Turbulent Liquid Jet Research Articles

Effects of radial fin arrangements on the thermal performance of turbulent liquid jet impingement heat sinks: Experimental and numerical approach

The combination of fins and jet impingement is a highly effective method for enhancing thermal performance in electronic cooling. The arrangement of fins significantly influences the thermohydraulic characteristics of heat sinks. A comprehensive analysis comparing the impact of different radial fin arrangements in confined jet impingement is crucial for a deeper insight into their thermohydraulic performance. This study presents experimental analyses of heat sinks with smooth surfaces and radial straight fins in the Reynolds number ranged from 4300 to 15,500 and the heat flux ranged from 45 to 105 W/cm2. Then a numerical study is conducted on the radial straight and curved fin arrangement to investigate the effects of different radial fin arrangements on the performance of jet impingement heat sinks. The results demonstrate that the use of fins improves heat sink performance by up to 27 % compared to smooth surfaces. The numerical model, employing the SST k-ω turbulent model, shows strong agreement with experimental findings, with a maximum deviation of 9.1 % for the heat transfer coefficient, well within the uncertainty range. Comparing radial straight and curved fin arrangements numerically, the straight fin arrangement exhibits superior thermal performance by up to 9 %. Additionally, the pressure drop for both configurations remains nearly identical. The findings of this research suggest the use of radial straight fin arrangements in annular jet impingement heat sinks over radial curved fin arrangements.

Modeling of primary breakup considering turbulent nozzle flow, internal turbulence and surface instability of liquid jet using turbulence decay theory

In heat engines utilizing fuel injection, the processes of atomization and spray formation have a significant impact on the combustion process, thereby determining both efficiency and emission characteristics. Accurate prediction and control of spray formation in fuel injection systems play a key role in improving the efficiency and environmental performance of thermal engines, especially with the emergence of carbon-neutral fuels. To achieve accurate prediction of spray mixture formation, it is imperative to refine the atomization model for the liquid jet within numerical simulations. This requires a phenomenological representation of the atomization process that avoids reliance on computational constants obtained from spray experimental results. Consequently, the present study attempts to mathematically model the turbulent nozzle flow and liquid jet atomization process, leading to the development of a novel primary breakup model. The construction of the primary breakup model involves an analysis of the turbulence at the nozzle inlet. By merging this turbulence with the turbulence resulting from wall shear flow within the nozzle, the model provides insight into the internal turbulence and surface instability of the liquid jet, encompassing the turbulence spectrum. Consequently, the influence of nozzle length on the turbulent flow within the nozzle can be understood, and the droplet formation characteristics of the liquid jet can be predicted along with its multi-wavelength dispersion characteristics. The model effectively captures the experimental results in terms of breakup length and droplet dispersion characteristics, thus adding a higher level of accuracy to numerical simulations. Ultimately, the in-depth study of this model, coupled with its comparison with experimental results, enhances the understanding of the liquid jet atomization process.

A numerical study of an atomizing jet in a resonant acoustic field

By applying state-of-the-art, high-fidelity methods for compressible multiphase flows, this work features a parametric study of a turbulent liquid jet atomizing under the influence of standing acoustic waves with five different amplitude values. We perform these simulations in a semi-periodic domain to isolate interactions between acoustic and atomization processes and to emulate a pure liquid fuel spray placed at the pressure node of a resonant acoustic field. Characterization of the flow includes measurements of the mean dimensions, droplet number, and surface area of the liquid distribution, alongside qualitative depictions of the interface evolution, breakup events, and hydrodynamic fields. As acoustic forcing is provided to the flow, the added energy provokes atomization when shear would otherwise be insufficient for breakup. The magnitude of the forcing determines the rate of droplet production, which accelerates as the evolving liquid surface modifies the induced motion of the gas phase. At the nodal plane, the acoustic field produces an organizing effect on the liquid structures.

