Heat Transfer and Thermodynamics

ОБУЧЕНИЕ ЧТЕНИЮ

И УСТНОЙ РЕЧИ

 НА АНГЛИЙСКОМ ЯЗЫКЕ

ПО СПЕЦИАЛЬНОСТИ

«ТЕПЛОФИЗИКА»

Учебное пособие

 

 

Москва

Издательство МГТУ имени Н.Э. Баумана

                                                                        2016

 

 

УДК - 802.0

ББК 81.  Англ. - 923

 

 

Издание доступно в электронном виде на портале ebooks. bmstu. ru                     

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Факультет «Лингвистика»

Кафедра «Английский язык для машиностроительных специальностей»

Рекомендовано Редакционно-издательским советом

МГТУ им. Н.Э. Баумана в качестве учебного пособия

 

Захарова С. С.

 Обучение чтению и устной речи на английском языке по специальности «Теплофизика»: учебное пособие / С.С. Захарова. – Москва: Издательство МГТУ им. Н.Э. Баумана, 2016. –        с.

ISBN 978-5-7038- -

Пособие «Обучение чтению и устной речи на английском языке по специальности «Теплофизика»Захаровой С. С., состоящее из трёх модульных блоков (Units), содержит современные оригинальные тексты на английском языке по изучаемой специальности, лексико-грамматические и коммуникативные задания и упражнения, позволяющие развить навыки чтения, аннотирования научно-технической литературы, а также профессионально-ориентированного общения и устной речи.   

Для студентов старших курсов, учащихся магистратуры и аспирантов, обучающихся по специальности «Теплофизика» на факультете «Энергомашиностроение» МГТУ им. Н.Э. Баумана.

                                                   УДК - 802.0

                                                        ББК 81.  Англ. - 923

                                                         © МГТУ им. Н.Э. Баумана, 2016

ISBN 978-5-7038- -    © Оформление. Издательство МГТУ им. Н.Э. Баумана, 2016

ПРЕДИСЛОВИЕ

 

 

        Учебное пособие «Обучение чтению и устной речи на английском языке по специальности «Теплофизика»Захаровой С. С., состоящее из трёх модульных блоков (Units), содержит учебные материалы: тексты из оригинальной научно-технической литературы на английском языке; словарные блоки, в которые включены термины и общетехническая лексика; лексико-грамматические упражнения, а также речевые упражнения, разговорные модели и клише, направленные на формирование у студентов навыков и умений вести профессионально-ориентированную беседу на английском языке и выступать с презентацией по соответствующей тематике.

    Целью работы является развитие и закрепление навыков чтения, говорения, аннотирования научно-технической литературы на английском языке по изучаемой специальности.

    Пособие предназначено для студентов старших курсов, учащихся магистратуры и аспирантов факультета «Энергомашиностроение» МГТУ им. Н.Э. Баумана, обучающихся по специальности «Теплофизика».

 

Unit 1

Heat Transfer and Thermodynamics

New words and word combinations to be memorized:

equilibrium n	равновесие
heat transfer	теплопередача
flow n	поток; расход
uninhabitable adj	необитаемый
tempt v	искушать
erroneous adj	ошибочный, неправильный
restrict v	ограничивать
primarily adv	главным образом
variable n	переменная (величина)
uniform adj	единообразный; однородный, постоянный
devise v	разрабатывать, придумывать
experience n,v	опыт; испытывать, знать по опыту
sole adj	единственный; исключительный
gross adj	общий
generalize v	обобщать, делать общие выводы
identify v	распознавать, устанавливать
quantify v	определять количество

Read the following nouns derived from Latin and Greek; mind the pronunciation in the singular and plural forms.

Hypothes is – hypothes es; bas is – bas es; dat um  – dat a; symbios is – symbios es; medi um – medi a; spectr um – spectr a; phenomen on – phenome na; ax is  –   ax es; analys is – analys es.

Translate the following words both as nouns and verbs.

Effect, contact, finish, pass, force, place, use, work, size, house, help, stop, cut, result, attempt, drive, range, design, form, experience, process, transport, transfer.

Text 1A.

Speak about the laws of thermodynamics, using the information obtained from Text 1A.

Read the following extracts with the Subjunctive Mood given in bold. Define the main idea of each extract. Could you continue the author’s idea? While speaking use as many sentences with the Subjunctive Mood as you can.

1. The average kinetic energy of the hot water particles gradually decreases; the average kinetic energy of the cold-water particles gradually increases; and eventually, thermal equilibrium could be reached at the point where the particles of the hot water and the cold water have the same average kinetic energy. At the macroscopic level, one would observe a decrease in temperature of the hot water and an increase in temperature of the cold water.

2. What would happen if the heat were transferred from hot water through glass to cold water? What would happen if the heat were transferred from hot water through Styrofoam to cold water? Answer: the rate of heat transfer would be different. Replacing the inner metal can (металлическая банка) with a glass jar would change the rate of heat transfer.

