Thermodynamics System Properties and Polymorphic Processes - Report Example

Thermodynamics System Properties and Polymorphic Processes Student Number: Date: Introduction Thermodynamics is majorly concerned with temperature and heat and how these two relate to energy and work. These quantities are all governed by the laws of thermodynamics. This work covers thermodynamic system properties and polymorphic processes, including different types of thermodynamic cycles and methods that are used to improve engine performance. Discussion TASK 1: A. Explain the following thermodynamics system properties and polytrophic processes: 1. Closed and open systems A closed system (also called control mass system) is one that allows energy transfer in the form of heat and work, but no mass crosses the system boundary. This means that the volume of a closed system does not remain constant. In an open system, both mass and energy are transferred across the system boundary. An open system is normally by a device that allow the flow of mass, such as a compressor, nozzle or turbines (Turns, 2006). 2. Intensive and extensive properties An intensive property is a physical property or a bulk property that is independent of the size of the system or the amount of mass. Examples of intensive properties are density, specific volume, temperature, pressure refractive index and hardness. On the other hand, an extensive property is a property that vary directly with mass and is additive for sub-systems. Examples of extensive properties include mass and volume. Thus, if a given amount of mater is divided into two equal parts, the intensive properties will remain the same in each piece as original, while the extensive properties will be half the original value of an extensive property (Arora, 2001). 3. Two property rule The two property rule states that if any two independent intensive properties of a system in equilibrium are specified, then it is possible to find the other intensive properties using thermodynamic relations. Two properties are considered to be independent if varying one property does not vary the second property (remains constant). For example, specific volume and temperature are properties that are independent. 4. Isochoric process An isochoric process (also known as an isometric process or a constant-volume process or iso-volumetric process) is a thermodynamic process in which the volume of a closed system that is undergoing the process remains constant. The process is exemplified by cooling or heating the closed system contents; and the condition of a constant volume is imposed by the inability of the container to expand or deform (Rajput, 2009). 5. Isobaric process An isobaric process is thermodynamic process in which the pressure of a system remains constant. The heat that is transferred into the system does work by changing the system’s internal energy to allow volume expansion and contraction to neutralize pressure changes due to the heat transfer (Arora, 2001). This maintains the pressure in the system to a constant amount. 6. Isothermal process An isothermal process is a change of a thermodynamic process in which there is no change in temperature. This occurs when a thermodynamic system comes into contact with an external heat bath, and the change typically occurs slowly enough to allow the system to adjust to the temperature of the heat bath through heat transfer and exchange. 7. Adiabatic process This is a thermodynamic process in which there is no heat gain or heat loss or mass transfer by the system to its surroundings. Energy is transferred as work. Thus, adiabatic condition can be used to obtain the expression the amount of work done in an adiabatic process. This process also provides a conceptual basis on which the first law of thermodynamics is derived, and as such it is a very important concept in thermodynamics (Nag, 2008). TASK 2: i. Carnot Cycle The Carnot cycle is the most efficient thermodynamic heat engine cycle that consist of two adiabatic processes and two isothermal processes. While the 2nd Law of Thermodynamics states that not all heat energy supplied is used to do work in a heat engine, the Carnot efficiency provides a limiting value that a classical heat engine can achieve when heat is converted to work, or vice versa. The Carnot cycle can be illustrated using a P-V diagram shown below: Figure 1: Carnot cycle illustrated on a P-V diagram to show the work done. The Carnot cycle consists of the following steps: 1. Isothermal heat addition or absorption at temperature T1: In this step (begins from 1 to 2 in figure 1 above), the gas expands and does work. During this process, the temperature remains constant, and thus, an isothermal expansion is achieved. Absorption of heat energy from the reservoir at a high temperature (Q1) propels gas expansion, resulting in increase of entropy of the gas by the amount: 2. Isentropic expansion of the gas: In this step (2-3 in figure 1), the mechanisms of the heat engine neither loses nor gain heat – an adiabatic process. Gas expansion continues and the system loses internal energy equal to the work done by expansion of the gas. The work that leaves the system due to gas expansion causes cooling of the system to a temperature T2; the entropy remains constant. 