High-speed Gas Stream Research Articles

Deformation and aerobreakup of RP-2 droplets from hypersonic shock waves

The shock-induced atomization and burning of liquid fuel droplets are paramount for hypersonic propulsion and detonation-based combustion systems. The present work experimentally explores the atomization and aerobreakup of liquid RP-2 droplets in a hypersonic shock tube. The shock tube is operated with normal shocks propagating between Mach 5–10 interacting with RP-2 droplets with diameters between 200 and 300 µm. The droplet deformation and breakup dynamics are characterized by ultra-high-speed 5 MHz shadowgraph imaging diagnostics. For all hypersonic shock-droplet interactions, the droplets first undergo structural deformations with subsequent breakup occurring from a shear-stripping mechanism along the periphery of the droplet. The structural changes and droplet deformation rate are found to be self-similar among all cases, and the rate of deformation collapsed to a unified non-dimensional timescale. The complete breakup time of the fuel droplets were analyzed, and theoretical scaling arguments were used to quantify the effects of the Mach number, Weber number, and Ohnesorge number. The experimental data is then used to develop a modified boundary layer stripping model to estimate the breakup time of liquid drops. The model leverages the concept of displacement and momentum thickness to provide a numerical solution to estimate droplet breakup times when exposed to a high-speed gas stream. The model is compared to the experimental RP-2 data and water data available in the literature. The model predicts the breakup time for a range of droplet sizes between 200 and 2000 µm within a reasonable accuracy. The results can be used to optimize liquid fuel injection for hypersonic and detonation-based combustion systems.

Dynamic behavior of droplet formation in dripping mode of capillary flow focusing

Experimental study on the liquid dripping in a capillary flow focusing process is performed. Due to the high-speed gas stream that drives the inner liquid co-flowing through an orifice, complex phenomena for the droplet formation in dripping regime can be found as the gas pressure drop and the liquid flow rate change. Periodic dripping mode can produce uniform droplets, and non-periodic ones can result in satellites and droplets of different diameters. The droplet-droplet coalescence in the core of co-flowing gas stream is also obtained. The size of resultant droplets is measured under different values of gas pressure drop and liquid flow rate. It can be seen that the droplet size tends to decrease as the gas pressure drop increases and keeps nearly the same as the liquid flow rate increases. The results also indicate that the dynamic behavior of droplet formation in dripping mode of capillary flow focusing is mainly dominated by the gas pressure drop, and the capillary flow focusing technique can produce droplets with high throughput even in the dripping regime. Cited as: Si, T. Dynamic behavior of droplet formation in dripping mode of capillary flow focusing. Capillarity, 2021, 4(3): 45-49, doi: 10.46690/capi.2021.03.01

Investigation of turbulent inflow specification in Euler–Lagrange simulations of mid-field spray

The process of atomization of a liquid jet by a parallel high-speed gas stream results in a spray, whose downstream development is of considerable interest to several applications. The round jet spray can be spatially divided into (i) a near-field (near-nozzle) region of liquid atomization and (ii) a downstream mid-field region of fully-dispersed droplets. In order to accurately model mid-field droplet dispersion, this work aims at developing a rigorous and robust injection model for Euler–Lagrange spray simulations. Results from experiments are used to obtain the relevant droplet number density, size distribution, and mean and standard deviation velocity distributions of the injection model, systematically in a step-by-step process. Two-phase large eddy simulations are performed by stochastically generating the Lagrangian droplets at the inlet of the mid-field region. Number flux, diameter distribution, mean velocity, and other time-averaged statistics at several downstream locations are shown to agree well with the corresponding experimental data.

