Transparent Nozzle Research Articles

Microscopic investigation and local standard deviation analysis of liquid propane jets in supercritical environment

To improve the thermal efficiency of engines, the temperature and pressure in the combustion chamber are continually increased, potentially reaching the critical points of the fuels. Additionally, light fuels such as liquid ammonia and propane are utilized to reduce carbon emissions. The relatively low critical temperature of light fuels facilitates their transition into a supercritical state, which greatly change the jet characteristics. To gain a deeper understanding of the supercritical transition of fuels, jets of liquid propane are investigated through high-speed microscopic imaging technology, covering subcritical to supercritical environmental conditions. Liquid propane is injected through a transparent flat nozzle to observe the internal flow states within the nozzle. For image and data processing, local standard deviation (LSD) analysis and a Residual Network (ResNet-50) model are employed to identify the structures within the mixing region. Droplets counting and frequency domain analysis are also performed to reveal the features of supercritical transition. The findings highlight the importance of mask size in the jet feature extraction. Large mask highlights large scale structures including jet surface and mixing region structures, while small mask brings about full-scale feature including droplets and all other features. A criterion for mixing regime classification based on reduced temperature (Tr) and reduced pressure (Pr) is established directly from image data using the ResNet-50 model, where a condition of Tr1.6Pr0.4 ≥ 1.067 indicates a transcritical mixing regime. Both droplet counting and frequency domain analysis corroborate this mixing regime criterion. These insights enhance our understanding of supercritical transitions and are useful for simulation verifications.

In-nozzle flow of superheated liquid release and the influence on the external flashing jet using transparent nozzles

Accident release of superheated liquid could produce a violent phase change and trigger a highly destructive flashing jet. In this work, release experiments are performed using a 20 L tank and transparent nozzles, with water as the fluid throughout. The experimental conditions included storage temperature (Tst = 100-160 °C), storage pressure (Pst = 6-16 bar), and nozzle sizes (D = 1-6 mm). High-speed image processing and capacitive void fraction sensor are used to quantify the two-phase flow of superheated liquid inside the nozzle and the downstream jet break-up regimes. The experimental results show that void fraction (α) goes through three phases during the release process: growth, stabilization and decay. It is also demonstrated that bubble nucleation, growth and burst determine the breakup of the downstream jet. The Tst and D are the main factors affecting the α, while the Pst only change the release rate. Moreover, it is verified that none of the existing flashing criteria (Ja-We, χ*) can accurately determine the boundary conditions of the flashing regimes. Instead, the parameters characterizing bubbles nucleation and growth, α, can unify the effects of (Tst, Pst, and D) to determine the physical mechanism of the flashing process. In this work, a novel method to distinguish between external flashing (α = 0), internal flashing (0 < α ≤ 25) and full flashing (α > 25), and a steady-state model k = f (Ja, We, α) is developed for the flashing jet.

Experimental study on the cavitation phenomenon effect on the efficiency of a turbulent premixed flame kerosene/air

In order to characterize the influence of cavitation in a nozzle on a turbulent premixed kerosene/air flame, an experimental study was conducted using a NexGen burner. This study was carried out in two distinct stages. The first stage was devoted to characterizing the necessary conditions for inducing the cavitation phenomenon, using a novel experimental device enabling fluidic study. The second stage enabled the characterization of the influence of cavitation on a flame, building upon the previously obtained results. To this end, three DELAVAN nozzles (2.25; 2.5; 3.0 80 W) were employed. The fluidic characterization stage involved the manufacture of transparent nozzles, facilitating the integration of piezoelectric pressure sensors. The results showed that cavitation has a significant influence on the temperature and density heat flux flame associated to the effect on the flame structure represented by three distinct zones: continuous flame, intermittent flame and plume flame. For the 2.25 80 W nozzle based on the smallest diameter, the risk of cavitation is the highest, and the results showed that for the lowest value of the cavitation number, the continuous flame was no longer visible proving the harmful effects of cavitation on the combustion of a premixed flame from a burner such as the NexGen burner.

