Droplet Entrainment Research Articles

Characterization and manufacturing of a paraffin wax as fuel for hybrid rockets

The hybrid propulsion performed with paraffin waxes exhibits most attractive capabilities compared to solid or liquid engines, e.g., throttleability and re-ignition, alongside higher regression rates compared to the conventional hydroxyl terminated polybutadiene (HTPB) hybrid fuel. This is because the paraffin wax forms a thin and hydro-dynamically unstable liquid layer, and then enhances the regression rate with the entrainment of droplets from the liquid-gas interface. Nevertheless, some critical open points on the manufacturing of the paraffin fuel grains still persist, because the paraffin wax exhibits high shrinkage during the solidification phase, leading to the formation of cavities, cracks and internal rips, which may be detrimental to the mechanical properties and the structural integrity of the fuel grain. In this context, this paper deals with a wide calorimetric, thermo-mechanical and physical characterization of the paraffin wax selected to manufacture the hybrid rocket engines (HRE) fuel grain, in order to gain a thorough knowledge of the material necessary to avoid the formation of critical defects. Several manufacturing methods were investigated, and it was found that only laboratory scale processes, based on the use of a heated circular mould-piston apparatus, are able to avoid the formation of critical defects, with the application of both high temperature and pressure.

On CCFL at a PWR Hot-Leg Pipe Geometry and Comparison Between Results in COLLIDER 1/4th- and Kobe 1/15th-Scaled Facilities

Countercurrent flow limitation (CCFL) in a pressurized water reactor hot-leg pipe geometry with a 190-mm pipe diameter was investigated experimentally and numerically at the COLLIDER test facility of the Technical University Munich in the past 3 years. This paper summarizes the most important CCFL findings learned from the COLLIDER test facility and tries to explain the reasons for obtaining different descriptions, results, and conclusions at different CCFL experimental investigations. The factors that can affect CCFL experimental results are explained in detail including some scale effects. The necessary preconditions to compare two sets of data from different CCFL experimental investigations are discussed in detail. The difference among CCFL-related limits/curves is clarified taking data at the COLLIDER as an example. The limits included the limit of the transition from a supercritical into a subcritical flow (SSTL); the onset of CCFL limit (iCCFL) inside the hot-leg pipe; the onset of CCFL limit (eCCFL) at the entrance of the steam generator; the deflooding limit (CCFLd); the CCFL characteristics curve (CCFLch), which predicts the water delivery rate after the onset of iCCFL; and the onset of hysteresis limit. It will be shown that among these limits only SSTL, CCFLch, and eCCFL are original limits while the rest are derivatives of them. In particular, it will be shown that the iCCFL limit is a combination of the SSTL and CCFLch limits. The effect of scale upon the eCCFL’s mechanism (whether a water accumulation or droplet entrainment at the entrance to the steam generator) is clarified via a comparison to a 50-mm CCFL facility at Kobe University. This paper tests the scalability of interface distribution at quasi-stationary conditions (i.e., points along the CCFLch curve) via a comparison of time-averaged interface distributions obtained at similar inlet conditions ( at the COLLIDER 190-mm and Kobe 50-mm channels. The comparison will show that interface distributions (which are directly linked to the pressure drop and interfacial momentum transfer) cannot be scaled at the bend/riser/entrance region because of the influence of the channel diameter upon occurring CCFL mechanism. Meanwhile, the water level gradient can be similar at the horizontal part, but not the relative water depth.

Cyclone separator for gas-liquid mixture with high flux density

The study is aimed at improvement of efficiency of a gas-liquid separator operating at the pressure of 2.5 MPa and high flux density in the cyclone body. The submitted schematic construction of the separator provides the reduced dynamic head of gas in the zones with increased liquid phase concentration and prevents the entrainment of separated liquid to the outlet receiver. The receiver consisting of a conical converging channel and an externally finned circular pipe is located in the middle of cylindrical cyclone body. A fin-and-tube flow conditioner is mounted upstream of the converging channel inlet. Detailed flow structure in the separator has been obtained from numerical study based on Reynolds-averaged Navier–Stokes equations with anisotropic Reynolds stress turbulence model. Simulation results were involved in estimation of the main performance characteristics of the separator and probability of droplet entrainment and upwards flowing film. Experimental research on a 1:2 separator model allowed estimation of separation efficiency and flow resistance coefficient of the separator.

