Droplet Size Spatial Distribution Model of Liquid Jets Injected into Subsonic Crossflow

Liquid jet injected into transverse subsonic gaseous flow has been widely utilized in many industrial applications. It is useful to determine the spatial distribution of generated droplets in the near-field region for high-efficiency combustion. In this paper, we propose a simplified model to predict droplet spatial distribution in transverse subsonic gaseous flow. Linear stability analysis has been used to determine the disturbance growth rate on the surface of a liquid column. When the amplitude of disturbance is of the same order of magnitude as jet radius, the liquid jet breaks up into ligaments. We can make an assumption that the generation rate of small droplet equals to liquid breakup rates, which varies with a spatial location under this circumstance. Combining these relations with the definition of SMD (Sauter mean diameter), a semitheoretical relation to evaluate droplet spatial distribution along the liquid column can be established. The present model has been compared with empirical relation based on experiments under different conditions. Results indicate that in the surface breakup region, the current model shows great consistency with experimental observations while there exists a relatively large discrepancy between the current model and experimental observation in the column breakup region because of its strong nonlinear effect near the breakup point. In addition, the effects of flow parameters on droplet size spatial distribution have been investigated

Dynamic behavior of droplets impacting cylindrical superhydrophobic surfaces with different structures

The dynamic behavior of droplets impacting cylindrical superhydrophobic surfaces with different structures (azimuthal groove, axial groove, pillar) is studied in this work. The rebound and splash thresholds with different structures were also proposed, which depended on D/D0 (where D is the cylinder diameter and D0 is the initial droplet diameter) and the surface structure of the substrate. Based on the energy conservation approach, a complete rebound threshold semi-empirical model is constructed for cylindrical superhydrophobic surfaces. The recovery coefficient is used to measure the energy loss during the droplet impacting the superhydrophobic cylindrical surface. At the same time, the energy loss was significant on the cylindrical superhydrophobic surface with different structures, and the surface structure of the substrate played a vital role in the energy loss of the collision process. Then, a prediction formula for the maximum spread diameter on the cylindrical superhydrophobic surface with different structures is presented to understand the droplet collision behavior further. In addition, a level wing-like splash morphology could reduce contact time on grooved superhydrophobic surfaces. Based on the contact time [(β a max / β z max) 1 / 2 τ] as a function of the Weber number, the azimuthal grooved structure surface has the least contact time. © 2023 Author(s).Dynamic behavior of droplets impacting cylindrical superhydrophobic surfaces with different structuresacceptedVersio

Molecular dynamics simulation of a nanoscale feedback-free fluidic oscillator

We present a molecular dynamics simulation study of a feedback-free fluidic oscillator model. Using the molecular dynamics simulations, it is demonstrated that the oscillation can be self-induced and sustained in a large range of flow rate and two very different jet directions. The oscillation mechanism of the nanoscale fluidic oscillator is physically similar to that in macroscale in which the dome vortex plays a crucial role. The thermal fluctuation is not significant enough to submerged the effect of hydrodynamics in the nanoscale feedback-free fluidic oscillator. The linear relationship between the oscillation frequency and the flow rate revealed by macroscopic experiments was also found in our simulations. Two of the three oscillation regimes found in macroscopic studies are shown to be able to be reproduced in our simulation. Our results show that molecular dynamics simulation is fully capable of studying the complicated flow in a feedback-free fluidic oscillator

Molecular Dynamics Simulation of a Jet in a Binary System at Supercritical Environment

With the development of large-thrust liquid rocket engines, the behavior of liquid in supercritical conditions arouses increasing public interest. Due to the high pressure and temperature of the combustion chamber, fuel reaches its critical point much more easily, and enters supercritical conditions. Due to the drastic changes in the physical properties of the fluid near the critical point, it is usually difficult to simulate the fluid motion using traditional computational fluid dynamic methods; but molecular dynamics (MD) can simulate fluid motion at the molecular level. In view of the engineering application, the physical properties of a binary system consisting of argon and nitrogen, and the stability of subcritical jets sprayed into supercritical environment, has been studied here using the MD method. First, the molecular dynamic simulation of the equation of state (EOS) of the mixture was put forward. Four conditions, with different mixing ratios of nitrogen, were designed. The results showed that the mixing ratio of nitrogen noticeably affected the results; these results were compared with the Soave-Redich-Kwong (SRK) EOS. Second, a simulation was conducted of subcritical nitrogen jet sprayed into a supercritical argon environment. After analyzing the results, the jet density and temperature distributions were obtained and the disturbance growth rate of the shear layer was analyzed

Molecular Dynamics Study on the Mechanism of Nanoscale Jet Instability Reaching Supercritical Conditions

