Droplet Entrainment Research Articles

垂直円管内の混合翼付きグリッドスペーサーを通過する気液二相環状流の圧力損失

This study investigates the effects of mixing vane (MV) attached to a grid spacer on pressure drop for air-water annular flows in a vertical circular pipe of 16 mm I.D. In order to know the effects of MV, grid spacers with or without MV were installed in the test pipe. Three types of grid spacers with MV which has four MVs inclined 20, 30 degree from the vertical flow axis (4-MV20, 4-MV30) and two MVs inclined 30 degree (2-MV30) were used. In addition, two liquid injection methods (wall and center jet injections) are adopted to control liquid droplet entrainment faction in gas core. In the experiment, pressure drop along the channel upstream and downstream from the spacer was measured with a pressure transducer. In the analysis, a correlation of the pressure drop across the spacer is newly developed based on the present data.

CFD prediction of droplet deposition in steam-water annular flow with flow obstacle effects

Recent model development on dryout prediction relies on the annular flow modeling, with three fields of gas, droplets and liquid film accounted for. Therefore one unified computational fluid dynamics (CFD) model for annular flow based on Lagrangian particle tracking approach was developed for dryout applications. On the other hand, it is well acknowledged that dryout performance could be improved if flow obstacles are placed in a flow channel. Therefore, to study and predict the dryout, the governing phenomena of droplet deposition and entrainment in annular flow with and without obstacles need to be investigated. The current work tested the CFD model against experimental data from a steam-water flow experiment. Both data with and without obstacles were employed to test the model capability on deposition calculation. The calculated deposition results without obstacles agree reasonably well with both the experimental data and the existing empirical correlations. In case of the flow with obstacles, the calculations also show reasonably good agreement with the experimental data. The current work laid a basis for further work on annular flow model development with dryout capabilities.

Modeling transient churn-annular flows in a long vertical tube

This work investigates the transient behavior of high gas fraction gas-liquid flows in vertical pipes (annular and churn flows). Hyperbolic balance equations for mass, momentum and entropy are written for the gas and liquid, which is split between a continuous film and droplets entrained in the gas core. Closure relationships to calculate the wall and interfacial friction and the rates of droplet entrainment and deposition were obtained from the literature. A finite-difference solution algorithm based on a coefficient matrix splitting method was implemented to deal with sharp variations in the spatial and temporal domains, such as pressure and phase holdup waves. The model results were compared with steady-state experimental data from eight different sources, totaling more than 1500 data points for pressure gradient, liquid film flow rate and void/core fraction. The absolute average deviation between the model and the data was 17% for the pressure gradient and 5.8% for the void fraction. A comparison of the model results with fully transient air-water data generated in a 49-mm ID, 42-m long vertical pipe is also presented. The experimental results consist of two outlet pressure-induced and two inlet mass flow rate-induced transient tests. Two main transient parameters are compared, namely the local void fraction and the pressure difference between selected points along the test section and the outlet (taken as a reference). The comparisons between the experiments and the numerical model indicate that the model was capable of describing the transient annular to churn flow transition with absolute average deviations of 14.5% and 7.9% for the pressure difference and void fraction, respectively.

Study of the Effect of Spinel Composition on Metallic Copper Losses in Slags

In both primary and secondary copper production, copper losses in slags are a decisive factor confining the process efficiency. An important cause for mechanical entrainment of metal droplets in slags is their attachment to solid spinel particles present in the slag phase, hindering sedimentation. To further optimize the production process, it is important to gain insights in the fundamental mechanisms governing this attachment. In the present study, the influence of the spinel composition on the attachment of copper droplets is investigated. First, the attachment is studied in an industrially relevant synthetic PbO–CaO–SiO2–Cu2O–Al2O3–FeO–ZnO slag system. Second, the wetting of copper on two spinel substrates (ZnFe2O4 and MgAl2O4) has been studied in the absence of a slag system, by sessile-drop experiments. Based on the results of both types of experiments, a clear influence of the spinel composition on the sticking behavior of copper droplets is noted. These observations might be transferred to industrial processes to adapt processing parameters to diminish copper losses in industrial slags.

