This paper describes the method of flow velocity measurement by use of correlation technique.
Since Butterfield developed the method of strip-speed measurement using random wave form correlation, several researchers have applied the Butterfield's method to the measurement of velocities of paper strip, air flow, and water flow.
In this paper, a new method of real-time measurement of the flow velocity in a pipe by use of random hot water injection is described. In this method, one utilizes essentially the symmetrical property of temperature distribution along the axis of the pipe at some instant after the impulsive injection of hot water. In practice, the hot water is injected into the flow according to the pseudorandom
m sequence, and small temperature fluctuations are detected at two downstream points. The difference between the two detected signals is crosscorrelated with the
m sequence. The shift pulse frequency of the
m sequence generator is controlled so as to make this crosscorrelation minimum. Thus, the controlled shift pulse frequency is directly proportional to the mean velocity of water flow.
Experiment showed that the standard deviation of relative accuracy of this method in comparison with the orifice flow metering is within 0.5% when
Re>1.3×10
4.
In connection with the dispersion of injected hot water, a three-dimensional diffusion model is proposed, assuming that the fluid flows with the mean velocity
u and has the virtual diffusion coefficients,
EZ for longitudinal direction, and
Er for radial direction. The temperature distribution along the longitudinal direction at a certain instant after the impulsive injection of hot water is given, and the impulse response, which is the signal detected at some distance downstream from the injecting point, is also given.
It is shown in the experiment that the three-dimensional diffusion model applies approximately in the neighbourhood of the point moving with the mean velocity of flow.
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