Annular Liquid Research Articles

Experimental Investigation of Breakup of Annular Liquid Sheet in a Hybrid Atomizer

Disintegration of annular liquid sheets is an important field of study that plays a crucial role in engine combustion. The focus of this study is to analyze different modes of sheets experimentally in an advanced hybrid injector, which uses both pressure swirl and air momentum to impart instability into the sheet. Apart from measurement of certain macroscopic spray physical parameters like breakup length and sheet width, the images for different sheet profiles were processed to obtain fractal dimensions, temporal frequency, and different orthogonal modes using proper orthogonal decomposition. A moderate airflow is seen to create a stable liquid sheet with increased breakup length, whereas higher airflows at low liquid flows create an air-blast type of spray with disintegration very close to the nozzle. The latter shows dominance of multiple smaller temporal frequencies with relatively lower power spectral densities. A spray with high liquid flow and relatively low airflows shows a longer and wider sheet with a single dominant frequency.

Comparison of cryogenic flow boiling in liquid nitrogen and liquid hydrogen chilldown experiments

This paper presents a comparison between experimental results from recent liquid hydrogen (LH2) transfer line chilldown experiments at high Reynolds (Re) numbers versus liquid nitrogen (LN2) experiments conducted at low Re numbers. Parasitic heat leak, inner wall temperatures, inner wall heat fluxes, and heat transfer coefficients are computed to compare between the two systems. Analysis of temperature traces and flow visualization indicates that the chilldown process evolves much more rapidly at higher Re numbers due to a quick transition from vapor flow to annular liquid flow and near immediate liquid contact along the pipe walls. The lower kinematic viscosity and surface tension of LH2, along with reduced parasitic heat leak and higher Re numbers relative to the LN2 experiments, causes chilldown to proceed almost immediately into the nucleate boiling regime, in comparison to low Re flows where >75% of the chilldown is spent in vapor film boiling.

An indirect criterion for the laminar to turbulent flow transition in shear-driven annular liquid films

An indirect method to identify the laminar to turbulent flow transition in shear-driven annular liquid films has been developed and used, together with literature measurements of the velocity profile, to approximately resolve the near wall structure in shear-driven annular liquid films. The limits between the laminar sublayer and the buffer layer and between the buffer layer and the turbulent layer have been found to correspond to about 9 and 40 wall units, respectively, which are higher than the corresponding limits of 5 and 30 wall units typical of single-phase boundary layers, thus indicating a weaker turbulence intensity in shear-driven annular liquid films with respect to single-phase wall-bounded flows. Additionally, a simple laminar to turbulent flow transition criterion and a prediction method for the average liquid film thickness have been developed for evaporation and condensation applications.

The growth of instabilities in annular liquid sheets

An annular liquid sheet surrounded by parallel co-flowing gas is an effective atomiser. However, the initial instabilities which determine the primary break-up of the liquid sheet are not well understood. Lack of agreement on the influence of the boundary conditions and the non-dimension scaling of the initial instability persists between theoretical stability analyses and experiments, since there is little experimental data available on the near-field behaviour of the instability. To address this matter, we have undertaken an experimental parametric study of an aerodynamically-driven, non-swirling annular water sheet. The effects of sheet thickness, inner and outer gas–liquid momentum ratio were investigated over an order of magnitude variation in Reynolds and Weber number. From high-speed image correlation measurements in the near-nozzle region, we propose new empirical correlations for the frequency of the instability as a function of the total gas–liquid momentum ratio, with good non-dimensional collapse. From analysis of the instability velocity probability densities, we find two persistent and distinct superimposed instabilities with different growth rates. The first is a short-lived, rapidly saturating sawtooth-like instability. The second is a slower-growing stochastic instability which persists through the break-up of the sheet. The presence of multiple instabilities whose growth rates do not strongly correlate with the shear velocities may explain some of the discrepancies between experiments and stability analyses.

Interface instability mechanism of an annular viscous liquid sheet exposed to axially moving inner and outer gas

The interface instability mechanism of an annular viscous liquid sheet exposed to axially moving inner and outer gas is investigated. Sheet instability and breakup have been investigated using a nonlinear spatial instability analysis. A perturbation expansion method is used with the initial amplitude of the disturbance as the perturbation parameter. The evolution of the two gas–liquid interfaces is tracked until the sheet breaks up and the breakup length is determined. The model is validated by comparison with available experimental data. To investigate the interface instability mechanism of an annular viscous liquid sheet, the effects of velocity difference between gas and liquid, annular curvature on the annular sheet instability have been studied. The results show that both first-order and second-order maximum disturbance growth rates increase with increase in velocity difference between gas and liquid, and annular liquid sheet curvature. Also, with increasing velocity difference between gas and liquid, large interface deformations occur and the breakup length of the annular viscous liquid sheet becomes shorter. The shear effect difference between the outer and inner sides of the annular liquid sheet is the source of instability.