Impact of the jet velocity on the turbulent liquid jet

Impact of the jet velocity on the turbulent liquid jet

Inhibiting flow-accelerated copper corrosion under liquid jet impingement by utilizing nanobubbles

Flow-accelerated corrosion can be very destructive, leading to rapid loss of component efficiency and eventual failure of the system; however, finding inexpensive, effective, and chemically benign inhibitors is still challenging. This paper studied for the first time the use of nanobubbles/ultrafine bubbles as a “green” functional nanostructured material for inhibiting flow-accelerated corrosion under the impingement of a turbulent liquid jet. We examined the corrosion inhibition of the copper specimen for 5 h in the flow regime at a Reynolds number of from 6.1 × 103 to 3.0 × 104, with adding ∼6 × 107 bubbles/mL of air-nanobubbles to the tested liquid of 0.25% CuCl2 solution at 40 °C. We measured weight losses, analyzed microstructure using SEM, and acquired maximum erosion depths and roughness curves of tested copper specimens. We found that nanobubbles mitigated flow-accelerated corrosion under jet impingement. The inhibition effectiveness was more significant at higher jet velocity and longer test time, with up to 43.9 ± 0.1% and 69.5 ± 1.8% based on weight losses and erosion depths, respectively. Nanobubbles play a role in reducing wall shear stress on the copper surface, likely by generating bubble mattresses. We suggest that nanobubbles inhibit the corrosion in highly erosive/corrosive turbulent conditions where most inhibitors could not work well.

Impact of Surrounding Gas Density on the Turbulent Liquid Jet

This numerical study aims to investigate the impact of surrounding gas density on liquid turbulent jet. Incompressible large eddy simulations (LES) with Wall-Adapting Local Eddy-Viscosity (WALE) sub-grid scale model in ANSYS-FLUENT were performed to capture the morphology of the breakup as well as the important flow field characteristics. A volume of fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Different variables have been examined to assess this impact. These variables are instantaneous velocity, liquid volume fraction, and turbulent kinetic energy. Also, the centerline values have been investigated to obtain the jet half-width and spray dispersion angle. The jet diameter and exit velocity are 0.2 mm and 100 m/s, respectively. Three different liquid-to-gas density ratios of 54.03, 32.42 and 23.49 are considered, corresponding to ambient gas density of 15, 25 and 34.5 kg/m3, respectively. It has been found that the jet speed and the liquid core length are inversely proportional to the density of the ambient gas. Regarding the liquid core length, it was noted that the effect of gas density on the liquid core length is considerably stronger in the gas density range between 15 - 25 kg/m³ (corresponding to density ratios of 54.03 - 32.42) than that in the range of 25 - 34.5 kg/m³ (corresponding to density ratios of 32.42 - 23.49). The results, also, showed that increasing the gas density leads to an increase in Weber number, which in turn causes a high instability, that makes the breakup initiates closer to the nozzle exit and occurs with higher rate. Likewise, it has been found that the jet width and the spray dispersion angle increase with increasing the gas density. HIGHLIGHTS Spray jets are included in many applications because its important in increasing the heat and mass transfer between the liquid and the surrounding fluid Large eddy simulations (LES) and volume of fluid (VOF) models are used to investigate the characteristics of turbulent liquid jet The investigation was characterized as a function of man variables such as turbulent kinetic energy, liquid potential core, the jet dispersion angle and more The investigation showed that the density of surrounding gas has an influence on the liquid jet characteristics GRAPHICAL ABSTRACT

Massive production of fibroin nano-fibrous biomaterial by turbulent co-flow

Among the different polymers (proteins, polysaccharides, etc.) that make up natural fibers, fibroin is a protein produced by silk spinning animals, which have developed an optimized system for the conversion of a highly concentrated solution of this protein into high-performance solid fibers. This protein undergoes a self-assembly process in the silk glands that result from chemical gradients and by the application of mechanical stresses during the last step of the process. In the quest for a process that could mimic natural spinning at massive scales, we have discovered that turbulence offers a novel and promising solution: a turbulent liquid jet can be formed by a chemically green and simple coagulating liquid (a diluted solution of acetic acid in etanol) co-flowing with a concentrated solution of fibroin in water by the use of a Flow Blurring nebulizer. In this system, (a) the co-flowing coagulant liquid extracts water from the original protein solution and, simultaneously, (b) the self-assembled proteins are subjected to mechanical actions, including splitting and stretching. Given the non-negligible produced content with the size and appearance of natural silk, the stochastic distribution of those effects in our process should contain the range of natural ones found in animals. The resulting easily functionalizable and tunable one-step material is 100% biocompatible, and our method a perfect candidate to large-scale, low-cost, green and sustainable processing of fibroin for fibres and textiles.