Experimental investigation of the heat transfer characteristics of spray/wall interaction in diesel selective catalytic reduction systems

This study presents an experimental investigation of the heat transfer characteristics of the spray/wall interaction in diesel selective catalytic reduction (SCR) systems. The work was performed with a commercial 3-Hole pressure-driven injector dosing into a flow channel under typical diesel exhaust flow conditions. Infrared thermography captured the surface temperature of the wall around the impingement area with high temporal and spatial resolution. The resulting temperatures have been used for assessing the heat extracted from the wall. Phase Doppler Anemometry (PDA) was applied to measure the droplet sizes and velocities prior to the wall impingement, providing information on the kinetic properties of the impinging droplets. Based on these, the influence of the gas flow conditions on the heat transfer characteristics is deduced.The spray impingement leads to a substantial and rapid temperature drop on the wall, resulting in a maximum heat flux of several MW/m2 during the injection duration. The spray cooling effect decreases with increasing exhaust gas flow rate due to the increased entrainment of spray droplets in the flow prior to impingement. Increase in gas flow temperature affects the heat transfer by increasing the wall temperature. At lower wall temperatures, the principal spray/wall interaction regime is deposition. With increasing wall temperature, there is a shift to rebound and thermal breakup. The shorter contact times in the rebound and thermal breakup regimes result in decreased spray/wall heat transfer.

A physically based, one-dimensional three-fluid model for direct contact condensation of steam jets in flowing water

A simulation model for the direct contact condensation of steam jets in flowing water is presented. In contrast to previous empirical approaches, the model takes into account the underlying physical phenomena governing the condensation process. Condensation at the interface between the steam jet and the surrounding water is calculated according to the surface renewal theory. Entrainment of water into the steam jet is modeled based on the Kelvin–Helmholtz and Rayleigh–Taylor instability theories. The resulting steam-water two-phase flow is simulated based on a one-dimensional three-fluid model. An interfacial area transport equation is used to track changes of the interfacial area density due to droplet entrainment and steam condensation on droplets. The simulation results are in good qualitative agreement with published experimental data. In particular, the dependency of the steam jet length on the flow Reynolds number is properly reproduced. This corroborates our theory that the heat transfer coefficient at the interface of a condensing steam jet can be linked to the water flow rate via the interfacial friction factor.

New models of droplet deposition and entrainment for prediction of liquid film flow in vertical annuli

Droplet deposition and entrainment significantly affect dryout occurrences in annular two-phase flow. Currently, most of the correlations for droplet deposition and entrainment are derived from flows in tubes. However, the main challenge in thermal-hydraulic analyses of BWRs is the prediction of dryout occurrence in the rod bundle geometry of fuel assemblies. In the work reported here, the applicability of several tube-based droplet deposition and entrainment correlations to the annular geometry (effectively a “one-rod” bundle) is checked. It is found that the prediction values from all those correlations deviate from the experimental measurements. Therefore, a set of simple and easy-to-implement models for deposition and entrainment rates in annuli is developed based on the tube-based Hewitt-Govan correlations (Hewitt and Govan, 1990), with considering the liquid film flow characteristics in annuli and the geometry effect of the annulus. The present models are verified by comparison with experimental data for developing liquid films in adiabatic annuli. By using these models to predict dryout in heated annuli, we find that these new relationships have good capability in predicting dryout occurrence in annuli with uniform as well as non-uniform axial power distribution.

Droplet entrainment analysis of three-phase low liquid loading flow

Most of the commonly used multiphase flow models neglect the amount of droplets entrained in stratified wavy flow. However, the experimental data presented in this study show high entrainment values, exceeding 50% in some cases. This shows that neglecting entrainment phenomenon can introduce a major source of discrepancy into multiphase flow modeling predictions. This study improves the entrainment fraction predictions in three-phase stratified flow.The droplet entrainment in three-phase stratified flow in horizontal pipelines is experimentally investigated using a 0.152-m ID facility. The experiments are conducted under low liquid loading conditions, which is very commonly observed in wet gas pipelines. The oil-aqueous-gas flow experiments are initially performed without mono-ethylene glycol (MEG), and then repeated with 50 wt% of MEG in the aqueous phase to analyze the effects of MEG presence on entrainment. MEG is a commonly used inhibitor in oil industry, applied to avoid hydrate formation in offshore systems. Its impacts on multiphase flow droplet entrainment are investigated.The experimental range of this study covers superficial gas velocity (vSg) values from 17 to 23m/s, superficial liquid velocity (vSL) values of 0.01 and 0.02m/s, and inlet liquid stream aqueous phase fraction (APFin) values between 0 to 100%. Similar test matrix is completed for both water and water and MEG solution as the aqueous phase. An isokinetic probe system is used to measure the entrained droplet flux at different vertical positions in the gas phase. Liquid entrainment fraction is then estimated by means of volumetric averaging. The trends of the data with respect to input parameters are investigated.The two and three-phase entrainment fraction data are used for a correlation evaluation study. Performances of ten correlations for two-phase entrainment fraction are compared to the experimental data, and best performing correlations are identified. The correlation of Pan and Hanratty (2002) is modified in an effort to improve the entrainment fraction estimations. The predictions of the modified correlation are compared with the acquired data and datasets from the literature. In addition, a simple empirical correlation is proposed to predict liquid phase entrainment fraction for three-phase flow systems.

