Experimental study on cavitation pattern and near-field spray characteristics of methanol in the scaled-up fuel injection nozzle

The objective of this paper is to investigate the flow characteristics of different cavitation flow patterns around a NACA (National Advisory Committee for Aeronautics) 66 hydrofoil by applying the BDIM (boundary data immersion method) and ILES (implicit large eddy simulation) with an artificial code. Meanwhile, an artificial compressibility method is also employed to consider the effects of compressibility on cavitating flow. The results present that the numerical method can effectively capture different cavitation patterns, which agrees well with the previous experimental data. Subsequently, the detailed analysis of vortex structures and dynamics for the non-cavitation (σ = 3.0), sheet cavitation (σ = 2.0), and cloud cavitation (σ = 1.6) cases with the Liutex method and the vortex enstrophy transport equation have been investigated. When cavitation occurs, the degree of turbulence and the enstrophy in the flow field have been enhanced, due to the disturbance of the velocity field. For sheet cavitation, complex vortex structures appear in the attached cavity region with high-intensity enstrophy causing by the highly intense velocity and density gradient. As the cavitation pattern transits from the sheet cavitation to the cloud cavitation, more complex vortex structures can be observed in the cavitation region. Furthermore, the value and the fluctuation amplitude of enstrophy intensity increase significantly under the effect of reentrant jet. Analysis of the enstrophy transport equation indicates that the vortex stretching term and dilatation term for cloud cavitation increase relatively significantly with the movement of the reentrant flow and are highly dependent on the cavitation evolution. In addition, the region affected by the baroclinic torque also increases.

Experimental study on the effect of nozzle hole-to-hole angle on the near-field spray of diesel injector using fast X-ray phase-contrast imaging

Experimental study on impingement spray and near-field spray characteristics under high-pressure cross-flow conditions

The present study investigates the influence of atomizing air-to-liquid mass ratio (ALR) on the near-field spray characteristics and stability of a novel twin-fluid injector that integrates bubble-bursting for primary atomization and shear-induced secondary atomization. Unlike conventional injectors, the novel design generates ultra-fine sprays at the exit with low sensitivity to liquid properties. The previous version improved secondary atomization even for highly viscous liquids, showing strong potential in hydrogel-based fire suppression. The current design improves primary atomization, leading to more stable and finer sprays. The near-field spray characteristics are quantified using a high-speed shadowgraph across ALRs ranging from 1.25 to 2.00. This study found that stable and finely atomized sprays are produced across all the tested ALRs. Increasing ALR reduces droplet size, while the spray is the widest at 1.25. Sauter Mean Diameter (SMD) contours show larger droplets at the edges and smaller ones toward the center, with ALR 2.00 yielding the most uniform size distribution. As per the atomization efficiency, ALR of 1.25 shows the best performance. Overall, an optimum ALR of 1.75 is identified, offering balanced droplet size distribution, stability, and atomization efficiency, making the injector potentially suitable for fire suppression and liquid-fueled gas turbines requiring high stability and fuel flexibility.

Experimental investigation of near-field spray characteristics of RP3 aviation fuel based on multivariable sensitivity and uncertainty analysis

A comprehensive methodology for characterizing sprinkler sprays

Experimental and numerical investigation of cavitating vortical patterns around a Tulin hydrofoil

In Diesel engines, the internal flow characteristics in the fuel injection nozzles, such as the turbulence level and distribution, the cavitation pattern and the velocity profile affect significantly the air-fuel mixture in the spray and subsequently the combustion process. Since the possibility to observe experimentally and measure the flow inside real size Diesel injectors is very limited, Computational Fluid Dynamics (CFD) calculations are generally used to obtain the relevant information. The work presented within this thesis is focused on the study of cavitation in real size automotive injectors by using a commercial CFD code. It is divided in three major phases, each corresponding to a different complementary objective. The first objective of the current work is to assess the ability of the cavitation model included in the CFD code to predict cavitating flow conditions. For this, the model is validated for an injector-like study case defined in the literature, and for which experimental data is available in different operating conditions, before and after the start of cavitation. Preliminary studies are performed to analyze the effects on the solution obtained of various numerical parameters of the cavitation model itself and of the solver, and to determine the adequate setup of the model. It may be concluded that overall the cavitation model is able to predict the onset and development of cavitation accurately. Indeed, there is satisfactory agreement between the experimental data of injection rate and choked flow conditions and the corresponding numerical solution.This study serves as the basis for the physical and numerical understanding of the problem. Next, using the model configuration obtained from the previous study, unsteady flow calculations are performed for real-size single and multi-hole sac type Diesel injectors, each one with two types of nozzles, tapered and cylindrical. The objective is to validate the model with real automotive cases and to ununderstand in what way some physical factors, such as geometry, operating conditions and needle position affect the inception of cavitation and its development in the nozzle holes. These calculations are made at full needle lift and for various values of injection pressure and back-pressure. The results obtained for injection rate, momentum flux and effective injection velocity at the exit of the nozzles are compared with available CMT-Motores Termicos in-house experimental data. Also, the cavitation pattern inside the nozzle and its effect on the internal nozzle flow is analyzed. The model predicts with reasonable accuracy the effects of geometry and operating conditions.

