Text b: the thermal conductivity

I. Прочитайте текст без словаря, перескажите его, давая при этом ответы на следующие вопросы:

What does the value of the thermal conductivity depend upon? What does the thermal conductivity vary with? Why does the author say, that the thermal conductivity is analogous to viscosity? Why are good heat conductors also good electric conductors?

II. Используйте следующие слова при пересказе:

Value То store Free electron transport

Energy exchange Specific heat Linearly

To impart Lattice vibration Average value

To transport Mass density Specimen

TEXT В

1. The thermal conductivity is analogous to viscosity, since its value depends upon the energy exchange between molecules in motion. Faster-moving molecules impart some of their energy to slower-moving ones in the collision process. An increase in temperature increases molecular motion, transferring energy from regions of higher temperature to regions of lower temperature. Thermal conductivity varies with temperature and pressure, being much more sensitive to temperature than pressure. For engineering purposes it is independent of pressure in solids, liquids, and most gases below the critical pressure.

2. It is frequently convenient to use the ratio of a material's ability to transport energy to its capacity to store energy. This is the thermal diffusivity, defined as, x=k/ А с where p is the mass density of the material and с is is its specific heat.

3. The energy transfer in solids is by lattice vibration and by free-electron transport. Since in metals, free-electron transport is more prominent than lattice vibration, good heat conductors are also good electric conductors.

4. For many materials thermal conductivity varies linearly with temperature, i.e., k=k₀ (1+aT), where k₀ is the value at zero temperature and a is a constant which depends upon the material. For such materials it is convenient to use an average value of thermal conductivity in making calculations of heat transfer.

5. Thermal conductivity can be measured in a variety of ways, all of which depend upon the observation of a temperature gradient across a specimen conducting a known amount of heat.

 

ТЕКСТ В

1. Теплопроводность аналогична вязкости, так как его величина зависит от обмена энергией между молекулами в движении. Молекулы, движущиеся быстрее, передают часть своей энергии более медленным из них в процессе столкновения. Повышение температуры увеличивает молекулярное движение, передавая энергию из областей с более высокой температурой к области с более низкой температурой. Теплопроводность зависит от температуры и давления, также более чувствительна к температуре, чем давлению. При решениях инженерных задач не зависит от давления в твердых телах, жидкостях и большинстве газов ниже критического давления.

2. Часто в роли определения удобно использовать отношение способности материала передачи энергии к её способности накапливать энергию. Теплопроводность, определяется как x=k/А*c, где “р” - плотность массы материала и “с” - его удельная теплоемкость.

3. Передача энергии в твердых телах происходит при помощи колебаний решетки и свободных электронов. Так как в металлах, свободный перенос электронов более заметен, чем колебание решетки, хорошие проводники тепла, также являются хорошими проводниками электричества.

4. Для многих материалов теплопроводность линейно зависит от температуры, т.е. к = k0 (1 + аТ), где k0 - значение при нулевой температуре и является константой, которая зависит от материала. Для таких материалов удобно использовать среднее значение теплопроводности при проведении расчетов теплообмена.

5. Теплопроводность может быть измерена различными способами, каждый из которых зависит от температурного градиента, зависящего от проводящей способности, выражающейся в количестве тепла.

UNIT IV

TEXT A: HEAT TRANSFER (INTRODUCTORY TEXT)

 

Термины, слова whereby - посредством чего rate of heat transfer - скорость

теплопередачи feature - особенность thermal motion - тепловое движение energy transport - перенос энергии surrounding space - окружающее пространство

и словосочетания

radiant emission - теплоизлучение relative motion - относительное

движении to include - включать в себя to employ - использовать mastery - совершенное владение therefore - поэтому

 

1. Прочитайте текст, найдите определения конвекции и теплоизлучения.

2. Сделайте письменный перевод 2-х последних предложений второго абзаца.

3. Вспомните возможные значения глагола "to involve" и дайте перевод причастия "involving" (абз.З).

4. Из приведенных ниже значений слова "technique" (абз.З) выберите наиболее подходящее для данного текста:

техника; совокупность технических приемов; технология; методика; метод, способ, процедура.

TEXT А

1. The study of heat transfer includes the physical processes whereby thermal energy is transferred as a result of difference or gradients of temperature.

The information generally desired is the way in which the rate of heat transfer depends upon the various features of the process.

