Chapter 9 Thermo, Nader, Helseth
In the P-v diagram for an ideal Otto cycle. From the list below, select all of the states that occur at bottom dead center.
1 and 4
What are the order of processes for a Turbojet engine?
1-2 Diffuser 2-3 Compresser 3-4 Combustion chamber 4-5 Turbine 5-6 Nozzle
In the Brayton Cycle what are the four internally reversible processes?
1-2 Isentropic Compression ( in a compressor ) 2-3 Constant-pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant-pressure heat rejection
The T-s Diagram provided represents an ideal turbojet cycle. Match the turbojet component to the corresponding process on the T-s diagram.
1. 1-2 Diffuser 2. 2-3 Compressor 3. 3-4 Combustion chamber 4. 4-5 Turbine 5. 5-6 Nozzle
Starting from gas entering the compressor, put the following processes in order so that they make up the Brayton cycle with regeneration.
1. Compression in compressor 2. Heat addition from regenerator. 3.Heat addiditon from combustion chamber 4.Expansion in turbine 5.Heat loss to regenerator. 6. Constant-pressure heat rejection.
The following Components are required for a gas-turbine engine with two-stage compression with intercooling, Two-stage expansion with reheating, and regeneration. Starting from the generator, put the components in the order that the working fluid would proceed through the components up to the point that they enter the high pressure turbine.
1. Compressor 1 ( low pressure ) 2. Intercooler 3. Compressor 2 (high pressure ) 4. Regenerator ( working fluid gains heat) 5. Combustion chamber 6. Turbine 1 (high pressure)
If the ideal stirling cycle begins with constant temperature expansion, put the following processes into order so that the complete the cycle.
1. Constant Volume Regeneration ( from the working fluid to the regenerator ) 2.Constant Temperature Compression 3.Constant Volume Regeneration ( from the regenerator to the working fluid )
Put the following processes into order so that they make up the ideal Diesel cyle. Being with the Isentropic compression Process.
1. Isentropic Compression 2.Constant-pressure heat addition 3.Isentropic Expansion 4.Constant volume heat rejection
Put the following processes into order so they make up the ideal Diesel cycle, Begin with the isentropic compression process.
1. Isentropic compression 2.Constant-Pressure heat addition 3. Isentropic expansion 4.Constant volume heat rejection
Put the following processes into order so that they make up the ideal Otto cycle. Begin with Isentropic compression.
1. Isentropic compression 2.Constant-volume heat addition 3.Isentropic expansion 4.Constant-volume heat rejection
From the list below, select all of the statements that are true regarding the ideal Brayton and Diesel cycles.
1. The Brayton cycle is executed in a closed loop if steady flow devices, while the Diesel cycle occurs in a reciprocating piston-cylinder device. 2. The heat rejection for the Brayton Cycle occurs at constant pressure, whereas the heat rejection from the Diesel cycle occurs at constant volume.
How does a turbofan engine work?
1. a large set of fan blades is set right in front of the inlet; the fan works like a propeller, thrusting the engine forward and pushing a large amount of air backwards 2. as the air is pushed back by the fan, some of it goes into the engine and some bypasses the engine; the engine that sits behind the fan is basically a turbojet 3. the air that goes into this engine receives the same treatment as air that goes through the turbojet 4. the air "jets" out the back of the engine
Put the following processes into order so that they make up the ideal Ericsson cycle. Begin with the Constant temperature expansion process.
1.Constant temperature expansion 2. Constant-pressure regeneration ( from the regenerator to the working fluid ) 3.Constant temperature compression 4.Constant-Pressure regeneration ( from the working fluid to regenerator )
Four stroke engine Process
1.Intake (air&fuel) TDC-BDC 2. Compression and ignition BDC-TDC 3.expansion (power) TDC-BDC 4.Exhaust BDC-TDC
The following Components are required for a gas-turbine engine with two-stage compression with intercooling, Two-stage expansion with reheating, and regeneration. Starting from the generator, put the components in the order that the working fluid would proceed through the components.
1.Regenerator (working fluid gains heat) 2.Combustion chamber 3.Turbine 1 (high-pressure) 4.reheater 5.turbine 2 (low-pressure) 6.Regenerator (working fluid loses heat)
Assuming that the ideal Brayton Cycle begins with isentropic compression, put the remaining processes in order so they complete the closed-loop Brayton cycle.
2 . Constant - Pressure Heat Addition 3. Isentropic Expansion 4. Constant - Pressure Heat Rejection
Which of the following statements is true?
A regenerator with a high effectivness can save on the amount of fuel used. The regenerator effectiveness measures the extent to which an actual regenerator approaches an ideal one. A regnerator preheats the air before it enters the combustion chamber.
Which of the following best describes the difference between an actual and ideal cycle.?
An ideal cycle always uses air as the working fluid, and it behaves like an ideal gas.
Which of the following best describes the difference between an actual and ideal cycle?
An ideal cycle has all of the internal irreversibilitities and complexities removed.
