Методические указания по обучению чтению технической литературы на английском языке по тепломассобмену

I. Прочитайте текст, найдите ответы на следующие вопросы
1. What definition of mass transfer can you give?

2. What mechanisms of mass transfer are mentioned in the text?

3. Can you explain the difference between the words “mechanism” and “mode”?

4. What examples of mass transfer does the author give?

5. Can you add any other examples?
II. Обратите внимание на перевод следующих словосочетаний:

Forced convection mass transfer

Interphase mass transfer

Molecular mass transfer

Convective mass transfer

Moisture laden air

TEXT A: MOLECULAR MASS TRANSFER

1. In this chapter another driving force, concentration gradient, is introduced. This driving force causes the transport of a component of a mixture from a region
of high concentration to a region of low concentration. The transport process
is known as mass transfer. The mechanisms of mass transfer are varied. They can
be classified into eight types: 1. Molecular (ordinary) diffusion, resulting
form a concentration gradient. 2. Thermal diffusion, arising from a temperature gradient. 3. Pressure diffusion, which occurs by virtue of a pressure gradient.
4. Forced diffusion, resulting from external forces other than gravity. 5. Forced-convection mass transfer. 6. Natural-convection mass transfer. 7. Turbulent mass transfer resulting from eddy currents in a fluid. 8. Interphase mass transfer occurring by virtue of non-equilibrium at an interface.
2. These types divide naturally into two distinct modes of transport. The first four are molecular mass transfer; the last four are convective mass transfer. Although the two modes often occur simultaneously, one mode usually dominates and we can understand the mechanisms better by considering them separately.
3. Examples of mass transfer in everyday life are legion: the diffusion of sugar in a cup of coffee; vaporization of water in a tea-kettle; the movement of moisture-laden air over the ocean with its subsequent precipitation on dry land; combustion and air-conditioning process, cloud formation; clothes drying. The chemical engineer is concerned with gas absorption, separation, crystallization and extraction, the mechanical engineer confronts the mass-transfer process in humidification, drying, cutting and welding metals, ablation of heat shields in high-speed flight, deaeration of feed water in steam boilers, and the production and heat treatment of metals; and civil engineers make use of mass transfer in waste treatment.


TEXT B: THE DIFFUSION MODE

Термины, слова и словосочетания.

I. Прочитайте текст, ответьте на следующие вопросы:

1. What practical application of thermal diffusion is mentioned in this text?

2. What example of forced diffusion does the author give?

3. Why is it possible to say that mass transfer by diffusion is analogous to conduction heat transfer?

4. Why is diffusion rate faster in gases than in liquids?

II. Напишите краткое содержание текста.

TEXT B: THE DIFFUSION MODE
1. This chapter will deal primarily with the molecular (ordinary) diffusion
of binary (two-component) mixtures, typifying the diffusion process and being
the most significant of the types of diffusion.
2. For the case of thermal diffusion in a binary mixture, the molecules
of one component travel toward the hot region while the molecules of the other component tend to move toward the cold region. The inverse is the tendency
to generate a thermal gradient with the development of a concentration gradient. Thermal diffusion has been successfully used in the separation of isotopes.
3. Pressure diffusion results when a pressure gradient exists in a fluid mixture, e.g., in a closed tube which is rotated about an axis perpendicular to the tube’q axis (centrifuge). The lighter component tends to move toward the low-pressure region.
4. An external force other than gravity in a mixture when it acts in a different manner on the different components, results in forced diffusion. The diffusion of ions in an electrolyte in an electric field is a classic example of forced diffusion.
5. When thermal, pressure, and/or forced diffusion occur, a concentration gradient is developed, casing ordinary diffusion in the opposite direction. Upon reaching a steady state, the fluxes from the two (or more types of diffusion) sometimes offset each other, resulting in properties at a point being constant with time. The effects of thermal, pressure, and forced diffusion will be ignored in the introductory treatment of this chapter.
6. Mass transfer by diffusion is analogous to conduction heat transfer. Mass
is transported by the movement of a species in the direction of its decreasing concentration, analogous to the energy exchange between molecules in the direction of decreasing temperature in conduction.
7. Ordinary diffusion may occur in gases, liquids or solids. Because
of the molecular spacing the diffusion rate is much faster in gases than in liquids;
it is faster in liquids than in solids.

