The Use of Pitot-Static Tubes and Hot-Film Anemometers in Dilute Polymer Solutions

Abstract

The operation of Pitot-static tubes in dilute polyacrylamide-water solutions was verified experimentally by using an independent method to measure the velocity. Measurement of the heat transfer characteristics of cylindrical hot-film anemometer probes in these solutions revealed reduced heat transfer in addition to the following anomalous results. In certain velocity ranges, the Nusselt number was independent of velocity. At high velocities, a form of vortex shedding from the probe occurred, and the Nusselt number was proportional to the square-root of velocity. The dependence of the Nusselt number on the angle of incidence was, for the most concentrated solution, opposite to that for pure water. The above results indicate that these probes cannot be used to measure velocity in certain cases, and interpretation of turbulence measurements would seem to be open to question. Calibration of a conical hot-film probe revealed only slightly reduced heat transfer. Measurement of the one-dimensional velocity spectrum in a polymer solution indicated that the cylindrical probe gave a power-law dependence on the wave number at high wave numbers; whereas the data obtained with the conical probe was qualitatively similar to Newtonian spectral data.