Simulation of a counter-current horizontal gas-liquid flow experiment at the WENKA channel using a droplet entrainment model

Usually droplet formation mechanisms are missing in standard fluid dynamics codes. However, for self-generating waves and slugs, the interfacial momentum exchange and the turbulence parameters ought to be modelled adequately. Furthermore, knowledge and thinking about the mechanism of droplet entrainment for heat and mass transfer processes is of high importance in the nuclear industry.Consequently a step of progress of modelling liquid/gas interfaces is the consideration of droplet entrainment model. The suggested model assumes that due to liquid turbulence the interface gets uneven and wavy leading to the creation of droplets. The model is validated against existing horizontal two-phase flow data from the WENKA (Water ENtrainment Channel KArlsruhe) channel. Experiments were taken out for an air/water system at ambient pressure and temperature. High speed videometry was used to get velocities from flow pattern maps of the counter current flow. In the horizontal part of the channel with partially reversed flow the fluid velocities were conducted by planar particle image velocimetry. An experiment with droplet generation at the reversed flow conditions was utilized to compare it with the simulation data. The agreement of the experimental findings and CFD results is more than satisfactory. Also the droplet mass flow was compared and showed the applicability of the droplet entrainment model. Further work is necessary to validate the model for different flow conditions.

Droplet entrainment and deposition rates in a horizontal annular flow for SPACE code

Correlations for the droplet entrainment and deposition rates were developed to accurately predict its behavior in a horizontal annular flow. The Lopez de Bertodano et al. droplet entrainment rate correlation was evaluated and then modified based on the experimental data for both vertical and horizontal annular flows. In addition to this, a new droplet deposition coefficient correlation was proposed for a droplet deposition rate. The droplet deposition coefficient is represented by a function of dimensionless parameters considering the effects of the gas and liquid superficial velocities and the pipe inner diameter. Finally, the SPACE code was assessed against various experimental data for droplet mass flow rate of horizontal annular flows obtained from the open literature. The improved SPACE code showed better prediction capability for the droplet behavior in the horizontal annular flow than the original one.

Investigation of falling-film plate wettability characteristics under dehumidification and regeneration conditions using LiCl-H2O

Liquid desiccant air conditioning system has unique advantages and prospects in both humidity control and low-grade heat driven cooling. The liquid film wettability is one of the significant factors governing the efficiency of dehumidification/regeneration processes in the system. In this paper, the effect of the LiCl solution properties on falling-film plate wettability characteristic is studied using a VOF method. This paper analyzes the dehumidification and regeneration processes respectively and gets the recommended amount of spray, which was 0.619 m3/(m·h) for dehumidification and 0.662 m3/(m·h) for regeneration. The recommended amount of spray can not only ensure the full-film flow, but also avoid the excessive solution flow rate which may increase the possibility of droplets entrainment. Since the wettability is determined by the coupled effects of the solution density, viscosity and surface tension, the effects of dynamic properties of liquid desiccant on the wettability are studied to explain the influence of the solution temperature and concentration on the wettability. Finally, least squares method is used to fit a formula to describe the effect of solution properties on falling-film plate wettability, which can guide the control of the amount of spray in actual operation.

