1. Lecture 4 Ideal cycles III Polytropic efficiencies Exercise
Reheat cycle
Intercool cycle
The WR21 engine
Polytropic efficiencies
Exercise
Problem 2.1, Problem from 2003 exam and Problem 2.9

2. Reheat cycle/Reheat with heat exchanger
Split expansion into a high pressure and a low pressure step and reheat in between

3. Selection of pressure ratio – reheat cycle

4. Theory 4.1 - Selection of optimal pressure ratio – reheat cycle
Introduce auxiliary variable
β according to:

5. Theory 4.1 - Selection of optimal pressure ratio – reheat cycle
Insert result into power formula:

6. Efficiency for reheat cycle (at pressure division for max
Efficiency for reheat cycle (at pressure division for max. power output)

7. Cycle changes due to reheat
Figure 2.5 or better from Cengel
You introduce an “additional cycle” operating at lower pressure
ratio. We have already derived what we want to know!!! Decreasing
pressure ratio in simple cycle => efficiency decreases.

8. Reheat/reheat with heat exchange compared to single cycle
Simple reheat
Power output increases
Decrease in efficiency (added cycle is worse than underlying cycle, since simple cycle efficiency decreases with pressure ratio)
Reheat with heat exchange
Increase in efficiency. Heat is added at a higher average temperature and removed at a lower temperature than in simple cycle. See figure to the right.
Simple cycle
Reheat with heat
exchange

9. Intercooling Bulky and requires large amounts of cooling water
Compactness and self-containedness of gas turbine is lost
What about efficiency and power output of cycle ?....
Try to draw a T-S diagram and make some arguments. Check with CRS.

10. The WR 21 ICR cycle - Intercooled Recuperated Cycle
Improved part load performance => 30% reduction in fuel burn for a typical operating profile
25 MW output
Fits in footprint of current naval engines of similar power.
LM2500 ηth=37. ICR ηth=43.
Starts in two minutes instead of 4 hours for comparable steam engine.
Greater power for given space when compared with steam/diesel.
The WR 21

11. The WR 21

12. Polytropic efficiencies - motivation
If we study multistage designs the isentropic efficiency for high pressure compressors tend to be lower than for low pressure compressors. Why?
Memorize these! You are going to be using them all the time.
Assume ηs (stage efficiency) constant, the overall temperature rise ΔT is obtained by:

13. Polytropic efficiency - motivation
Thus, the total efficiency is always
less than the stage efficiency.
But:
Remember that ΔT is proportional to the inlet temperature for compressors and turbines.
Preheat effect: as you go through the stages you move to the right in the T-s diagram. Isobars diverge in that direction!

14. Polytropic efficiency – “preheat independence”
Define the polytropic efficiency (differential stage efficiency) as:
We have (second revision question – lecture 1 – before integrating):
Divide with T in (1) and use differential isentropic formula
(1) + (2) produces:

15. Polytropic efficiency – “preheat independence”
Similarly for a turbine:
First guess for preliminary
design work
Polytropic efficiencies are useful for preliminary design, when many compressor concepts with different pressure ratios may be evaluated for a given application.

16. Learning goals
Know how to show (by arguments or T-S diagrams) how the efficiency of the reheat cycle with and without heat exchange changes in comparison with the simple cycle (ideal case)
Be able to derive the optimal pressure division in ideal reheat cycles
Be familiar with the polytropic efficiency concept and state reasonable loss levels for turbine and compressors