Increase In Liquid Velocity Research Articles

Phase split characteristics of slug and annular flow in a dividing micro-T-junction

The characteristics of slug and annular gas–liquid two-phase flow when moving through a rectangular micro-channel dividing T-junction were investigated with experiments focusing on the relationships between the phase split and the two-phase superficial velocity, liquid surface tension, and liquid effective viscosity. The effects of the pipe sizes and pipe cross section shapes on phase split were compared and analyzed. Phase split was tested using a 100×800μm rectangular dividing micro-T-junction. Air was used as the gas medium and the liquid mediums were water, 0.01% SDS solution, 0.5% SDS solution, 0.1wt% PAAm, 0.25wt% PAAm, and 0.5wt% PAAm. Results showed that whether the inlet flow pattern was slug or annular flow, the liquid taken off from side branch at the dividing T-junction decreases with an increase in liquid superficial velocity and that there is little effect of inlet gas flow rate on the liquid taken off. It was also found that the liquid taken off from side branch decreases with a decrease in liquid surface tension and increases with an increase in liquid effective viscosity. When results were compared to those from studies using channels of other diameters, it was seen that the phase split of the slug flow was similar at different sized dividing T-junction, while the phase split of annular flow varied at different pipe sizes. It was also seen that the phase split characteristics of a dividing micro-T-junction are dependent on the aspect ratios of the channel’s cross section. A predictive model of phase split characteristics studied in this investigation was formulated with the regression analysis of the experimental data and verified by calculations showing little error.

Interfacial oscillation of liquid jets discharging from non-circular orifices

This work presents an experimental study on the oscillation behavior of liquid jets discharging from elliptical, triangular, and square shaped non-circular orifices. The measurements of wavelength and amplitude of jet oscillation are extracted from digital images of the liquid jets captured via still photography technique at different flow conditions using three different liquids (pure water, water-glycerol mixture, and Jet A-1 fuel). The measured wavelength of jet oscillation (λ), irrespective of the shape geometry of the tested non-circular orifices, increases linearly with increase in liquid jet velocity (Uo) and scales with the liquid jet Weber number (We) as λ/Deq ∝ We0.5. The experimental measurements of λ recorded at different flow conditions exhibit good agreement with the theoretical predictions obtained using the mathematical model proposed by Rayleigh (1879) for liquid jet undulating in its cross section. For a given Uo, the liquid jet of Jet A-1 displays higher value of λ and undergoes a larger increase in λ along the axis of the jet compared to the jets of other two liquids. Further the trends on the variation of maximum and minimum amplitudes of the first wave segment of liquid jets discharging from the elliptical and square orifices with Uo are discussed.

Toward gas-phase controlled mass transfer in micro-porous membrane contactors for recovery and concentration of dissolved methane in the gas phase

Abstract A micro-porous hollow fibre membrane contactor (HFMC) operated in sweep-gas mode has been studied to enable the recovery of dissolved methane from water in concentrated form. At high sweep-gas flow rates, up to 97% dissolved methane removal efficiency is achievable which is sufficient to achieve carbon neutrality (around 88%). An increase in methane composition of the recovered sweep-gas was achievable through two primary mechanisms: (i) an increase in liquid velocity which improved dissolved methane mass transfer into the gas phase; and (ii) a reduction in gas flow which lowered dilution from the receiving gas phase. It was posited that further refinement of the methane content was provided through counter-diffusion of the nitrogen sweep-gas into the liquid phase. Within the boundary conditions studied, the methane composition of the recovered gas phase exceeded the threshold for use in micro-turbines for electricity production. However, reducing the gas-to-liquid ratio to enhance gas phase methane purity introduced gas-phase controlled mass transfer which constrained removal efficiency. Whilst this reduction in removal efficiency can be compensated for by extending path length (i.e. more than one module in series), it is suggested that the gas-phase controlled conditions encountered were also a product of poor shell-side dispersion rather than an approach toward the limiting theoretical gas-to-liquid ratio. This implies that further optimisation can be ascertained through improved membrane contactor design. Importantly, this study demonstrates that micro-porous hollow fibre membrane contactors provide a compact process for recovery of dissolved methane in sufficient concentration for re-use.

