EML4450L9

EML4450L9 - Sustainable Energy Science and Engineering...

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S ustainable E nergy S cience and E ngineering C enter Thermodynamics Fundamentals for Energy Conversion Systems (Continued)

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S ustainable E nergy S cience and E ngineering C enter Open Brayton Power Cycle Joule or Brayton cycles are used either in open or closed systems in heat engines or in power plants exclusively with gas turbines. The Brayton cycle is also known as the gas turbine cycle since it uses gases (other than steam) which can be compressed but not liquefied by a condenser.
S ustainable E nergy S cience and E ngineering C enter Open Brayton Power Cycle The air-standard Brayton cycle is an ideal cycle that approximates the processes incorporated within the standard gas-turbine engine. In the following description of the ideal Brayton cycle, the initial state is taken where atmospheric pressure air enters the inlet of a steady flow compressor. This cycle is shown for constant specific heats on P-v, and T-s diagrams. Process 1 - 2: an isentropic compression of atmospheric air from the inlet to the compressor to the maximum pressure in the cycle, Process 2-3: a constant-pressure combustion process (heat addition), Process 3-4: an isentropic expansion of the products of combustion from the inlet to the turbine to the exhaust of the turbine at atmospheric pressure, Process 4-1: a constant-pressure heat rejection process until the temperature returns to initial conditions. The thermal efficiency of this cycle is found as the net work delivered by the cycle divided by the heat added to the working substance. From this definition of the cycle thermal efficiency, we may write: η = W net Q added = 1 Q rejected Q added

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S ustainable E nergy S cience and E ngineering C enter Open Brayton Power Cycle Since the constant pressure heat rejection is equal to the change of enthalpy in process from state 4 to state 1, and the heat added in a constant pressure process from state 2 to state 3 is the change of enthalpy between these two states, we may write for the case of constant specific heats: Note that the process from state 1 to state 2 is an isentropic compression and the process from state 3 to state 4 is an isentropic expansion, and that P3 = P2 and thatP4 = P1 . Hence, we may write: where γ is the ratio of specific heats. Canceling through the appropriate terms yields an expression for the ideal Brayton cycle thermal efficiency for constant specific heats as: In this expression, the ratio P 2 / P 1 , is the pressure ratio for the cycle. η = 1 T 4 T 2 T 1 T 2 T 3 T 2 1 T 2 T 1 = p 2 p 1 γ 1 ( ) = p 3 p 4 1 ( ) = T 3 T 4 = 1 T 1 T 2 = 1 p 1 p 2 1 ()
S ustainable E nergy S cience and E ngineering C enter Open Brayton Power Cycle The following figures were produced using an ideal Brayton cycle with constant specific heats, with the working substance consisting a mixture of oxygen and nitrogen in the ratio of 1.0 kmol of O 2 to 3.773 kmols of N 2 . The initial pressure in the cycle is 100.0 kPa, the initial temperature

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This note was uploaded on 10/22/2011 for the course EML 4450 taught by Professor Greska during the Fall '06 term at FSU.

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EML4450L9 - Sustainable Energy Science and Engineering...

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