Optical analysis of the liquid layer combustion of paraffin-based hybrid rocket fuels

Liquefying hybrid rocket fuels (e.g. paraffin) enable higher regression rates due to the presence of an unstable melt layer on the fuel surface during combustion, which causes entrainment of liquid droplets into the oxidizer gas flow. In order to better understand the mechanism responsible for the droplets entrainment, the combustion behaviour of paraffin-based hybrid rocket fuels in combination with gaseous oxygen (GOX) was investigated in the framework of this research. Tests were performed in a 2D slab burner configuration at atmospheric conditions. High-speed videos were recorded and analysed with two different decomposition techniques, applied to the scalar field of the flame luminosity (the flame front is assumed to follow the liquid layer). The fuel slab composition and configuration and the oxidizer mass flow have been varied in order to study the influence of these parameters on the liquid layer instability process. The main focus of the research is to understand the relation between the unstable waves which enable the droplets entrainment process and the regression rate. The results show that the combustion is dominated by periodic, wave-like structures for all the analysed fuels. The frequencies and the wavelengths characterizing the liquid melt layer depend on the fuel viscosity and geometry and on the oxidizer mass flow. Moreover, a dependency of the regression rate on the most excited frequencies and longitudinal wavelengths was found. This is important to better understand the relation between the increased regression rate and the onset and development of the entrainment process, which is connected to the amplification of longitudinal unstable waves caused by the high velocity gas flow over the fuel surface.

Hydrodynamics and Thermal Interaction with Air at Water Jet Blow Against Its Surface in the Limited Volume with Scattering of the Received Drops

Background. Air heating in winter in the range of ambient temperatures below -5°С is possible with heat of the phase transition of water into ice. Heated air reduces the energy consumption in ventilation systems and air–water and air–air heat pumps during peak loads on the heating system reducing the installed capacity of the heat generating equipment. For water cooling and nozzle irrigation the water should be dispersed into droplets, and through contact with fast airflow drops shouldn’t be carried away by the flow, that is achieved by increasing the diameter of drops.Objective. The aim of the paper is determination of parameters of jet flowing and crushing on drops and evaluation of the maximum radius of droplet flight depending on the geometric characteristics of the vessels (diameter and depth) and the water flow through the nozzle from which the jet emerges. It is also planned to study the possibility of heating and cooling water with air sucked by the jet.Methods. The air temperature below -5°С is heated in the conditions of forced flow with no entrainment of droplets by a jet. The lower the air temperature, the greater the economic and energy effect.Results. We experimentally investigated the destruction processes of the water jet on the primary drop with the definition of the jet destruction beginning and the diameter of primary drops. Moreover, we have carried out experimental studies on the dependence of the maximum radii of the secondary droplets from the destruction of air bubbles formed by the water jet impact on the water surface saturated with air, depending on the diameters and heights of the water vessels.Conclusions. It was found that the decrease in the diameter of the vessels and the liquid layer, as well as the increase in the water flow through the nozzle, contribute to the drop spread to large radii. From a physical point of view, this effect should be explained by a smaller loss of jet energy with an increase of the horizontal and vertical dimensions of the vessel and an increase of air pressure in the bubbles.

Wet resuspension modelling and validation

Abstract An empirical correlation for the estimation of the droplet entrainment from boiling water pools is under development. The correlation is validated on the basis of experiments, taken from test facilities of different scale, which investigate the effect of several boundary conditions on the droplet entrainment. The calculated values for the droplet entrainment are in good agreement with the experimental data, indicating that relevant influences are considered in the developed correlation with an appropriate quantity. Nevertheless, the application of the correlation on scenarios with flashing pools and non-boiling pools with gas flows through the pool surface have to be investigated in the ongoing validation process. Furthermore, other experiments are needed, which consider pools containing surface-active substances to depict the influence of surface tension on the droplet entrainment.

