Liquid Impingement Research Articles

ELUCIDATION OF INTERNAL FLOW AND ITS EFFECT ON THE JET OF A TWIN-FLUID ATOMIZER FOR GAS TURBINE CROSSFLOW COMBUSTORS

Conventional liquid fuel jet injection into a crossflow is widely used for large-scale power generation in gas turbine combustors. The authors proposed a new twin-fluid injection technique as a potential alternative and conducted a series of studies on the internal flow of the atomizer and liquid jet/spray characteristics. In this study, experimental and numerical studies were first performed on internal flow behaviors and liquid and atomizing air interactions. The important behaviors of the internal flow of the atomizer, such as the dynamic change in the liquid film thickness, the rapidly changing velocity of the wave front, and the liquid impingement distance on the inner wall of the atomizer, were measured by both the experiment and numerical analysis. Atomizing air generates a high-speed oscillating annular liquid film flow that flows at the exit of a twin-fluid atomizer. This dynamic of the annular liquid film is considered fairly advantageous for achieving good atomization, which is investigated by analyzing only the liquid's behaviors in jet/spray and twin-fluid injection in a crossflow. High-speed photography and image processing were applied to observe the liquid jet/spray phenomena in the crossflow and reveal the atomization characteristics such as jet/spray trajectories and breakup length. Furthermore, the atomizing air can dominate the trajectory of the liquid jet/spray in the crossflow. The twin-fluid injection provides improved atomization properties compared to conventional liquid-only injection.

Development of a hybrid Lagrange–Euler transition model for the film formation and dynamics of an impinging liquid spray

This paper proposes a hybrid Lagrange–Euler transition model that fully describes the injection and film formation process with different transition sub-models to balance accuracy and efficiency under the incompressible framework for spray cooling of electric machines. When the liquid phase is far away from a wall or film, a Lagrange discrete particle modeling method is employed to track liquid phase motion, which then transforms into a Eulerian droplet when a particle is close to a wall or film. Thereafter, the volume-of-fluid (VOF) method is adopted to simulate the processes of impingement, wall film formation, and dynamics. The transition model is implemented with the CONVERGE code package and validated against several available experimental data ranging from single droplets to spray impingement. The results indicate that the hybrid Lagrange–Euler transition model exhibits excellent mass and momentum conservation under different mesh resolution conditions and can well reproduce film formation and dynamics behaviors. In addition, detailed comparisons between the transition model and particle-based thin film modeling (TFM) are made for wall film formed by spray impingement. The transition model predicts a smaller film area and a better film shape than the TFM model compared to the measurements. Overall, the transition model coupled with the VOF method shows a higher sensitivity to mesh resolution, whereas the TFM model is more easily affected by semi-empirical splash models.

Heat transfer at the stagnation point of a free-falling impinging liquid jet

Heat transfer at the stagnation point of a free-falling impinging liquid jet

A Liquid Jet Impinging onto Rotating Convex Superhydrophobic and Hydrophilic Surfaces: Reflection or Deflection?

A liquid jet impinging on stationary and rotating superhydrophobic and hydrophilic convex surfaces is experimentally investigated. The effects of the rotation and wettability of the surface and the inertia and impingement rate of the jet on the flow, and the reflection and deflection behavior of the impinging jet are examined. This study examines the effect of air film formation at the constantly regenerating interface between a superhydrophobic surface and a liquid jet. For this purpose, two copper pipes and one plexiglass pipe, which had outer diameters of 8, 22, and 50 mm, were used for the convex surfaces. The copper pipes were coated with a superhydrophobic coating with a 157° apparent contact angle. The uncoated plexiglass pipe had a 73° apparent contact angle. The Reynolds and Weber numbers ranged from 1082 to 3443 and from 3.90 to 35.12, respectively. The liquid jet was sent to the rotating convex surfaces at different impingement rates. The experimental results show that the impinging liquid jet is reflected off the stationary superhydrophobic surface. This reflection behavior is not nearly distributed from the rotation of the superhydrophobic convex surface. The distribution increases slightly with an increase in the Reynolds or Weber numbers, the diameter of the convex surface, and the impingement rate. Nevertheless, the impingement liquid jet is deflected off the stationary hydrophilic surface. This deflection increases considerably with the rotation of the convex surface. The renewal of the air film between the superhydrophobic surface and the liquid significantly reduces the viscous drag force. Therefore, the impinging liquid jet cannot be dragged by the rotating superhydrophobic convex surface.