Design and performance of the rotating liquid sheet contactor

The Rotating Liquid Sheet (RLS) contactor is a new gas-liquid contacting device that is based on the formation of continuous liquid sheets in the shape of multiple blades or helices issuing from slots in a centrally rotating tube. The resulting rotating liquid sheets provide a high interfacial area of contact with the gas and are shaped in such a way that the rotation of the liquid sheet is able to pump the gas through the otherwise empty column at rates comparable to that seen in packed bed contactors. The centrifugal action of the device significantly reduces the entrainment of droplets, while the control of the generation of the liquid surface also makes the device suitable for solvents that are too viscous to be utilized in a packed bed or similar conventional contactors.This paper sets out the design equations for the RLS contactor, provides a basis for comparison with conventional contactors including packed beds consisting of random or structured packing. Experimental evidence for the pumping capacity and Computational Fluid Dynamics (CFD) modelling are applied to justify the performance of the RLS contactor and the choice of the design approach.

Investigation of High-Temperature Slag/Copper/Spinel Interactions

An important cause for the mechanical entrainment of copper droplets in slags during primary and secondary copper production is their interaction with solid spinel particles, hindering the sedimentation of the copper droplets. In the present study, the interactions between the three phases involved (slag–Cu droplets–spinel solids) were investigated using an adapted sessile drop experiment, combined with detailed microstructural investigation of the interaction zone. An industrially relevant synthetic PbO-CaO-SiO2-Cu2O-Al2O3-FeO-ZnO slag system, a MgAl2O4 spinel particle, and pure copper were examined with electron microscopy after their brief interaction at 1523 K (1250 °C). Based on the experimental results, a mechanism depending on the interlinked dissolved Cu and oxygen contents within the slag is proposed to describe the origin of the phenomenon of sticking Cu alloy droplets. In addition, the oxygen potential gradient across the phases (i.e., liquid Cu, slag, and spinel) appears to affect the Cu entrainment, as deduced from a microstructural analysis.

Heat and mass transfer performance comparison between a direct-contact liquid desiccant packed bed and a liquid-to-air membrane energy exchanger for air dehumidification

Entrainment of liquid desiccant droplets into the airstream and flow maldistribution are two challenges for liquid desiccant packed beds. Liquid-to-air membrane energy exchangers are novel liquid desiccant exchangers that have the potential to overcome these challenges by using membranes to separate the air and solution flows but have higher heat and moisture transfer resistances. As a new contribution of this study, the heat mass transfer performance of these two exchangers with the same volume and operating condition are compared at two conditions: (1) with the same pressure drop on the air side and (2) with the same total heat/mass transfer area. Results show that liquid-to-air membrane energy exchanger gets up to 13 and 20% higher latent and total effectiveness respectively than the packed bed at the same air pressure drop. Reversely, the packed bed achieves up to 16% higher mass transfer performance than the liquid-to-air membrane energy exchanger with the same heat/mass transfer area. The flow maldistribution and its influences in the packed bed and liquid-to-air membrane energy exchanger are also discussed. Finally, the impact of membrane on the heat mass transfer in the liquid-to-air membrane energy exchanger is evaluated. The membrane accounts for 6∼12% and 26∼43% of the overall heat and mass transfer resistance, respectively, depending on the width of the air channel.

The effect of boundary conditions and droplet entrainment on severe slugging using a Lagrangian slug tracking model

This study presents a Lagrangian slug tracking model for prediction of slug flow in multiphase flow pipelines. Simulations are compared with severe slugging experiments from the SINTEF Large Scale Multiphase Laboratory with 0.189m inner diameter and a length of about 1000m. The fluids used in the experiments are nitrogen and naphtha, and the system pressure is 25bar. The model is shown to give fast and accurate results within the experimental accuracy without the need for a sub-grid model, as long as the grid size is not too large around the riser bottom where the liquid blockage occurs. It is also shown that simulations using a constant pressure boundary condition at the outlet are not always adequate, as the pressure fluctuations in the receiving facilities can significantly affect the flow. Furthermore, we show that the inclusion of a droplet field has a significant impact on the predicted slugging periods.