Measurement of needle and armature dynamics in a gasoline direct injector by high-speed neutron imaging

In modern spark ignition engines, precise delivery of fuel with gasoline direct injection has become increasingly important in the effort to meet ever stricter efficiency and emissions regulations. Use of multiple small close-coupled injections has become more common in attempt to precisely control fuel distribution in the cylinder, but these strategies are hindered by nonlinear injection effects due to operation in the ballistic region of the solenoid-operated valve and due to armature bounce at the end of injection. Understanding the internal dynamics of the injector is crucial to minimizing and controlling non-linearity and shot-to-shot variation, but the experimental techniques available to date are capable only of tracking the position of either the top of the needle (via laser sensors or by monitoring current and voltage in the solenoid coil) or the bottom of the needle (via transparent nozzles or high-speed x-ray imaging). A complete picture of the axial and radial motion of valve needle has until now remained elusive. In this work, we present high-speed ensemble neutron transmission imaging of an entire 8-hole gasoline direct injector operating at 200 bar, allowing for both visualization and quantification of the actuation dynamics including lift and wobble of the valve ball, oscillation and bending of the valve needle, lift, rocking, and bounce of the armature, compression of the springs, and radial swelling of the solenoid during energization. Because neutrons offer high penetration through the metal injector while also being sensitive to the 1H nuclei in fuel molecules, it is also possible to simultaneously see the fluid dynamics of the injection process, including filling of the sac volume, emanation of the spray through the nozzle holes and into the downstream gas, and the formation and evolution of fuel films on the tip of the injector and the walls of the spray container.

Investigation of cavitation phenomenon with different fuel temperatures in diesel nozzles

Two-phase cavitating flow in the diesel nozzle has been widely concerned by researchers for a long time. Most of the attention has been paid to irregular geometry-induced cavitation, while the research on string cavitation induced by vortex is relatively less. The two cavitating patterns often occur simultaneously in the nozzle, which are still not understandable for their interaction characteristics. In this paper, it is analyzed that the characteristics of string cavitation and geometry-induced cavitation in the transparent nozzle based on the visual scale-up test bench. In the study of hole-to-hole string cavitation, it is found that when needle lifts elevate, geometry-induced cavitation and hole-to-hole string cavitation are observed all together under the same working condition. The development and stability of hole-to-hole string cavitation are further improved with the increase of fuel temperature. The geometry-induced cavitation and string cavitation interact with each other, that is, geometry-induced cavitation at the nozzle inlet interrupts the hole-to-hole string cavitation inception. The tail of hole-to-hole string cavitation is disturbed by cloud cavitation shedding after its developing into the nozzle hole, leading to fracture in serious cases. At high temperatures, there is a periodic transformation between the needle-originated string cavitation and hole-to-hole string cavitation.

Flow characteristics and phase transition of subcritical to supercritical kerosene injections in a convergent nozzle

The internal flow characteristics of near- and supercritical RP-3 aviation kerosene injected into an atmospheric pressure environment were experimentally investigated. The internal flow structure was visualized in a two-dimensional transparent convergent nozzle using shadowgraph imaging. A series of images of the RP-3 flow inside a convergent nozzle varying from subcritical to supercritical injection conditions were obtained for the first time. Under subcritical, critical, and supercritical pressures, different RP-3 flow structure transitions with decreasing injection temperature were examined. In addition, the associated phase transition was analyzed using the thermodynamic phase diagram of a 10-component RP-3 surrogate based on the adiabatic isentropic expansion hypothesis. Six distinct phase state regions governing the flow structure were identified, considering the phase transitions and density variations of the fuel injection from different regions during their respective expansion processes. The experimentally observed phase transition boundary with respect to injection conditions was further determined, which can be utilized to predict the onset of a drastic change in mass flow rate. Furthermore, the axial length between the nozzle exit and the onset location of observable phase transition was detected, which initially increases and then decreases with decreasing injection temperature. Further analysis showed that the unique evolution behaviour of the specific heat ratio during injection process has a strong effect on flow characteristics near critical point.