Read the text paying attention to the sentences with the Subjunctive Mood. Could you suggest your own heading to this text? Think of as many questions to the text as you can and let your fellow students answer them.

        

Translate into English.

1. Инженеры и учёные должны знать основные законы термодинамики. 2. Теплопередача – это процесс, с помощью которого происходит перемещение энергии. 3. С первого взгляда можно было бы предположить, что принципы теплопередачи можно вывести из основных законов термодинамики. 4. Поскольку поток тепла является результатом отсутствия температурного равновесия, его количественная обработка должна основываться на других отраслях науки. 5. Если мы будем неправильно использовать энергию, которая существует в настоящее время, то мир может стать необитаемым. 6. Теплопередача широко применяется в работе (действии) многочисленных устройств. 7. Первый закон термодинамики гласит, что энергию нельзя ни создать, ни разрушить, но можно только преобразовать из одного вида в другой. 8. Понимание процесса теплопередачи очень важно (crucial) для анализа термодинамических процессов, которые происходят в тепловых двигателях и тепловых насосах.9. Термодинамика – это область физики, которая связана с соотношением тепла и других свойств, таких как давление, плотность, температура и.т.д. в веществе (substance). 10. Теплопередача управляется (to guide) некоторыми основными принципами, которые стали известны как законы термодинамики.

 

Look through the text and say in what branches of engineering heat transfer processes can be found. Can you give your own examples of heat transfer processes in different branches of engineering? Share your opinion with your fellow students.

Text 1 B

Engineering Heat Transfer.

At one time or another every engineer is likely to be confronted with a heat transfer problem. In the design of computer circuits electrical engineers may be concerned with temperature variations owing to electrical heating; civil and mechanical engineers may need to assess the importance of thermal stresses and strains in the structural design of high-speed aircraft and nuclear reactors; and chemical engineers are often required to design chemical reactors that operate at temperatures high enough so that the reaction rate is reasonably fast, but low enough so that product degradation or reactor burnout is not a problem. Agricultural engineers are interested in the radiative heat transfer that often leads to frost formation when the ambient air temperature is above the freezing point, and the energy transport processes are associated with micro-meteorology. The ecologist is concerned with a variety of heat transfer processes such as the "greenhouse" effect caused by the increasing carbon dioxide concentration in our atmosphere.

To estimate the cost, the feasibility, and the size of equipment necessary to transfer a specified amount of heat in a given time, a detailed heat transfer analysis must be made. The dimensions of boilers, heaters, refrigerators, and heat exchangers depend not only on the amount of heat to be transmitted but also on the rate at which the heat is to be transferred under given conditions. The successful operation of equipment components such as turbine blades, or the walls of combustion chambers, depends on the possibility of cooling certain metal parts by continuously removing heat from a surface at a rapid rate. A heat transfer analysis must also be made in the design of electric machines, transformers, and bearings to avoid conditions that will cause overheating and damage the equipment. The listing in Table 1, which by no means is comprehensive, gives an indication of the extensive significance of heat transfer and its different practical applications. These examples show that almost every branch of engineering encounters heat transfer problems, which shows that they are not capable of solution by thermodynamic reasoning alone but require an analysis based on the science of heat transfer.

In heat transfer, as in other branches of engineering, the successful solution of a problem requires assumptions and idealizations. It is almost impossible to describe physical phenomena exactly, and in order to express a problem in the form of an equation that can be solved, it is necessary to make some approximations. In electrical circuit calculations, for example, it is usually assumed that the values of the resistances, capacitances, and inductances are independent of the current flowing through them. This assumption simplifies the analysis but may in certain cases severely limit the accuracy of the results.

 

TABLE 1Significance and diverse practical applications of heat transfer

Chemical, petrochemical, and process industry	Power generation and distribution	Aviation and space exploration	Electrical machines and electronic equipment:	Transportation	Comfort heating, ventilation, and air-conditioning
Heat exchangers, reactors, reboilers, etc.	Boilers, condensers, cooling towers, feed heaters, transformer cooling, transmission cable cooling, etc.	Gas turbine blade cooling, vehicle heat shields, rocket engine/nozzle cooling, space suits, space power generation, etc.	Cooling of motors, generators, computers and microelectronic devices, etc.	Engine cooling, automobile radiators, climate control, mobile food storage, etc.	Air conditioners, water heaters, furnaces, chillers, refrigerators, etc.

20. Write a summary of Text 1B. The following verbs in Passive and phrases are to help you to make a summary: are described; are summarized; are emphasized; are analysed; attention is given to…; a study of... was performed; it is concluded that….