Phenomenological modelling of CHF in annular flow in annuli using new models of droplet deposition and entrainment

In this paper, we present a phenomenological model to predict the CHF (critical heat flux) for upward annular flow in heated vertical annuli. In present model, a new set of correlations of droplet deposition and entrainment in annuli was used which were verified by comparison with the data of Moeck (1970) for developing liquid films in adiabatic annuli. In the results presented here, these new correlations have been used to predict 2249 independent data on critical heat flux (CHF) obtained both regarding internal heating of the rod as well as simultaneous heating of the rod and the outer tube in six heated vertical annuli under various mass flow rate, pressure and inlet quality and where the conditions were such that (as is most common) the CHF condition occurred in the annular flow regime. The comparisons between the calculated and measured CHFs showed that the present model has good predictive capability in predicting CHF.

New models of droplet deposition and entrainment for prediction of CHF in cylindrical rod bundles

In this paper, we present a new set of model of droplet deposition and entrainment in cylindrical rod bundles based on the previously proposed model for annuli (effectively a “one-rod” bundle) (2016a). These models make it possible to evaluate the differences of the rates of droplet deposition and entrainment for the respective rods and for the outer tube by taking into account the geometrical characteristics of the rod bundles. Using these models, a phenomenological model to predict the CHF (critical heat flux) for upward annular flow in vertical rod bundles is described. The performance of the model is tested against the experimental data of Becker et al. (1964) for CHF in 3-rod and 7-rod bundles. These data include tests in which only the rods were heated and data for simultaneous uniform and non-uniform heating of the rods and the outer tube. It was shown that the predicted CHFs by the present model agree well with the experimental data and with the experimental observation that dryout occurred first on the outer rods in 7-rod bundles. It is expected that the methodology used will be generally applicable in the prediction of CHF in rod bundles.

Rotor dynamics behavior of turbo-expander involving droplet impact

ABSTRACTThis paper dedicates on the rotor dynamics behaviour research on the turbo-expander rotor system involving droplet impact. A stochastic model based on Beta distribution and Bernoulli distribution of droplet generation is established and the formulations of droplet impact forces are deduced, which is applied on the rotor dynamics equations of the tilting pad bearing supported turbo-expander considering the temperature gradient for a further analysis. A time domain research is carried out and a conclusion that continuously droplet impact will perturb the steady vibration of the turbo-expander rotor system is obtained. Monte Carlo method is implemented for a statistics dynamics research and the results suggests that in the design of expander impellers, in order to decrease the uncertainty brought by droplet impact, the number of channels should be as few as possible, the droplet impact should be controlled to occur uniformly, and the collision, entrainment of the primary droplets and the stripping of the liquid film on the blade should be strictly restrained.

Prediction of the critical heat flux for saturated upward flow boiling water in vertical narrow rectangular channels

A study, on the theoretical method to predict the critical heat flux (CHF) of saturated upward flow boiling water in vertical narrow rectangular channels, has been conducted. For the assessment of this CHF prediction method, 608 experimental data were selected from the previous researches, in which the heated sections were uniformly heated from both wide surfaces under the high pressure condition over 41bar.For this purpose, representative previous liquid film dryout (LFD) models for circular channels were reviewed by using 6058 points from the KAIST CHF data bank. This shows that it is reasonable to define the initial condition of quality and entrainment fraction at onset of annular flow (OAF) as the transition to annular flow regime and the equilibrium value, respectively, and the prediction error of the LFD model is dependent on the accuracy of the constitutive equations of droplet deposition and entrainment. In the modified Levy model, the CHF data are predicted with standard deviation (SD) of 14.0% and root mean square error (RMSE) of 14.1%. Meanwhile, in the present LFD model, which is based on the constitutive equations developed by Okawa et al., the entire data are calculated with SD of 17.1% and RMSE of 17.3%. Because of its qualitative prediction trend and universal calculation convergence, the present model was finally selected as the best LFD model to predict the CHF for narrow rectangular channels.For the assessment of the present LFD model for narrow rectangular channels, effective 284 data were selected. By using the present LFD model, these data are predicted with RMSE of 22.9% with the dryout criterion of zero-liquid film flow, but RMSE of 18.7% with rivulet formation model. This shows that the prediction error of the present LFD model for narrow rectangular channels is similar with that for circular channels.