Which of the following statements are true?
An ideal cycle is made up of internally reversible processes. An ideal cycle is less complex than an actual cycle. An ideal cycle is an actual cycle stripped of all the internal irreversibilities.
The ideal Brayton cycle T-s and P-v diagrams are provided. Match the corresponding states from each diagram.
B-F C-E G-A H-D
The ------ ------ ratio is the ratio of the work consumed by the compressor to the total work produced by the turbine.
Back Work ( The back work ratio is the ratior of the compressor work to the turbine work. )
The -------- is a diameter of the cylinder in a reciprocating engine.
Bore
In gas-turbine engines, the temperature of the exhaust has leaving the turbine is often considerably higher than the temperature of the air leaving the compressor.
Brayton Cycle with Regeneration
How are Aircraft propelled?
By accelerating a fluid in the opposite direction to motion.
From the list below, select all that are isentropic processes in the ideal Brayton cycle. Heat rejection Compression Heat addition Expansion
Compression Expansion
In the Stirling cycle, which of the following process involves heat transfer from an external source?
Constant temperature expansion
In the ideal Otto cycle, which process takes the working fluid from the end of the Expansion process and returns it to the intital state before the process begins again with compression?
Constant-volume heat rejection
The ratio of the cylinder volumes after and before the combustion process of the ideal Diesel cycle is called the -------- ratio.
Cutoff ( The ratio of the volumes at state 3 to the volume at state 2 is called the cutoff ratio)
The attached figure presents the T-s diagram for an ideal Otto cycle. From the list below, select all the states that occur at top dead center
D A
The attatched figure presents the T-s diagram for an ideal Otto cycle. Which state represents the end of compression?
D ( Second highest T )
Modern diesel engines, where the fuel begins to ignite during the compression stroke, are best modeled using the ------- cycle, which combines elements of the Otto and Diesel cycles.
Dual ( The dual cycle has a constant-volume heat addition, followed by constant-pressure heat addition. it can be used to model modern diesel engines )
What is the measure of performance of a regenerator. that is the level to which it approaches an ideal regenerator, called?
Effectivness.
Match the properties on the left with their corresponding changes on the right during the compression stroke of an ideal Otto cycle. Entropy: Temperature: Volume:
Entropy: Decreases Temperature: Increases Volume: Remains constant
Mathematical formula for Thrust(Propulsive force)
F=ṁ(Vexit-Vinlet).....ṁ = massflow rate
The ideal Brayton cycle T-s and P-v diagrams are provided. Which processes, as illustrated on the P-V diagram, are Isentropic.
H-F , E-G
In a air standard assumption the combustion process is replaced by a what? What is the exhaust process replaced by?
Heat addition process. Heat rejection.
Common gasses to use in the Stirling engine
Helium, Hydrogen,Air
Select all of the following strokes from the actual four-stroke spark-ignition cycle that are not included in the ideal Otto cycle. Intake Compression Power (expansion) Exhaust
Intake, Exhaust ( The ideal Otto cycle is a closed system, so there is no intake or exhaust)
What is intercooling and reheating?
It always decreases the thermal efficiency unles they are accompanied by regeneration.
What happens when the number of compression and expansion stages increase in the Brayton cycle (with intercooling, Reheating and Regeneration)?
It approaches being similar to the Ericsson Cycle.
Why does the Ideal jet-Propulsion Cycle differ from the simple Ideal Brayton cycle?
It differs in that the gases are not expanded to the ambient pressure in the turbine. Instead, they are expanded to a pressure such that the power produced by the turbine is just sufficient to drive the compressor and the auxillary equpiment.
What is propulsive power?
It is the thrust acting on the aircraft through a distance per unit times.
What is Multistage expansion with reheating?
It keeps the specific volume of the working fluid as high as possible during an expansion proccess, thus maximizing work output.
What type of cycle to aircraft gas turbines operate on?
Jet-Propulsion cycle.
What would gas turbine engines be used in frequently?
Military aviation commercial aviation Electric generation ( peak-Load duty) Gas-Pipeline transmission Transportation(Ships, Tanks) Industrial Processes
Define an ideal cycle analysis.
No friction Quasi-equilibrium expansion and compression Heat transfer negligible when not intended (like pipes connecting components. etc...) Heat transfer will be through a finite temperature differential. ( thus not externally reversible.)
What is the measure of a fuel's resistance to autoignition called?
Octane rating
Which of the following properties is constant during the heat-addition process of an ideal Diesel cycle?
Pressure
What is the ratio of the pressures after and before the compressor in a Brayton cycle engine called?
Pressure ratio (The compression ratio is not used with the Brayton cycle engines.)
Under the cold-air assumption, the thermal efficiency of an ideal Brayton cycle is dependent on which parameter(s) below? Pressure ratio of the gas turbine Specific heat ratio of the working fluid Pressure ratio across the throttle valve Gas constant of the working fluid
Pressure ratio of the gas turbine. Specific heat ratio of the working fluid
On the P-v Diagram for an ideal Otto cycle. What processes are Isentropic?
