Trans/supercritical injection has great potential for improving the formation of combustible gas mixtures and emission performance. In our work, based on the sufficient validation of the current numerical framework, we compared the PR EoS with the RK-PR EoS and applied them to simulate the injection events under trans/supercritical conditions. The effects of different nozzle diameters and chamber conditions on jet characteristics and pseudo-boiling were investigated. The results show that RK-PR EoS has higher prediction accuracy. The nozzle diameter has a significant effect on fuel injection characteristics, and the time required to undergo the pseudo-boiling process becomes longer with increasing nozzle diameter. Compared with chamber temperature, chamber pressure has a greater effect on the fuel breakup. The high pseudo-boiling intensity caused by the low supercritical pressure significantly increases the jet length and reduces the mixing layer thickness. In addition, unlike the single-component case, the occurrence of mixing affects both pseudo-boiling and its intensity.

Ducted fuel injection (DFI) is a newly established technology showing great potential in breaking the soot/NOx trade-off. Multiple-injection strategy is also seen as a possible way to reduce soot and NOx emissions simultaneously. However, the applicability of these approaches is not fully understood. The present study investigated the spray characteristics of DFI technology coupled with a dual-injection strategy. Results showed that during the injection interval (the interval between two injections), ambient gas continues to flow into the duct due to the pressure difference between the duct inside and outside, which increases the velocity of the second injection. The spray penetration length development of ducted spray is faster than that of free spray in both the first and the second injection events. During the first injection, the distribution of the equivalence ratio of ducted spray is more uniform than free spray, and the peak value of the equivalence ratio of ducted spray is lower than that of free spray. Compared to the first injection, in the second injection, when the radial equivalence ratio distribution of ducted spray is equivalent to the that of free injection, the path of spray traversed is shorter. These indicate that DFI technology and dual injection strategy couples well in case of spray and mixing process.

Computational simulations of spray flows typically start with bulk liquid flow, bulk-to-droplet conversion algorithm for primary atomization, then tracking of discrete particle motion. The key step is the atomization criterion and subsequent drop size conversion. To facilitate this process, we consider the Weber number, based on strain rate (West), as the local atomization condition in computational simulations of spray flows. This atomization criterion is tested within the computational protocol developed in this laboratory, which uses the integral theory as the primary atomization algorithm. Based on this definition, West ~ 107 appears to work quite well in specifying the location of primary atomization, across different spray geometries. Therefore, the conservation equations of mass and energy in integral forms can be effectively coupled with the CFD-based momentum solver to simulate spray flows, by using the current atomization criterion.

To study the influence of ducts other than circular shapes on spray characteristics of ducted fuel injection (DFI), experimental and simulation methods were used to study the impacts of elliptical ducts on DFI macroscopic spray characteristics. Two elliptical ducts, small and large, were used, with a circular duct for comparison. The small elliptical duct had the same hole cross-sectional area as the circular duct, and the large elliptical duct had a larger cross-sectional area. The DFI spray configuration with the elliptical duct promoted the axial dispersion and weakened the radial dispersion of spray with respect to free spray. Different spray-duct interactions caused differences in spray characteristics, manifested in changes in velocity field and pressure field. The spray velocity field of the circular duct had the best effect on promoting spray dispersion, followed by the small elliptical duct, and finally by the large elliptical duct compared to that of free spray. Furthermore, the pumping effect caused by the pressure differences between inside and outside the duct promoted the thorough mixing of spray and ambient gas inside the duct. From the radial velocity of ambient gas flowing to spray around the duct inlet, the pumping effect of the circular duct was the strongest, followed by the small elliptical duct and finally by the large elliptical duct.