An experimental study on the in-nozzle cavitating flow and near-field breakup of spirally grooved hole nozzles

To improve liquid atomization for pressure atomizer, a spirally grooved hole (SGH) nozzle was designed, which produces swirling flow inside the nozzle hole and thereby promotes the disturbance in liquid jets, enhancing breakup of liquid jets. In this paper, the optical visualization of the cavitating flow inside nozzles and primary breakup behaviors in the near-nozzle field for three scaled-up transparent nozzles, including two SGH nozzles with different spiral angles, and a non-spiral hole (NSH) nozzle, were performed to clarify the effects of the spirally grooved structures on in-nozzle cavitating flow and near-field fuel breakup. The results show that with an identical K-factor, the cavitation was largely restrained in the SGH nozzles, and the discharge coefficients of the SGH nozzles with spiral angles of 63° and 43° were approximately 13% and 25% lower than that of the NSH nozzle, respectively. Moreover, a liquid column was observed for each of the jets, surrounded with scattering droplets. And the SGH nozzle yields 9–10 times higher number density, 9%–10% smaller Sauter mean diameter of the droplets, and 4–7 times wider spreading angle than those of the NSH nozzle. These facts verified that the use of spirally grooved hole enhanced primary breakup of liquid jets.

Quantitative Expression of a Slight Deviation of the Impact Angle in a Collision Atomizer

In the processing of colliding atomizers, a small change in the inclination of each orifice will double the impact angle, seriously affecting its atomization performance. In this paper, the influence of slight impact angle deviation on atomization performance was studied in steps of 1°, quantitatively, for the first time. The cavitation effect of the flow field was combined with the shape and parameters of the atomization field. FLUENT was used to simulate the internal flow field, and an independently designed atomizer with transparent nozzles was used to detect the internal flow field in real time. The collision atomization experimental platform and the laser interference particle measurement platform were built independently, and the collision angle was adjusted through a high-precision rotating table to establish the relationship between collision-angle deviation (60° ± 5°) and the atomization field performance (Sauter mean diameter, atomization cone angle, and spatial distribution of droplets). The experimental results showed that under the same injection pressure, the increase in the collision angle led to an decrease in the Sauter mean diameter and an increase in the atomization cone angle. Taking 60° as the benchmark, the particle size distribution was concentrated at ~150 μm to 300 μm within the variation range of ±2°, and the peak positions were very similar.

Flash-Boiling Characterization During Ingress of Coolant Event for Dust Issue in ITER

Abstract Dust resuspension inside the vacuum vessel (VV) is one of the safety issues of the fusion reactor ITER. Plasma interaction with the plasma facing components (PFC) leads to their erosion, generating dust. One of the accident scenarios leading to dust resuspension is the ingress of coolant event (ICE) where a leak of the coolant pipes inside the VV conducts to injection and flash atomization of the cooling water. The metallic dust, produced by the erosion, is then oxidized by water throughout an exothermic reaction that produces hydrogen leading to a loss of confinement risk due to hydrogen and dust combustion. The steam flow, produced by the flash atomization of the liquid leaking from the breach, is considered to be the main source of dust resuspension. Therefore, experimentations about the two-phase flow generated by the flashing liquid jet are important to identify the main physical phenomena involved in the aerosol particles resuspension for ITER-like conditions that impose in particular low pressure level. Flash-boiling experiments were conducted under primary vacuum conditions. We studied the behavior and the structure of the flow resulting from superheated water injection into low pressure environment. Using shadowgraphy and particle image velocity (PIV), qualitative information and quantitative measurements on the two-phase flow that develops for different superheat conditions were gathered. The measured spray lateral spreading and droplets velocity are shown to increase with the superheat level. The use of a transparent nozzle also confirmed the strong coupling between the external structure of the atomized spray and the two-phase flow that develops upstream of the coolant circuit breach.

Impacts of Micro-Deviations of Aperture on the Characteristics of Collision Atomization Field

As the final flow channel of the liquid rocket engine, the manufacturing of impinging atomization nozzles has become a critical link in the manufacture of impinging atomization components. At present, the high-precision machining of millimeter nozzles in large-scale production is quite difficult, which inevitably leads to the diversity of internal flow field and atomization field parameters. In this investigation, the influence of the diameter deviation of impinging nozzles on the atomization field is analyzed by experiment. The transparent nozzle is installed in a typical colliding atomizer. The ultra-precision measurement is carried out by the optical fiber measurement system and the flow field in the nozzle is visualized. The information of atomized droplets in the atomization field is assembled by the laser interferometric particle imaging technology (IPI). The experimental results indicate that the micro-deviations of the collision aperture have a profound influence on the cavitation state of the flow field in the nozzles and the atomization characteristics (droplet diameter and atomization cone angle) of the atomization field.