Effect of back pressure on nozzle inner flow in fuel injector

The internal nozzle flow has great influence on fuel injection and spray. This work investigates the impacts of the injection back pressure on the nozzle inner cavitation developing, especially the flow characteristic during choking process. Based on the theoretical analysis, a three-dimension numerical model is developed to investigate the details of the inner nozzle cavitation flow. The numerical model is verified by experiments on a diesel fuel system test rig under different boundary pressures, including varied injection pressure and back pressure. The investigation shows that for a given injection pressure, cavitation occurs with the drop in the back pressure. The incipient bubbles appear just at the upper corner of nozzle inlet hole, and the cavitation area continues to expand until it reaches to the nozzle outlet. The nozzle inner cavitation is divided into three periods: no cavitation period; local cavitation developing period and super cavitation period. Before the super cavitation, as the back pressure drops, bubbles at the entrance of the nozzle hole cover more area, but the average liquid velocity increases and as a result the mass flow increases. Once the liquid velocity reaches to its maximum, the cavitation of the nozzle inlet hole maintains stable, so does the vapor–liquid mixture cross section, and the effective cross section of the liquid phase is minimum; by now the super cavitation forms. Under the condition of the super cavitation, the nozzle inner flow becomes choking and the back pressure has little influence on the mass flow, but the discharge coefficient declines as the back pressure decreases. During the choking flow period, when the back pressure declines, along the nozzle hole central axis from the inlet to the outlet, the effective flow cross section maintains stable in sequence and the liquid velocity also reaches its maximum in order. Moreover, at the outlet of the nozzle hole, super cavitation induces the increase of the turbulence kinetic energy and the outlet velocity. This increased outlet velocity makes up for the loss of the contracting effective flow passage due to super cavitation, thus the total mass flow maintains the same value after choke even the back pressure decreases.

Numerical simulation of flow behavior of particles in an inverse liquid–solid fluidized bed

Flow behavior of particles is simulated by means of two-fluid model combining with kinetic theory of granular flow in an inverse liquid–solid fluidized bed. The Huilin–Gidaspow drag model is used to obtain the interphase interaction of liquid and solid phases. The virtual mass force is considered in simulations. A detailed description of the model equations used has been presented. Predictions are compared with experimental data measured by Renganathan (2005) in an inverse liquid–solid fluidized bed. This comparison shows that the present model can capture flow behavior of liquid and solids phases in an inverse fluidized bed. The distributions of velocity and volume fraction are predicted at the different superficial liquid velocities. Simulations indicate that axial velocities of particles and the bed expansion height are increased with an increase of liquid velocity. The granular temperature as a function of solids concentrations is computed from simulations. Roughly, the granular temperatures increase, reach maximum, and then decrease with the increase of solids volume fraction. The effects of liquid viscosity and temperature on flow behavior of particles are generally scarce using CFD. Present simulations show the temperature is not main effect on flow of particles in an inverse liquid–solid fluidized bed. The bed height and velocity increase with the increase of fluid viscosity which effects on flow behavior of particles in the bed.

EXPERIMENTAL STUDY OF BUBBLE DYNAMICS DURING NUCLEATE FLOW BOILING ON HORIZONTAL AND VERTICAL SURFACES

Knowledge of the physical mechanisms governing bubble dynamics and two-phase heat transfer is required to develop mechanistic models and to accurately predict and scale the performance of two-phase systems in reduced gravity environments. To better understand the effect of the magnitude of gravity on flow boiling, especially the dynamics of single bubbles under different levels of bulk liquid velocity, surface orientation at earth normal gravity and contact angle effects were studied in this work. A microfabricated nucleation site (10 mm in diameter) at the center of a flat heating surface was used to generate bubbles. Silicon and aluminum were used as substrate materials, with static contact angles of 56° and 19°, respectively, and water as the test liquid. Bulk liquid velocities ranged from 0 to 0.25 m/s, for horizontal and vertical surface orientations, in upflow conditions. Bulk liquid temperature was set close to saturation temperature, with bulk liquid subcooling less than 1°C, and wall superheat was maintained between 5.0°C and 6.0°C. The experimental results for bubble growth, departure diameter and time for departure, bubble sliding distance and velocity, bubble lift-off diameter and time for lift-off, and bubble base diameter are reported for vertical and horizontal orientations, as the system variables were varied parametrically. It is found that irrespective of orientation and contact angle, departure and lift-off diameters decrease with increase in bulk liquid velocity. At a given velocity, departure diameter decreases with increase in the component of gravity parallel to the heater surface. For a given bulk liquid velocity, increase in contact angle results in increase in bubble departure and lift-off diameters, for all orientations. For given conditions, bubble lift-off diameter increases with decrease in the component of gravity normal to the heater surface. The effect of gravity on lift-off diameter is reduced at high liquid bulk velocities. For given orientation and flow velocity, sliding distance decreases monotonically with decrease in contact angle. The bubble velocity at lift-off increases with increase in flow velocity and increase in the component of gravity parallel to heater surface.

