Some Observations of Flow Patterns and Statistical Properties of Three Component Flows

Vertical air-water flows, solids-water flows and three component air-solids-water flows were investigated in a Three Component Flow Facility. Visual observations of the flow patterns show that three component flows undergo transition and can exhibit strong unsteady vortical motions. Measurements of the fluctuations in cross-sectionally averaged volume fraction measurements were made. The statistical properties of the fluctuations are presented in terms of their amplitude and coherent time scale in the form of the Signal To Noise Ratio (STNR) and the Time Constant (symbol), respectively. Remarkably, the solids-water flows and the dispersed bubbly air-water flows exhibit almost identical values of STNR for the same volume fraction. Equally remarkable in the linear relationship between the Time Constant and the mean bubble or particle velocity; this relationship is found to have the same constant of proportionality for both species in the well behaved disperse regime. In the two-component churn-turbulent and the three-component agitated vortical regimes, the variables (symbol) and STNR significantly deviate from their dispersed regime values. The onset of large coherent structures characteristic of these regimes is reflected by a rise in the amplitude of the fluctuations and a marked increase in their coherent time scale. The results of this study demonstrate the large information content in the fluctuations of the measured quantity, both as a flow regime indicator and as a measure of flow quantities in two- and three-component flows

Some Observations of Flow Patterns and Statistical Properties of Three Component Flows

Air-water flows, solids-water flows and three component air-solids-water flows in a vertical pipe have been investigated in a Three Component Flow Facility. Visual observations of the patterns show that the three component flow exhibits strong unsteady vertical motions which do not occur in the two phase flows studied. Quantitative results of the fluctuating component of the cross-sectionally averaged volume fraction measurements are presented, and related to the nature of the flows. The ratio of the steady component to the r.m.s of the fluctuating component of the volume fraction measurement (Signal To Noise Ratio) is found to be a good flow structure indicator. Remarkably, the solids-water flows and the bubbly air-water flows exhibit almost identical signal to noise ratios for the same volume fraction. However, the corresponding values for the three component flows reflect greater fluctuations corresponding to the vertical structures

Measurement of Friction Pressure Drops in Vertical Slurry and Bubbly Flows

A Three Component Flow Facility (TCFF) was used to study friction pressure drops in vertical two component flows of both air bubbles in water and polyester particle-water mixtures. Friction factors of up to two orders in magnitude higher than those at zero volume fraction were observed for both bubbly and slurry flows. This deviation is shown to decrease with increased liquid Reynolds number. Bubbly and slurry flow friction factors were comparably large in magnitude and displayed the same decreasing trend as a function of Reynolds number. The two phase friction multiplier for bubbly flow was shown to attain values up to one order of magnitude higher than the prediction given by Lockhart and Martinelli. Two phase multiplier data is presented for the dispersed flow regime

Small amplitude kinematic wave propagation in two-component media

Experimentally determined attenuation and propagation characteristics are presented for small amplitude concentration waves in vertical bubbly and particulate flows. These were studied up to concentrations of 44.3 and 58%, respectively, in a 10 cm pipe. The wave propagation was studied in terms of the time delay, phase lag and loss of coherence of naturally occurring volume fraction fluctuations by means of simultaneous impedance measurements at two separate locations. Small amplitude natural kinematic waves were confirmed to be non-dispersive, as has previously been shown by other investigators. In this system configuration, bubbly flows undergo a regime transition to churn-turbulence, and not to slug flows as is typically observed in smaller diameter pipes. A dramatic drop in the attenuation time constant of small kinematic waves was found prior to the transition to churn-turbulence in gas-liquid flows, indicating that the regime change is the consequence of a loss of kinematic stability. The solid-liquid mixtures studied were found to always remain stable, with a range of greatest stability between 15–20%, as indicated by a maximum in the kinematic wave attenuation constant. The idea of a stable intermediate range of concentrations is consistent with the observations by Homsy et al. [Int. J. Multiphase Flow 6, 305–318 (1980)], who first observed structure formation above and below such a range. At concentrations above 40%, gradual transition to plug flow occurs, in which the particles execute little or no motion relative to one another

Small amplitude kinematic wave propagation in two-component media

The speed and attenuation of small amplitude kinematic waves were measured in vertical bubbly and particulate flows in a continuous medium of water. This was done by evaluating the time delay and phase lag of coherent random fluctuations in the volume fraction signal at two measuring locations. The volume fraction was monitored using two closely spaced Impedance Volume Fraction Meters (Kytomaa (1986)). Using the broad-band volume fraction perturbations yields the dependence of the kinematic speed and attenuation of wave number from a single experiment for one set of conditions. The kinematic waves were found to be non-dispersive. Bubbly flows are observed to undergo a change in flow regime at an approximate volume fraction of 45%. Prior to onset of churn-turbulence, a sharp drop in kenematic wave attenuation is observed above volume fractions of 40%. When further increase in volume fraction is attempted, the homogeneous dispersion suddenly becomes unstable. The particulate flows remain uniformly dispersed for all volume fractions, but above a value of ~55%, the mixture flows like a solid plug. The volume fraction fluctuations become incresingly persistent as the volume fraction approaches the solidification value, but no instability is observed. It is argued that the inability of air-water flows to withstand bubble-bubble forces without break-up may account for the differences between the bubbly and particulate flow results