURE 10-3

FIGURE 10-4

工程热力学与传热学

第十章对流换热习题

1.何谓表面传热系数？请写出其定义式并说明其物理意义。

2.用实例简要说明对流换热的主要影响因素。

3.对流换热微分方程组由几个方程组成，各自导出的理论依据是什么？

4.何谓流动边界层和热边界层？他们的厚度是如何规定的？

5.简述边界层理论的基本内容。

6.边界层理论对求解对流换热问题有何意义？

7.层流边界层和湍流边界层在传热机理上有何区别？

8.何谓两个物理现象相似？

9.简述相似理论的主要内容。

10.如何判断两个现象是否相似？

11.相似理论对解决对流换热问题有何指导意义？

12.分别写出努塞尔特数Nu，雷诺数Re，普朗特熟Pr，格拉晓夫数Gr的表达式，并说明他们的物理意义。

13.努塞尔特数Nu和毕渥数Bi的表达式的形式完全相同，二者有何区别？

14.水和空气均以u∞=1m/s的速度分别平行流过平板，边界层的平均温度均为50℃，试求距平板前沿100mm处流动边界层及热边界层的厚度。