Lasted edited by Andrew Munsey, updated on June 14, 2016 at 9:44 pm.
[[Image:ThermalEnergy.PNG|frame|right|1. Particles (atoms or molecules) are moving towards each other. This is kinetic energy.
2. The particles are close to each other and their kinetic energy has been converted into the potential energy of electromagnetic repulsion between the electons of the atoms.
3. The particles have accelerated away from each other and their energy is again kinetic.]]
Thermal energy is the internal energy of a thermodynamic system at equilibrium. The flow of thermal energy from one system to another is called heat. If the systems are at different temperatures then part of the thermal energy flow can be converted into work. Thermal energy is a measure of the total vibrational energy in all the molecules and atoms in a certain substance. Thermal energy is composed of both kinetic and potential energy. The kinetic energy is from the random motion of the particles, and the potential energy originates from the repulsive electromagnetic force between the electrons of atoms that are close to each other.
Energy, in the distant past, was discussed in terms of easily observable effects it has on the properties of objects or changes in state of various systems. It was generally construed that all changes can in fact be explained through some sort of energy. Soon the idea, that energy could be stored in objects took its roots in scientific thought and the concept of energy came to embrace the idea of the potential for change as well as change itself. Such effects (both potential and realized) come in many different forms. While in spiritualism they were reflected in changes in a person, in physical sciences it is reflected in different forms of energy itself. For example, electrical energy stored in a battery, the chemical energy stored in a piece of food, the thermal energy of a water heater, or the kinetic energy of a moving train.
The development of steam engines required engineers to develop concepts and formulas that would allow them to describe the mechanical and thermal efficiencies of their systems. Engineers such as Sadi Carnot and James Prescott Joule, mathematicians such as Émile Claperyon and Hermann von Helmholtz , and amateurs such as Julius Robert von Mayer all contributed to the notions that the ability to perform certain tasks, called work, was somehow related to the amount of energy in the system. The nature of energy was elusive, however, and it was argued for some years whether energy was a substance (the caloric) or merely a physical quantity, such as momentum.
Thermal internal energy is present in all macroscopic objects in the universe. Although some heat transfer is mediated by the kinetic energy of a system's constituent particles, this kinetic energy exhibits Brownian motion, a highly disorganized state. Potential thermal energy is the part of thermal energy stored in "deformation" of atomic bonds during thermal motion of atoms (as atoms oscillate around position of equilibrium they not only have kinetic energy of motion but also potential energy of displacement from equilibrium). This energy is significant portion of thermal energy for strongly bonded systems (=solids and liquids) and practically nonexistent for gasses. Thermal motion is motion on the scale of molecules caused by heat. Brownian motion is an example of a phenomenon caused by thermal motion.
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Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames. Since not every oxidation process results in the production of heat (for example, corrosion), the term combustion can only be applied to exothermic processes that occur at a rate fast enough to produce heat. In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. When air is the source of the oxygen, nitrogen is by far the largest part of the resultant flue gas.
Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. This often occurs as a fire. This is used in a form of machinery, such as internal combustion engines, and in thermobaric weapons. Combustion is double replacement, on the other hand a chemical reaction is single replacement. Slow combustion is a form of combustion which takes place at low temperatures. Respiration is an example of slow combustion. Turbulent combustion is a combustion characterized by turbulent flows. It is the most used for industrial application (e.g. gas turbines, diesel engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer. Incomplete combustion happens when there is an inadequate supply of oxygen for combustion to occur completely. The reactant will burn in oxygen, but will produce numerous products. When a hydrocarbon burns in air, the reaction will yield carbon dioxide, water, carbon monoxide, and various other compounds such as nitrogen oxides. Incomplete combustion is much more common and will produce large amounts of byproducts, and in the case of burning fuel in automobiles, these byproducts can be quite unhealthy and damaging to the environment.
In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen. When elements such as carbon, nitrogen, sulfur, and iron are burned, they will yield the most common oxides. Carbon will yield carbon dioxide. Nitrogen will yield nitrogen dioxide. Sulfur will yield sulfur dioxide. Iron will yield iron(III) oxide. It should be noted that complete combustion is impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present. For example, the combustion of methane in air will yield, in addition to the major products of carbon dioxide and water, the minor products which include carbon monoxide, hydroxyl, nitrogen oxides, monatomic hydrogen, and monatomic oxygen.
