Surface Of Liquid Jet Research Articles

Flow and spray characteristics of a gas–liquid pintle injector under backpressure environment

Both the flow and spray characteristics of a gas–liquid pintle injector element under the backpressure environment were investigated experimentally and numerically. The cold atomization tests were conducted with the backpressure range from 0.50 to 1.54 MPa. Both the interaction process between gas film and liquid jets and the detail distribution of the spray were obtained by the verified volume of fluid to discrete phase model. Results showed that there is a local high-pressure zone at the root of the liquid jets resulted by the collision of gas film and liquid jet. A semi-empirical model for predicting the discharge coefficient of the orifices is proposed considering the effect of local high-pressure zones based on the experiments. It was found that the discharge coefficient is mainly affected by the local momentum ratio (LMR) and ambient pressure. The discharge coefficient increases with the increase in LMR and ambient pressure. Before the primary breakup occurs, the liquid jets deform from rectangle jets to bow-shaped liquid films under the effect of the gas film. Then, both the gas and liquid mix in the range included by the gas passed by the windward surface and side of liquid jets. The droplet size is larger at the edges of the spray and the Sauter mean diameter (SMD) is beyond 100 μm. On the contrary, it is relatively small and uniform at the spray central, and the corresponding SMD is about 50 μm.

Cooling enhancement for engine parts using jet impingement

A computational study has been performed to evaluate the use of jet impingement for cooling applications in the automotive industry. The current study uses an entire internal combustion engine cylinder with its components as a computational domain. An unsteady numerical solution for the Navier-Stokes equations was carried out using Improved Delayed Detached Eddy Simulation (IDDES). The volume of fluid approach is proposed to track and locate the liquid jet surface that is in contact with the air. The conjugate heat transfer approach is used to link the heat transfer solution between the fluid and the solid. The boundary conditions that are employed in the study are provided from lab experiments and one-dimensional simulations. The cooling jet in this study targets the hottest region in the piston, i.e., the region underneath the exhaust valve. Three nozzle sizes with flows at different Reynolds numbers are chosen to examine the thermal characteristics of the cooling jet. The computational study reveals that for a specific Reynolds number, the smaller diameter nozzle provides the highest heat transfer coefficient around the impingement point. The maximum relative velocity location at the impingement point slightly leads the location of the maximum Nusselt number. The maximum temperature in the piston decreases by 7% to 11% as the nozzle diameter changes from 1.0 to 3.0 mm for a jet Reynolds number of 4,500. If a correct selection is made for the nozzle size, the cooling jet can be efficiently used to reduce the temperature and alleviate the thermal stresses in the piston in the region underneath the exhaust valve where the maximum temperature occurs.

Numerical Study of the Dynamics of a Gas Bubble Near a Wall Under Ultrasound Excitation

A numerical study of the dynamics of a gas bubble in water near a plane rigid wall under ultrasound excitation has been performed. The bubble dynamics is excited at the natural frequency of the radial oscillations of the bubble in the absence of the wall. The dynamics of the bubble with the formation on its surface of a cumulative liquid jet directed to the wall is considered. A regime of the ultrasound excitation is investigated, at which the cumulative jet impact against the bubble surface part closer to the wall is realized at the moment of the beginning of the second expansion of the bubble. At this moment two known mechanisms of emitting pressure pulses into liquid by a bubble start working simultaneously. The first one is due to the bubble expansion. The second one results from the impact of the cumulative jet. The boundary element method is used. The study has revealed the dependencies of the ultrasound excitation amplitude, at which the indicated regime is realized, on the distance between the bubble and the wall, as well as the influence of the bubble-to-wall distance on the jet impact velocity and the pressure inside the bubble corresponding to that regime.

