Critical Liquid Velocity Research Articles

Experimental study on distribution parameter characteristics in vertical rod bundles

The drift-flux model is important for rod bundle two-phase flow analysis. A 5 × 5 rod bundles experiment has been performed under air-water two-phase flow conditions. The superficial liquid velocity ranges from 0.00 to 1.50 m/s while the superficial gas velocity ranges from 0.02 to 6.00 m/s. The void fraction has been measured by the impedance meter. Four typical drift-flux models developed for rod bundles have been compared with present data. By analyzing the experimental data and visualization, the distribution parameter may be influenced by the flow recirculation and turbulence significantly. In order to consider these two effects, a critical liquid velocity has been defined to determine which effect is dominant as the increase of liquid velocity, and a new critical void fraction correlation related to the flow-regime transition from cap-bubbly to cap-turbulent flow is incorporated to determine the distribution parameter. The new drift-flux model developed from existing models shows a good prediction capability.

Evaluation of the approach based on the concept of hyperbolicity breaking for prediction of flooding velocity of both room temperature and cryogenic fluids

The theoretical approach for the prediction of flooding velocity based on the concept of hyperbolicity breaking was evaluated in the counter-current two-phase flow. Detailed mathematical derivations of neutral stability condition together with the correlation of the void fraction are presented. The flooding velocity is obtained by assuming that the wavelength at flooding is proportional to the wavelength of the fastest-growing wave at Helmholtz instability. Some available experimental data for different fluid pair flow in inclined tubes is adopted for comparison with the theoretical calculations, which includes the data of water/air, aqueous oleic acid natrium solution/air, Aq. butanol 2%/air and kerosene/air in the published papers, as well as the liquid nitrogen/vapor nitrogen by the present authors. The comparison of flooding velocity proves that the approach can predict the flooding velocity with accepted accuracy for the water/air and liquid nitrogen/vapor nitrogen flow if the tube diameter is greater than 9 mm. While the diameter is smaller than 9 mm, regardless of the inclinations and the fluid pairs, the error becomes larger relative to the cases of diameter larger than 9 mm. The calculations for small diameter cases also fail to predict the critical liquid velocity at which the flooding velocity of gas reaches the maximum value, as revealed by the experiments. The reasons for the increased errors were qualitatively explained.

Effects of a long pipeline on severe slugging in an S-shaped riser

Severe slugging is an undesired multiphase phenomenon in offshore pipeline/riser systems. Previous experimental studies have been limited to relatively short pipelines (no more than 400m) compared with the subsea fluid transportation system. Here, a much longer pipeline/riser system (46mm ID) was built to investigate severe slugging. The test loop consists of a 1657m long horizontal pipeline; 30m long, 7-degree downward inclined pipeline; and 11.2m high S-shaped riser. Based on the pressure cycling characteristics, the flow patterns were classified into three broad categories. It was found that the long pipeline resulted in a longer slug production stage during severe slugging flow, eventually leading to longer liquid slugs. The critical gas velocities for the severe slugging region were larger compared with short pipeline/riser systems, while the critical liquid velocities were barely influenced by the long pipeline. The main factors that influenced the flow pattern transitions on the critical gas and liquid velocities for the severe slugging region were different. Based on the existing models, two criteria for severe slugging and unstable flows were developed, and more accurate predictions were obtained when compared with experimental results in the present study and the literature.

Investigation of Critical Sand-Deposition Velocity in Horizontal Gas/Liquid Stratified Flow

SummaryExperimental and theoretical investigations have been conducted on gas/liquid/sand stratified flow in horizontal pipes at low sand concentrations. A 4-in. experimental facility was designed and constructed, and data were acquired using air/water/glass-bead flow. The data include measurements of critical sand-deposition velocities—namely, the transition between moving and stationary beds. The data reveal that for a constant superficial liquid velocity, the critical sand-deposition mixture and liquid velocities increase with increasing sand concentrations. Moreover, for a given sand concentration, the critical liquid velocity is nearly the same for different superficial liquid velocities. The sand-deposition correlations of Oroskar and Turian (1980) for single-phase flow and Salama (2000) for two-phase flow were modified and extended to develop a new correlation. The developed correlation enables the prediction of critical sand-deposition mixture velocity for horizontal stratified flow as a function of sand concentration, along with other parameters. Comparison between the predictions of the developed correlation and experimental data from the literature (Angelsen et al. 1989; Najmi et al. 2015) reveals a good agreement, especially for a pipe diameter of 0.1 m.

Effects of nonlocal elasticity and slip condition on vibration of nano-plate coupled with fluid flow

Fluid–structure interaction (FSI) problem of a nano-plate under fluid flow is addressed in this article, where this nano-plate together with two rigid walls and a solid roof, is transformed to a fluid-carrying channel. We would analyze static instability (divergence) and dynamic instability for this system. To consider the effect of nano-size of the plate, the nonlocal elasticity theory and the effect of Knudsen number (Kn) in fluid boundary are used. Kn directly affects the velocity correction factor (VCF) defined as the ratio of average sliding velocity of fluid over channel wall to average no-sliding fluid velocity. Classical plate theory (CPT) and Navier–Stokes' continuum equation are used for modeling the FSI problem. The results show that nonlocal parameter could have a higher effect for liquid nano-flow than Kn, in reducing the divergence and flutter critical flow velocities. It is due to small range of variations of Kn for a liquid. Nonlocal parameter could have more effect on reducing the divergence critical liquid velocity of higher modes, specifically coupled-mode flutter. However, the results would be somewhat different from gas nano-flows. The range of variations of Kn for nano-gas flow would be higher and regarding the nonlocal parameter, the effect of Kn would be quite considerable for reducing critical gas velocities.