Experimental study on the dryout point and post-dryout heat transfer in square channel

Blanket is one of the most important safety components for heat transfer in fusion reactor. Under accident conditions, two-phase fluid boiling flow in square channel of blanket is extremely efficient for cooling of fusion reactor core. The second kind of heat transfer deterioration usually occurs when the quality is high such as annular flow in the boiling process. The liquid film is depleted for the entrainment of liquid droplets and the evaporation, and no longer covers the wall. The heat transfer between the fluid and the channel wall deteriorates, leading to the boiling crisis called dryout (DO) and a rapid rise in wall temperature when the wall heat flux reaches critical heat flux (CHF), which could damage the channel wall. Therefore, accurate prediction of dryout occurrence and the post-dryout heat transfer are crucial to the optimized design and safety operation of fusion reactor. For square channel in blanket, the experimental study of dryout has been conducted. Experimental results show that the dryout heat flux increases with the increase of mass velocity and with the decrease of inlet vapor quality. Furthermore, with the increase of heat flux, the location of dryout point move to inlet of test section and the wall temperature in dryout point increases gradually. Based on the obtained experimental data, the correlation for prediction of critical vapor quality (XDO) in the square channels is proposed, which is compared with the predictions obtained by other correlations.

Investigation of the Role of Droplet Transport in Mitigating Top of the Line Corrosion

The entrainment of liquid droplets, occurring in a limited range of gas and liquid flow conditions within the stratified flow region, could represent an effective way to transport a non-volatile liquid corrosion inhibitor through the gas phase and combat top of the line corrosion (TLC). However, such an approach is only viable if the inhibitor can reach the top of the pipe and deposit at a rate higher than the local rate of condensing water can dilute it. This work presents a combined modeling and experimental methodology to determine the onset of droplet entrainment from the bottom and deposition at the top of the line. A modeling approach predicting the droplet entrainment onset is proposed and validated against new multiphase flow data recorded in a large scale flow loop, at operating conditions similar to those encountered in gas-condensate production facilities. Additionally, TLC experiments were performed in the same flow loop under simulated water condensation conditions to measure the actual corrosion at different rates of inhibiting droplet deposition. The results confirm that the droplet entrainment/deposition can effectively mitigate TLC when operating parameters are accurately controlled.

Understanding Kelvin\u2013Helmholtz instability in paraffin-based hybrid rocket fuels

Liquefying fuels show higher regression rates than the classical polymeric ones. They are able to form, along their burning surface, a low viscosity and surface tension liquid layer, which can become unstable (Kelvin–Helmholtz instability) due to the high velocity gas flow in the fuel port. This causes entrainment of liquid droplets from the fuel surface into the oxidizer gas flow. To better understand the droplets entrainment mechanism, optical investigations on the combustion behaviour of paraffin-based hybrid rocket fuels in combination with gaseous oxygen have been conducted in the framework of this research. Combustion tests were performed in a 2D single-slab burner at atmospheric conditions. High speed videos were recorded and analysed with two decomposition techniques. Proper orthogonal decomposition (POD) and independent component analysis (ICA) were applied to the scalar field of the flame luminosity. The most excited frequencies and wavelengths of the wave-like structures characterizing the liquid melt layer were computed. The fuel slab viscosity and the oxidizer mass flow were varied to study their influence on the liquid layer instability process. The combustion is dominated by periodic, wave-like structures for all the analysed fuels. Frequencies and wavelengths characterizing the liquid melt layer depend on the fuel viscosity and oxidizer mass flow. Moreover, for very low mass flows, no wavelength peaks are detected for the higher viscosity fuels. This is important to better understand and predict the onset and development of the entrainment process, which is connected to the amplification of the longitudinal waves.

