Air Volume Fraction Research Articles

A numerical analysis and experimental investigation of three oil grooves sleeve bearing performance

PurposeThis paper aims to reveal the cavitation characteristics of three oil wedges sleeve bearing and set the theoretical and experimental basis for defining the oil film boundary condition.Design/methodology/approachComputational fluid dynamics model of three oil wedges sleeve bearings based on the Navier–Stokes equation is set using Fluent considering turbulent situation and two-phase flow theory. The cavitation characteristics of bearing is investigated by taking pictures of experiment.FindingsThe rupture region of oil film and the contours of air volume fraction increase distinctly with the increase of rotating speed and the decrease of input pressure. The critical rotating speed of cavitation occurrence and oil film pressure increases with the increase of input pressure. The change trend of experiment cavitation with the rotating speed and input pressure is consistent with theoretical cavitation in general.Originality/valueThe finite element model of three oil wedges sleeve bearings is established based on the Navier-Stokes calculation equation of the fluid, two-phase flow theory and turbulent model. Sleeve bearing is transparent, the pictures of cavitation can be easily taken by high-speed camera, the cavitation characteristics of bearing is studied by experiment. The cavitation performance of three oil wedges bearings is studied with the change of input pressure and rotating speed, the change trend is basically consistent for theory and experiment. The study on critical rotating speed of cavitation occurrence is benefit for defining the oil film boundary condition.

Pseudo-diffusion effects in lung MRI.

Magnetic resonance imaging of lung tissue is strongly influenced by susceptibility effects between spin-bearing water molecules and air-filled alveoli. The measured lineshape, however, also depends on the interplay between susceptibility effects and blood-flow around alveoli that can be approximated as pseudo-diffusion. Both effects are quantitatively described by the Bloch-Torrey-equation, which was so far only solved for dephasing on the alveolar surface. In this work, we extend this model to the whole range of physiological relevant air volume fractions. The results agree very well with in vivo measurements in human lung tissue.

Wearable UHF RFID Sensor-Tag Based on Customized 3D-Printed Antenna Substrates

The combination of radiofrequency identification (RFID) technology with 3D-printed structures is very appealing to design new RFID-based smart but cost-effective devices. In this paper, two different 3D-printed devices in the RFID framework are designed, realized, and tested. In particular, since permittivity and loss tangent of the printed substrate depend on the volume fraction of air in the host material, the first proposed device is purposely a T-resonator-based system suitable for the dielectric characterization of 3D-printed substrates in UHF RFID band, realized in 3D-printing technology itself. Based on the obtained results, the second device is a compact 3D-printed wearable antenna feeding a UHF RFID sensor tag designed to passively sense and retrieve the human body temperature. Both devices have been realized by combining 3D-printed plastic elements and conductive parts and then tested so to demonstrate the aptness of 3D-printing technology in designing RFID devices.

Numerical simulation of micro-bubble drag reduction of an axisymmetric body using OpenFOAM

The two-phase micro-bubble flow over an axisymmetric body is investigated using the OpenFOAM framework. The numerical model consists of an Eulerian-Eulerian two-fluid model with closure relationships for the interfacial momentum transfer to capture the multiphase flow, a standard k – e model for the continuous phase and one turbulence model inside the OpenFOAM for the dispersed phase. The bubble sizes are calculated based on the solution of the transport equation of the interfacial area density. The simulations in this work are carried out with different air injection rates and different flow velocities. The effects of bubble size on drag reduction are analyzed. The numerical results are compared against some available experiments and other numerical simulations. The numerical results indicate that the airflow rate and air volume fraction within the boundary layer near the body play important roles in micro-bubble drag reduction. The frictional drag reduction effect by micro bubbles is larger for lower water speed, and the presence of the micro bubbles can increase the pressure resistance of the body. Drag reduction rates are generally higher when the bubble diameter is smaller.

Numerical estimation of air core length in two-phase free surface vortex

ABSTRACTThe free-surface vortex is simulated with the homogeneous mixture approach. In order to define suitable boundary conditions in the two-phase flow field, a numerical strategy is proposed on the air mass conservation in a recirculating flow system. The numerical approach is verified from an experiment. Numerical results agreed well with the experiment not only in the vortex circulation but also in the axial velocity gradient. On the homogeneous mixture assumption, the compressibility corrections for the turbulence modelling had almost no effect on the result in this case study. In order to estimate the continuous air–core length, the visible air–core interface was assumed to be characterized by the constant void fraction. With validated numerical results, vortex air core was visualized by the air volume fraction, based on the air–core length in not only the experiment but also the analytical model in the applicable range.