Breakup process of liquid jet in gas film

In order to study the breakup process of liquid jet in gas film, the backlit photography technique and the VOF TO DPM method are used for experimental and simulation research respectively. Water and air are used as simulant media. Grid adaptive technology is used to refine the gas-liquid interface grid and improve the capture accuracy of the gas-liquid interface. The results show that there are two main breakup processes of liquid jet in gas film: column breakup and surface breakup. The local high-pressure zone in front of the liquid jet makes the jet have a large normal velocity gradient, which causes R-T instability. The surface wave that is generated by the R-T instability is mainly responsible for the liquid column breakup. When the thin liquid film reaches a column breakup point, the airflow penetrating the trough of the surface wave causes the jet column to break. The tangential velocity gradient is generated when the gas film bypasses the liquid jet surface, which causes K-H instability. The K-H surface waves cause ligaments and droplets to strip from the surface of the liquid jet. The local momentum ratio has an important influence on the breakup process of the liquid jet in gas film. When the local momentum ratio is low, the breakup of liquid jet is dominated by the K-H instability. As the local momentum ratio increases, the breakup of liquid jet is gradually dominated by R-T instability. The local momentum ratio plays an important role in the distribution range of the liquid jet in gas film. When the local momentum ratio is low, the ligaments and droplets caused by the liquid jet are mainly distributed within the range of gas film. As the local momentum ratio increases, part of the ligaments and droplets escape from the range of the gas film. The liquid jet penetrates the gas film when the local momentum ratio is greater than 0.74. The breakup length and the penetration depth are both affected by the local momentum ratio. The breakup length increases with the local momentum ratio increasing. The penetration depth also increases with the local momentum ratio, and the penetration depth increases significantly when the liquid jet penetrates the gas film.

Effect of elevated pressure on air-assisted primary atomization of coaxial liquid jets: Basic research for entrained flow gasification

Highly resolved numerical simulations have been conducted for a generic, coaxial air-blast atomizer designed for fundamental research of entrained flow gasification processes. Objective of the work is to gain a detailed knowledge of the influence of elevated reactor pressure on the primary atomization behaviour of high-viscous liquid jets. In agreement with measured breakup morphology and breakup regimes proposed in literature, the simulations yield a pulsating mode instability of liquid jet, along with disintegrations of fibre-type liquid fragments for different pressures. From the mechanism point of view, the breakup process has been shown to be triggered by concentric, axisymmetric ring vortices, which disturb the liquid jet surface in a first stage and penetrate further into the intact core, leading to interfacial instabilities and pinch-off of liquid ligaments. The liquid jet breaks up faster at elevated pressure, leading to a shorter core length LC. The calculated exponent (b≈−0.5) of the power law for fitting the decrease of LC with p agrees well with measured correlations from literature in terms of varied momentum flux ratio M and Weber number WeG, although water jets, atmospheric pressure and different air-assisted, external mixing nozzles were used in these works. Therefore, the effect of elevated pressure is equivalent to that of increased M or WeG, which scale linearly with p or the gas density for the current setup. The specific kinetic energy of liquid kL has been found to be increased with p, which is particularly pronounced in the high frequency range. A first-order estimate has been proposed, which can be used for the evaluation of liquid kinetic energy or droplet velocity within the spray. The results have been validated by simulations with twice-refined resolution, yielding a grid-independence behaviour with respect to the primary breakup characteristics. However, the follow-up processes with secondary breakup and spray dispersion are reproduced better by using the finer grid.

Comparative study on interfacial oscillations of rectangular and elliptical liquid jets

The instability characteristics and flow structures of water jets injected from rectangular and elliptical nozzles with aspect ratios varying from 2 to 6 were experimentally studied and compared. Shadowgraph technique was employed for flow visualization, and structures on the liquid jet surface were captured using high speed photography. It was found that disturbances originating from the nozzle geometry initially perturbed the liquid column, and then, at high jet velocities, disturbances generated within the flow dominated the jet surface. It was also found that rectangular nozzles introduced more disturbances into the flow than the elliptical ones. The characteristic parameters of axis-switching phenomenon including wavelength, frequency, and amplitude were measured and compared. Axis-switching wavelength was found to increase linearly with Weber number. Also, the wavelengths of rectangular jets were longer than the elliptical jets. Further, the frequency of axis-switching was shown to be reduced with increase of both Weber number and aspect ratio. It was observed that the axis-switching amplitude increased monotonically, reached a peak, and then decreased gradually. It was also found that the axis-switching amplitude varied with Weber number. At lower values of Weber number, the rectangular nozzles had higher amplitude than the elliptical nozzles. However, at higher values of Weber number, this relation was reversed, and the elliptical nozzles had the higher axis-switching amplitudes. This reversal Weber number decreased with the orifice aspect ratio. The reversal Weber number for aspect ratio of 4 was about 289, and it had decreased to 144 for the aspect ratio of 6.

