Transient heating effects in high pressure Diesel injector nozzles

The tendency of today’s fuel injection systems to reach injection pressures up to 3000 bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (C_d), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1 − C_d²) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration. Increasing the inlet pressure from 2000 bar, which is the pressure utilized in today’s fuel systems to 3000 bar, results to significantly increased fluid temperatures above the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified.     

Numerical research of diesel spray and atomization coupled cavitation by Large Eddy Simulation (LES) under high injection pressure

A special spray model is applied to study the spray behavior with high injection pressure and micro-hole nozzle. To reveal the cavitation in diesel nozzle and its influence on spray and atomization, the Large Eddy Simulation (LES) turbulence model is adopted to detect the cavitation, and then the special spray model coupling the cavitation is build. From research results, three important conclusions can be drawn. Firstly, the cavitation flow can raise the effective velocity at the nozzle exit and such effect become even more obvious with higher injection pressure, e.g.180 MPa. Secondly, the applied spray model is in good agreement with the spray characteristics and images obtained from the EFS8400 spray test platform. Thirdly, the cavitation with high injection pressure and micro-hole nozzle can increase the spray cone angle and reduce the spray penetration; the cavitation intensity has a great impact on the spray velocity field and vorticity intensity, especially at the initial spray field under the condition of high injection pressure.     

Atomization of glycerin with a twin-fluid swirl nozzle

Atomization of liquids with high viscosity is always a challenge, especially when small diameter droplets and high liquid flow rates are simultaneously required. In the present research, the performance of a Venturi–vortex twin-fluid swirl nozzle is examined, attending to its capabilities to generate droplets with diameters below 20 µm when atomizing pure glycerin at room temperature. In this nozzle, air is injected tangentially in a central convergent section, and discharges suctioning the liquid fed to a coaxial chamber, here using a gear pump. The resulting spray is visualized and analyzed. Droplet size distributions are measured with a laser diffractometer. As expected, droplet diameter increases with liquid flow rate, and quickly diminishes when air flow rate is increased. Sauter mean diameters (SMD) below 15 µm can be obtained even when atomizing pure glycerin. However, these values are obtained for relatively low glycerin flow rates (∼5 l/h), and with rather wide distributions. For 10 l/h and an air-to-liquid mass flow rate ratio (ALR) of 13.7 more than 26% of the glycerin volume is atomized in droplets smaller than 20 µm. Liquid ligaments are observed near the nozzle exit, but they tend to break up while moving downstream.     

Numerical analysis of unsteady cavitating turbulent flow and shedding horse-shoe vortex structure around a twisted hydrofoil

Cavitating turbulent flow around hydrofoils was simulated using the Partially-Averaged Navier–Stokes (PANS) method and a mass transfer cavitation model with the maximum density ratio (ρ_l/ρ_v,clip) effect between the liquid and the vapor. The predicted cavity length and thickness of stable cavities as well as the pressure distribution along the suction surface of a NACA66(MOD) hydrofoil compare well with experimental data when using the actual maximum density ratio (ρ_l/ρ_v,clip = 43391) at room temperature. The unsteady cavitation patterns and their evolution around a Delft twisted hydrofoil were then simulated. The numerical results indicate that the cavity volume fluctuates dramatically as the cavitating flow develops with cavity growth, destabilization, and collapse. The predicted three dimensional cavity structures due to the variation of attack angle in the span-wise direction and the shedding cycle as well as its frequency agree fairly well with experimental observations. The distinct side-lobes of the attached cavity and the shedding U-shaped horse-shoe vortex are well captured. Furthermore, it is shown that the shedding horse-shoe vortex includes a primary U-shaped vapor cloud and two secondary U-shaped vapor clouds originating from the primary shedding at the cavity center and the secondary shedding at both cavity sides. The primary shedding is related to the collision of a radially-diverging re-entrant jet and the attached cavity surface, while the secondary shedding is due to the collision of side-entrant jets and the radially-diverging re-entrant jet. The local flow fields show that the interaction between the circulating flow and the shedding vapor cloud may be the main mechanism producing the cavitating horse-shoe vortex. Two side views described by iso-surfaces of the vapor volume fraction for a 10% vapor volume, and a non-dimensional Q-criterion equal to 200 are used to illustrate the formation, roll-up and transport of the shedding horse-shoe vortex. The predicted height of the shedding horse-shoe vortex increases as the vortex moves downstream. It is shown that the shape of the horse-shoe vortex for the non-dimensional Q-criterion is more complicated than that of the 10% vapor fraction iso-surface and is more consistent with the experiments. Further, though the time-averaged lift coefficient predicted by the PANS calculation is about 12% lower than the experimental value, it is better than other predictions based on RANS solvers.     

