Application of an electromagnetic wall velocimeter to velocity measurement in liquid films.

The electromagnetic well velocimeter works on the principle that a conductor moving through a magnetic field has a potential gradient established in it which is directly proportional to the local cross product of the magnetic field strength and the velocity. Therefore, measurement of the induced voltage can be related to the liquid velocity. A water film flowing down an inclined plane was used to experimentally calibrate and test this instrument. Measurements were performed over an average velocity range of 0.1 and 2.5 m/s and film thickness varying from 0.5 to 4 mm. Within this range the meter output was linear with the average film velocity, independent of whether the film was smooth or rough. Using this device in conjunction with a film thickness measurement it is possible to determine the volumetric flow in the liquid film. The device proved useful only for steady flows, and modifications to the transducer and electronics are required for its application to unsteady flows. (from authors' abstract)