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THERMODYNAMICS
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HEAT AND THERMODYNAMICS
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Prior to the 19th century, it was believed that the sense of how hot or cold an object felt was determined by how much "heat" it contained. Heat was envisioned as a liquid that flowed from a hotter to a colder object; this weightless fluid was called "caloric", and until the writings of Joseph Black (1728-1799), no distinction was made between heat and temperature. Black distinguished between the quantity (caloric) and the intensity (temperature) of heat.
Benjamin Thomson, Count Rumford, published a paper in 1798 entitled "an Inquiry Concerning the Source of Heat which is Excited by Friction". Rumford had noticed the large amount of heat generated when a cannon was drilled. He doubted that a material substance was flowing into the cannon and concluded "it appears to me to be extremely difficult if not impossible to form any distinct idea of anything capable of being excited and communicated in the manner the heat was excited and communicated in these experiments except motion."
But it was not until J. P. Joule published a definitive paper in 1847 that the the caloric idea was abandoned. Joule conclusively showed that heat was a form of energy. As a result of the experiments of Rumford, Joule, and others, it was demonstrated (explicitly stated by Helmholtz in 1847), that the various forms of energy can be transformed one into another.
When heat is transformed into any other form of energy, or when other forms of energy are transformed into heat, the total amount of energy (heat plus other forms) in the system is constant.
This is the first law of thermodynamics, the conservation of energy. To express it another way: it is in no way possible either by mechanical, thermal, chemical, or other means, to obtain a perpetual motion machine; i.e., one that creates its own energy (except in the fantasy world of Maurits Escher's "Waterfall"!)
A second statement may also be made about how machines operate. A steam engine uses a source of heat to produce work. Is it possible to completely convert the heat energy into work, making it a 100% efficient machine? The answer is to be found in the second law of thermodynamics:
No cyclic machine can convert heat energy wholly into other forms of energy. It is not possible to construct a cyclic machine that does nothing but withdraw heat energy and convert it into mechanical energy.
The second law of thermodynamics implies the irreversibility of certain processes - that of converting all heat into mechanical energy, although it is possible to have a cyclic machine that does nothing but convert mechanical energy into heat!
Sadi Carnot (1796-1832) conducted theoretical studies of the efficiencies of heat engines (a machine which converts some of its heat into useful work). He was trying to model the most efficient heat engine possible. His theoretical work provided the basis for practical improvements in the steam engine and also laid the foundations of thermodynamics. He described an ideal engine, called the Carnot engine, that is the most efficient way an engine can be constructed. He showed that the efficiency of such an engine is given by
efficiency = 1 - T"/T',
where the temperatures, T' and T" , are the hot and cold "reservoirs" , respectively, between which the machine operates. On this temperature scale, a heat engine whose coldest reservoir is zero degrees would operate with 100% efficiency. This is one definition of absolute zero, and it can be shown to be identical to the absolute zero we discussed previously. The temperature scale is called the absolute, the thermodynamic , or the kelvin scale.
The way that the gas temperature scale and the thermodynamic temperature scale are shown to be identical is based on the microscopic interpretation of temperature, which postulates that the macroscopic measurable quantity called temperature is a result of the random motions of the microscopic particles that make up a system.
by
missnits
on
December 31, 1999
I'm 18 years old,
from mississauga,CA.
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FIRST LAW OF THERMODYNAMICS:
One of the characteristics of any system is the total amount of energy it contains. This characteristic is called the total internal energy (symbol: U). The total internal energy is a state function, and must necessarily be so if we accept the fact that energy cannot be created or destroyed. The energy of a system can, of course, be changed by moving energy in from the surroundings or by moving energy out to the surroundings. We do this using devices such as furnaces and refrigerators daily. But energy is only transferred and not changed in quantity by such processes. The type of energy can be changed either inside or ouside a system -- but energy is transformed in type only, and its quantity is not changed, whatever transformation procedure is employed. Even in nuclear reactions energy is conserved, because mass and energy are considered equivalent. Total internal energy U is not a particularly useful state function, since we have no method of measuring U for any system and no simple one of measuring changes in it, DU. Heat flow, as from a boiler to a condenser, and work flow, as in the work output of a steam engine, are both measurable and highly significant quantities. Heat flow (symbol: q) is the change in heat, and work flow (symbol: w) is the change in work. Heat and work are not the only forms of energy which exist, of course, and this is indicated by the ... in the equation form of the first law of thermodynamics:
q + w + ... - U = 0
The first law of thermodynamics says that all of the energy changes in a system must add up to zero -- no net change, or no creation or destruction of energy, is possible. Any change in heat which is not exactly equal and opposite to a change in work must appear as a change in the total internal energy. Chemists usually find it easier to assume, as we will do from now on, that all of the other forms of energy such as light energy and electrical energy do not change at all. Then their changes are zero, and under these conditions we write the first law of thermodynamics as q + w - U = 0, or U = q + w.