TEXT C: TYPES OF MOTION

Термины, слова и словосочетания

I. Прочитайте текст, ответьте на следующие вопросы:

1. What types of flow are described in the text?

2. What experiment helped Reynolds to observe laminar and turbulent flow?

II. Обратите внимание на форму сослагательного наклонения в последнем предложении 3-го абзаца. Переведите предложение.

III. Переведите письменно 6-ой абзац текста. Какое значение имеет глагол would в 1-ом предложении этого абзаца?

TEXT C: TYPES OF MOTION
1. Steady and unsteady flow. If the local acceleration is zero, the motion is steady. The velocity does not change with time, although it may change from point in space. On the other hand, a flow which is time-dependent is unsteady.
2. Often an unsteady flow can be transformed to steady flow by changing
the reference axis. Consider for example, an airplane moving through the atmosphere at a constant speed of V_o. The fluid velocity at a point (x_o, y_o) is unsteady, being zero before the plane reaches the point, varying widely as it passes due to he wake
and waves produced by disturbing he air, and finally becoming zero again
as the plane disappears.
3. Uniform and non-uniform flow. If motion is uniform, the convective acceleration is zero. In uniform flow the velocity vector is identical, in magnitude
and direction, at every point in the flow field, that is, V/r=0 where
“r” is a displacement in any direction. This definition does not require
that the velocity itself be constant with respect to time; it requires that any change occur at every point simultaneously; the streamlines must be straight.
4. A frictionless liquid flowing through a long straight pipe is an example
of uniform flow. Non-uniform flow is typified by the flow of a frictionless liquid through a pipe of changing cross section or through a pipe which is curved.
5. Laminar and turbulent flow. In 1883, while injecting dyes into flows fed
by constant-head tanks Reynolds observed two distinct types of flow. At relatively low velocities fluid particles move smoothly, everywhere parallel. Because the fluid moves in a laminated form, it is termed laminar. For laminar flow the dye moves
in a thin, straight line.
6. At relatively high velocities, Reynolds noted that the dye would abruptly break up, diffusing throughout the tube. At higher velocities the breaking point moves upstream until it is finally turbulent throughout. Turbulent flow is always unsteady flow by our prior definition.

SUPPLEMENTARY TEXTS

FUNDAMENTAL CONCEPTS FROM THERMODYNAMICS
In the transfer processes we seek the relationships between fluxes and field intensities in terms of field properties, physical properties of the transfer media,
and the dimensions of space and time. Thermodynamics deals with energy quantities which are transferred during the processes – work and heat. Its principles and laws apply to all fields of engineering. This chapter sets forth some fundamental concepts necessary for subsequent study of the transfer processes, unifying the definitions
and symbols of thermodynamics and the rate processes.
In its broadest sense the science of thermodynamics considers thee conversion and transfer of energy. Classical, or macroscopic, thermodynamics is based upon man’s observations. Its laws were developed inductively. No observable violations have occurred. Media are viewer from a continuum standpoint. Probabilistic,
or microscopic, thermodynamics is based upon the interactions of molecules
and the probability of their behaving in accordance with a set of laws which
are identical to those developed in the classical approach. The two approaches
are complementary in that the microscopic viewpoint describes fundamental behavior while the macroscopic viewpoint guarantees repeatability.
Equilibrium. Thermodynamics is based upon an equilibrium condition or a series
of equilibrium states. Equilibrium is that state which is characterized by no change.
In the preceding chapter we noted that change occurs when the field intensity – any field intensity – varies throughout a region. Therefore, for equilibrium the intensity
of all fields must be identical; no potential gradient can exist.
System and control volume. A thermodynamic system is a fixed quantity of matter.
It does not vary in mass or identity. Everything outside the system is termed
the surroundings. The system and surroundings are separated by boundaries. Consider, for example, filling an automobile gasoline tank from a large tank truck. We may define the system as that amount of gasoline which will be transferred
into the smaller tank.
The thermodynamics problem then becomes that of determining what happens to the gasoline between the initial equilibrium state and the final equilibrium; it is a “book-keeping process” of tabulating observable quantities initially and finally.
An alternative method of solving the same problem involves focusing attention on a fixed region in space, say the automobile tank. The fixed region is the control surface (analogous to the system boundary) and observing the gasoline as it crosses.
All thermodynamic problems can be solved by using one of these two concepts, control volume or system. We shall use whichever is more convenient
in any given problem. In some cases it will be more feasible to think in terms
of a deformable control volume, typified by a balloon. At this point the student should ponder the analogy between the sulerian method of describing field properties and the thermodynamic concept of the control volume.

