Étude sur le fil chaud et le film chaud dans l'eau

¹ Institut deMécanique Statistique de la Turbulence de la Faculté des Sciences de Marseille, Laboratoire associé du C.N.R.S

Résumé

The research carried out on this subject has helped in the development of methods of measurïng mean water flow velocities and turbulent fluctuations by means of hot wire and film anemometers to within a similar degree of accuracy to that attainable in air. For this type of measurement, it is ncecssary to know the cooling relationship for the sensitive element, which is a relationship between dimensionless Nusselt, Reynolds, Prandtl and Grashof numbers. Various difIiculties arise in the use of this type of probe in water, which can only be suceessfully oversome by special manufacturing methods, appropriate geometrical design features and by designing for low heating coefficients. Fluid temperature variations are liable to cause considerable error. The experimental rig comprised a tubular water tunnel for calibration and check measurements in turbulent flow of know characteristics, pressure measuring instrumentation, a constant-temperature anemometer and various items of auxiliary equipment. Fairly satisfactory results were obtained with bare hot wire pick-ups with low regular drift thanks to a specially developed manufacturing method. Various types of industrially produced hot-film anemometer with quartz insulation were also studied and calibrated and drift correction methods suggested. The relationship between the cooling of a conical film, flow velocity and water temperature was investigated. In the range of values covered, the Nusselt number seemed to solely depend on the Péclet number, and approximately on velocity. The relationship appears to be of the form P = D + C Pén = D + C' Vn, with the exponent n working out at 0.265. Turbulence strength and spectrum measurements were carried out to check the method in fully developed flow discharging from the cylindrical water tunnel with Reynold numbers varying between 15 000 and 150 000. The results agreed well with those of previous measurements in dynamically similar air f1ows. Summing up it seems that, with careful use, a hot wire or hot film anemometer can produce comparably accurate measurement results in water as in air.