Gas Density Ratio Research Articles

Prediction of interfacial shear stress and pressure drop in vertical two-phase annular flow

The interfacial shear stress and pressure drop of an upward vertical annular flow of nitrogen–water, HFC134a–water, and nitrogen–95 % ethanol solution were comprehensively investigated considering the effect of the liquid–gas density ratio and surface tension. A direct link between the disturbance wave height and equivalent sand-grain roughness was noted through the analogy with the famous Moody chart for single-phase turbulent flows. A predictive model of the interfacial friction factor was developed based on this finding. To predict the pressure drop of the annular flow, a new model with good predictive performance for annular flows of various working fluids including steam–water under boiling water reactor operating condition (286 °C and 7 MPa) was proposed.

3D modeling of positive streamers in air with inhomogeneous density

We study the effect of an inhomogeneous gas density on positive streamer discharges in air using a 3D fluid model with stochastic photoionization, generalizing earlier work with a 2D axisymmetric model by Starikovskiy and Aleksandrov (2019 Plasma Sources Sci. Technol. 28 095022). We consider various types of planar and (hemi)spherical gas density gradients. Streamers propagate from a region of density towards a region of higher or lower gas density , where corresponds to and . We observe that streamers can always propagate into a region with a lower gas density. When streamers enter a region with a higher gas density, branching can occur at the density gradient, with branches growing in a flower-like pattern over the gradient surface. Depending on the gas density ratio, the gradient width and other factors, narrow branches are able to propagate into the higher-density gas. In a planar geometry, we find that such propagation is possible up to a gas density slope of , although this value depends on a number of conditions, such as the gradient angle. Surprisingly, a higher applied voltage makes it more difficult for streamers to penetrate into the high-density region, due to an increase of the primary streamer’s radius.

On the velocity and frequency of disturbance waves in vertical annular flow with different surface tension and gas–liquid density ratio

In this study, the characteristics of disturbance waves, including velocity, longitudinal size, and frequency, are comprehensively investigated by conducting gas–liquid upward annular flow experiments in a 5 mm tube through the constant electric current method (CECM). The film thickness time trace is recorded with respect to time, and the information on disturbance waves is subsequently extracted. The effect of liquid and gas flow rate, density ratio, and surface tension on the disturbance wave is quantitatively analyzed. A predictive model of the wave velocity based on the shear stress balance of the liquid film is proposed. A physical model for wave frequency prediction is derived on the basis of the mass conservation equation. A simple empirical model with good prediction accuracy of wave frequency is also derived. Compared with existing correlations available in the literature, the newly derived models show better performance under a wide range of flow conditions.

Investigation of Zero-/High-Field Ion Mobility Orthogonal Separation Using a Hyphenated DMA–FAIMS System and Validation of the Two-Temperature Theory at Arbitrary Field for Tetraalkylammonium Salts in Nitrogen

Toward greater separation techniques for ions, a differential mobility analyzer (DMA) has been coupled with field asymmetric waveform ion mobility spectrometry (FAIMS) to take advantage of two mobility-related but different methods of separation. The filtering effect of the DMA allows ions to be selected individually based on low-field mobility and studied in FAIMS at variable electric field, yielding mobility separations in two dimensions. Because spectra fully describe ion mobility at variable field strength, results are then compared with a two-temperature theory-predicted mobility up to the fourth-order approximation. The comparison yields excellent results up to at least 100 Td, beyond which the theory deviates from experiments. This is attributed to two effects, the enlargement of the structure due to ion heating and the inelasticity of the collisions with the nitrogen bath gas. The corrected mobility can then be used to predict the dispersion plot through a newly developed implicit equation that circumvents the possible issues related to the more elaborate Buryakov equation. Our results simultaneously show that the DMA-FAIMS coupling yields complete information on ion mobility versus the field-strength to gas-density ratio and works toward predicting such spectra from ion structures and gas properties.