Physical parameters of the energy-carrying phase during crushing of a jet of slag melt

In this paper, the possibility of introducing slag in the fiery-liquid state into the metallurgical cycle with the production of different products depending on the processing technology (binders, slag, pumice, etc.) is considered. The scientific substantiation of the processing of slag melts by crushing a slag stream in order to obtain granules of a certain size is considered, and the process of deformation and crushing (decay) of a slag stream during interaction with the energy-carrying phase - high-speed gas (air) stream is analyzed. It has been established that under conditions of dynamic interaction with an air high-speed flow, the slag jet in the deformable section behaves like a quasi-elastic body, exhibiting properties characteristic of a solid, which explains the destruction of the jet under the action of a longitudinal tensile and deforming slag stream due to the pressure of the gas stream. Taking into account it the sizes of destroying pressure and corresponding for this purpose speed of current of air are set depending on a temperature and diameter of stream of slag fusion. It was found that the pressure and air flow rate increase with increasing diameter of the slag stream and decrease with increasing temperature of the slag. The got performances hired can be used for the choice of optimal parameters of slag fusion and energy-carrying phase at development of technological processes of processing of metallurgical slags with the forecast receipt of friction composition of eventual products. Mathematical models are neat for the got dependences with approximation of dependence of maximum pressure of current of air from the physical parameters of slag fusion and the quadratic is got and sedate to dependence with the high coefficient of determination, exceeding 0,98

Investigation of the hydraulic efficiency of gathering gas pipelines

The current state of two gathering gas pipelines was analyzed in the article: the first – from the gas gathering station (GGS) of the Narizhnianske oil and gas condensate field (OGCF) to the complex gas treatment unit-2 (CGTU-2) of the Yuliivske OGCF; the second – from the CGTU-1 of the Skvortsivske OGCF to the CGTU-2 of the Yuliivske OGCF. At different times of the year the hydraulic efficiency of gathering gas pipelines was determined, which was in a wide range from 75 % to 98 % on the first, and from 69 % to 96 % on the second gathering gas pipeline respectively. The operating modes of gathering gas pipelines were studied. The volume of contamination in the fields was determined using calculations. The industrial research to measure the volume of contamination, transported together with gas on the CGTU-2 measuring lines, was performed in separators and segregators. It was established that the volume of contamination, determined based on the results of experimental investigations, was higher than the calculated value. The main factors that negatively influence the hydraulic efficiency of gas pipelines were defined. The monitoring of these main factors will prevent the reduction of the hydraulic efficiency coefficient and decrease in the volume of transported gas, it will also help to achieve the planned production figures. The authors determined the main reasons for the reduction in the hydraulic efficiency of gathering gas pipelines resulting from the accumulation of liquid contaminants in the inner cavity and hydrates formation. The proposed solutions to these problems were introduced and are still used in production. In particular, a surfactant solution is fed using a mobile pumping unit, a high-speed gas stream is created to remove the accumulated liquid from the internal cavity of field gas pipelines operating with the CGTU-2 of the Yuliivske OGCF and methanol is supplied using the available pumps from the GGS of the Narizhnianske OGCF and CGTU-1 of the Skvortsivske OGCF. The authors proposed a series of measures to prevent the emergencies and failures and ensure the reliable operation of gathering gas pipelines. The consistency of performed investigations will help to estimate the occurrence probability of complications caused by the accumulation of liquid and hydrates formation.

Destabilization of a planar liquid stream by a co-flowing turbulent gas stream

In Airblast atomization, the disintegration of a liquid stream is assisted by a co-flowing high speed gas stream. The shear instability develops at the interface, forming interfacial waves that propagate and eventually break into droplets downstream. In the present study, the destabilization of a planar liquid stream by a co-flowing gas stream with different turbulence intensities is investigated through direct numerical simulation. A parametric study is conducted to investigate the effect of gas inlet turbulence intensity on the interfacial instability near the nozzle exit and the development of two-phase mixing layer downstream. The gas-liquid interface is resolved by a momentum-conserving volume-of-fluid method. A digital filter approach is used to generate temporally and spatially correlated turbulent velocity fluctuations at the gas inlet. The interfacial stability is absolute for all cases considered. The dominant frequency and the spatial growth rate corresponding to the most unstable mode are measured and compared with experiments and spatial-temporal linear stability analysis. The numerical results of the dominant frequencies agree well with the experimental data. Both the dominant frequency and the spatial growth rate increase with the gas inlet turbulence intensity, after passing a threshold value. The linear stability analysis with the turbulent eddy viscosity model captures the increasing trends of the dominant frequency and the spatial growth rate but significantly underpredicts the values. Two-phase turbulence statistics, including Reynolds stresses and turbulent kinetic energy dissipation, are also presented. It is shown that the gas inlet turbulence tends to enhance interaction between the two streams near the nozzle exit and to reduce the interaction downstream.

