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Bài giảng Hóa lý - Chương 6: Thermodynamic cycles

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HÓ A LÝ 1  
Dr. Ngo Thanh An  
Chapter 6 – Thermodynamic cycles  
Classification of thermodynamic cycle:  
• Power / refrigeration cycle  
• Gas / vapor cycle  
• Closed / open cycle  
Chapter 6 – Thermodynamic cycles  
Otto cycle  
Internal  
combustion  
Diesel cycle  
engine  
Gas turbine  
Jet engine  
Brayton cycle  
Power cycle  
Rankine  
cycle  
Steam turbine  
External  
combustion  
Stirling  
cycle  
engine  
Ericsson  
cycle  
Chapter 6 – Thermodynamic cycles  
Carnot cycle  
Gas cycle  
Stirling cycle  
Rankine  
Vapor cycle  
cycle  
Chapter 6 – Thermodynamic cycles  
Chapter 6 – Thermodynamic cycles  
Ideal Diesel cycle  
Ideal Otto cycle  
Brayton cycle  
Chapter 6 – Thermodynamic cycles  
Ideal Rankine cycle  
Chapter 6 – Thermodynamic cycles  
Diagrams for a Carnot Cycle  
1 – 2 isothermal expansion (in contact with TH)  
2 – 3 isentropic expansion to TC  
3 – 4 isothermal compression (in contact with TC)  
4 – 1 isentropic compression to TH  
Chapter 6 – Thermodynamic cycles  
• Thermodynamic cycle for heat engines  
• Describes the thermodynamic energy conversion process for  
the most efficient heat engine.  
• The cycle has 4 states.  
• Q1 is the heat (i.e., energy) provided to the Carnot engine  
• Q2 is the heat that the engine returns to the environment (heat  
rejection)  
• W is the work (i.e., energy) produced in one cycle  
Chapter 6 – Thermodynamic cycles  
W = Q1 - Q2  
Q
dS =  
Q =
TdS  
Since  
then  
T
S2  
S4  
W = Q −Q2 = T dS − T2dS  
1
1
S S  
1
3
W = Q −Q2 = (T −T2 )(S2 − S1)  
1
1
W
Q
(T −T2 )(S2 − S1)  
T2  
1
=  
=1−  
=  
Efficiency:  
T (S2 − S1)  
T
1
1
1
• Observation #1: The efficiency increases as T1 increases (higher quality heat) and  
T2 (typically the ambient temperature) decreases.  
• Observation #2: Since T2 can never be zero, the efficiency can never be 1.  
• Observation #3: Stirling engines operation approximates a Carnot Cycle.  
Chapter 6 – Thermodynamic cycles  
The Carnot engine is useful as an idealized  
model.  
All of the heat input originates from a source  
at a single temperature, and all the rejected  
heat goes into a cold reservoir at a single  
temperature.  
Since the efficiency can only depend on the  
reservoir temperatures, the ratio of heats can  
only depend on those temperatures.  
QC  
eCarnot =1− =1−  
QH  
TC  
TH  
Chapter 6 – Thermodynamic cycles  
• Vapor power cycle: the one in which the working fluid is  
alternatively vaporized and condensed.  
• Steam is the most common working fluid used in vapor  
power cycles because of its many desirable  
characteristics, such as low cost, availability, and high  
enthalpy of vaporization.  
Chapter 6 – Thermodynamic cycles  
Carnot cycle is the most efficient cycle operating between two specified  
temperature limits. However, the Carnot cycle is not a suitable model for  
power cycles.  
Several impracticalities are associated with this cycle:  
• Temperature can not be higher than the critical temp. (374oC).  
• Process 2-3: quality of steam is low → corrosion for turbine blade.  
• Process 4-1: compress a two phase fluid is impossible  
Chapter 6 – Thermodynamic cycles  
Ideal cycle for vapor power cycles, consists of the following  
four processes:  
• 1-2 Isentropic compression in a pump  
• 2-3 Constant pressure heat addition in a boiler  
• 3-4 Isentropic expansion in a turbine  
• 4-1 Constant pressure heat rejection in a condenser  
Chapter 6 – Thermodynamic cycles  
Chapter 6 – Thermodynamic cycles  
wpump ,in = h2 − h  
Pump (q = 0)  
1
(
)
wpump ,in = v P − P  
2
1
h = hf @ P and v v1 = v f @ P  
1
1
1
qin = h3 −h2  
Boiler (w = 0):  
wturb,out = h3 − h4  
Turbine (q = 0)  
Condenser (w = 0):  
qout = h4 −h  
1
Chapter 6 – Thermodynamic cycles  
The thermal efficiency of the Rankine cycle is determined from  
wnet  
th = =1−  
qin  
qout  
qin  
where :wnet = qin −qout = wturb,out − wpump ,in  
Chapter 6 – Thermodynamic cycles  
Chapter 6 – Thermodynamic cycles  
Chapter 6 – Thermodynamic cycles  
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