Theoretical analysis on macro-mesoscopic gas flow performances in gas dynamic bearing with three pads

The Reynolds equation based on the continuum medium assumption fails to meet the accuracy requirements of numerical simulation for mesoscale gas flow. In this research, the gas flow performances and bearing performances of gas dynamic bearing with three pads (GDBTPs) are theoretically analyzed from macroscopic to mesoscopic perspectives. A modified lattice Boltzmann equation is exploited considering the wall effect ψ(y/λ) with gas density ratio ρ/ρref, and the dimensionless gas flow velocity is analyzed for smooth, square cavity, half-sine asperity, triangular asperity, and a combination of surface morphologies. A modified Reynolds equation considering the gas compressibility and gas rarefaction effect is developed to study the static bearing performances of GDBTPs. Results show that the relative roughness Δh and asperities geometries are key factors to affect the mesoscale gas flow characteristics. The load-carrying capacity of GDBTPs increases with the growth of length-to-diameter ratio L/D, rotational speed ω, and eccentricity ratio ɛ and decreases with the increase of gas film thickness hg.

Effect of gas density and surface tension on liquid film thickness in vertical upward disturbance wave flow

Gas–liquid annular flows, such as the flow attached to the fuel rods of boiling water reactors, is a prevalent occurrence in industrial processes. At the gas–liquid interface of such flows, disturbance waves with diverse heights commonly arise. Because the thin liquid film between two successive disturbance waves leads to the dryout on the heating surface and limits the performance of the heating components, complete knowledge of the film thickness and disturbance waves is crucial. The properties of the base film and disturbance waves have been studied extensively through experimental and analytical approaches. However, most experimental data and analysis available in the literature are limited to the near atmospheric condition, and the liquid–gas density ratio under near atmospheric conditions is much higher than in practical nuclear applications. In this study, we use nitrogen gas and water under the system pressures of 0.2 and 0.4 MPa, as well as HFC134a gas and water under the system pressure of 0.7 MPa. Therefore, the density ratio can be varied between 32 and 434. Based on our experimental data, we report a direct link between the film thickness, wave height, and dimensionless numbers. The analysis indicates the Weber number which represents the ratio of the drag force to the surface tension force is critical for the film thickness behavior. To examine this, we perform the experiments using nitrogen gas and 95% (v/v) of aqueous ethanol solution under the system pressure of 0.2 MPa. Finally, two models are proposed to predict the averaged film thickness and wave height, which are then compared with previous data.

An effective pseudo-potential lattice Boltzmann model with extremely large density ratio and adjustable surface tension

The pseudo-potential lattice Boltzmann (LB) model is versatile in modeling multiphase flows since the mesoscopic interaction potential enables it to directly describe the nonideal effect evading the tracking or integrating of phase interface. In this paper, we develop an effective pseudo-potential lattice Boltzmann model to simultaneously realize the thermodynamic consistency, the extremely large density ratio, and the adjustable surface tension. Decoupling the mesh space from the momentum space by a scale factor, denser lattice nodes depict the transition region more accurately. The high-precision explicit finite difference method (EFM) further enhances the calculation accuracy of interaction force. The present model is validated to satisfy thermodynamic even at very low temperature, where the liquid–gas density ratio exceeds 1010. The spurious current can be suppressed to a very low level (<0.0007) despite the density ratio reaching tens of thousands. A modified pressure tension is introduced to tune the surface tension free from the influence of the density ratio. The numerical stability of multiphase simulations is significantly improved, and the droplet splashing is successfully reproduced at Reynolds number 25 000, while the density ratio is more than 10 000.

Study of the secondary droplet breakup mechanism and regime map of Newtonian and power law fluids at high liquid–gas density ratio