Effect of nanofluids on heat transfer characteristics of an aerodynamic swirl nozzle for isothermal and isoflux conditions

ABSTRACT Achieving axisymmetric boundary conditions in turbulent swirl flows and predicting thermo-fluidic behaviours within a nozzle is often a challenge due to the difficulties of measurement diagnostics. Adding nanoparticles to ase fluids exacerbates the complexity because of the regeneration of effective properties of the fluid medium. As such, this study numerically investigates the flow and thermal characteristics for the effect of swirl and nanofluids of incompressible turbulent liquid jets. The results show that the axial and tangential velocity profiles for water-based nanofluids resemble to those for water in no-to-high swirl flows. The surface Nusselt number for strong swirl flow reduces with axial distances at isothermal wall condition, and the Nusselt number is strongly dependent on types of nanofluid for isoflux condition. Water-based nanofluids cause higher heat transfer compared to Dowtherm+Al2O3 and Syltherm800+ Al2O3. The average Nusselt number and the pressure drop are found to be the largest for 10% volume fraction. The increase of thermal-hydraulic performance factor for nanofluids after nozzle convergence is steeper for highly swirling flow than for non-swirling and medium swirling cases. A strong turbulent kinetic energy is observed immediately after the nozzle converging section for all nanofluid cases and Dowtherm+Al2O3 shows relatively uniform distributions across the nozzle.

A conservative level set method for liquid-gas flows with application in liquid jet atomisation

In this paper, a methodology for modelling two-phase flows based on a conservative level set method in the framework of finite volume method is presented. The novelty of the interface capturing method used here lies on the advection of level set which is solved with a WENO scheme and corrected with a novel re-initialisation method for retaining its signed distance function character. The coupling with the volume of fluid method is done with a simple algebraic approach, and with the new algorithm the accumulated mass conservation errors remain reasonably low. The paper presents a unique coupling between the level set method and the Eulerian-Lagrangian Spray Atomisation approach for modelling spray dispersion in liquid atomisation systems. The method is shown to have good accuracy providing similar results to other numerical codes for the classical tests presented. Preliminary results are also shown for three-dimensional simulations of the primary break-up of a turbulent liquid jet obtaining results comparable to direct numerical simulations. Consequently, the coupled method can be used for simulating various two-phase flow applications offering an accurate representation of the interface dynamics.

Implementation of a probabilistic surface density volume of fluid approach for spray atomisation

Eulerian Stochastic methods in Large Eddy Simulation context have been successfully used in reactive flows. They have been recently applied to spray atomisation, using surface and liquid volume as independent variables. These methods involve the solutions of Stochastic Partial Differential Equations, which are continuous in space and discontinuous in time. The implementation of these equations into a conventional finite volume solver can be challenging due to both the numerical diffusion of the scalar fields and the numerical integration of the stochastic source terms. This work presents the implementation and validation of the Eulerian Stochastic Fields surface/liquid volume atomisation method within the OpenFOAM framework. The paper discusses how algebraic compression methods can be used for the stochastic fields to minimise the numerical diffusion and the effects of the stochastic source term implementation. The model targets a-priori spray atomisation at large Weber numbers, although the implementation recovers the correct solution at low Weber model and captures correctly the capillary effects on liquid–gas interfaces. A turbulent liquid jet is used for validation and discussion of the accuracy and relative cost of the model implementation. The approach is then applied to a multi-hole gasoline direct injector where the agreement with experimental data showcases the capabilities of the solver for realistic injectors and atomisation regimes.

