Thermodynamics Thermodynamics describes the physics of matter using the concept of the thermodynamic system, a region of the universe that is under study. All quantities, such as pressure or mechanical work, in an equation refer to the system unless labeled otherwise. As thermodynamics is fundamentally concerned with the flow and balance of energy and matter, systems are distinguished depending on the kinds of interaction they undergo and the types of energy they exchange with the surrounding environment. Isolated systems are completely isolated from their environment. They do not exchange heat, work or matter with their environment. An example of an isolated system is a completely insulated rigid container, such as a completely insulated gas cylinder. Closed systems are able to exchange energy (heat and work) but not matter with their environment. A greenhouse is an example of a closed system exchanging heat but not work with its environment. Whether a system exchanges heat, work or both is usually thought of as a property of its boundary. Open systems may exchange any form of energy as well as matter with their environment. A boundary allowing matter exchange is called permeable. The ocean would be an example of an open system. Boundary A system boundary is a real or imaginary volumetric demarcation region drawn around a thermodynamic system across which quantities such as heat, mass, or work can flow. [1] In short, a thermodynamic boundary is a division between a system and its surroundings. Boundaries can also be fixed (e.g. a constant volume reactor) or moveable (e.g. a piston). For example, in an engine, a fixed boundary means the piston is locked at its position; as such, a constant volume process occurs. In that same engine, a moveable boundary allows the piston to move in and out. Boundaries may be real or imaginary. For closed systems, boundaries are real while for open system boundaries are often imaginary. A boundary may be adiabatic, isothermal, diathermal, insulating, permeable, or semipermeable. Surroundings Interactions of thermodynamic systems Type of system Mass flow Work Heat Open Closed Isolated The system is the part of the universe being studied, while the surroundings is the remainder of the universe that lies outside the boundaries of the system. It is also known as the environment, and the reservoir. Depending on the type of system, it may interact with the system by exchanging mass, energy (including heat and work), momentum, electric charge, or other conserved properties. The environment is ignored in analysis of the system, except in regards to these interactions.
System Work When work is done by a thermodynamic system, it is ususlly a gas that is doing the work. The work done by a gas at constant pressure is: Example For non-constant pressure, the work can be visualized as the area under the pressurevolume curve which represents the process taking place. The more general expression for work done is: Thermodynamics Thermodynamics is the study of the conversion of energy between heat and other forms, mechanical in particular. There are many processes that convert energy from one form to another. For example burning wood converts chemical energy (in the wood) to heat; turning a hydroelectric generator converts the kinetic (motion) energy of the water into electrical energy. There are two laws which we can use to help understand processes like these: The first law of thermodynamics says that energy is conserved, it is neither created nor destroyed but can change form. The second law of thermodynamics says that systems always tend to states of greater disorder -- this is another way to say that the entropy always increases. In terms of energy conversions this means that they can never be 100% efficient. Some portion of the energy involved in a conversion will inevitably be lost to the surroundings as heat. System The first concept which must be understood in applying thermodynamics is the necessity to begin with the definition of what is called a "system". In thermodynamics this is any region completely enclosed within a well defined boundary. Everything outside the system is then defined as the surroundings. Although it is possible to speak of the subject matter of thermodynamics in a general sense, the establishment of analytical relationships among heat, work, and thermodynamic properties requires that they be related to a particular system.
