Mechanisms of Heat Transfer.

    Heat is a form of energy that can cross the boundary of a system. Heat can, therefore, be defined as ‘the form of energy that is transferred between a system and its surrounding as a result of a temperature difference.’ There can only be a transfer of energy across the boundary in the form of heat if there is a temperature difference between the system and its surroundings. Conversely, if the system and surroundings are at the same temperature there is no heat transfer across the boundary.

  The term ‘heat” is a name given to the particular form of energy crossing the boundary. However, heat is more usually referred to in thermodynamics through the term ‘heat transfer’, which is consistent with the ability of heat to raise or lower the energy within a system.

  The literature of heat transfer generally recognizes three distinct modes of heat transmission: conduction, radiation, and convection. Strictly speaking, only conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of the three, convection, does not strictly comply with the definition of heat transfer because its operation also depends on mechanical mass transport. But since convection also accomplishes transmission of energy from regions of higher temperature to regions of lower temperature, the term “heat transfer by convection” has become generally accepted.

  All three are different. Convection relies on movement of a fluid. Conduction relies on transfer of energy between molecules within a solid or fluid. Radiation is a form of electromagnetic energy transmission and is independent of any substance between the emitter and receiver of such energy. However, all three modes of heat transfer rely on temperature difference for the transfer of energy to take place.

 The greater the temperature difference the more rapidly will the heat be transferred. Conversely, the lower the temperature difference, the slower will be the rate at which heat is transferred.When discussing the modes of heat transfer it is the rate of heat transfer Q that defines the characteristics rather than the quantity of heat. Although three modes of heat transfer may be combined in any particular thermodynamic situation, the three are quite different and are generally introduced separately.

Put questions to Text 1C and answer them.

Translate the following sentences paying attention to the Emphatic construction.

1. It is the temperature difference between the two neighboring objects that causes this heat transfer. 2. It is the fraction of solar radiation that is reflected by the surface and not absorbed by it. 3. It is in this scientific journal that you will find necessary information on heat transfer. 4. It is the particles of solid that vibrate more vigorously about their fixed positions. 5. It is the particles of liquid and gas that move about their container more rapidly. 6. It is for an ideal gas that the heat transfer rate is proportional to the average molecular velocity and the mean free path (средняя длина пути свободного пробега (молекулы). 7. It is the radiation that is in the infrared region of the electromagnetic spectrum.

 

Unit 2

Convective Heat Transfer

New words and word combinations to be memorized:

effect v, n	воздействовать, влиять; воздействие, влияние
bulk n	объём; большие размеры; масса; большая часть чего-л.
enhance v	повышать; увеличивать, усиливать
buoyancy n	плавучесть; способность держаться на поверхности воды
induce v	вызывать; cтимулировать
stirrer n	устройство для перемешивания, мешалка
fluid n	жидкость, жидкая или газообразная среда
linear adj	линейный
abound v	изобиловать; иметься в большом количестве
layer n	слой
impose v	зд. воздействовать
specify v	точно определять, устанавливать
parcel n	зд. сгусток
extraneous adj	внешний
diminish v	уменьшать(ся); убавлять(ся)

 1. Match the words in A with their definitions in B.

A	B
1. gradient	(a) a substance that can flow freely, as gases and liquids do
2. conduction	(b) the relationship between two different living creatures that live close together and depend on each other in particular ways
3. process	(c) a substance or an object that is solid, not a liquid or a gas
4. motion	(d) (technical) the rate at which temperature, pressure, etc. changes, or increases and decreases, between one region and another
5. flow	(e) an area within which the force mentioned has an effect
6. solid	(f) a series of things that happen, especially ones that result in natural changes
7. liquid	(g) the process by which heat or electricity passes through a material
8. сonvection	(h) the steady and continuous movement of sth in one direction
9. field	(i) the act or process of moving or the way sth moves
10. density	(j) a substance that flows freely and is not a solid or a gas, for example water or oil
11. symbiosis	(k) the process in which heat moves through a gas or a liquid as the hotter part rises and the cooler, heavier part sinks.
12. fluid	(l) the thickness of a solid, liquid or gas measured by its mass (=weight) per unit of volume