GAS Power Cycles - Lecture notes 2 PDF

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APPLIED THERMODYNAMICS II (EHATH3A/EHTTA3C GAS POWER CYCLES 1. Air-Standard Analysis • Air-Standard Analysis  Air-standard analysis makes the following assumptions: • The working fluid is air, and is an ideal gas • Compression and expansion processes are adiabatic and reversible, so they are isentropic • Combustion will be treated as a heat transfer process from an external source • Exhaust transfers heat and does no work

• Cold-Air Standard  The cold-air standard additionally assumes that the specific heats are constant.  We use the specific heats at 27 ºC.  The assumptions of the air-standard and cold-air standard analyses introduce some error, but allow simplified analysis of gas cycles. 2. Reciprocating Engine Terminology • Reciprocating Engine Terminology  The position where the piston provides the minimum volume is top dead centre (TDC).  The position where the piston provides the maximum volume is bottom dead centre (BDC).  The bore is the inside diameter of the cylinder.  The stroke is the distance of travel of the piston from BDC to TDC

• Reciprocating Engine Terminology  The displacement is the volume equal to the stroke times the cross-sectional area:

 The clearance volume is the volume between the cylinder head and the top of the piston when the piston is at top dead centre.  The compression ratio is the ratio of the maximum volume (piston at BDC) to the minimum volume (piston at TDC).  The intake valve lets air or an air-fuel mixture into the cylinder.  The exhaust valve lets combustion products exit the cylinder • Reciprocating Engine Terminology  A spark plug ignites the fuel-air mixture in spark ignition.  The high temperature during compression ignites the fuel-air mixture in compression ignition.  The mean effective pressure (MEP) is the ratio of the net work done during the cycle to the displacement

3. The Brayton Cycle  The gas turbine is used in most aircrafts as well as some power plants.

 It is a continuous flow engine: there is a constant flow of air into the engine and combustion products out of the engine.

The open-cycle gas turbine  The top heat exchanger accounts for the energy input that would occur during combustion of the fuel.  The lower heat exchanger emits the heat associated with the products of combustion and does no work.

The closed-cycle gas turbine

• The Efficiency of the Brayton Cycle  The pressure ratio is

 𝑃�2 is at the exit of the compressor, 𝑃�1 is at the entrance.  The efficiency is:

• The Brayton Cycle with Regenerative Heating  In the regenerative Brayton cycle, the exhaust gases are used to heat the air before it enters the combustion chamber using a regenerator.

• The Efficiency of the Regenerative Brayton Cycle  The efficiency is:



The efficiency of the regenerative cycle will actually decrease as the pressure ratio increases

• The Brayton Cycle with Regeneration, Intercooling, and Reheat  An intercooler reduces compressor work by cooling the air between compressor stages.  A reheater increases turbine power output by heating the air in between turbine stages

• The Brayton Cycle with Regeneration, Intercooling, and Reheat  Intercooling and reheat are only effective when combined with a regenerator. • The Turbojet Engine  Perhaps the primary use of the gas turbine that operates on the Brayton cycle has been in the aircraft propulsion.

• The Turbojet Engine  Operation of an turbojet engine.

4. The Combined Brayton-Rankine Cycle • Cogeneration Cycles  The combined-cycle power plant is an example of cogeneration.  The exhaust gases from the gas turbine are used to generate steam in boiler

•

Cogeneration Cycles  The efficiency of an actual combined-cycle power plant may exceed 60%.  This efficiency is possible because the secondary Rankine cycle recovers much of the rejected heat from the turbine.

• Gas Properties Properties of Air

Properties of Combustion Gas

R = 0.287 kJ/kg·K

R = 0.287 kJ/kg·K

CP = 1.003 kJ/kg·K k = 1.4

CP = 1.003 kJ/kg·K k = 1.4...