Rico and the jets: Direct numerical simulations of turbulent liquid jets

This paper is associated with a poster winner of a 2019 American Physical Society's Di- vision of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0020 The breakup of an interface into a cascade of droplets and their subsequent coalescence is a generic problem of central importance to a large number of industrial settings. Examples of these applications include the atomisation of propellants in engines, the formation of droplets in injectors, mixers, separators, and the generation of droplets in multiphase flow regime transitions [2, 3, 12]. In all of these situations, it is important to predict the evolving droplet size distribution that results from a competition between breakup and coalescence, which are influenced by a range of multi-scale physics; this includes the interaction of turbulence with interfaces, capillarity, viscosity, and gravity. Therefore, it is unsurprising that the breakup of liquid jets during injection (i.e. atomisation) has received great scientific interest [4, 8, 10, 11, 13]. To the best of our knowledge, the transient dynamics of turbulent liquid/liquid systems have not been reported in the literature. Temporal instabilities and the resulting spatio-temporal interfacial structures are predicted by solving the full three-dimensional two-phase Navier-Stokes system in the context of a hybrid front-tracking/level-set method [14-16].

An analysis of surface breakup induced by laser-generated cavitation bubbles in a turbulent liquid jet

The breakup of turbulent liquid jets by cavitation bubbles was investigated by artificially introducing them by focusing laser light into the jet. The induced surface deformations and ejected liquid structures were characterized using shadowgraphy with a high-speed video camera. The flow velocity of the liquid jets, which were ejected from a 6 mm nozzle, was varied by adjusting the injection pressure from 1 to 5 bar. Deionized water and a dipropylene glycol–water mixture were used to compare the breakup of liquid jets with different surface tension and viscosity. Surface deformation and breakup were found to occur in two stages. One was early breakup of liquid strings into tiny droplets. This was followed by the formation of a larger structure separating into ligaments and larger drops. Averaged time-resolved one-dimensional plots were introduced and implemented to analyze breakup statistically, to address the problem of shot-to-shot variations in the breakup due to the turbulent condition of the jets. Bubble-induced breakup could easily be distinguished from spontaneous breakup with this method. Both the position of bubble formation and the injection pressure had an influence on the scale of the breakup. The deformation of the jet surface was highly affected by shear. The structure of the deformation became less intact when the surface tension was lower. The sizes of the drops produced during the second stage of breakup were analyzed. The bubble-induced breakup produced smaller drops than the spontaneous breakup at lower injection pressure. As expected, lower surface tension favored droplet detachment and smaller sized drops.Graphic abstract

DNS and LES of primary atomization of turbulent liquid jet injection into a gaseous crossflow environment

In this paper, we study the primary atomization characteristics of liquid jet injected into a gaseous crossflow through direct numerical simulations (DNS) and large eddy simulations (LES). The DNS use a coupled level set volume of fluid (CLSVOF) sharp interface capturing method resolving all relevant scales to predict the drop size distribution (DSD) for drops larger than the grid spacing. The LES use a volume of fluid (VOF) diffused interface method modelling the sub grid droplets. The purpose of this paper is to provide a comparison of the results of drop data between DNS and LES. The simulations are performed for a liquid jet injection with liquid-gas momentum flux ratio of 6.6, liquid jet Reynolds number of 14,000 injected into a crossflowing air with Reynolds number 570,000 and Weber number of 330 at a liquid-to-gas density ratio of 10. Two distinct and simultaneous atomization/breakup mechanisms have been observed in the simulations: column/bag breakup and ligament/surface breakup. It was found that the DSDs obtained from the DNS and LES each follow a log-normal distribution based on their respective droplet diameter data. An overlap region exists between the individual DSDs from the DNS and LES when combined. The width of this overlap region decreases along the downstream direction. A log-normal distribution is found to be a good fit to the combined DSD incorporating both resolved and sub-grid droplets. This information is relevant for the secondary atomization simulations and modeling.