Experimental study of the hydrodynamic behaviour of slug flow in a vertical riser

This paper presents an investigation of the hydrodynamics of slug flow in a vertical 67mm internal diameter riser. The slug flow regime was generated using a multiphase air–silicone oil mixture over a range of gas (0.42<USG<1.35m/s) and liquid (0.05<USL<0.38m/s) superficial velocities. Electrical capacitance tomography (ECT) was used to determine: the velocities of the Taylor bubbles and liquid slugs, the slug frequencies, the lengths of Taylor bubbles and the liquid slugs, the void fractions within the Taylor bubbles and liquid slugs and the liquid film thicknesses. A differential pressure transducer was used to measure the pressure drops along the length of the riser. It was found that the translational velocity of a Taylor bubble (the structure velocity) was strongly dependent on the mixture superficial velocity. As the gas superficial velocity, was increased, the void fraction and the lengths of the liquid slugs and the Taylor bubbles were observed to increase. The increase in gas superficial velocity causes an increase in the frictional pressure drop within the pipe, whilst the total pressure drop (which is a sum of the hydrostatic and frictional pressure drop) along the length of the riser decreases. In addition, the frequencies of the liquid slugs were observed to increase as the liquid superficial velocity increases, but to be weakly dependant on the gas superficial velocity. The manual counting method for the determination of slug frequency was found to be in good agreement with the power spectral density (PSD) computed method.

Two-dimensional numerical simulation of single bubble rising behavior in liquid metal using moving particle semi-implicit method

Gas-lift pump in liquid metal cooling fast reactor (LMFR) is an innovative conceptual design to enhance the natural circulation ability of reactor core. The two phase flow characteristics of gas–liquid metal make significant improvement of the natural circulation capacity and reactor safety. It is important to study bubble flow in liquid metal. In present study, the rising behaviors of a single nitrogen bubble in 5 kinds of common stagnant liquid metals (lead bismuth alloy (LBE), liquid kalium (K), sodium (Na), potassium sodium alloy (Na–K) and lithium lead alloy (Li–Pb)) and in flowing lead bismuth alloy have been numerically simulated using two-dimensional moving particle semi-implicit (MPS) method. The whole bubble rising process in liquid was captured. The bubble shape, rising velocity and aspect ratio during rising process of single nitrogen bubble were studied. The computational results show that, in the stagnant liquid metals, the bubble rising shape can be described by the Grace's diagram, the terminal velocity is not beyond 0.3 m/s, the terminal aspect ratio is between 0.5 and 0.6. In the flowing lead bismuth alloy, as the liquid velocity increases, both the bubble aspect ratio and terminal velocity increase as well. This work is the fundamental research of two phase flow and will be important to the study of the natural circulation capability of Accelerator Driven System (ADS) by using gas-lift pump.

Experimental Studies on Hydrodynamics Characteristics of Co-current Three Phase Fluidization Using Different Sizes of Glass Beads and its Prediction by Fuzzy Logic

The experiment was carried out with different sizes of Glass beads. That is Glass beads of diameter 0.2 cm, 0.4 cm, 0.6 cm. Water was used as a continuous phase and air was used as a dispersed phase. These three different sized Glass beads were used as solids for co-current studies. The three sections of the experimental setup viz. gas liquid disengagement section, test section and gas liquid distributed section. The experiment was conducted with Glass beads with varying liquid velocities and keeping constant gas velocity 0.16985138 cm/sec, 0.212314225 cm/sec, 0.304317056 cm/sec, 0.346779901 cm/sec. The graphs were plotted and their effects on individual phase holdups for various particle sizes were studied. Pressure drop of fluids in the fluidized bed was measured by using mercury manometer and by using ethyl acetate manometer. The phase holdups like solid, liquid, gas were determined, liquid holdup and solid holdup decreases with increase in liquid velocity whereas gas holdup increases with increases in liquid velocity. With the experimental data, simulation studies were carried out using Fuzzy Logic.

