Lesson test: Control volumes and mass flow

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Control volumes and mass flow

1.[2p]
Why is a closed system the wrong model for a steam turbine?

Matter streams through the hardware, so no fixed parcel of fluid ever coincides with the machine

Correct

Turbines exchange heat with their surroundings, which a closed system forbids

The answer is: Matter streams through the hardware, so no fixed parcel of fluid ever coincides with the machine

The First Law does not apply to rotating machinery

The answer is: Matter streams through the hardware, so no fixed parcel of fluid ever coincides with the machine

Steam is not a pure substance, so no system boundary can be drawn round it

The answer is: Matter streams through the hardware, so no fixed parcel of fluid ever coincides with the machine

2.[2p]
Steam with $v = 0.09056$ m³/kg flows down a pipe of internal diameter $200$ mm at $45$ m/s. What is the mass flow rate, in kg/s?

CorrectNot quite: 15.61

3.[3p]
A turbine passes $15.61$ kg/s of steam whose exit specific volume is $11.92$ m³/kg. If the exhaust velocity is $180$ m/s, what flow area is needed, in square metres?

CorrectNot quite: 1.034

4.[1p]
Under steady flow the mass inside the control volume is constant, even though the fluid's properties change enormously between inlet and outlet.

True

Correct

False

The answer is: True

5.[3p]
Which of these follow from steady flow through a device with one inlet and one outlet?

Select all that apply

The mass flow rate is the same at both ports

Correct

$A_{1} V_{1} / v_{1} = A_{2} V_{2} / v_{2}$

Correct

The properties at each port are constant in time

Correct

The specific volume is the same at both ports

The answer is: The mass flow rate is the same at both ports, $A_1V_1/v_1 = A_2V_2/v_2$, The properties at each port are constant in time

6.[2p]
A mixing chamber receives $2.0$ and $5.0$ kg/s of water and discharges through a pipe where $v = 0.001043$ m³/kg at $5.0$ m/s. What exit area is needed, in square centimetres?

CorrectNot quite: 14.6

7.[2p]
Match each term to what it means.

Control volume
Control surface
Port
Steady flow

a place where mass crosses
the boundary mass crosses
nothing at a point changes in time
a chosen region of space

Show the answer

Control volume: a chosen region of space Control surface: the boundary mass crosses Port: a place where mass crosses Steady flow: nothing at a point changes in time

8.[2p]
The velocity $V$ in $\dot{m} = A V / v$ is which velocity?

The mass-averaged velocity over the cross-section, which is what makes the formula exact

Correct

The centreline velocity, which is the largest in the pipe

The answer is: The mass-averaged velocity over the cross-section, which is what makes the formula exact

The velocity at the wall, which is zero

The answer is: The mass-averaged velocity over the cross-section, which is what makes the formula exact

The speed of sound in the fluid at that state

The answer is: The mass-averaged velocity over the cross-section, which is what makes the formula exact

9.[2p]
Steam leaves a turbine occupying over $130$ times the volume per kilogram that it entered with. What does that force on the hardware?

A very large exhaust flow area, which is why low-pressure turbine cylinders are enormous

Correct

A very high exhaust pressure, which is why condensers are pressure vessels

The answer is: A very large exhaust flow area, which is why low-pressure turbine cylinders are enormous

A much smaller mass flow at exit than at inlet

The answer is: A very large exhaust flow area, which is why low-pressure turbine cylinders are enormous

A reduction in exhaust velocity to keep the mass flow constant

The answer is: A very large exhaust flow area, which is why low-pressure turbine cylinders are enormous

1.[2p] Why is a closed system the wrong model for a steam turbine?

2.[2p] Steam with v=0.09056 m³/kg flows down a pipe of internal diameter 200 mm at 45 m/s. What is the mass flow rate, in kg/s?

3.[3p] A turbine passes 15.61 kg/s of steam whose exit specific volume is 11.92 m³/kg. If the exhaust velocity is 180 m/s, what flow area is needed, in square metres?

4.[1p] Under steady flow the mass inside the control volume is constant, even though the fluid's properties change enormously between inlet and outlet.

5.[3p] Which of these follow from steady flow through a device with one inlet and one outlet?

6.[2p] A mixing chamber receives 2.0 and 5.0 kg/s of water and discharges through a pipe where v=0.001043 m³/kg at 5.0 m/s. What exit area is needed, in square centimetres?

7.[2p] Match each term to what it means.

8.[2p] The velocity V in m˙=AV/v is which velocity?

9.[2p] Steam leaves a turbine occupying over 130 times the volume per kilogram that it entered with. What does that force on the hardware?

1.[2p]
Why is a closed system the wrong model for a steam turbine?

2.[2p]
Steam with $v = 0.09056$ m³/kg flows down a pipe of internal diameter $200$ mm at $45$ m/s. What is the mass flow rate, in kg/s?

3.[3p]
A turbine passes $15.61$ kg/s of steam whose exit specific volume is $11.92$ m³/kg. If the exhaust velocity is $180$ m/s, what flow area is needed, in square metres?

4.[1p]
Under steady flow the mass inside the control volume is constant, even though the fluid's properties change enormously between inlet and outlet.

5.[3p]
Which of these follow from steady flow through a device with one inlet and one outlet?

6.[2p]
A mixing chamber receives $2.0$ and $5.0$ kg/s of water and discharges through a pipe where $v = 0.001043$ m³/kg at $5.0$ m/s. What exit area is needed, in square centimetres?

7.[2p]
Match each term to what it means.

8.[2p]
The velocity $V$ in $\dot{m} = A V / v$ is which velocity?

9.[2p]
Steam leaves a turbine occupying over $130$ times the volume per kilogram that it entered with. What does that force on the hardware?