File Name: vapour and gas power cycles .zip
General Energy Equation. In Carnot suggested a particular cycle of operation for a CHPP which avoided all irreversibilities.
Thermodynamic cycles can be divided into two general categories: power cycles, which produce a net power output, and refrigeration and heat pump cycles, which consume a net power input. The thermodynamic power cycles can be categorized as gas cycles and vapor cycles. In gas cycles, the working fluid remains in the gas phase throughout the entire cycle. In vapor cycles, the working fluid exits as vapor phase during one part of the cycle and as liquid phase during another part of the cycle. Steam power plants run vapor power cycles with water as the working fluid.
General Energy Equation. In Carnot suggested a particular cycle of operation for a CHPP which avoided all irreversibilities. The process take place between a heat source at temperature T h and a heat sink at temperature T c. The system is a mass of gas behind a piston. The cycle on a p-v diagram is shown below:. Adiabatic expansion A to B.
The gas expands adiabatically and very slowly, i. Isothermal Compression B to C. When the gas reaches temperatures T c the heat reservoir at temperature T c is brought into contact with the cylinder end. The gas is then compressed quasi statically from state B to C and at constant temperature with heat transfer from the gas to the reservoir through a negilible temperature difference. Thus process B to C is also reversible. Adiabatic Compression C to D. At C the heat reservoir at temperature T c is removed and the insulation put back.
Then slow compression from C to D. At D the temperature reaches T h. Again process is reversible. Isothermal expansion D to A. At D the heat reservoir at temperature T h is brought into contact with the cylinder and as a result slow isothermal expansion occurs from state D to A, there by completing the cycle. It is possible to show that the efficiency of this reversible CHPP depends only on temperature. Thermodynamic temperature scale. Theorem All reversible cyclic engines operating between the same two temperature level have the same Maximum efficiency.
Proof: Assume opposite is true and show before that PMM2 is produced. Because all reversible engines, whatever their internal processes or materials, operating between the same two thermal reservoirs have the same efficiency, then there must be some common factor which determines the efficiency.
Clearly the only common factor is the temperature of the two reservoirs. Therefore the efficiency of a reversible heat engine depends upon the temperature of the thermal reservoirs that it exchanges energy with. We shall show this more rigorously for a carnot cycle, with a perfect gas as working substance.
We mentioned earlier that the zeorth law of TD provides the basis for temperature measurements, but that a temperature scale must be defined in terms of particular thermometer and device.
Thus a temp scale that is independent of any particular substance is most desirable. Now the efficiency of a carnot cycle is independent of the working substance and depends only on temperature. This fact may be used to define the TD temperature scale.
Now consider the following diagram:. T1 is the highest temperature, T3 the lowest and T2 is the intermediate temperature. Since efficiency of Carnot cycle is a function of temperature only then:. Applying these to the different engines we get: But Lord Kelvin proposed the Linear relationship ie.
With temperature so defined, the efficiency of a carnot cycle may be expressed as 2. Violates the second law. Units Equation 1 only defines a ratio of absolute temperature but does not give information about the scale. In this case we need to define only one temperature point, since there is zero temperature. Mark triple point of water The maximum efficiency obtainted if we make T C as low as possible and T H as high as possible. T C is practically limited by the temperature of the sea or ocean which is nearby the power plant ie o C or K.
The upper limit is set by the metallurgical properties and at present it is limited to about o C for alloyed steel. Deal with systems that produce power in which the working fluid remains a gas throughout the cycle in other words, there is no change in phase.
Spark Ignition gasoline engines, Compression ignition diesel engines and conventional gas turbine engines generally refer to as Internal Combustion engines or IC Engines are some examples of engines that operate on gas cycles. Internal combustion engines: Combustion of fuel is non-cyclic process. Hence, the working fluid doesn't undergo a thermodynamic cycle.
In order to analyze this complex gas power cycles, air standard cycles are conceived. In air standard cycle a certain mass of air operates in a complete thermodynamic cycle where the heat is added and rejected using external reservoirs , and all the processes in the cycle are reversible. Summary of assumptions made during such analysis:The working fluid, air behaves like an ideal gas and specific heats are assumed to be constant Combustion process is replaced by heat addition and exhaust process by heat rejection All the processes are reversible.
Internal combustion engines There are two types of reciprocating engines: Spark Ignition- Otto cycle Compression Ignition-Diesel cycle. Assumptions for Air standard cycle, as describe before: Fixed amount of air ideal gas for working fluid Combustion process replaced by constant volume heat addition with piston at TDC Intake and exhaust not considered, cycle completed with constant volume heat removal with piston at BDC.
