Radiation Heat Transfer

Slides:



Advertisements
Similar presentations
Mathematical Models for Modes of Heat Action P M V Subbarao Professor Mechanical Engineering Department Tools to select Means of Heat Interactions between.
Advertisements

The Atmosphere: Structure and Temperature
MET 112 Global Climate Change
Radiation Heat Transfer
Chapter 12 : Thermal Radiation
Radiant Exchange Heat Transfer at the Speed of Light (3 x cm/sec) No medium required - can occur in vacuum Not dependent on air temperature Net transfer.
Chapter 22 Heat Transfer.
RADIATIVE HEAT TRANSFER Thermal radiation is the electromagnetic radiation emitted by a body as a result of its temperature. There are many types of electromagnetic.
Energy Ability to do work Many different forms Conservation of energy (Law) Transformed: example: – Radiant to Thermal – Kinetic to Thermal (friction)
Heat Transfer Introduction
Radiation Heat Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Select a Suitable Geometry to meet the industrial.
What are the 3 ways heat can be transferred? Radiation: transfer by electromagnetic waves. Conduction: transfer by molecular collisions. Convection: transfer.
MET 60: Chapter: 4 (W&H) and 2 (Stull) Radiative Transfer Dr. Craig Clements San José State University.
MET 61 1 MET 61 Introduction to Meteorology MET 61 Introduction to Meteorology - Lecture 7 “Warming the Earth and Atmosphere” Dr. Eugene Cordero San Jose.
Radiation Heat Transfer. The third method of heat transfer How does heat energy get from the Sun to the Earth? There are no particles between the Sun.
Modes of Heat Transfer P M V Subbarao Professor Mechanical Engineering Department Accounting of Natural Happenings …..
Quantum physics. Quantum physics grew out failures of classical physics which found some quantum remedies in the Planck hypothesis and wave-particle duality.
MECHANISMS OF HEAT TRANSFER
© 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their.
Copyright © 2010 Pearson Education, Inc. Lecture Outline Chapter 16 Physics, 4 th Edition James S. Walker.
Radiation Definitions and laws Heat transfer by conduction and convection required the existence of a material medium, either a solid or a.
Laws of Radiation Heat Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Macro Description of highly complex Wave.
Chapter 7 Light.
Chapter 18 Bose-Einstein Gases Blackbody Radiation 1.The energy loss of a hot body is attributable to the emission of electromagnetic waves from.
Radiation Heat Transfer
Sun path diagrams For practical purposes, instead of using the preceding equations, it is convenient to have the sun’s path plotted on a horizontal plane,
98/02 國立台北科技大學能源與冷凍空調 工程研究所 施陽正 老師 1 高等熱傳學 (Advanced Heat Transfer) 能源與冷凍空調工程研究所 九十八年二月.
Heat – Thermal Energy ISCI What is Heat? Place your finger on the handle of a ‘hot’ pan. Ouch! Heat is energy that is transferred from one ‘system’
Heat Transfer Carlos Silva December 9 th Energy transference Energy can be transferred between a system and its surroundings Work Heat Mass flow.
Climate Long time, Large Area. Weather short term, small area.
Photon Statistics Blackbody Radiation 1.The energy loss of a hot body is attributable to the emission of electromagnetic waves from the body. 2.The.
Radiation Fundamental Concepts EGR 4345 Heat Transfer.
