Transient aerodynamic atomization model to predict aerosol droplet size of pressurized metered dose inhalers (pMDI)

Pressurized metered dose inhalers (pMDI) produce large numbers of droplets with smaller sizes than 5 μm to treat asthma and other pulmonary diseases. The mechanism responsible for droplet generation from bulk propellant liquid is poorly understood, mainly because the small length scales and short time scales make it difficult to characterize transient spray formation events. This article describes the development and findings of a numerical atomization model to predict droplet size of pharmaceutical propellants from first principles. In this model, the velocity difference between propellant vapor and liquid phase inside spray orifice leads to formation of wave-like instabilities on the liquid surface. Two variants of the aerodynamic atomization model are presented based on assumed liquid precursor geometry: (1) cylindrical jet-shaped liquid ligaments surrounded by vapor annulus; (2) annular liquid film with vapor flow in the core. The growth of instabilities on the liquid precursor surfaces and the size of the subsequently formed droplets are predicted by numerical solutions of dispersion equations. The droplet size predictions were compared with phase doppler anemometry (PDA) data and the predictions were in good agreement with the number mean diameter D₁₀, which is representative of the respirable droplets. The temporal behavior of droplet size production was captured consistently well during the period of the first 95% of the propellant mass emission. The outcome of our modeling activities also suggests that, in addition to saturated vapor pressure of the propellant, its viscosity and surface tension are also key properties that govern pMDI droplet size.

© 2017 American Association for Aerosol Research     

A model of transient internal flow and atomization of propellant/ethanol mixtures in pressurized metered dose inhalers (pMDI)

This article reports the extension to binary propellant/excipient mixtures of the multiphase model of transient internal flow and atomization in pressurized metered dose inhalers (pMDIs) of Gavtash and colleagues for propellant-only flows. The work considers different accounts of the effect of less volatile ethanol on the saturated vapor pressure (SVP), viscosity and surface tension of HFA-based pMDI formulations. Representation of the SVP of HFA/ethanol mixtures by Raoult's law is compared with the empirical model developed by Gavtash and colleagues as well as different theoretical mixing rules for surface tension and viscosity. For initial ethanol contents ranging from 0 to 20% by mass, the temperature, pressure and spray velocity were predicted to be almost independent of ethanol concentration when using the empirical SVP model of Gavtash and colleagues. The predicted aerosol droplet size increases with increasing concentration of ethanol. These model predictions compare favorably with phase Doppler anemometry (PDA) measurements of pMDI sprays. Exploration of model predictions with different mixing rules suggest that variations of the dynamic viscosity could result in 0.7 µm droplet size change, and different surface tension models yield around 1.5 µm droplet size change. The findings of this work challenge the view that the increase of droplet size is caused by the low volatility of excipients such as ethanol. Instead, attention is focused on composition-dependent viscosity and surface tension as potential controlling parameters with significant effect on the droplet size of HFA/ethanol sprays.

Copyright © 2018 American Association for Aerosol Research     

Experimental investigations of particle formation from propellant and solvent droplets using a monodisperse spray dryer

Experimental studies of particle formation from solution droplets were conducted using a newly developed monodisperse spray drying process. Solutes beclomethasone dipropionate and caffeine were dissolved in ethanol, pressurized hydrofluoroalkane propellant 134a, and mixtures thereof. Solutions were atomized into monodisperse microdroplets using a custom droplet generator installed in a laboratory scale spray dryer, enabling drying and collection of the resulting monodisperse microparticles. The effects of droplet diameter, solution concentration, solvent composition, and drying rate on the physical properties of the dried particles were evaluated. Particle morphology and size were assessed using ultramicroscopy and image analysis of micrographs. Extent of crystallinity and polymorphism were investigated using Raman spectroscopy. The drying temperature was found to have a large effect on the morphology of amorphous beclomethasone dipropionate particles. Particles dried near room temperature were spheroidal to ellipsoidal with prevalent surface concavities and evidence of shell buckling; increasing the drying temperature for fixed droplet size and composition resulted in a transition to more spherical, smooth-surfaced particle morphologies. Crystalline caffeine microparticles were made up of assemblies of multiple crystallites. The measured length and breadth of these crystallites was found to be correlated with the time available for crystal nucleation and growth as calculated using a particle formation model. The results highlight the abilities and limitations of currently available particle formation models in elucidating the relationships between the size, composition, and evaporation rate of drying solution droplets and the physical properties of the resulting particles. The work demonstrates the suitability of monodisperse spray drying as an experimental technique for investigating the fundamentals of particle formation from solution droplets.