Assessment of liquid and gas impingement cooling fluids with numerical solution for better steel austempering

Impingement jet heat transfer was studied using liquid and gas fluids to determine better cooling fluid. The materials used were rectangular steel plates of 230 mm by 120 mm by 12 mm, single jet diameters of 10–40 mm with impingement gaps of 115–155 mm. A computational fluid dynamics software application ANSYS 2020R1 was employed for simulation, and lumped thermal mass analysis was used for experimental modeling. The experimental results showed an increase in heat transfer coefficient with increased pipe diameters and with a corresponding increase in impingement gaps and flow rate. This revealed 265.4–383.9 W/m2 K and 85.3–109 W/m2 K at diameter 10 mm and 336.5–365 W/m2 K and 109.0–137.5 W/m2 K at diameter 40 mm for both water and air, respectively. Numerical simulation revealed heat flux of 22518–38.94 W/m2 and 7570.2–4.25 W/m2 and 6742.8–27.1 W/m2 and 4155.6–6.1 W/m at diameters 10 mm and 40 mm, respectively. This confirmed that water remains a better cooling fluid with a 6.2% difference at the diameter of 10 mm and a 0.1% difference at the diameter of 40 mm. An acceptable error margin of 4 to 18% upon the comparison of empirical analysis with numerical simulation is obtained. The above suggests a better cooling rate for the microstructure of the steel using water against air.

Efficiency and sensitivity optimization of a protocol to quantify indoor airborne SARS-CoV-2 levels

BackgroundDevelopment of methodologies to quantify airborne microorganisms is needed for the prevention and control of risk infections. It is difficult to conclude which is the most efficient and sensitivity strategy to assess airborne SARS-CoV-2 RNA levels given the disparity of results reported in clinical settings.AimTo improve our previously reported method of measuring SARS-CoV-2 RNA levels, was based on bioaerosol collection with a liquid impinger and RNA quantification with droplet digital PCR (ddPCR).MethodsAir samples were collected in COVID-19 patient rooms to assess efficiency and/or sensitivity of different air samplers, liquid collection media and reverse transcriptases (RT).FindingsMineral oil better retains airborne RNA than hydrophilic media without impairing integrity. SARS-CoV-2 ORF1ab target was detected in 80% of the air samples using BioSampler® with mineral oil. No significant differences in effectiveness were obtained with MD8® sampler provided with gelatine membrane filters, but the SARS-CoV-2 copies/m3 air obtained with the latter were lower (28.4±6.1 vs 9±1.7). SuperScript II RT allows the detection of one single SARS-CoV-2 genome RNA molecule by ddPCR with a high efficiency. This was the only RT that allowed the detection of SARS-CoV-2 N1 target in air samples.ConclusionOptimizing collection efficiency and detection sensitivity for quantification od SARS-CoV-2 RNA in indoor air is important to improve our understanding of the microbiological safety of indoor air.

Evolution mechanism of diesel ignition with injection pressure under different spray wet-wall conditions at low temperatures

The effect of injection pressure on diesel ignition has been widely studied, but the conclusions are different or even contradictory. Thus, the 3-D simulation was conducted at injection pressures of 40–280 MPa under different spray wet-wall conditions. The results show that as the injection pressure increases, the fuel injection ends earlier, so the mass fraction of the mixture suitable for ignition (0.8 < φ < 1.6) reaches its maximum earlier and starts to decrease. Its value is sharply reduced shortly after the cool flame reaction, especially for the liquid-phase spray impingement. Accordingly, the ignition delay under liquid impingement increases with the injection pressure, and misfire occurs at 160 MPa due to the too low mixture concentration (φ = 0.6) in the hot flame stage. In the case of vapor impingement, as the injection pressure increases from 40 MPa to 280 MPa, the ignition delay shortens and then extends, and the transition occurs at 220 MPa. Ignition delay in free spray decreases with increasing injection pressure. The above variation depends mainly on the transition from a cool flame to a hot flame. In addition, with the increase of injection pressure, more than 90 % of the fuel film remains even after intense combustion. Although the instantaneous heat release rate increases, the total heat release decreases significantly, which is caused by the mixture concentration being too lean due to excessive fuel diffusion and thus incomplete combustion. In conclusion, low-pressure injection is preferred for liquid impingement, while moderately elevated injection pressures under vapor impingement and free spray facilitate fast ignition and optimized heat release.

