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Engineering thermodynamics

Thermodynamics is a subject that studies the properties of the material system in an equilibrium state in thermal phenomena, establishes the energy balance relationship, and interacts with the outside world when the state changes.

Engineering thermodynamics is the first branch of thermodynamics, which mainly studies the law and application of mutual conversion of thermal energy, mechanical energy and other energy, and is one of the important basic disciplines of mechanical engineering.

The basic task of engineering thermodynamics is to improve and perfect the working cycle of heat engine, refrigerator and heat pump through the analysis and research of thermal system, heat balance, thermal state, thermal process, thermal cycle and working medium, and to improve the utilization rate of heat energy and the conversion efficiency of heat and work.

Therefore, it is necessary to explore the characteristics of various thermodynamic processes from the basic laws of thermodynamics; Study the thermophysical properties of gases and liquids, as well as the laws of phase transition such as evaporation and condensation; The analysis of some types of refrigerators also needs to study the characteristics of the solution. Modern engineering thermodynamics also includes chemical reaction processes such as combustion, physical and chemical processes such as dissolution, absorption or desorption, which involve the basic knowledge of chemical thermodynamics.

Engineering thermodynamics is a macroscopic theory about thermal phenomena, and its research method is macroscopic. Based on the first law, second law and third law of thermodynamics, it studies macroscopic phenomena and thermodynamic processes through macroscopic parameters such as pressure, temperature and specific volume and overall behaviors such as heating, cooling, expansion and contraction.

In this method, the specific properties related to the internal structure of matter are confirmed as macroscopic real physical data, and there is no need to make any assumptions about the microstructure of matter, so the results of analysis and reasoning are highly reliable and clear. This is its unique advantage.

Ancient humans learned to use fire for a long time, but later they paid attention to exploring the essence of cold and hot phenomena. But until the end of 17, people could not correctly distinguish the essence of the two basic concepts of temperature and heat. Under the popular "heat theory" at that time, people mistakenly thought that the high temperature of an object was due to the storage of a large amount of "heat mass" With the establishment of Fahrenheit temperature scale 1709 ~ 17 14 and centigrade scale 1742 ~ 1745, temperature measurement has become an accepted standard. Subsequently, calorimetry technology was developed, which provided experimental means for scientific observation of thermal phenomena and made thermodynamics embark on the road of modern experimental science.

In 1798, langford observed that when drilling the barrel with a drill bit, mechanical work was consumed, which led to the heating of the drill bit and barrel. 1799, the Englishman David rubbed two pieces of ice against each other and caused the surface to melt, which obviously cannot be explained by heat theory. 1842, Meyer put forward the theory of conservation of energy, holding that heat is a form of energy, which can be converted by mechanical energy, and the mechanical equivalent of heat can be calculated from the difference between the specific heat capacity of constant pressure and that of constant volume air.

British physicist Joule established the concept of electrothermal equivalent in 1840, and measured the mechanical equivalent of heat in different ways after 1842. 1850, Joule's experimental results made the scientific community completely abandon the "heat theory". The recognized first law of thermodynamics, the conservation of energy and the interchangeability of forms of energy, is an objective natural law. The energy unit Joule is named after him.

The formation of thermodynamics was related to the urgent need to find a reasonable large and efficient heat engine in production practice at that time. 1824, the Frenchman Carnot put forward the famous Carnot theorem, pointed out the efficiency limit that a heat engine can achieve when it works in a given temperature range, and essentially established the second law of thermodynamics. However, influenced by the "heat theory", his proof method is still wrong. 1848, British engineer Kelvin calculated the thermodynamic temperature scale according to Carnot theorem. 1850 and 185 1 year, Clausius and Kelvin of Germany put forward the second law of thermodynamics, and on this basis, they re-proved Carnot's theorem.

From 1850 to 1854, Clausius proposed and developed the concept of entropy according to Carnot's theorem. The confirmation of the first law and the second law of thermodynamics made a scientific conclusion that the two perpetual motion machines could not be realized, and formally formed the macroscopic theoretical thermodynamics of thermodynamic phenomena. At the same time, it has also formed the technical science of "engineering thermodynamics", which has become the theoretical basis for studying the working principle of heat engines, and has made rapid development in internal combustion engines, steam turbines, gas turbines, jet propellers and so on.

At the same time, in the process of applying thermodynamic theory to study the properties of matter, the mathematical theory of thermodynamics has been developed, the corresponding thermodynamic functions reflecting various properties of matter have been found, and various laws followed by matter in phase transition, chemical reaction and solution characteristics have been studied. 1906, Nernst discovered the thermodynamic theorem by observing low temperature phenomena and chemical reactions. In 19 12, this theorem is modified into the expression of the third law of thermodynamics.

Since the beginning of the twentieth century, new achievements have been made in the study of physical properties such as ultra-high pressure, ultra-high temperature water vapor and ultra-low temperature. With people's attention to energy issues, the research on the compound cycle and new compound working fluids related to energy conservation has aroused great interest.