Analysis of the results of an experimental study of the process of vapor condensation on the surface of the cylindrical free-draining liquid jet (Part 2)

Analysis of the results of an experimental study of the process of vapor condensation on the surface of the cylindrical free-draining liquid jet (Part 2)

Interferometric velocity measurements through a fluctuating interface using a Fresnel guide star-based wavefront correction system

To improve optical measurements, which are degraded by optical distortions, wavefront correction systems can be used. Generally, these systems evaluate a guide star in transmission. The guide star emits well-known wavefronts, which sample the distortion by propagating through it. The system is able to directly measure the distortion and correct it. There are setups, where it is not possible to generate a guide star behind the distortion. Here, we consider a liquid jet with a radially open surface. A Mach–Zehnder interferometer is presented where both beams are stabilized through a fluctuating liquid jet surface with the Fresnel guide star (FGS) technique. The wavefront correction system estimates the beam path behind the surface by evaluating the incident beam angle and reflected beam angle of the Fresnel reflex with an observer to control the incident angle for the desired beam path. With this approach, only one optical access through the phase boundary is needed for the measurement, which can be traversed over a range of 250 μm with a significantly increased rate of valid signals. The experiment demonstrates the potential of the FGS technique for measurements through fluctuating phase boundaries, such as film flows or jets.

Experimental investigation of spray characteristics of a liquid jet in a turbulent subsonic gaseous crossflow

Two perforated plates with different solidity ratios, S=50% and 67%, were used to investigate the effect of the velocity fluctuations of a subsonic gaseous crossflow on the spray characteristics of a liquid jet including droplet size and velocity distributions. The experiments were conducted over a range of jet-to-crossflow momentum flux ratio of q=16.5-172, and two gas Weber numbers of Weg=2.7 and 5.9, corresponding to the enhanced capillary breakup and bag breakup regimes, respectively. The experimental results of this study revealed that the distribution of droplets size associated with a turbulent and a uniform crossflow for each specific breakup regime were approximately identical. The bimodal and single peak distributions of droplets size, respectively, associated with enhanced capillary and bag breakup regimes were generally consistent with the literature reports. However, the transition of the liquid primary breakup regime from enhanced capillary to bag breakup mode was delayed in a turbulent crossflow compared to its uniform counterpart. The general behavior of droplets size-velocity profiles were also consistent with the literature reports. Nonetheless, complex variations in the distribution of droplets velocity when changing the crossflow turbulence intensity were observed and linked with the presence of instabilities on the liquid jet's surface. Finally, the present experiments allowed shedding more light on the reason why the breakup mechanisms of a liquid jet in a conventional uniform crossflow should not be generalized to predict the distinct breakup process of a liquid jet in a turbulent crossflow.

Temporal analysis of breakup for a power law liquid jet in a swirling gas

The breakup mechanism and instability of a power law liquid jet are investigated in this study. The power law model is used to account for the non-Newtonian behavior of the liquid fluid. A new theoretical model is established to explain the breakup of a power law liquid jet with axisymmetric and asymmetric disturbances, which moves in a swirling gas. The corresponding dispersion relation is derived by a linear approximation, and it is applicable for both shear-thinning and shear-thickening liquid jets. Analysis results are calculated based on the temporal mode. The analysis includes the effects of the generalized Reynolds number, the Weber number, the power law exponent, and the air swirl strength on the breakup of the jet. Results show that the shear-thickening liquid jet is more unstable than its Newtonian and shear-thinning counterparts when the effect of the air swirl is taken into account. The axisymmetric mode can be the dominant mode on the power law jet breakup when the air swirl strength is strong enough, while the non-axisymmetric mode is the domination on the instability of the power liquid jet with a high We and a low Re n . It is also found that the air swirl is a stabilizing factor on the breakup of the power law liquid jet. Furthermore, the instability characteristics are different for different power law exponents. The amplitude of the power law liquid jet surface on the temporal mode is also discussed under different air swirl strengths.

On the stability of capillary waves on the surface of a space-charged dielectric liquid jet moving in a material medium

We have derived and analyzed the dispersion equation for capillary waves with an arbitrary symmetry (with arbitrary azimuthal numbers) on the surface of a space-charged cylindrical jet of an ideal incompressible dielectric liquid moving relative to an ideal incompressible dielectric medium. It has been proved that the existence of a tangential jump of the velocity field on the jet surface leads to a periodic Kelvin–Helmholtz- type instability at the interface between the media and plays a destabilizing role. The wavenumber ranges of unstable waves and the instability increments depend on the squared velocity of the relative motion and increase with the velocity. With increasing volume charge density, the critical value of the velocity for the emergence of instability decreases. The reduction of the permittivity of the liquid in the jet or an increase in the permittivity of the medium narrows the regions of instability and leads to an increase in the increments. The wavenumber of the most unstable wave increases in accordance with a power law upon an increase in the volume charge density and velocity of the jet. The variations in the permittivities of the jet and the medium produce opposite effects on the wavenumber of the most unstable wave.

