Flashcards
10 - Radiation heat transfer
Cards in this deck (33)
- 1What is radiation?Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
- 2What is different about radiation compared to conduction and convection?Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an intervening medium.
- 3Examples of thermal radiation- Solar energy: Solar thermal (cooker, heating, hot water…), Solar PV, Solar power station
- Furnace / combustion chamber designs
- Combined modes of heat transfer: convection + radiation - 4What can cold surface cause?Cold surfaces cause excessive heat loss from the body by radiation, and thus discomfort on the side of body.
- 5What is the emissive power for a black body?
- 6What is Stefan-Boltzmann constant when T is the absolute temperature [K]?
![What is Stefan-Boltzmann constant when T is the absolute temperature [K]?](https://examtimeassets.s3.amazonaws.com/uploads/media/image/23525214/desktop_cd6235c2-a584-4e21-aabb-6b10e877e92e.png)
- 7Graph of values of emissive power from black body at typical temperatures?
- 8What is the spectral emissive power/monochromatic emissive power black body?distribution of radiative flux with regard to the wavelength λ.
- 9Spectral emissive power is integrated over the full spectrum to give the emissive power:
- 10What is assumed as a black body?Radiation heat exchange between two parallel infinitely large plates that are assumed as a black body, and thermally insulated on the back.
- 11What happens when all thermal radiation leaves 1 object?All thermal radiation leaving object 1 arrives at object 2, and all thermal radiation arriving at object 1 comes from object 2.
- 12Equation of net rate of radiation heat transfer:
- 13What is a so-called black body? What does it absorb/reflects, so...The model for the perfect thermal radiator is a so-called black body. This is a body which absorbs all energy that reaches it and reflects or transmits nothing, so:
- 14How can radiation striking a body be absorbed/transmitted? What is it called? In general?
- 15For most liquid and solids in engineering, what can we say?For most solids and liquids in engineering the amount of radiation transmitted through the substance is negligible, so it is possible to write:
- 16Do total black bodies exist?No totally black body exists in practice, but many surfaces can be approximated to the definition due to the surface property or shape.
- 17Emissivity of a real body:A real body (non-black body) emits less emissive power than a black body at the same temperature, the ratio between them is called the emittance/emissivity of a real body:
- 18What is this emittance determined by?The emittance is determined entirely by the properties of the surface of a body and its temperature, independent of the surroundings of the body.
- 19What is the emittance dependent of?The emittance is generally dependent of wavelength λ, i.e., has a spectral distribution
- 20What is a grey body?One particular kind of surface behaviour is that for which the emittance is independent of wavelength λ. We call such a surface as a grey body.
- 21What does Kirchhoff’s law state?Kirchhoff’s law states that a body in thermodynamic equilibrium emits as much energy as it absorbs in each direction and at each wavelength.
- 22What is Kirchhoff’s law for a grey body?
- 23What is the proof for the Kirchhoff’s law for a grey body?
- 24What else does radiation heat exchange also need to consider?Radiation heat exchange between surfaces also needs to consider geometry configuration factors, i.e., view factors.
- 25Thermal radiation for 2 infinite plates?For two infinite plates, all thermal radiation leaving object 1 arrives at object 2, and all radiation arriving at object 1 comes from object 2,
- 26What occurs for radiation heat exchange between finite plat and an enclosure?For radiation heat exchange between a finite plate and an enclosure, all thermal radiation leaving object 1 arrives at object 2, but all radiation arriving at object 1 from object 2 is almost negligible, so
- 27Radiation heat exchange considering view factors: for example, between two black bodies:
- 28Proof for the above?
- 29For radiation heat exchange between a finite plate and an enclosure that are both a black body, for example, a roof and the sky,
- 30For radiation heat exchange between a roof as a grey body and the sky as a black body,
- 31Also, for the roof as a grey body, Kirchhoff’s law is applied, so... Therefore...
- 32Intensity of radiation? What happens when one moves away from a source? How much radiation does the sun emit?Intensity of radiation, W/m2 ·per solid angle (steradian)
When one moves away from a source, the intensity of radiation diminishes following the Inverse Square Law.
The Sun emits about 2.2 billion times the amount of radiation that is caught by the Earth. - 33What does irradiation refer to? Solar irradiance?Irradiation refers to radiation from all directions incident on a surface
Solar irradiance: solar energy irradiates on the earth
I, W/m2
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- Thermodynamics 2 Flashcards on 10 - Radiation heat transfer, created by Alice K on 03/12/2019.
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