3. Isothermal heat rejection at temperature T2: In this step (3-4 in figure 1), the system surroundings do work on the gas, resulting in the leaving of heat energy Q2 to the reservoir at low temperature. The entropy reduces by the amount: (Equal to the amount absorbed in step 1) 4. Isentropic work input: In this step (4-1 in figure 1), the surroundings do some work on the system, increasing the internal energy of the gas and compressing it. This results in rise in temperature to T1, but the entropy remains constant. Thus, the gas goes back to the same state as in step 1 and; Thermal Efficiency of the Carnot cycle, = 1- ii. Otto Cycle This cycle describes how a typical spark ignition internal combustion engine works. It describes what happens to a gas subjected to changes of temperature, pressure, volume, and addition or removal of heat. The processes involved in an ideal Otto cycle are two isentropic and two isochoric processes as illustrated in figure 2 below: Figure 2: P-V diagram illustrating an ideal Otto cycle. 1. Intake stroke at constant pressure (process 1-2): A mass of air at near atmospheric pressure is drawn into the piston. The gas volume increases as air is drawn into the piston through the intake valve. 2. Compression stroke (process 2-3): As the piston moves from BDC to TDC, the air is compressed in an isentropic process. The volume of the gas decreases and the pressure increases, some work is done. 3. Air combustion (process 3-4): This is an isochoric process in which heat from an external source is transferred to the gas. The volume remains constant during the combustion process. The temperature and pressure increases due to heat released from the burning gas. Heat supplied () = Where: – Mass – Specific heat (at constant volume) 4. The power stroke (process 4-5): This is an adiabatic expansion process. As the piston moves towards the crankshaft, the gas volume increases and the pressure drops as work is done on the piston. 5. Valve exhaust (process 5-6): The cycle is completed by a constant-volume process in which a valve opens to release the heated gas to the surrounding. The pressure adjusts to near atmospheric pressure and the volume remains constant. Heat rejected () = 6. Exhaust stroke (process 6-1): In this process, the piston moves to the BDC where the process began. The volume drops and the pressure remains constant and the process repeats itself. Thermal Efficiency of the Otto cycle = iii. Diesel cycle This is a combustion process of a typical reciprocating internal combustion engine. In contrast to the spark ignition internal combustion engine in the Otto cycle, the fuel is ignited when heat is generated in the combustion chamber. It is assumed that a diesel cycle has constant pressure in the initial stages of the combustion phase (Invernizzi, 2013). The diagram in figure 3 below shows an ideal P-V diagram of a diesel cycle. Figure 3: P-V diagram of an ideal diesel cycle The cycle has the following four processes: 1. Isentropic compression process: In this process (1-2 in figure 3), the gas is compressed by the movement of the piston from BDC to TDC. The pressure increases fromP1 to P2, the temperature increases from T1 to T2 while the volume increases from V1 to V2. Entropy remains constant (S1=S2). The work done on the system is denoted by Win. 2. Reversible constant-pressure heating: This is process 2-3, heat from an external source is added into the system at constant pressure. The gas volume increases from V2 to V3 and the temperature increases from T2 to T3. Entropy increases from S1 to S3. Heat supplied (Qin) = Where: – Mass of air (kg) – Specific heat (at constant pressure) kJ/kgK – Temperature at point 2&3 respectively (K) 3. Isentropic Expansion: In this process (3-4 in figure 3), the piston moves from TDC to BDC. The compressed heated air expands isentropically in the cylinder, causing decrease of pressure from P3 to P2, temperature drop from T3 to T4, and increase in volume from V3 to V4. Entropy does not change (S3 = S4). The work done during this process is denoted Wout. 4. Reversible constant-volume heat rejection process: This process is denoted 4-1 in figure 3. Here, the system rejects heat at constant volume, V4 = V1. Pressure drops from P4 to P1, while the temperature drops from T4 to T1, and there is a decrease in entropy from S4 to S1. Heat rejected (out) = Thermal efficiency of the diesel cycle = ; iv. Dual Cycle This thermal cycle combines the Otto cycle and the Diesel cycle. Heat is supplied at partly constant pressure and partly constant volume, creating more time for complete combustion of the fuel. The cycle consists of two isochoric and one constant pressure processes. A P-V diagram of the cycle is shown in figure 4 below. There are five operations in dual cycle: 1. Adiabatic compression: This is the process 1-2 on figure 4, in which isentropic compression causes a rise in temperature and pressure, while the volume decreases. 2. Heat supply at constant volume: This process (2-3) is like in the Otto cycle where heat from an external source is added into the system. 3. Heat supply at constant pressure: In this process (3-4), heat continues to be supplied, but now at a constant pressure, like in the diesel cycle. The volume and temperature of the gas increases. 