The Effect of Ink Supply Pressure on Piezoelectric Inkjet.

Experimental and numerical analysis of the drop-on-demand inkjet was conducted to determine the jetting characteristics and meniscus motion under the control of the ink supply pressure. A single transparent nozzle inkjet head driven by a piezoelectric actuator was used to eject droplets. To control ink supply pressure, the pressure of the air in the reservoir was regulated by a dual valve pressure controller. The inkjet performance and the motion of the meniscus were evaluated by visualization and numerical simulation. A two-dimensional axisymmetric numerical simulation with the dynamic mesh method was performed to simulate the inkjet dynamics, including the actual deformation of the piezoelectric actuator. Numerical simulation showed good agreement with the experimental results of droplet velocity and volume with an accuracy of 87.1%. Both the experimental and simulation results showed that the drop volume and velocity were linearly proportional to the voltage change. For the specific voltages, an analysis of the effect of the ink supply pressure control was conducted. At the maximum negative pressure, −3 kPa, the average velocity reductions were 0.558 and 0.392 m/s in the experiment and simulation, respectively, which were 18.7 and 11.6% less than those of the uncontrolled case of 0 kPa. Therefore, the simulation environment capable of simulating the entire inkjet dynamics, including meniscus movement regarded to be successfully established. The average volume reductions were 18.7 and 6.97 pL for the experiment and simulation, respectively, which were 21.7 and 9.17% less than those of the uncontrolled case. In the results of the meniscus motion simulation, the damping of the residual vibration agreed well with the experimental results according to the ink supply pressure change. Reducing the ink supply pressure reduced the speed and volume, improved the damping of residual vibrations, and suppressed satellite drops. Decreasing ink supply pressure can be expected to improve the stability and productivity of inkjet printing.

Benchmark study of 2D and 3D VOF simulations of a simplex nozzle using a hybrid RANS-LES approach

In this study, a simplex nozzle is tested with water for the benchmarking of different flow simulation models. A large scale Plexi-glass transparent nozzle is used to reduce the influence of production tolerances on the performance. Experiments are conducted at different flow rates and CD, spray angle and film thickness parameters are evaluated. 2D and 3D hybrid RANS-LES multiphase flow simulations of simplex nozzle are validated against the experimental data. Multiphase nature of the flow is modelled by volume of fluid method. The main goal is to assess the capabilities and drawbacks of 2D axisymmetric and full sector 3D modeling approaches. It is observed that although full sector 3D simulations require HPC cluster systems, accuracies in validation parameters are quite satisfying. Conversely, 2D axisymmetric simulations which can be run on a single core and give a general outlook of the flow field, they show an overshoot of CD and film thickness over the selected range of flow rate. It is shown that this overshoot is mostly related with the inlet boundary condition, which can not take the flow contraction and/or separation at the inlet slots into account. After correcting the inlet velocity 2D simulations by using the 3D results, it is shown that the predictions can be quite close to the experimental data.

Experimental circuit for the generation of cavitation in oil flow

The paper deals with the flow of oil through an experimental hydraulic circuit with a convergent divergent nozzle with a circular cross-section. Under different physical conditions, i.e. changes in pressure, flow and temperature, the formation and development of cavitation is monitored. As it is a highly dynamic flow in a transparent nozzle, cavitation is monitored using a high-speed camera and the frequency of formation and dissolution of the cavitation bubble and the movement of the cavitation cloud is then determined. The authors deal with the issue of the amount of dissolved or undissolved air in the hydraulic oil. Both variants of air influence the formation, development and size of the cavitation area. This cavitation is in this case called air cavitation. In technical practice, the issue of air cavitation is relevant, especially in the pump suction, where vacuum, leaks and hence air intake may occur.

Numerical Simulation and Experimental Study for the Impact of In-Flow Nozzle on Spray Characteristics.

