Chapter 9 Flashcards
Most power-producing devices operate on _____
Cycles
A cycles that resembles the actual cycle closely but it made up totally or internally reversible processes
Ideal cycle
Have the highest efficiency of all heat engines operating between the same temperature levels
Reversible cycles such as Carnot cycle
Reversible cycles vs. ideal cycles
Totally reversible
Unsuitable as a realistic model
_____ is a powerful engineering tool that provides great insight and simplicity at the expense of some loss in accuracy
Modeling
Ideal cycles
Internally reversible
Not necessarily externally reversible like Carnot Cycles
The thermal efficiency of an ideal cycle is ____ than that of a totally reversible cycle operating between the same temperature limits
Less
The thermal efficiency of an ideal cycle is considerably ____ than the thermal efficiency of an actual cycle because of the idealization utilized
Higher
The idealization and simplifications in the analysis of power cycles
- 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
- All expansion and compression processes take place in a quasi-equilibrium manner
- The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible
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 ____
Thermal efficiency of the cycle
Increase ration —> increase thermal efficiency
The Carnot cycle is composed of four totally reversible processes:
Isothermal heat addition
Isentropic expansion
Isothermal heat rejection
Isentropic compression
For both ideal and actual cycles:
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
Air-standard Assumptions
- The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas
- All the processes that make up the cycles are internally reversible
- The combustion process is replaced by a heat-addition process from an external source
- The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state
When the working fluid is considered to be air with constant specific heats at room temperature (25C)
Cold-air-standard assumptions
A cycle for which the air-standard assumptions are applicable
Air-standard cycle
The combustion process is replaced by a _____ in ideal cycles
Heat-addition process
Braydon Cycle: the ideal cycle for gas-turbine engines
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
The two major application area of gas-turbine engines are
Aircraft propulsion
Electric power generation
The fraction of the turbine work used to drive the compressor
Backwork ratio
Gas-tubing engines
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
Development of Gas Turbines
- Increasing the turbine inlet (or firing) temperatures
- Increasing the efficiencies of turbomachinery components (turbines, compressors)
- Adding modifications to the basic cycle (intercooling regeneration or recuperation, and reheating)
Deviation of actual gas-turbine cycles from idealized ones (Brayton Cycle)
Irreversibilities in turbine and compressors, pressure drops, heat losses
Reciprocating engines
Spark ignition (SI) engines
Compression-ignition (CI) engines
Displacement volume
Between TDC and BDC
Clearance volume
Above TDC
TDC
Top dead center
BDC
Bottom dead center
Can be used as a parameter to compare the performances of reciprocating engines of equal size
Mean effective parameter
The engine with a ____ value of MEP delivers more net work per cycle and thus performs better
Larger
Wnet —> MEP
MEP x Displacement Volume
Four-stroke cycle
SI engine
1 cycle = 4 stroke = 2 revolution
Two stroke cycle
1 cycle = 2 stroke = 1 revolution
Less efficient than 4 stroke but simple and inexpensive
High power-to-weight and power-to-volume ratios
Ideal cycle for spark ignition engines
Otto cycle
Otto cycle
1-2 isentropic compression
2-3 constant-volume heat addition
3-4 isentropic expansion
4-1 constant-volume heat rejection
2-stroke engines are commonly used in ____
Motorcycles and lawn mowers
Air enters the cylinder through the open intake valve at atmospheric pressure P0 during process 0-1 as the piston moves from
TDC to BDC
The intake valve is closed at _____
State 1
Work interactions during intake and exhaust _____
Cancel each other
In SI engines, the compression ration is limited by ____
Auto ignition or engine knock (combusts on own when too high)
The thermal efficiency of the Otto cycle increases with…
The specific heat ratio k of the working fluid
