Lesson test: Charging and emptying

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Charging and emptying

1.[3p]
A rigid insulated evacuated bottle is filled from a line. What does the energy balance reduce to?

$u_{2} = h_{l i n e}$ , so the final internal energy equals the line enthalpy

Correct

$h_{2} = h_{l i n e}$ , so the process is isenthalpic

The answer is: $u_2 = h_{line}$, so the final internal energy equals the line enthalpy

$u_{2} = u_{l i n e}$ , since no work crosses the boundary

The answer is: $u_2 = h_{line}$, so the final internal energy equals the line enthalpy

$T_{2} = T_{l i n e}$ , since no heat crosses the boundary

The answer is: $u_2 = h_{line}$, so the final internal energy equals the line enthalpy

2.[2p]
Air at $300$ K fills a rigid insulated evacuated bottle. Taking $k = 1.400$ , what is the final temperature inside, in kelvin?

CorrectNot quite: 420

3.[2p]
That bottle has a volume of $0.500$ m³ and is filled to $700$ kPa. With $R = 0.287$ kJ/(kg K), what mass of air does it hold, in kg?

CorrectNot quite: 2.90

4.[2p]
A cylinder charged to its rated pressure while hot is still at rated pressure once it has cooled to ambient.

True

The answer is: False

False

Correct

5.[3p]
The same $0.500$ m³ bottle, holding air at $700$ kPa and $420$ K, is vented to $100$ kPa. Treating the residual gas as expanding reversibly and adiabatically with $k = 1.400$ , what is its final temperature, in kelvin?

CorrectNot quite: 241

6.[3p]
Why can the gas remaining in a vessel after blowdown be treated as a closed system?

It never crossed the boundary, so it is a fixed parcel that has simply expanded

Correct

The vessel is rigid, so no work is done on anything

The answer is: It never crossed the boundary, so it is a fixed parcel that has simply expanded

Blowdown is slow enough for the pressure to be uniform

The answer is: It never crossed the boundary, so it is a fixed parcel that has simply expanded

Its temperature stays constant throughout

The answer is: It never crossed the boundary, so it is a fixed parcel that has simply expanded

7.[3p]
What does the uniform-flow model assume?

Select all that apply

Each port has one fixed state for the whole process

Correct

The contents of the control volume are uniform in space at any instant

Correct

The contents of the control volume do not change with time

The answer is: Each port has one fixed state for the whole process, The contents of the control volume are uniform in space at any instant

No mass crosses the boundary

The answer is: Each port has one fixed state for the whole process, The contents of the control volume are uniform in space at any instant

8.[2p]
Match each quantity to the energy it is counted with in a transient balance.

Contents of the control volume
Mass crossing a port
Shaft crossing the surface
Boundary heat transfer

enthalpy
internal energy
heat
work

Show the answer

Contents of the control volume: internal energy Mass crossing a port: enthalpy Shaft crossing the surface: work Boundary heat transfer: heat

9.[2p]
A steel cylinder is depressurised rapidly and its contents reach $- 32$ degrees Celsius. Why is that a safety concern rather than a curiosity?

Carbon steel loses toughness as it cools, so the vessel may become brittle

Correct

The cold gas expands and raises the pressure again

The answer is: Carbon steel loses toughness as it cools, so the vessel may become brittle

The valve seat melts at low temperature

The answer is: Carbon steel loses toughness as it cools, so the vessel may become brittle

The gas liquefies and the vessel overfills

The answer is: Carbon steel loses toughness as it cools, so the vessel may become brittle

1.[3p] A rigid insulated evacuated bottle is filled from a line. What does the energy balance reduce to?

2.[2p] Air at 300 K fills a rigid insulated evacuated bottle. Taking k=1.400, what is the final temperature inside, in kelvin?

3.[2p] That bottle has a volume of 0.500 m³ and is filled to 700 kPa. With R=0.287 kJ/(kg K), what mass of air does it hold, in kg?

4.[2p] A cylinder charged to its rated pressure while hot is still at rated pressure once it has cooled to ambient.

5.[3p] The same 0.500 m³ bottle, holding air at 700 kPa and 420 K, is vented to 100 kPa. Treating the residual gas as expanding reversibly and adiabatically with k=1.400, what is its final temperature, in kelvin?

6.[3p] Why can the gas remaining in a vessel after blowdown be treated as a closed system?

7.[3p] What does the uniform-flow model assume?

8.[2p] Match each quantity to the energy it is counted with in a transient balance.

9.[2p] A steel cylinder is depressurised rapidly and its contents reach -32 degrees Celsius. Why is that a safety concern rather than a curiosity?

1.[3p]
A rigid insulated evacuated bottle is filled from a line. What does the energy balance reduce to?

2.[2p]
Air at $300$ K fills a rigid insulated evacuated bottle. Taking $k = 1.400$ , what is the final temperature inside, in kelvin?

3.[2p]
That bottle has a volume of $0.500$ m³ and is filled to $700$ kPa. With $R = 0.287$ kJ/(kg K), what mass of air does it hold, in kg?

4.[2p]
A cylinder charged to its rated pressure while hot is still at rated pressure once it has cooled to ambient.

5.[3p]
The same $0.500$ m³ bottle, holding air at $700$ kPa and $420$ K, is vented to $100$ kPa. Treating the residual gas as expanding reversibly and adiabatically with $k = 1.400$ , what is its final temperature, in kelvin?

6.[3p]
Why can the gas remaining in a vessel after blowdown be treated as a closed system?

7.[3p]
What does the uniform-flow model assume?

8.[2p]
Match each quantity to the energy it is counted with in a transient balance.

9.[2p]
A steel cylinder is depressurised rapidly and its contents reach $- 32$ degrees Celsius. Why is that a safety concern rather than a curiosity?