Physics for Scientists and Engineers
Physics for Scientists and Engineers
6th Edition
ISBN: 9781429281843
Author: Tipler
Publisher: MAC HIGHER
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Chapter 19, Problem 83P

(a)

To determine

To sketch the cycle on PV diagram.

(a)

Expert Solution
Check Mark

Explanation of Solution

Introduction:

The Brayton cycle consists of various thermodynamic process, an adiabatic compression, an isobaric expansion, an adiabatic expansion and an isobaric compression.

The figure below shows the PV curve of Brayton cycle,

  Physics for Scientists and Engineers, Chapter 19, Problem 83P

Figure (1)

From the range of temperature T1 to T2 , the process called an adiabatic compression, for the temperature range of T2 to T3 , the process is isobaric process, for the temperature T3 to T4 , the process is adiabatic expansion and for the temperature T4 to T1 . The process is isobaric compression.

Conclusion:

Therefore, the diagram of PV curve on Brayton cycle is shown in figure (1).

(b)

To determine

Show that the efficiency of the cycle as ε=1(T4T1)(T3T2)

(b)

Expert Solution
Check Mark

Answer to Problem 83P

The efficiency of the cycle as ε=1(T4T1)(T3T2) .

Explanation of Solution

Formula used:

The expression for the efficiency is,

  E=1QcQh

Here, the heat absorbed from the heat reservoir during isobaric expansion is Qh and heat delivered to cold reservoir during isobaric compression is Qc .

The expression for change in enthalpy from first law of thermodynamics is,

  Q=dEin+W

The expression for ideal gas law is,

  pV=nRT

The expression for work done throught the cycle is,

  W=WAB+WBC+WCD+WDA

Calculation:

The heat absorbed from the heat reservoir during isobaric expansion is calculated as,

  Qh=Cp(T3T2)+Ph(V3V2)=Cp(T3T2)+PhV3PhV2

By using the ideal gas law we get,

  Qh=Cp(T3T2)+nRT3nRT2=Cp(T3T2)+nR(T3T2)=(Cp+nR)(T3T2)

Similarly,

The delivered to cold reservoir during isobaric compression is calculated as

  Qc=[Cp( T 1 T 4)+PL( V 1 V 4)]=[Cp( T 1 T 4)+PLV1PLV4]

By using the ideal gas law we get

  Qc=[Cp( T 1 T 4)+nRT1nRT4]=[Cp+nR](T4T1)

The efficiency is calculated as,

  E=1[ C p+nR]( T 4 T 1 )[ C p+nR]( T 3 T 2 )=1( T 4 T 1 )( T 3 T 2 )

Conclusion:

Therefore, the efficiency of the cycle as ε=1(T4T1)(T3T2) .

(c)

To determine

To show the efficiency is ε=1r(1γ)/γ .

(c)

Expert Solution
Check Mark

Answer to Problem 83P

The efficiency is ε=1r(1γ)/γ .

Explanation of Solution

Formula used:

The expression for adiabatic process is given as,

  p1γTγ=constant

Calculation:

The relation between pressure and temperature for adiabatic process AB is calculated as,

  PL1γT1γ=Ph1γT2γPL 1γγT1=Ph 1γγT2 ........(1)

The relation between pressure and temperature for adiabatic process CD is calculated as,

  PL1γT4γ=Ph1γT3γPL 1γγT4=Ph 1γγT3 ........(2)

Subtracting equation (1) with equation (2) we get,

  PL 1γγT4PL 1γγT1=Ph 1γγT3Ph 1γγT2( P h P L ) 1γγ=( T 4 T 1 )( T 3 T 2 )(r) 1γγ=( T 4 T 1 )( T 3 T 2 )

By using efficiency from part (a) we get,

  E=1( T 4 T 1 )( T 3 T 2 )E=1(r) 1γγ

Conclusion:

Therefore, the efficiency is ε=1r(1γ)/γ .

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