The impact of the in-flow characteristics inside the injection nozzle on atomization has been experimentally and computationally studied. Measurements are carried out using a transparent glass nozzle. Pulsed laser sheet with a synchronized charge-coupled device (CCD) camera and image processing, together with a particle image velocimetry (PIV) setup have been used as measuring techniques. Images and relevant image processing are used to visualize and quantify the rate of generation of cavitation bubbles inside the nozzle, the spray particle size distribution, and cone angle. Velocities inside and outside the injection nozzle are measured using PIV. The experimental investigation has been extended to include a wider range of the injection nozzle geometrical aspect ratios and working parameters. The computational model is a three-dimensional, two-phase, turbulent model to solve both the in- and out-nozzle flows. A novel coupling mathematical model is proposed for the definition of the probability density function of the issuing droplet size distribution, based on the in-flow developed conditions. A good agreement between both the experimental and computational results has been found under all conditions. According to both the experimental and computational results, it has been found that the onset of cavitation inside the injection nozzle, its location, collapse, and consequently the issuing spray configurations depend on the flow cavitation number, the nozzle geometrical characteristics, the liquid temperature, and the injection and back pressures. According to the quality of the obtained results from the model, it can be used to extend the study to cover a wider range of spray applications.

Spectral analysis of gaseous cavitation in water through multiphase mathematical and acoustic methods

Mathematical modeling is applied as an effective tool for prediction of cavitation in hydraulic components and systems. A multiphase mathematical model based on the change in phase between water and vapor is typically used to investigate the cavitation flow. However, dissolved air can significantly affect the cavitation. This study proposes a new approach based on a multiphase turbulent mathematical model by adding the air into the mixture to solve the dynamics of cavitation. To clearly assess the significance of air in the multiphase model, four variants of the mixture are investigated (water; water and vapor; water and air; and water, vapor, and air together). The software of the computational fluid dynamics ANSYS Fluent was applied to numerically solve the proposed mathematical models. The influence of gaseous components is analyzed through evaluation of hydraulic parameters and spectral characteristics of the cavitation bubble. To verify the proposed mathematical models, a hydraulic water circuit was built to generate cavitation in a transparent Venturi nozzle. Cavitation in the experiment was identified by measuring the flow rate, static pressure, and noise and visualized with a camera. The numerical results of the extended multiphase flow confirmed very good agreement with experimentally obtained basic hydraulic parameters and frequency-related characteristics. Knowledge obtained from the multiphase mathematical model of cavitation can be applied to cavitation in the oil flow (pump suction and flow through the valve) in future research, where the effect of the air on cavitation is more important than the effect of vapor.

Effects of nozzle configuration on flash boiling fuel sprays of twin-orifice nozzle with aviation kerosene

To investigate the effects of nozzle configuration on flash boiling fuel sprays of the twin-orifice nozzle with aviation kerosene, five different two-dimensional transparent slit nozzles with various orifices’ diameter ratio and expansion chamber's aspect ratio are fabricated and studied. Both internal two-phase flow and near-nozzle fuel jet under subcooled and superheated conditions are studied via a high-speed camera with backlit measurement and image processing methods. The experimental results indicate that the bubbles nucleation from cavitation at the inlet-orifice wall can accumulate within the expansion chamber with enhanced bubble growth and aggregation under higher-level superheated conditions, which can significantly promote the fuel atomization and evaporation. In addition, a smaller orifices’ diameter ratio leads to larger in-nozzle bubble volumes within the twin-orifice nozzle, while the internal bubble inception may be inhibited by increasing the expansion chamber's aspect ratio. Furthermore, decreasing the orifices’ diameter ratio can promote the flash boiling process, while better-quality spray at flare flash boiling state may be facilitated by the near-nozzle mechanical breakup. The nozzle with a lower expansion chamber's aspect ratio has a larger spray cone angle and weaker jet instability, benefited from larger bubble volume and higher effective bubble rate, which accelerates the bubble disruption and fulfills the potential of in-nozzle flash boiling on near-nozzle jet disintegration.