The thermal efficiency is given by:. The specific heats are assumed to be constant. However, increase in pressure ratios, would increase the air-fuel temperature above the temperature at which the mixture can auto-ignite.
This would result in 'engine-knock', reducing the performance of the engine. In order to avoid such situations, additives are generally added which increases the auto-ignition temperature. Here the fuel is injected when the piston approaches TDC, ie when the air is at maximum temperature due to compression.
The combustion process starts now. The fuel is injected after the piston starts moving down The volume increases, on the other hand, the fuel evaporates to fill the volume.
Thus keeping the pressure inside roughly the same. Three Volume Ratios From previous definition:. Otto and Diesel cycle comparison. Therefore, the efficiency of the diesel cycle is less than that of the otto cycle for the same compression ration.
However, the advantages of Diesel over petrol engines is that we can operate at higher compression ratios without auto ignition and fuel is less expensive. Gas turbine power plants are lighter and compact when compared to power plants running on vapour cycles. The power to weight ratios are generally high for high throughout Gas turbine power plants and hence are favoured for the aviation and also for power generation.
A simple GT power plant is shown in the image below. Air is first compressed The compressed air enters the combustion chamber where fuel is injected and burned, essentially at constant pressure The combustion products expand in turbine to the ambient pressure and thrown out to the surroundings.
Air expands reversibly, adiabatically in the turbine The heat is removed from the system reversibly at constant pressure to bring it to original state Brayton cycle therefore consists of two isobars and two reversible adiabatics isentropics :.
Where rv is the pressure ratio. We also know that a high work ratio is desirable in order to minimize the effect of irreversibilities in real gas turbines. This depends on the temperature limits and the pressure ratio for constant gamma. The two dotted cycles show the limits of operation. Consider left hand dotted cycle. Unfortunately the net work output is approaching zero. It can be shown that for an ideal cycle with fixed T1 and T3, the value of T2 for maximum work output is:.
Irreversibilities in heaters and coolers who up as pressure drops and are not considered here. The two T-S diagrams, show the effect on compression and expansion processes in general from state 1 to state 2.
These are analogous to the similar diagrams for the Rankine cycle except that they are processes of a perfect gas. Then for the steady flow compression process:- For the steady flow compression process: For the steady flow expansion process:.
Note that Celsius temperatures may also be used in these expressions. Mech Engineering: Thermodynamics. Pages Blog. Child pages. Gas Power Cycles. Browse pages. A t tachments 95 Page History. Dashboard Home. Jira links. Created by Unknown User zcemb63 , last modified on May 14, Isothermal Compression B to C When the gas reaches temperatures T c the heat reservoir at temperature T c is brought into contact with the cylinder end.
Again process is reversible Isothermal expansion D to A At D the heat reservoir at temperature T h is brought into contact with the cylinder and as a result slow isothermal expansion occurs from state D to A, there by completing the cycle.
Thermodynamic temperature scale Theorem All reversible cyclic engines operating between the same two temperature level have the same Maximum efficiency. With temperature so defined, the efficiency of a carnot cycle may be expressed as 2 Note: i It is impossible to attain negative temperatures on the absolute scale. Air standard cycles Internal combustion engines: Combustion of fuel is non-cyclic process.
The combustion process starts now The fuel is injected after the piston starts moving down The volume increases, on the other hand, the fuel evaporates to fill the volume. Hence the combustion can be considered to occur at constant pressure. Otto and Diesel cycle comparison Therefore, the efficiency of the diesel cycle is less than that of the otto cycle for the same compression ration.
(ii) Steam locomotives in which the water is taken in the boiler from a tank and steam is exhausted into the atmosphere. Page 2. Gas Power Cycles. Prof. U.S.P..
Engineering Thermodynamics by Jones, J. 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.
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines , such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat source and heat sink. Heat energy is supplied to the system via a Boiler where the working fluid typically water is converted to a high pressure gaseous state steam in order to turn a Turbine. After passing over the turbine the fluid is allowed to condense back into a liquid state as waste heat energy is rejected before being returned to the boiler, completing the cycle.
Which one of the following is NOT a rotating machine? If the inlet temperature to the turbine is K and the isentropic efficency is 0. The temperatures of the gas at turbine entry and exit are K and K, respectively.
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actual devices and is not a realistic model for vapor power cycles. Ideal Rankine Cycle. The Rankine cycle is the ideal cycle for vapor power plants; it includes the gas-turbine cycle at high-temperature and to use the high temperature.Joy C. 21.05.2021 at 06:12
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