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Flashcards in Chapter 9 Deck (40)
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1
Q

Most power-producing devices operate on _____

A

Cycles

2
Q

A cycles that resembles the actual cycle closely but it made up totally or internally reversible processes

A

Ideal cycle

3
Q

Have the highest efficiency of all heat engines operating between the same temperature levels

A

Reversible cycles such as Carnot cycle

4
Q

Reversible cycles vs. ideal cycles

A

Totally reversible

Unsuitable as a realistic model

5
Q

_____ is a powerful engineering tool that provides great insight and simplicity at the expense of some loss in accuracy

A

Modeling

6
Q

Ideal cycles

A

Internally reversible

Not necessarily externally reversible like Carnot Cycles

7
Q

The thermal efficiency of an ideal cycle is ____ than that of a totally reversible cycle operating between the same temperature limits

A

Less

8
Q

The thermal efficiency of an ideal cycle is considerably ____ than the thermal efficiency of an actual cycle because of the idealization utilized

A

Higher

9
Q

The idealization and simplifications in the analysis of power cycles

A
  1. The cycle does not involve any friction. Therefore, the working fluid does not experience any pressure drop as it flows in pipes or devices such as heat exchangers
  2. All expansion and compression processes take place in a quasi-equilibrium manner
  3. The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible
10
Q

On a T-s diagram, the ratio of the area enclosed by the cyclic curve to the area under the heat-addition process curve represents the ____

A

Thermal efficiency of the cycle

Increase ration —> increase thermal efficiency

11
Q

The Carnot cycle is composed of four totally reversible processes:

A

Isothermal heat addition

Isentropic expansion

Isothermal heat rejection

Isentropic compression

12
Q

For both ideal and actual cycles:

A

Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system

13
Q

Air-standard Assumptions

A
  1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas
  2. All the processes that make up the cycles are internally reversible
  3. The combustion process is replaced by a heat-addition process from an external source
  4. The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state
14
Q

When the working fluid is considered to be air with constant specific heats at room temperature (25C)

A

Cold-air-standard assumptions

15
Q

A cycle for which the air-standard assumptions are applicable

A

Air-standard cycle

16
Q

The combustion process is replaced by a _____ in ideal cycles

A

Heat-addition process

17
Q

Braydon Cycle: the ideal cycle for gas-turbine engines

A

The combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air

Isentropic compression (in a compressor)
Constant-pressure heat addition
Isentropic expansion (in a turbine)
Constant-pressure heat rejection

18
Q

The two major application area of gas-turbine engines are

A

Aircraft propulsion

Electric power generation

19
Q

The fraction of the turbine work used to drive the compressor

A

Backwork ratio

20
Q

Gas-tubing engines

A

The air supplies the necessary oxidant for the combustion of the fuel, and it serves as a coolant to keep the temperature of various components within safe limits. An air-fuel ratio fo 50 or above is not uncommon

The highest temperatures in the cycle is limited by the maximum temperature that the turbine blades can withstand

21
Q

Development of Gas Turbines

A
  1. Increasing the turbine inlet (or firing) temperatures
  2. Increasing the efficiencies of turbomachinery components (turbines, compressors)
  3. Adding modifications to the basic cycle (intercooling regeneration or recuperation, and reheating)
22
Q

Deviation of actual gas-turbine cycles from idealized ones (Brayton Cycle)

A

Irreversibilities in turbine and compressors, pressure drops, heat losses

23
Q

Reciprocating engines

A

Spark ignition (SI) engines

Compression-ignition (CI) engines

24
Q

Displacement volume

A

Between TDC and BDC

25
Q

Clearance volume

A

Above TDC

26
Q

TDC

A

Top dead center

27
Q

BDC

A

Bottom dead center

28
Q

Can be used as a parameter to compare the performances of reciprocating engines of equal size

A

Mean effective parameter

29
Q

The engine with a ____ value of MEP delivers more net work per cycle and thus performs better

A

Larger

30
Q

Wnet —> MEP

A

MEP x Displacement Volume

31
Q

Four-stroke cycle

A

SI engine

1 cycle = 4 stroke = 2 revolution

32
Q

Two stroke cycle

A

1 cycle = 2 stroke = 1 revolution

Less efficient than 4 stroke but simple and inexpensive

High power-to-weight and power-to-volume ratios

33
Q

Ideal cycle for spark ignition engines

A

Otto cycle

34
Q

Otto cycle

A

1-2 isentropic compression
2-3 constant-volume heat addition
3-4 isentropic expansion
4-1 constant-volume heat rejection

35
Q

2-stroke engines are commonly used in ____

A

Motorcycles and lawn mowers

36
Q

Air enters the cylinder through the open intake valve at atmospheric pressure P0 during process 0-1 as the piston moves from

A

TDC to BDC

37
Q

The intake valve is closed at _____

A

State 1

38
Q

Work interactions during intake and exhaust _____

A

Cancel each other

39
Q

In SI engines, the compression ration is limited by ____

A

Auto ignition or engine knock (combusts on own when too high)

40
Q

The thermal efficiency of the Otto cycle increases with…

A

The specific heat ratio k of the working fluid