Decrease In Density Ratio Research Articles

Impact of Fluid Rheology and Density Ratio in Droplet Collision: A Numerical Investigation

Abstract This research delves into the intricate interplay of fluid rheology, characterized by the power-law model, and density ratio ρr=ρlρg in the context of droplet collision dynamics. The power-law index (n) is systematically varied within the range of 0.5 to 1.5, while the density ratio spans two orders of magnitude, ranging from 101-103. Comprehensive investigations are conducted across various impact parameters (B = 0 - 0.75) and Weber numbers (We = 40 - 160). A noteworthy finding is the cessation of droplet coalescence at elevated Weber numbers (We = 160), revealing a critical threshold beyond which coalescence is no longer sustained. The impact of fluid rheology on internal fluid flow dynamics within the complex droplet structure is substantial. The variation in viscous dissipation with (n) contributes to observable changes in the critical wavelength of the complex droplet rim structure, consequently influencing the size of child droplets. Furthermore, the density ratio is a pivotal factor influencing the deformation rate during collision events. A decrease in density ratio correlates with a reduction in the deformation ratio, shedding light on the significant role of density ratio in shaping the dynamics of droplet collisions.

Gas-phase transient effects on droplet evaporation and ignition

In order to achieve high efficiency and power density, modern combustors are generally operated at elevated pressures. As the pressure and gas-phase density increase, classical quasi-steady (QS) models of droplet evaporation, ignition, and burning introduce non-negligible errors. In this paper, we extend the QS model to incorporate the gas-phase transient (GT) effect by examining the development of the evaporating boundary layer near the droplet surface. Specifically, the evaporation and ignition of single droplets in an infinite, hot, and oxidizing environment are analyzed. The theoretical result agrees well with the transient numerical simulation for an extensive range of parameters. The proposed theory successfully predicts the GT effect on evaporation lifetime and critical ignition limit of droplets. It is demonstrated that the GT effect increases the droplet evaporation rate while tends to suppress ignition. In comparison with pure evaporation, the ignition of droplets is more severely affected. Additionally, the GT effect increases as the liquid-to-gas density ratio decreases or evaporation intensity increases.

Modeling of droplet dynamics with soluble surfactant by multi-relaxation-time phase-field lattice Boltzmann method

The multiphase fluid system in the presence of surfactant is frequently encountered in numerous scientific and engineering applications. Developing a model for accurately simulating such a complex system is of great significance. In this work, we propose a multi-relaxation-time phase-field lattice Boltzmann model for simulating droplet dynamics with soluble surfactants. The accuracy and validity of the model are verified by benchmark cases including static droplet and Rayleigh–Taylor instability tests. The effects of surfactant, capillary number, and density ratio on single-droplet deformation and two-droplet interaction under shear flow are investigated. Simulation results indicate that the Marangoni stress generated by the inhomogeneous distribution of surfactant at the interface plays the role of promoting droplet deformation and hindering droplet coalescence. Within the studied range, it tends to be much easier for droplets to deform with the decrease in density ratio. The increase in the capillary number and surfactant concentration is conducive to promoting the deformation and breakup of droplets. In addition, a higher surfactant concentration is found to result in greater liquid film thickness between droplets, which would hinder the coalescence of the droplets.

Study of bubble growth and microchannel flow boiling heat transfer characteristics using dynamic contact angle model

