Introduction

Background

Spirals, screws and windmills have a long history of use for speed measurement. Robert Hook proposed a small windmill in 1681 for measuring air velocity, and later one for use as a ship's log (distance meter). A Captain Phipps, in 1773, employed the principle that a spiral, in turning, moves through the length of one turn of the spiral to create a ship's log. Many centuries earlier than this, it appears that a Roman architect, Vitruvius, suggested a more basic form of this device.

In 1870 Reinhard Woltmann developed a multi-bladed fan to measure river flows (Medlock 1986). The device was a forerunner of the long helical screw–type meter still called after him and used widely for pipe flows in the water industry. The first modern meters, of the type with which we are mainly concerned, were developed in the United States in 1938 (Watson and Furness 1977; cf. Furness 1982). These were attractive for fuel flow measurement in airborne applications. They consisted of a helically bladed rotor and simple bearings. Improved sleeve bearings were developed for longer life with hardened thrust balls or endstones to withstand the axial load. An alternative developed over several years and patented by Potter (1961) was to profile the hub of the rotor. The pressure difference caused by the hub shape, rather than the thrust on the bearings, may have held the rotor against the axial drag forces due to:

1. • the pressure balance across the rotor and/or:

2. • the spinning of the rotor on a film of the liquid, flowing upstream through the annular passage between the stationary axle and the moving rotor.

This allowed the rotor to run on a single journal bearing.

Qualitative Description of Operation

The turbine consists of a bladed rotor which turns due to the flow in the pipe. In most of the designs to be discussed, the rotor is designed to create the minimum disturbance as the oncoming flow passes round it. Ideally it cuts perfectly through the fluid in a helix so that every revolution of the helix represents one complete axial length of the screw and hence a calculable volume of the fluid. In practice drag forces slightly retard the rotation.