Gas Phase Velocity Field Research Articles

Numerical Simulation of the Oil Droplet Size Distribution Considering Coalescence and Breakup in Aero-Engine Bearing Chamber

In order to improve the inadequacy of the current research on oil droplet size distribution in aero-engine bearing chamber, the influence of oil droplet size distribution with the oil droplets coalescence and breakup is analyzed by using the computational fluid dynamics-population balance model (CFD-PBM). The Euler–Euler equation and population balance equation are solved in Fluent software. The distribution of the gas phase velocity field and the volume fraction of different oil droplet diameter at different time are obtained in the bearing chamber. Then, the influence of different initial oil droplet diameter, air, and oil mass flow on oil droplet size distribution is discussed. The result of numerical analysis is compared with the experiment in the literature to verify the feasibility and validity. The main results provide the following conclusions. At the initial stage, the coalescence of oil droplets plays a dominant role. Then, the breakup of larger diameter oil droplet appears. Finally, the oil droplet size distribution tends to be stable. The coalescence and breakup of oil droplet increases with the initial diameter of oil droplet and the air mass flow increasing, and the oil droplet size distribution changes significantly. With the oil mass flow increasing, the coalescence and breakup of oil droplet has little change and the variation of oil droplet size distribution is not obvious.

Experimental investigation of intermittent airflow separation and microscale wave breaking in wavy two-phase pipe flow

We perform an experimental analysis of co-current, stratified wavy pipe flow, with the aim of investigating the effect of small scale wave breaking (microscale breaking) on the airflow. Particle image velocimetry is applied simultaneously in the gas and liquid phases. Active wave breaking is identified by high levels of vorticity on the leeward side of individual waves, and the statistics of the airflow above breaking and non-breaking waves are extracted from the gas-phase velocity fields. Keeping the liquid superficial velocity constant ($U_{sl}=0.1~\text{m}~\text{s}^{-1}$), we consider two experimental cases of different gas flow rates. The lowest flow rate ($U_{sg}=1.85~\text{m}~\text{s}^{-1}$) is slightly higher than the onset of microscale breaking, while the higher flow rate ($U_{sg}=2.20~\text{m}~\text{s}^{-1}$) is within the regime where wave breaking is observed to be frequent, and the root-mean-square interface elevation $\unicode[STIX]{x1D702}_{rms}$ is independent of gas flow rate. Results show that for the lowest gas flow rate considered, active wave breaking has a stabilizing effect on the airflow above the waves, reducing the sheltered region on the leeward side of the wave and the turbulence above the wave crest compared with non-breaking waves at similar steepness. At the higher gas flow rate the effect of active wave breaking is found to be small, and the main geometrical properties of the waves are found to dominate the evolution of the separated flow region.

Large Eddy Simulation of a Novel Gas-Assisted Coal Combustion Chamber

In this work a recently presented combustion chamber that is specifically designed for the investigation of gas-assisted coal combustion and the validation of models is simulated under reactive conditions for the first time. In the configuration coal combustion is assisted and stabilized by a methane flame. In the course of the investigation, the configuration’s complexity is increased successively. Results of the isothermal single-phase flow are discussed first. Subsequently, reproducibility of the single-phase methane flame by means of the applied modeling approach is evaluated. In a further step, coal particles having the same thermal power as the methane flame are injected into the configuration. Particle histories, the conversion of the coal particles as well as its retroactive effect on the gas phase are investigated. Experimental results based on laser diagnostics are provided for all operating points and used for comparison with numerical results. Gas phase velocity fields for all operating points are available. In order to identify the reaction in the reactive single-phase case planar laser induced fluorescence of the OH-radical (OH-PLIF) was applied. Overall good agreement between numerical and experimental results could be obtained. In the Large Eddy Simulation (LES) a Flamelet Generated Manifold (FGM) based model is utilized. The four-dimensional manifold is spanned by two mixture fractions, a reaction progress variable and the enthalpy on which the gas phase chemistry gets mapped onto. Thereby, the model accounts for both, volatiles reaction and char conversion. Furthermore, finite rate chemistry effects as well as non-adiabatic physics are considered.