In high heat flux electronic cooling applications, a moderate pressure drop with minimal sub-cooling at the entrance is desired to reduce the pumping power. With these flow conditions, saturated critical heat flux (CHF) is encountered. The present work focuses on the understanding of bubble lift-off dynamics and the influence of flow and geometrical parameters and fluid properties on saturated critical heat flux (CHF) in microchannel flow boiling. A three-dimensional DGLSM (Dual-Grid Level Set Method) based numerical model is proposed using dynamic contact angle model considering water, FC72, R113, R245fa as the working fluids. The continuity, momentum and energy equations are solved for the two phases using Finite Volume Method. A bubble growth model is proposed to predict the bubble lift-off diameter within ±2% accuracy. The contact diameter to bubble diameter ratio is found to increase with time, which is correlated within 5.5% accuracy. The study further explores the influence of latent heat of vapourization, density ratio of the two phases, Reynolds number, Weber number, Laplace number and channel length to diameter ratio on CHF in microchannel flow boiling. It is found that the decrease in latent heat of vaporization by 2.6% increases the bubble growth and Nusselt number by 3.5% and 2.8%, respectively, while a decrease in density ratio of 10% increases the bubble growth and Nusselt number by 15.8% and 12.9%, respectively. For cooling application, it is therefore suggested that a fluid with high value of latent heat of vaporization, high density ratio of the two phases, low Weber number and low Laplace number with high Reynolds number should be employed keeping low value of channel length to diameter ratio to avoid early onset of critical heat flux.

Bioprinting multidimensional constructs: a quantitative approach to understanding printed cell density and redistribution phenomena

Currently, bioprinting high cellular fractions in bio-inks represents a critical step towards engineered tissue models with enhanced biological relevance. However, precise spatiotemporal mapping of high cell densities within hydrogel materials represents a significant challenge. To address these needs and challenges, this paper advances a systematic multidimensional analytical framework to quantitatively determine the spatial variations in cell density and temporal evolution of cell redistribution phenomena within bioprinted cell-laden fiber-based constructs as a function of the complex interplay of the various processing parameters for an alginate-gelatin hydrogel material system. For a one dimensional analysis, fibers printed at high relative viscosities exhibit large contact angles in the cross-sectional plane with a high degree of cell dispersion (Dd = 96.8 ± 6.27%). Conversely, fibers printed at low relative viscosities exhibit small contact angles with cells coalescing towards the fiber midline (Dd = 76.3 ± 4.58%). The observed in-process redistribution of cells is attributed to the flow topology inside the needle tip. The precise mechanism is governed by the temperature-dependent viscoplastic material properties as demonstrated numerically using the Herschel–Bulkley model. In the 2D analysis, the printed grid structures yield differences in local cell densities between the strut and cross regions within the fibers. At low relative viscosities, cells aggregate in regions where two overlapping fibers fuse together, yielding a high cell density ratio (1.79 ± 0.04) between the cross and strut regions. However, at high relative viscosities, the cell density ratio decreases (0.96 ± 0.03). In the 3D analysis, due to gravity and extrusion process effects, initially uniform bio-ink cell distributions within bio-inks are redistributed as a function of printing time, yielding quantitative increases in cell density for the initial layers, followed by quantitative decreases in the subsequent layers. Finally, a time course incubation study of the 3D bioprinted cell-laden construct reveals a time-dependent increase in cell density owing to proliferation that accelerates the material degradation.

Bulk viscosity for pion and nucleon thermal fluctuation in the hadron resonance gas model

We have calculated microscopically bulk viscosity of hadronic matter, where equilibrium thermodynamics for all hadrons in medium are described by Hadron Resonance Gas (HRG) model. Considering pions and nucleons as abundant medium constituents, we have calculated their thermal widths, which inversely control the strength of bulk viscosities for respective components and represent their in-medium scattering probabilities with other mesonic and baryonic resonances, present in the medium. Our calculations show that bulk viscosity increases with both temperature and baryon chemical potential, whereas viscosity to entropy density ratio decreases with temperature and with baryon chemical potential, the ratio increases first and then decreases. The decreasing nature of the ratio with temperature is observed in most of the earlier investigations with few exceptions. We find that the temperature dependence of bulk viscosity crucially depends on the structure of the relaxation time. Along the chemical freeze-out line in nucleus-nucleus collisions with increasing collision energy, bulk viscosity as well as the bulk viscosity to entropy density ratio decreases, which also agrees with earlier references. Our results indicate the picture of a strongly coupled hadronic medium.

A similarity solution for phase change of binary alloy with shrinkage or expansion

A similarity solution for solidification of an undercooled binary alloy is developed including the effect of shrinkage or expansion due to change in the density. The proposed similarity solution takes into account the change in density of the alloy while undergoing a phase change. Thermo-physical properties are assumed to be constant. An analytical solution (which is an outcome of the similarity solution) for temperature and concentration distribution has been established. The effect of the density ratio and Lewis number on both interface motion and conjugate heat and mass transfer is studied. It is found that the interface moves faster with the decrease in density ratio and initial temperature during solidification.