Experimental study on circulatory flash speed of aqueous NaCl solution circulatory flash evaporation

In present paper, a series of experiments on circulatory flash evaporation of 15% aqueous NaCl solution was carried out under various main experimental parameters. Circulatory flash speed, which represented the superheated energy consumed in unit time in flash chamber, was defined as average change rate of the non-equilibrium fraction during the residence time in flash chamber. It indicated the intensity of flash evaporation. The experiment results showed that circulatory flash speed quickly decreased to a lowest point, then increased monotonously with the increasing superheat and finally became flat. Influence of parameters, such as flow rate, initial height of water film and equilibrium pressure on circulatory flash speed was also investigated. As circulatory flash speed increased, NEF had a minimum value. At the same circulatory flash speed, NEF decreased when superheat degree and equilibrium pressure increased. And comparative study on flash speed between static and circulatory flash evaporation was also conducted. Both theoretical and experimental results of volumetric heat transfer coefficient showed a linear relationship with circulatory flash speed. However, the experimental values were greater than the calculated values due to the liquid droplets entrainment.

Simultaneous Analysis of the Dispersed Liquid and the Bulk Gas Phase of Water Sprays Using Raman Spectroscopy.

Currently, a comprehensive physical description of sprays is not possible, as the involved heat- and mass-transport mechanisms have not yet been understood completely. Therefore, we here show and apply a straightforward Raman evaluation technique which simultaneously probes spatially resolved (i) droplet temperature, (ii) evaporation progress and (iii) entrainment of air into the spray. First, the working principle of the Raman technique and the calibration of the sensor are described. Then, the applicability of the Raman technique is demonstrated showing example measurement results obtained from a superheated water spray. The plausibility of the obtained measurement results is demonstrated comparing them with computations for thermodynamic equilibrium conditions. Information about the droplet temperature, the evaporation progress and the entrainment of air might complement the insights into heat- and mass-transport mechanisms which can already be provided applying other existing optical spray diagnostic techniques.

Entrained liquid fraction calculation in adiabatic disperse-annular flows at low rate in film

In this work, we continue our study [1] and extend further an approach to low reduced pressures. An approximate model of droplets entrainment from the laminar film surface and an equation for calculating entrainment intensity are proposed. To carry out direct verification of this equation using experimental data is extremely difficult because the integral effect—liquid flow rate in a film at a dynamic equilibrium between entrainment and deposition—is usually measured in the experiments. The balance between flows of droplets entrainment and deposition corresponds to the dynamic equilibrium because of turbulent diffusion. The transcendental equation, which was obtained on the basis of this balance, contains one unknown numerical factor and allows one to calculate the liquid rate. Comparing calculation results with the experimental data for the water–air and water–helium flows at low reduced pressures (less than 0.03) has shown their good agreement at the universal value of a numerical constant, if an additional dimensionless parameter, a fourth root of vaporliquid densities ratio, is introduced. The criterion that determines the boundary of using methods of this work and that of [1] in calculations and that reflects effect of pressure and state of film surface on distribution of the liquid in the annular flow is proposed; the numerical value of this criterion has been determined.

Investigation of Bubble-Slag Layer Behaviors with Hybrid Eulerian–Lagrangian Modeling and Large Eddy Simulation

In ladle metallurgy, bubble–liquid interaction leads to complex phase structures. Gas bubble behavior, as well as the induced slag layer behavior, plays a significant role in the refining process and the steel quality. In the present work, a mathematical model using the large eddy simulation (LES) is developed to investigate the bubble transport and slag layer behavior in a water model of an argon-stirred ladle. The Eulerian volume of fluid model is adopted to track the liquid steel–slag–air free surfaces while the Lagrangian discrete phase model is used for tracking and handling the dynamics of discrete bubbles. The bubble coalescence is considered using O’Rourke’s algorithm to solve the bubble diameter redistribution and bubbles are removed after leaving the air–liquid interface. The turbulent liquid flow that is induced by bubble–liquid interaction is solved by LES. The slag layer fluactuation, slag droplet entrainment and spout eye open–close phenomenon are well revealed. The bubble diameter distribution and the spout eye size are compared with the experiment. The results show that the hybrid Eulerian–Lagrangian–LES model provides a valid modeling framework to predict the unsteady gas bubble–slag layer coupled behaviors.