Characteristics of flash boiling spray of aviation kerosene in the twin-orifice nozzle

To investigate the flash boiling fuel spray characteristics of the twin-orifice nozzle with aviation kerosene, a two-dimensional transparent slit nozzle is designed and studied under a wide range of superheated conditions. Both internal bubble development and near-nozzle jet atomization under subcooled and superheated conditions are studied via a high-speed camera with backlit measurement and image processing methods, accompanied with numerical simulation on in-nozzle two-phase flow for auxiliary analysis. The results indicate the expansion chamber of twin-orifice can increase the time for bubble nucleation and growth and enhance the bubble collision and aggregation. Also, under a higher superheat degree, the bubble volume fraction in the liquid-bubble interaction zone increases, and the bubbles exhausted from discharge-orifice can initiate from the interior to the edge of fuel jet at transitional flash boiling state. Furthermore, the bubble disruption occurs at the jet edge, which greatly enhances the atomization of fuel jet, leading to narrower liquid-core and wider spray distribution of near-nozzle fuel jet. In addition, the mechanisms of in-nozzle bubble development and near-nozzle spray atomization in the twin-orifice nozzle are concluded, which can be elucidated with more fundamental understanding of flash boiling fuel sprays.

Influence of nozzle geometry on spray and combustion characteristics related to large two-stroke engine fuel injection systems

As emission regulations for large two-stroke marine Diesel engines increase as well, the manufacturers face similar problems as small-sized engine builders. The fuel injection, spray formation and subsequent combustion process remain among the main drivers for emissions of direct-injection, compression-ignition internal combustion engines. Since the fuel injection nozzle design of large two-stroke marine Diesel engines differs significantly, not only in size but especially regarding their non-symmetricity and eccentrical orifice arrangement, compared with four-stroke engines, the bulk of research available related to this topic is very limited.To further deepen the understanding of how in-nozzle cavitation flow during the fuel injection process affects the combustion behaviour in large two-stroke marine Diesel engines, transparent nozzle geometries have been used to link the cavitation phenomena with spray and combustion characteristics. A total of six single-orifice nozzles based on the original five-orifice nozzle design of large two-stroke marine Diesel fuel injectors, with and without hydro-erosive grinding, have been experimentally investigated under realistic engine conditions using highspeed optical measurement techniques and a unique constant-volume spray chamber that geometrically represents a combustion chamber of a large two-stroke marine Diesel engine at top dead centre.The spray morphology and combustion results reveal significant differences between the non- and hydro-erosive ground nozzle geometries. While the standard and angled nozzle versions remain similar, the eccentrical nozzle with its distinctive in-nozzle swirl cavitation pattern behaves differently, leading to a very reliable start of ignition behaviour and extreme wide spray angle.

Vortex Flow and Cavitation in Liquid Injection: A Comparison between High-Fidelity CFD Simulations and Experimental Visualizations on Transparent Nozzle Replicas

Experimental instantaneous shadowgraph visualizations on transparent glass nozzle replicas of high-pressure fuel injectors have been used to validate a novel in-house high-fidelity LES-VOF multiphase solver, to study the evolution of vortex flow and fuel cavitation. Both experiments and simulations capture the formation of an unsteady vapor structure inside the nozzle volume, which is referred to as ‘string-cavitation’; strings are found at the core of the recirculation zones. High-fidelity simulations provide a very detailed insight into the vortex generation in the injector nozzle; strings appear within the time scales that are relevant for fast injection events (on the order of 0.1 milliseconds) and, for the problem under consideration, their generation seems mostly related to the flow pattern in the sac. It is also shown that vortexes interact, merge till they disrupt and favor the temporary inception of shear cavitation.

Study on Cavitating Flow Inside Orifice and Spray Angle Near Nozzle Tip According to the Position of Needle Using Enlarged Transparent Acrylic Nozzle

This study aims to analyze effect of needle position inside nozzle on the internal and external flow characteristics. To visualize cavitating flow inside nozzle, the transparent acrylic nozzle was used. We tested five needle positions that simulated the needle movement of injector. The cross-section of test nozzle is rectangular with a very narrow width instead of circular shaped for a better visualization of cavitating flow. The inside of the nozzle was visualized by the shadowgraphic visualization using a high-speed camera and a metal-halide lamp. From the experiments, development of the cavitating flow according to the needle positions depends on the flow velocity and the cavitation number. The cavitation length, cavitation width, and spray angle are relatively symmetrically shown when the needle was at the center. However, they were asymmetrical when the needle position was biased. When the needle positions were in the same on the vertical line, the minimum cavitation length, minimum cavitation width, and minimum spray angle were larger when the upper level. The needle position affects the development timing and cavitation growth. When the needle position was located at the upper level, the hydraulic flip occurred at a higher injection pressure than the lower level.