© 2018 American Association for Aerosol Research     

Mathematical modeling and numerical simulation of surfactant delivery within a physical model of the neonatal trachea for different aerosol characteristics

Surfactant aerosol delivery in conjunction with a noninvasive respiratory support holds the potential to treat neonatal respiratory distress syndrome in a safe manner. The objective of the present study was to gain knowledge in order to optimize the geometry of an intracorporeal inhalation catheter and improve surfactant aerosol delivery effectiveness in neonates. Initially, a mathematical model capable of predicting the aerosol flow generated by this inhalation catheter within a physical model of the neonatal trachea was implemented and validated. Subsequently, a numerical study was performed to analyze the effect of the aerosol liquid droplet size and mass flow rate on surfactant delivery and on the required aerosolization time period. Experimental validation of the mathematical model showed a close prediction of the air axial velocity at the distal end of the physical model, with an absolute error between 0.01 and 0.15 m/s. Furthermore, an admissible absolute error between 0.2 and 2 µm was attained in the prediction of the aerosol mean aerodynamic diameter and mass median aerodynamic diameter in this region. The numerical study highlighted the beneficial effects of generating an intracorporeal aerosol with a mass median aerodynamic diameter higher than 4 µm and a surfactant mass flow rate above 8.93 mg/s in order to obtain effective surfactant delivery in neonates with minimal airway manipulation.

Copyright © 2017 American Association for Aerosol Research     

R134a闪蒸喷雾过程中喷管内流动形态对喷雾特性的影响

闪蒸喷雾过程中喷管内流动状态对喷雾形态及雾化效果具有重要的影响。为了观察喷管内流动状态及其对应的喷雾形态,使用高速相机对不同喷射压力条件下制冷剂R134a在石英玻璃直喷管中的流动状态及喷雾形态进行可视化研究,同时分析不同喷射压力条件下喷雾半径的动态变化情况。研究发现,喷管内的气化可以促进喷管出口处喷雾半径的迅速扩大,但同时限制了喷雾半径在喷雾过程中的进一步发展。喷管内流动状态在一定喷射压力下具有极强的不稳定性,在喷管内形成均匀的泡状流有利于形成均匀、稳定的喷雾形态。     

Photomicrographic analysis of a spray: Determination of droplet size and velocity spectra by means of diamant''s rotating mirror microscope

This paper presents a new application of the rotating mirror microscope described by S. W. Diamant for the study of sprays.

One predicts the theoretical picture of a spherical droplet moving in the focal plane at a velocity different from that of synchronism with the rotating mirror. This study makes it possible to infer from the photograph not only a local droplet size spectrum but also local droplet velocity distributions.     

Analysis by droplet size classes of the liquid flow structure in a pressure swirl hollow cone spray

In this work the structure of a spray resulting from the break-up of a conical liquid sheet was investigated through experimental techniques. The disperse and continuous phase velocities and the size of droplets were measured using a phase Doppler particle analyzer. A data post-processing, applying the generalized integral method, was used to evaluate liquid volume fluxes for different droplet size classes.     

Relationship between droplet size and fluid flow characteristics in miniemulsion polymerization of methyl methacrylate

Static mixer (SM) can be applied for emulsification, but the fundamental understanding of the nature of fluid flow and mixing in static mixers, is however poor. Droplet size is a very important parameter in miniemulsion systems and affects strongly the mechanism of particle formation in polymerization reactions. In this study, static mixer was used as homogenization device for emulsification of methyl methacrylate (MMA). Re number (Re) was obtained for SM inserted tube in different flow rates. It was demonstrated the nature of fluid flow was turbulent under our experimental conditions. The relationship between droplet size—the most important variable in our study—and Weber number (We) was investigated. The results showed that the ratio of the droplet size to the pipe diameter was fit as an exponential function with an order of −0.35. The polymerization of created droplets under certain We values by SM showed that it is possible to obtain a reasonable 1 : 1 copy of droplets to the particles. All these, indicate that using relationship between We and droplet size allow one to obtain acceptable condition of droplet nucleation in miniemulsion polymerization. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of co- and counter-swirl on the droplet characteristics in a spray flame

This paper reports measurements of droplet characteristics and flow field in a spray flame with inner and outer swirling air streams. The spatial distribution of droplet characteristics produced by the burner's airblast atomizer was measured using dual-phase Doppler anemometry (PDA). The spray flame was operated near the lean blow-out limit at two flow conditions: co-swirling (flow rotation in the same direction) and counter-swirling (flow rotation in opposite directions). In both cases, the flame exhibited a U-shaped form and was marked by a large central recirculation zone. Based on the measurements of the droplet velocity components, differences between both configurations appeared for the counter-rotational setup mainly in the near burner region, where the decrease of total swirl causes deeper penetration of the droplets from the inner duct into the combustion chamber, resulting in a much more homogeneous distribution than the other one. The droplet size in terms of the Sauter mean diameter (SMD) shows little variation in the change of the direction swirl condition. Application of counter-swirl results in more turbulent droplet motion.     