Mathematical Modeling of the Ejected Droplet Size Distribution in the Vicinity of a Gas–Liquid Impingement Zone

The controlled splashing of metal droplets plays a decisive role in the kinetics of the basic oxygen furnace (BOF) process. In this work, a mathematical model was developed for predicting the size distribution of spherical droplets ejected at an impingement zone. Harmonic oscillators are used to describe the ejection sites, and the upper limit for the droplet population is calculated through a force balance. The model was validated against literature data from high-temperature crucible experiments involving different supply pressures and lance heights as well as both single-hole and multihole lances. The predicted size distribution of the metal droplets was found to be in good agreement with the droplet size distributions measured from outside the crucible. The model was also applied for predicting the size distribution parameters of the Rosin–Rammler–Sperling (RRS) size distribution function. The model developed is computationally light and is suitable to be used as a part of offline and online simulation tools for the BOF process.

Simulation of Erosion Behavior of a Paraffin Plate with Particle Method

There are many mechanisms in a reactor shutdown function of MSR and inherent self-stabilization. One of those mechanisms is the fuel-salt drain system. The present study focused on the melting and solidification phenomenon that occurs in the freeze valve. An experiment was performed to investigate the erosion behavior of a solid plate by an impinging liquid to time and the effects of fluid viscosity. In addition, numerical modeling based on the MPS method to visualize the heat distribution in the plate will also be carried out. The experiment will be conducted by varying the parameters such as the liquids, temperature, and diameter. Hot water (90°C), molten paraffin, and cooking oil will be used with molded pure paraffin wax will serve as the target plates. The dimension of the target plate is cylindrical, with 44 mm in thickness and 144 mm in length for paraffin wax. The data will then be compared to the MPS simulation. The radial dispersion of the heated liquid and the temperature of the impinged liquid will affect the penetration time, hence making the formation of a mushy zone more likely and promoting the pool effect. The noticeable difference of penetration time between simulation and experiment is likely caused by the changing value of kinematic viscosity of the liquids used in different temperatures. The kinematic viscosity is set to be a constant value in the simulation.

Estimating the number of fingers and size of ejected droplets in droplet impingement processes on solid substrates

The splashing of droplets impinging on solid substrates includes complex behavior that is dependent on numerous factors, including the liquid properties, impingement velocity, and solid surface properties, such as wettability and roughness. Quantitative understanding of the behavior of the unstable liquid film during the splashing process remains elusive. Here we propose new models in which the number of fingers and the size of the ejected secondary droplets can be characterized taking into account the liquid film thickness and the wettability and roughness of the surface. The experimental data regarding the size of the secondary droplet and the number of fingers is found to be in good agreement with the predictions of the proposed model. The results of the present study provide insight into the mechanism behind rim or liquid film fragmentation.

Effect of wall parameters on impinging combustion and soot emission characteristics of heavy-duty diesel engine at low temperature

The effect of wall parameters on combustion and soot emission characteristics remains to be revealed so far, especially in the case of spray impingement at low temperatures. Therefore, the visualization experiments are carried out in a constant volume combustion chamber using the Mie scattering and direct photography techniques, and the two-color method was used to extract the flame temperature and soot volume. The results show that increased wall distance and wall angle contribute to fast and stable ignition, but result in more soot production. In addition, the lower the ambient temperature, the more prominent the above characteristics are. At Tamb = 820 K, as the wall distance increases from 40 mm to 60 mm, the liquid spray impingement changes to the vapor impingement, so the ignition delay shortens, and the flame area and natural luminance increase significantly. At lower Tamb = 770 K, a complete misfire is observed at Lw = 40 mm but the ignition remains stable at Lw = 60 mm. The variation of the ignition characteristic parameters with the wall angle is similar to that of the wall distance. Under Pinj = 40 MPa, as the wall angle increases from 0° to 70°, the time to reach luminance saturation advances from 0.60 ms to 0.27 ms. Under higher Pinj = 100 MPa, a complete misfire occurs at θ =0° but bright flames are observed at θ =30–70°. With the increase of wall distance, the mean flame temperature increases due to reduced wall cooling. Coupled with the expanded flame area and combustion duration, the high-temperature region (>1800K) of soot distribution and KL factor increase significantly, especially in the range of Lw = 40–50 mm.

A three dimensional simulation of spray cooling and its evaporating liquid film generated on patterned surfaces

Liquid impingement cooling in the form of jet or spray is an appropriate method to remove heat from small surfaces progressively utilized in industry. In this study, numerical simulations are conducted to investigate the effect of surface structure on heat transfer characteristics. Five different three-dimensional structured surfaces are investigated using Euler-Lagrangian approach. The effect of surface structures is discussed on the liquid film thickness, the liquid film velocity, and the heat flux with its uniformity for a given spray and temperature characteristics. The results show that the convective heat transfer strongly depends on both liquid film thickness and velocity. While the liquid film thickness always grows in the grooved surfaces, its velocity depends on the form and direction of the fins. The averaged values of the film thickness and velocity magnitude are almost independent of the surface temperature. Among the investigated surfaces, the parallel and circular patterns showed the best and worst performance on heat transfer characteristics, respectively.