Friction power analysis and improvement for a tilting-pad journal bearing considering air entrainment

This paper analyses the effects of air on oil–air distribution and energy characteristics of a tilting-pad journal bearing via computational fluid dynamics. With a gaseous cavitation model, air entrainment from an outlet boundary is analysed for the TPJB at a 3000 rpm rotation speed under a 180 kN load. The simulated bearing friction power is consistent with the experimental data, which indicates that the air entrainment from the outlet boundary is near the actual working conditions. According to the analyses, the shear stress of the loaded area is mainly influenced by velocity gradient in the normal direction to the rotor-side wall, whereas the shear stress of the unloaded area is mainly influenced by air volume fraction. With air cavitation and entrainment, the air volume fraction increases and affects the viscosity of a mixture considerably, thereby eventually influencing the shear stress on the rotor-side wall and bearing friction power. On the basis of this relationship, an improvement, which closes two oil inlet holes of the unloaded area whilst increasing the oil-supplied pressure of the loaded area, is proposed to remarkably decrease the bearing friction power whilst keeping load capacity. Simulation results validate the effectiveness and feasibility of the improvement.

Dynamics of Liquid Oil that Flows Inside Aqueous Wet Foam

Wet soap foam spontaneously imbibes liquid oil without defoaming when it is brought into contact. The kinetics behind this recently observed phenomenon was studied experimentally, with focus on the origin of the suction force and on the oil front dynamics. Using an aqueous foam with an air volume fraction slightly greater than the critical valueϕC, we show that the pumping pressure of oil and/or miscible liquid into the wet foam is attributed to the interfacial distortion of the bubble surfaces. Two distinct regimes along time t were observed in the oil imbibition dynamics. The proceeding oil front evolves with t1/2 dependency in the early imbibition time in accordance with the classical theory of penetration of a porous medium, whereas it departs into t1/3 at late imbibition time. The latter process is attributed to the elongation of an oil branch trapped inside the foam when pumping of the exterior oil has ceased.

An experimental investigation of the flow and mass transfer behavior in a vertical aeration process with orifice ejector

Vertical two-phase jets are widely used in environmental and industrial applications for their capacities to enhance fluid mixing, reaction and mass transfer rates. This paper presents an experimental investigation on the flow and mass transfer characteristics of vertical aeration process with the operational and design parameters of orifice ejector. The primary and suction flow rates, dissolved oxygen concentration and electric input power were measured, and a direct visualization technique with a high-speed camera was used to capture the image of the vertical two-phase jet. The experimental data were used to calculate the air volume fraction, specific input power, volumetric mass transfer coefficient and mass transfer efficiency. It was found that the air volume fraction and the mass transfer efficiency decreased with Reynolds number, whereas the input power and volumetric mass transfer coefficient increased. The air volume fraction, volumetric mass transfer coefficient and mass transfer efficiency were higher for the orifice ejector configuration with a smaller throat area ratio. The experimental data was used to propose empirical correlations to predict the volumetric mass transfer coefficient.

Simultaneous determination of soil bulk density and water content: a heat pulse‐based method

SummarySoil bulk density (ρb) and volumetric water content (θ) determine the volume fractions of soil solids, water and air, and influence mass and energy transfer in soil. It is desirable to monitor ρb and θ concurrently and non‐destructively. We present a heat pulse‐based method for simultaneous determination of ρb and θ from soil thermal properties. The method uses equations that relate ρb and θ to soil volumetric heat capacity (C) and to soil thermal conductivity (λ). We developed a three‐step procedure to calculate ρb and θ from C and λ measured by a heat pulse sensor, with soil texture and specific heat of soil solids known a priori. Laboratory evaluation of soil samples with various textures showed that the three‐step method provided reliable estimates of ρb and θ at θ values greater than the critical water content (θc) when λ started to respond notably to increases in θ. This method provides a new way to determine ρb and θ simultaneously with heat pulse sensors.Highlights We developed an approach to determine soil bulk density (ρb) and water content (θ) simultaneously with a heat pulse sensor. We estimated ρb and θ from soil thermal properties based on heat capacity and thermal conductivity models. The new approach provided reliable ρb and θ values at water contents >θc, the critical value. At θ < θc, the approach gave unstable results because soil thermal conductivity was insensitive to ρb.

A soft sensor based on online biomass measurements for the glucose estimation and control of fed-batch cultures of Bacillus thuringiensis.