Smouldering combustion is a flameless form of combustion, deriving its heat from heterogeneous reactions occurring on the surface of a solid fuel when heated in an oxidizing environment. The fundamental difference between smouldering and flaming combustion is that in smouldering, the oxidation of the reactant species occurs on the surface of the solid rather than in the gas phase. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion of a solid. Typical values in smouldering are around 600 °C for the peak temperature and 5 kJ/g-O2 for the heat released typical values during flaming are around 1500 °C and 13 kJ/g-O2 respectively. These characteristics cause smoulder to propagate at low velocities, typically around 0.1 mm/s, which is about two orders of magnitude lower than the velocity of flame spread over a solid. In spite of its weak combustion characteristics, smouldering is a significant fire hazard.
Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature, its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.
In the combustion of solid fuels, the act of combustion consists of three relatively distinct but overlapping phases:
Preheating phase: when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
Distillation phase (gaseous phase): when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light. Flames are often visible.
Charcoal phase or solid phase:when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore but just glows and later only smoulders.
Assuming perfect combustion conditions, such as an adiabatic (no heat loss and no heat gain) and complete combustion, the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the flue gas).
In the case of fossil fuels burnt in air, the combustion temperature depends on
the heating value
the stoichiometric air to fuel ratio ?
the heat capacity of fuel and air
the air and fuel inlet temperatures
The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.
Typically, the adiabatic combustion temperatures for coals are around 1500 °C (for inlet air and fuel at ambient temperatures and for ? = 1.0), around 2000 °C for oil and 2200 °C for natural gas. In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used. Combustion analysis is a process used to determine the composition of organic compounds.
Combustion instabilities are typically violent pressure oscillations in a combustion chamber. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage of the combustion chamber and surrounding components. This problem was solved by re-designing the fuel injector. In liquid jet engines the droplet size and distribution can be used to attenuate the instabilities. Combustion instabilities are a major concern in ground-based gas turbine engines because of NOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the NOx emissions however, running the combustor lean makes it very susceptible to combustion instabilities, especially in premixed combustors. The Rayleigh Criteron is the basis for analysis of combustion instabilities. It states that the pressure oscillations will be amplified as long as the heat release oscillations are in phase with them. These pressure oscillations can be as high as 180dB, and long term exposure to these cyclic pressure and thermal loads reduces the life of engine components.
Thermodynamics (from the Greek thermos meaning heat and dynamis meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. Roughly, heat means "energy in transit" and dynamics relates to "movement" thus, in essence thermodynamics studies the movement of energy and how energy instills movement. Historically, thermodynamics developed out of the need to increase the efficiency of early steam engines.
Classical thermodynamics is the original early 1800s variation of thermodynamics concerned with thermodynamic states, and properties as energy, work, and heat, and with the laws of thermodynamics, all lacking an atomic interpretation. In precursory form, classical thermodynamics derives from physicist Robert Boyle’s 1662 postulate that the pressure P of a given quantity of gas varies inversely as its volume V at constant temperature i.e. in equation form: PV = k, a constant. From here, a semblance of a thermo-science began to develop with the construction of the first successful atmospheric steam engines in England by Thomas Savery in 1697 and Thomas Newcomen in 1712. The first and second laws of thermodynamics emerged simultaneously in the 1850s, primarily out of the works of William Rankine, Rudolf Clausius, and William Thomson (Lord Kelvin). The latter coined the term thermodynamics in his 1849 publication An Account of Carnot's Theory of the Motive Power of Heat. The first thermodynamic textbook was written in 1859 by William Rankine, a civil and mechanical engineering professor at the University of Glasgow.
Chemical thermodynamics is the study of the interrelation of heat with chemical reactions or with a physical change of state within the confines of the laws of thermodynamics. During the years 1873-76 the American mathematical physicist Willard Gibbs published a series of three papers, the most famous being On the Equilibrium of Heterogeneous Substances, in which he showed how thermodynamic processes could be graphically analyzed, by studying the energy, entropy, volume, temperature and pressure of the thermodynamic system, in such a manner to determine if a process would occur spontaneously. During the early 20th century, chemists such as Gilbert Lewis, Merle Randall, and E. A. Guggenheim began to apply the mathematical methods of Gibbs to the analysis of chemical processes.
An important concept in thermodynamics is the “system?. A system is the region of the universe under study. A system is separated from the remainder of the universe by a boundary which may be imaginary or not, but which by convention delimits a finite volume. The possible exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. There are five dominant classes of systems:
# Isolated Systems – matter and energy may not cross the boundary.
# Adiabatic Systems – heat may not cross the boundary.
# Diathermic Systems - heat may cross boundary.
# Closed Systems – matter may not cross the boundary.
# Open Systems – heat, work, and matter may cross the boundary.
For isolated systems, as time goes by, internal differences in the system tend to even out pressures and temperatures tend to equalize, as do density differences. A system in which all equalizing processes have gone practically to completion, is considered to be in a state of thermodynamic equilibrium.