Text 1C

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

 

Text 2 A

 Convective Heat Transfer

      Convective heat transfer, or simply, convection, is the study of heat transport processes effected by the flow of fluids. The very word convectionhas its roots in the Latin verb convecto-are, which means to bring together or to carry into one place. Convective heat transfer has grown to the status of a contemporary science because of our need to understand and predict how a fluid flow acts as a ‘‘carrier’’ or ‘‘conveyor belt’’ for energy and matter.

 Convective heat transfer, or convection, is the transfer of heat from one place to another by the movement of fluids, a process that is essentially the transfer of heat via mass transfer. Bulk motion of fluid enhances heat transfer in many physical situations, such as, for example, between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming. The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river. In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.

 Free, or natural, convection occurs when bulk fluid motions (streams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. Forcedconvection is a term used when the streams and currents in the fluid are induced by external means - such as fans, stirrers, and pumps - creating an artificially induced convection current.

 Convective heating or cooling in some circumstances may be described by Newton's law of cooling: "The rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings." However, by definition, the validity of Newton's law of cooling requires that the rate of heat loss from convection be a linear function of ("proportional to") the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case. In general, convection is not linearly dependent on temperature gradients, and in some cases is strongly nonlinear. In these cases, Newton's law does not apply.

 Convective heat transfer is clearly a field at the interface between two older fields: heat transfer and fluid mechanics. To study the interdisciplinary is valuable, but it must come after one possesses the disciplines, not the other way around. For this reason, the study of any convective heat transfer problem must rest on a solid understanding of basic heat transfer and fluid mechanics principles.

 It is worth reexamining the historic relationship between fluid mechanics and heat transfer. During the past 100 years, heat transfer and fluid mechanics have enjoyed a symbiotic relationship in their development, a relationship where one field was stimulated by the curiosity and advance in the other field. Examples of this symbiosis abound in the history of boundary layer theory and natural convection. The field of convection grew out of this symbiosis, and if we are to learn anything from history, important advances in convection will continue to result from this symbiosis. Thus, the student and the future researcher would be well advised to devote equal attention to fluid mechanics and heat transfer literature.

  The convection mode of heat transfer actually consists of two mechanisms operating simultaneously. The first is the energy transfer due to molecular motion, that is, the conductive mode. Superimposed upon this mode is energy transfer by the macroscopic motion of fluid parcels. The fluid motion is a result of parcels of fluid, each consisting of a large number of molecules, moving by virtue of an external force. This extraneous force may be due to a density gradient, as in natural convection, or due to a pressure difference generated by a pump or a fan, or possibly to a combinationof the two.

 The principal difference is that in forced convection the velocity far from the surface approaches the free-stream value imposed by an external force, whereas in natural convection the velocity at first increases with increasing distance from the heat transfer surface and then decreases. The reason for this behavior is that the action of viscosity diminishes rather rapidly with distance from the surface, while the density difference decreases more slowly. Eventually, however, the buoyant force also decreases as the fluid density approaches the value of the unheated surrounding fluid. This interaction of forces will cause the velocity to reach a maximum and then approach zero far from the heated surface. The temperature fields in natural and forced convection have similar shapes, and in both cases the heat transfer mechanism at the fluid-solid interface is conduction.

  Convection heat transfer depends on the density, viscosity, and velocity of the fluid as well as on its thermal properties (thermal conductivity and specific heat). Whereas in forced convection the velocity is usually imposed on the system by a pump or a fan and can be directly specified, in natural convection the velocity depends on the temperature difference between the surface and the fluid, the coefficient of thermal expansion of the fluid (which determines the density change per unit temperature difference), and the body force field, which in systems located on the earth is simply the gravitational force.

 The evaluation of the convection heat transfer coefficient is difficult because convection is a very complex phenomenon. It is sufficient to note that the numerical value of a system depends on the geometry of the surface, on the velocity as well as the physical properties of the fluid, and often even on the temperature difference. In view of the fact that these quantities are not necessarily constant over a surface, the convection heat transfer coefficient may also vary from point to point. For this reason, we must distinguish between a local and an average convection heat transfer coefficient. For most engineering applications, we are interested in average values.

4. Answer the questions:

1. What is convective heat transfer? 2. What enhances heat transfer in many physical situations? 3. What kind of process is known as advection? 4. When does free, or natural, convection occur? 5. When is the term ‘forced convection’ used? 6. What does Newton's law of cooling state? 7. What must the study of any convective heat transfer problem rest on? 8. What is the principal difference between forced convection and natural convection? 9. What does сonvection heat transfer depend on? 10. Why is the evaluation of the convection heat transfer coefficient difficult? 11. What does the numerical value of a system depend on?

Translate into English.

