Contd...
Hence the energy balance equation for the wire reads as
Figure 2.25: Transient Response of a Hot-wire Probe. |
Here  and  are reference wire resistance and temperature respectively. The time constant of the wire is the reciprocal of the term multiplying  . It is desirable to reduce the time constant to a minimum value for improving the transient as well as the frequency response. This is accomplished under the following conditions:
When a hot wire is operated under constant temperature conditions through a feedback circuit  and it becomes a zeroth order system. In principle the hot wire attains an infinite frequency response. In practice it is limited by the frequency response of the feedback circuit. This is in the range 10 to 100 kHz and is usually adequate for flow studies.
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