We must always distinguish clearly between energy changes taking place within a system and energy transferred across the system boundary. We must likewise distinguish between properties of material within a system and properties of its surroundings. In accordance with their definition, thermodynamic properties apply to systems which must contain a very large number of ultimate particles. Other than this there are no fundamental restrictions on the definition of a system. The boundary may be either rigid or movable. It can be completely impermeable or it can allow energy or mass to be transported through it. In any given situation a system may be defined in several ways; although with some definitions the computations to be performed are quite simple, with others they are difficult or even impossible. For example, it is often impossible by means of thermodynamic methods alone to make heat transfer calculations if a system is defined so that both heat transfer and diffusional mass transfer occur simultaneously through the same area on the boundary of the system. For processes in which mass transfer takes place only by bulk stream flow this problem can be avoided easily by a proper definition of the system. In a flow process of this type the system is defined so that it is enclosed by moveable boundaries with no stream flows across them. Heat transfer then always occurs across a boundary not crossed by mass. An open system is one where both matter and energy can freely cross from the system to the surroundings and back. eg an open test tube A closed system is one where energy can cross the boundary, but matter cannot. eg a sealed test tube An isolated system is one where neither matter nor energy can cross between the system and the surroundings. The universe itself is an isolated system (as there are no surroundings to exchange matter or energy with). Classical thermodynamics Classical thermodynamics is the description of the states of thermodynamical systems at near-equilibrium, using macroscopic, empirical properties directly measurable in the laboratory. It is used to model exchanges of energy, work and heat based on the laws of thermodynamics. The qualifier classical reflects the fact that it represents the level of
knowledge in the early 19th century. An atomic interpretation of these principles was provided later by the development of statistical mechanics. A thermodynamic system is characterized and defined by a set of thermodynamic parameters associated with the system. The parameters are experimentally measurable macroscopic properties, such as volume, pressure, temperature, electric field, and others. Thermodynamics: Thermodynamics is the branch of science or physics that studies various forms of energies and their conversion from one form to the other like electrical energy to mechanical energy, heat to electrical, chemical to mechanical, wind to electrical etc. The study of thermodynamics is comprised of important laws of thermodynamics namely first law of thermodynamics, second law of thermodynamics, third law of thermodynamics and Zeroth law of thermodynamics. Thermodynamics is an important subject area studied under Mechanical Engineering. System: A quantity of the matter or part of the space which is under thermodynamic study is called as system. There are three types of system: closed system, open system and isolated system. Surroundings or environment: Everything external to the matter or space, which is under thermodynamic study is called surroundings or environment. Boundary: The boundary that separates the system and surrounding is called as system boundary. The system boundary may be fixed or moving. Closed system: The system of fixed mass across the boundary of which no mass transfer can take place is called as closed system. However, across the closed system the energy transfer may take place. An example is fluid being compressed by the piston in cylinder. Open system: The system across the boundary of which transfer of both mass as well as energy can take place across the boundary is called as open system. An example is an air compressor. Isolated system: The system in which both the mass as well as energy content remains constant is called an isolated system. In this system no mass or energy transfer takes place across the boundary. State of the system: The present status of the system described in terms of properties such as pressure, temperature, and volume is called the state of system. Properties of the system: The characteristics by which the physical condition of the system is described are called as properties of system. Some examples of these characteristics are: temperature, pressure, volume etc and are called as properties of system. The system properties are of two types: extensive and intensive properties.
Extensive properties of system: The properties of the system that depend on the mass or quantity of the system are called extensive properties. Some examples of extensive properties are: mass, volume, enthalpy, internal energy, entropy etc. Intensive properties of the system: These properties do not depend on the quantity of matter of the system. Some of the examples of intensive properties are: freezing point temperature, boiling point, temperature of the system, density, specific volume etc. Some important terms and types of systems Here are some more important terms which are found frequently in the subject area of thermodynamics and various types of system. Change of state of system: When one or more properties of the system like pressure, temperature, volume changes, the state of system changes. Path of change of state: The succession of states through which the system undergoes change to reach the final state is called as the path of change of state of the system. Homogeneous system: The system that has single or uniform phase such as like solid or liquid or gaseous is called as homogeneous system. Heterogeneous system: The system that has more than one phase i.e. the combination of solid, liquid and gaseous state is called as heterogeneous system. Thermodynamics process: When the system changes from one thermodynamic state to the final thermodynamic state due to change in pressure, temperature, volume etc, the system is said to have undergone thermodynamic process. The various types of thermodynamic processes are: isothermal process, adiabatic process, isochoric process, isobaric process and reversible process. Cyclic process: When the system undergoes a number of changes in states and returns back to the initial state, the system is said to have undergone cyclic process. Isothermal process: The process during which the temperature of the system remains constant is called as isothermal process. Adiabatic process: The process during which the heat content of the system remains constant i.e. no flow of heat takes place across the boudaries of system, the process is called as adiabatic process. Isochoric process: In this process the volume of system remains constant.