Vortical structure evolutions and spreading characteristics of a plane jet flow under anti-symmetric long-wave excitation

The vortical evolutions and spreading characteristics of a low-speed plane jet under anti-symmetric long-wave excitations are investigated experimentally. The perturbation is introduced with two oscillating strips located at the nozzle exit. The experiments were operated at Reynolds number of 8.2 × 10³ based on the nozzle exit height. Mixing and spreading properties are influenced obviously by long-wave excitation after the end of potential core. The increments of half-width, momentum thickness, and volume flow rate depend on the excitation frequency. The results of flow visualization also reveal the dependence of excitation frequency. The power spectra of fluctuating velocities shows that the evolution of coherent structure is significantly influenced by the long-wave excitation in the downstream, but it is similar to the natural jet in the near field. The long-wave excitation at certain frequencies can promote large-scaled anti-symmetric vortical structures in the far field.     

Study on the radial jet velocity distribution of two closely spaced opposed jets

The experimental and theoretical researches on the radial jet of two opposed jets have been carried out in this paper. The radial velocities of opposed jets with various exit velocities, nozzle diameters and nozzle separations were measured experimentally by a hot-wire anemometer (HWA). The results show that, the normalized radial velocities are self-similar across various radial sections at r ? 1.5D and the radial velocity profiles can be described by a Gaussian distribution function. The half-width increases linearly with increasing radial distance at r ? 1.5D, and spreading rates of radial jet are about 0.121. The normalized radial velocity at impingement plane increases firstly, and then decreases with the increasing normalized radial distance. The normalized radial velocity is independent on nozzle diameter, nozzle separation and exit velocity. The maximum radial velocity at impingement plane is proportional to the exit velocity, and it is inversely proportional to the 0.551th power of the normalized nozzle separation. The position of the maximum radial velocity increases with the nozzle separation at L/D < 1, and keeps invariant at L/D ? 1.     

Air flow and pressure inside a pressure-swirl spray and their effects on spray development

Air flow and pressure inside a pressure-swirl spray for direct injection (DI) gasoline engines and their effects on spray development have been analyzed at different injector operating conditions. A simulation tool was utilized and the static air pressure at the centerline of the spray was measured to investigate the static pressure and flow structure inside the swirl spray. To investigate the effect of static air pressure on swirl spray development, a liquid film model was applied and the Mie-scattered images were captured. The simulation and experiment showed that recirculation vortex and air pressure drop inside the swirl spray were observable and the air pressure drop was greater at high injection pressure. At high fuel temperature, the air pressure at the nozzle exit showed higher value compared to the atmospheric pressure and then continuously decreased up to few millimeters distance from the nozzle exit. The pressure drop at high fuel temperatures was more than that of atmospheric temperature. This reduced air pressure was recovered to the atmospheric pressure at further downstream. The results from the liquid film model and macroscopic spray images showed that the air pressure started to affect the liquid film trajectory about 3 mm from the nozzle exit and this effect was sustained until the air pressure recovered to the atmospheric pressure. However, the entrained air motion and droplet size have more significant influence on the spray development after the most of the liquid sheet is broken-up and the spray loses its initial momentum.     

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers (Re = 1 × 10⁴, 2 × 10⁴ and 3 × 10⁴) and four different inclination angles between the wall and the flow velocity at the nozzle (β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.     

Dynamics of heavy particles in two swirling jets

We report flow visualisations and laser Doppler anemometry (LDA) velocity measurements in the near field of two swirling jets. The Reynolds number based on jet diameter and bulk velocity at the nozzle exit is 1.4 × 10⁵. In the first jet, a small recirculation region is formed around the jet axis, while, in the second, the streamwise velocity remains positive and overshoots near the jet centre. In both cases, flow visualisations show that the vortex core of the jets is depleted of seeding particles. By using time-averaged distributions of the streamwise and tangential velocities measured at the nozzle outlet, the dynamics of the particles is simulated, by integrating their simplified equations of motion. The particles trajectory thus computed agrees well with that observed in the flow visualisations. Although the turbulence intensity is substantially different in the core of the two jets, its effect on the seeding concentration is localised near the edge of the core.     