Procces 1-2 (Isentropic Compression) Procces 3-4 (Isentropic Expansion)
What are two types of Modifications made to Turbojet Engines?
Propjet engine and the Turbofan Engine.
A process during which heat is transferred to a thermal energy storage device in one part of a cycle then used again in another part of the cycle is called -----.
Regeneration
In the Brayton Cycle when is regeneration most effective?
Regeneration is most effective at Lower Pressure ratios and low minimum-to-maximum temperature ratios.
What engines are internal combustion engines?
Spark-Ignition Diesel Gas turbines
What engines are external combustion?
Stirling engine Ericsson engine.
Two Stroke engine Process
TDC-BDC Exhaust and Intake BDC-TDC Compression and Ignition Chainsaws, maybe snowmobiles, Engines where you have to mix oil into fuel.
In the Stirling cycle, which property is constant during the compression process?
Temperature
What is the most widely used engine in aircraft propulsion?
The Turbofan ( or fanjet) Engine
Which of the following provides the best statement of an air-standard assumption?
The combustion process is replaced by heat addition from an external source.
Which of the following is an example of an idealization or simplification commonly employed in the analysis of power cycles?
The cycle does not involve friction.
Which of the following provides the best statement of an air-standard assumption?
The exhaust process is replaced by a heat rejection process that restores the working fluid to its inital state.
How are improved materials for turbine blade construction related to an increase in Brayton Cycle efficiency?
The improved materials allowed for increased turbine inlet temperatures. (which means increased turbine inlet temperatures, which increaases cycle efficiency.)
What makes the Thermal efficiency rise in the Brayton Cycle?
The thermal efficiency of the Brayton cycle increases as a result of regeneration since less fuel is used for the same work output
What is Multistage compression with intercooling?
The work required to compress a gas between two specified pressures can be decreased by carrying out the compression process in stages and cooling the gas in between. This keeps the specific volume as low as possible.
From the following list select all the differences between compression strokes of the actual four-stroke spark-ignition engine and the ideal Otto cycle.
The working fluid in the actual cycle is a fuel-air mixture, while it is only air in the ideal cycle. The ideal Otto cycle has Isentropic compression, while compression in the actual cycle is not isentropic.
Which of the following provides the best statement of an air-standard assumption?
The working fluid is air,which circulates in a closed loop and always behaves like an ideal gas.
In the Brayton Cycle with regeneration what does thermal efficiency depend on?
Thermal efficiency depends on the ratio of the minimum to maximum temperatures as well as the pressure ratio.
In jet engines what does the high temperature and high pressure gases do after leaving the turbine?
They are accelerated in a nozzle to provide thrust.
What are Gas-Turbine engines widely used for and why?
They are widely used to power aircrafts. Because they are light and compact and have a higher power to weight ratio.
How are Propeller-Driven engines Propelled?
This is accomplished by either slightly accelerating a large mass of fluid
How are Jet or Turbo jet engines or turboprop engines propelled?
This is accomplished by greatly accelerating a small mass of fluid.
----- ------- ---- is the position of the piston when it forms the smallest volume in the cylinder.
Top Dead Center
The Stirling is a reversible process.
True
True or False. Carnot thermal efficiency relation implies that for actual cycles, the thermal efficiency increases with an increase in the average temperature at the heat source, or with a decrease in the average temperature at the heat sink.
True
True or False? The Ericsson cycle is totally reversible?
True
The Otto and Diesel Cycles are not internally reversible? True or false
True because of heat transfer through a finite temperature difference.
What are various other engine types?
Turbofan propjet ramjet sacramjet rocket
Compression ratio is
Vmax/Vmin or Vbdc/Vtdc
What is the displacement volume?
Volume displaced by piston as it goes from TDC to BDC
What is clearance Volume?
Volume of gas In cylinder when piston is at TDC
In a Cold air Standard assumption what is Cp and Cv?
We assume the air is at 25*C and take the constants in the book.
MEP =
Wnet/(vmax-Vmin)
Mathematical formula for Propulsive power
Wp=F(Vaircraft) = ṁ(Vexit-Vinlet)Vaircraft
Is the Brayton cycle internally reversible?
Yes
What is the net work output of a jet-propulsion cycle?
Zero, The gases that exit the turbine at a relatively high pressure are subsequently accelerated in a nozzle to provide the thrust to propel the aircraft.
Air standard assumptions are considered to be a open or closed loop? reversible or irreversible?
closed loop reversible
Air cooled engines use what to cool it
fins
What happens to the High-Pressure air leaving the compressor in the Brayton cycle?
it is Heated by the exhaust gasses in a counter-flow heat exchanger.
Propulsive efficiency Formula
np=Propulsive power/energy input rate= Wp/Qin
How do you find total engine displacement?
pie/4*bore^2*stroke*#of cylinders