Studies on Regime Transition, Operating Range and System Stability in a Liquid‐Solid Circulating Fluidized Bed

AbstractIn the present work, the variations in the solids circulation rate and solids holdup were analyzed to study the behavior of a liquid‐solid circulating fluidized‐bed (LSCFB) regime. The results confirm the existence of two regions in the regime of LSCFB. A new concept of critical liquid velocity, jlc, is proposed in the present work for demarcation between region 1 and region 2, which is found to be a constant value of about 1.3 ut for all particles considered. The operating range of the LSCFB regime is obtained for the various particles and a correlation is developed from the data to estimate the maximum total liquid velocity. The predicted maximum liquid velocity was compared with the experimental values and found to be in good agreement within ±9 %. The effects of total liquid velocity, particle size and density on the stable operating range are discussed. Analysis of the experimental results shows that stable operation prevails both in region 1 and region 2.

Measurement and Correlation of Critical Gas and Liquid Velocities for Complete Circulation of Solid Particles in External Loop Airlift Bubble Columns

AbstractThe external loop airlift bubble column provides an easy way of good contacting among gas, liquid and solid phases due to a relatively high recirculating liquid velocity UL. The critical gas and liquid velocities for complete circulation of solid particles, UG,C and UL,C, were measured in two different scales of columns with air, tap water and aqueous CMC solutions, and ion exchange resin and glass beads (155∼3834 µm) were employed. The UG,C was determined as the inflection point on the plot of the pressure drop due to the suspended solid particles in the downcomer as a function of the gas velocity UG. The critical liquid velocity UL,C corresponding to the UG,C was obtained from the measured relationship between UL and UG. As a result, a unified dimensionless empirical correlation of UL,C was obtained within an error of ±20% and a dimensionless empirical relationship between UL and UG was developed within an error of ±15%.

Evaluation of Critical Gas Flow Rate for the Entrapment of Slag Using a Water Model.

Cold model experiments were performed to make clear the critical condition causing the entrapment of slag under bottom gas injection. Entrapment of slag was judged from visual observation. The following empirical correction of the critical liquid velocity on the centerline of the vessel. ucl, c was derived.ucl, c/V=1.2(νs/νm)0.068(Hs/D)-0.11 dB/D<Hs/D<1/2, 0.6<ρs/ρm<1, 0.3<νs/νm<120, 45 mN/m<σms<63 mN/m ucl, c=1.2 urP-0.28 ur=(gQa, c/Hm)1/3 P={Q2a, c/(gHm5)}1/5 V=(σmsg/ρs)1/4 where, g: the acceleration due to gravity, Qa, c: the critical gas flow rate for the entrapment of slag, Hm: the thickness of metal layer, σms: the interfacial tension between slag and metal, ρs', ρm: the densities of slag and metal, νs', νm: the kinematic viscosities of slag and metal, Hs: the thickness of slag layer, D: the bath diameter, dB: the bubble diameter. This correlation could predict the critical gas flow rate Qa, c for previous hot model experiments as well, provided that the flow pattern in the baths were similar to the present case.

Flow characteristics in a bubble column with enlarged section at bottom.

The critical operation condition for complete countercurrent flow may be extended by enlarging the bottom of the bubble column. Several cylindrical enlarged sections of different diameter and length were examined and the critical liquid velocities were measured.It was found that the critical liquid velocity increased with increasing diameter or length of the enlarged section. An experimental equation for the critical liquid velocity was deduced.Higher gas holdup and liquid-phase overall mass transfer coefficient were obtained in the bubble column with enlarged bottom and correlating equations were derived.

Hydrodynamic properties of pulses in two-phase downflow operated packed columns

In a packed column with downward cocurrent two-phase flow, under certain circumstances pulsing flow can be observed. Several properties of these pulses were determined: pulse frequency, pulse velocity, liquid holdup in a pulse, liquid holdup outside the pulses and the pulse height. These parameters were measured as functions of gas and liquid flow rate, column diameter and packing diameter. All experiments were carried out with a water/ air system. It was found that the pulse frequency is completely governed by the real liquid velocity in excess of a critical liquid velocity at the pulsing onset. The relationship is worked out in terms of flow rates and is then shown to correlate pulse frequencies for several systems; data from the literature are also included. Pulse velocity is shown to be completely determined by the real gas velocity in the bed. The ratio between the maximum liquid holdup at the front of apulse and the liquid holdup between pulses is found to be nearly constant at a value of 1.6. Finally, the pulse height is practically independent of gas and liquid flow rates, tending to be slightly higher in a system with larger packing particles.