Determination of gun propellants erosivity: Experimental and theoretical studies

Requirements for higher and higher performances of contemporary guns intensify investigations concerning influence of various factors on barrel erosion. The most important parameter for the life of the gun is the type of the gun propellant. The device based on modification of 37 mm M39 gun for investigating influence of gun propellant on barrel erosion is used. The nozzle (main part of device) mass loss during firing was the measure of gun propellant erosivity. The theory of nozzle erosion includes basic thermal, chemical and metallurgical factors. The main thermal and chemical factors are maximum nozzle inside surface temperature and gun propellant composition. The maximum nozzle surface temperature was determined theoretically by developed interior ballistic model with heat transfer, and experimentally by micro thermocouples measurements and solution of inverse heat conduction problem. The coefficients of gun propellants erosivity are determined in experimental and theoretical way.     

IR measurements of the thermodynamic effects in cavitating flow

The understanding of the thermodynamic effects of cavitating flow is crucial for applications like turbopumps for liquid hydrogen LH2 and oxygen LOx in space launcher engines. Experimental studies of this phenomenon are rare as most of them were performed in the 1960s and 1970s. The present study presents time resolved IR (Infra-Red) measurements of thermodynamic effects of cavitating flow in a Venturi nozzle.Developed cavitating flow of hot water (95 °C) was observed at different operating conditions – both conventional high speed visualization and high speed IR thermography were used to evaluate the flow parameters.Both the mean features of the temperature distributions and the dynamics of the temperature field were investigated. As a result of evaporation and consequent latent heat flow in the vicinity of the throat a temperature depression of approximately 0.4 K was measured. In the region of pressure recuperation, where the cavitation structures collapse, the temperature rise of up to 1.4 K was recorded. It was found that the temperature dynamics closely follows the dynamics of cavitation structures.Finally experimental results were compared against a simple model based on the Rayleigh–Plesset equation and the thermal delay theory and plausible agreement was achieved.Experimental data is most valuable for further development of numerical models which are, due to poor ensemble of existing experimental results, still at a very rudimentary level.     

Hydraulic characterization of high temperature hydrocarbon liquid jets

An experimental study has been conducted to investigate the hydraulic characteristics of a plain orifice nozzle issuing pressurized high-temperature liquid hydrocarbon, in order to simulate injection of aviation fuel after being used as coolant in an active cooling system in a hypersonic flight vehicle. The fuel was heated to 553 K (280°C) using an induction heater, at an upstream pressure of up to 1.0 MPa, and injected to atmospheric pressure conditions through a sharp-edged orifice of diameter 0.7 mm and length 4.3 mm. It has been observed that the isothermal lines on the plane of the mass flow rate versus the square root of the pressure drop (ΔP) were clearly affected by increased fuel temperatures, and the discharge coefficient (C_d) decreased sharply with increasing fuel injection temperature (T_inj) above the fuel boiling point of 460 K. The Reynolds number (Re) for three ΔPs with respect to T_inj reached maxima and then began to decrease as T_inj increased for each ΔP case, and the fuel temperature of maximum Re at a given pressure condition increased as ΔP increased. The effects of cavitation on the hydraulic characteristics of the high temperature fuel were explored by representing C_d with respect to three cavitation numbers and dissipation efficiency. The behaviors of C_d showed a clear dependency on cavitation number, and all of the results collapsed to a single curve, regardless of ΔP. In addition, the curve indicated that the C_d characteristics was divided into non-cavitating and cavitating regions by the critical cavitation numbers near the fuel boiling point, and a sharp decrease in C_d was found to be typical in the cavitating region. The relationship between C_d and Re showed that when T_inj exceeded the boiling point the high temperature liquid jets experienced a sharp decrease in C_d at a determined Reynolds number, due to the collapse of the mass flow rate induced by the choked cavitaiton.     