This work reports the numerical investigation of the secondary breakup of non-Newtonian droplets at different Weber Weg and Ohnesorge Ohl numbers. As part of this work, an in-house coupled level set volume of fluid solver is developed based on OpenFOAM libraries. It uses improved curvature calculation techniques like smoothening and the closest point search method. Flow is assumed to be axisymmetric. Approximately 95 different cases were simulated to investigate the effect of Weg and Ohl numbers on secondary breakup for Newtonian, shear-thinning, and shear-thickening fluids. Ohl varies from 10−3 to 101, and, correspondingly, Weg varies from 101 to 103. The non-Newtonian rheology is modeled as a power-law fluid, and the power-law index n ranged from 0.2–1.8. The present work describes the flow field near the droplet and the effects of non-Newtonian parameters and viscosity on the flow field. The various aspects of droplet dynamics like droplet deformation ratio y, deformation rate ẏ, and coefficient of pressure Cp are studied and compared with the internal flow theory. A generalized relation for critical Weber number Weg,cr is proposed for both Newtonian and non-Newtonian fluids and is shown in a phase diagram plot to map the different regimes of secondary droplet breakup.

A self-similar solution for shock waves in conducting rotating non-ideal dusty gas medium with monochromatic radiation and magnetic field

Abstract In this paper, the cylindrical shock wave propagation in a perfectly conducting rotating mixture of micro size dust particles and van der Waal gas with magnetic field either axial or azimuthal and monochromatic radiation is investigated. The effect of thermal radiation is included in the energy equation of the governing system. In our study, it is assumed that the flux of radiation moves in the mixture of particles and real gas with invariable intensity and the shock wave is moving appositive to the direction of radiation heat flux and the energy is engrossed behind the cylindrical shock only. In the present model, dusty gas is assumed to be a mixture of micro size dust particles and van der Waal gas in which solid particles are continuously distributed and the equilibrium flow conditions are assumed to hold in the entire flow-field region. The effects of the particles’ density to the initial gas density ratio, the real gas effect, rotational parameter, the concentration of mass of the micro size dust particles in the conducting mixture, Alfven-Mach number and the adiabatic exponent on shock and on the physical variables such as velocity, density etc. are discussed. It is found that due to the rotating medium consideration or by an increase in small particles density to the initial gas density ratio, the shock wave strength increases. Also, it is significant to memorize that the strength of the shock wave decreases by an increase in the strength of initial magnetic field or gas non-ideal parameter or the adiabatic index.

CFD Based Non-Dimensional Characterization of Energy Dissipation Due to Verticle Slosh

We present the CFD based non-dimensional characterization of violent slosh induced energy dissipation due a tank under vertical excitation. Experimentally validated CFD is used for this purpose as an ideally suited and versatile tool. It is thus first demonstrated that a weakly compressible VoF based CFD scheme is capable of computing violent slosh induced energy dissipation with high accuracy. The resulting CFD based energy analysis further informs that the main source of energy dissipation during violent slosh is due liquid impact. Next, a functional relationship characterising slosh induced energy dissipation is formulated in terms of fluid physics based non-dimensional numbers. These comprised contact angle and liquid–gas density ratio as well as Reynolds, Weber and Froude numbers. The Froude number is found the most significant in characterising verticle violent slosh induced energy dissipation (in the absence of significant phase change). The validated CFD is consequently employed to develop scaling laws (curve fits) which quantify energy dissipation as a function of the most important fluid physics non-dimensional numbers. These newly developed scaling laws show for the first time that slosh induced energy dissipation may be expressed as a quadratic function of Froude number and as a linear function of liquid–gas density ratio. Based on the aforementioned it is postulated that violent slosh induced energy dissipation may be expressed as a linear function of tank kinetic energy. The article is concluded by demonstrating the practical use of the novel CFD derived non-dimensional scaling laws to infer slosh induced energy dissipation for ideal experiments (with exact fluid physics similarity to the full scale Aircraft) from (non-ideal) slosh experiments.

Novel methodology for turbine gas meters error curve modelling across a wide range of operating parameters

Abstract In this paper, performance of turbine flowmeters was investigated for different flowmeter ranges and working gas operating pressures. Variation of these parameters was represented in dimensionless form as a function of Reynolds Number and gas density ratio, while the relative flow measurement error was selected as the most important operating characteristic. A novel error curve model based on turbine machine theory and dimensionless analysis was introduced for the purpose of error data fitting across a wide range of gas flow rates and operating pressures. The main advantage of the presented model is the capability of accurate error data fitting with a single continuous equation, as demonstrated by high R 2 {R^{2}} values for the vast majority of flowmeters analyzed in this study. The acceptability criterion was designed based on the fact that the expanded measurement uncertainty of the relative error must not exceed 0.5 %. Besides an accurate interpolation, our model can also be utilized for prediction of turbine flowmeter performance at modified flow conditions (pressure and flow rate, working gas properties), and for assessment of the drift of flowmeter performance over time. The novel error curve model is demonstrated to outperform the standard polynomial-based model regardless of the independent variable used.