Measurement of liquid film thickness in micro tube annular flow

Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per unit volume. At large flow rate and high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the heat transfer phenomena. In the present study, time averaged annular liquid film thickness is measured by laser confocal displacement meter (LCDM), and the gas–liquid interface profile is observed by a high-speed camera. Glass tubes with inner diameters of D=0.3, 0.5 and 1.0mm are used. Degassed water and air are used for working fluids, and the total mass flux is varied from G=100 to 500kg/m2s. Flow patterns are observed and flow pattern map based on Reynolds numbers of gas and liquid flows is suggested. Pressure drop is measured and compared with the prediction using Lockhart and Martinelli parameter. Pressure drop is well predicted with Lockhart–Martinelli correlation. Dimensionless mean film thickness is then plotted against quality, and compared with the annular film model assuming flat gas–liquid interface. Flat interface model overestimated the experimental data. It is considered that the shear stress on the gas–liquid interface in the real annular flow is larger than that estimated in the ideal flat interface model. Prediction using new empirical correlation considering the effect of the non-flat gas–liquid interface showed good agreement with the experiment data.

Experimental and computational investigation of vertical downflow condensation

This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.

Spatial instability in annular swirling viscous liquid sheet

The spatial linear instability of an annular swirling viscous liquid sheet moving in an inviscid gas medium was investigated. With both para-sinuous and para-varicose modes analyzed, results show that under certain conditions the mode difference for the maximum disturbance growth rate is small. However, the para-sinuous mode is always the dominant mode for an annular swirling viscous liquid sheet in the parameter range studied here. Liquid swirls have a stabilizing effect at low swirling strengths. Two regions were identified: the initial region, where the maximum disturbance growth rate decreases with increasing dimensionless liquid swirling strength, and a region of monotonic increase of maximum disturbance growth rate with increasing liquid swirling strength. The non-axisymmetric mode is the dominant mode when swirling strength is large in an annular liquid sheet. The drop in axial size of non-axisymmetric mode is larger than that for the axisymmetric mode. Liquid viscosity suppresses increases in the maximum disturbance growth rates and the dominant wavenumber. Surface tension has a destabilizing effect when liquid swirling strength is absent or small, subject to a certain criterion, and a stabilizing effect irrespective of the presence of liquid swirling strength otherwise. Curvature in the annular sheet stabilizes the breakup of the liquid sheet. This effect is more obvious if the liquid swirling strength is large. For different liquid swirling strengths, the dominant disturbance modes of annular liquid sheets become axisymmetric modes as the annular liquid curvature decreases. The dominant wavenumber increases as annular liquid curvature decreases; this trend is more obvious with larger liquid swirling strengths.

Evaluation of Lyapunov-based Adaptive Observer using Low-Order Lumped Model for Estimation of Production Index in Under-balanced Drilling

A distributed drift-flux model and a low-order lumped model describing a multiphase (gas-liquid) flow in the well during Under-Balanced Drilling (UBD) has been presented. This paper presents a novel nonlinear adaptive observer to estimate the total mass of gas and liquid in the annulus and production constant of gas and liquid from the reservoir into the well during UBD operations. Furthermore, it describes a joint unscented Kalman filter to estimate parameters and states for both the distributed drift-flux and lumped model by using real-time measurements of the choke and the bottom-hole pressures. The performance of the adaptive observers are evaluated for typical drilling scenarios. The results show that all adaptive observers are capable of identifying the production index, although the adaptive observers based on the low-order lumped model achieves better convergence rate than adaptive observer based on the drift-flux model. The results show that the LOL model is sufficient for the purpose of estimating the production parameters.

