JSSC- JE Mechanical Engg. Question Bank (Chapter – 14)

Mechanical

Which instrument is commonly used to measure the discharge of liquid flowing through a closed pipe?

Venturimeter
Thermometer
Barometer
Hygrometer

Explanation:

A venturimeter measures the flow rate of a fluid through a pipe by using the pressure difference between the inlet and throat.
It works on the principles of continuity and Bernoulli’s equation.

The basic principle used in a venturimeter is:

Bernoulli’s theorem and continuity equation
Newton’s law of cooling
Archimedes’ principle only
Pascal’s law only

Explanation:

As the fluid enters the reduced-area throat, its velocity increases and its static pressure decreases.
The pressure difference is related to the discharge using Bernoulli’s equation and the continuity equation.

Which section of a venturimeter has the minimum cross-sectional area?

Throat
Inlet
Outlet pipe
Diffuser inlet only

Explanation:

The throat is the section of minimum area in a venturimeter.
Velocity is maximum and static pressure is minimum at the throat, subject to losses.

When water flows through a venturimeter, the velocity at the throat is generally:

Greater than the velocity at the inlet
Always zero
Less than the inlet velocity
Exactly equal to the inlet velocity in every case

Explanation:

From the continuity equation, Q = AV.
Since the throat area is smaller than the inlet area, the velocity at the throat increases for steady incompressible flow.

According to Bernoulli’s principle, when the velocity of an ideal incompressible fluid increases, its static pressure generally:

Decreases
Increases proportionally
Remains unchanged in all cases
Becomes atmospheric

Explanation:

For steady, incompressible, frictionless flow at the same elevation, an increase in velocity head is accompanied by a decrease in static pressure head.
This pressure difference is utilized in flow-measuring devices such as venturimeters.

The pressure difference in a venturimeter is generally measured between:

Inlet and throat
Two points in the straight pipe far upstream
Only the outlet and atmosphere
Two points on the diffuser outlet

Explanation:

Pressure taps are commonly provided at the inlet section and throat.
The pressure difference between these locations is used to determine discharge.

The coefficient of discharge of a venturimeter is defined as:

Actual discharge divided by theoretical discharge
Theoretical discharge divided by actual discharge
Pressure head divided by velocity head
Velocity divided by pressure

Explanation:

Coefficient of discharge is:
C_d = Q_actual / Q_theoretical.
It accounts mainly for losses and deviations from ideal flow.

For an ideal venturimeter, the coefficient of discharge is:

1
0
Less than 0
Greater than 10

Explanation:

For ideal flow with no losses, actual discharge equals theoretical discharge.
Therefore, C_d = 1.
Actual venturimeters have C_d values slightly below unity.

The convergent portion of a venturimeter is used to:

Increase fluid velocity and reduce static pressure
Decrease velocity and increase pressure only
Stop the fluid flow
Measure temperature

Explanation:

The converging section reduces the flow area.
For steady incompressible flow, velocity increases as area decreases.
Consequently, static pressure decreases according to Bernoulli’s equation, subject to elevation and losses.

The diffuser portion of a venturimeter is provided mainly to:

Recover pressure by reducing velocity gradually
Increase velocity indefinitely
Increase friction deliberately
Stop the flow

Explanation:

The diffuser is the gradually expanding section after the throat.
It converts part of the kinetic energy back into pressure energy.
A gradual expansion helps minimize flow separation and energy losses.

Why is the diffuser angle of a venturimeter kept relatively small?

To reduce flow separation and energy loss
To increase turbulence intentionally
To reduce the throat velocity to zero
To increase fluid viscosity

Explanation:

A sudden or excessively rapid expansion can cause flow separation.
Flow separation increases energy loss.
A gradual diffuser provides better pressure recovery.

Which flow-measuring device generally has comparatively low permanent head loss?

Venturimeter
Orifice meter
Sudden expansion
Sharp-edged obstruction

Explanation:

A well-designed venturimeter has relatively low permanent pressure loss because the diffuser allows substantial pressure recovery.
This is one of its major advantages over an orifice meter.

Which flow meter is generally cheaper and more compact than a venturimeter?

Orifice meter
Pitot tube
Large venturi tube
Rotameter only

Explanation:

An orifice meter consists of a relatively simple thin plate with an opening installed between flanges.
It is compact and inexpensive but causes a larger permanent pressure loss than a venturimeter.