1. Существуют разные способы, которыми энергия может передаваться из одного места в другое. 2. Если теплопроводность очень низкая, может образоваться большой температурный градиент и конвекция могла бы усилиться. 3. Поскольку конвекция очень сложное явление, трудно математически вычислить (evaluate) коэффициент конвективной теплопередачи. 4.Изучение проблемы конвективной теплопередачи должно основываться на понимании основных принципов теплопередачи. 5. Стоит пересмотреть исторически сложившиеся взаимоотношения между механикойжидкостей (fluid mechanics) и теплопередачей. 6.Конвективная теплопередача зависит от плотности, вязкости и скорости жидкой среды, а также от тепловых характеристик. 7. Коэффициент конвективной теплопередачи (heat transfer coefficient) может меняться от точки к точке, учитывая тот факт, что геометрия поверхности, скорость, физические характеристики жидкой среды могут также меняться.

 

Look through the text and say what you know about Fourier’s law of heat conduction. What kind of materials are called insulators? Can you give your own examples of heat conduction processes in different branches of engineering? Share your opinion with your fellow students.

Text 2 B

Heat Conduction

 The most efficient method of heat transfer is conduction. This mode of heat transfer occurs when there is a temperature gradient across a body. In this case, the energy is transferred from a high temperature region to low temperature region due to random molecular motion (diffusion). Heat flows through a solid by a process that is called thermal diffusion, or simply diffusion or conduction. In this mode, heat is transferred through a complex submicroscopic mechanism in which atoms interact by elastic and inelastic collisions to propagate the energy from regions of higher to regions of lower temperature. From an engineering point of view there is no need to delve into the complexities of the molecular mechanisms, because the rate of heat propagation can be predicted by Fourier’s law, which incorporates the mechanistic features of the process into a physical property known as the thermal conductivity.

Conduction occurs similarly in liquids and gases. Althoughconduction occurs in liquids and gases, it is rarely the predominant transport mechanism in fluids — once heat begins to flow in a fluid, even if no external force is applied, density gradients are set up and convective currents are set in motion. In convection, thermal energy is thus transported on a macroscopic scale as well as on a microscopic scale, and convection currents are generally more effective in transporting heat than conduction alone, where the motion is limited to submicroscopic transport of energy.

Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions. In metals, a significant portion of the transported thermal energy is also carried by conduction-band electrons. Different materials have varying abilities to conduct heat. Materials that conduct heat poorly (wood, styrofoam- пенополистирол) are often called insulators. However, materials that conduct heat well (metals, glass, some plastics) have no special name.

The simplest conduction heat transfer can be described as “one-dimensional heat flow”. The rate of heat flow from one side of an object to the other, or between objects that touch, depends on the cross-sectional area of flow, the conductivity of the material and the temperature difference between the two surfaces or objects.

Mathematically, it can be expressed as

 

where q is the heat transfer rate in watts (W), k is the thermal conductivity of the material (W/m.K), A is the cross-sectional area of heat path, and is the temperature gradient in the direction of the flow (K/m).

The above equation is known as Fourier’s law of heat conduction. Therefore, the heat transfer rate by conduction through the object can be expressed as

 

where L is the conductor thickness (or length), ∆ T is the temperature difference between one side and the other (for example, ∆ T = T ₁ – T₂ is the temperature difference between side 1 and side 2). The quantity (∆T/L) in equation is called the temperature gradient: it tells how many 0C or K the temperature changes per unit of distance moved along the path of heat flow. The quantity L/kA is called the thermal resistance.

 

Thermal resistance has SI units of kelvins per watt (K/W). Notice that the thermal resistance depends on the nature of the material (thermal conductivity k and geometry of the body d/A). From the above equations we realize the heat transfer rate as a flow, and the combination of thermal conductivity, thickness of material and area as a resistance to this flow.

Considering the temperature as a potential function of the heat flow, the Fourier law can be written as

 

If we define the resistance as the ratio of potential to the corresponding transfer rate, the thermal resistance for conduction can be expressed as

 

It is clear from the above equation that decreasing the thickness or increasing the cross-sectional area or thermal conductivity of an object will decrease its thermal resistance and increase its heat transfer rate.

Conduction heat transfer can readily be modeled and described mathematically. The associated governing physical relations are partialdifferential equations, which are susceptible to solution by classical methods. Famous mathematicians, including Laplace and Fourier, spent part of their lives seeking and tabulating useful solutions to heat conduction problems. However, the analytic approach to conduction is limited to relatively simple geometric shapes and to boundary conditions that can only approximatethe situation in realistic engineering problems. With the advent of the high-speed computer, the situation changed dramatically and a revolution occurred in the field of conduction heat transfer. The computer made it possible to solve, with relative ease, complex problems that closely approximate real conditions.

As a result, the analytic approach has nearly disappeared from the engineering scene. The analytic approach is nevertheless important as background.

Text 2 C

Heat Transfer by Radiation

 

Radiation heat transfer differs from that by convection and conduction because the driving potential is not the temperature, but the absolute temperature raised to the fourth power. Furthermore, heat can be transported by radiation without an intervening medium.