Large Eddy Simulation of CO2 diluted oxy-fuel spray flames

We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.

Numerical analysis of the flue gas-coal particles mixture flow in burner's distribution channels with regulation shutters at the TPP Nikola Tesla - A1 utility boiler

Pulverized coal particles concentration distribution across the burner's distribution channels, especially where plasma torches are installed, is one of the key issues for efficient implementation of plasma system for liquid fuel free combustion support at the pulverized coal fired boilers. The possibility of pulverized coal particles concentration increase at the lower burner channels of TPP Nikola Tesla - A1 boiler using regulation shutters is analyzed experimentally and numerically. Subject of present work is two-phase flue gas-particles mixture flow in burner's distribution channels with regulation shutters installed at the TPP Nikola Tesla - A1 boiler. Aim of this work is to optimize position of implemented system of shutters to achieve desired concentration and velocity distribution in channels with plasma torches, using numerical modelling. Experimental investigation was performed for the verification of proposed mathematical model for the prediction of the analyzed two-phase flow. Based on verified model, numerical parametric analysis was done. Obtained results of gas phase velocity field, coal particles concentration field, velocity and concentration profiles clearly show the dependence between shutters position and the coal particles mass flow rate and concentration distribution at the outlet cross-section of the burner's distribution channels. According to the numerical optimization results suitable modification of the shutter system is proposed.

Measurement of Fuel Cell Flowfields Using Particle Image Velocimetry

Because the parasitic load on most fuel cell systems is dominated by pressure losses across the flow fields, understanding the dynamics in a flow channel has garnered significant recent attention. The current work is aimed at enabling advanced design of reactant flow channels by elucidating the details of the velocity fields within various reactant flow channel geometries. Gas-phase measurement is critical to ensuring that the results are valid for real fuel cell systems, and for application of this technique to the study of flow channels with two-phase obstruction phenomena. In the current research, gas-phase velocity fields have been measured using particle image velocimetry (PIV) in representative geometries, including a straight channel (for technique validation) and a 180å switchback with channel-to-land ratio of 1:1. The accuracy of the measurement depends on the ability of the seed particles to follow the flow. Numerical simulations have been performed to develop a criterion for identifying appropriate seed particles. The research presented is a first step towards measuring velocity fields in novel flow reactant channels in operational cells.

Gas-phase particle image velocimetry (PIV) for application to the design of fuel cell reactant flow channels

Because the parasitic load on most polymer electrolyte fuel cell systems is dominated by pressure losses across the flow fields, understanding the flow channel dynamics has garnered significant recent attention. The current work is aimed at enabling advanced design of reactant flow channels by elucidating the details of the velocity fields within various reactant flow channel geometries. Gas-phase measurement is critical to ensuring that the results are valid for real fuel cell systems, and for application of this technique to the study of flow channels with two-phase obstruction phenomena. In the current research, gas-phase velocity fields have been measured using particle image velocimetry (PIV) in representative geometries, including a straight channel (for technique validation) and a 180° switchback with channel-to-land ratio of 1:1. The accuracy of the measurement depends on the ability of the seed particles to follow the flow. Numerical simulations have been performed to develop a criterion for identifying appropriate seed particles. The research presented is a first step toward measuring velocity fields in novel flow reactant channels in operational cells.

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail.

Gas-phase velocity field measurements in dense sprays by laser-based flow tagging

It is demonstrated in this work that gas-phase velocity measurements can be performed in dense sprays by using a new 2D laser-based flow tagging technique. Velocity measurements in dense sprays, such as automotive direct injection (DI) Otto and Diesel sprays, are difficult with conventional techniques because of the high number densities of droplets, the optical thickness of the medium, and multiple light scattering effects. The present flow tagging technique is performed by two consecutive laser pulses, i.e., ‘write’ and ‘read’ lasers. The write laser creates a grid of tracer molecules (NO) by inducing a photodissociation process. The tracer molecules are convected with the flow and probed by the read laser after a certain delay. The instantaneous velocity field is determined by time-of-flight analysis.

