Non-evaporating Diesel Spray Research Articles

Numerical study on turbulent dispersion of diesel sprays under ultrahigh injection pressure using large eddy simulation

ABSTRACT The modern diesel engines equipped with an advanced fuel injection system up to ultrahigh injection pressure of 300 MPa are under development at present. The impact of injection pressure on fuel spray characteristics such as spray penetration and spray cone angle has been widely concerned in previous studies. However, it is still lack of the detailed investigation on the turbulent dispersion of small droplets produced by a high-pressure injector due to the limited capability of optical diagnostic techniques to simultaneously measure the two phases. Droplet dispersion due to turbulence plays a major role in high-velocity fuel spray mixing and combustion processes involving group evaporation of droplets. Thus, the effect of different injection pressures on droplet dispersion in nonevaporating diesel sprays was investigated using the Euler-Lagrange framework with large eddy simulation (LES) implemented in an open-source code OpenFOAM. Size-dependent instantaneous radial motion and dispersion characteristics of droplets dominated by large-scale turbulent structures within a polydisperse downstream spray were analyzed in detail. Results show that increasing injection pressure can modestly promote the radial dispersion of most droplets with diameter of 2 ~ 12 µm further downstream at the late stage of injection. This could be the result of the increasing interaction time of droplets with larger turbulent structures due to the increase of spray penetration with increasing injection pressure. However, increasing injection pressure has no significant impact on the strength of droplet-turbulence interaction in terms of slip velocity between droplet and gas phases and this could be quantitatively explained by the droplet Stokes number (St) estimated in this study. The most droplets with 0.1 < St < 10 tend to move and concentrate further radially outward, leading to droplet clustering. Thus, the effect of injection pressure on droplet-turbulence interaction is much less than its influence on droplet size for high-velocity fuel sprays, indicating that the ultrahigh injection pressure has much less important impact on the spatial inhomogeneity of droplet distribution than its influence on the evaporation rate of individual droplets.

Towards a deterministic composite collision outcome model for surface-tension dominated droplets

The focus of the present study is on the modeling and simulation of coalescence of surface-tension dominated liquid droplets in a gaseous environment. In the framework of a four-way coupled Euler-Lagrange approach using the large-eddy simulation technique, an improved composite collision outcome model identifying the four regimes of binary collisions of such droplets (bouncing, fast coalescence, reflexive and stretching separation) is assembled based on well-known correlations. A specific feature is that a hard-sphere model in combination with a deterministic collision detection is applied and hence the stochastic parcel approach often used for spray simulations is obsolete. In this composite model the bounding curves between the regimes mentioned before are taken from available experiments and described by well-known analytic correlations. The main objective is to provide a compilation of the different modeling assumptions resulting in a consistent model for the entire regime. Furthermore, the composite model is improved by introducing an additional condition to consider the overlapping region of the stretching and reflexive separation regimes at high Weber numbers. The outcome of a binary collision is identified based on the collision Weber number, the dimensionless impact parameter and the droplet size ratio. The results of each collision is treated separately with regard to the collision regime found, whereas the formation of satellite droplets during stretching and reflexive separation is currently not taken into account. Additionally, a droplet injection model for spray systems is introduced to specify the injected mass, the position, the velocity and the diameters of the released droplets. In this model the initial droplet diameters mimicking the primary break-up (atomization process) of the jet are modeled by means of a number distribution function. The implementation of the composite collision outcome model is verified using the test case of an inter-impingement spray system consisting of two crossing conical water sprays. Besides correctly reproducing the experimental correlations the composite model predicts a coalescence rate which is within the experimental range found in the literature. Then, the composite collision outcome model is applied to simulate the injection process of a solid-cone non-evaporating diesel spray into a quiescent nitrogen environment and validated against experimental data. The validation study is carried out in terms of the spray tip penetration, since it is one of the most important characteristics of sprays. A detailed comparative study is carried out to investigate the effect of different simulation parameters on the spray tip penetration. The results clearly show that a four-way coupled simulation using the enhanced composite model leads to the best agreement with the experiment.

Spray flow structure from twin-hole diesel injector nozzles

Two techniques were used to study non-evaporating diesel sprays from common rail injectors which were equipped with twin-hole and single-hole nozzles for comparison. To characterise the sprays, high speed optical imaging and X-ray radiography were used. The former was performed at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University, while the latter was performed at the 7-BM beamline of the Advanced Photon Source (APS) at Argonne National Laboratory. The optical imaging made use of high temporal, high spatial resolution spray recordings on a digital camera from which peripheral parameters in the initial injection phase were investigated based on edge detection. The X-ray radiography was used to explore quantitative mass distributions, which were measured on a point-wise basis at roughly similar sampling rate. Three twin-hole nozzles of different subtended angles and a single-hole nozzle were investigated at injection pressure of 1000bar in environments of 20bar back pressure. Evidence of strong cavitation was found for all nozzles examined with their CD ranging from 0.62 to 0.69. Penetration of the twin-hole nozzles was found to lag the single-hole nozzle, before the sprays merged. Switching in hole dominance was observed from one twin-hole nozzle, and this was accompanied by greater instability in mass flow during the transient opening phase of the injectors.