Radiation is the process of transferring heat by emitting electromagnetic energy in the form of waves or particles. Radiation can transfer heat through empty space, while the other two methods require some form of matter-on-matter contact for the transfer.

Radiant heat is simply heat energy in transit as electromagnetic radiation. All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. In this case, heat moves through space as an electromagnetic radiation without the assistance of a physical substance. All objects that contain heat emit some level of radiant energy. The amount of radiation is inversely proportional to its wavelength (the shorter the wavelength the greater the energy content) which is, in turn, inversely proportional to its temperature (in °K).

When a body is placed in an enclosure whose walls are at a temperature below that of the body, the temperature of the body will decrease even if the enclosure is evacuated. The process by which heat is transferred from a body by virtue of its temperature, without the aid of any intervening medium, is called thermal radiation.

The physical mechanism of radiation is not completely understood yet. Radiant energy is envisioned sometimes as transported by electromagnetic waves, at other times as transported by photons. Neither viewpoint completely describes the nature of all observed phenomena. It is known, however, that radiation travels with the speed of light c, equal to about in a vacuum.

From the viewpoint of electromagnetic theory, the waves travel at the speed of light, while from the quantum point of view, energy is transported by photons that travel at that speed.Although all the photons have the same velocity, there is always a distribution of energy among them.

Radiation phenomena are usually classified by their characteristic wavelength. Electromagnetic phenomenon encompasses many types of radiation, from short-wavelength gamma-rays and x-rays to long-wavelength radio waves. The wavelength of radiation depends on how the radiation is produced. For example, a metal bombarded by high-frequency electrons emits x-rays, while certain crystals can be excited to emit long-wavelength radio waves. Thermal radiationis defined as radiant energy emitted by a medium by virtue of its temperature. In other words, the emission of thermal radiation is governed by the temperature of the emitting body.

The Sun’s heat is an example of thermal radiation that reaches the Earth. Radiative heat is transferred directly into the surface of any solid object it hits (unless it is highly reflective), but passes readily through transparent materials such as air and glass. An ideal thermal radiator or a blackbody, will emit energy at a rate proportional to the forth power of its absolute temperature and its surface area. Mathematically, that is

 

 

where σ is a proportionality constant (Stefan-Boltzmann constant = 5.669 × 10^-8 W/m².K⁴). The above equation is called the Stefan-Boltzmann law of thermal radiation and it applies only to the blackbodies. The fourth-power temperature dependence implies that the power emitted is very sensitive to temperature changes. If the absolute temperature of a body doubles, the energy emitted increases by a factor of 2⁴ = 16.

For bodies not behaving as a blackbody a factor known as emissivity e, which relates the radiation of a surface to that of an ideal black surface is introduced. The equation becomes

 

 

The emissivity ranges from 0 to 1; e = 1 for a perfect radiator and absorber (a blackbody) and e = 0 for a perfect radiator. Human skin, for example, no matter what the pigmentation, has an emissivity of about 0.97 in the infrared part of the spectrum. While a polished aluminum has an emissivity of about 0.05.

Thermal radiation from a body is used as a diagnostic tool in medicine. A thermogram shows whether one area is radiating more heat than it should, indicating a higher temperature due to abnormal cellular activity. Thermography or thermovision in medicine is based on the natural thermal radiation of the skin. Most advantage is the radiance free of the measuring principle.

Certain body regions have different temperature levels. If one exposes the body e.g. to a cooling attraction, then the body zones of the skin react, in order to repair the heat balance of the body. Thereby the thermal regulation of diseased body regions and organs is different to healthy one. The so-called "regulation thermography" is based on this principle.

Thermal radiation always encompasses a range of wavelengths. The amount of radiation emitted per unit wavelength is called monochromatic radiation; it varies with wavelength, and the word “spectral»is used to denote this dependence. The spectral distribution depends on the temperature and the surface characteristics of theemitting body. The sun, with an effective surface temperature of about 5800 K(10,400°R), emits most of its energy below 3 µm, whereas the earth, at a temperatureof about 290 K (520°R), emits over 99% of its radiation at wavelengths longerthan 3µm. The difference in the spectral ranges warms a greenhouse inside evenwhen the outside air is cool because glass permits radiation at the wavelength of thesun to pass, but it is almost opaqueto radiation in the wavelength range emitted bythe interior of the greenhouse. Thus, most of the solar energy that enters the greenhouseis trapped inside. In recent years, the combustion of fossil fuels has increased the amount of carbon dioxide in the atmosphere. Since carbon dioxide absorbs radiation in the solar spectrum, less energy escapes. This causes global warming, which is also called the “greenhouse effect.”

Consequently, the integration of radiation heat transfer into an overall thermal analysis presents considerable challenges, including the need for carefully stated boundary conditions and assumptions necessary for the appropriate inclusion in the thermal circuit of a system.

Unit 3.