Effects of liquid and air swirls on the three-dimensional instability of an annular viscous liquid sheet

A three-dimensional temporal linear instability model is proposed for an annular viscous liquid sheet moving between two inviscid gas streams. The model is numerically solved by Chebyshev collocation method. Effects of liquid swirl, inner and outer gas swirls, liquid viscosity, radius ratio, and density ratio on the liquid sheet instability are systematically investigated. The energy budget is calculated to provide insight into the instability driving mechanism. Results show that without gas swirl the liquid swirl has a dual effect on the sheet instability mainly through the axisymmetric mode. For a non-swirling liquid sheet, inner gas swirl induces the maximum wave growth rates of non-axisymmetric modes about 2–3 times higher than those induced by outer gas swirl, suggesting that inner gas swirl is more effective than outer one in promoting the sheet instability. For a swirling liquid sheet, simultaneous inner and outer gas swirls have the greatest destabilizing effect; outer gas swirl shows a slightly greater destabilizing effect than the inner one. Regarding the relative rotating direction, the combination of both co-rotating inner and outer gases is more favored to producing finer droplets. Liquid viscosity is found to have a evident stabilizing effect when Reynolds number is smaller than 500, it dampens the disturbance by dissipating the disturbance energy. Increasing radius ratio decreases the maximum wave growth rates of modes with n=0 and 5⩽n⩽7 but increases those of modes with 1⩽n⩽4. Maximum wave growth rate and optimal wave number are found to first decrease and then increase with increasing density ratio.

Simulations of the merging galaxy cluster Abell 2034: what determines the level of separation between gas and dark matter

ABSTRACT Cluster mergers are an important laboratory for studying the behaviour of dark matter (DM) and intracluster gas. There are dissociative collisions that can separate the intracluster gas from the DM. Abell 2034 presents clear dissociative features observed by X-rays and gravitational lensing. The cluster, at z = 0.114, consists of two substructures with mass ratio of about 1:2.2, separated by ∼720 kpc. The X-ray emission peak is offcentred from the south DM peak by ∼350 kpc. Using N-body hydrodynamical simulations, we aim to reconstruct the dynamic history of the collision, reproducing the observed features, and also to explore the conditions that led to the dissociation. Our best model assuming that the collision is close to the plane of the sky, with a small impact parameter, observed 0.26 Gyr after central passage, reproduces the observed features of this cluster, such as the offset between X-ray and DM peaks, X-ray morphology, and temperatures. We explored several variations using different gas and DM concentrations for each cluster. The level of dissociation was quantified by the distances between X-ray and DM peaks, and also by the gas retention in the cluster cores. We found that the ratio of central gas densities is more important than the ratio of central DM densities in determining the level of dissociation.