Low frequency azimuthal stability of the ionization region of the Hall thruster discharge. I. Local analysis

Results based on a local linear stability analysis of the Hall thruster discharge are presented. A one-dimensional azimuthal framework is used including three species: neutrals, singly charged ions, and electrons. A simplified linear model is developed with the aim of deriving analytical expressions to characterize the stability of the ionization region. The results from the local analysis presented here indicate the existence of an instability that gives rise to an azimuthal oscillation in the +E × B direction with a long wavelength. According to the model, the instability seems to appear only in regions where the ionization and the electric field make it possible to have positive gradients of plasma density and ion velocity at the same time. A more complex model is also solved numerically to validate the analytical results. Additionally, parametric variations are carried out with respect to the main parameters of the model to identify the trends of the instability. As the temperature increases and the neutral-to-plasma density ratio decreases, the growth rate of the instability decreases down to a limit where azimuthal perturbations are no longer unstable.

Effects of N2 Dilution on Laminar Burning Characteristics of Propane−Air Premixed Flames

Effects of nitrogen dilution on laminar burning velocities and Markstein lengths of propane-air mixtures were determined at the atmospheric pressure and room temperature based on the spherically expanding flames. The results show that, with the increase of the nitrogen dilution ratio, the burning velocity decreases and, for equivalence ratio less than 1.4, Markstein length increases with the increase of the dilution ratio, indicating that nitrogen addition decreases the preferential diffusion instability. The density ratio decreases, and the laminar flame thickness increases, which indicates the decrease of hydrodynamic instability. The ratio of unstretched laminar burning velocity with and without diluent is only related to the dilution ratio and is not influenced by the equivalence ratio. A linear correlation is found between the ratio of unstretched laminar burning velocity with and without the diluent and the dilution ratio.

Solar wind composition changes across the Earth's magnetopause

ISEE 1 and Active Magnetosphcric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) composition measurements are used to investigate the change in solar wind composition across the Earth's magnetopause. The combined data set of 59 magnetopause crossings covers 0500 to 1900 LT and −20° to +40° magnetic latitude. For each magnetopause crossing, the ratio of the He2+/H+ density in the magnetosheath and in the low latitude boundary layer (LLBL) were determined. In addition, some crossings on the duskside flank of the magnetosphere were sufficiently long to determine this density ratio in the current layer. Averaged over all local time, the He2+/H+ density ratio is observed to decrease by 40% from the magnetosheath to the low latitude boundary layer. The density ratio decreases by its greatest amount (47%) in the dawnside LLBL and decreases by its smallest amount (16%±14%) in the duskside LLBL. In addition, there is essentially no reduction in the density ratio (7%±12%) in the duskside current layer. Finally, even those events that show clear signatures of magnetic reconnection at the magnetopause exhibit an average decrease in the density ratio that is comparable to the decrease seen in the entire data set. This decrease in the He2+/H+ density ratio across the magnetopause is not due to the presence of a significant magnetospheric H+ population in the LLBL. One possible explanation for this composition change is a mass dependent reflection coefficient at the magnetopause. To be consistent with the observations, the reflection coefficient for He2+ at the magnetopause must be 40–50% higher than that for H+. If true, then the He2+ concentration must increase by about 20% in the magnetosheath boundary layer (MSBL) just sunward of the magnetopause.

The Prediction of the Optimum Performance of Ejectors

A simple, incompressible flow, ejector model was formulated and analysed. It was found that the results obtained analytically agreed closely with those of an experimentally conducted optimization carried out by other workers. The model was also used to compare the relative merits of constant static pressure versus constant area mixing: it was shown that the latter was superior to the former when nozzle, mixing tube and diffuser friction losses were taken into account. The work also indicated that the efficiency of an ejector tends to increase as the secondary-to-primary stream density ratio decreases.