Simulation of liquid micro-jet in free expanding high-speed co-flowing gas streams

We present the development of an experimentally validated computational fluid dynamics model for liquid micro jets. Such jets are produced by focusing hydrodynamic momentum from a co-flowing sheath of gas on a liquid stream in a nozzle. The numerical model based on laminar two-phase, Newtonian, compressible Navier–Stokes equations is solved with finite volume method, where the phase interface is treated by the volume of fluid approach. A mixture model of the two-phase system is solved in axisymmetry using ~ 300,000 finite volumes, while ensuring mesh independence with the finite volumes of the size 0.25 µm in the vicinity of the jet and drops. The numerical model is evaluated by comparing jet diameters and jet lengths obtained experimentally and from scaling analysis. They are not affected by the strong temperature and viscosity changes in the focusing gas while expanding at nozzle outlet. A range of gas and liquid-operating parameters is investigated numerically to understand their influence on the jet performance. The study is performed for gas and liquid Reynolds numbers in the range 17–1222 and 110–215, and Weber numbers in the range 3–320, respectively. A reasonably good agreement between experimental and scaling results is found for the range of operating parameters never tackled before. This study provides a basis for further computational designs as well as adjustments of the operating conditions for specific liquids and gases.

Review of domestic designs in the field of protecting carbonaceous materials against gas corrosion and erosion in high-speed plasma fluxes

The article is a continuation of the publication cycle of the authors the on the theme “Multifunctional Protective Coatings for Especially Thermally Loaded Elements of Constructions of Hypersonic Systems.” Modern domestic approaches to the formation of single-layer and multilayered high-temperature protective coatings of various classes such as glass–ceramics ones obtained by various methods, coatings based on oxide ceramics, oxygen-free reaction-coupled coatings, and microcomposition coatings of the synergetic type are analyzed. Attention is paid to the fact that the scientific-and-applied investigation directed at the development of the structural and physicochemical models of the coating architecture and substantiation of their functioning with high-speed gas streams are almost absent. The selection of the chemical composition of coatings and production procedures of their formation is performed mainly based on the empirical basis. A series of coatings claimed as workable in conditions in a medium of tranquil or weakly perturbed air up to temperatures of 1600–2000°C is presented; however, their protective ability in conditions of the effect of high-enthalpy supersonic and hypersonic gas streams cannot be judged because of the insufficiency or absence of required data. Microcompositional coatings of the synergetic type based on the Si–TiSi2–MoSi2–B–Y system, which were developed in terms of an original conceptual approach to the formation of a multilevel system of heat protection of thermally stressed constructional elements made of heat-resistant materials of hypersonic aircrafts and their propulsion systems, are especially emphasized. A detailed analysis of their functioning in the composition of a unique construction wall with protected carbonaceous materials will make it possible to determine the tendencies and directions of subsequent investigations by the authors with the purpose of improving the developments.

Steady cone-jet mode in compound-fluidic electro-flow focusing for fabricating multicompartment microcapsules

A compound-fluidic electro-flow focusing (CEFF) process is proposed to produce multicompartment microcapsules. The central device mainly consists of a needle assembly of two parallel inner needles and one outer needle mounted in a gas chamber with their tips facing a small orifice at the bottom of the chamber. As the outer and the inner fluids flow through the needle assembly, a high-speed gas stream elongates the liquid menisci in the vicinity of the orifice entrance. An electric field is further integrated into capillary flow focusing to promote the formation of steady cone-jet mode in a wide range of operation parameters. The multiphase liquid jet is broken up into droplets due to perturbation propagation along the jet surface. To estimate the diameter of the multiphase liquid jet as a function of process parameters, a modified scaling law is derived and experimentally validated. Microcapsules of around 100 μm with an alginate shell and multiple cores at a production rate of 103–105 per second are produced. Technical feasibility of stimulation triggered coalescence and drug release is demonstrated by benchtop experiments. The proposed CEFF process can be potentially used to encapsulate therapeutic agents and biological cargos for controlled micro-reaction and drug delivery.