Primary atomization of a turbulent liquid jet in crossflow: a comparison between VOF and FGVT methods

The primary atomization of a turbulent liquid jet in crossflow was investigated using a mathematical, numerical and computational model. A comparison between the standard volume of fluid (VOF) method and the fine grid volume tracking (FGVT) method was reported. The FGVT method advects the interface between two fluids using a finer grid than the employed by the standard VOF method, and according to the literature, it provides a better interface resolution. Simulations were performed using adaptive mesh refinement in a three-dimensional domain subjected to gravitational field using the in-house code MFSim. The flow was modeled using an Eulerian–Lagrangian approach to capture the interface. The interface was tracked initially with VOF or FGVT methods until the initial breakup. Broken off, small-scale nearly spherical drops were transferred into the Lagrangian point particle description. Column breakup and shear breakup modes were observed on the liquid jet. Drops were small as one-hundredth the size of the injector diameter. The model was validated against experimental correlations for the liquid jet column trajectory, and the droplet size distribution was compared to a previous numerical study from the literature. In addition, the breakup mechanisms predicted were qualitatively compared to those in previous reports. The results of the liquid column trajectory from the simulations performed presented low differences with the literature for both methods tested. According to the numerical results obtained from the computational simulations, the liquid column trajectory was well captured and the droplet size distribution was similar to the literature; however, the FGVT method provided higher accuracy compared to VOF method. The two main breakup modes were identified, namely the column breakup with Kelvin–Helmholtz instabilities and the surface breakup with the formation of multiple ligaments which later lead to droplet formation. The FGVT method provided a more detailed interface contour and improved the number of droplets converted from the Eulerian to the Lagrangian approach compared to the standard VOF method. On the other hand, the FGVT method presented relatively higher computational costs compared to VOF. Therefore, the FGVT method presented a higher interface quality and allowed a larger number of droplet conversion to the Lagrangian approach compared to the VOF method, even though the simulation run time using VOF was lower than with FGVT.

A 3D Moment of Fluid method for simulating complex turbulent multiphase flows

This paper presents the moment of fluid method as a liquid/gas interface reconstruction method coupled with a mass momentum conservative approach within the context of numerical simulations of incompressible two-phase flows. This method tracks both liquid volume fraction and phase centroid for reconstructing the interface. The interface reconstruction is performed in a volume (and mass) conservative manner and accuracy of orientation of interface is ensured by minimizing the centroid distance between original and reconstructed interface. With two-phase flows, moment of fluid method is able to reconstruct interface without needing phase volume data from neighboring cells. The performance of this method is analyzed through various transport and deformation tests, and through simple two-phase flows tests that encounter changes in the interface topologies. Exhaustive mesh convergence study for the reconstruction error has been performed through various transport and deformation tests involving simple two-phase flows. It is then applied to simulate atomization of turbulent liquid diesel jet injected into a quiescent environment. The volume conservation error for the moment of fluid method remains small for this complex turbulent case.

Visualization of penetration of a high-speed focused microjet into gel and animal skin

Realization of a needle-free drug delivery device can solve needle-caused problems. Recently, attention has been given to the use of a focused liquid jet instead of the use of the existing needle-free device that generates a turbulent liquid jet. This study experimentally visualized the penetration into a gel and animal skin of a focused liquid jet with a very high speed around 200 m/s. We first examined high-speed images to evaluate the dynamics of the jet penetration into a gel. We found that the penetration depth is determined within a short time t_p (on the order of 0.01 ms). t_p is similar to L/V_j, where L and V_j are, respectively, the penetration depth and the impact speed of the jet tip. We then investigated the overall shape of the penetration site represented by the length ratio alpha =L/W, where W is the width of the penetration site. Remarkably, the ratio alpha is greater than unity for a gel, as well as for the animal skin. This suggests that the focused liquid jet can create a narrow penetration site into the target.Graphical abstract

Dominant factor and incremental depth formula for self-aerated flow in open channel

The presence of air in open channel flows will increase the bulk of the flow, and is of great importance in the design of spillway and chute sidewalls. Hydraulics in the developed region is investigated in this paper systematically by a series of model and prototype investigations. It is verified that the velocity in the aeration region is the dominant factor. For the flow with an identical velocity but a different flow depth, the air concentration distributions are nearly the same. From the theory based on the formation of drops on the surface by the turbulent liquid jets and the vortex deformation, a formula to calculate the incremental depth is obtained by best correlating the model and prototype investigations. The formula is reasonable with less mean error than those obtained by other methods, which could recommended for the use in engineering designs.