In this study, inert liquid sprays are generated by impinging two symmetric jets of water with a central jet of water, ethanol, or n-dodecane. This configuration, referred to as an unlike triplet injector, can be used in rocket engines to atomize liquid storable propellants, for instance, hydrogen peroxide oxidizer combined to a fuel. Here, the inert sprays are investigated in the so-called impact waves regime, which corresponds to jets a Weber number of higher than 1000. The atomization process is characterized using high-magnification shadowgraphy (HMS) from the impinging point of the jets into a sheet until it breaks up into ligaments and droplets. The HMS technique enables 10 kHz visualizations with an interframe of 4 μs and a spatial resolution up to 6.4 μm/pixel (1024 × 1024 pixels). Characteristic lengths of the primary atomization are measured: breakup length, apparent wavelength, and ligaments size. Similarly, the droplet populations are described based on arithmetic and Sauter mean diameters, shape, and velocity. Statistics of large droplet distributions are analyzed regarding the injection conditions and distance to the impingement point. Compared to like-doublet spray, the like-triplet evidences a slower atomization (longer breakup distance) and generates larger drops that require more distance to stabilize in size, centricity, and velocity. Unlike-triplet sprays exhibit a similar behavior to like-triplet spray while producing larger droplets, probably because of the fuel properties that stabilize the liquid sheet.

Numerical simulation and experiment are the two main methods in the investigation of spray and atomization. Some crucial parameters of simulation models are supposed to be calibrated using corresponding experimental data. However, direct comparisons between simulation data and experimental results might be confusing when focusing on spray boundaries or penetration, as the light scattering physics during imaging is always likely to be ignored in computational fluid dynamics (CFD) post-processing. In many cases, CFD provides invisible droplets, resulting in variance in the boundary confirming process. Previous studies discussed backlit conditions in Euler-based simulations to identify spray boundaries, but for most commonly used Lagrangian-based simulations, which are often coupled with Mie scattering experiments, this topic remains undiscussed. In Lagrangian-based methods, droplets are treated as discrete particles, where scattering plays a more crucial role. In this study, light intensity analysis based on Mie scattering theory and intensity integration focusing on Lagrangian field has been presented, aiming to adjust simulation data of spray coincides with Mie scattering image as much as possible on the theoretical base. It is found that particle size and in-parcel numbers are related to the scattering intensity of droplet particles. the correlated CFD data using Mie scattering theory are tested to be theoretically similar with Mie scattering imaging results compared with raw simulation data, making the comparison between datasets reasonable, which makes adequate preparations for the calibration of spray models.

Pressure swirl atomizers are widely applied in daily life and production activities. The spray angle is one of the most important parameters for the pressure swirl atomizers and mainly affects the spatial distribution of sprays and droplets. The quick prediction of the spray angle is important for the design and optimization of a pressure swirl atomizer. In the present study, the effects of length of swirl chamber, liquid viscosity and pressure differential on the hollow cone spray angle are studied by experiments. An available semi-empirical correlation for hollow cone spray angle prediction is derived by proper assumptions and simplification. This available correlation is validated by comparison with the experimental results. Besides, other correlations based on inviscid flow and viscous flow are also introduced to predict the spray angle for comparison. Based on the experimental results, the measured hollow cone spray angle decreases with the increasing of length of swirl chamber and liquid viscosity nonlinearly. The measured hollow cone spray angle shows no obvious growth with the increasing of the pressure differential when the pressure differential reaches a critical value. Further, the hollow cone spray angles predicted by the present available correlation agree well with the experimental results. The prediction uncertainties can be within ±15% for all cases. While, the prediction uncertainties are no less than ±40% by other correlations. Compared with previous correlations, the present available correlation can achieve better prediction accuracy. This available correlation is capable for the quick prediction of the viscous hollow cone spray angle.

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.

The flash-boiling spray collapse has been widely investigated for multi-hole gasoline direct injection (GDI) injectors. No consensus has been achieved on the collapse process and its mechanism. Herein, the flash-boiling spray behaviors were carefully examined using a multi-hole GDI injector with a larger envelop angle to better observe the jet-to-jet interactions from two view directions. From the front view, the spray collapse occurred as superheat degree (Rp) increased while the sprays under relatively high ambient pressures showed stronger collapse given similar Rp. From the bottom view, the adjacent jets with the closest hole-to-hole distance firstly merged with the increase in Rp, then more jets with relatively larger distance started to merge, and finally the sprays collapsed as Rp further increased. This sort of spatially orderly collapse with the increase in Rp occurred at the beginning of injection and near-nozzle region, contributing to the spray collapse under flash-boiling conditions. It was also found that the merged jets could attract the adjacent jets to some extent with the spray developing. This attraction occurred in the far field and influenced the spray morphology, but had a limited role in the spray collapse. The spatially orderly collapse in such a nozzle configuration delivers a more general understanding of the flash-boiling induced collapse and it will be of help in injector design optimization.