4. Adiabatic expansion: In this process (4-5), isentropic expansion of the heated air occurs. There is a drop in pressure and temperature as the volume increases. 5. Heat rejection at constant volume: Here (5-1), the cycle goes back to where it began. Heat is rejected to the surrounding at constant volume. There is also a drop in temperature and pressure. Figure 4: P-V diagram of a dual cycle. The efficiency of dual a dual cycle lies between Diesel and Otto cycle. v. Joules or Brayton cycle The Joules cycle represents an ideal air standard cycle operation of a gas turbine engine. The cycle operates in four process as shown in the P-V diagram in figure 5. 1. Isentropic compression process: This process is represented as 1-2 in figure 5. Here, the compressor draws in air and increases its pressure. There is a drop in the volume and the temperature increases. 2. Isobaric process (2-3): In this process, the compressed air/fuel mixture is combusted at constant pressure. The heat supplied to the cycle is given by: Figure 5: P-V diagram of Brayton cycle 3. Isentropic expansion (3-4): The heated air releases energy as it expands through the turbine. Some of the work is used to drive the compressor, and some is used to accelerate the fluid to turn a generator or propel a jet. 4. Isobaric process (4-1): This is a cooling process in which heat rejection to the atmosphere occurs at constant pressure. The gas volume and temperature drops back to the initial point. The heat rejected is given by: Thermal Efficiency of Brayton cycle, =   TASK 4: The second law of thermodynamics is a principle which puts constraints on the direction of heat transfer in a system and the efficiencies that can be attained by heat engines. The law states that the entropy of a closed system increases over time, or tends to remain constant in an ideal steady state or during a reversible process. Irreversibility of process that occur naturally and the asymmetry between the past and future is accounted by the increased entropy (Boles & Cengel, 2014). Rudolf Clausius states the second law of thermodynamics that heat cannot be transferred from a colder body to a warmer body without another change connected to the system occurring at the same time (see figure 6). Figure 6: An isolated thermal system Thermal efficiency indicates how good a closed system can convert the heat energy supplied to do work. The second law of thermodynamics states that it is not possible to have an isolated system that can change all the heat energy to work. Some energy is lost during the conversion process as illustrated in figure 6 above. TASK 5: 1. Supercharging A supercharger is a mechanical air compressor that increases the air pressure supplied to the internal combustion engine’s intake manifold. This ensures more oxygen during each intake, causing more fuel combustion and more work done. Thus, increasing the engine power. Figure 7: A two-stroke engine with a supercharger 2. Turbocharging The efficiency of an engine is directly related to its volumetric efficiency. Turbo chargers uses the force exerted by exhaust gases to force more air into the cylinder, increasing the cylinder pressure. Hence, there is increased combustion due to increased air supply. This improves the volumetric efficiency, which in turn improves the engine’s overall efficiency. Figure 8: A turbocharger system 3. Intercooling This is a technique in which compression of the gas is done in multiple stages and in-between the stages, it is cooled to its initial temperature by passing the gas in a heat exchanger. The heat exchanger acts as an intercooler. Intercoolers make charged air denser, promoting more intake into the engine due to forced induction. Intake of denser air also eliminates the risk of pre-detonation of air/fuel charge prior to spark ignition (Granet & Bluestein, 2014). This increases the efficiency of the engine’s ignition system. The overall result is increased engine performance. 4. Cooling system Cooling is done by passing coolant liquid (water) mixed with antifreeze agent through cooling passages. In some engines, cooling is done by air flow over cylinder fins. The diagram below illustrates the cooling system of an automobile. Figure 9: Automobile cooling system A cooling system helps to save fuel economy and reduce emissions at low temperatures. At high temperatures, the cooling system reduces the danger of self-destruction of the engine. 5. Exhaust gas heat recovery (EGHR) systems These systems are used to turn thermal losses that would otherwise be wasted in the exhaust into energy. A diagrammatic representation of an EGHR system is shown below: Figure 10: Automobile EGHR system It improves cabin warm-up and enhances coolant warming via exhaust gas energy. EGHR also promotes fuel efficiency through rapid warming of powertrain fluids, thus, reducing friction (Boles & Cengel, 2014). Conclusion To conclude, a given variable may be held constant in five common processes of a thermodynamic system. These are isothermal process, adiabatic process, isobaric process, isentropic process and isovolumetric process. The state of a thermodynamic system is described by various intensive and extensive variables that are independent of these processes.   Read More