Erratum to: Turned trochoidal disturbance on a liquid jet surface

Erratum to: Turned trochoidal disturbance on a liquid jet surface

Turned trochoidal disturbance on a liquid jet surface

This paper shows that a turned trochoidal function disturbance may lead to peripheral drops production. The resulting model is used to describe that a turned trochoidal disturbance leads to peripheral drops production on the liquid jet surface without the necessity for superimposed disturbances. The trochoid is a non-unique parametric function. Only non-unique parametric functions disturbances may lead to peripheral drops production. The trochoidal function disturbance is decomposed to Fourier series. Every Fourier element receives an amplification factor in accordance to the Rayleigh inviscid jet model. Peripheral drops are received on the jet surface. The paper shows that all trochoidal disturbance functions, prolate cycloid, cycloid and curtate cycloid have a capability of peripheral drops producing. A limited capability of peripheral drops production is introduced for the trochoidal curtate cycloid. Produced drops size are reduced for increasing the jet velocity and wave number. Smaller drops are also received by transition from the prolate cycloid to curtate cycloid disturbance.

Turned trochoidal disturbance on a liquid jet surface

This paper shows that a turned trochoidal function disturbance may lead to peripheral drops production. The resulting model is used to describe that a turned trochoidal disturbance leads to peripheral drops production on the liquid jet surface without the necessity for superimposed disturbances. The trochoid is a non-unique parametric function. Only non-unique parametric functions disturbances may lead to peripheral drops production. The trochoidal function disturbance is decomposed to Fourier series. Every Fourier element receives an amplification factor in accordance to the Rayleigh inviscid jet model. Peripheral drops are received on the jet surface. The paper shows that all trochoidal disturbance functions, prolate cycloid, cycloid and curtate cycloid have a capability of peripheral drops producing. A limited capability of peripheral drops production is introduced for the trochoidal curtate cycloid. Produced drops size are reduced for increasing the jet velocity and wave number. Smaller drops are also received by transition from the prolate cycloid to curtate cycloid disturbance.

Liquid entrainment behavior at the nozzle exit in coaxial gas–liquid jets

The near-field flow behavior of coaxial gas–liquid jets has a significant influence on the atomization performance and life cycle of atomizers, and the present study focused on this issue. The flow visualization technique was employed to examine liquid entrainment behavior near the nozzle. The thick central tube affected both gas and liquid flow fields. Liquid entrainment could be observed in the near-field with the increase in gas velocity, and a liquid “bulge” structure was induced on the surface of the near-field liquid jet for the first time. The formation mechanism and dynamic characteristics of the “bulge” were studied. Two critical situations in the liquid entrainment process to the surface of the central tube were identified as “initial entrainment” and “full entrainment”. The two critical situations were studied in a convective gas–liquid layer in the recirculation region based on total pressure conservation theory. The theoretical models for the two situations were obtained, and these models agreed well with the experimental results.

LES of turbulent liquid jet primary breakup in turbulent coaxial air flow

A robust two-phase flow Large Eddy Simulation (LES) algorithm has been developed and applied to predict the primary breakup of an axisymmetric water jet injected into a surrounding coaxial air flow. The high liquid/gas density and viscosity ratios are known to represent a significant challenge in numerical modelling of the primary breakup process. In the current LES methodology, an extrapolated liquid velocity field was used to minimise discretisation errors, whilst maintaining sharp treatment of fluid properties across the interface. The proposed numerical approach showed excellent robustness and high accuracy in predicting coaxial liquid jet primary breakup. Since strong turbulence structures will develop inside the injector at high Reynolds numbers and affect the subsequent primary breakup, the Rescaling and Recycling Method (R2M) was implemented to facilitate generation of appropriate unsteady LES inlet conditions for both phases. The influence of inflowing liquid and gas turbulent structures on the initial interface instability was investigated. It is shown that liquid turbulent eddies play the dominant role in the initial development of liquid jet surface disturbance and distortion for the flow conditions considered. When turbulent inflows were specified by the R2M technique, the predicted core breakup lengths at different air/water velocities agreed closely with experimental data.