This paper investigates the characteristics of a nitrogen jet (the thermodynamic conditions ranging from subcritical to supercritical) ejected into a supercritical nitrogen environment using the molecular dynamics (MD) simulation method. The thermodynamic properties of nitrogen obtained by molecular dynamics show good agreement with the Soave-Redlich-Kwong (SRK) equation of state (EOS). The agreement provides validation for this nitrogen molecular model. The molecular dynamics simulation of homogeneous nitrogen spray is carried out in different thermodynamic conditions from subcritical to supercritical, and a spatio-temporal evolution of the nitrogen spray is obtained. The interface of the nitrogen spray is determined at the point where the concentration of ejected fluid component reaches 50%, since the supercritical jet has no obvious vapor-liquid interface. A stability analysis of the transcritical jets shows that the disturbance growth rate of the shear layer coincides very well with the classical theoretical result at subcritical region. In the supercritical region, however, the growth rate obtained by molecular dynamics deviates from the theoretical result

Influence of Flow Rate Distribution on Combustion Instability of Hypergolic Propellant

Combustion instability is the biggest threat to the reliability of liquid rocket engines, whose prediction and suppression are of great significance for engineering applications. To predict the stability of a combustion chamber with a hypergolic propellant, this work used the method of decoupling unsteady combustion and acoustic system. The turbulence is described by the Reynolds-averaged Navier–Stokes technique, and the interaction of turbulence and chemistry interaction is described by the eddy-dissipation model. By extracting the flame transfer function of the combustion field, the eigenvalues of each acoustic mode were obtained by solving the Helmholtz equation, thereby predicting the combustion stability for the combustion chamber. By predictions of the combustion chamber instability with different flow rate distributions, it was found that the increasing of inlet flow rate amplitude will improve the stability or instability of combustion. The combustion stability of the chamber was optimized when the flow rate distribution for the oxidant was set more uniform in the radial direction. The heterogeneity of the flow rate distribution in the circumferential direction is not recommended, considering that a homogeneous flow rate distribution in the circumferential direction is beneficial to the combustion stability of the chamber

Pressure Characteristics and Vortex Observation in Chiral-Symmetric Space Orthogonal Bifurcation

In aerospace engine delivery systems, “one-in-two-out” bifurcation structures are commonly used for flow distribution to downstream pipelines. There are two common “one-in-two-out” bifurcation structures in aircraft engines: the planar orthogonal bifurcation and the spatial orthogonal bifurcation. By adjusting the flow supply upstream and the cross-sectional diameter downstream, the flow distribution in the two branches can be adjusted, i.e., the “splitting ratio” changes. In this paper, a dismantling and flexible experimental system is constructed to measure the pressure signals in each channel and use non-linear dynamic analysis methods to extract pressure characteristics. The particle image velocimetry (PIV) technique combined with the fine rope tracing technique is creatively used to observe the vortex structure in the cross section of the downstream branch. The study found that for spatial orthogonal bifurcation, the pressure signal characteristics in each channel are basically the same at larger splitting ratios, regardless of the chirality. As the splitting ratio decreases, the difference in pressure signal characteristics between the two branches gradually becomes evident and becomes related to the chirality. Moreover, unlike the planar orthogonal bifurcation structure, a complete large vortex structure has not been found in the downstream branch of the spatial orthogonal bifurcation structure, regardless of changes in the splitting ratio, and it is unrelated to the chirality

Origin of the Thickness-Dependent Oxidation of Ultrathin Cu Films on Au(111)

Ultrathin Cu films deposited on a metal substrate have been used as a model system to understand the structure function relationship in electro-catalysis, heterogeneous catalysis, and microelectronics. The stability of ultrathin Cu films against oxidation has been of particular interest, but there is a lack of microscopic understanding. We report here an atomic-level study on the thickness dependent oxidation kinetics of Cu layers on Au(111), from ultrahigh vacuum to near ambient conditions. Ultrathin Cu films on Au(111) were found to exhibit a superior oxidation resistance over Cu(111), and their oxidation resistances increase in the order of Cu(111) < 2.4 ML Cu < 0.4 ML Cu. For 0.4 ML Cu, the spontaneous subsurface diffusion of Cu at 300 K and the formation of a Au-rich surface alloy inhibit the formation of copper oxides at the O-2 pressure below 10(-4) mbar. However, at near ambient conditions, 0.4 ML Cu would be partially oxidized to the CuO phase directly. In contrast, multilayer Cu or bulk Cu(111), though oxidized more rapidly, forms only Cu2O surface layers under the same oxidation conditions. We analyzed further the atomic process of alloying at elevated temperatures. An intermediate Au3Cu alloy phase was suggested at the subsurface at 400 K. The diffusion of Cu into bulk Au(111) at 600 K prevents the formation of copper oxides at 300 K even under near-ambient conditions. Our study could thus provide insight for the rational design of a highly efficient Cu-based oxidation catalyst