A Fractal Model for the Maximum Droplet Diameter in Gas-Liquid Mist Flow

Distribution characteristics of liquid droplet size are described using the fractal theory for liquid droplet size distribution in gas-liquid mist flow. Thereby, the fractal expression of the maximum droplet diameter is derived. The fractal model for maximum droplet diameter is obtained based on the internal relationship between maximum droplet diameter and the droplet fractal dimension, which is obtained by analyzing the balance between total droplet surface energy and total gas turbulent kinetic energy. Fractal model predictions of maximum droplet diameter agree with the experimental data. Maximum droplet diameter and droplet fractal dimension are both found to be related to the superficial velocity of gas and liquid. Maximum droplet diameter decreases with an increase in gas superficial velocity but increases with an increase in liquid superficial velocity. Droplet fractal dimension increases with an increase in gas superficial velocity but decreases with an increase in liquid superficial velocity. These are all consistent with the physical facts.

Analytical and numerical modelling of Newtonian and non-Newtonian liquid in a rotational cross-flow MBR

Fouling is the main bottleneck of the widespread use of MBR systems. One way to decrease and/or control fouling is by process hydrodynamics. This can be achieved by the increase of liquid cross-flow velocity. In rotational cross-flow MBR systems, this is attained by the spinning of, for example, impellers. Validation of the CFD (computational fluid dynamics) model was made against laser Doppler anemometry (LDA) tangential velocity measurements (error less than 8%) using water as a fluid. The shear stress over the membrane surface was inferred from the CFD simulations for water. However, activated sludge (AS) is a non-Newtonian liquid, for which the CFD model was modified incorporating the non-Newtonian behaviour of AS. Shear stress and area-weighted average shear stress relationships were made giving error less that 8% compared with the CFD results. An empirical relationship for the area-weighted average shear stress was developed for water and AS as a function of the angular velocity and the total suspended solids concentration. These relationships can be linked to the energy consumption of this type of systems.

3D transient model of vapour chamber: Effect of nanofluids on its performance

This paper presents three-dimensional transient model for vapour chamber (flat heat pipe) and the effect of nanofluids on its performance. The vapour chamber consists of two copper plates, two copper/water wick regions and a vapour region sandwiched between the two wick regions. The numerical model consists of 3D transient thermal model coupled with a 3D transient hydrodynamic model. The 3D transient model takes into consideration the return liquid between the two wick regions. An implicit finite difference method is used to solve the numerical model and the numerical model is validated by an experimental work. The effect of Cu, CuO and Al2O3 nanofluids on the vapour chamber performance is investigated for different wick porosities. Three volume fractions of nanoparticles added to the working fluid of the vapour chamber 0.03%, 0.06% and 0.09% are used in the study. A good agreement is found between the results of the numerical and experimental work and the numerical results illustrate well the heat transfer and the fluid flow for the vapour chamber. The results show that the nanoparticles added to the working fluid decrease vapour chamber temperature, liquid velocity and liquid pressure and increase mass evaporated. They also show that Cu nanofluid has the greatest effect on the vapour chamber temperature compared with CuO and Al2O3 nanofluids. The increase of volume fraction of nanoparticles decreases vapour chamber temperature and liquid velocity and increases mass evaporated. At volume fraction 9% of Cu nanoparticles, the maximum temperature gradient of vapour chamber decreases by about 19.5% for wick porosity 0.75 and 15.7% for wick porosity 0.35.

Simulation of flow behavior of liquid and particles in a liquid–solid fluidized bed

Flow behavior of liquid and solid phases is simulated by means of DEM-CFD in a liquid–solid fluidized bed. The lubrication force is considered. A detailed description of the model equations used has been presented. The distributions of velocity and volume fraction are predicted at the different superficial liquid velocities, liquid viscosity and solids densities in the bed. The granular temperature is computed from simulated particle velocity. Predicted solid axial velocities are in agreement with experiments. Simulations indicate that axial velocities of particles increase with the increase in the superficial liquid velocity. The bed expansion height is increased with an increase of superficial liquid velocity and liquid viscosity and decreases with the increase of particle density. The lubrication force reduces granular temperature in the liquid–solid fluidized beds.