On bioprocess engineering, experimental measurements are always a costly part of the modeling effort; therefore, there is a constant need to develop cheaper, simpler, and more efficient methodologies to exploit the information available. The aim of the present work was to develop a soft sensor with the capacity to perform reliable substrate predictions and control in microbial cultures of the fed-batch type, using mainly microbial growth data. This objective was achieved using dielectric spectroscopy technology for online monitoring of microbial growth and hybrid neural networks for online prediction of substrate concentration. The glucose estimator was integrated to a fuzzy logic controller to control the substrate concentration in a fed-batch experiment. Dielectric spectroscopy is a technology sensitive to the air volume fraction in the culture media and the turbulence generated by the agitation; however, the introduction of a polynomial function for the calibration of the permittivity signal allowed biomass estimations with an approximation error of 2%. The methodology presented in this work was successfully implemented for the glucose prediction and control of a fed-batch culture of Bacillus thuringiensis with an approximation error of 6%.

Numerical study on hydraulic and self-priming performance of a double-stage self-priming pump

Vertical self-priming pump has numerous advantages and has been widely used in industrial and agricultural areas. Nevertheless, impeller in single-stage self-priming pump bears large diameter and huge friction loss in the delivery of high-lift fluid, and enhanced requirement for sealing capacity meanwhile generates greater increment of power loss from sealing auxiliary-impeller and sealing ring. Both of these have resulted in low efficiency in operation, to the extent of imposing restriction on the further application of self-priming pump. In this paper, a new design of double-stage vertical self-priming pump with two impellers settled back to back was put forward to cover the shortfalls above. The internal flow field was calculated adopting realizable k-ε turbulence model in commercial CFD software Fluent and performance prediction was conducted. Concurrently, two-phase flow inside this self-priming pump without backflow hole was calculated by gas-liquid two-phase unsteady simulation to explore the self-priming characteristics. In order to consist with practical self-priming situation, a section of suction pipe filled with air was installed as the initial condition, pressure inlet and pressure outlet was set as boundary condition. Base on the results, the variation of pressure distribution and the fluctuation of air volume fraction on the specific cross-section of impeller and volute were obtained to study the self-priming process. The simulation and test results showed distinct uniformity, which indicated that the optimizing design provided impacts great effect on efficiency improvement as well as meeting the purpose of energy-saving and expense-reduce in the aspect of self-priming performance. The research here has provided the instruction to the design of high-head vertical self-priming pump.

A dynamic model for level prediction in aerated tanks

Stirred aerated tanks are a key unit operation in many industries, including froth flotation. Reliable and robust level control is of great importance in maintaining steady operation for successful implementation of higher level optimising control strategies, particularly when such tanks are arranged in series. When changes are made to the rate of aeration, there is a corresponding change in the pulp bubble size and gas holdup (the volume fraction of air in the tank), and consequently the pulp height. Stable operation of flotation tanks must, therefore, include the effect of air rate on pulp height in level control systems, especially if air rate is being actively controlled. In this paper, a model is developed from first principles to link the change in gas holdup with variation in air rate under dynamic conditions, accounting for the variability in gas holdup with height that results from differences in gas compressibility. This is validated experimentally.In order to test the model, experiments were carried out using a 70 L laboratory tank comprising water and reagent systems. For both simple and complex changes in air rate, the model showed good agreement with the experimental results when predicting the change in pulp height at steady state. Under dynamic conditions, the experimental system exhibited a slightly slower response than is predicted by the model; this is likely to be due to the well mixed assumption not being adequately met.This model provides a method to improve the operating stability of aerated tanks through better modelling of the dynamic pulp height changes that result from changes in air flowrate. In flotation tanks, this will enable greater control over froth height, which has been found to affect significantly mass pull, froth stability and flotation performance.

Investigation on centrifugal pump performance degradation under air-water inlet two-phase flow conditions

In order to study the flow characteristics of centrifugal pumps when transporting the gas-liquid mixture, water and air were chosen as the working medium. Both numerical simulation and experimental tests were conducted on a centrifugal pump under different conditions of inlet air volume fraction (IAVF). The calculation used URANS k-epsilon turbulence model combined with the Euler-Euler inhomogeneous two-phase model. The air distribution and velocity streamline inside the impeller were obtained to discuss the flow characteristics of the pump. The results show that air concentration is high at the inlet pressure side of the blade, where the vortex will exist, indicating that the gas concentration have a great relationship with the vortex aggregation in the impeller passages. In the experimental works, pump performances were measured at different IAVF and compared with numerical results. Contributions to the centrifugal pump performance degradations were analyzed under different air-water inlet flow condition such as IAVF, bubble size, inlet pressure. Results show that pump performance degradation is more pronounced for low flow rates compared to high flow rates. Finally, pressure pulsation and vibration experiments of the pump model under different IAVF were also conducted. Inlet and outlet transient pressure signals under four IAVF were investigated and pressure pulsation frequency of the monitors is near the blade passing frequency at different IAVF, and when IAVF increased, the lower frequency signal is more and more obvious. Vibration signals at five measuring points were also obtained under different IAVF for various flow rates.