Heat Exchangers

  New words and word combinations to be memorized:

 

shell-and-tube (heat) exchanger	кожухо-трубный теплообменник (с комбинацией параллельного и перекрёстного токов)
double-pipe exchanger	двухтрубный теплообменник
stacked-plate structure	уложенная рядами пластинчатая структура
plate-fin exchanger	пластинчатый ребристый теплообменник
plate-and-frame exchanger	пластинчатый каркасный теплообменник
conspicuous adj	видный, заметный; обращающий на себя внимание
boiler feed-water heater	водонагреватель котла
gas-fired hot water heater	газовый водонагреватель
condenser coil	охлаждающий змеевик
energy payback	окупаемость энергетических затрат
counter flow	противоток, обратное течение
cross flow	перекрёстный ток; поперечный ток
extruded	прессованный
the log (logarithmic)  mean temperature difference (LMTD)	логарифмическая средняя разность температур
fin n	тех. ребро, ребристый выступ
contact heat exchanger	смесительный теплообменник
core n	сердцевина; внутренность; ядро; тех. стержень
immiscible adj	не поддающийся смешению, несмешивающийся
annular adj	кольцеобразный, кольцевой
inlet header	впускной коллектор
doughnut baffle	кольцеобразная отражательная перегородка
orifice n	отверстие, сопло, насадка
tube bundle	пучок труб

 

Text 3A

Heat Exchangers

Heat exchangers are generally devices or systems in which heat is transferred from one flowing fluid to another. The fluids may be liquids or gases, and in some heat exchangers more than two fluids might flow. These devices may have a tubular structure, of which the double - pipe and shell - and - tube exchangers are perhaps the most prevalent, or a stacked - plate structure, which includes the plate - fin and plate - and - frame exchangers, among some other configurations. Perhaps the most conspicuous and historically the oldest applications can be found in a power plant. The steam generator or boiler, water - cooled steam condenser, boiler feed - water heater, and combustion air regenerator, as well as several other types of equipment are all heat exchangers. In most homes, common heat exchangers are the gas - fired hot water heater, and the evaporatorand condenser coils of a central air - conditioning unit. All automobiles have a radiator and oil cooler, along with a few other heat exchangers.

When a heat exchanger is placed into a thermal transfer system, a temperature drop is required to transfer the heat. The magnitude of this temperature drop can be decreased by utilizing a larger heat exchanger, but this will increase the cost of the heat exchanger. Economic considerations are important in engineering design, and in a complete engineering design of heat exchange equipment, not only the thermal performance characteristics but also the pumping power requirements and the economics of the system are important. The role has taken an increasing importance recently as engineers have become energy conscious and want to optimize designs not only in terms of a thermal analysis and economic return on the investment but also in terms ofthe energy payback of a system. Thus economics, as well as such considerations as the availability and amount of energy and raw materials necessary to accomplish a given task, should be considered.

 A heat exchanger is a device in which heat is transferred between a warmer and a colder substance, usually fluids. Common types of heat exchanger flows include parallel flow, counter flow, and cross flow. In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions; and in cross flow, the fluids move at right angles to each other. Common constructions for heat exchangers include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate.

When engineers calculate the theoretical heat transfer in a heat exchanger, they must contend with the fact that the driving temperature difference between the two fluids varies with position. To account for this in simple systems, the log mean temperature difference (LMTD) is often used as an "average" temperature. In more complex systems, direct knowledge of the LMTD is not available, and the number of transfer units (NTU) method can be used instead.

There are three basic types of heat exchangers:recuperators, regenerators and direct contact heat exchangers.

In arecuperator the hot and cold fluids are separated by a wall and heat is transferred by a combination of convection to and from the wall and conduction through the wall. The wall can include extended surfaces, such as fins, or other heat transfer enhancement devices.

In a regenerator the hot and cold fluids alternately occupy the same space in the exchanger core. The exchanger core or “matrix” serves as a heat storage device that is periodically heated by the warmer of the two fluids and then transfers heat to the colder fluid. In a fixed matrixconfiguration, the hot and cold fluids pass alternately through a stationary exchanger, and for continuous operation two or more matrices are necessary. Another approach is the rotary regeneratorin which a circular matrix rotates and alternately exposes a portion of its surface to the hot and then to the cold fluid.

In a direct contact heat exchanger the hot and cold fluids contact each other directly. An example of such a device is a cooling tower in which a spray of water falling from the top of the tower is directly contacted and cooled by a stream of air flowing upward. Other direct contact systems use immiscible liquids or solid-to-gas exchange. The Direct contact heat exchanger is used to transfer heat between the molten salt and air. The direct contact approach is still in the research and development stage.

The simplest arrangement of this type of heat exchanger consists of a tube within a tube. Such an arrangement can be operated either in counterflow or in parallel flow, with either the hot or the cold fluid passing through the annular space and the other fluid passing through the inside of the inner pipe.