Parameter dependences of the onset of turbulent liquid-jet breakup

Previous studies have predicted $We^{-2/5}$ dependence of the streamwise location at which primary breakup of turbulent liquid jets begins and $We^{-3/5}$ dependence of the Sauter mean diameter (SMD) of droplets released at that location, where $We$ is the jet Weber number. Measured deviations from these predictions were attributed to measurement uncertainties and to the simplicity of the analysis, which invoked turbulence inertial-range phenomenology. Here, it is proposed that breakup onset is instead controlled by the residual presence of the boundary-layer structure of the nozzle flow in the near field of the jet. Assuming that the size of the breakup-inducing eddy is within the scale range of the log-law region, $We^{-1}$ dependence of both the onset location and the SMD at onset is predicted. These dependences agree with the available measurements more closely than those previously predicted. To predict the dependences on the Reynolds number $Re$, either the friction velocity in conjunction with the Blasius friction law or the bulk velocity can be used, where the former yields $Re^{3/8}$ and $Re^{1/4}$ dependence of the onset location and the SMD at onset respectively, while the latter implies no $Re$ dependence of either. The latter result is consistent with the available measurements, but the boundary-layer analysis indicates that the velocity scaling should be based on the friction velocity rather than the bulk velocity, so the origin of the measured lack of $Re$ dependence merits further investigation. A plausible hypothesis is that pressure effects associated with the transition from wall-bounded nozzle flow to jet free-slip boundary conditions induce a transient large-scale flow modification that counteracts the $Re$ dependence of the nozzle flow while preserving the logarithmic flow structure near the jet surface. Notwithstanding the absence of direct evidence supporting this hypothesis, the new analysis and comparisons of its predictions with measurements suggest that transient effects such as the residual influence of the nozzle-flow structure are the likely explanations of the observed parameter dependences.

Numerical investigation of turbulent-jet primary breakup using one-dimensional turbulence

Primary breakup to form droplets at liquid surfaces is an important fundamental process to study as it determines the initial properties of the dispersed phase, which affect mixing rates, secondary breakup, droplet collisions, and flow separation within the dispersed flow region. Primary breakup can be regarded as one of the least developed model components for simulating and predicting liquid jet breakup. However, it is of paramount importance in many technical applications, e.g. fuel injection in engines and spray painting. This paper presents a numerical investigation of primary breakup of a turbulent liquid jet in still air at standard conditions using the one-dimensional turbulence (ODT) modeling framework. ODT is a stochastic model that simulates turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. An important feature of ODT is the resolution of all relevant scales, both temporal and spatial. The restriction to one spatial dimension in ODT permits affordable high resolution of interfacial and single-phase property gradients, which is key to capturing the local behavior of the breakup process and allows simulations at high Reynolds and Weber numbers that are currently not accessible to direct numerical simulations (DNS).This paper summarizes our extensions of the ODT model to simulate geometrically simple jet breakup problems, including representations of Rayleigh wave breakup, turbulent breakup, and shear-driven breakup. Each jet breakup simulation consists of a short temporal channel section to initialize a turbulent velocity profile at the nozzle exit followed by an adjacent jet section. The simulations are carried out for jet exit Reynolds number of 11,500, 23,000, 46,000 and 92,000 while the Weber number is varied within the range 102–107. We present results on breakup statistics including spatial locations of droplet release, droplet sizes and liquid core length. The results on primary breakup are compared to experimental results and models.

Stereo-PIV measurements of vapor-induced flow modifications in confined jet impingement boiling

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10°C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.