This paper presents a detailed mapping of flow-blurring (FB) and flow-focusing (FF) atomization as relevant to Newtonian fluids and non-Newtonian fluids. Two Newtonian fluids with different viscosities are tested, along with a non-Newtonian fluid, where the properties are similar to those of human saliva. Images featuring the fragmentation characteristics are presented with regime diagrams describing the transition from flow-focusing to flow-blurring. Flow-blurring refers to a mode of atomization where the fluid is partially aerated with gas bubbles to assist breakup, while flow-focusing has similarities to air-blast atomization (AB). The regime transition map developed for FF/FB atomizers reveals the parameters, which define transitions from the flow-focusing to the flow-blurring regime. Along with the transition identification, the breakup regime map also details the fragment morphology of the atomizing liquid stream as a function of the governing dimensionless groups.

The present study investigates, numerically, the spill-return atomizer's (SRa) internal flow features within a 3D geometry by the use of a commercial code, ANSYS FLUENT. Experimental measurements, from the literature, are used to validate the numerical results of spray-cone angle (SCA), discharge coefficient (CD), and mass flow rates. The unsteady flow is solved as two-phase flow using the volume of fluid (VOF) method. The turbulence is captured using the K−ω shear-stress transport (SST) turbulence model. The geo-reconstruct scheme is used to capture the gas-liquid interface. An adaptive mesh refinement (AMR) is applied to refine the regions featuring high gradients in space. The simulations manage to capture the overall flow characteristics of a SRa with the formation of an air core and a thin liquid film in the exit region of the swirl chamber. Profiles of axial and tangential mean velocities are obtained. Furthermore, pressure measurements are conducted and pictures of the air core, velocity, and pressure field are taken for qualitative analysis. The tangential velocity profile resembles a Rankine vortex. The results show that air cores behave differently (size and shape) when changing the spill to feed ratio (SFR) due to a significant rise in the velocity profiles inside the swirl chamber, which directly affect the SRa performances, such as SCA and breakup process. The results show an important influence of the SFR variation on the gas-liquid volume fraction. A brief overview at the end is devoted to creation of the liquid spray cone outside of the injector, as well as the liquid sheet breakup process.

The characterization of the shape of liquid elements in a spray is a good way to analyze atomization processes. Experimental approaches based on common imaging techniques suffer from partial information given by 2D images to characterize intrinsically 3D objects such as liquid droplets. In this paper, we address the question, To what extent can the shape parameters measured on 2D images reveal the 3D content of the droplet shape? An analytical approach is first adopted to determine relationships between 2D and 3D shape parameters for two families of objects, the oblate and prolate spheroids. Two numerical databases obtained from direct numerical simulation of two-phase flows are then explored for their ability to give complete 3D shape information and extrapolate 2D parameter values from projections on planes selected according to flow characteristics. Focus is put on one shape parameter particularly sensitive to 3D to 2D projection effects, namely the uniformity parameter introduced by Blaisot and Yon (2005). Statistics obtained from the numerical databases are used to guide the analysis of results extracted from experimental images. It is shown that statistics on a 3D shape parameter could be induced from the ones for the 2D parameters. Two conditions are necessary: (1) the 2D projection is performed perpendicularly to the main flow direction, which is always the case in experiments; and (2) a particular care must be put on the determination of the statistics of orientations of the main axis of the liquid elements. This last point should be tackled in future experimental analyzes to estimate 3D shape parameter statistics from 2D images.

Flash boiling spray is a promising method to improve atomization, which subsequently affects the combustion phenomenon in engines. Therefore, an investigation on both macroscopic and microscopic characteristics of the flash boiling spray should be done to understand this behavior and mechanism. A mini-sac injector with a single hole was used under different injection pressures among 10, 15, and 20 MPa. Experiments were performed in a constant volume chamber with low ambient pressures varying from 5 to 100 kPa. First, macroscopic behaviors including morphology, spray tip penetration, spray angle, and spray cone angle were compared by a Mie scattering method. Then, microscopic characteristics of flash boiling spray were obtained with the help of particle image analysis technology. In order to analyze the droplet behaviors along the spray axis completely, a spray "slicer" was introduced to filter the spray. The influences of measurement location, injection pressure, and ambient pressure were discussed in detail. Results showed that although the injection pressure and measurement location have influences on the spray and atomization in both macroscopic and microscopic cases, the ambient pressure plays a more critical effect on it, indicating the low ambient pressure-induced flash boiling spray could improve the atomization much more efficiently.

