Stability Of Flow Patterns Research Articles

Numerical simulation of flow boiling in an orthogonally rotating duct

Numerical predictions of flow boiling in a square tube rotating in vertical direction at a uniform rotating speed are presented. There-dimensional physical model and numerical model were established for simplified problem. Realizable k−e (RKE) turbulent model was employed to study vapor–liquid two-phase turbulent flow. Heat and mass transfer between vapor and liquid were solved based on volume of fluid multiphase flow model combined with user-defined function. The transient results show that wall superheat at onset of nucleate boiling (ONB) condition in rotating tube is obviously higher than that in stable tube. Based on Jens–Lottes formula of Bowring model, empirical formulas for wall superheat at ONB condition were modified with consideration of Rossby number. As the rotation speed increased, location of ONB shifts to the tube exit. Flow pattern in tube is determined by coupling effect of heating and rotation. Stability of flow pattern is gradually weakened during the increase in rotation speed. The numerical results are in fair agreement with two sets of experimental data for same physical model and working conditions.

Dynamics of flow transitions from bubbly to churn flow in high viscosity oils and large diameter columns

The dynamic behaviour of the gas-liquid two phase flows and in particular the flow pattern stability and transition between the flow regimes are influenced significantly by both the properties of the liquid and gas as well as the pipe diameter. The majority of the studies reported in the literature on the dynamics of gas-liquid flow transitions focus only on low viscosity liquids (e.g. water) and small diameter pipes. In the present work a series of experiments were carried out to study the dynamics of flow transitions (bubbly to slug and slug to churn) of gas rising through very viscous oils (330 Pa s and 360 Pa s) in two large diameter columns (290 and 240 mm, respectively), using Electrical Capacitance Tomography (ECT) and pressure sensors. The experiments aimed to imitate a number of realistic flow conditions that might be encountered, for examples, in; bitumen, crude oil, viscous liquids in food processing and volcanic magmatic flows. Observation and quantification of bubbly to slug and slug to churn flow transitions for gas-high viscous liquids in large pipe diameters are presented for the first time. Flow parameters and characteristics including; void fraction, pressure gradient, Probability Density Function, structure velocity, lengths of large/Taylor bubbles and liquid slugs and the effect of liquid temperature on the void fraction and Taylor bubble lengths, were measured and analysed. It was found that transition to slug and churn flow occurs gradually. Transition to slug flow occurs at a gas superficial velocity of 0.011 m/s–0.016 m/s, while transition to churn appears in the range of 0.127–0.243 m/s in both columns.

Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics

Gas-liquid flows are affected strongly by both the liquid and gas properties and the pipe diameter, which control features and the stability of flow patterns and their transitions. For this reason, empirical models describing the flow dynamics can be applied only to limited range of conditions. Experiments were carried out to study the behaviour of air passing through silicone oil (360 Pa.s) in 240 mm diameter bubble column using Electrical Capacitance Tomography and pressure transducers mounted on the wall. These experiments are aimed at reproducing expected conditions for flows including (but not limited to) crude oils, bitumen, and magmatic flows in volcanic conduits. The paper presents observation and quantification of the flow patterns present. It particularly provides the characteristics of gas-liquid slug flows such as: void fraction; Taylor bubble velocity; frequency of periodic structures; lengths of liquid slugs and Taylor bubbles. An additional flow pattern, churn flow, has been identified. The transition between slug and churn has been quantified and the mechanism causing it are elucidated with the assistance of a model for the draining of the liquid film surrounding the Taylor bubble once this has burst through the top surface of the aerated column of gas-liquid mixture. It is noted that the transition from slug to churn is gradual.

Kinetics and stability of the mixing flow patterns found in champagne glasses as determined by laser tomography techniques: likely impact on champagne tasting

Laser tomography techniques were used in order to make visible the flow patterns induced by ascending bubbles in flutes poured with champagne. The stability of flow patterns was investigated in flutes showing natural (without any specific surface treatment) as well as artificial effervescence (i.e., engraved at their bottom), all along the first 15 min after pouring. Engravement conditions were found to strongly influence the kinetics and the stability with time of the mixing flow phenomena found in champagne glasses.

The effect of geometry on the stability of flow patterns in stirred tanks

The stability of circulation patterns in stirred tanks was examined using tuft flow visualization. Tufts were placed throughout the tank and their direction was recorded continuously for 20 min. A direction coefficient, which allowed comparison of the stability between various tank geometries, was defined and measured. Using a two-level factorial design, the influence of four different geometric variables (impeller type, impeller diameter, off-bottom clearance and number of baffles) was examined. The impeller diameter, the number of baffles, and the interactions of the type of impeller with the number of baffles, and the type of impeller with off-bottom clearance affected the directional stability of the flow. This study has confirmed our understanding of time-varying phenomena in stirred tanks, and clarified the more complex interactions between elements of the geometry.

Stability of flow patterns in the in vivo post-stenotic velocity field

Pulsed Doppler velocimetry was used to construct velocity profiles which demonstrated jet flow and recirculation in the poststenotic velocity field of canine peripheral arteries in vivo. Maximal spectral broadening in near midline velocity waveforms was used to assess turbulent type flow disturbance. A doubling or halving of mean volume flow, in iliac and carotid arteries, respectively, had no effect on the spatial relationship of these flow patterns. Peak jet velocity was changed less, and end diastolic velocity was changed more than expected. Magnitude of turbulent type flow disturbance was reduced by halving but not doubling flow. Effects of changes in pulsatility and heart rate are discussed. It is suggested that similar variation in maximum and end diastolic jet velocities and poststenotic spectral broadening will occur in patients undergoing noninvasive diagnostic tests for arterial disease. This will reduce the resolution of methods based on absolute levels of these parameters.

Structure and Stability of Flow Patterns in a Rotating Fluid Annulus

Structure and Stability of Flow Patterns in a Rotating Fluid Annulus