Molecular Dynamics Simulation of Ti and Ni Particles on Ti Substrate in the Cold Gas Dynamic Spray (CGDS) Process

This paper presents simulation of molecular dynamics for the deposition of Titanium (Ti) and Nickel (Ni) particles on Ti substrate during Cold Gas Dynamic Spray (CGDS) process. The influencing factors of the deposition process, such as particle incident velocity, particle size and particle temperature are taken into consideration. Ti and Ni were selected because of their potential applications in the aerospace, marine and bio-medical industries. CGDS is preferred because it is a state of the art technique by which coatings are created without significant heating of the sprayed powder. In CGDS, particles are accelerated to supersonic velocities using a high speed gas stream. However, there are inherent difficulties in relating particle deposition characteristics with influencing factors of the deposition process. Moreover, there is limited literature on molecular dynamics simulation of CGDS process. For this reason, this paper develops a simulation process for Ti and Ni particles under influence of many factors using molecular dynamics. In this process, particles are allowed to interact for a short time, giving a view of their motion. The trajectories of these particles are determined by numerically solving the Newton's equations of motion for a system of interacting particles, in which the forces between the particles are defined. The results of the simulation process show that higher incident velocities and larger particle sizes result in stronger interface between the particle and the substrate. Further, higher temperatures of the substrate and particles improve the bond strength.

Melt-Blown and Electrospun Drug-Loaded Polymer Fiber Mats for Dissolution Enhancement: A Comparative Study

Melt blowing (MB) was investigated to prepare a fast dissolving fibrous drug-loaded solid dispersion and compared with solvent-based electrospinning (SES) and melt electrospinning (MES). As a conventional solvent-free technique coupled with melt extrusion and using a high-speed gas stream, MB can provide high-quality micro- and nanofibers at industrial throughput levels. Carvedilol, a weak-base model drug with poor water solubility, was processed using a common composition optimized for the fiber spinning and blowing methods based on a hydrophilic vinylpyrrolidone-vinyl acetate copolymer (PVPVA64) and PEG 3000 plasticizer. Scanning electron microscopy combined with fiber diameter analysis showed diameter distributions characteristic to each prepared fibrous fabrics (the mean value increased toward SES<MB<MES). Differential scanning calorimetry and X-ray diffraction studies revealed that the incorporated drug was in amorphous form regardless the preparation method. The HPLC studies demonstrated that all of the materials produced by the different techniques passed the regulatory purity requirements. The fibers exhibited ultrafast drug release tested under neutral pH conditions; the melt-blown sample dissolved within 2 min owing to its large specific surface area. The presented results confirm the applicability of MB as a novel formulation technique for polymer-based drug delivery systems.

Formation of steady compound cone-jet modes and multilayered droplets in a tri-axial capillary flow focusing device

A novel method for forming steady cone-jet modes and multilayered droplets is developed based on capillary flow focusing geometry using tri-axial needles. The needles manufactured by a laser beam welding process can be readily cleaned, assembled and aligned. A tri-axial capillary flow focusing (TCFF) device is assembled in order to focus a combination of different liquids in the core of a high-speed gas stream co-flowing through a small orifice. The effects of main control process parameters on the characteristics of compound cone-jet mode are studied. Under appropriate working conditions, steady cone-jet configurations with three layers of liquids are obtained, resulting in multilayered droplets with outer shell, middle layer and inner core. The process can also produce monodisperse droplets at smaller scales by releasing the outer shell and the middle liquid. The new design of tri-axial needles and TCFF devices enables reliable encapsulation of multiple components in one shell for potential applications in multimodal imaging, drug delivery, material processing and biomedicine.

Temporal instability of coflowing liquid-gas jets under an electric field

Temporal instability of an electrified liquid jet in the core of a high-speed gas stream is studied to better understand the electro-flow focusing (EFF) technique. Two types of physical models with and without viscosities of fluids are considered. One utilizes uniform basic flow and an axial electric field, while the other one considers both the axial and radial electric fields and employs appropriate velocity profiles based on the pipe flow for inner liquid and the error function for outer gas stream. Both models demonstrate that the axisymmetric instability and the helical instability are two most unstable modes in the EFF problem. The significance of free charge initially imposed on the interface is highlighted and the effects of surface tension and liquid viscosity on the jet instability are also studied. It is shown that the increase of free charge density can definitely promote both the axisymmetric and helical instabilities and the transition between them arises for sufficiently large free charge densities when the axial electric field intensity increases. Finally, the EFF experiments are carried out to compare with theoretical predictions in the temporal instability analysis and a good agreement between them is achieved.