An orifice meter measures flow rate mainly by measuring:

Pressure difference across an orifice plate
Temperature difference only
Fluid density only
Pipe wall thickness

Explanation:

An orifice plate creates a restriction in the pipe.
The restriction causes a change in velocity and pressure.
The measured pressure difference is related to flow rate using the appropriate discharge equation and coefficient of discharge.

Which of the following is an important disadvantage of an orifice meter compared with a venturimeter?

Higher permanent head loss
Very high installation cost
Very large physical size
No pressure difference is produced

Explanation:

The abrupt contraction and subsequent flow expansion around an orifice plate cause considerable energy loss.
Therefore, its permanent pressure loss is generally higher than that of a venturimeter.
Its low cost and compactness are important advantages.

What is the primary purpose of an orifice plate in an orifice meter?

To create a known restriction and pressure drop in the flow
To eliminate all turbulence
To increase pipe diameter
To measure fluid temperature

Explanation:

The orifice plate has a precisely machined opening.
The restriction changes the flow velocity and pressure, producing a measurable differential pressure.
This pressure difference is used to calculate the discharge.

The discharge through an ideal differential-pressure flow meter is approximately proportional to:

The square root of the measured pressure difference
The square of pressure difference
The cube of pressure difference
The inverse of pressure difference

Explanation:

For incompressible flow through devices such as venturimeters and orifice meters, the theoretical discharge has the general relationship Q ∝ √Δp.
The exact equation also depends on areas, density and the coefficient of discharge.

Which instrument measures local fluid velocity rather than directly measuring the total pipe discharge?

Pitot tube
Venturimeter
Orifice meter
Weir only

Explanation:

A Pitot tube measures the local flow velocity at its measurement point.
Velocity is determined from the difference between stagnation pressure and static pressure.
To determine total discharge in a pipe, the velocity distribution across the section must be considered or an appropriate calibrated arrangement must be used.

The working principle of a Pitot tube is based mainly on:

Conversion of kinetic energy into pressure energy at a stagnation point
Archimedes’ principle
Pascal’s law only
Heat conduction

Explanation:

At the stagnation point, the fluid velocity is reduced to zero.
The kinetic energy associated with the flow is converted into pressure head.
The difference between stagnation and static pressure gives the velocity head.

For an ideal Pitot tube, the velocity of flow is given by:

V = √[2(p₀ − p)/(ρ)]
V = (p₀ − p)ρ/2
V = 2ρ(p₀ − p)
V = ρ/[2(p₀ − p)]

Explanation:

Applying Bernoulli’s equation between the flowing point and the stagnation point gives:
p₀ − p = ρV²/2.
Therefore, V = √[2(p₀ − p)/ρ].
For a real Pitot tube, a velocity coefficient may be included.

At the stagnation point of a Pitot tube, the fluid velocity is:

Zero
Maximum
Equal to the average pipe velocity
Infinite

Explanation:

The stagnation point is a point where the local fluid velocity becomes zero.
The kinetic energy is converted into pressure energy, producing stagnation pressure.

The pressure measured by a Pitot tube at the stagnation point is called:

Stagnation pressure
Vacuum pressure
Vapour pressure
Critical pressure

Explanation:

Stagnation pressure is the pressure obtained when the flowing fluid is brought to rest isentropically in the ideal case.
The difference between stagnation pressure and static pressure is related to the velocity head.

A Pitot-static tube measures:

Static pressure and stagnation pressure
Temperature and density only
Viscosity and density only
Discharge and temperature directly

Explanation:

A Pitot-static tube has openings arranged to sense both static and stagnation pressures.
Their difference provides the velocity head.
It is commonly used for measuring airspeed and local fluid velocity.

For a Pitot tube, if the difference between stagnation and static pressure increases, the flow velocity:

Increases
Decreases
Becomes zero
Remains unchanged in all cases

Explanation:

For incompressible ideal flow:
p₀ − p = ρV²/2.
Therefore, a larger pressure difference corresponds to a higher velocity.

Which flow-measuring device is based on the measurement of stagnation pressure?

Pitot tube
Orifice meter
Venturimeter
Rotameter

Explanation:

A Pitot tube determines velocity using stagnation pressure.
Venturimeters and orifice meters determine flow rate using pressure differences created by area restrictions.

Which device causes the greatest permanent pressure loss among a well-designed venturimeter and an orifice meter for comparable measurement duty?