On derivation of the analytical expression for damping ratios in a low-viscosity approximation

The damping ratios of waves and oscillations in nonlinear dispersion equations are found for planar, cylindrical, and spherical geometries as applied to finite-volume liquids. For a cylindrical jet and a plane interface between viscous liquids, the damping ratios are determined for the first time. When the radius of curvature of the liquid jet surface decreases, so does the damping ratio of capillary waves. In a system of immiscible liquids, the damping ratio may be both larger and smaller than that for the pure liquid depending on the viscosity of the liquids and the ratio of their densities. This is because the damping ratio depends on the kinematic viscosities of pure liquids. The damping ratio is also estimated for waves arising at the liquidgas interface due to a tangential discontinuity of the velocity field.

A coupled level-set and volume-of-fluid method for the buoyant rise of gas bubbles in liquids

This paper investigates some fundamental aspects of rising gas bubbles in stagnant Newtonian liquids using a coupled level-set and volume-of-fluid (CLSVOF) method. The coupled method not only calculates the geometric properties (normal and curvature) of the bubble surface accurately but also satisfies compliance of mass conservation very well. The method solves a single set of governing equations for both phases using variable transportive properties on a fixed Eulerian two-dimensional mesh in axisymmetric coordinates. We computed the terminal shapes and Reynolds numbers (Re) of isolated gas bubbles rising in stagnant liquids for low to high Eötvös number (Eo<340) and Morton number (M<44). In addition, the drag co-efficients for a single bubble rising in liquid with M⩾10−4 are computed. The rise and shape of two co-axial gas bubbles with same size in stagnant liquid and the evolution history of the bubble coalescence process are examined. Furthermore, the mechanisms of bubble-bursting at a free surface and formation of liquid jets are also studied qualitatively. The computational results are compared with the experimental results available in literature. It is observed that the predicted results are in good agreement with experimental counterpart. Finally, the formation of gas bubbles from a submerged orifice in quiescent and co-flowing liquids for low Reynolds number flows is discussed where the viscosity ratio of liquid to gas is about 16, 474.

Combined aerodynamic and electrostatic atomization of dielectric liquid jets

The electrical and atomization performance of a plane–plane charge injection atomizer using a dielectric liquid, and operating at pump pressures ranging from 15 to 35 bar corresponding to injection velocities of up to 50 m/s, is explored via low current electrical measurements, spray imaging and phase Doppler anemometry. The work is aimed at understanding the contribution of electrostatic charging relevant to typical higher pressure fuel injection systems such as those employed in the aeronautical, automotive and marine sectors. Results show that mean-specific charge increases with injection velocity significantly. The effect of electrostatic charge is advantageous at the 15–35 bar range, and an arithmetic mean diameter D 10 as low as 0.2d is achievable in the spray core and lower still in the periphery where d is the orifice diameter. Using the data available from this higher pressure system and from previous high Reynolds number systems (Shrimpton and Yule Exp Fluids 26:460–469, 1999), the promotion of primary atomization has been analysed by examining the effect that charge has on liquid jet surface and liquid jet bulk instability. The results suggest that for the low charge density Q v ~ 2 C/m3 cases under consideration here, a significant increase in primary atomization is observed due to a combination of electrical and aerodynamic forces acting on the jet surface, attributed to the significantly higher jet Weber number (We j) when compared to low injection pressure cases. Analysis of Sauter mean diameter results shows that for jets with elevated specific charge density of the order Q v ~ 6 C/m3, the jet creates droplets that a conventional turbulent jet would, but with a significantly lower power requirement. This suggests that ‘turbulent’ primary atomization, the turbulence being induced by electrical forces, may be achieved under injection pressures that would produce laminar jets.

Stability of capillary waves with arbitrary symmetry on the surface of a jet in a longitudinal electric field periodically varying with time

The Mathieu differential equation for the evolution of the amplitudes of arbitrarily symmetric capillary waves (with arbitrary azimuthal numbers) propagating over the surface of a incompressible dielectric cylindrical liquid jet is analyzed. The jet is placed in a time-periodic uniform electric field that is parallel to the symmetry axis of the jet unperturbed by the wave flow. It is found that the time-varying electric field pressure parametrically builds up both axisymmetric waves on the jet surface, flexural waves, and flexural deformation waves. At a fixed frequency of the external field, waves with different wavelengths and symmetries (different azimuthal numbers) may build up simultaneously in the main demultiplication resonance, as well as in secondary and tertiary resonances. The parametric buildup of flexural deformation waves has a threshold relative to the external field frequency: it takes place at the field frequency exceeding a certain value depending on the jet radius and physicochemical properties of the liquid.