Stability of an annular power-law liquid sheet

Because of the mathematical difficulties dealing with the nonlinear viscous stress term in momentum equation for power-law liquid, the study of stability analysis of a power-law liquid sheet has been lacking. In the present study, a temporal stability analysis has been carried out for an annular power-law liquid sheet exposed to both inner and outer-gas streams, by integrating the governing equations for the power-law liquid sheet. The dimensionless dispersion equation that governs the instability of liquid sheet is obtained by considering the velocity profile of liquid sheet. It is found that the instability of liquid sheet can be enhanced by independently increasing either the outer gas stream velocity, or the inner gas stream velocity. The liquid sheet is more unstable when both inner and outer gas streams are applied. To promote the instability of annular liquid sheet, a gas stream applied to the outer interface is more effective than when applied to the inner surface. The effects of rheological parameters on the instability of the liquid sheet are actually determined by the relative velocity across the gas–liquid interfaces. The surface tension, liquid sheet thickness, and outer surface radius of annular liquid sheet have been tested for their influence on the instability of annular power-law liquid sheet.

Effect of gravity levels on the flow pattern modulation by the phase separation concept

Suspending a mesh cylinder in a tube modulates flow patterns with gas bubbles in the annular region to form thin liquid film on the wall. We investigate the effect of gravity levels on the modulated flow patterns. The volume of fluid (VOF) method simulates the slug bubble train flow in bare tube and modulated flow sections. The bubble population density along the flow direction (β) and averaged liquid film thickness (δα) synthesize a parameter β/δa to characterize the enhancement of phase change heat transfer. It is found that at the normal gravity on earth, counter-flow appears with fast upward flow in the annular region and downward liquid flow in the core region. A lower β due to sparsely populated bubbles and thin liquid film form a larger β/δa to enhance the phase change heat transfer. Convective heat transfer in liquid plugs is enhanced by the fast fluid movement in the annular region and liquid circulations at three length scales. At the miniature gravity, quasi-co-current flow happens with upward flows in both annular region and core region, except that a liquid layer inside mesh cylinder flows downward. Bubbles are more densely populated than those at the normal gravity. Liquid circulation occurs only at the bubble length scale. At the micro gravity, the two-phases are thoroughly separated with co-current flows in both annular region and core region. Gas flows slowly and the residence time of gas is increased to result in β=1. Τhe liquid film is ultra-thin. These two factors create a significantly large β/δa to enhance the phase change heat transfer. We demonstrate the effectiveness of the phase separation concept at miniature and micro gravity environment.

Films in narrow tubes

AbstractWe consider the axisymmetric arrangement of an annular liquid film, coating the inner surface of a narrow cylindrical tube, in interaction with an active core fluid. We introduce a low-dimensional model based on the two-phase weighted residual integral boundary layer (WRIBL) formalism (Dietze & Ruyer-Quil, J. Fluid Mech., vol. 722, 2013, pp. 348–393) which is able to capture the long-wave instabilities characterizing such flows. Our model improves upon existing works by fully representing interfacial coupling and accounting for inertia as well as streamwise viscous diffusion in both phases. We apply this model to gravity-free liquid-film/core-fluid arrangements in narrow capillaries with specific attention to the dynamics leading to flooding, i.e. when the liquid film drains into large-amplitude collars that occlude the tube cross-section. We do this against the background of linear stability calculations and nonlinear two-phase direct numerical simulations (DNS). Due to the improvements of our model, we have found a number of novel/salient physical features of these flows. First, we show that it is essential to account for inertia and full interphase coupling to capture the temporal evolution of flooding for fluid combinations that are not dominated by viscosity, e.g. water/air and water/silicone oil. Second, we elucidate a viscous-blocking mechanism which drastically delays flooding in thin films that are too thick to form unduloids. This mechanism involves buckling of the residual film between two liquid collars, generating two very pronounced film troughs where viscous dissipation is drastically increased and growth effectively arrested. Only at very long times does breaking of symmetry in this region (due to small perturbations) initiate a sliding motion of the liquid film similar to observations by Lister et al. (J. Fluid Mech., vol. 552, 2006, pp. 311–343) in thin non-flooding films. This kickstarts the growth of liquid collars anew and ultimately leads to flooding. We show that streamwise viscous diffusion is essential to this mechanism. Low-frequency core-flow oscillations, such as occur in human pulmonary capillaries, are found to set off this sliding-induced flooding mechanism much earlier.

Reprint of: A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.

A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.

Studying the Interaction of Liquid and Solid in the Flow of Liquid in Annulus by CFD Analysis

This article is aimed to define the flow in the annulus. The shaft is deposited in a case which has at inlet and outlet recess. The rotor moves in the case. This movement can be divided into the rotation and precession. It is situation where the rotor rotates around own axis and together moves along the circle. The solution of the problem was the initiative in practice where this type of movement occurs in many components, e.g. sliding journal bearings. The fluid flows in the annulus due to the pressure gradient between inlet and outlet and together takes place rotational movement due to movement of the rotor. Between the liquid and the solid of an interaction which results in a change in the forces. The problem is solved by mathematical CFD solver ANSYS Fluent. Based on the characteristics of the problem has been defined corresponding to a mathematical model for the selected types of liquids - VG 32 mineral oil, VG 150 mineral oil and water. Individual variants of used liquids are compared with each other courses of forces acting on the rotor under the same boundary conditions.

