fLUID AND THERMAL SYSTEMS FOR MECHANICAL ENGINEERS.ppt

FLUID AND THERMAL SYSTEMS FOR MECHANICAL ENGINEERS

1. ES 202
Fluid and Thermal Systems
Lecture 24:
Power Cycles (II)
(2/6/2003)

2. Lecture 24 ES 202 Fluid & Thermal Systems 2
Assignments
•Homework:
–8-132, 8-133 in Cengel & Turner (try them but
don’t need to hand them in on Monday)
•Study for Exam 2

3. Lecture 24 ES 202 Fluid & Thermal Systems 3
Announcements
•Problem session this evening at 7 pm
•Review for Exam 2 on Saturday from 3 to 5 pm in GM Room
•What can you expect?
–3 problems:
•one property table lookup (similar to in-class exercises)
•two problems (require analysis and calculations)
•What can you bring to the exam?
–textbook
–1 equation sheet (cannot consist of worked out problems)
–computer (cannot use EES)
•My advice to you on exam
–always work out logic in symbols and substitute numbers at the end

4. Lecture 24 ES 202 Fluid & Thermal Systems 4
Road Map of Lecture 24
•Finish up power cycle
–property table supplies enthalpy values at various states
–approximation on pump work
–non-ideal cases
•Caution with property tables/computer programs
–absolute versus relative
•Address common questions/concerns
•Refrigeration cycles

5. Lecture 24 ES 202 Fluid & Thermal Systems 5
Energy Conversion
•With reference to the T-s diagram on previous slide, a few
observations are noteworthy:
–the divergence of constant pressure lines at high temperatures implies that the
mechanical power extracted from the turbine outweighs that required by the
pump
–in most situations, a fraction of the turbine work output is used to drive the pump
and this fraction is called the back work ratio:
–the Rankine cycle can be viewed as an energy conversion process from thermal
energy to mechanical energy
–the ratio between the net power output (turbine power – pump power) and the
heat addition at the boiler is termed the thermal efficiency:
boiler
pumpturbine
thermal
Q
WW




turbine
pump
W
W
BWR




6. Lecture 24 ES 202 Fluid & Thermal Systems 6
 
12inpump, hhmW 
Summary of Energy Analysis (Rankine Cycle)
inQ

inW

balanceenergy reduced
 
43outturbine,
hhmW 
pump
boiler
turbine
condenser
0
0
0
0
0 
0 
0 
0 
 
23inboiler, hhmQ 
 
14outcondenser, hhmQ 

7. Lecture 24 ES 202 Fluid & Thermal Systems 7
How to find the h’s?
•Since the Rankine cycle operates around the two-
phase region, the ideal gas and incompressible
substance models are not applicable in general.
•The values of enthalpy can be acquired from property
tables or computer programs.
–recall the information you know about each state
–needs two independent, intensive properties to specify the
enthalpy (State Principle)

8. Lecture 24 ES 202 Fluid & Thermal Systems 8
Recall the States
•State 1: known pressure, saturated liquid (x = 0).
•State 2: isentropic from State 1 to State 2 (i.e. s
2
= s
1
), known
pressure
•State 3: isobaric from State 2 to State 3 (i.e. P
3 = P
2), exact
location depends on total heat transfer during heating process
(specific enthalpy obtained from energy balance)
•State 4: isentropic from State 3 to State 4 (i.e. s
4 = s
3), known
pressure (same as State 1 due to isobaric cooling from State 4
to State 1

9. Lecture 24 ES 202 Fluid & Thermal Systems 9
Approximation on Pump Work
•The isentropic compression from State 1 to State 2 occurs in
the compressed liquid region.
•In case the values of enthalpy is not available,
approximation is commonly done as follows:
–due to the divergence of constant pressure lines at high temperatures, the
temperature difference between State 1 and State 2 is usually very small in
the compressed liquid region (it is exaggerated on the process diagram for
identification)
–the enthalpy difference can be simplified using incompressible assumption:
  
1122
negligible
assumed
1212
vPvPuuhh 
  vPPhh
1212 
assume incompressible

10. Lecture 24 ES 202 Fluid & Thermal Systems 10
Non-Ideal Turbine/Pump
•For non-ideal cases, there are irreversibilities in the pump
and turbine:
–separate isentropic efficiencies for turbine and pump
–relate actual work to ideal work through isentropic efficiency
–important points to remember:
•pressure is the same for both ideal
and actual states
•temperature is different between
ideal and actual states
•entropy at actual state is always
larger than ideal states
•be careful when you can use the
isentropic relationships
T
s
1
3
2s
4s
4a
2a

11. Lecture 24 ES 202 Fluid & Thermal Systems 11
Absolute Versus Relative
•In doing cycle analysis, using computer programs (i.e. EES)
is a common practice.
•But you need to be aware of the facts that
–presure, temperature and specific volume are absolute measures
–specific internal energy, specific enthalpy and specific entropy
depend on the reference state (don’t pull their values from one table
and use them with another table or computer program, they may
have different reference states)
•Usually in energy balance and entropy balance, the changes
in specific enthalpy, specific internal energy and specific
entropy are quantities of interests, not their absolute values.