Dynamic flow measurement/special feature

摘要

Flow measurement is practised in many areas of mechanised activity. Examples are: fuel flow into engines, cooling flows in power stations, water distribution through irrigation channels, and an enormous range of liquid, gas and slurry flows within the food, beverage and petrochemical industries. There is also a wide range of flow measurement devices; some are long established, e.g. the orifice plate, used on the natural gas national grid, and newer technologies, e.g. ultrasound, now being used by water authorities for water distribution. Measurements are usually made for one of the following purposes: control of a process (e.g. energy transfer in a heat exchanger), control of product composition (e.g. soup), delivery of product for fiscal purposes (e.g. tax on North Sea product coming on-shore), sale to a customer as a packaged item (e.g. bottle of perfume), and medical diagnosis and monitoring (e.g. cardiac function). In many applications, measurements are required to deliver mean flow rate information only or totalised flow, i.e. the mass delivered over an identified time period. There is a relatively long history (70 years or so) of flow metering research aimed towards achieving an accurate indication of mean flow in the presence of pulsations. There has been much less apparent interest in looking at flow measurements in order to determine flow pulsation waveform and amplitude information (i.e. dynamic features). There is, however, some history of interest in determining the details of time varying flows. In the mid-twentieth century there was a relatively small number of areas in which there were attempts to make measurements of the time histories of unsteady flows (i.e. dynamic measurements). Examples include: measurement of pulsating gas flows in relation to internal combustion engine research, in which the time-varying pressure differential across a sharp-edged orifice was recorded (Earles and Zarek). There were also the beginnings of what subsequently became a major measurement activity: that of determination of blood flow pulsation time histories (frequency range 1 to 3 Hz). This became an important feature in the diagnosis of cardiovascular disease and the evaluation of congenital abnormalities. The development of an electromagnetic flow meter with an adequate dynamic performance was closely associated with this application. Significant work was contributed by Wyatt, who developed cuff instruments for application around blood vessels exposed during surgery. At a similar time Mills developed a catheter-mounted electromagnetic flow meter which could be inserted into major arteries and the heart via a peripheral artery, which necessitated only the use of a local anaesthetic. It continues to be true that large numbers of dynamic flow measurements are made daily in hospitals throughout the developed world. In the measurement of cardiovascular flows, ultrasonic techniques have largely replaced the electromagnetic flow meter. Dynamic air flow measurements are also widely used in the evaluation of lung function (respiration frequency range 0.2 to 1 Hz). There appears to be very little published work on dynamic flow measurement. However, a recent paper (Wiklund and Peluso) from one of the major meter manufacturers reported work in which the dynamic performance of four types of flow meter - differential pressure (orifice plate), vortex, electromagnetic and Coriolis - were studied through their response to very low frequency (maximum 2 Hz) pulsations. Somewhat surprisingly, these authors reported that it would be both too difficult and inappropriate to use a step response test for these meters. Their meter response results were reported in terms of meter "dead" time and time constant (the latter for various user selected values of damping), assuming either a first or second order system. A conversion of these results to an equivalent step response for the fastest meter of each type was presented by Henry et al. These results le