Isobaric process: The process during which the pressure of the system remains constant, is called as isobaric process. Reversible process: When the system undergoes changes infinitesimally slowly the changes can be reversed back, such a process is called as reversible process. During reversible process the system remains in equilibrium during the change of state of the system. Conditions for Thermodynamic Equilibrium The system is said to be in thermodynamic equilibrium if the conditions for following three equilibrium is satisfied: 1) Mechanical equilibrium 2) Chemical equilibrium 3) Thermal equilibrium 1) Mechanical equilibrium: When there are no unbalanced forces within the system and between the system and the surrounding, the system is said to be under mechanical equilibrium. The system is also said to be in mechanical equilibrium when the pressure throughout the system and between the system and surrounding is same. Whenever some unbalance forces exist within the system, they will get neutralized to attain the condition of equilibrium. Two systems are said to be in mechanical equilibrium with each other when their pressures are same. 2) Chemical equilibrium: The system is said to be in chemical equilibrium when there are no chemical reactions going on within the system or there is no transfer of matter from one part of the system to other due to diffusion. Two systems are said to be in chemical equilibrium with each other when their chemical potentials are same. 3) Thermal equilibrium: When the system is in mechanical and chemical equilibrium and there is no spontaneous change in any of its properties, the system is said to be in thermal equilibrium. When the temperature of the system is uniform and not changing throughout the system and also in the surroundings, the system is said to be thermal equilibrium. Two systems are said to be thermal equilibrium with each other if their temperatures are same. For the system to be thermodynamic equilibrium it is necessary that it should be under mechanical, chemical and thermal equilibrium. If any one of the above condition are not fulfilled, the system is said to be in non-equilibrium. The zeroth law of thermodynamics is a generalization principle of the thermal
equilibrium among bodies, or thermodynamic systems, in contact. Systems are in thermal equilibrium if they do not exchange energy in the form of heat. The zeroth law states that if two systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This means that thermal equilibrium is transitive and it affords the definition of an empirical physical parameter, called temperature, which is the same for all systems in thermal equilibrium. The law permits the construction of a thermometer to measure this property. Zeroth Law of Thermodynamics If a system A is in thermal equilibrium with another system B and also with a third system C, then all of the systems are in thermal equilibrium with each other. This is called the zeroth law of thermodynamics. This is how a thermometer works. If a thermometer is placed in a substance for temperature measurement, the thermometer's glass comes into thermal equilibrium with the substance. The glass then comes into thermal equilibrium with the liquid (mercury, alcohol, etc...) inside the thermometer. Because the substance is in thermal equilibrium with the glass and the glass is in thermal equilibrium with the inner liquid, the substance and liquid must be in thermal equilibrium by the zeroth law. And because they are thermally equivalent, they must have the same temperature. The Ideal Gas Recall that a thermodynamic system may have a certain substance or material whose quantity can be expressed in mass or moles in an overall volume. These are extensive properties of the system. If the substance is evenly distributed throughout the volume in question, then a value of volume per amount of substance may be used as an intensive property. For an example, for an amount called a mole, volume per mole is typically called molar volume. Also, a volume per mass for a specific substance may be called specific volume. In such cases, an equation of state may relate the three intensive properties, temperature, pressure, and molar or specific volume. A simple but very useful equation of state is for an ideal gas. The ideal gas is a useful notion in thermodynamics, as it is a simple system that depends on two independent properties. An ideal gas is one that has no intermolecular interactions except for completely elastic collisions with other molecules. For a closed system containing an ideal gas, the state can be specified by giving the values of any two of pressure, temperature, and molar volume. Consider a system, an ideal gas enclosed in a container. Starting from an initial state 1, where the temperature is T 1, its temperature is changed to T 2 through a constant pressure process and then a constant molar volume process, then the ratio of pressures is found to