Effects of Fuel Staging on the Hydrodynamic Stability of Multinozzle Swirl Flows

Abstract Hydrodynamic instability in lean premixed gas turbine combustors can cause coherent flow velocity oscillations. These can in turn drive heat release oscillations that when favorably coupled with combustor acoustic modes can result in combustion instability. The aim of this paper is to understand the impact of fuel staging on the characteristics of hydrodynamic modes in multinozzle combustors. We extend our recent numerical study on the hydrodynamic stability characteristics of a multinozzle combustor having three nozzles in a straight line with uniform fuel–air ratio in each nozzle, to the nonuniform fuel–air ratio case. As before, we construct the base flow model for this study by superposing contributions from individual nozzles, determined using a base flow model for a nominally axisymmetric single nozzle, at every point in the computational domain. The impact of fuel staging is captured by changing the burnt to unburnt gas density ratio parameter in the individual contribution from each nozzle. We investigate the characteristics of the most locally absolutely unstable mode for two cases. The first one is when the middle nozzle is made fuel rich when compared to the side nozzles and the second is when the side nozzles are made fuel rich relative to the middle nozzle. The impact of nonuniform fuel/air ratio on the local absolutely unstable temporal eigenvalues is seen to be small. However, significant changes in the spatial structure of the flow oscillations associated with the hydrodynamic eigenmodes are observed. In the first case, the flow oscillations with a different locally azimuthal nature on the middle nozzle when compared to the side nozzles emerge as the middle nozzle is made richer. In the second case, the oscillations on the two side nozzles are suppressed leaving the middle nozzle in a state that closely matches that of a single unconfined nozzle with the same nominal base flow velocity field. These types of internozzle variations in flow oscillation characteristics can explain the emergence of nonuniformity in heat release oscillation characteristics between individual nozzles in multinozzle combustors.

An all-oxide electrolysis cells for syngas production with tunable H2/CO yield via co-electrolysis of H2O and CO2

High-rate production of syngas with tunable H2/CO and coke-free operation is achieved in a solid-oxide electrolysis cell (SOEC). Prior to operation, controlled pre-reduction of La0.7Sr0.3Fe0.9Ni0.1O3-δ(LSFNi) cathode is used to trigger the in-situ exsolution of Ni-Fe alloy nanoparticles with an average size of ~45 nm uniformly distributed and socketed on LSFNi backbone, enabling efficient co-electrolysis of H2O and CO2 to H2 and CO. At 1.5 V, the current density reaches ~1.0 A cm−2 at 750 °C and ~2.4 A cm −2 at 850 °C with near 100% Faradaic Efficiency. We demonstrate the feasibility of tuning the output H2/CO ratio by nearly two orders of magnitude (from ~0.1 to ~7) by manipulating H2O/CO2 ratio of feed gas, operating temperature, and current density. Stable operation for >100 h is obtained without evidence of carbon deposition, although high current density operation leads to observable deterioration of anode/electrolyte interface due to the rapid oxygen evolution.

Detailed numerical simulation of primary atomization by crossflow under gas turbine engine combustor conditions

The present study concerns a detailed numerical simulation of primary atomization by crossflow in the flow of a gas turbine engine combustor. At high Weber numbers, such as in a gas turbine engine, the scale ratio of the atomization phenomenon becomes large, and it is necessary to use a relatively fine grid to obtain reliable results. The present study conducts the first numerical analysis in a gas turbine engine condition with a sufficiently fine grid to give a result that can be discussed reliably, and details of the atomization mechanism are clarified for the first time. In a gas turbine engine combustor, Rayleigh–Taylor (RT) instability dominates at the edge of the liquid column. On the other hand, when the liquid–gas density ratio or the Weber number are low, RT instability is not significant. The process of ligament formation changes depending on the dominant instability mechanism, and the ligaments becomes smaller when RT instability is dominant. Also, the size distribution of the atomized droplets clearly changes when RT instability becomes significant.

Droplet impact onto a solid sphere in mid-air: Effect of viscosity, gas density, and diameter ratio on impact outcomes

Collision of a droplet and a hydrophobic particle in mid-air was investigated. To study the impact outcomes, specifically the lamella formation, a numerical simulation tool was developed and verified with impact experiments (water droplets and glass particles, ρrel = 0.41). The velocity field within the lamella showed that the flow inside the liquid film moves in two opposite directions along the lamella axis of symmetry: one is generated through the momentum transfer from the particle, and the other is due to the droplet initial velocity. This causes the lamella to be stretched in the same direction as the particle moves and forms a rim at the end of the lamella. Although a larger droplet-to-particle diameter ratio (Dr) increased the impact duration, it did not change the collision outcomes and two opposite flows still exist inside a thicker liquid film. However, the liquid viscosity affects impact outcomes; as viscosity increased, a thicker film remained on the particle, the liquid film became shorter, and the lamella formation was hindered accordingly. The pressure of the ambient gas also affects the liquid film formation. Unlike the literature of the drop impact on a flat surface, our results indicate that by increasing the ambient pressure, the lamella formation will be suppressed (hence chance of splashing). The pressure gradient around the liquid film creates a downward force that hinders the stretching of the liquid film. The effect of the ambient pressure on lamella formation is only significant for relatively higher gas pressures (i.e., Pamb > 2 Patm).