Orifice meter
Venturimeter
Both always have exactly the same loss
Neither causes any loss

Explanation:

An orifice meter causes significant irreversible losses due to flow separation and turbulence downstream of the restriction.
A venturimeter has a gradual expansion section that recovers much of the pressure.

Which device generally requires the shortest straight length and is suitable for point velocity measurement?

Pitot tube
Venturimeter
Orifice meter
Large nozzle meter

Explanation:

A Pitot tube is a local velocity-measuring instrument.
It is inserted into the flow and aligned with the local flow direction.
It does not require a long converging and diverging body like a venturimeter.

Which equation represents the continuity principle for steady incompressible flow?

Q = AV
Q = A/V
Q = V/A
Q = A + V

Explanation:

For steady incompressible flow, discharge Q equals cross-sectional area A multiplied by mean velocity V.
Thus, Q = AV.
This principle explains the increase in velocity when the flow area decreases in a venturimeter.

If the pipe area is reduced to half while discharge remains constant, the average velocity will:

Double
Become half
Remain unchanged
Become four times

Explanation:

From Q = AV, for constant Q, velocity is inversely proportional to area.
If A becomes A/2, then V becomes 2V.

Which quantity is directly measured by a differential-pressure manometer connected across a venturimeter?

Pressure difference
Discharge directly
Velocity directly
Pipe diameter directly

Explanation:

A differential manometer measures the pressure difference between two pressure taps.
The pressure difference is then used with the venturimeter geometry and fluid properties to calculate discharge.

If the pressure difference across a venturimeter increases, the measured flow rate generally:

Increases
Decreases
Becomes zero
Remains exactly constant regardless of pressure difference

Explanation:

For a given venturimeter and fluid, Q is approximately proportional to √Δp.
Therefore, an increase in differential pressure corresponds to an increase in discharge.

Which flow meter is most suitable when low permanent head loss is an important consideration?

Venturimeter
Orifice meter
Sharp-edged restriction only
Sudden contraction

Explanation:

The venturimeter has a smooth converging section and gradual diffuser.
This design provides relatively good pressure recovery and low permanent head loss compared with an orifice meter.

Which flow meter is generally preferred when low initial cost and compact construction are important?

Orifice meter
Venturimeter
Long-throat venturi only
Large open channel weir

Explanation:

An orifice meter is relatively simple, compact and inexpensive to install.
Its disadvantage is comparatively high permanent pressure loss.

Which instrument can be used to determine the velocity profile at different locations across a pipe cross-section?

Pitot tube
Orifice plate alone
Venturimeter alone
Pressure relief valve

Explanation:

A Pitot tube can be positioned at different points across a pipe section to measure local velocity.
The resulting measurements can be used to study the velocity distribution.

What is the main limitation of using a simple Pitot tube for measuring total discharge in a pipe?

It measures local velocity rather than the average velocity directly
It cannot measure velocity
It always causes complete blockage
It measures only temperature

Explanation:

A simple Pitot tube gives the velocity at its measurement location.
Since velocity varies across a pipe section, multiple measurements or suitable integration/calibration may be required to determine total discharge accurately.

Which statement about a venturimeter is correct?

It has a converging section, throat and diverging section
It consists only of a sharp-edged plate
It has no pressure taps
It works only with gases

Explanation:

A conventional venturimeter consists of a converging cone, throat and diverging cone.
Pressure taps are generally provided at the inlet and throat.
It can be used for liquids and gases with appropriate analysis and design.

Which statement about an orifice meter is correct?

It uses a thin plate with a precisely sized opening to create a pressure drop
It has a long diffuser for complete pressure recovery
It measures only temperature
It requires an impeller

Explanation:

An orifice meter uses an orifice plate installed in a pipe.
The restriction creates a differential pressure related to flow rate.
Compared with a venturimeter, it is simpler and cheaper but generally has greater permanent energy loss.

Which three instruments are commonly based on fluid-flow principles for measuring velocity or discharge?

Venturimeter, Pitot tube and orifice meter
Thermometer, barometer and hygrometer
Lathe, milling machine and shaper
Boiler, condenser and turbine

Explanation:

Venturimeter and orifice meter use differential pressure generated by a restriction to determine flow rate.
A Pitot tube uses stagnation and static pressure to determine local velocity.
All three are important fluid-mechanics instruments for flow measurement.