High Mixture Velocities Research Articles

PREDICTION OF PRESSURE GRADIENT AND HOLDUP IN HORIZONTAL LIQUID–LIQUID SEGREGATED FLOW WITH SMALL EÖTVÖS NUMBER

This experiment utilized two-phase oil-water flows in a 26.1 mm diameter horizontal steel pipe to investigate segregated flow pattern. Data on pressure gradient and in situ phase distribution were obtained to come up with different combinations of phase superficial velocities, which ranged from 0.05 to 0.96 m/s. For the current liquid–liquid system having a small Eötvös number, we focused on identifying the effect of the interfacial tension and wall-wetting properties. Direct observation showed that oil and water started to mix when mixture velocity was increased. Also, at higher mixture velocities wherein the flows of two phases were initially separated, the droplets of one phase appeared in the mixed layer with negligible thickness. A closure relationship, which describes the interfacial mass transfer between liquid phases, was introduced into the framework of the two-fluid model. The model predictions agree well with the measured data. Similarly, an approach performance was evaluated based on the experimental data reported in previous literature.

Imaging stratifying liquid–liquid flow by capacitance tomography

Electrical capacitance tomography offers a non-intrusive technique for on-line visualisation of two-phase liquid–liquid flows. It has been applied on a facility which provides metered flows of water and kerosene to a test section at the start of which they pass through a dispersing multi-hole orifice plate. The test section consists of a sudden expansion with an internal diameter of 63 mm inlet and 100 mm outlet and which can be inclined. Beyond this the mixture is separated into the two constituents and returned to their individual tanks. Tomography measurements were made using a PTL-300 electronic system coupled to a 12-electrode sensor which was built in-house. The sensor is fitted on the outside of one of the plastic pipe lengths of the test section. By varying the input oil fractions from 20% to 70%, using mixture velocities of 0.2, 0.3, and 0.4 m/s and positioning the pipe at angles of +6 ∘,+3 ∘,0 ∘, −4 ∘ and −7 ∘ to the horizontal, different flow patterns were established in the test section. A specially developed calibration method is used in all experiments and tomographic images of the stratifying liquid–liquid flow were obtained. These images show clearly that the spatial distribution in a pipe cross-section is strongly dependent on the mixture velocity and the distance from expansion in the range studied. Concave interfaces were observed in horizontal and downward inclination flow for all cases while convex interfaces were identified only in an upward inclination flow at the high input oil fractions and high mixture velocities. This application illustrates very clearly the capability of the ECT for on-line imaging of liquid–liquid two-phase flows.

Feasibility analysis of two-phase MHD energy conversion for liquid metal cooled reactors

A two-phase MHD energy conversion unit is proposed to a liquid metal cooled fast reactor. Using supercritical CO 2 as the working fluid in the gas cycle without considering friction and heat losses, the optimized cycles efficiency is obtained, which is about 5% higher than that of the gas turbine Brayton cycle with the same regenerator/compressor configurations. Based on a simple MHD power analysis and the two-phase homogeneous flow model, the important system operational conditions were estimated. The results suggest that a liquid lead pump of at least 20% of the MHD power output is needed in order to convert the 400 MW reactor heat into electricity at the specified thermal efficiency, unless a mixture foam flow of void fraction greater than 80% is achievable at very high mixture velocity.

Upward and downward inclination oil–water flows

The effect of upward (+5°, +10°) and downward (−5°) pipe inclinations on the flow patterns, hold up and pressure gradient during two-liquid phase flows was investigated experimentally for mixture velocities between 0.7 and 2.5 m/s and phase fractions between 10% and 90%. The investigations were performed in a 38 mm ID stainless steel test pipe with water and oil as test fluids. High-speed video recording and local impedance and conductivity probes were used to precisely identify the different flow patterns. In both positive and negative inclinations the dispersed oil-in-water regime extended to lower mixture velocities and higher oil fractions compared to horizontal flow. A new flow pattern, oil plug flow, appeared at both +5° and +10° inclination while the stratified wavy pattern disappeared at −5° inclination. The oil to water velocity ratio was higher for the upward than for the downward flows but in the majority of cases and all inclinations oil was flowing faster than water. At low mixture velocities the velocity ratio increased with oil fraction while it decreased at high velocities. The increase became more significant as the degree of inclination increased. The frictional pressure gradient in both upward and downward flows was in general lower than in horizontal flows while a minimum occurred at all inclinations at high mixture velocities during the transition from dispersed water-in-oil to dual continuous flow.

Phase inversion in dispersed liquid–liquid flows

Phase inversion and its effect on pressure gradient during the dispersed flow of two immiscible liquids have been studied for two pipe materials (steel and acrylic) and two pipe sizes (60 and 32 mm ID). Water and oil (796 kg m −3 density and 2.19 mm 2 s −1 viscosity) were used as test fluids while the appearance of phase inversion in the acrylic pipes was confirmed with the use of impedance ring probes. In the large pipes (steel and acrylic with 60 mm ID) it was found that the phase inversion point (oil volume fraction where inversion appears) depended on whether the inversion was from oil to water continuous mixture or from water to oil. The difference in the phase inversion points between the two dispersion initialisation conditions increased with increasing mixture velocity. No effect of initial conditions on the inversion point was found for the small acrylic pipe. Phase inversion was in all cases preceded by a large increase in pressure gradient, which was sharply reduced immediately after the new continuous phase was established. The pressure gradient peak was sharper and larger at high mixture velocities than at low ones and in the acrylic pipe compared to the steel one. The change in phase continuity lasted a few minutes and was accompanied by large fluctuations in pressure gradient and mixture impedance.