A cavitation flow can greatly impact a vehicle's attitude and stability when exiting water. This paper adopts an improved delayed detached eddy turbulence model and a Schnerr–Sauer cavitation model as well as the volume-of-fluid method and an overlapping grid technique to investigate this effect. In addition, the experimental system of the underwater launch is designed and built independently, which the numerical results are in good agreement with the experimental results. The transient cavitation flow structure and motion characteristics of the projectiles successively launched underwater are studied. When the axial spacing ranges from 0 to 1.0 times the diameter of the projectile, both projectiles are severely affected to various extents in cavitation pattern, vortex structure, and motion characteristics. It is worth noting that the internal cavity of the secondary projectile is disturbed by the wake of the primary projectile, resulting in large-scale fractures and detachment of the internal cavity, but its motion stability is good.

Ball Motion and Near-Field Spray Characteristics of a Gasoline Direct Injection Injector using an X-ray Phase-Contrast Imaging Technique under High-Injection Pressures

The present study investigates the effect of the atomizing air-to-liquid mass ratio (ALR) on the spray stability and near-field spray properties of a recently developed transparent novel twin-fluid injector. It involves primary atomization by bubble-bursting and secondary atomization by shear layer instabilities. A previous design integrated swirling flows and successfully enhanced secondary atomization. This design resulted in clean, compact lean-premixed combustion of distinct fuels, potentially enabling small-core fuel-flexible combustors. The current design enhances the primary atomization that leads to fine droplets immediately at the injector exit, rather than the typical liquid jet-core from a conventional air-blast atomizer. The effect of injector on the near-field spray characteristics and dynamics were quantitatively investigated using high-speed laser-driven shadowgraph imaging for ALR of 1.25 to 2.00. Results show that at all the tested ALRs, stable and finely atomized sprays were achieved. Increase in ALR resulted in finer sprays. An optimum ALR of 1.75 was suggested based on the droplet size distribution and stability for future application of the injector’s optimal range of operation in liquid-fueled gas turbine engines that can be expected to have high stability and fuel flexibility.

An experimental study is carried out in a cavitation tunnel on a propeller operating downstream of a non-uniform wake. The goal of this work is to establish quantitative correlations between the near pressure field and the cavitation pattern that takes place on the propeller blades. The pressure field is measured at the walls of the test section and in the near wake of the propeller and is combined with quantitative high-speed image recording of the cavitation pattern. Through harmonic analysis of the pressure data and image processing techniques that allow retrieving the cavitation extension and volume, we discuss the potential sources that generate the pressure fluctuations. Time correlations are unambiguously established between pressure peak fluctuations and cavitation collapse events, based on the Rayleigh collapse time. Finally, we design a model to predict the cavitation-induced pressure fluctuations from the derivation of the cavitation volume acceleration. A remarkable agreement is observed with the actual pressure field.

Near-field spray characteristics and steadiness of a novel twin-fluid injector with enhanced primary atomization

The spatially resolved spray scanning system (4S) is a new technology providing high-fidelity near-field spray characteristics generated by sprinklers, capturing all the physical parameters necessary to completely describe the sprinkler’s spray. In this study, a fixed spray plate sprinkler (FSPS) using three different deflector plates and two orifice sizes has been successfully characterized using the 4S. The current research demonstrates the potential of 4S technology to support irrigation sprinkler design optimization. The influence of the deflector plate design on spray characteristics can be clearly observed in the 4S measurements. It has been shown that deflector plate design (smooth, grooved, flat or two-level) generates deep changes in the important near-field spray properties. The baseline smooth-flat deflector plate generates a homogenous spray mainly concentrated at a polar angle of 90°. While the grooved-flat deflector plate presents a fluctuating spatial distribution, with similar volume flux and reference velocity, and larger drop sizes (~ 15%) as compared with baseline, two-stage deflector plates tend to redirect the main water flow from a polar angle of 90° to 110°, doubling the volume flux and resulting in an important increase in drop size (> 20%) and reference velocities (~ 7%) as compared with the baseline. In addition, measurements show frame arms induce local decreases in volume flux and drop velocity by as much as 60% compared to average values. While the ability of the 4S to provide high-fidelity near-spray descriptions of FSPS has been demonstrated, application to other sprinklers types such as impact, spinners and off-center wobbling could be limited and will be explored in future research.

In order to study the near-field target characteristic of the radio fuse, an accurate near-field echo characteristics modeling algorithm based on Physical optics (PO) and physical theory of diffraction (PTD) and shooting and bouncing rays (SBR) methods is proposed to calculate the echo characteristics of fuse, including echo power and echo signal and RCS. This method fully considers the characteristics of near-field, such as the influence of antenna pattern, spherical wave irradiation, distance, polarization and so on. The main research work in this paper involves the followings: (1) Firstly, the expression of near-field irradiation field is presented as spherical wave propagation mode, which takes into count the influence of the near-field distance and spherical wave irradiation. (2) Establishing the system coordinate of antenna and the system coordinate of antenna local polarization, and then define the directivity function of near-field antenna to characterize the antenna pattern, local irradiation and near-field polarization conversion characteristics. (3) Introducing near-field polarization scattering matrix to unify the scattering characteristic components of different scattering mechanisms, and then calculate the echo characteristic of radio fuse by integrating those of the geometrical elements which are illuminated by antenna beam during missile target encounter. In addition, the proposed theoretical model in this paper is calibrated by actually-measured data. And the emulation results are with a good agreement with measured results. Finally, we consider a simplified missile model, and compute its echo characteristics under different antenna pattern and different distance and different polarization. The results show that scattering peaks correspond to the points of the wings of the missile. In addition, the results change obviously when using different antenna pattern. At the same time, the results are different under the different distance. Numerical results prove the proposed method high efficiency and preciseness. It would be especially valuable in engineering application.