Influence of single rectangular groove on the flow past a circular cylinder

In the present study, flow control mechanism of single groove on a circular cylinder surface is presented experimentally using Particle image velocimetry (PIV). A square shaped groove is patterned longitudinally on the surface of the cylinder with a diameter of 50 mm. The flow characteristics are studied as a function of angular position of the groove from the forward stagnation point of the cylinder within 0° ≤ θ ≤ 150°. In the current work, instantaneous and time-averaged flow data such as vorticity, ω streamline, Ψ streamwise, u/U_o and transverse, v/U_o velocity components, turbulent kinetic energy, TKE and RMS of streamwise, u_rms and transverse, v_rms velocity components are utilized in order to present the results of quantitative analyses. Furthermore, Strouhal numbers are calculated using Karman vortex shedding frequency, f_k obtained from single point spectral analysis. It is concluded that a critical angular position of the groove, θ = 80° is observed. The flow separation is controlled within 0° ≤ θ < 80°. At θ = 80°, the flow separation starts to occur in the upstream direction. The instability within the shear layer is also induced on grooved side of the cylinder with frequencies different than Karman vortex shedding frequency, f_k.     

Self-sustained oscillation and cavitation characteristics of a jet in a Helmholtz resonator

Self-sustained oscillations resulting from a sudden expansion in geometry, as encountered in cavities, occur in a broad array of engineering applications. In the present study, the turbulent flow past a 120°-impinging edge Helmholtz nozzle was investigated. A modified theoretical model accompanied by numerical simulation was proposed to obtain the range of the oscillation frequency and was verified using experimental results. It was found that the cavitation clouds in the chamber dominate the oscillating frequency under the low pressure-high flowrate condition. Based on the simulation results, the details of cavitation development, the motion of vortex structures, and the fluid injection and reinjection were investigated in one typical cycle. The interaction between the cavitation and the vortex formation was analyzed with the vortex transport equation. The dilatation term, which is related to the mass transfer rate through the linkage of velocity divergence, has a high value only around the bulk flow; while the baroclinic torque is predominant due to the unremitting collapse and coalescence of the cavitation clouds.     

The effect of sidewalls on rectangular jets

An experimental study is presented regarding the influence of sidewalls on the turbulent free jet flow issuing from a smoothly contracting rectangular nozzle of aspect ratio 15. “Sidewalls” are two parallel plates, flush with each of the slots’ short sides, practically establishing bounding walls extending the nozzle sidewalls in the downstream direction. Measurements of the streamwise and lateral velocity mean and turbulent characteristics have been accomplished, with an x-sensor hot wire anemometer, up to an axial distance of 35 nozzle widths, for jets with identical inlet conditions with and without sidewalls. Centreline measurements for both configurations have been collected for three Reynolds numbers, Re_D = 10,000, 20,000 and 30,000. For Re_D = 20,000 measurements in the transverse direction were collected at 13 different downstream locations in the range, x = 0–35 nozzle widths, and in the spanwise direction at three different downstream locations, x = 2, 6 and 25 nozzle widths.Results indicate that, the two jet configurations (with and without sidewalls) produce statistically different flow fields. Sidewalls do not lead to the production of a 2D flow field as undulations in the spanwise mean velocity distribution indicate. They do increase the two-dimensionality of the jet increasing the longevity of 2D spanwise rollers structures formed in the initial stages of entrainment, which are responsible for the convection of longitudinal momentum towards the outer field, establishing larger streamwise mean velocities at the jet edges. In the near field, up to 25 nozzle widths, lower outward lateral velocities in the presence of the sidewalls are held responsible for the decrease of turbulent terms including rms of velocity fluctuations and Reynolds stresses. Skewness factors increase monotonically across the shear layers from negative values to positive forming sharp peaks at the outer edges of the jet, illustrative of the presence of well defined 2D roller structures in the jet with sidewalls.     

Separated flow structures around a cylindrical obstacle in a narrow channel

A detailed experimental study is performed on the separated flow structures around a low aspect-ratio circular cylinder (pin-fin) in a practical configuration of liquid cooling channel. Distinctive features of the present arrangement are the confinement of the cylinder at both ends, water flow at low Reynolds numbers (Re = 800, 1800, 2800), very high core flow turbulence and undeveloped boundary layers at the position of the obstacle. The horseshoe vortex system at the junctions between the cylinder and the confining walls and the near wake region behind the obstacle are deeply investigated by means of Particle Image Velocimetry (PIV). Upstream of the cylinder, the horseshoe vortex system turns out to be perturbed by vorticity bursts from the incoming boundary layers, leading to aperiodical vortex oscillations at Re = 800 or to break-away and secondary vorticity eruptions at the higher Reynolds numbers. The flow structures in the near wake show a complex three-dimensional behaviour associated with a peculiar mechanism of spanwise mass transport. High levels of free-stream turbulence trigger an early instabilization of the shear layers and strong Bloor–Gerrard vortices are observed even at Re = 800. Coalescence of these vortices and intense spanwise flow inhibit the alternate primary vortex shedding for time periods whose length and frequency increase as the Reynolds number is reduced. The inhibition of alternate vortex shedding for long time periods is finally related to the very large wake characteristic lengths and to the low velocity fluctuations observed especially at the lowest Reynolds number.     