Numerical analysis of bubble bursting at the liquid surface by wave propagation

Bubble bursting at the liquid surface can generate the jet drops and satellite bubbles, which is widely encountered in boiling, ocean engineering, bioengineering etc. This process is affected by quite a few parameters, such as surface capillarity, gravity, temperature, density, viscosity and bubble size. In this paper, the axisymmetric numerical model with level set method is built up to study the bubble bursting process. It is found that the bubble bursting process can be analyzed through the capillary wave propagation in terms of the tangential angle along the gas-liquid interface. The capillary wave propagation method can describe well the effect of Ohnesorge number, Marangoni number, density and viscosity ratios of gas over liquid on the bubble bursting. The critical Ohnesorge number for jet drop formation during bubble bursting with negligible gravity, gas inertia and viscosity is around 0.045, and it will decrease dramatically with increasing ratios of gas density or viscosity over liquid. The jet formation during bubble bursting can be suppressed by applying the negative temperature gradient across the bubble, or by increasing density and viscosity ratio of gas over liquid, mainly because they can damp the capillary waves. These findings may help get insight in the bubble bursting when the gas properties are transiting towards the liquid under pressurized atmosphere.

Instantaneous mixing characteristics of binary mixtures differing in density in a gas‒solid fluidised bed

Characterisations of instantaneous mixing behaviours in fluidised beds with binary mixtures has many challenges. We studied the instantaneous mixing characteristics of binary mixtures in a 2-D quasi gas‒solid fluidised bed using a capacitance probe method. This method enabled the quantitative assessment of instantaneous mixing behaviours, including mixing index, rate, and time. Three kinds of binary mixtures of similar size but different density were used for the transient and steady fluidisation experiments in a bubbling fluidised bed. The mixing curves of initially segregated binary mixtures were acquired, and the instantaneous characteristics for the entire process were analysed. An equation was proposed to describe the mixing process and predict the mixing degree over time. By comparing experiments with different working conditions, the effects of superficial gas velocity and density ratio on the mixing in the centre and at the wall of the fluidised bed were addressed. During the stable fluidisation stage, the axial concentration profiles of the mixtures were obtained, and the mechanisms of mixing were discussed. Results showed that the mixing level increased logarithmically with time and eventually reached a relatively stable value. Increasing the superficial gas velocity and reducing the density ratio promoted mixing and reduced mixing time. High superficial gas velocities tended to mix the mixtures well and quickly, regardless of the density ratio of the two components.

C-type shock modelling – the effect of new H2–H collisional rate coefficients

ABSTRACTWe consider collisional excitation of H2 molecules in C-type shocks propagating in dense molecular clouds. New data on collisional rate coefficients for (de-)excitation of H2 molecule in collisions with H atoms and new H2 dissociation rates are used. The new H2–H collisional data are state of the art and are based on the most accurate H3 potential energy surface. We re-examine the excitation of rotational levels of H2 molecule, the para-to-ortho-H2 conversion, and H2 dissociation by H2–H collisions. At cosmic ray ionization rates ζ ≥ 10−16 s−1 and at moderate shock speeds, the H/H2 ratio at the shock front is mainly determined by the cosmic ray ionization rate. The H2–H collisions play the main role in the para-to-ortho-H2 conversion and, at ζ ≥ 10−15 s−1, in the excitation of vibrationally excited states of H2 molecule in the shock. The H2ortho-to-para ratio of the shocked gas and column densities of rotational levels of vibrationally excited states of H2 are found to depend strongly on the cosmic ray ionization rate. We discuss the applicability of the presented results to interpretation of observations of H2 emission in supernova remnants.