The method of measuring velocity with the help of the pulsed thermal anemometer is based on measuring the transit time of particles heated by the wire to which electric pulses are sent up to the wire transducers operating in the resistance thermometer mode. The advantage of the method is its utilization for the flows with a high degree of turbulence including flows with sign-variable averaged velocity, i.e., in conditions when the traditional method of diagnosis of turbulent flows, a hot-wire anemometer, cannot be used. Furthermore, the velocity is measured directly and is absolutely independent of the properties of a fluid.
A sensing element of transducer of a pulsed thermal anemometer consists of three wires (see Figure 1). The central wire is the source of thermal pulses, and on each side of it, the receiving wires are arranged at distance 1 apart, their axes being perpendicular to the pulse source axis. For the central wire, platinum or nickel wires 10 μm in diameter are used, and 5 μm wires L/I = 5-6 long and spaced 1.5 ≤ l ≤ 2.5 mm apart are used for the peripheral wires. The temperature of the heated wire must not exceed 500-600°C in order to maintain its strength. The pulse length of the voltage supplied to the central wire for receiving warmed gas particles is 5-30 μs, the front rise time being not less than 0.5 μs and with a current of 4-9A. The receiving wires are supplied with a 0.5-3 mA constant current and are connected to the bridge arms. The polarity of a signal depends on the transducer conductors which receive a thermal pulse and determine the flow direction.
At currents lower than 0.5 mA the influence of electron noise becomes significant and at currents higher than 5 mA the transducer starts to respond to velocity fluctuations as a hot-wire anemometer. In an ideal case the temperature of a pulse wire rises instantaneously when the temperature of the surrounding medium increases to a higher value. The particles of the air warmed by it are carried away with an instantaneous velocity of the flow streamlining the transducer. With the effects of diffusion and thermodiffusion being neglected, the time t required for the front of warmed particles to reach the receiving wire is t = i/u cos φ, where φ is the angle formed by the instantaneous velocity vector u andl the normal to the transducer plane. The use of two receiving wires on each side of the pulse wire ensures an unambiguous definition of the flow direction at the time moment being considered. If a sufficient number of measurements of the transit time for a given position of a transducer are made, then the averaged set of realizations will allow us to determine the time average and the fluctuation components of the velocity vector. The advantage of the method is that the transducer does not respond to turbulent pulsations which have typical dimensions smaller than the dimension of the transducer. The effect of the thermal diffusion causes an additional barrier in applying the method; the limit is Pe = yl/κ = 50.
Bradbury, L. J. S. and Castro, I. P. A. (1971) A Pulsed-Wire Technique for Velocity Measurement in Highly Turbulent Flows, J. of Fluid Mechanics, 4.