Velocity and mass flow by. pressure measurements

Abstract: This chapter will address the measurement of velocity in a steady flow, in magnitude and direction, and of mass flow in pipes, obtained from pressure measurements.

Key words: direction probes, Pitot tube, Prandtl tube, Venturi tube.

2.1 Introduction

Speed at a point in a fluid flow can be measured in the following ways:

■ By measuring the distance, ds, travelled by fluid particles in a known time interval, dt (particle image velocimetry, PIV), or by measuring the time taken by particles to travel a fixed distance (laser two focus anemometer, L2F). Because air is transparent to light, it must be seeded with visible solid or liquid particles assumed to have the same speed as the air stream. These methods require fairly sophisticated equipment and data processing.

■ More frequently by a measure of other parameters (pressures, Doppler effect produced by particles carried by the stream, heat exchange between the stream and a metal wire heated by the Joule effect, speed of rotation of a windmill, etc.) from which the speed can be calculated from theoretical considerations and/or a preliminary calibration. The most commonly used method, when speed is constant in time, is based on pressure measurements; other methods such as hot-wire or laser – Doppler anemometers, are particularly effective when the flow is turbulent. In supersonic flows another method is used based on the measurement of the inclination of Mach waves or of shock waves on wedges and cones.