be the same as the ratio of molar volumes. Suppose the initial value of the pressure and molar volume are p 1 and V 1 respectively, and final value of pressure and molar volume are p 2 and V 2 respectively. Note that we haven't chosen a specific scale for the temperature (like say, the Celsius scale). Now, suppose we were to choose a scale such that T 1 /T 2 = p 1 /p 2, we can show that the value of pv/t is constant for an ideal gas, so that it obeys the gas equation pv = RT, where p is the absolute pressure, V is the molar volume, and R is a constant known as the universal gas constant. The temperature T is the absolute temperature in the ideal gas scale, and the scale is found to be the same as the thermodynamic temperature scale. The thermodynamic temperature scale will be defined after the statement of the second law of thermodynamics. This equation pv = RT is called the equation of state for an ideal gas, and is known as the ideal gas equation. Most common gases obey the ideal gas equation unless they are compressed or cooled to extreme states, so this is a very useful relation. A similar equation may be written where, for the specific type of gas, specific volume is used instead of molar volume and a specific gas constant is used instead of the universal gas constant. This then writes as pv = mrt. The Laws of Thermodynamics Alternative statements that are mathematically equivalent can be given for each law. Zeroth law: Thermodynamic equilibrium. When two systems are put in contact with each other, there will be a net exchange of energy and/or matter between them unless they are in thermodynamic equilibrium. Two systems are in thermodynamic equilibrium with each other if they stay the same after being put in contact. The zeroth law is stated as If systems A and B are in thermodynamic equilibrium, and systems B and C are in thermodynamic equilibrium, then systems A and C are also in thermodynamic 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. There is still some discussion about its status. Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential). 1st Law: Conservation of energy. This is a fundamental principle of mechanics, and more generally of physics. In thermodynamics, it is used to give a precise definition of heat. It is stated as follows:
The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process. or The net sum of exchange of heat and work of a system with the environment is a change of property. The amount of property change is determined only by the initial and final states and is independent on the path through which the process takes place. or The heat flowing into a system equals the increase in internal energy of the system plus the work done by the system. or Energy cannot be created or destroyed, only modified in form. 2nd Law: A far reaching and powerful law, it is typically stated in one of two ways: It is impossible to obtain a process that, operating in cycle, produces no other effect than the subtraction of a positive amount of heat from a reservoir and the production of an equal amount of work. (Kelvin-Planck Statement) or It is impossible to obtain a process that, operating in cycle, produces no other effect than a positive heat flow from a colder body to a hotter one. (Clausius Statement) Entropy entropy is a measure of the uncertainty associated with a random variable. Clausius Inequality The Clausius theorem (1854) states that in a cyclic process The equality holds in the reversible case [1] and the '<' is in the irreversible case. The reversible case is used to introduce the state function entropy. This is because in cyclic process the variation of a state function is zero.
Proof Proving Clausius Inequality Suppose a system absorbs heat δq at temperature T. Since the value of does not depend on the details of how the heat is transferred, we can assume it is from a Carnot engine, which in turn absorbs heat δq 0 from a heat reservoir with constant temperature T 0. According to the nature of Carnot cycle, Therefore in one cycle, the total heat absorbed from the reservoir is
RANKINE CYCLE Since after a cycle, the system and the Carnot engine as a whole return to its initial status, the difference of the internal energy is zero. Thus according to First Law of Thermodynamics, Q 0 = ΔU + W + W 0 = W + W 0 = W total According to the Kelvin statement of Second Law of thermodynamics, we cannot drain heat from one reservoir and convert them entirely into work without making any other changes, so Combine all the above and we get Clausius inequality Steam Power Plant 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.
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 THERMAL EFFICIENCY Boiler/Superheater Condenser Turbine
Pump Internal Combustion Engine The Spark Ignition (SI) engines work on the principle of cycle of operations invented by Nicolaus A. Otto in the year 1876. The Compression Ignition (CI) engines work on the principle founded by Rudolf Diesel in the year 1892. For the engine to work properly it has to perform some cycle of operations continuously. The principle of operation of the spark ignition (SI) engines was invented by Nicolaus A. Otto in the year 1876; hence SI engine is also called the Otto engine. The principle of working of compression ignition engine (CI) was found out by Rudolf Diesel in the year 1892, hence CI engine is also called the Diesel engine. The principle of working of both SI and CI engines are almost the same, except the process of the fuel combustion that occurs in both engines. In SI engines, the burning of fuel occurs by the spark generated by the spark plug located in the cylinder head. The fuel is compressed to high pressures and its combustion takes place at a constant volume. In CI engines the burning of the fuel occurs due to compression of the fuel to excessively high pressures which does not require any spark to initiate the ignition of fuel. In this case the combustion of fuel occurs at constant pressure. Both SI and CI engines can work either on two-stroke or four stroke cycle. Both the cycles have been described below:
1) Four-stroke engine: In the four-stroke engine the cycle of operations of the engine are completed in four strokes of the piston inside the cylinder. The four strokes of the 4- stroke engine are: suction of fuel, compression of fuel, expansion or power stroke, and exhaust stroke. In 4-stroke engines the power is produced when piston performs expansion stroke. During four strokes of the engine two revolutions of the engine's crankshaft are produced. 2) Two-stroke engine: In case of the 2-stroke, the suction and compression strokes occur at the same time. Similarly, the expansion and exhaust strokes occur at the same time. Power is produced during the expansion stroke. When two strokes of the piston are completed, one revolution of the engine's crankshaft is produced. In 4-stroke engines the engine burns fuel once for two rotations of the wheel, while in 2- stroke engine the fuel is burnt once for one rotation of the wheel. Hence the efficiency of 4-stroke engines is greater than the 2-stroke engines. However, the power produced by the 2-stroke engines is more than the 4-stroke engines. Read more:
Compression stroke: In this one, both valves should be closed. The piston starts to move upward to compress the fuel, until it reaches the top dead center. By compressing the fuel, the fuel temperature and pressure increases.figure 5b Power Stroke: As the piston reaches the top dead center, the spark plug ignites a spark, allowing the fuel to burn. The combustion yields a high power that is transmitted through the crankshaft mechanism. It should be noted that in order for the combustion energy to be consumed efficiently in moving the piston, both valves should be closed.figure 5c Exhaust Stroke: After reaching to the maximum displacement of the piston, most of the energy liberated is transferred. Accordingly, the pistons starts it back upward motion to get rid of the exhaust gases that result from combustion. At that moment, the exhaust valve is opened to allow it to go outside the cylinder.figure 5d It should be clear from the above argument that for one complete cycle to be done, the crankshaft has to finish two revolutions.
CYCLE CATEGORIZATION: This is one of the important points to discuss, which is the thermodynamics of the combustion process. There are two main cycles based on which we can categorize internal combustion engines, which are: Otto cycle and Diesel cycle. OTTO CYCLE: Otto cycle is the typical cycle for most of the cars internal combustion engines, that work using gasoline as a fuel. Otto cycle is exactly the same one that was described for the four-stroke engine. It consists of the same four major steps: Intake, compression, ignition and exhaust. Figure 6 PV diagram for Otto cycle On the PV-diagram, 1-2: Intake: suction stroke 2-3: Isentropic Compression stroke 3-4: Heat addition stroke 4-5: Exhaust stroke (Isentropic expansion) 5-2: Heat rejection The distance between points 1-2 is the stroke of the engine. By dividing V2/V1, we get: where r is called the compression ratio of the engine. The efficiency is taken to be:
DIESEL CYCLE: In the Diesel Cycle, named after Rudolf Christian Karl Diesel (1858-1913), only air is admitted in the intake stroke. The air is then adiabatically compressed, and fuel is injected into to the hot air in the form of many small drops (not a vapor). Each drop burns over a small time, giving an approximation of a isobaric explosion. The explosion pushes the cylinder outwards. The power stroke, valve exhaust, and exhaust stroke which follow are identical to those in the Otto Cycle. A - 1 to 2: Isentropic compression B - 2 to 3: Reversible constant pressure heating C - 3 to 4: Isentropic expansion D - 4 to 1: Reversible constant volume cooling In other words, the only difference between is the Otto engine and diesel engine is that the latter does not require a spark plug to ignite the fuel; the fuel here is ignited under the effect of increase in pressure and temperature. In Diesel engines, compression ratios are as high as 22.5 to 1, where for Otto engines it normally does not reach even one fifth that number. The four cycles of the diesel engine are: 1 - The piston is moved away from the cylinder head by the
crankshaft, drawing only air into the cylinder. 2 - The piston moves towards the cylinder head, compressing the air. At the end of the stroke vaporized fuel is injected into the cylinder and is ignited by the high temperature of the air. 3 - The piston is forced away from the cylinder head by the gas, expanding after the ignition of the fuel. 4 - The exhaust valve is opened and the piston moves towards the cylinder head, driving the exhaust gases from the cylinder. CONCLUSION: Internal combustion engines are among the most important engineering applications. The theory of application either depends on Diesel or Otto cycles. They are categorized either according to the operating cycle, or due to the mechanism of working. Each type of engines has some advantages over the other one. Thus, the selection of the appropriate engine requires determining the conditions of application.