PROPRTIES AND STATE OF A SUBSTANCE

A thermodynamic property is any measurement or quantity which serves
to describe a system. Thermodynamic properties are either intensive or extensive. Intensive properties are independent of mass. Temperature, pressure and density
are intensive properties. Extensive properties vary directly with mass. Mass and total volume are extensive properties.

A property of a pure, simple, compressive substance can always be defined
in terms of two independent intensive properties. For example, the pressure of a gas can be expressed in terms of its temperature and specific volume: P = p(T,v) (3-5).
A pure substance is also homogeneous and of fixed chemical composition.
We sometimes speak of air as being pure: however, thermodynamically
it is a mixture of several gases and vapors.

A phase is a quantity of matter which is homogeneous throughout. A substance may exist in any one or a combination of three phases – solid, liquid and vapor. Two or more phases may coexist when in a common state, identified by two or more observable properties such as temperature and pressure. Change of phase and phase equilibrium can be understood by considering water. At a pressure of 14.7 psia water is a solid (ice) when below 32^oF solid, vapor and liquid water can coexist. Further increases in temperature cause the liquid water to vaporize (turn to steam) until it is 100 percent water above 212^oF. During this transition the quality x, ratio of the mass of vapor to the total mass changes from 0 to 1.00.

Work. Work, one of the basic quantities transferred during a thermodynamic process, is defined from elementary mechanics as a force F acting through a displacement
x, where x is positive in the direction of the force; i.e., W = (3-6).

This basic relation enables us to determine the work required to raise weights, propel missiles, etc. But this definition of work is too limited for thermodynamics, where
the concern is with the interactions between a system and its surroundings. Therefore, we shall define work compatible with our concepts of systems, properties
and processes. Hence, work is done by a system if the sole effect external
to the system (on the surroundings) could be the raising of a weight. Work done
by a system in assumed to be positive and work done on a system is considered negative. This definition does not state that a weight is raised or that a force actually acts through a distance. This definition is necessary because of the need to distinguish between work and heat in the second law of thermodynamics.
The term “sole effect” in the definition of work implies that another effect might be external to the system.

The term “external” in the definition of work suggests that work is defined only with reference to a system boundary.

Heat. The other form of energy of significance in transfer processes, heat,
is defined in terms of temperature. Heat is the energy which is transferred across the boundaries of a system interacting with the surroundings by virtue
of a temperature difference.

THE FIRST LAW OF THERMODYNAMICS

Since the first law of thermodynamics is a relation between the fundamental quantities of heat and work, let us look further at their distinctions and similarities.
Neither heat nor work is a property of the system. They are boundary phenomena, path-dependent, inexact differentials. Both are forms of energy in transit and have meaning when a system undergoes a change of state.

The conventional units of work are foot-pounds force; of heat, the British thermal unit. Btu was originally defined as that quantity of heat required to raise1 lb_m of water from 59.5 to 60.5^oF, which is referred to as the 60^oFBtu.
To understand the first law of thermodynamics we must understand a cycle, defined as the passing of a system through a series of states but returning to its initial condition. Consider an ice-cream freezer. The ingredients, milk, eggs, sugar, etc., are contained in the system chosen. Work is transferred to the system by paddle, causing the temperature of the system to rise, but the heat resulting from the increased temperature is transferred to the surrounding brine. Work goes in; heat comes out.
What happens when all the energy added by work is extracted by the heat transfer? The system returns to its initial state, passing through a cycle. Note
that for the system chosen the work is negative and the heat is negative.
The total work and heat transferred in the cycle is different from zero, i.e.,

 W#0,  Q#0 (3-7)

As a matter of fact, for the system in question  W<0, Q<0 (3-18). With a little ingenuity we can measure the work and heat transferred. Equipping the input shaft with a pulley and weight will give the work, while the heat transfer can be measured by ice meltage. Before leaving this example, we should observe that more heat must be extracted than added by the work if we are to freeze the ice cream.

