In radiation, the transfer of heat from one body to another body requires no transmitting medium. There are several types of electromagnetic radiations such as cosmic rays, gamma rays, x- rays, ultraviolet rays, infrared rays, etc. However, in calculation of radiation heat transfer between two bodies only thermal radiation is considered. What could be the reason? and what would be the limitations?
Hi Pisipaty,
certainly, energy can be transferred between bodies by em radiation of any frequency. Which part of the spectrum is mainly involved depends on the nature of the bodies. If they are not devices specifically designed to emit radiation then - in the temperature range we usually deal with, say below 3000° K - nearly all of the energy radiated by them is located in the infrared domain, i. e. it's thermal radiation (see Plank's black-body radiation law).
Of course, this is not true if we deal with devices like microwave ovens or mobile phones. If you want to estimate the increase of temperature in your body caused by a mobile phone you have to take the frequency band used for transmission into account.
Hi Pisipaty,
certainly, energy can be transferred between bodies by em radiation of any frequency. Which part of the spectrum is mainly involved depends on the nature of the bodies. If they are not devices specifically designed to emit radiation then - in the temperature range we usually deal with, say below 3000° K - nearly all of the energy radiated by them is located in the infrared domain, i. e. it's thermal radiation (see Plank's black-body radiation law).
Of course, this is not true if we deal with devices like microwave ovens or mobile phones. If you want to estimate the increase of temperature in your body caused by a mobile phone you have to take the frequency band used for transmission into account.
Thank you very much, Joerg Fricke and Attallah for your answers. Equations are available for thermal radiation in the wave band of (10-1 to 102 microns) and are used to calculate emissive power. Are there any equations for calculating for other radiations like ultraviolet, infrared microwave etc.?
Can we have more answers on thermal radiation phenomena between two bodies?
Radiation is natural. Radiation can occur in the form of thermal, X-rays, cosmic rays, radio waves, ultraviolet rays etc. In studying radiation heat transfer between two bodies thermal radiation is considered for heat transfer for change in temperature effect which will exist in a particular wave band. The other radiations neglected.
One uses Stephan-Boltzmann type of radiation heat transfer equations to calculate the coefficient of heat transfer via radiation from a hot body and a cold body. In cases of photons that do not cause much heating (such as the lower wavelengths of visible light, UV and lower wavelengths), one can still calculate the individual photon energy and using the photon flux and the fraction of the flux caricatured by the second body as well as its absorption characteristics one can estimate the energy received (absorbed) by the second body.
Evaluation of heat transfer by radiation depends on temperature and thermal radiation is considered for design and performance of thermal equipment. Thermal radiation is considered occurs in a particular wave band.
Thermal radiation is one which involves temperatures which leads to radiation heat transfer in a particular wave band. Most of the design of radiation heat transfer equipment are based on thermal radiation and is also appropriate for study.
Thermal radiation is evaluated and assessed by difference in temperatures. This is got by the radiation emitting by a body on to the other body. The most important thing is, as and when the steady state is reached, the thermal radiation still continues.
One can sense or feel thermal radiation. This radiation will be getting in a particular wave band. As the emissive power is calculated based on the temperature, hence, thermal radiation is considered for radiation between two bodies.
Thermal radiation is a form of electromagnetic radiation which mostly depends on nature of surface and its temperature. They do not heat the space unless it is obstructed. The radiation phenomena does not cease with zero thermal potential between two bodies. The nature of heat rays as well as visible light rays are same the only difference lies in their wave length. Hence thermal radiation is considered.
The radiation will be carried in the form of electromagnetic waves at different wave lengths and frequency. All of the rays emit same radiation since one which is having high wave length will travel at low frequency and one at small wave length travel at high frequency. If a body is obstructed to the source, then thermal radiation is felt. As it is a function of temperature, thermal radiation is studied.
Thermal radiation is considered between two bodies participating in radiation, since radiation heat transfer is a function of temperature. This phenomena of this radiation is same as light rays but differes in wave length.
No idea, but few days ago, july 2019, in an hotel in Frankfurt, Germany, having double glass windows, and a generous blanket, I felt cold while sleeping, that dissapeared when I sent down the outer window curtain, supposedly being there just to block light in daytime. Heat radiation is more important than I guessed
Salut +
Thank you Joe Graymer for your nice and practical way of answering. I really liked.
To evaluate radiation heat transfer between two bodies, temperature is considered. The source is considered to be high temperature and the receiver can be at a very low temperature compared to source. Hence thermal radiation is taken.
Thermal radiation is easy to sense, involves temperature and behaviour is equivalent to that of light rays. Since it involves the high temperature flowing to a low temperature, radiation heat transfer dominates and hence calculated. It involves radiation emitting from a body and receiving by other body called absorption.
Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required. Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation.
Infrared radiation and thermal radiation are not same. They will be at different wave lengths. Thank you Ali Alhayany for your answer.
In dealing with radiation heat transfer, thermal radiation is considered since radiation properties are almost same as light rays, one can sense heat, involves temperatures and travels in a band of wave length.
Radiation from sun travels through space by means of electromagnetic waves to earth. The radiation felt is thermal radiation in a certain wave band. Since this involves temperatures in evaluation of emissive power, thermal radiation is considered.
When the temperature of one of the heat transferring surfaces is high, then radiation heat transfer becomes prominent as it is governed by the Stefan Boltzmann type of relation with fourth order temperature dependence. At lower temperatures radiation heat transfer is not that significant compared to convective heat transfer. Design of furnaces should take radiation heat transfer into account.
Thermal radiation involves temperature and occurs in a wave band of 10 to 100 microns. The equations are developed in this region considering energy and frequency. One can sense thermal radiation.
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