5 Experimental Fluid Mechanics
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the temperature of the wire to drop. According to the theory of forced convection, heat exchange in an infinitely long cylinder, the relationship between
the heat taken away by the hot wire and the velocity U of the fluid can be
derived. This relationship is called the formula of L.V. King (1914), which is
I
2 R = (T s − T 0 )(A + B
√
U )
where R and I are, respectively, the resistance of the hot wire and the current
intensity flowing, Ts is the operating temperature of the hot wire, T0 is the
ambient temperature of the fluid to be measured, and A and B are physical
constants related to the fluid and the hot wire. The hot wire anemometer can
measure the velocity in real time, dynamically and continuously without the
addition of tracer particles. It has a high-speed response frequency and can
be used to measure opaque fluid and study the frequency response range of
all turbulence fluctuations.
The length of the hot wire is generally in the range of 0.5 to 2 mm, and
the diameter is in the range of 1 to 10 μm. The material is platinum, tungsten or platinum-rhodium alloy. If a thin metal film (thickness less than 0.1
micron) is used instead of the metal wire, it is a hot film anemometer, which
functions similarly to the hot wire, but is mostly used to measure the liquid
flow rate. In addition to the ordinary single-wire type, the hot line can also be
a combined two-wire or three-wire type to measure the velocity component
in all directions. The electric signal output from the hot wire is amplified,
compensated and digitized and input into the computer, which can improve
the measurement accuracy, automatically complete the data post-processing
process, and expand the speed measurement function, such as simultaneously
completing measurement on the instantaneous value and the time-average
value, the combined speed and the sub-speed, and the turbulence and other
turbulence parameters (as shown in Fig. 5.22). Compared with the Pitot tube,
the hot wire anemometer has the advantages of a small probe volume and
small interference to flow field, fast response, measuring unsteady flow rate,
measuring flow with very low speed (such as low as 0.3 m/s).
The hot wire anemometer converts the flow rate signal into an electrical
signal, using the formula of L.V. King (as shown in Fig. 5.21) and can also be
used to measure fluid temperature or density. It has two working modes: ➀
constant electric current. The electric current through the hot wire remains
unchanged. When the temperature changes, the heat line resistance changes,
so the voltage at both ends changes, thereby measuring the flow rate; ➁
constant temperature type. The temperature of the hot wire remains the same,
such as maintaining 150 °C, the flow velocity can be measured according to
363
the temperature of the wire to drop. According to the theory of forced convection, heat exchange in an infinitely long cylinder, the relationship between
the heat taken away by the hot wire and the velocity U of the fluid can be
derived. This relationship is called the formula of L.V. King (1914), which is
I
2 R = (T s − T 0 )(A + B
√
U )
where R and I are, respectively, the resistance of the hot wire and the current
intensity flowing, Ts is the operating temperature of the hot wire, T0 is the
ambient temperature of the fluid to be measured, and A and B are physical
constants related to the fluid and the hot wire. The hot wire anemometer can
measure the velocity in real time, dynamically and continuously without the
addition of tracer particles. It has a high-speed response frequency and can
be used to measure opaque fluid and study the frequency response range of
all turbulence fluctuations.
The length of the hot wire is generally in the range of 0.5 to 2 mm, and
the diameter is in the range of 1 to 10 μm. The material is platinum, tungsten or platinum-rhodium alloy. If a thin metal film (thickness less than 0.1
micron) is used instead of the metal wire, it is a hot film anemometer, which
functions similarly to the hot wire, but is mostly used to measure the liquid
flow rate. In addition to the ordinary single-wire type, the hot line can also be
a combined two-wire or three-wire type to measure the velocity component
in all directions. The electric signal output from the hot wire is amplified,
compensated and digitized and input into the computer, which can improve
the measurement accuracy, automatically complete the data post-processing
process, and expand the speed measurement function, such as simultaneously
completing measurement on the instantaneous value and the time-average
value, the combined speed and the sub-speed, and the turbulence and other
turbulence parameters (as shown in Fig. 5.22). Compared with the Pitot tube,
the hot wire anemometer has the advantages of a small probe volume and
small interference to flow field, fast response, measuring unsteady flow rate,
measuring flow with very low speed (such as low as 0.3 m/s).
The hot wire anemometer converts the flow rate signal into an electrical
signal, using the formula of L.V. King (as shown in Fig. 5.21) and can also be
used to measure fluid temperature or density. It has two working modes: ➀
constant electric current. The electric current through the hot wire remains
unchanged. When the temperature changes, the heat line resistance changes,
so the voltage at both ends changes, thereby measuring the flow rate; ➁
constant temperature type. The temperature of the hot wire remains the same,
such as maintaining 150 °C, the flow velocity can be measured according to