Simulation of lateral jet formation in high-speed liquid droplet impingement and its impact on crater side wall

The formation of a high-velocity lateral jet has been shown to impact the erosion characteristics of high-speed liquid droplet impingement (LDI) erosion. In this study, the dynamics of LDI on a planar rigid surface with a side wall were discussed. This setup serves as a typical representation of a crater, which is formed in the advanced stages of high-speed LDI erosion. In this setup, the lateral jet that is ejected from the contact edge of the droplet interacts with the crater side wall. As the lateral jet impacts the side wall, an axial flow develops generating extremely high pressure on both the target surface wall and the side wall. The potential damage to the wall surfaces was quantified by measuring the pressure on the wall. Lastly, we computed the impact pressure for various standoff distances (i.e., the distance between the droplet and the side wall).

Predicting the splash of a droplet impinging on solid substrates

The impingement behaviours of droplets towards solid substrates depend on the liquid properties, impingement velocity and solid surface conditions, such as wettability and roughness. However, the prediction regarding whether the droplet splashes after the impingement, is still an open question. Here we show that the splashing can be predicted by the pressure balance of the liquid film appearing beneath the impingement droplet coupled with the modified energy balance equation. Hydrodynamic and hydrostatic pressures are the driving forces for the droplet’s radial spreading, while the capillary pressure at the rim edge and viscous stress oppose the driving forces. Thus, splashing occurs when the driving forces overcome the opposing forces. Moreover, the splashing condition is affected by various surface factors, such as wettability and surface roughness. Our work would pave the way to understand the basic physics for rim or liquid film fragmentation and enabling advances in important for engineering field such as printing, sprays for cooling and pesticide.

Electrochemical Evaluation of the Effect of Different NaCl Concentrations on Low Alloy- and Stainless Steels under Corrosion and Erosion-Corrosion Conditions

The main objective of this study was to assess the influence of salt concentration on the corrosion behaviour, including the role of hydrodynamic conditions, of two broad classes of ferrous engineering materials. These are comprised of alloys, typified by a low-alloy steel (UNS G43400) that corrodes actively in aqueous conditions and a range of passive-film-forming stainless steels (UNS S31600, UNS S15500 and UNS S32760). Corrosion monitoring employed electrochemical (potentiodynamic polarisation) techniques. Three concentrations of aerated sodium chloride were utilised: 0.05 wt% NaCl, 3.5 wt% NaCl and 10 wt% NaCl. In quiescent, liquid impingement and solid/liquid impingement conditions, the corrosion rate of the low-alloy steel was observed to peak at 3.5 wt% NaCl, followed by a reduction in 10 wt% NaCl solution. These findings expand the range of previously reported trends, focused on static conditions. Such corrosion rate/salinity trends were observed to be dictated by the progress of the anodic reaction rather than influence on the cathodic reaction. Detailed studies were undertaken using segmented specimens to facilitate comparisons of the influence of hydrodynamic variations on corrosion behavior; these revealed that such variations influence the corrosion rates of low-alloy steel to a much lesser extent than the effect of changes in salinity. For the stainless steels, in quiescent and flowing conditions, when surface passive films are stable, there was a constant increase in corrosion rate with salinity. In solid-liquid conditions, however, the periodic film-destruction/repassivation events resulted in a similar corrosion rate/salinity trend to that displayed by the low-alloy steel, but with a much larger effect of hydrodynamic conditions. Additonally, the study revealed an underlying influence of stainless steel composition that mirrored, to an extent, the corrosion behaviour in pitting/re-passivation situations

Time-resolved 3D visualization of liquid jet breakup and impingement behavior in a shallow liquid pool

A new three-dimensional laser-induced fluorescent (3D-LIF) technology to obtain the hydrodynamic behavior of liquid jets in a shallow pool were developed. In this technology, firstly, a refractive index matching was applied to acquire a clear cross-sectional image. Secondly, a series of cross-sectional images was obtained by using a high-speed galvanometer scanner. Finally, to evaluate the unsteady 3D interface shape of liquid jet, a method was developed to reconstruct 3D shapes from the series of cross-sectional images obtained using the 3D-LIF method. The spatial and temporal resolutions of measurement were 0.21 × 0.21 × 1.0 mm/pixel and 2 ms, respectively. The shape of a 3D liquid jet in a liquid pool and its impingement, spreading and atomization behavior were reconstructed using the proposed method, successfully. The behaviors of atomized particles detached from the jet were obtained by applying data processing techniques. Diameters distribution and position of atomized droplets after detachment were estimated from the results.

