Showing posts with label Thermodynamics Cycles. Show all posts
Showing posts with label Thermodynamics Cycles. Show all posts
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.
Rankine cycleSaturated 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 ts1.gif (9k) the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.
Typical Modificationsts
Rehaeat
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.
Refrigeration is the withdrawl of heat from a substance or space so that temperature lower than that of the natural surroundings is achieved.
Refrigeration may be produced by
- thermoelectric means
- vapor compression systems
- expansion of compressed gases
- throttling or unrestrained expansion of gases.
Vapor compression systems are employed in most refrigeration systems. Here, cooling is accomplished by evaporation of a liquid refrigerant under reduced pressure and temperature. The fluid enters the compressors at state 1 where the temperature is elevated by mechanical compression (state 2). The vapor condenses at this pressure, and the resultant heat is dissipated to the surrounding. The high pressure liquid (state 3) then passes through an expansion valve through which the fluid pressure is lowered. The low-pressure fluid enters the evaporator at state 4 where it evaporates by absorbing heat from the refrigerated space, and reenters the compressor. The whole cycle is repeated.
Internal Combustion (IC) engines have completely revolutionized transportation, power generation and have perhaps altered the way the society operates forever. Typical IC engines are classified as Spark and Compression ignition engines.
The simplest model for IC engines is the air-standard model, which assumes that:
The simplest model for IC engines is the air-standard model, which assumes that:
- The system is closed.
- Air is the working fluid and is modeled as an ideal gas throughout the cycle.
- Compression and expansion processes are isentropic.
- A reversible heat transfer process characterizes the combustion of fuel and air.
- Heat rejection takes place reversibly and at constant volume.
The Otto cycle is used to model a basic Spark Ignition engine, while the Diesel cycle is the basic model for the Compression Ignition engine.
Spark Ignition Engines (Otto Cycle)
The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:
Spark Ignition Engines (Otto Cycle)
The spark-ignition engines are the most common type used in cars. Larger engines operate using a four-stroke cycle, while smaller engines operate on a two-stroke cycle. In a simple four-stroke cycle, a combustible mixture of air and fuel is drawn into a cylinder during the intake stroke, and the temperature and pressure of the mixture is raised during the compression stroke. At near the maximum compression, a spark initiates combustion of the mixture, raising its temperature and forcing expansion. The expanding gases do work on the piston during the power stroke and then the burnt gases are purged during the exhaust stroke. Typically 3000 or more such cycles are repeated in a minute.
The Otto cycle is an air-standard model of the actual cycle. In addition to the air-standard assumptions listed above, the combustion process is modelled as a reversible constant volume heat addition process. The four steps of the air-standard Otto cycle are outlined below:
- (1-2) Isentropic compression (Compression Stroke)
- (2-3) Constant-volume, reversible heat addition (Ignition)
- (3-4) Isentropic expansion (Power Stroke)
- (4-1) Reversible, constant-volume heat rejection (Exhaust)
Typical pv and Ts diagrams for an Otto cycle are shown below where steps (1-2) and (3-4) are isentropic, and (2-3) and (4-1) are isochoric.
Compression Ignition engines are mostly used in marine applications, power generation and heavier transportation vehicles. Here, in a typical four-stroke cycle, air is drawn into the cylinder in the intake stroke and then compressed during the Compression Stroke. At near maximum compression, finely atomized diesel fuel is sprayed into the hot air, initiating auto-ignition of the mixture. During the subsequent power stroke, the expanding hot mixture does work on the piston, then the burnt gases are purged during the exhaust stroke.
The Diesel Cycle is an air-standard model of the actual cycle described above. The Diesel Cycle differs from the Otto Cycle only in the modeling of the combustion process: In a Diesel Cycle, it is assumed to occur as a reversible constant pressure heat addition process, while in an Otto Cycle, the volume is assumed constant. The four steps of the air-standard Diesel Cycle are outlined below:
- (1-2) Isentropic Compression (Compression Stroke)
- (2-3) Reversible, constant pressure heat addition (Ignition)
- (3-4) Isentropic expansion to initial volume (Power Stroke)
- (4-1) Reversible constant-volume heat rejection (Exhaust)
Typical pv and Ts diagrams for Diesel Cycle are shown below where steps (1-2) and (3-4) are Isentropic and step (2-3) is Isobaric while (4-1) is Isochoric
The gas turbine is used in a wide range of applications. Common uses include power generation plants and military and commercial aircraft. In Jet Engine applications, the power output of the turbine is used to provide thrust for the aircraft.
In a simple gas turbine cycle, low pressure air is drawn into a compressor (state 1) where it is compressed to a higher pressure (state 2). Fuel is added to the compressed air and the mixture is burnt in a combustion chamber. The resulting hot products enter the turbine (state 3) and expand to state 4. Most of the work produced in the turbine is used to run the compressor and the rest is used to run auxiliary equipment and produce power.
Air standard models provide useful quantitative results for gas turbine cycles. In these models the following assumptions hold true.
- The working substance is air and treated as an ideal gas throughout the cycle
- The combustion process is modeled as a constant pressure heat addition
- The exhaust is modeled as a constant pressure heat rejection process
In cold air standard (CAS) models, the specific heat of air is assumed constant at the lowest temperature in the cycle.
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
- (1-2) Isentropic Compression
- (2-3) Reversible Constant Pressure Heat Addition
- (3-4) Isentropic Expansion
- (4-1) Reversible Constant Pressure Heat Rejection
The pv and Ts diagrams are shown below.
Subscribe to:
Posts (Atom)