A novel micro-channel based rotary (MCR) atomizer has been developed, which produces spiraling micro-jets leading to the formation of a well-dispersed fine spray. A co-swirl air stream envelops the liquid jets to aid in the process of atomization. Initially, the jet breakup process is studied using highspeed shadowgraphy. The liquid jets deform into elliptical and eventually sheet-type cross sections at higher rotational speeds, due to air drag. Also, a hydrodynamic instability develops in the form of wavelets consisting of bag-like surfaces pinned between thick crests. At higher rotational speeds or air-flow rates, the wavelets break down to form small droplets. Subsequently, secondary spray atomization characteristics of droplets due to interaction with air stream have been investigated using phase Doppler particle analyzer. At axial distances of about 5 nozzle diameters, a hollow cone spray is observed, consisting of relatively larger droplets formed during jet breakup. The region close to the spray axis consists of smaller droplets entrained by the recirculatory air flow. At higher swirl, the recirculation zone widens radially and as a result, peak droplet velocities and droplet sizes shift away from the axis. At an axial distance of about 10 nozzle diameters, the droplet sizes and droplet number densities become more uniform spatially, as a result of spray breakup and dispersion phenomena, aided by the swirling air flow. The present novel MCR atomizer provides a nearly uniform fine spray, for a range of flow conditions.

In the present study, the contact angle model and the origin of the parasitic current, specifically the parasitic currents relation with grid distribution have been studied to provide accurate prediction of droplet impact on static and moving walls in Volume of Fluid (VOF) framework. It has been quantitatively shown that the number of neighboring cells sharing the same faces influences the gradient calculations associated with the formation of parasitic current. It has been observed that the polyhedral cell structure delivers smoother the interface gradient distribution than the cartesian cell structure. After implementing, modified Kistler contact angle model in OpenFoam and using ployhedral cells for the simulations, we could succesfully validate transient droplet shapes formed upon impact with those obtained from experiments. Droplet outcomes, such as deposition, partial rebound and split deposition on stationary and moving smooth surfaces are obtained consistent with experimental results.

The influence of nozzle parameters on the atomization performance of aviation kerosene is not clear. To solve this problem, the kerosene atomization characteristics under different nozzle hole diameters (d), nozzle hole numbers (N), nozzle hole cone angle (φ), as well as nozzle hole tilt angle (α) were investigated by numerical simulation. The results showed that when the nozzle hole diameter increased from 0.16 mm to 0.2 mm, the liquid penetration length and spray cone angle increased from 36 mm and 13° to 41 mm and 15°, respectively. Due to the influence of cross interference, the liquid penetration length of aviation kerosene presented a non-linear relationship with the nozzle hole number. In addition, for multi-hole nozzles, the cross-interference phenomenon reduced significantly as the spray hole cone angle decreased. The decrease in nozzle hole tilt angle facilitated the increase of turbulent kinetic energy, thus increasing the fuel evaporation rate. On this basis, the simulation analysis of the atomization and combustion process in aviation kerosene rotary engine was carried out. The findings indicated that the liquid penetration length and particle distribution of kerosene were more suitable for a small-scaled kerosene rotary engine with a narrow combustion chamber when the d of the two-hole nozzle is 0.18 mm, and φ and a are 12° and 15°, respectively. Compared to the original nozzle parameters, the average pressure at the optimized nozzle parameters increased from 1.83 MPa to 4.16 MPa, showing an increase of 127%, and the total heat release rate increased by 107%.