2. There are two basically different processes whereby thermal energy is transported: conduction and radiation. Energy is conducted through a material in which a temperature gradient exists by the thermal motion of various of the microscopic particles of which the material is composed; energy is diffused through the material by these thermal motions. Radiation is an energy transport from material into the surrounding space by electromagnetic waves. Radiant emission is also due to the thermal motion of microscopic particles but the energy is transmitted electromagnetically. If conduction occurs in a fluid in motion, the diffusion of thermal energy will be affected by the relative motion within the fluid. Conduction processes affected by relative motion are called convection processes.

3. Since the field of heat transfer includes processes involving thermal diffusion, electromagnetic radiation, and fluid motion, the study includes theories from many branches of science and employs many different types of analytic techniques. Therefore the study of heat transfer requires the mastery of many concepts and methods of analysis.

TEXT B: HEAT CONDUCTION AND THERMAL CONDUCTIVITY Термины, слова и словосочетания

 

rate - скорость, степень, величина,

расход, производительность directly proportional - прямо

пропорционально Inversely proportional - обратно

пропорционально thermal conductivity - коэффициент

теплопроводности face - грань, сторона, поверхность unit cube - элементарный куб to maintain - поддерживать transport property - характеристики

переноса wide range - широкий диапазон to encounter - встречать psia/pound per square inch absolute - абсолютное давление в фунтах на кв. дюйм ratio - соотношение, пропорция, коэффициент

1. Прочитайте текст и ответьте

negligible - очень малый,

незначительный conduction characteristics -

характеристики теплопроводности grain - фибра, волокно prediction - расчет, прогнозирование random motion - беспорядочное

движение similar - сходный, подобный net energy - полезная мощность, мощность нетто, эффективная мощность space density- пространственная

плотность diffusion of momentum - рассеивание импульса to store - хранить, накапливать monatomic - одноатомный diatomic - двухатомный

следующие вопросы:

1. What does the rate of heat conduction depend upon? 2. What definition of thermal conductivity is given in this text? 3. In what units is thermal conductivity expressed? 4. How is heat conducted in gases?

2. Обратите внимание на перевод причастного оборота "followed by good insulators" (3 абз.). Почему невозможен перевод на русский язык посредством определительного причастного оборота?

3. Напишите резюме текста.

TEXT В

1. The rate of heat conduction through a solid material (of fluid without relative motion) is proportional to the temperature difference across the material and to the area perpendicular to heat flow and inversely proportional to the length of the path of heat flow between the two temperature levels. This dependence was established by Furier and is analogous to the relation for the conduction of electricity, called Ohm's law. The constant of proportionality in Fourier's low, denoted by K, is called thermal conductivity and is a property of the conducting material and of its state.

2. The thermal conductivity is analogous to electric conductivity. It is equivalent to the rate of heat transfer between opposite faces of a unit cube of the material which are maintained at temperatures differing by one degree. In engineering units in the English system, k is pressed in Btu/h ft2 F/ft =Btu/h ft F.

In metric units, k may be expressed as cal/sec cm°C or watts/cm°C.

3. The transport property, thermal conductivity, varies over a wide range for the various substances commonly encountered. For example, for air at 14,7 psia and 60°F it is 0,015 and for silver it is 240 in English units. This is a ratio of 1:16000. Gases generally have the lowest thermal conductivities, followed by good insulators, nonmetallic liquids, nonmetallic solids, liquid metals, metal alloys, and, finally, the best conductors, pure metals.

4. Thermal conductivity for a given material depends upon its state and may vary with temperature, pressure, and ets. For moderate pressure levels the effect of pressure is small. However for many substances the effect of temperature upon К is not negligible.

5. Many materials have different conduction characteristics in different directions. For example, wood and other fibrous materials have higher thermal conductivities parallel to the grain than perpendicular to it.

6. Theoretical predictions have been made of the value of thermal conductivity for several types of substances. In gases heat is conducted (i.e. thermal energy is diffused) by the random motion of molecules. Higher-velocity molecules from higher-temperature regions move about randomly, and some reach regions of lower temperature. By a similar random process lower-velocity molecules reach higher-temperature regions. Thereby net energy is exchanged between the two regions. The thermal conductivity depends upon the space density of molecules, upon their mean free path and upon the magnitude of the molecular velocities. The net result of these effects for gases having very simple molecules is a dependence of К upon T where T is the absolute temperature. This is a result of the kinetic theory of gases.

7. A similar temperature dependence is found for the viscosity of gases. The viscosity ":" is a measure of the diffusion of momentum. It may be shown that there is a simple relation between к and: involving the specific heat c_v and a factor i, where the value of i depends upon the way in which energy is stored in the gas molecules, к = ic_v where c_v is the specific heat at constant volume.

