传递现象 Transport phenomenon Lesson 17 - Energy Transport by Radiation (1)

Contents of the Course Consists of 5 parts:
1. Preparative knowledge
2. Fundamental concepts & Simple illustrations 3. Foundation of transport phenomena
c /
(16.1-1)
There is a very wide range of wave length for electromagnetic waves. So far people have measured a range of order of magnitude by 1020. A rough classification of electromagnetic waves according to wave length and producing mechanism is given in Fig.16.1-1 (p. 489) .
§16.2 Absorption and Emission at Solid Surfaces (1)
Most opaque solid are strongly absorbing media of thermal radiation. The thermal ray impinge on the surface of such a solid is absorbed or reflected. So, solid thermal radiation may be thought of a surface behavior. To describe the solid thermal radiation, a few of concepts are introduced.
§16.2 Absorption and Emission at Solid Surfaces (2)
1. Basic Concepts
1) Absorptivity Absorptivity of a solid surface is defined as the ratio of absorbed energy to incident energy at equilibrium of radiation.
h
(16.1-2)
§16.1 Spectrum of Electromagnetic radiation (3)
The wave property of electromagnetic radiation may also be represented by its wave length ,
radiation
§16.1 Spectrum of Electromagnetic Radiation (1)
Any electrically charged particle, which is moving periodically, should emit electromagnetic energy because of the quantum distribution of energy. Almost all substances consist of charged particles, e.g. electrons and protons, and these particles always are moving periodically provided temperature is not at absolute zero. Therefore, the electromagnetic radiation always exists.
(Tab.11.4-1.I)
(19.1-7)
Introduction (1)
In this lesson, we discuss the long-range transport of internal energy by thermal radiation. In the sense of long-range transport by a physical field, radiation is analogous to gravitation force field in momentum transport. But the gravitation force acts on whole volume of a body, on the other hand, thermal radiation acts apparently on the surface of a strongly absorbing media such as a solid and on the whole volume of a weakly absorbing media such as a gas.
radiation
Introduction (3)
The way of transport
conduction Direct transport of internal energy radiation Indirect transport in a way as follows
emission
internal energy
§16.1 Spectrum of Electromagnetic radiation (2)
From modern physics we have learnt that the electromagnetic radiation has dual images of wave and particles. On one hand, it could be taken as a transversal wave propagated in space with a speed very fast (light speed), and its property of wave may be represented by its frequency . On the other hand, it could be thought of moving particles, named photons, each of which has energy . The two images are associated through the famous Planck’s quantum formula:
aq
a
/ q i
(16.2-1,2)
For any real material,
a 1; a 1
a depends considerably
on frequency.
§16.2 Absorption and Emission at Solid Surfaces (3)
eq
e
/ q b ; e q / q b , (16.2-3,4)
i
e
i
§16.2 Absorption and Emission at Solid Surfaces (4)
2. Cavity Radiation
A cavity is a chamber enclosed by isothermal walls. It is an important apparatus for thought experiments in radiation study. The radiation at equilibrium in a cavity has following properties: 1) The radiation flux (intensity) in a cavity is uniform and unpolarized. Proof By the second law of thermodynamics,
It can be seen that not whole range of electromagnetic radiation is connected with internal energy which involves in the thermal motion of molecules or atoms related to the thermodynamic temperature.
1. 2. 3. 4.
D v Dt
(Tab.3.5-1.A)
Dv p g Dt
(Tab.3.5-1.B)
DT p ˆ CV q T v : v Qp Dt T
D v j r Dt
4. Typical solutions in transport phenomena
5. Selected topics in transport phenomena
Selected Topics in Transport Phenomena
Summary of Equations of Change
electromagnetic wave
propagation
internal energy
absorption
electromagnetic wave
Introduction (4)
Apparent form
conduction The heat flux at a point depends on temperature gradient at the point. Internal energy is transported in space point by point. The heat flux between two points depends on temperature difference between the two points. Internal energy is transported in space point to point.