Radiation Heat Transfer EGR 4345 Heat Transfer. Blackbody Radiation Blackbody – a perfect emitter & absorber of radiation Emits radiation uniformly in.
CBE 150A – Transport Spring Semester 2014 Radiation.
CHAPTER 12 RADIATION HEAT TRANSFER. Electromagnetic Radiation – electromagnetic waves (brought upon by accelerated charges or changing electric currents)
Warm Up 3/6/08 More solar energy reaches the equatorial regions than the polar regions because the equatorial regions a. are covered by a greater area.
Introduction to Thermal Physics
Chapter 21 RADIATION HEAT TRANSFER Copyright © 2012 The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fundamentals of Thermal-Fluid.
Chapter 16 MECHANISMS OF HEAT TRANSFER Copyright © 2012 The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fundamentals of.
Introduction to Thermal Radiation and Radiation Heat Transfer.
Introduction to Thermal Radiation
Unit 42: Heat Transfer and Combustion
Radiation Heat Transfer
Radiation (Ch 12 YAC) Thermal energy is emitted by matter as a result of vibrational and rotational motion of molecules, atoms and electrons. The energy.
1 MET 112 Global Climate Change MET 112 Global Climate Change - Lecture 3 The Earth’s Energy Balance Dr. Eugene Cordero San Jose State University Outline.
Chapter 12: Fundamentals of Thermal Radiation
Electromagnetic Energy Topic 5 Energy in Earth Processes.
Warm-Up What would happen if there was no more ozone? What would happen if there was no more ozone? –We would die. What are the four layers of the atmosphere?
1 Teaching Innovation - Entrepreneurial - Global The Centre for Technology enabled Teaching & Learning D M I E T R, Wardha DTEL DTEL (Department for Technology.
Lecture 2: Heat and radiation in the atmosphere. TEMPERATURE… is a measure of the internal heat energy of a substance. The molecules that make up all.
Topic 5 Energy. Energy is the ability to do work or cause change Kinetic energy: energy of motion  faster objects have more kinetic energy Temperature.
Blackbody. Kirchhoff’s Radiation  Radiated electromagnetic energy is the source of radiated thermal energy. Depends on wavelengthDepends on wavelength.
Remote sensing: the collection of information about an object without being in direct physical contact with the object. the collection of information about.
Damian Luna Yetziel Sandoval – Alberto Gonzales – 80546
Heat transfer mechanism Dhivagar R Lecture 1 1. MECHANISMS OF HEAT TRANSFER Heat can be transferred in three different ways: conduction, convection, and.
Heat Transfer RADIATION HEAT TRANSFER FUNDAMENTALS.
The temperature of a lava flow can be estimated by observing its color
AHMEDABAD INSTITUTE OF TECHNOLOGY
NURADIBAH BINTI MOHD AMER
Radiation Heat Exchange Between System & Surroundings
ERT 216 HEAT & MASS TRANSFER
Radiation Process and Properties
Radiation Thermal energy emitted by matter as a result of vibrational and rotational movements of molecules, atoms and electrons. The energy is transported.
Chapter 2C: BASIC THERMAL SCIENCES: RADIATION HEAT TRANSFER
17.2 Heating the Atmosphere
Radiation Thermal energy emitted by matter as a result of vibrational and rotational movements of molecules, atoms and electrons. The energy is transported.
Chapter 2 Energy in transit
Heat.
Radiation Heat Transfer
Chapter 2C: BASIC THERMAL SCIENCES: RADIATION HEAT TRANSFER
Presentation transcript:

Radiation Heat Transfer Chapter14 Radiation Heat Transfer

Radiation Definition: It may be considered to be energy streaming through space at the speed of light, may originate in various ways.

Some types of materials will emit radiation when they are treated by external agencies. All substances at temperatures above absolute zero emit radiation that is independent of external agencies.

Radiation that is the result of temperature only is called thermal radiation.

Fundamental facts concerning radiation Radiation moves through space in straight lines, or beams, and only substances in sight of a radiating body can intercept radiation from that body.

Radiation as such is not heat, and when transformed into heat on absorption, it is no longer radiation.

Some definitions: The fraction that is absorbed is called absorptivity α. The fraction that is transmitted is called transmissivity τ. The fraction of the radiation falling on a body that is reflected is called reflectivity ρ.

The sum of these fractions must be unity, or ρ+ α + τ =1 (14-1)

The maximum possible absorptivity is unity The maximum possible absorptivity is unity. A body which absorbs all incident radiation is called a black body. The maximum possible reflectivity is unity. A body which reflects all incident radiation is called a white body.

Emission of radiation The radiation emitted by any given mass of substance is independent of other material in sight of , or in contact with the mass.

The net energy gained or lost by a body is the difference between the energy emitted by the body and that absorbed by it from the radiation reaching it from other bodies.

When bodies at different temperatures are placed in sight of one another inside an enclosure, the hotter bodies loss energy by emission of radiation faster than they receive energy by absorption of radiation from the cooler bodies, and temperatures of hotter bodies decrease.

Wavelength of radiation Known electromagnetic radiations cover an enormous range of wavelengths, from the short cosmic rays to long wave broadcasting wave.

Although radiation of any wavelength is, in principle, convertible into heat on absorption by matter, the portion of the electromagnetic spectrum that is of importance in heat flow lies in the wavelength range between 0.5 and 50µm.

Visible light covers a wavelength range of about 0.38 to 0.78 µm At temperature above about 5000ºC heat radiation in the visible spectrum become significant. The higher the temperature of the radiating body, the shorter the predominant wavelength of the thermal radiation emitted by it.

Emissive power The monochromatic energy emitted by a radiating surface depends on the temperature of the surface and on the wavelength of the radiation.

At constant surface temperature, a curve can be plotted showing the rate of energy emission as a function of the wavelength.

The monochromatic radiation emitted in this manner from unit area in unit time, divided by the wavelength, is called the monochromatic radiating power Wλ.

For the entire spectrum of the radiation from a surface, the total radiating power W is the sum of all the monochromatic radiations from the surface, or , mathematically, (14-2)

Blackbody radiation A blackbody has the maximum attainable emissive power at any given temperature. The ratio of the total emissive power W of a body to that of a blackbody Wb is by definition the emissivity ε of the body, thus

Emissivities of solids Emissivity usually increases with temperature. Emissivities of polished metals are low, in the range 0.03 to 0.08. Emissivities of most oxidized metals range from 0.6 to 0.85, those of nonmetals from 0.65 to 0.95.

Practical source of blackbody radiation No actual substance is a blackbody, although some materials, such as certain grades of carbon black, do approach blackness.

Laws of blackbody radiation A basic relationship for blackbody radiation is the Stefan-Boltzmann law, which states that the total emissive power of a blackbody is proportional to the fourth power of the absolute temperature, or Wb=σT4 (14-3) Where σ is a universal constant

The distribution of energy in the spectrum of a blackbody is known accurately. It is given by Planck’s law (14-7) Where C1 and C2 are constant.

Planck’s law can be shown to be consistent with the Stefan-Boltzmann law by substituting Wb,λ from Eq(14-7) into Eq(14-2) and integrating.

Absorption of radiation by opaque solids Kirchhoff’s law at temperature equilibrium, the ratio of the total radiating power of any body to the absorptivity of that body depends only upon the temperature of the body.

Thus, consider any two bodies in temperature equilibrium with common surroundings. Kirchhoff’s law states that

If the first body is blackbody, α1=1, and Thus

By definition, the emissivity of the second body ε2 is

Thus, when any body is at temperature equilibrium with its surroundings, its emissivity and absorptivity are equal. Kirchholff’ law applies whether or not the two surfaces are at same temperature.

Radiation between surfaces The total radiation from a unit area of an opaque body of area A1, emissivity ε1, and absolute temperature T1 is (14-14)

Qualitatively, the interception of radiation from an area element of a surface by another surface of finite size can be visualized in terms of the angle of vision.

The equation for two bodies radiating each other can be written in the form (14-25) The factor F is called the view factor or angle factor; it depends upon the geometry of the two surface

If surface A1 is chosen for A, Eq(14-25) can be written (14-26) If surface A2 is chosen (14-27)

In general, for gray surfaces, Eq(14-26) and Eq(14-27) can be written F12 and F21 are the overall interchange factor and are functions of ε1 and ε2.

Two large parallel planes (14-39)

One gray surface completely surrounded by another (14-40)

Combined heat transfer by conduction- convection and radiation The total heat transfer from hot bodies to its surroundings is as follows Or

Where hr is a radiation heat transfer coefficient, Defined by