A more common type of heat exchanger that is widely used in the chemical and process industry is the shell-and-tube arrangement. In this type of heat exchanger one fluid flows inside the tubes while the other fluid is forced through the shell and over the outside of the tubes. The fluid is forced to flow over the tubes rather than along the tubes because a higher heat transfer coefficient can be achieved in cross-flow than in flow parallel to the tubes. To achieve cross-flow on the shell side, baffles are placed inside the shell. These baffles ensure that the flow passes across the tubes in each section, flowing downward in the first, upward in the second, and so on. Depending on the header arrangements at the two ends of the heat exchanger, one or more tube passes can be achieved. For a two-tube-pass arrangement, the inlet header is split so that the fluid flowing into the tubes passes through half of the tubes in one direction, then turns around and returns through the other half of the tubes to where it started. Three- and four-tube passes can be achieved by rearrangement of the header space. There are three types of baffles used in shell-and-tube heat exchangers: orifice baffle; disk-and-doughnut baffle and segmental baffle. A variety of baffles have been used in industry, but the most common kind is the disk-and-doughnut baffle.

The shell-and-tube heat exchanger has fixed tube sheetsat each end, and the tubes are welded or expanded into the sheets. This type ofconstruction has the lowest initial cost but can be used only for small temperaturedifferences between the hot and the cold fluids because no provision is made to prevent thermal stresses due to the differential expansion between the tubes and the shell. Another disadvantage is that the tube bundle cannot be removed for cleaning.

These drawbacks can be overcome by modification of the basic design. In the improved arrangement one tube sheet is fixed but the other is bolted to a floating-head cover that permits the tube bundle to move relative to the shell. The floating tube sheet is clamped between the floating headand a flange so that it is possible to remove the tube bundle for cleaning. This heat exchanger has one shell pass and two tube passes.

In the design and selection of a shell - and - tube heat exchanger, the power requirement and the initial cost of the unit must be considered.

In gas heating or cooling it is often convenient to use a cross-flow heat exchanger. The cross-flow heat exchanger is widely used in the heating, ventilating, and air-conditioning industry. In such a heat exchanger, one of the fluids passes through the tubes while the gaseous fluid is forced across the tube bundle. The flow of the exterior fluid may be forced by natural convection.

In this type of exchanger the gas flowing across the tube is considered to be mixed, whereas the fluid in the tube is considered to be unmixed. The exterior gas flow is mixed because it can move about freely between the tubes as it exchanges heat, whereas the fluid within the tubes is confined and cannot mix with any other stream during the heat exchange process. The mixed flow implies that all of the fluid in any given plane normal to the flow has the same temperature. The unmixed flow implies that although temperature differences within the fluid may exist in at least one direction normal to the flow, no heat transfer results from this gradient.

In the design of heat exchangers it is important to specify whether the fluids are mixed or unmixed, and which of the fluids is mixed. It is also important to balance the temperature drop by obtaining approximately equal heat transfer coefficients on the exterior and interior of the tubes. If this is not done, one of the thermal resistances may be unduly large and cause an unnecessarily high overall temperature drop for a given rate of heat transfer, which in turn demands larger equipment and results in poor economics.

 

5. Answer the questions:

1. What kind of device is a heat exchanger? 2. Are economic considerations important in a complete engineering design of heat exchange equipment and why? 3. Where can the oldest applications of heat exchangers be found? 4. What common heat exchangers are used in most homes? 5. Why has the role of heat exchangers increased recently? 6. What basic types of heat exchangers are mentioned in the text? 7. What more common type of heat exchanger is widely used in the chemical and process industry? 8. How many types of baffles are used in shell-and-tube heat exchangers? 9. Where is it convenient to use a cross-flow heat exchanger?

Speak about various types of heat exchangers and describe the improved modification of the shell-and-tube heat exchanger using the information from Text 3A or your own information; words and word combinations from the box are to help you.

Energy payback of a system, economic return on the investment, to optimize designs, increasing importance, the most conspicuous applications, recuperators, regenerators, immiscible liquids, annular space, shell-and-tube arrangement, inlet header, a variety of baffles, gaseous fluid, in turn, the lowest initial cost, tube bundle, floating-head cover, heat transfer enhancement devices, drawbacks, basic design, improved arrangement, flange, shell pass.

Text 3B

Make a presentation in Power Point. You may use the information from Texts 3A, 3B and get the information from the Internet or from any other source to characterize various types of heat exchangers, their thermal analysis, structural and geometry features. Prove the diversity of practical applications of heat exchangers. You may choose any heat exchanger for a detailed analysis as well.

Read Text 3С to find out the description of different types of heat sinks, as well as scientific principles that explain how they work. Say in what branch of engineering the heat transfer phenomenon can be applied.