Numerical investigation of gravitational effects in horizontal annular liquid–gas flow

In this work, exploratory numerical simulations of liquid–gas flows in horizontal pipes are conducted for three different sets of conditions in the annular and stratified-annular flow regimes. Careful dimensional analysis is used to choose governing parameters in a way that yields flows that are relevant to realistic engineering applications, while remaining computationally tractable. Statistics of the velocity field and height of the liquid film are computed as a function of circumferential location in the pipe, demonstrating the existence of a viscous sublayer within the liquid film, as well as a viscous layer near the interface and a log law region within the gas core. The probability of dry-out conditions at the wall in upper regions of the pipe is shown to increase as gravitational effects increase. Circumferential motion of the liquid and gas phases within the pipe cross section are analyzed, informing possible mechanisms for sustainment of the liquid film. A simple model is developed that helps to characterize the dynamics of the liquid annulus and aids in understanding the effect of secondary gas flow on the circumferential motion of the film. Void fraction, film height, and film asymmetry are compared with experimental correlations available in the literature.

Electro-viscous instability of a swirling compound jet

The aim of this work is to study the effects of a combination of axial and swirl velocity on the instability of an annular liquid jet. A linear instability analysis yields an analytical dispersion relation, which governs the temporal growth of perturbations to an electrified streaming annular viscous liquid jet coated by two cylindrical shells of streaming swirling inviscid gases. The three-layer system is subjected to an electric field normal to the interfaces. In our study, the electric body force vanishes, and the electric field and fluid dynamics are coupled only at the interfaces through the tangential and normal interfacial stress balance equations. The governing electrohydrodynamic equations, along with the hydrodynamic and electrical boundary conditions, are solved, and then the corresponding dispersion relation is obtained. The instability analysis of a inviscid jet is discussed as a special case. Numerical results on the effect of the various problem parameters on the instability and breakup of a liquid jet are presented. It is observed, for instance, that the swirling ratio of the outer gas layer always enhances the stability of flow in the case of axisymmetric disturbances, whereas it has a dual effect (destabilizing effect) corresponding to the case of asymmetric disturbances. It has been found that the higher electrical potential, the larger the maximum growth rate and critical wave number of the disturbances and, consequently, the initial applied electric potential tends the instability and breakup of the liquid jet.

Extended depth of field using a liquid crystal annular spatial light modulator.

A detailed investigation is presented on the tunable extended depth of field (EDOF) method, proposed recently by Klapp etal. [Opt. Lett.39, 1414 (2014)]. This method is based on temporal multiplexing of phase masks, using an annular liquid crystal spatial light modulator possessing a small number of rings. Examples of 3D simulations used to determine the phase profiles in the pupil plane are presented, as well as more detailed experimental results. Both the experimental and numerical results include comprehensive analysis of contrast dependence on both the spatial spectrum of the object and the amount of defocus. In addition, for the first time, we present the EDOF order inversion in the experimental and simulated data. The results show a profound performance of the proposed system and method.

Investigation of the Effect of Liquid Viscosity on the Primary Break Up Lengths and Droplet Diameter and Ligament Diameter and Instability of a Swirling Annular Liquid Sheet

Investigation of the Effect of Liquid Viscosity on the Primary Break Up Lengths and Droplet Diameter and Ligament Diameter and Instability of a Swirling Annular Liquid Sheet

Intermittent dewetting and dryout of annular flows

Flow visualisation of saturated flow boiling of refrigerant R245fa in a silicon parallel multi-microchannel evaporator, at low mass flux and moderate uniform heat flux, has been carried out with a high-speed digital camera. Using the time-strip technique to post-process the recorded image sequences has revealed profound details regarding the intermittent dryout mechanism of annular flows. Features of these observed phenomena have been successfully correlated with the dynamics expected from a ruptured metastable liquid film under shear. These observations can act as a basis for the development of new mechanistic critical heat flux prediction models and the design of future high heat flux devices. This study also reveals details concerning the nature of the droplet entrainment-deposition process and nucleate boiling in very thin annular liquid films.