An inverse method for estimating air volume fraction of sea foam from emissivity data

ABSTRACTThe air volume fraction (AVF) of a sea foam layer is a key parameter that affects the microwave emissivity and brightness temperature of the sea surface. To extract AVF of a sea foam layer from the measured emissivity data at microwave frequencies, a novel retrieval method is developed based on the spectral representation of effective dielectric constant of sea foam composites media, with the volume fraction of constituents and the geometry of foam microstructure included in the spectral function. In the retrieval method, the cost function of emissivity is constructed with two constraints: one is the first two moments of the spectral function and the other is derived from the spectral representation of coated spherical composites. We tested the inverse method using the simulation data of a known emissivity model with Gaussian noise. Good agreement is obtained between the inverse results and the known AVF of sea foam from the simulated emissivity data with varying sea surface temperature, salinity, and incident angle. Finally, the proposed method was used to estimate the AVF of the sea foam layer from the measured emissivity data at frequencies 1.4, 6.8, 10.8, 35, and 36.5 GHz. Generally, the retrieved values of the AVF fall in the range of 0.85–0.98. This study implies that our method can be extended to retrieve the volume fraction of constituents of other composites media from the passive microwave remote-sensing data.

Effect of two-phase flow on transmission characteristics of oil film between friction pair

PurposeThe purpose of this paper is to use the viscosity of fluid to transmit power known as the hydro-viscous drive (HVD) to research the effect of two-phase flow on transmission characteristics.Design/methodology/approachIn this paper, a 3D computational model of oil film between friction pair was built to study the transmission characteristics of a two-phase oil film, and the distribution contours of pressure and temperature of oil film were investigated using the computational fluid dynamics technology.FindingsThe finding of the paper suggests that the distribution law of pressure and temperature of two-phase oil film is almost linear along the radial direction. However, since the physical phenomena near the outlet of the oil film are entirely different, there exists fluctuation. Meanwhile, the volume fraction of air was obtained at different rotation speeds, and the maximum value is 10.55 percent. Compared to the single-phase oil film, the torque transferred by the oil film is not linear with the rotation speed, its value decreases gradually.Originality/valueThis paper’s conclusions are very important for the study of HVD and its applications, which provide a new idea to further study the mechanism of oil film transmission and its cavitation. The development of fluid viscous speed clutch is dedicated to a large industrial fan and water pump speed regulation and energy saving. With the successful application of the technology, it will have more wide applications in different fields, such as, in steel, water, petrochemical, power plant of slag pump and exhaust fan.

Single-Fed Wide-Beamwidth Circularly Polarized Patch Antenna Using Dual-Function 3-D Printed Substrate

This letter presents a novel single-fed microstrip patch antenna using three-dimensional (3-D) printed substrate for both circular polarization and wide 3 dB axial-ratio beamwidth (ARBW). Here, the substrate consisting of a photosensitive resin and air with a square mesh-grid structure is dual-function. On the one hand, the permittivity of the substrate in two orthogonal directions is tuned to be slightly different by inserting two resin chunks. Hence, the asymmetry of the substrate can split the two degenerate modes of the patch antenna so as to generate circularly polarized waves. On the other hand, the effective permittivity of the substrate can be freely controlled by the volume fraction of air, and thus influences the 3 dB ARBW. As a result, wide ARBW can be realized by properly configuring substrate through a 3-D printing process. A prototype is fabricated and measured. Measured results are found in reasonably good agreement with the simulated ones, exhibiting a 3 dB ARBW as wide as 120°.

New developed burner towards stable lean turbulent partially premixed flames

A new burner was developed in order to achieve very stable lean flames at lower equivalence ratio and higher level of turbulence intensity. The air-fuel mixing process of the current burner was controlled either by using different levels of partially premixed or by changing the turbulence generator disk slit diameter, ds. Initially, the distributions of turbulent intensity and air volume fraction inside the burner were numerically investigated using three-dimensional computational fluid dynamic (CFD) modelling. Then the lean flame stability limits corresponding to the lean natural gas (NG)/air mixture at an equivalence ratio of φ = 0.6, under five degrees of partially premixed and two turbulent generators disk slit diameters were delineated. Based on the stability limits map, laser induced breakdown spectroscopy (LIBS) technique was employed for further quantitative measurements of the mixture fraction or the equivalence ratio distributions of NG/air mixture. The results indicated that the maximum burner stability for smaller ds was achieved at mixing length to diameter ratio (L/D) of 1:1, whilst for larger ds the maximum stability was achieved at L/D ratio of 2:1. Furthermore, the largest disk slit diameter yielded a homogeneous mixture fraction distribution and lower rms fluctuation, compared to that of lower disk slit diameter. Consequently, this improved the conduction effectiveness in preheating the unburned gases layers resulting in higher flame propagation speed.

