The development of a nozzle for absolute airflow measurement by pitot-static traverse

Abstract

A special nozzle has been made with associated ducting and instruments in order to provide an absolute measure of airflow. The equipment could form a portable self-contained assembly, enabling calibrations to be made of meters installed in test rigs. In order to give an absolute measurement, the nozzle has been designed to generate an idealised flow amenable both to theoretical prediction and to experimental survey. Firstly, the nozzle produces a uniform velocity profile across the mainstream, which can easily be traversed for pitot and static pressure to a high order of accuracy. Secondly , the boundary layer is fairly thin in the traverse plane, thus minimising the effect of variations within it on nozzle C D. The discharge coefficient has been determined by traversing within the flow range from 6 to 18 lb/sec. The theoretical calculation of C D is in good agreement with the 'traverse' results. The uncertainty of an airflow measurement in a steady flow is estimated as ± 0.17 per cent due to known random errors. By far the biggest error is that resulting from circumferential variation of the boundary-layer profiles. It is felt that much of the error from the boundary layer could be removed with the manufacture of a new nozzle. An extensive subsidiary experimental programme has been carried out to solve the problem of the accurate measurement of static pressure. The effect of static-hole size was found to agree with Shaw's correlation.