Heat can flow into, and out of, a system to its surroundings. Work can be done by the system on the surroundings, and vice versa. The internal energy of a system can increase or decrease by any of these processes. Thus energy as heat, work, total internal energy, or any other form must be an algebraic quantity -- a quantity which has a positive sign (for increase) or a negative sign (for decrease). Increases or decreases in heat and work are more easily visualized as heat or work flows between system and surroundings and are looked at from the point of view of the system:
If q is negative, heat flows OUT of the system (to the surroundings).
If q is positive, heat flows INTO the system (from the surroundings).
If w is negative, work is done BY the system (on the surroundings).
If w is positive, work is done ON the system (by the surroundings).
SECONDLAWOFTHERMODYNAMICS:
Thermodynamics is the science of energy. Although scientists have argued over the proper meaning of the laws of thermodynamics, since they were fully developed in the mid 19th century, the basic significance of these laws are well understood in just about every culture on earth. Sayings such as "You can't get something for nothing" and "Its no good crying over spilt milk", are familiar expressions of the two laws of thermodynamics. The second law of thermodynamics has such a profound impact upon our current economic system and way of life that many eminent physicists have tried over the years to prove that it does not necessarily apply in all situations (Figure 1). Unfortunately for industrialized cultures, the second law is one of the most universal of all scientific laws and may prove to be our downfall if its implications are ignored for much longer.
The second law of thermodynamics says that: "Every time energy is transformed from one state to another, a penalty is exacted. The penalty is a reduction in the amount of energy available to perform useful work in the future." The term for this penalty is "entropy". The two laws of thermodynamics can be stated together in one simple sentence: "The total energy content of the universe is constant, and the total entropy is continually increasing." In other words, "there is no such thing as a free lunch; and there are always crumbs under the table." Entropy is thus a measure of the amount of energy which is no longer capable of performing work.
An increase in entropy means the reduction of energy available to do work in the future. Increasing entropy can also be considered as a change from an ordered state to a disordered state. Every event in the natural world results in an increase in entropy. Time is not symmetrical, it only moves in one direction, therefore entropy will always increase. One example of increasing entropy is water falling over a dam. When the water is above the dam it has some potential energy due to gravity, which can be used to generate electricity or turn a wheel to perform some useful task. Once the water has fallen to the level below the dam, its total energy is the same - as the fall warms the water increasing its thermal energy - but it no longer has the same capacity to do work. The water has moved from what is referred to as an "available" or "free" energy state (high grade energy) to an "unavailable" or "bound" energy state (low grade energy). This change in the energy state of the water as it falls over the dam is an increase in entropy.
Unavailable energy is most often in the form of thermal energy. Thermal energy is the result of random and disorderly motions of huge numbers of individual atoms and molecules. Whereas kinetic energy, such as that of a moving object is the result of the orderly motion of all the atoms in the object. Conversion of other forms of energy (kinetic, electric etc.) into thermal energy is simple but converting thermal energy into other forms is difficult and can never be performed completely.
In order to harness thermal energy, two reservoirs of heat at different temperatures are required. In such a 'heat engine', heat from the high temperature reservoir (high grade energy) flows to the low temperature reservoir (low grade energy). The maximum possible efficiency at which the heat engine can work is defined by the equation E=(T2-T1)/T2, where E is the efficiency, T1 is the temperature of the cold reservoir (Kelvin) and T2 the temperature of the hot reservoir. From the equation, it can be seen that unless the cold reservoir is at a temperature of absolute zero (and no such reservoirs exist), the efficiency of the heat engine will always be less than 1, meaning there can never be a complete conversion of thermal energy into another form of energy. This is illustrated in a thermal electric power plant (Figure 2). In this case, the hot reservoir has a temperature of about 773 K (500� C) and the cold reservoir 293 K (20� C), resulting in a maximum efficiency of about 60%, although in actual power plants the efficiency is rarely above 40% due to frictional and other losses.
