Mechanical Engineering Principles

FLUID FLOW 253

Applications of electromagnetic flowmeters are
found in the measurement of speeds of slurries,
pastes and viscous liquids, and they are also widely
used in the water production, supply and treatment
industry.

22.12 Hot-wire anemometer

A simple hot-wire anemometer consists of a small
piece of wire which is heated by an electric current
and positioned in the air or gas stream whose veloc-
ity is to be measured. The stream passing the wire
cools it, the rate of cooling being dependent on the
flow velocity. In practice there are various ways in
which this is achieved:

(i) If a constant current is passed through the

wire, variation in flow results in a change of

temperature of the wire and hence a change

in resistance which may be measured by a

Wheatstone bridge arrangement. The change

in resistance may be related to fluid flow.

(ii) If the wire’s resistance, and hence tempera-

ture, is kept constant, a change in fluid flow

results in a corresponding change in current

which can be calibrated as an indication of

the flow rate.

(iii) A thermocouple may be incorporated in the
assembly, monitoring the hot wire and record-
ing the temperature which is an indication of
the air or gas velocity.

Advantages of the hot-wire anemometer

(a) Its size is small

(b) It has great sensitivity

22.13 Choice of flowmeter

Problem 1. Choose the most appropriate

fluid flow measuring device for the following

circumstances:

(a) The most accurate, permanent

installation for measuring liquid

flow rate.

(b) To determine the velocity of low-speed

aircraft and ships.

(c) Accurate continuous volumetric

measurement of crude petroleum

products in a duct of 500 mm bore.

(d) To give a reasonable indication of the

mean flow velocity, while maintaining a

steady pressure difference on a

hydraulic test rig.

(e) For an essentially constant flow rate

with reasonable accuracy in a large pipe

bore, with a cheap and simple

installation.

(a) Venturimeter

(b) Pitot-static tube

(c) Turbine flowmeter

(d) Float and tapered-tube flowmeter

(e) Orifice plate

Now try the following exercise

Exercise 114 Further problems on the

measurement of fluid flow

For the flow measurement devices listed 1

to 5, (a) describe briefly their construction,

(b) state their principle of operation, (c) state

their characteristics and limitations, (d) state

typical practical applications, (e) discuss their

advantages and disadvantages.

1. Orifice plate


2. Venturimeter


3. Pitot-static tube


4. Float and tapered-tube meter


5. Turbine flowmeter

22.14 Equation of continuity

The calibrations of many of the flowmeters des-

cribed earlier are based on the equation of continuity

and Bernoulli’s equation.

The equation of continuity states that for the

steady flow of a fluid through a pipe of varying

cross-section the rate of mass entering the pipe must

be equal to the rate of mass leaving the pipe; this is

really a statement of theprinciple of conservation

of mass. Thus, for an incompressible fluid:

a 1 v 1 =a 2 v 2