Numerical analysis of the effects of air on light distribution in a bubble column photobioreactor

Light distribution inside photobioreactors (PBR) is a crucial parameter for the determination of growth of phototropic microorganisms and reactor productivity. In order to compute the light propagation inside PBR, scattering due to the presence of microorganisms is often neglected, since it is difficult to measure experimentally and it is not trivial to handle numerically. Moreover, absorption is usually assumed constant, but it is affected by the concentration of microorganisms and the presence of gas bubbles. In the present contribution we study how the flow hydrodynamics and local gas fractions inside a bubble column PBR affect the light distribution. First, we perform numerical simulations of a bubble column flow at different gas superficial velocities. Afterwards, we use instantaneous air volume fractions to calculate the effective scattering and absorption coefficient of the mixture, as well as the effective scattering phase function. Finally, we compute the polychromatic light distribution inside the PBR by means of a Lattice-Boltzmann solver. On the one hand, we find that gas bubbles affect both spatial distribution and magnitude of the light intensity field and their impact increases at higher gas superficial velocity. On the other hand, we also observe that the biomass counteracts these effects already at concentrations less than 1 kg/m3 so that the role of the gas phase on light fields seems to be of minor importance in PBR.

Experimental investigations of a two-phase gas/liquid flow in a diverging horizontal channel

In the present study the flow properties of a turbulent two-phase (air/water) flow through a horizontal, diverging channel are investigated experimentally. The mixture flows from a rectangular channel of 40 × 44 mm through a diffuser to a rectangular channel of 100 × 44 mm, corresponding to a hydraulic diameter ratio of 1.45. The diffuser is designed with an increasing opening angle, ensuring flow separation within the studied range of flow conditions. The main objectives are (1) to investigate the possible gas accumulation due to flow separation, and (2) to deliver a complete experimental database allowing later validation of computational models. The superficial Reynolds number of water and air phases has been varied in the range of ReL = 50,130–87,730, and ReG = 3–18.5, respectively. The results show large gas accumulation even at very low air volume fraction (0.05%), which significantly affects the velocity field and the pressure recovery of the diffuser. When increasing ReG, the accumulated gas always increases. However, when ReL is increased, the accumulated gas first increases up to a certain limit, then decreases again. This can be explained by noticing that, at very high ReL, the turbulence intensity in the liquid phase becomes very high, leading to a break-up of the large gas pocket into smaller bubbles that are transported away with the liquid phase; in this manner, the huge gas accumulation is prevented. The flow in such a divergent channel is somewhat similar to the flow found in a centrifugal pump impeller transporting gas/liquid mixtures. However, since it does not involve any rotating part, the investigated configuration allows a far more accurate and complete experimental characterization. Therefore, this study provides a first step toward understanding the complex flow patterns occurring into centrifugal pumps transporting gas/liquid mixtures, and opens the door for validation studies.

Numerical Visualization of Plunging Water Jet using Volume of Fluid Model

A plunging liquid jet is defined as a moving column of liquid passing through a gaseous headspace, air in our case, before impinging a free surface of receiving liquid pool. The mechanism of air entrainment due to plunging liquid jets is very complex and the complete mechanism of air entrainment is not fully understood so far. The present paper is an unsteady numerical simulation of air entrainment by water jet plunging, using the Volume Of Fluid (VOF) model. The piece wise linear interface construction algorithm (PLIC) for interface tracking is used, to describe the phase distributions of entirely immiscible air and liquid phases. The aim of this work is to investigate the performance and accuracy of the VOF method in predicting the initial impact between the descending jet and water free surface, air entrainment and the developing flow region under free surface. Three scale models based on geometric similarities (Froude number and dimensionless free jet length) are used for validation according to Chanson (2004) experience. The simulations show with accuracy, the air cavity formation steps, caused by the initial jet impact, its deep stretching under the pool free surface, until breakdown due to the shear created by a toroidal vortex. In terms of, air entrainment estimation, bubble dispersion and radial distribution of air volume fraction, large-scale models present a good agreement with the experience. However, for the smallest scale model, the results lead to suggest that air entrainment is governed by more parameters than the geometric similarities.