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence-particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption-scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.     

Reduction of a model for single droplet drying and application to CFD of skim milk spray drying

In this work, a novel methodology for the development of a high-accuracy computational fluid dynamics (CFD) model for the spray-drying process is described. Starting point is an own spatially resolving model of droplet/particle drying, which was developed and validated on the basis of a series of single droplet drying (SDD) experiments. This sophisticated model is transformed to a much simpler version: the characteristic drying curve approach, after running the full SDD model in a wide range of operating conditions. Then, the obtained reduced model is implemented into the CFD solver. The CFD spray-drying model takes into account the hydrodynamics of the continuous phase, particle drying kinetics, changes in the particle diameter, and the heat loss from the drying chamber to the environment. Validation of the entire procedure is provided by data obtained from drying experiments performed in a co-current laboratory spray tower. High accuracy of the developed CFD model of skim milk spray drying has been found for both phases, for the mean outlet temperature of the continuous phase (air) and for the change in average particle moisture content along the spray tower (discrete phase).     

Combustion characteristics of a hydroxylammonium nitrate based liquid propellant. Combustion mechanism and application to thrusters

A new composition of hydroxylammonium nitrate based solution containing ammonium nitrate, methanol, and water was developed for monopropellant in a reaction control system (RCS) as an alternative to conventional hydrazine. In comparison with hydrazine, this solution has a 20% higher specific impulse, 1.4 times higher density, and lower freezing point and toxicity. The linear burning rate of the solution is moderate at the operating pressures of RCS thrusters. It was found that the linear burning rate had some characteristics whose mechanisms had not been understood. The combustion mechanism of this solution was investigated. The burning behavior was observed using a medium speed camera, and a temperature profile for the combustion wave was measured with a 2.5 μm diameter thermocouple. From these results, the instability of the liquid-gas interface may trigger a sudden increase in the burning rate, and methanol was found to be effective in reducing the bubble growth rate in the solution. The reactivity of several catalysts was evaluated in an open-cup test, and the S405 catalyst for hydrazine showed the best performance among them. Thruster tests were conducted using the S405 catalyst with variation in the propellant mass flow rate, catalyst bed configuration, and length-to-diameter ratio of the combustor. As a result, parameters were determined that ensured long operating time. The model thruster operated stably for up to 100 sec with a specific impulse I _sp = 240 sec, which corresponds to a 90% efficiency. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 109–120, July–August, 2009. An erratum to this article can be found at     

Experimental evaluation and modeling of liquid jet penetration to estimate droplet size in a three-phase riser reactor

In this work,the effects of injecting an evaporating liquid jet into solid–gas flow are experimentally investigated.A new model(SHED model) and a supplementary model(spray model) have also been proposed to investigate some flow-field characteristics in three-phase fluidized bed with the mean relative error 4.3% between model and measured results.Some experiments were conducted to study the influences of flow-field parameters such as liquid volumetric flow rate,injection velocity,jet angle and gas superficial velocity as well as solid mass flux on the jet penetration depth(JPD).In addition,independent variables were experimentally employed to propose two empirical correlations for JPD by using multiple regression method and spray cone angle(SCA) by using dimensional analysis technique.The mean relative errors between the JPD and SCA correlations versus experimental data were 7.5% and 3.9%,respectively.In addition,in order to identify the variable effect,a parametric study was carried out.Applying the proposed model can avoid direct use of expensive devices to measure JPD and to predict droplet size.     

Transition to unsteady flow and intensity of velocity fluctuations in a porous medium

Direct measurements of local velocity and the intensity of velocity fluctuations in a pore of a number three rhombohedral array of spheres indicate a region of increasing intensity beginning at Reynolds number of about 80 and reaching a maximum intensity of approximately 0.6 at a Reynolds number of about 200. At higher Reynolds numbers the intensity falls to a constant value between 0.4 and 0.5.     

