EFFECTS OF PULSATING FLOW ON AN ULTRASONIC GAS FLOWMETER

A series of tests was carried out to assess the effects of pulsating flow on a small-dimension ultrasonic gas flowmeter. Three different parameters were investigated: pulsation amplitude, pulsation frequency and mean flow velocity. The measurements were carried out in air using a 100D straight pipe upstream of the ultrasonic gas flowmeter. The performance of the flowmeter was tested at mean flow rates from 1.2 to 9.6 m s-1, using four different pulsation frequencies (2, 5, 10 and 20 Hz) with pulsation amplitudes from 0 to approximately 70% r.m.s. Our measurements reveal three major phenomena affecting the ultrasonic flowmeter performance. (1) Sampling phenomena, i.e. aliasing: it is shown that a mismatch between the setting of the sampling parameters in the flowmeter and the frequency of the pulsations can result in large positive and negative errors due to aliasing. (2) Errors due to changes of the flow profile caused by an imaginary high Reynolds number due to the pulsating flow: it is shown that these errors are proportional to the pulsation amplitude and inversely proportional to the pulsation frequency. Further, the error caused by a change in the flow profile generally appears in the laminar and transition areas. (3) Non-linearities when averaging very high or low flow velocities, respectively: very large pulsation amplitudes result in instantaneous flow velocities, sometimes exceeding both the maximum and the minimum limits of the calibrated range of the meter. For the ultrasonic meter tested, this results in large negative errors due to non-linearities on the extremes of the error curve of the meter.