In 1843 a British scientist, Joule, carried out a number of experiments similar to the preceding example with various configurations. In all cases, he observed that the work done on the system was directly proportional to the quantity of heat removed from the system. Mathematically, (3-19) cycle, where the proportionality constant J is the mechanical equivalent of heat the value of which depends upon the units chosen. Equation (3-19)
is the mathematical statement of the first law of thermodynamics. This law, which
is the basic law of the conservation of energy, was deduced from observations.
It is given the status of a law only because no contradiction to it has ever been found.
It is evident from Eq. (3-19) that work and heat can be expressed in equivalent units. Expressing work in foot-pounds force and heat in Btu, J = 778 ft-lb_f/Btu. Equation (3-19) does not suggest that heat and work is the same thing, but it does establish the relationship between the two. While discussing units, recall that power is work rate, or work per unit time. Therefore, the following conversion factors will be useful 1 hp – 33,000 ft-lb_f/min = 2545 Btu/hr, 1 kw = 44,200 ft-lb_f/min = 3412 Btu/hr.
Most of our thermodynamic problems are concerned with processes rather
than cycles. Systems rarely return to their initial state. Therefore, to be useful the first law should be formulated for easy application to processes.
Specific heats. If a red hot iron ingot of 20-lb_m is quenched in a 20-lb_m pail of cold water, we know intuitively that the iron will cool and the water will become hot. Experience has shown that the temperature change of the iron is not equal
to the temperature change of the water. Furthermore, this is the case for all materials. This characteristic is due to a property of the material known as specific heat
c. It is the amount of heat required to change the temperature of a unit mass
by 1^o under certain conditions.
The third law of thermodynamics. The second-law relationship for entropy can account only for changes in entropy – one state relative to another. Although this
is adequate for thermodynamic calculations, it is sometimes advantageous to speak
in terms of absolute entropy, which requires the third law of thermodynamics.

Simply stated, it is that the entropy of a pure substance is zero at absolute zero.

In a probabilistic sense, entropy is a measure of the disorder of a system.
At absolute zero there is no translational molecular activity, hence no disorder,
or zero entropy.
The second law of thermodynamics. The first law of thermodynamics establishes
a relationship between heat and work but places no conditions on the direction
of transfer. The second law of thermodynamics is the directional law. It may
be formulated thus: Heat cannot, of itself, pass form a colder to hotter body.
Limitations of the first law. To illustrate the directional characteristic of the second law, let us return to the example of the ice cream freezer. We added work
to the system and extracted heat. Now let us reverse the process – add heat and get work out of the system. There is no conceivable way in which a weight might
be returned to its original position by reversing the process. It is impossible to fully convert all heat into work. The process is irreversible.
Consider another example. A flywheel in stopped by a friction brake.
In the process of stopping the flywheel the brake gets hot, and its internal energy
is increased by an amount equal to the loss if kinetic energy of the flywheel. The first law would be satisfied if the hot brake gave up its energy to the flywheel causing
it to resume rotation. But there is no conceivable way in which this can happen.
The process is irreversible.
Two bodies at different temperatures are placed in thermal contact
in an insulated box. Heat in transferred from the high temperature body in accordance with the first law, causing the low temperature body to get warmer. The energy given up by the high temperature body is gained by the low temperature body in coming
to thermal equilibrium. Letting the process be reversed would not violate the first law since it is concerned with the conservation of energy, but the same amount of energy cannot be transferred from the low temperature body to the high temperature body. Heat has never been conserved to “flow uphill”. The process is irreversible.
Some factors which cause irreversibility are (1) friction (2) finite temperature difference, (3) unrestrained expansion, and (4) mixing of different substances.
In a cyclic process it is possible to convert all work into heat, but it is impossible
to convert all the heat into work.
Heat engine. A heat engine is any device which operates cyclically and has
as its primary purpose the conversion of heat into work. For example, a steam power plant has its working fluid, water, returning periodically to its initial state. Liquid water is pumped into the boiler, where it is vaporized and drives the turbine, producing work, some of which may be used to drive the condensate pump. Choosing the system as shown, only heat and work cross the boundary.
The system can be simplified as receiving heat from a high temperature reservoir (source) and rejecting heat to a low temperature reservoir (sink). A thermal reservoir is a body which can receive or reject heat indefinitely without having