In thermodynamic equilibrium, a system's properties are, by definition, unchanging in time. Systems in equilibrium are much simpler and easier to understand than systems which are not in equilibrium. Often, when analyzing a thermodynamic process, it can be assumed that each intermediate state in the process is at equilibrium. This will also considerably simplify the situation. Thermodynamic processes which develop so slowly as to allow each intermediate step to be an equilibrium state are said to be reversible processes.
A thermodynamic process may be defined as the energetic evolution of a thermodynamic system proceeding from an initial state to a final state. Typically, each thermodynamic process is distinguished from other processes, in energetic character, according to what parameters, as temperature, pressure, or volume, etc., are held fixed. Furthermore, it is useful to group these processes into pairs, in which each variable held constant is one member of a
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The laws of thermodynamics, in principle, describe the specifics for the transport of heat and work in thermodynamic processes. Since their conception, however, these laws have become some of the most important in all of physics and other branches of science connected to thermodynamics. They are often associated with concepts far beyond what is directly stated in the wording.
When two systems are put in contact with each other, there will be a net exchange of energy between them unless they are in thermal equilibrium. While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use, hence the zero numbering. The Zeroth Law asserts that thermal equilibrium, viewed as a binary relation, is an equivalence relation.
More simply, the First Law states that energy cannot be created or destroyed, rather, the amount of energy lost in a process cannot be greater than the amount of energy gained. This is the statement of conservation of energy for a thermodynamic system. It refers to the two ways that a closed system transfers energy to and from its surroundings - by the process of heating (or cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of a system is determined by the rates of these two processes. In open systems, the flow of matter is another energy transfer mechanism, and extra terms must be included in the expression of the first law. The First Law clarifies the nature of energy. It is a stored quantity which is independent of any particular process path, i.e., it is independent of the system history. If a system undergoes a thermodynamic cycle, whether it becomes warmer, cooler, larger, or smaller, then it will have the same amount of energy each time it returns to a particular state. Mathematically speaking, energy is a state function and infinitesimal changes in the energy are exact differentials. All laws of thermodynamics but the First are statistical and simply describe the tendencies of macroscopic systems. For microscopic systems with few particles, the variations in the parameters become larger than the parameters themselves, and the assumptions of thermodynamics become meaningless. The First Law, i.e. the law of conservation, has become the most secure of all basic laws of science. At present, it is unquestioned.
This law is what is the so-called Kelvin-Planck Statement. In a simple manner, the Second Law states that energy systems have a tendency to increase their entropy (heat transformation content) rather than decrease it. The entropy of a thermally isolated macroscopic system never decreases (see Maxwell's demon), however a microscopic system may exhibit fluctuations of entropy opposite to that dictated by the Second Law (see Fluctuation Theorem). In fact the mathematical proof of the Fluctuation Theorem from time-reversible dynamics and the Axiom of Causality, constitutes a proof of the Second Law. In a logical sense the Second Law thus ceases to be a "Law" of Physics and instead becomes a theorem which is valid for large systems or long times. Stephen Hawking described this using time as an entropy base. For example, when time moves in a foward direction and one, say, breaks a cup of coffee on the floor, no matter what happens, in our universe, one will never see the cup reform. Cups are breaking all the time, but never reforming. Since the Big Bang, the entropy of the universe has been on the rise, and so the Second Law states that this process will continue to increase.
The Third Law deals with the fact that there is an absolute constant in the universe known as absolute zero. Derived from the Gibb's free energy equation, where ?G = ?H - T?S (where ?G is the change in free energy, ?H is the change in enthalpy (or total heat), T is Temperature and ?S is the change in entropy (or unusable heat), as the temperature reaches 0 or a very low value, ?S naturally will also approach 0 or a very small value. Put another way, if one imagines atoms flying around in a box, hitting each other randomly, all the time, one can imagine a lot of chaos. Then, imagine what would happen if the temperature begins to decrease. The atoms slow down, hit each other less frequently, begin to settle as gravity has more effect on them the chaos decreases.
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Hydrocarbon combustion Simple applet that illustrates the Chemical equation.
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There was an error working with the wiki: Code, Wikipedia: The Free Encyclopedia. Wikimedia Foundation.
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