Ablation mechanism and properties of in-situ SiAlON reinforced BN–SiO2 ceramic composite under an oxyacetylene torch environment

Ablation mechanism and properties of in-situ SiAlON reinforced BN–SiO2 ceramic composites have been carefully studied under different ablation time. Results showed that the ablation properties depend on the ablation time. When the ablation time increases from 10s to 60s, the mass ablation rate increases from 0.20mg/s to 6.24mg/s and the linear ablation rate increases from 9.0μm/s to 28.1μm/s. The phases of ablation products mainly consist of amorphous SiO2, Mullite, Corundum and B2O. Four time-dependent ablation behaviors contributed to the various ablation regions: oxidation of matrix which includes BN and Si2Al3O7N; evaporation of B2O3(l) and softening of SiO2(s); scouring away of SiO2(l) and Al2O3(l) by high-speed gas stream; evaporation of SiO2(l) and Al2O3(l).

Laws governing the fragmentation of a droplet in a high-speed stream

A common solution of the equations of ablation and motion of a droplet undergoing fragmentation in a homogeneous high-speed gas stream is obtained. Based on the approximate solution of the equation for the quantity of torn off droplets, an expression for the droplet size distribution function corresponding to these laws has been found. A comparison of the results of calculations carried out for the obtained distribution function with the same calculations for the earlier found function with the use of the empirical law of droplet motion points to their satisfactory agreement.

Effect of flow conditions on spray cone angle of a two-fluid atomizer

A visual study is conducted to determine the effects of operating conditions on the spray cone angle of a two-fluid atomizer. The liquid (water) jets exit from peripheral inclined orifices and are introduced into a high-speed gas (air) stream in the gravitational direction. Using a high-speed imaging system, the spray cone angle is determined for Reynolds numbers ranging from 4×104 to 9×104 and different Weber numbers up to 140. The droplet sizes (Sauter mean diameter) and their distributions are determined using a Malvern Mastersizer X. The results show that the spray cone angle depends on the operating conditions, especially in lower values of Reynolds and Weber numbers. An empirical correlation is also obtained to predict the spray cone angle in terms of these two parameters.

Two-Dimensional Simulation of Stripping Breakup of a Water Droplet

The breakup of a liquid droplet induced by a high-speed gas stream is a known multiphase flow problem. Among various breakup modes of liquid droplets impinged by shock waves, the stripping breakup takes place over a wide range of Weber numbers, from 100 to approximately 20,000. In this study, the evolution of the stripping breakup of a water droplet is simulated by using a multiphase flow solver with a five-equation model. Several test cases such as gas-gas shock tube, water-air shock tube, and underwater explosion problems are performed to validate the present numerical methods. To compare with the experimental results, the water droplets with diameters of 6.4 and 4.8 mm and Mach numbers of 1.3 and 1.47 are chosen in this study. The stripping breakup of a water droplet, including the shape deformation, vortex shedding, unsteady drag force, and flow instability, is investigated. The computed displacement, acceleration, and volume change of the water droplet are in agreement with the experimental data in dimensionless form. The evolution of a water droplet during the stripping breakup for an inviscid flow is presented by flow visualization.

Flow visualization of shock/water column interactions

The breakup of a liquid droplet induced by a high speed gas stream is a typical multiphase flow problem. The shock/droplet interaction is the beginning stage of the droplet breakup. Therefore, investigation of the shock/droplet interactions would be a milestone for interpreting the mechanism of the droplet breakup. In this study, a compressible multiphase solver with a five-equation model is successfully developed to study shock/water column interactions. For code validation, interface-only, gas–gas shock tube, and gas–liquid shock tube problems are first computed. Subsequently, a planar shock wave interacting with a water column is simulated. The transmitted wave and the alternative appearances of local high- and low-pressure regions inside the water column are observed clearly. Finally, a planar shock wave interacting with two water columns is investigated. In this work, both horizontal and vertical arrangements of two water columns are studied. It is found that different arrangements can result in the diversity of the interacting process. The complex flow structures generated by shock/water column interactions are presented by flow-visualization techniques.