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LAWS OF THERMODYNAMICS:
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APPLICATIONS OF THERMODYNAMICS:
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REFRIGERATION CYCLE:
DESCRIPTION
Refrigeration is the withdrawl of heat from a substance or space so that temperature lower than that of the natural surroundings is achieved.
Refrigeration may be produced by
thermoelectric means
vapor compression systems
expansion of compressed gases
throttling or unrestrained expansion of gases.
Vapor compression systems are employed in most refrigeration systems. Here, cooling is accomplished by evaporation of a liquid refrigerant under reduced pressure and temperature. The fluid enters the compressors at state 1 where the temperature is elevated by mechanical compression (state 2). The vapor condenses at this pressure, and the resultant heat is dissipated to the surrounding. The high pressure liquid (state 3) then passes through an expansion valve through which the fluid pressure is lowered. The low-pressure fluid enters the evaporator at state 4 where it evaporates by absorbing heat from the refrigerated space, and reenters the compressor. The whole cycle is repeated.
GAS TURBINE CYCLE:
The gas turbine is used in a wide range of applications. Common uses include power generation plants and military and commercial aircraft. In Jet Engine applications, the power output of the turbine is used to provide thrust for the aircraft.
In a simple gas turbine cycle, low pressure air is drawn into a compressor (state 1) where it is compressed to a higher pressure (state 2). Fuel is added to the compressed air and the mixture is burnt in a combustion chamber. The resulting hot products enter the turbine (state 3) and expand to state 4. Most of the work produced in the turbine is used to run the compressor and the rest is used to run auxiliary equipment and produce power.
Air standard models provide useful quantitative results for gas turbine cycles. In these models the following assumptions hold true.
The working substance is air and treated as an ideal gas throughout the cycle
The combustion process is modeled as a constant pressure heat addition
The exhaust is modeled as a constant pressure heat rejection process
In cold air standard (CAS) models, the specific heat of air is assumed constant at the lowest temperature in the cycle.
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
(1-2) Isentropic Compression
(2-3) Reversible Constant Pressure Heat Addition
(3-4) Isentropic Expansion
(4-1) Reversible Constant Pressure Heat Rejection
STEAM TURBINE CYCLE:
Power plants generate electrical power by using fuels like coal, oil or natural gas. A simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler and superheater, heats the water to generate steam. The steam is then heated to a superheated state in the superheater. This steam is used to rotate the turbine which powers the generator. Electrical energy is generated when the generator windings rotate in a strong magnetic field. After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurized by the pump prior to going back to the boiler A simple power plant is described by a Rankine Cycle.
RANKINE CYCLE
Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature to a saturated liquid, state 3. The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure (state 4), where the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.
TYPICAL MODIFICATIONS
REHEAT
When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine (state 2), and then it is reheated in the boiler (state 2') and sent back to the low pressure turbine.
REGENERATION
Regeneration helps improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open feedwater heaters or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feedwater at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feedwater, and therefore, the two streams can be at different pressures.
COMPONENTS
Boiler/Superheater
Condenser
Turbine
Pump
THERMAL EFFICIENCY
INTERNAL COMBUSTION ENGINE:
Internal Combustion (IC) engines have completely revolutionized transportation, power generation and have perhaps altered the way the society operates forever. Typical IC engines are classified as Spark and Compression ignition engines.
The simplest model for IC engines is the air-standard model, which assumes that:
The system is closed.
Air is the working fluid and is modeled as an ideal gas throughout the cycle.
Compression and expansion processes are isentropic.
A reversible heat transfer process characterizes the combustion of fuel and air.
Heat rejection takes place reversibly and at constant volume.
The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.
Spark Ignition Engines (Otto Cycle)
The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:
(1-2) Isentropic compression (Compression Stroke)
(2-3) Constant-volume, reversible heat addition (Ignition)
(3-4) Isentropic expansion (Power Stroke)
(4-1) Reversible, constant-volume heat rejection (Exhaust)
Typical pv and Ts diagrams for an Otto cycle are shown below where steps (1-2) and (3-4) are isentropic, and (2-3) and (4-1) are isochoric.
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