Thermal efficiency  th is defend as  th = energy effect sought

energy input required

For the heat engine, the energy effect sought is the work output W, and the energy input required to produce it is the heat input Q_H; therefore, ή th = W .

Q_H

There are two classic statements of the second law, both of which are negative statements and cannot be proved. However, since neither has ever been experimentally violated, we shall accept them as law. They are: Kelvin – Planck.
It is impossible to construct a device which will operate in a cycle and produce
no effect other then the raising of a weight and the exchange of heat with a single reservoir.
Clausius: Heat cannot pass spontaneously from a low temperature body. Proof of the equivalence of these two statements can be establishes by contradiction
and is included in any complete treatise on thermodynamics.

DESCRIPTION OF A FLOW FIELD

1. A streamline is an imaginary line in a flow field at an instant of time taken such that the fluid velocity at any point is tangent to it. Since the velocity vector
is tangent to the streamline, no matter can cross it. A streamline is analogous
to a heat-flow line in the case of heat transfer.

2. A stream filament is a fanaly of streamlines forming a cylindrical passage
of infinitesimal cross section. A stream tube is bounded by an infinite number
of streamlines forming a finite surface across which there is no flow. If there
is no creation, storage, or destruction of mass within the stream tube, all fluid which enters must leave.

ISOTHERMAL FLOW

The basic differences between laminar and turbulent flow were discussed
in Chap 9. The fundamental difference between laminar and turbulent types of flow
is the existence of completely random fluctuations in the velocity components
for the turbulent case. In addition to purely laminar and purely turbulent flow,
we find that transition flow usually exists whenever we have the turbulent case.
In the development of any boundary layer, internal or external, there normally exists a laminar leading section which becomes turbulent as the fluid moves downstream. This results in a flow regime between the completely laminar and completely turbulent areas in which the fluid motion is highly unstable, fluctuating between laminar and turbulent characteristics.

FLUID MOTION

1. In the dynamics of solids we are accustomed to describing the motion
of particles or rigid bodies by their velocities and accelerations or more exactly
by the velocities and accelerations of their centers of mass. For a finite number
of particles, the velocity of the i-th particle can be given by the scalar equations

where the subscript “i” identifies the particle. In a fluid, however, there is an infinite number of particles whose character may change continuously, making this approach unfeasible. This technique of describing motion of discrete particles with respect
to a fixed set of axes, the lagrangian approach, is not normally used for fluids.

2. In the lagrangian method the specification of velocity applies only at a given time, location the particle at some point (a, b, c). Location of the same particle
at a subsequent time requires a set of equations:

x_i = F_i(t) Y_i = G_i(t) Z_i = H_i(t) (9-4)

3. The more common approach, the eulerian method, permits us to focus attention on a fixed region in space without regard to the identity of the particles which occupy it a given time. An observation is an instantaneous picture
of the velocities and accelerations of every particle. To accomplish this it is necessary only to take the space coordinates as independent variables, rather than dependent
as in the lagrangian method. The eulerian velocity field is given by

V = iu _* jv_* kw (9-5)

Where the respective velocities, in Cartesian coordinates, are

Similarly, in the cylindrical and spherical coordinate systems, respectively,
the velocity is V = V(r,  ,z,t) (9-7)

V = V(r, ,  ,t) (9-8)

4. With the eulerian approach differential changes in velocities must
be expressed in terms of partial derivatives, since each component is affected by both space and time.