Very-near-field dynamics in the injection of two-dimensional gas jets and thin liquid sheets between two parallel high-speed gas streams

A numerical investigation of the velocity, pressure and vorticity fields very near the injection of flat and thin two-dimensional gas jets or liquid sheets between two parallel high-speed gas coflows is performed. The motivation of this research is to uncover some basic physical mechanisms underlying twin-fluid atomization. Conservation equations and boundary and initial conditions are presented for both single-phase jets and two-phase liquid sheet/gas-stream systems. Both infinitely thin and thick solid walls are considered. Apart from the gas Strouhal and Reynolds numbers appearing in the dimensionless single-phase flow equations, the liquid Reynolds number, the momentum flux ratio, the gas/liquid velocity ratio and the Weber number enter the two-phase flow dimensionless formulation. The classical numerical techniques for single-phase jets are supplemented with the volume-of-fluid (VOF) method for interface tracking and the continuum surface force (CSF) method to include surface tension in two-phase flow systems. Ad hoc convection algorithms in combination with a developed version of the fractional-step scheme allows a significant reduction of the numerical diffusion, maintaining localized and sharp interfaces. The action of the surface tension is correctly found via the CSF with a smoothed scalar-field approximation.Results for single-phase jets with thin-wall injectors indicate qualitatively correct features and trends when varying the Reynolds number and the coflow/jet ratios: thick-wall injectors significantly modify the vorticity and pressure near fields; increasing the Reynolds number leads to larger flow disturbances; larger coflow/jet velocity ratios yield more perturbed near flow fields. For single-phase jets the Strouhal number as a function of the Reynolds number follows the usual trends of flows behind a circular cylinder.For two-phase flows, increasing the gas Reynolds number leads to larger liquid-sheet deformations and to a reduction of the breakup length; a plot of the gas Strouhal number, in the presence of a liquid sheet, as a function of the gas Reynolds number displays a monotonically decreasing curve, contrary to that for a gas jet. This observation strongly suggests that the gas vortex shedding mechanism is modified by the liquid-sheet motion. The gas vortex shedding frequency as a function of the liquid-sheet oscillation frequency follows a straight line with a slope of approximately for momentum flux ratios greater than roughly 0.45; for values below 0.45 the gas vortex shedding frequency remains constant while the liquid sheet varies its oscillation frequency. Increasing the surface tension leads to a larger breakup length. Thin trailing edges almost double the sheet oscillation frequency and more than halve the perturbation wavelength compared to thick trailing edges.

Initial breakup of a small-diameter liquid jet by a high-speed gas stream

The situation of a small-diameter liquid jet exposed to a large-diameter high-speed gas jet (gas-to-liquid nozzle area ratio of order 100 to 1000) is investigated experimentally. Flow visualization and particle-sizing techniques are employed to examine the initial jet breakup process and primary liquid atomization. Observations of the initial breakup of the liquid jet in the near-nozzle region, combined with droplet-size mea-surements, are used in an effort to elucidate the dominant mechanism of primary breakup of the liquid. It is shown that for large aerodynamic Weber numbers, the bulk of the liquid atomization is completed within a few gas-jet diameters of the nozzle exit, inside of the potential cone of the gas flow. Breakup is therefore completed within the zone of constant ambient gas velocity. It is argued that the mechanism of initial jet breakup is similar to that of a liquid drop suddenly exposed to a high-speed gas stream. A phenomenological breakup model is proposed for the initial droplet size, based upon the accelerative, secondary destabilization (via Rayleigh–Taylor instability) of the liquid wave crests resulting from the primary Kelvin–Helmholtz instability of the liquid jet surface. Primary mean droplet sizes are shown to scale well on the most unstable Rayleigh–Taylor wavelength, and the dependence of the droplet diameter on both the atomizing gas velocity and the liquid surface tension are successfully captured by the proposed breakup model.