De-colorization of textile wastewater by adsorption in a fluidized bed of locally available activated carbon

The competency of a locally available granular activated carbon (GAC) in removing color from textile wastewater was studied in a fluidized bed. The characteristics of the fluidized bed with different GAC particles were obtained. A reactive dye was used to prepare the colored water of different dye concentrations. The effects of the GAC particle size, velocity and initial concentration of the colored water on dye removal were investigated. Equilibrium adsorption isotherm and breakthrough curves were generated to determine adsorption characteristics of activated carbon as well as mass transfer coefficients of the fluidized bed. The results of these experiments showed that an increase in liquid velocity and a decrease in particle size increased the color removal. The experiments were also carried out with industrial effluent and the de-colorization achieved was satisfactory. The mass transfer coefficient calculated for the fluidized bed was very low, which suggests that the GAC used in the local industries, although less expensive, may not be economically viable for de-colorization of industrial effluent.

Gas Hold-up in Three Phase Co-current Bubble Columns

Abstract Bubble columns are used in a large number of applications in chemical engineering. The important variables that affect the gas holdup, bubble dynamics and flow regime in a bubble column are gas and liquid velocities, liquid viscosity, liquid surface tension, design of the gas distributor, solid concentration and column diameter. Experiments have been performed in a 15 cm diameter co-current slurry bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35 μm have been used as solid phase. Solid loading up to 9% has been used. The superficial gas velocity varies from 1.0 to 16.28 cm / s and superficial liquid velocity varies from 0 to 12.26 cm / s. Effects of liquid height, liquid velocity, gas velocity and solid concentration over gas holdup for both two and three phase co-current flows have been studied. For batch case the liquid height didn’t affect the gas holdup. The gas holdup increases with increase in gas velocity for both two and three phase co-current columns. For two phase and three phase flow up to 1% solid loading; at low superficial gas velocity i.e. in the homogeneous regime, the increase in liquid velocity doesn’t show any change in the gas holdup. For higher gas velocities i.e. in the heterogeneous regime, increase in liquid velocity decreases the gas holdup rapidly. Above 1% solid loading, liquid velocity effect over gas hold-up is negligible. With increase in solid concentration for co-current bubble column the gas holdup slightly increases or remains constant up to 5% loading; beyond this loading there is a significant decrease in gas holdup

High Pressure Experiments and Simulations in Cocurrent Bubble Columns

Abstract Bubble column is a multiphase reactor in which gas is passes through the liquid in two phase flow. When liquid is stationary the operation is called as batch mode and co-current when both gas and liquid are moving in the same direction. The gas hold-up increases with increase in pressure for both batch & co-current column. This increment in gas holdup is very sharp initially, but soon it becomes insignificant. The increase in liquid velocity decreases the gas hold-up at all pressures. As we increase the pressure, the effect of liquid velocity is less. Three dimensional Euler-Euler two-phase fluid model has been used to simulate two-phase up-flow in bubble column (15cm diameter) using ANSYS 12.1. These experiments and simulations were operated over a range of superficial gas velocities (1 to10 cm/s) at ambient conditions as well as high pressures. The liquid velocity range was 0 to 16 cm/s. The turbulence in the liquid phase has been modeled using the standard k-ɛ model. The interactions between the two phases are described through Schiller Neumann drag coefficient formulation. The objectives are to find the effect of pressure and liquid velocity over gas holdup and to validate the CFD simulations with experimental data. Quantitatively good agreements are obtained between experimental data for hold-up and simulation values. Radial gas holdup profiles and axial liquid velocity profiles were also obtained for the simulation.

CFD simulation of gas-solid-liquid fluidised bed reactor

An Eulerian multi-fluid computational fluid dynamics (CFD) model was used to simulate a gas-solid-liquid fluidised bed to analyse the behaviour of fluidised bed with varying inputs on the pressure drop, bed expansion and also gas hold up in the column. Simulated predictions were also plotted against the experimental results which showed they agree with each other. Results plotted for velocity vector of solid particles, i.e., glass beads reveal that the direction of magnitude is upwards in the centre column and near the wall zone there is downward movement due to absence of bubble regime. The velocity vector behaviour of gas and liquid has also been explained. In addition to this, investigations were also carried out to analyse the bed expansion in the column; it shows that bed expansion increases as liquid velocity increases. Gas hold up and pressure drop distribution were also simulated for the three phase fluidised bed; results reveal that pressure drop decreases as air velocity is increased and vice-versa for gas hold up, i.e., gas hold up increases as air velocity is increased.

