Page 119 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT
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& Ideal conditions include medium- to high-speed flow 4.1.8.3 Anemometers
because the formation of vortices is irregular at low
flow rates. . What is the principle of operation of a hot-wire ane-
& Density variations affect the strength of the shed mometer? Does it measure velocity or flow? For what
vortex and this places a lower limit on fluid density, type of applications, it is well suited?
which is based on the sensitivity of the vortex shed- & The measuring element of a hot-wire anemometer
ding detection equipment. consists of an electrically heated wire, generally
& It is necessary to eliminate swirl and distorted flow made of platinum or tungsten.
patterns upstream of the meter. & Rate of heat flow from the wire to the fluid (gas) is a
. Under what circumstances vortex flow meters are most function of fluid velocity.
accurate? & Temperature of the wire is a function of its resistance
& High-speed flows involving clean and low viscosity
(cf. principle involved in platinum-resistant ther-
liquids, free from swirl and distorted flow patterns. mometer), which in turn is a function of fluid veloc-
. What are the advantages and disadvantages of vortex ity. Thus, fluid velocity is calibrated in terms of wire
shedding flow meters? resistance.
Advantages & It measures velocity of gases/air in conduits/ducts
& Relatively wide rangeability with linear output. into which the probe is inserted with the wire element
& On clean fluids (liquids and gases), the meters have facing flow direction. Velocity measured is point
long-term stable proofs. velocity.
& Frequency output can be read directly into electronic & Another type of anemometer is constant resistance
readout systems. type, in which the current is varied to keep wire
& Installation is simple and costs are moderate. temperature constant.
& Current is measured and expressed as a function of
& Effects of viscosity, pressure, and temperature are
minimal over wide range of Reynolds numbers. velocity.
& Used for gas velocity measurements in the range of
& No moving parts in contact with the flowing stream.
0.15 m/s to supersonic velocities.
Disadvantages
. What are the characteristic features and applications of
& Flow into a meter must be swirl-free requiring
hot-wire anemometers?
straightening vane and/or long, straight piping.
& A positive consequence of their small mass is fast
& Output may have frequency instability and/or fade in
speed of response. They are widely used in HVAC
certain areas of operation, which affect readout
and ventilation applications.
requirements.
& Larger and more rugged anemometers are also avail-
& Not available in sizes above 20 cm.
able for more demanding industrial applications.
& Pulse train is irregular and proving requires a
& To ensure the proper formation of the velocity
long test time to obtain a representative average pulse
profile, a straight duct section is usually provided
rate.
upstream of the anemometer station (usually 10
& Pulse resolution is the same for all meter sizes,
diameters long).
meaning a low pulse rate with larger meters yields
& A conditioning nozzle is used to eliminate boundary
low volume resolution.
layer effects.
& Subject to range limitations at lower Reynolds
& If there is no room for the straight pipe section, a
numbers.
honeycomb flow straightener can be incorporated
. Give typical applications of vortex meters.
into the sensor assembly.
& Cryogenic fluids.
& Typically, the anemometer wire is made of platinum
& Steam measurement.
or tungsten and is 4–10 mm in diameter and 1 mm in
& Condensate measurement.
length.
& Ultra pure and deionized water.
& Because of the small size and fragility of the wire,
& Acids. hot-wire anemometers are susceptible to dirt buildup
& Solvents. and breakage.
. “Vortex meters with integral reducing flanges are a good . What is a heated thermocouple anemometer?
choice for measuring low flows”. True/False? & Heated thermocouple anemometer measures gas ve-
& True. These provide flexibility after installation. locity from cooling effect of the fluid (gas) stream