SARS-CoV-2 detection in bioaerosols using a liquid impinger collector and ddPCR.

The airborne route is the dominant form of COVID‐19 transmission, and therefore, the development of methodologies to quantify SARS‐CoV‐2 in bioaerosols is needed. We aimed to identify SARS‐CoV‐2 in bioaerosols by using a highly efficient sampler for the collection of 1–3 µm particles, followed by a highly sensitive detection method. 65 bioaerosol samples were collected in hospital rooms in the presence of a COVID‐19 patient using a liquid impinger sampler. The SARS‐CoV‐2 genome was detected by ddPCR using different primer/probe sets. 44.6% of the samples resulted positive for SARS‐CoV‐2 following this protocol. By increasing the sampled air volume from 339 to 650 L, the percentage of positive samples went from 41% to 50%. We detected five times less positives with a commercial one‐step RT‐PCR assay. However, the selection of primer/probe sets might be one of the most determining factor for bioaerosol SARS‐CoV‐2 detection since with the ORF1ab set more than 40% of the samples were positive, compared to <10% with other sets. In conclusion, the use of a liquid impinger collector and ddPCR is an adequate strategy to detect SARS‐CoV‐2 in bioaerosols. However, there are still some methodological aspects that must be adjusted to optimize and standardize a definitive protocol.

Risks and limitations of steam curtains protection in case of flammable gases leakage

AbstractRefinery and petrochemical production facilities process and produce gaseous and liquid hydrocarbons. In case of any failure, improper design, or human error, flammable or toxic substances can be released into the environment. In such cases, vapor barriers of steam curtains are used in the chemical industry to primarily dilute the released gases as well as to divert the gases to the safe area away from a potential source of ignition. When steam is ejected from a nozzle in the pipe wall, the medium rubs against the edge of the nozzle, liquid water impinges on the wall of the nozzle, and an electrostatic charge can therefore be expected. From the above description, it can be deduced that if the operation of the steam curtains is associated with the generation of electrostatic charge and the possibility of its discharge and the generation of electrostatic sparks, the activation of the steam curtains during the discharge of flammable gases paradoxically represents a certain risk. From the point of view of assessing the potential risks in the operation of vapor barriers, the magnitude of the electric charge energy generated is a crucial issue. Measuring the electric charge on the flowing steam from the nozzles is complicated with a large scatter of measured values. However, with reference to the experience gained from repeated measurements, the experimentally measured values and the obtained results can at least be seen as an expert estimate. This paper focuses on the evaluation of experimental measurements of the magnitude of the generated electrostatic charge on real steam curtains of an ethylene plant. Based on the measured results, risks are defined and appropriate measures are taken.

Analysis of Material Loss Behavior According to Long-Term Experiments on LDIE-FAC Multiple Degradation of Carbon Steel Materials

Recently, damage caused by liquid droplet impingement erosion (LDIE) in addition to flow-accelerated corrosion (FAC) has frequently occurred in the secondary side steam piping of nuclear power plants, and the damage-occurring frequency is expected to increase as their operating years’ increase. In order to scrutinize its causes, therefore, an experimental study was conducted to understand how the behavior of LDIE-FAC multiple degradation changes when the piping of nuclear power plants is operated for a long time. Experimental results show that more magnetite was formed on the surface of the carbon steel specimen than on the low-alloy steel specimen, and that the rate of magnetite formation and extinction reached equilibrium due to the complex action of liquid droplet impingement erosion and flow-accelerated corrosion after a certain period of time. Furthermore, it was confirmed at the beginning of the experiment that A106 Gr.B specimen has more mass loss than A335 P22 specimen. After a certain period of time, however, the mass loss tends to be the opposite. This is presumed to have resulted from the magnetite formed on the surface playing a role in suppressing liquid droplet impingement erosion. In addition, it was confirmed that the amount of erosion linearly increases under the conditions in which the formation and extinction of magnetite reach equilibrium.

Experimental and Numerical Investigations on the Spreading Dynamics of Impinging Liquid Droplets on Diverse Wettable Surfaces

Experimental and Numerical Investigations on the Spreading Dynamics of Impinging Liquid Droplets on Diverse Wettable Surfaces