Droplets entrainment ratio in a PWR hot-leg pipe geometry

Droplets entrainment was found to contribute to the occurrence of countercurrent flow limitation (CCFL) at COLLIDER test facility which has a PWR hot-leg pipe geometry with a large diameter value of 190 mm. Previous investigations showed that droplets entrainment starts to become observable at medium water inlet velocities (In Wallis parameters Jw,in∗0.5>0.2). In current article, the occurrence of droplets entrainment at COLLIDER test facility is discussed along with a detailed descriptions of visual observations that corresponds to entrainment mechanisms at the junction between the hot-leg and the steam generator. These mechanisms are compared to typical mechanisms found in literature for annular flows. The significance of COLLIDER large-diameter and pipe geometry against rectangular and narrow channel geometry is explained. Experimental measurements of the droplet entrainment ratio at COLLIDER test facility (using air/water at atmospheric pressure) are introduced and discussed. Measurements cover a range of air superficial velocities between 10.6 and 15.2 m/s, and water superficial velocities between 0.05 and 0.12 m/s. A correlation of the critical air velocity at which the droplets entrainment start to occur is given. An empirical correlation of the entrainment ratio was developed for obtained data. The correlation shows a stronger dependency upon the water inlet velocity in comparison to correlations found in literature for annular flows. A comparison between the prediction of available entrainment ratio correlations in literature and COLLIDER experimental data is made. The comparison shows the need of a new correlation for the droplets entrainment that takes place at the entrance of a hot-leg pipe geometry. The obtained correlations are useful to predict the onset of entrainment and to quantify its possible contribution to the reduction of water delivery into the reactor core during the occurrence of an SBLOCA accident. The last result becomes of a special importance knowing that CCFL caused by droplets entrainment occurs at lower air velocities than those at which a typical CCFL occurs (caused by large roll-waves at the bend), and that droplets entrainment does not require a transition from a supercritical into a subcritical flow condition in order to take place.

Flow dependence of steel corrosion in supercritical CO2 environments with different water concentrations

The corrosion behavior of mild steel in water contaminated supercritical CO2 fluids was investigated by weight-loss measurements and surface morphology observations. It is found that a higher flow rate induces a higher corrosion rate in various water containing conditions, which becomes much obvious with increasing the water concentration. By a detailed comparison of the surface corrosion morphologies formed in different supercritical CO2 fluids, the flow dependence is attributed to a mechanism of water droplet entrainment and its impact on steel surfaces. This observation is helpful for the understanding and the control of steel corrosion in CO2 transportation and injection conditions.

Development of droplet entrainment and deposition models for horizontal flow

Abstract Models for the rate of atomization and deposition of droplets for stratified and annular flow in horizontal pipes are presented. The entrained fraction is the result of a balance between the rate of atomization of the liquid layer that is in contact with air and the rate of deposition of droplets. The rate of deposition is strongly affected by gravity in horizontal pipes. The gravitational settling of droplets is influenced by droplet size: heavier droplets deposit more rapidly. Model calculation and simulation results are compared with experimental data from various diameter pipes. Validation for the suggested models was performed by comparing the Safety and Performance Analysis Code for Nuclear Power Plants calculation results with the droplet experimental data obtained in various diameter horizontal pipes.

Pressure drop for gas-liquid two-phase annular flow across grid spacer with mixing vane in vertical circular pipe

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, the correlation of the pressure drop across the spacer is newly developed based on the present data.

Prediction of friction pressure drop for low pressure two-phase flows on the basis of approximate analytical models

Wide use of natural circulation loops operating at low redused pressures generates the real need to develop reliable methods for predicting flow regimes and friction pressure drop for two-phase flows in this region of parameters. Although water–air flows at close-to-atmospheric pressures are the most widely studied subject in the field of two-phase hydrodynamics, the problem of reliably calculating friction pressure drop can hardly be regarded to have been fully solved. The specific volumes of liquid differ very much from those of steam (gas) under such conditions, due to which even a small change in flow quality may cause the flow pattern to alter very significantly. Frequently made attempts to use some or another universal approach to calculating friction pressure drop in a wide range of steam quality values do not seem to be justified and yield predicted values that are poorly consistent with experimentally measured data. The article analyzes the existing methods used to calculate friction pressure drop for two-phase flows at low pressures by comparing their results with the experimentally obtained data. The advisability of elaborating calculation procedures for determining the friction pressure drop and void fraction for two-phase flows taking their pattern (flow regime) into account is demonstrated. It is shown that, for flows characterized by low reduced pressures, satisfactory results are obtained from using a homogeneous model for quasi-homogeneous flows, whereas satisfactory results are obtained from using an annular flow model for flows characterized by high values of void fraction. Recommendations for making a shift from one model to another in carrying out engineering calculations are formulated and tested. By using the modified annular flow model, it is possible to obtain reliable predictions for not only the pressure gradient but also for the liquid film thickness; the consideration of droplet entrainment and deposition phenomena allows reasonable corrections to be introduced into calculations. To the best of the authors' knowledge, it is for the first time that the entrainment of droplets from the film surface is taken into consideration in the dispersed–annular flow model.