The process is exemplified by cooling or heating the closed system contents; and the condition of a constant volume is imposed by the inability of the container to expand or deform (Rajput, 2009). 5. Isobaric process An isobaric process is thermodynamic process in which the pressure of a system remains constant. The heat that is transferred into the system does work by changing the system’s internal energy to allow volume expansion and contraction to neutralize pressure changes due to the heat transfer (Arora, 2001).

This maintains the pressure in the system to a constant amount. 6. Isothermal process An isothermal process is a change of a thermodynamic process in which there is no change in temperature. This occurs when a thermodynamic system comes into contact with an external heat bath, and the change typically occurs slowly enough to allow the system to adjust to the temperature of the heat bath through heat transfer and exchange. 7. Adiabatic process This is a thermodynamic process in which there is no heat gain or heat loss or mass transfer by the system to its surroundings.

Energy is transferred as work. Thus, adiabatic condition can be used to obtain the expression the amount of work done in an adiabatic process. This process also provides a conceptual basis on which the first law of thermodynamics is derived, and as such it is a very important concept in thermodynamics (Nag, 2008). TASK 2: i. Carnot Cycle The Carnot cycle is the most efficient thermodynamic heat engine cycle that consist of two adiabatic processes and two isothermal processes. While the 2nd Law of Thermodynamics states that not all heat energy supplied is used to do work in a heat engine, the Carnot efficiency provides a limiting value that a classical heat engine can achieve when heat is converted to work, or vice versa.

The Carnot cycle can be illustrated using a P-V diagram shown below: Figure 1: Carnot cycle illustrated on a P-V diagram to show the work done. The Carnot cycle consists of the following steps: 1. Isothermal heat addition or absorption at temperature T1: In this step (begins from 1 to 2 in figure 1 above), the gas expands and does work. During this process, the temperature remains constant, and thus, an isothermal expansion is achieved. Absorption of heat energy from the reservoir at a high temperature (Q1) propels gas expansion, resulting in increase of entropy of the gas by the amount: 2.

Isentropic expansion of the gas: In this step (2-3 in figure 1), the mechanisms of the heat engine neither loses nor gain heat – an adiabatic process. Gas expansion continues and the system loses internal energy equal to the work done by expansion of the gas. The work that leaves the system due to gas expansion causes cooling of the system to a temperature T2; the entropy remains constant. 3. Isothermal heat rejection at temperature T2: In this step (3-4 in figure 1), the system surroundings do work on the gas, resulting in the leaving of heat energy Q2 to the reservoir at low temperature.

The entropy reduces by the amount: (Equal to the amount absorbed in step 1) 4. Isentropic work input: In this step (4-1 in figure 1), the surroundings do some work on the system, increasing the internal energy of the gas and compressing it. This results in rise in temperature to T1, but the entropy remains constant. Thus, the gas goes back to the same state as in step 1 and; Thermal Efficiency of the Carnot cycle, = 1- ii. Otto Cycle This cycle describes how a typical spark ignition internal combustion engine works.

It describes what happens to a gas subjected to changes of temperature, pressure, volume, and addition or removal of heat. The processes involved in an ideal Otto cycle are two isentropic and two isochoric processes as illustrated in figure 2 below: Figure 2: P-V diagram illustrating an ideal Otto cycle. 1. Intake stroke at constant pressure (process 1-2): A mass of air at near atmospheric pressure is drawn into the piston. The gas volume increases as air is drawn into the piston through the intake valve. 2.

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