Unsteady Two-Phase Flow Instrumentation and Measurement

Abstract

The performance of a transverse field electromagnetic flowmeter in a steady two-phase flow was investigated analytically for a disperse and an annular flow regime. In both cases the flowmeter output voltage was found to be proportional to the mean velocity of the liquid phase. Experiments in a steady air-water mixture showed good agreement with the analysis. An impedance void fraction meter was designed and built to conduct measurements of unsteady void fractions. Short electrodes excited by voltages of opposite polarity were used in combination with a highly sensitive signal processor. The steady state calibration indicated that the meter was somewhat sensitive to the void fraction distribution for the bubbly flow regime. However, the transition to a churn turbulent regime greatly affected the meter steady state response. The dynamic capability of the void fraction meter was estimated by comparison of the statistical properties of the voltage fluctuations in a nominally steady bubbly flow with those of a shot-noise process. The filter function associated with the finite volume of the electric field within the fluid cell could be determined from the measured autocorrelation function and was shown to be mainly a function of the velocity of the disperse phase. Also some properties of the disperse phase could be inferred from the statistical analysis. Two void fraction meters were used to measure the propagation speed of kinematic shocks in an air-water bubbly mixture for various void fractions and water flow rates. The relative velocity of the disperse phase calculated from these measurements decreased with an increase in the disperse phase concentration. However, this effect disappeared at higher water flow rates and the relative velocity became independent of void fraction. Measurements of the propagation speed of shocks of decreasing strength provided a good verification of the kinematic wave theory. The shock thicknesses could also be determined leading to the conclusion that an important diffusion mechanism was responsible for arresting the steepening of the wave. Cross-correlations of the fluctuating voltage of two void fraction meters in a steady bubbly flow were determined. The speed measured by this technique was identified as the infinitesimal wave speed of the void fraction and not the velocity of the dispersed phase as postulated by some authors. The normalized cross-correlation maxima showed that the small amplitude void fraction disturbances were short-lived structures, which were created and diffused on a continuous basis. The cross spectral density revealed that the waves present in these disturbances were nondispersive.