This study examines the sonic twin-fluid atomizer based on gas-dynamic effects and atomization behavior with two distinct configurations: converging and converging-diverging (CD) atomizers. The atomization characteristics are compared by employing a 280-μm annular liquid sheet with central core air. CD atomizer exhibited the sheet rupture breakup mechanism, whereas perforated wavy sheet disintegration was observed in the converging atomizer with both atomizers exhibiting a bursting phenomenon. Sauter mean diameter (D32) slightly varied with increased axial locations in the turbulent region. In comparison, D32 drastically increased with an increase in radial locations in the aerodynamic region, with more increment in the converging atomizer. Drop size distribution (DSD) showed unimodal distribution with a narrower range for CD atomizer in the turbulent region. In the aerodynamic region, DSD becomes more dispersed with an increase in radial location. The relative span factor (Δ) value sharply decreases for the converging atomizer with the axial location in the turbulent region. In comparison, the RSF (Δ) value remains in a narrow range ( ~ 2−4) for both atomizers in the aerodynamic region. Sauter mean diameter (SMD), when plotted against the air-to-liquid mass ratio for the turbulent and aerodynamic region, exhibited a near-inverse relationship. The relative span factor (Δ) displayed a similar trend except for the aerodynamic region with slight variation for the CD atomizer case.

Water was sprayed through a full-cone nozzle with a droplet Sauter mean diameter of 35 µm onto stainless-steel meshes with pore sizes of 178, 457, or 787 µm that were held either horizontally or vertically. The mass flow rate of liquid penetrating the mesh was measured continuously by capturing droplets that passed through pores and weighing them using an analytical balance. High-speed videos were taken of droplets landing on the mesh. Droplets accumulated rapidly on mesh wires and blocked pores so that the mass flux of water passing through the mesh decreased rapidly after the spray was started. Droplets landing near an unblocked pore were leached by neighbouring pools of liquid, delaying the blockage of that pore. Water captured on vertical meshes drained downwards due to gravity, unblocking pores, so that eventually the mass flux penetrating the vertical mesh reached a steady state. A stochastic model was developed to predict the spray mass that penetrates a vertical mesh before it becomes fully blocked. The volume required to block the mesh was found to be a function of both the open-area ratio and the number of surrounding blocked pores. Predictions from the model of the mass fraction penetrating the mesh and the time required for blockage agreed well with experimental measurements.

In this study, the atomization of an effervescent atomizer with an elliptical and circular orifice was investigated experimentally in the gaseous crossflow. The shadowgraph technique was used to visualize the near field of the spray atomization. To measure the spray penetration height in crossflow, image processing was performed on the shadowgraph images based on two different approaches: (i) a threshold analysis of average spray images and (ii) standard deviation images of spray. By comparing these two approaches, it is found that the penetration heights obtained from the standard deviation images were much higher since these images were able to capture small droplets more accurately. Moreover, based on the standard deviation images of spray, an empirical correlation for the spray penetration height was developed as a function of gas-liquid mass flowrate ratio, liquid-air momentum flux ratio, orifice aspect ratio, and downstream location. The laser diffraction technique was used to analyze the particle size of the aerated elliptical and aerated circular jets in a gaseous crossflow. For each orifice shape, the effect of gas-liquid ratio and downstream location on the Sauter mean diameter (SMD) was studied. The results show that the aerated circular jet penetrates higher into the gaseous crossflow than the aerated elliptical jet. Besides, the aerated circular jet in crossflow mainly generates smaller drop sizes compared to the aerated elliptical jet.

The transcritical transition of the nonpolar hydrocarbon and highly polar alcohol mixtures from the subcritical to supercritical regime has not been yet well understood. In the present paper, based on molecular dynamics simulation, the nonequilibrium evaporation and transcritical transition processes of the n-heptane/ethanol mixtures are comprehensively investigated under various ambient conditions, and the development of the vapor-liquid interface is deeply analyzed. Under the sub-critical condition, the increased ethanol concentration elevates the evaporation rate because of the decreased vapor pressure and increased thermal conductivity. However, under the supercritical condition, the effect of ethanol addition on the mixture temperature becomes slight due to the decreased difference of thermal conductivity between ethanol and n-heptane, but the preferential diffusion of ethanol is observed because of its higher diffusivity. In addition, it is found that the increase of ethanol broadens the vapor-liquid interface at moderately high pressure due to its higher volatility and thermal conductivity. Since the interfacial thickness affects the interfacial resistivities and evaporation characteristics in the hydrodynamic framework, the thickened interface indicates the more significant nonequilibrium effect in the vicinity of interface for the mixtures with large amounts of ethanol. Finally, the transcritical transition time decreases rapidly with the increased ambient pressure, and the descent gradient gradually decreases with the further increased pressure because the thermal conductivity becomes less sensitive to the pressure.