TEXT C: CONVECTION Термины, слова и словосочетания

in the absence - при отсутствии forced convection - вынужденная

to result —зд. возникать(в результате ч.-л.) конвекция

to result in - приводить к ч.-л. to introduce - вызывать

resulting - получающийся в результате to modify - изменять

buoyant effect - эффект подъемной силы to aid in - помогать, buoyancy - подъемная сила способствовать

natural convection - естественная displacement - перемещение,

конвекция смещение

1. Прочитайте текст. Найдите определение вынужденной конвекции.

2. Обратите внимание на перевод сочетания "The natural convection heat transfer process ".

TEXT С

Energy is conducted through fluids, as through solids. However the heat transfer process in the air is not simple conduction. Even in the absence of wind a flow process results. The buoyant effect in the heated layers of air near the surface causes them to rise and move away from the surface. These layers are replaced by cooler air from below and from farther out from the surface. This effect results in temperature distribution. The resulting heat-transfer process in the outside air is called natural convection. Convection processes in which the fluid motion is induced by heat-transfer are called natural convection.

A wind velocity would further modify the temperature distribution by aiding in the displacement of the heated air layers by cooler air. The effect of a wind velocity, which is imposed upon the natural convection heat-transfer process is called forced convection. For sufficiently high wind velocities, buoyancy effects would be negligible, and the process would be pure forced convection.

TEXT D: THERMAL RADIATION AND EMISSIVE POWER

 

to distinguish - зд. отличаться от presence - наличие

intermediate carrier - промежуточный носитель

to impede - препятствовать, мешать space between - зд. промежуточное

пространство as a consequence- как

последовательность to emit - испускать energy content - энергосодержание a quantity - какое-то количество,

величина microscopic arrangement -

микроскопическая структура rate of emission of energy - скорость

излучения энергии surroundings - окруж. среда to promote - способствовать means - способ, средство occurrence - случай, явление incidence - падение, наклон particular wavelength-

определенная длина волн thermal motion - тепловое

движение thermal radiation - теплоизлучение amenable - поддающийся,

подчиняющийся to be dependent upon - зависеть от relation - зависимость, связь,

соотношение incident radiation - падающее

излучение

 

Прочитайте текст, ответьте на следующие вопросы:

1. What definition of radiation energy-transfer process is given in the text?

2. What are the possible uses of radiant discharge processes?

3. What surface is called "black"?

II. Переведите следующие сочетания слов:

1. Radiant energy transfer process Energy carrying electromagnetic waves Net energy transfer rate

The temperature and spatial relationships

2. Radiant energy discharge High energy particles Heat-transfer phenomena Radiant exchange process

The rate of thermal energy emission Energy emission rate

TEXT D

1. One of the basic mechanisms by which energy is transferred between regions of different temperature is called radiation. This mechanism is distinguished from conduction by the fact that it does not depend upon the presence of intermediate material to act as a carrier of energy. On the contrary, a radiation transfer process between two regions is usually impeded by the presence of a material in the space between. The radiation energy- transfer process is explained as a consequence of energy-carrying electromagnetic waves. These waves are emitted by atoms and molecules of matter as the result of various changes in their energy content. The amount and characteristics of the radiant energy emitted by a quantity of material depends primarily upon the nature of the material, its microscopic arrangement,

and its absolute temperature. The rate of emission of energy is assumed to be independent of the surroundings. However, the net energy-transfer rate depends upon the temperature and spatial relationships of the various materials involved in the radiation-transfer process.

2. A wide variety of radiant energy-discharge processes are known. The various kinds of discharge are promoted by many means - for example, by bombardment with high-energy particles by the occurrence of a chemical reaction, by an electric discharge, or by the incidence of relatively low energy radiation of particular wave-lengths. One type of discharge process of special interest in connection with heat-transfer phenomena is that which arises as the result of the thermal motion of molecules. This type of radiant energy is called thermal radiation. Thermal radiation is composed of waves of many wave-lengths and is amenable to relatively simple laws. Many of the radiant-exchange processes by which appreciable amounts of energy are transferred between surfaces are thermal in nature.

3. The rate of thermal radiant energy emission by a surface is directly dependent upon its absolute temperature. The relation between the energy-emission rate and the temperature is very simple if the surface is "black". A surface is called "black" if it will absorb all incident radiation.

UNIT V