Text 3С

How Heat Sinks Work

Though the term heat sink¹ probably isn't one most people think of when they hear the word computer, it should be. Without heat sinks, modern computers couldn't run at the speeds they do. Heat sinks cool down your computer's processor after it runs multiple programs at once. And without a quality heat sink, your computer processor is at risk of overheating, which could destroy your entire system, costing you hundreds, even thousands of dollars.

But what exactly is a heat sink and how does it work? Simply put, a heat sink is an object that disperses heat from another object. They are most commonly used in computers, but are also found in cell phones, DVD players and even refrigerators. In computers, a heat sink is an attachment for a chip that prevents the chip from overheating and, in modern computers, it is as important as any other component.

If you are not very tech-savvy² think of the heat sink like a car radiator. The same way a radiator draws heat away from your car's engine, a heat sink draws heat away from your computer's central processing unit (CPU). The heat sink has a thermal conductor that carries heat away from the CPU into fins that provide a large surface area for the heat to dissipate throughout the rest of the computer, thus cooling both the heat sink and processor. Both a heat sink and a radiator require airflow and, therefore, both have fans built in.

Before the 1990s, heat sinks were usually only necessary in large computers where the heat from the processor was a problem. But with the introduction of faster processors, heat sinks became essential in almost every computer because they tended to overheat without the aid of a cooling mechanism.

Heat can be transferred in three different ways: convection, radiation and conduction. Conduction is the way heat is transferred in a solid, and therefore is the way it is transferred in a heat sink. Conduction occurs when two objects with different temperatures come into contact with one another. At the point where the two objects meet, the faster moving molecules of the warmer object crash into the slower moving molecules of the cooler object. When this happens, the faster moving molecules from the warmer object give energy to the slower moving molecules, which in turn heats the cooler object. This process is known as thermal conductivity, which is how heat sinks transfer heat away from the computer's processor.

Heat sinks are usually made of metal, which serves as the thermal conductor that carries heat away from the CPU. However, there are pros and cons³ to using every type of metal. First, each metal has a different level of thermal conductivity. The higher the thermal conductivity of the metal, the more efficient it is at transferring heat.

One of the most common metals used in heat sinks is aluminum. Aluminum has a thermal conductivity of 235 watts per Kelvin per meter (W/mK). The thermal conductivity number, in this case 235, refers to the metal's ability to conduct heat. Simply put, the higher the thermal conductivity number of a metal, the more heat that metal can conduct. Aluminum is also cheap to produce and is lightweight. When a heat sink is attached, its weight puts a certain level of stress on the motherboard.

One of the best and most common materials used to make heat sinks is copper. Copper has a very high thermal conductivity of 400 W/mK. It is, however, heavier than aluminum and more expensive. But for operating systems that require an extensive amount of heat dissipation, copper is frequently used.

So where does the heat go once it has been conducted from the processor through the heat sink? A fan inside the computer moves air across the heat sink and out the computer. Most computers also have an additional fan installed directly above the heat sink to help properly cool the processor. Heat sinks with these additional fans are called active heat sinks, while those with the single fan are called passive heat sinks. The most common fan is the case fan, which draws cool air from outside the computer and blows it through the computer,expelling the hot air out of the rear.

________________________________

¹ Heat sink¹ – теплоприёмник

² Tech - savvy – зд. технически подкованный, грамотный.

³ Pros and cons – доводы за и против.

Round-table discussion. Speak about the main reasons for using different types of heat sinks in computer engineering. Say whether any type of heat sinks described is used in your computer. Is it worth having a heat sink in personal computers and why?

Play the part of a computer specialist or heat transfer specialist to prove the necessity of using heat sinks. Use the following words and phrases:

As some of you know I am a specialist in charge of computer maintenance. I am here in my function as a heat transfer specialist. The general opinion is that heat sinks are…. From what I know of heat sinks…. Heat sink seems to be a kind of….   My process of reasoning is like this. Today’s topic is of particular interest to those of you who…. I’d like to focus your attention on….I think you’ll be surprised to see that…. Let me point out that…. My talk is particularly relevant to those of us who…. Let’s look more closely at…. Let’s move on/turn to…. Before we go on, let me clarify one point. Today I’d like to give you an overview of…. I’d like to share an amazing fact/figure with you. In view of all these details…. Before I go on, let me summarize the key issues. I’d like to stress/highlight/emphasize the following points. Before I stop, let me go over the key issues again. That’s all I wanted to say about…. To sum up (then), we…. This brings me to the end of my report.

Supplement.

I. To be done after Unit 1.

DIALOGUE 1

Mr Clark, a British scientist, is talking to Oleg Smirnov, his Russian counterpart (коллега), at an international conference during a break.

Clark: Your recent experiments have been a great success, Mr. Smirnov. Congratulations!

Smirnov: Thank you very much. You’ve read my last article, then, haven’t you?

C: Of course I have. I’m very interested in your research, and I hardly ever miss your publications. By the way, when are you going to give a talk on your work?

S. Some time next