Analysis of an organized turbulent structure using a pattern recognition technique in a drag-reducing surfactant solution flow

This work presents the investigation for an organized turbulent structure in a drag-reducing flow of dilute surfactant solution by utilizing a particle image velocimetry system to perform the pattern recognition technique on a trajectory in four quadrants of streamwise and wall-normal velocity fluctuations. The pattern recognition is added to a new algorithm in order to directly capture the spatial rotation motion. The Reynolds number based on the channel height and bulk mean velocity was set to 1.5 × 10⁴. Surfactant solution with a weight concentration of 150 ppm was employed and the drag reduction rate was 65%. In the drag-reducing flow, we observe increased frequencies of occurrence of the flow events that correspond to a meandering motion in the wall-normal direction of the high-and low-speed regions. Three findings from investigation of the ensemble-averaged Reynolds shear stress and vortex structure are as follows: (i) the Reynolds shear stress in the large fluctuation range occurs in the narrow region; (ii) Size, strength, arrangement and inclination in the spatial vortex structure in the drag-reducing flow differ from those of the water; and (iii) all trajectory contributions for the wall friction coefficient decrease. Finally, we interpreted that the viscoelasticity characterizing the viscoelastic stress and relaxation time in rheological properties of the flow changes specific elementary vortex for the drag-reducing flow, and the trajectories of each flow pattern change drastically.     

URANS vs. experiments of flow induced motions of multiple circular cylinders with passive turbulence control

Two-dimensional Unsteady Reynolds-Average Navier–Stokes equations with the Spalart–Allmaras turbulence model are used to simulate the flow induced motions of multiple circular cylinders with passive turbulence control (PTC) in steady uniform flow. Four configurations with 1, 2, 3, and 4 cylinders in tandem are simulated and studied at a series of Reynolds numbers in the range of 30 000<Re<120 000. Simulation results are verified by experimental data measured in the Marine Renewable Energy Laboratory. Good agreement was observed between the values of vorticity, amplitude ratio, and frequency ratio predicted by numerical simulations and experimental measurements. The amplitude and frequency response show the initial and upper branches in vortex induced vibration (VIV), transition from VIV to galloping, and galloping branch for all PTC-cylinders. The maximum amplitude of 2.9 diameters for the first cylinder is achieved at Re=104 356 in the numerical results. Compared with the first cylinder, the VIV initial branch starts at higher Re for the downstream cylinders due to the presence of the upstream cylinder(s). 2P and 2P+2S vortex patterns are observed at Re=62 049 and Re=90 254 for the single PTC-cylinder. Furthermore, the shed vortices of the downstream cylinders are strongly disrupted and modified by the vortices shed from the upstream one in the cases of multiple PTC-cylinders.     

Investigation of slip boundary conditions in the T-shaped microchannel

Flow regimes and mixing performance in a T-type micromixer at high Reynolds numbers were studied by numerical solution of the Navier–Stokes equations. The Reynolds number was varied from 1 to 1000. The cross section of the mixing channel was 100 μm × 200 μm, and its length was 1400 μm. The transverse inlet channels were symmetric about the mixing channel, and their cross-section was 100 μm × 100 μm, and the total length was 800 μm. Five different flow regimes were identified: (i) stationary vortex-free flow (Re < 5); (ii) stationary symmetric vortex flow with two horseshoe vortices (5 < Re < 150); (iii) stationary asymmetric vortex flow (150 < Re < 240); (iv) non-stationary periodic flow (240 < Re < 400); and (v) stochastic flow (Re > 400). Maximum mixing efficiency is obtained for nonstationary asymmetric vortex flow. In this case, an S-shaped vortex structure is formed in the flow field. The effect of the slip conditions on the flow pattern and mixing efficiency is studied. The slip length varied from 1 to 70 μm in the calculations. It was shown that the mixing can be controlled by hydrophobic coating.