Investigation of Paraffin Deposition During Multiphase Flow in Pipelines and Wellbores—Part 1: Experiments

Results are presented from two-phase flow wax deposition tests using a state-of-the-art, high-pressure, multiphase flow test facility. Wax deposition was found to be flow pattern specific and dependent on the flow velocities of the two-phase fluids. Wax deposition occurs only along the pipe wall in contact with a waxy crude oil. An increase in mixture velocity results in harder deposits, but with a lower deposit thickness. The wax buildup trend at low mixture velocities is similar to that observed in laminar single-phase flow tests. The wax buildup trend at high mixture velocities is similar to that observed in turbulent single-phase flow tests. Thinner and harder deposits at the bottom than at the top of the pipe were observed in horizontal and near-horizontal intermittent flow tests. For annular flow tests, thicker and harder deposits were observed at low superficial liquid velocity than at high superficial liquid velocity. In stratified flow tests, no wax deposition was observed along the upper portion of the pipe.

Pressure gradient in horizontal liquid–liquid flows

Pressure gradients were measured during the cocurrent flow of a low viscosity oil (1.6 mPa s viscosity and 801 kg/m 3 density) and water in two 1-inch nominal bore horizontal test sections made from stainless steel and acrylic resin, respectively. Measurements were made for mixture velocities from 0.3 to 3.9 m/s and for water volume fractions from 0 to 100%. The main finding is the large difference between the results for the respective tube materials which cannot be explained only in terms of the difference in tube roughness. It is postulated that the different wettability characteristics of the two pipe materials are also responsible for this disparity. Furthermore, it was found that at high mixture velocities, where dispersed flow patterns prevail, there is a peak in pressure gradient during phase inversion and an apparent drag reduction effect when oil is the continuous phase.

The Flow of Slugs in Horizontal, Two-Phase Pipelines

The flow characteristics in horizontal slug flow are studied experimentally in the Harwell Laboratory 150-mm-dia pipeline. If a frame of reference is taken as moving with the translational velocity of the slug, measurements of the Froude number in the liquid film ahead of the slug were always greater than unity while the Froude number within the slug was in general less than unity. This illustrates a change in flow from super to subcritical flow and the presence of a hydraulic jump. Different types of flow are noticed using high-speed video equipment and these types closely resemble those reported by open-channel hydraulics tests. The distribution of gas in the slug body is only homogeneous at high-mixture velocities and the effect of buoyancy on the gas is more noticeable at low gas velocities. The liquid fraction in the slug is shown to be directly dependent on the Froude number in the liquid film. The ratio of the translational velocity of the slug to the mixture velocity decreases continuously from 2.0 at low-mixture velocities to 1.25 and a mixture velocity of approximately 3m/s. After this point, it remains constant at 1.25.

The flow of sand/water slurries in horizontal pipes with internal spiral ribs — effect of rib height

AbstractThe effect of secondary flows created by an internal spiral rib in an otherwise conventional smooth 2‐inch pipe on the transportability of “settling” suspensions of sand in water was investigated with emphasis on the effect of rib height.While the ribs were found to be a disadvantage at relatively high average mixture velocities because of increased pressure gradients, they were advantageous at low velocities. The deposit velocity and the pressure gradient at the deposit velocity in ribbed pipes of several pitch to diameter ratios and rib heights were correlated with respect to the corresponding smooth pipe values. These correlations are useful in pipeline design. The optimum rib geometry was estimated to be a pitch to diameter ratio of about 8 and a rib height in the range of 10–15% of the pipe diameter for the solid‐liquid mixtures investigated.

The flow of “settling” slurries in tubes with internal spiral ribs

AbstractThe secondary flows created by an internal spiral rib in an otherwise conventional smooth tube are illustrated by reference to the axial, and combined tangential and radial velocity components for a fluid flowing in a 2‐inch diameter tube containing a rib with a pitch‐to‐diameter ratio of 3The effect of such secondary flows on the transportability of “settling”(suspensions)was investigated by measuring pressure gradients for the flow of two sand/water slurries in a smooth 2‐inch tube and in 2‐inch diameter tubes with ribs of pitch‐to‐diameter ratio 5.15, 3.32 and 1.80. Average flow velocities ranged from 0.5 to 8 feet/sec. and delivered concentrations ranged from 5 to 18 per cent by volumeWhile the ribs were found to be a disadvantage at relatively high average mixture velocities because of increased pressure gradients, they were found to be advantageous at relatively low velocities, i.e. velocities less than the critical deposit velocity for flow in the smooth tube, in that the pressure gradients, for a given sand/water slurry with a given delivered concentration and average velocity, were lower in the ribbed tubes than in the smooth tube. Because of this, the power consumption per unit mass of solid transported was reduced by the presence of a rib. The optimum pitch‐to‐diameter ratio was estimated to be about 5Pressure gradients measured for one of the slurries with air added as a third phase showed the presence of air to be a definite disadvantage