Numerical Computation and Validation of Two-Phase Flow Downstream of a Gas Turbine Combustor Dome Swirl Cup

The two-phase axisymmetric flow field downstream of the swirl cup of an advanced gas turbine combustor is studied numerically and validated against experimental Phase-Doppler Particle Analyzer (PDPA) data. The swirl cup analyzed is that of a single annular GE/SNECMA CFM56 turbofan engine that is comprised of a pair of coaxial counterswirling air streams together with a fuel atomizer. The atomized fuel mixes with the swirling air stream, resulting in the establishment of a complex two-phase flow field within the swirl chamber. The analysis procedure involves the solution of the gas phase equations in an Eulerian frame of reference using the code CONCERT. CONCERT has been developed and used extensively in the past and represents a fully elliptic body-fitted computational fluid dynamics code to predict flow fields in practical full-scale combustors. The flow in this study is assumed to be nonreacting and isothermal. The liquid phase is simulated by using a droplet spray model and by treating the motion of the fuel droplets in a Lagrangian frame of reference. Extensive PDPA data for the CFM56 engine swirl cup have been obtained at atmospheric pressure by using water as the fuel (Wang et al., 1992a). The PDPA system makes pointwise measurements that are fundamentally Eulerian. Measurements have been made of the continuous gas phase velocity together with discrete phase attributes such as droplet size, droplet number count, and droplet velocity distribution at various axial stations downstream of the injector. Numerical calculations were performed under the exact inlet and boundary conditions as the experimental measurements. The computed gas phase velocity field showed good agreement with the test data. The agreement was found to be best at the stations close to the primary venturi of the swirler and to be reasonable at later stations. The unique contribution of this work is the formulation of a numerical PDPA scheme for comparing droplet data. The numerical PDPA scheme essentially converts the Lagrangian droplet phase data to the format of the experimental PDPA. Several sampling volumes (bins) were selected within the computational domain. The trajectories of various droplets passing through these volumes were monitored and appropriately integrated to obtain the distribution of the droplet characteristics in space. The calculated droplet count and mean droplet velocity distributions were compared with the measurements and showed very good agreement in the case of larger size droplets and fair agreement for smaller size droplets.

Structure of a swirl-stabilized spray flame by imaging, laser doppler velocimetry, and phase doppler anemometry

Data are presented which describe the mean structure of a steady, swirl-stabilized, kerosene spray flame in the near-injector region of a research furnace. The data presented include ensemble-averaged results of schlieren, luminosity, and extinction imaging, measurement of the gas phase velocity field by laser Doppler velocimetry, and characterization of the condensed phase velocity by phase Doppler anemometry. The results of these studies define six key regions in the flame: the dense spray region; the rich, two-phase, fuel jet; the main air jet; the internal product recirculation zone; the external product recirculation zone; and the gaseous diffusion flame zone. The first five of these regions form a conical mixing layer which prepares the air and fuel for combustion. The air and fuel jets comprise the central portion of this mixing layer and are bounded on either side by the hot product gases of the internal and external recirculation zones. Entrainment of these product gases into the air/fuel streams provides the energy required to evaporate the fuel spray and initiate combustion. Intermittency of the internal recirculation and spray jet flows accounts for unexpected behavior observed in the aerodynamics of the two phases. The data reported herein are part of a database being accumulated on this spray flame for the purpose of detailed comparison with numerical modeling.

Application of Advanced Diagnostics to Airblast Injector Flows

Experimental data on the characteristics of the spray produced by a gas turbine engine airblast fuel injector are reported. The data acquired include the mass-flux distribution, measured by use of a high-resolution spray patternator; the gas-phase velocity field, measured by use of a two-component laser-Doppler velocimeter; and the liquid droplet size and velocity distributions, measured by use of a single-component phase-Doppler anemometer. The data are intended for use in assessments of two-phase flow computational methods as applied to combustor design procedures.