Liquid droplet entrainment in two-phase oil-gas low-liquid-loading flow in horizontal pipes at high pressure

An experimental study is conducted in a 152.4-mm inner diameter (ID) high-pressure facility to investigate the liquid droplet entrainment in two-phase oil-gas flow in horizontal pipe under low-liquid loading condition. Total of 247 tests are carried out at five different operating pressures, namely, 1.48, 1.82, 2.17, 2.51, and 2.86 MPa. The test fluids are nitrogen and mineral oil (Isopar-L). Superficial oil velocity (vSL) ranges from 0.01 to 0.05 m/s, while superficial gas velocity (vSg) ranges from 1.6 to 16.7 m/s depending on the operating pressures. An isokinetic sampling system is used to measure the entrained droplet flux at three different vertical positions in the gas phase. A high-speed video camera is used to visually observe and estimate the onsets of entrainment. This paper provides a unique entrainment data set for two-phase flow in a large diameter pipe at high operating pressure.The experimental data are analyzed to evaluate the effects of high operating pressure and superficial velocities. It is found that the entrainment fraction increases with the operating pressure. In addition, predictions of ten correlations for two-phase entrainment fraction are compared with the experimental data to evaluate the performances. The results show that these correlations over-predict the entrainment fraction data from this study.

A prediction method for entrained liquid fraction in adiabatic gas-liquid flow at high reduced pressure

A goal of the study is development of an approximate but well-grounded model of entrainment/deposition processes in annular two-phase flow at high reduced pressures (p/pcr>0.45). Nakazatomi and Sekoguchi (1996) have presented the unique experimental data on liquid distribution between the core and the film in air/water two-phase flow at high pressures, up to 20 MPa; at pressures higher 10 MPa the data feature with abnormally high fraction of entrained liquid and manifest very strong deviation from any known empirical correlations, including the recent one by Cioncolini and Thome (2012). In deducing the approximate model of droplets entrainment, we used the experimental observations, according to which a liquid film becomes thin and smooth at high reduced pressures. A plenty of tiny droplets detach from the liquid film surface at the points, which spacing is determined as a length scale in Weber number for gas flow. This spacing and the liquid film thickness are assumed being the parameters controlling a droplet departure diameter. These assumptions allow developing an equation for calculating entrainment intensity at high reduced pressures. A balance between the flows of droplets entrainment and deposition due to turbulent diffusion corresponds to the dynamic equilibrium. The equation based on this balance contains one unknown numerical factor and allows one to calculate liquid distribution in a channel cross section. Comparing the calculation results with the experimental data for the water–air flows at high reduced pressures (more than 0.45) has shown their good agreement at the universal value of the numerical constant.

Dryout quality prediction for boiling two-phase flow in vertical helically coiled tubes

An analytical model for the prediction of the onset of dryout quality in annular flow regime in helically coiled tubes is present in this paper. According to the liquid film dryout mechanism, film dryout is primarily dependent on the competition between droplet entrainment and deposition with a certain heat flux. To this effect, it is necessary to define the net entrainment rate in the helical tube. The factors significantly affecting the droplet entrainment and deposition characteristics in the helical tube are analyzed theoretically. Three dimensionless numbers are used to modify the net entrainment rate in straight tube. A total of 252 experimental points from 6 different datasets are used to test the performance of the model. Comparisons between the calculated and experimental results in literatures indicate that the present model has good predictive capability in predicting onset of dryout in helically coiled tubes.