Hydrodynamic Studies on Three-Phase Combined (Internal &amp; External) Loop Airlift Fluidized Bed Reactor Using Newtonian and non-Newtonian Liquids: Minimum Fluidization Velocity and Liquid Holdup

A novel combined airlift loop fluidized bed reactor was proposed in this work. The internal and external loops were combined and the hydrodynamic parameters like minimum fluidization velocity and liquid holdup were studied for Newtonian and non-Newtonian fluids. Studies were conducted using Newtonian fluids of water, n-butanol, 60% and 80% glycerol and non - Newtonian fluids such as 0.25%, 0.6% and 1.0% Carboxy Methyl Cellulose (CMC) aqueous solutions were employed in the liquid phases. Spheres, Bearl saddle and Raschig ring with different sizes were used as solid phase. The experimental results indicated that the increase in particle size and sphericity increased minimum fluidization velocity whereas increase in superficial gas velocity decreased minimum fluidization velocity. In addition, the liquid holdup increased with increase in particle size and superficial liquid velocity. Furthermore, an increase in superficial gas velocity decreased the liquid holdup for Newtonian and non-Newtonian systems. Two separate correlations were developed to predict the minimum fluidization velocity and liquid holdup based on the experimental results for both Newtonian and non-Newtonian liquids for a wide range of operating conditions. The capability of the proposed correlation for minimum fluidization velocity and liquid holdup was examined and reasonable agreement between predicted and experimental results of Newtonian and non-Newtonian liquids suggested the applicability of the proposed correlations.

Studies on Minimum Fluidization Velocity and Liquid Hold in an Internal Loop Airlift Fluidized Bed Reactor

The influence of superficial gas and liquid velocities, particle diameter and sphericity, physical and rheological properties of liquids on minimum fluidization velocity and liquid holdup were studied in an internal loop airlift fluidized bed reactor. Spheres, Bearl saddles and Raschig rings were used as solid phases. Water, n-Butanol, two concentrations of Glycerol (60 and 80%) were used as Newtonian liquids and three concentrations (0.25%, 0.6% and 1.0%) of Carboxy Methyl Cellulose (CMC) solutions were used as non-Newtonian liquids. Superficial gas velocity was varied from 0.142 x 10-3 m/s to 5.662 x 10-3 m/s and superficial liquid velocity was varied from 0.001 to 0.12 m/s. The experimental results showed that increase in particle size and sphericity increased minimum fluidization velocity whereas increase in superficial gas velocity decreased minimum fluidization velocity. The liquid holdup increased with increase in particle size and superficial liquid velocity. An increase in superficial gas velocity decreased the liquid holdup for Newtonian and non-Newtonian systems. Based on the experimental results separate correlations were developed for the prediction of minimum fluidization velocity and liquid holdup for both Newtonian and non-Newtonian liquids for a wide range of operating conditions.

The effect of flow pattern on split of two-phase flow through a micro-T-junction

An experiment has been carried out to study the phase split of water–nitrogen two-phase flow through a horizontal T-junction with a square cross section (500 μm × 500 μm), focusing on the effect of flow pattern. By comparing the results of slug, slug–annular and annular flows, it is shown that phase split characteristic of micro-T-junction highly depends on inlet flow pattern. When the inlet flow is a slug flow, it takes on gas rich in side branch. But when the inlet flow shifts to an annular flow, the side branch is rich in liquid. For an slug–annular flow, the curves show transitional characteristics. The effect of superficial velocities on the phase split of each flow pattern is considered as following: the liquid taken off decreases with an increase of liquid superficial velocity and increases with an increase of gas superficial velocity. The results have been compared to that of a 500 μm T-junction with a circular cross section. It is found that the square T-junction shows more liquid taken off at slug flow and slug–annular flow. While, at annular flow, the liquid taken off increases slower at high gas taken off in square T-junction due to cross section effect.