Numerical probing of a low velocity concurrent pilot scale spray drying tower for mono-disperse particle production – Unusual characteristics and possible improvements

A pilot scale micro-fluidic-spray-dryer (MFJSD-II) utilizing a single-stream atomization to produce mono-disperse particles has been developed at Monash University. A unique feature of this unit is the use of relatively low air velocities in the range of 10⁻¹-10⁻² m s⁻¹, which increases the residence time of the particles during the drying process, in comparison with conventional spray dryers. In this study, Computational Fluid Dynamics (CFD) simulation revealed that the effects of natural convection, caused by heat loss from the wall, were significant in deflecting the flows of air from the central jet-side recirculation pattern, causing them to spread towards the wall. The flow patterns formed a layer of relatively high velocity region adjacent to the wall. The understanding of these flow patterns would be crucial for future designs of low-velocity spray dryers. The spreading of the central jet towards the wall also created recirculation regions in the center of the tower, thus affecting the residence times particularly for smaller particles. More numerical simulations are required to optimize the design of the bottom bustle to be effectively used as a particle-separator.     

Predictions of flow behaviors and entrance pressure drop characteristics of a rubber compound in a capillary die using various rheological models

Rubber compounds have highly viscoelastic properties. The viscoelastic behaviors that have been exhibited during die extrusion include die swell and vortices in regions of sudden contraction. In this study, the application of rheological models to the capillary die extrusion process is investigated. Experiments and simulations were conducted using a fluidity tester and finite element analysis, respectively. The velocity distributions, velocity profiles, pressure drops, and vortices at the capillary die entrance were analyzed through computer simulations for various viscoelastic models [i.e., Phan‐Thien and Tanner (PTT), Giesekus, POMPOM, simplified viscoelastic, and generalized Newtonian models]. Different models exhibited different pressure drops and different velocity profiles in the capillary die. Only the full viscoelastic models (PTT, Giesekus, and POMPOM) predicted the vortex at the corner of the reservoir that is the capillary die entrance. However, the simplified viscoelastic and generalized Newtonian models did not predict the vortex. All the viscoelastic models studied in this article predicted the die swells in various ways, and these were compared with the experimental results. The PTT and simplified viscoelastic models exhibited good agreement with the experimental results of the die swells. POLYM. ENG. SCI., 54:2441–2448, 2014. © 2013 Society of Plastics Engineers     

Numerical simulation and experimental study of the characteristics of packing feature size on liquid flow in a rotating packed bed

Rotating packed bed has high efficiency of gas–liquid mass transfer. So it is significant to investigate fluid motion in rotating packed bed. Numerical simulations of the effects of packing feature size on liquid flow characteristics in a rotating packed bed are reported in this paper. The particle image velocimetry is compared with the numerical simulations to validate the turbulent model. Results show that the liquid exists in the packing zone in the form of droplet and liquid line, and the cavity is droplet. When the radial thickness of the packing is less than 0.101 m, liquid line and droplets appear in the cavity. When rotational speed and radial thickness of the packing increase, the average diameter of the droplets becomes smaller, and the droplet size distribution becomes uniform. As the initial velocity of the liquid increases, the average droplet diameter increases and the uniformity of particle size distribution become worse. The droplet velocity increases with the radial thickness of the packing increasing, and gradually decreases when it reaches the cavity region. The effect of packing thickness is most substantial through linear fitting. The predicted and simulated values are within ±15%. The cumulative volume distribution curves of the experimental and simulated droplets are consistent with the R-R distribution.     

Numerical modeling study for the analysis of transient flow characteristics of gas, oil, water, and hydrate flow through a pipeline

This study presents the development of a four-phase, four-fluid flow pipeline simulator to describe simultaneous flow of gas, oil, water, and hydrate through a pipeline. The model has been equipped with a phase behavior model and hydrate equilibrium model to efficiently estimate thermodynamic and hydrodynamic properties of multicomponent mixtures. The governing equations are formulated for describing the physical phenomena of mass, momentum, and heat transfers between the fluids, and the wall. The equations are solved by utilizing the implicit finite-difference method on the staggered-grid system which can properly describe the boundary conditions as well as phase appearance or disappearance. The developed pipeline simulator has been validated against the field data presented by a previous investigator, and their matches are found to be relatively excellent. The model also has been applied to a multi-component, four-phase flow system in order to examine the transient flow characteristics in pipeline. Also, the potential and the location of hydrate formed in the pipeline have been studied by analyzing the flow characteristics. As a result, it was found that a pipeline system flowing gas, oil, water, and hydrate could be optimized by systematically investigating the hydrodynamic variables for the prevention of hydrate formation.