Modeling of multi-component droplet coalescence in evaporating and non-evaporating diesel fuel sprays

A new coalescence model for droplets with different mixture composition is developed and implemented in spray calculations. This model is based on an extension of Brazier-Smith et al.’s model (1972) for two colliding droplets with different densities. Based on the rotational kinetic and surface energies of a system of two droplets, coalescence efficiency for two colliding droplets with different densities is formulated. In the case of grazing collision, the effect of density of droplets is also included. The new droplet coalescence model is examined for evaporating and non-evaporating diesel sprays. Validation studies were carried out for both cases and the results were compared with available experimental data. The model shows good predictive capability and was demonstrated to improve the accuracy of multi-phase flow simulations.

G071054 2台のカメラとLIF-PlV法による燃料噴霧液擲と雰囲気流動流速の同時計測([G071-05]エンジンシステム部門,一般セッション(5))

Velocity distributions of the spray droplets and ambient gas in a non-evaporating Diesel spray were measured by the LIF (Laser Induced Fluorescence) - PIV technique which adopts two CCD cameras. Simultaneous measurement of the both velocity distributions inside the spray could be made in the particular region 4-5mm inside from the spray tip. The axial velocity of the spray droplets is much larger than that of the ambient gas at 4-5mm inside from the spray tip. The axial velocity of the spray droplets becomes the similar value as the ambient gas in the region l-2mm inside from the spray tip. In this region, the spray droplets velocity vector changes its direction to the radial (outward) direction, whereas the ambient gas velocity vector keeps the axial direction (downward).

EFFECT OF NOZZLE HOLE GEOMETRY ON NON-EVAPORATING DIESEL SPRAY CHARACTERISTICS AT HIGH-PRESSURE INJECTION

Non-evaporating diesel sprays from a common-rail injection system were characterized to investigate the effect of nozzle hole geometry. Two valve covered orifice (VCO) injectors with cylindrical and tapered nozzle holes were used. The injection rate was measured using an injection rate meter via the Bosch tube method. The spray tip penetration was analyzed from macroscopic spray images in a constant volume chamber using a high-resolution charge-coupled device camera and a spark light. Spray characteristics near the nozzle hole exit were observed using a microscopic visualization technique. The velocity and size of the droplets were measured downstream using a phase Doppler anemometer system. At the initial stage, the slope of the injection velocity from the tapered nozzle hole was larger than that from the cylindrical nozzle hole at the same injection pressure. The maximum injection velocity from the tapered nozzle hole was also higher. Until the micro-spray cone angle reached a maximum value, the spray tip penetration of the two nozzles increased with the change of injection rate. Subsequently, the spray tip penetration increased linearly with time. At the initial stage, the length of the liquid column injected from the tapered nozzle hole was about twice as long as that from the cylindrical nozzle hole. During the injection period, the oscillation in the micro-spray angle of the tapered nozzle hole was smaller than that of the cylindrical nozzle hole. The velocity and size of a droplet downstream with the tapered nozzle hole were slower and smaller than the velocity and size of the cylindrical nozzle hole.

QUANTITATIVE ANALYSES OF FUEL SPRAY-AMBIENT GAS INTERACTION BY MEANS OF LIF-PIV TECHNIQUE

The in-cylinder fuel-ambient gas mixing property in a direct injection (D.I.) diesel engine significantly influences the ensuing combustion and exhaust emission performance. In this study, the interaction of nonevaporating diesel spray with the surrounding gas was analyzed quantitatively in the quiescent condition at room temperature and with ambient gas pressure of 1 MPa by means of the laser induced fluorescence-particle image velocimetry (LIF-PIV) technique. Particularly, this study focused on the calculation of gas mass flow rate entrained through the entire spray region (spray side periphery and tip region) and total entrained gas-fuel ratio by using the gas velocity data obtained by the LIF-PIV technique. Another focus of this study was the gas entrainment characteristics of diesel spray under a wide range of injection pressures (100, 200, and 300 MPa) and the micro-hole nozzle (0.08mm) condition. The results indicate that the entrained gas mass flow rate at the spray tip region is prominent in the whole periphery and the proportion of gas entrainment at the side surface region increases as the spray develops Higher injection pressure significantly enhances the total entrained gas mass; however the increase of ambient gas/fuel mass ratio becomes moderate gradually as the injection pressure increases. The calculation model proposed by this work is capable of illustrating the ambient gas flow characteristics of the diesel spray accurately.

Time resolved measurements of the initial stages of fuel spray penetration

The period of diesel fuel spray injection from the start of injection up to 0.5 ms is investigated using ultra-high speed digital imaging. Contrary to the widely accepted idea of tip penetration exhibiting a linear dependence with time during the initial stages of spray development, the spray tip penetration is observed to follow a functional fit of the form S( t) = At 3/2 right up until the tip velocity reaches its maximum value. After this, the spray tip penetration correlates well with previously established correlations found in the literature. Furthermore, the spray tip velocity immediately after start of injection is not observed to be constant but is found to be proportional to t . A collapse of the tip velocity curves is obtained when they are scaled by the peak velocity and corresponding time. Results are presented for non-evaporating diesel sprays injected into a constant volume chamber at various injection and chamber pressures.

Evaporating and non-evaporating diesel spray simulation: comparison between the ETAB and wave breakup model

Evaporating and non-evaporating diesel sprays have been simulated using the Enhanced Taylor Analogy Breakup (ETAB) and the wave atomisation and breakup models implemented in STAR-CD and compared with experiments. The model validation was carried out by means of experimental data of sprays under controlled conditions in a constant-volume bomb. The model constants were calibrated for a specific test case low injection pressure, non-evaporating conditions and a small nozzle diameter and subsequently their robustness with respect to changes in boundary conditions was assessed for different injection pressures, evaporating conditions and larger nozzle diameters. The results suggest that, for suitable mesh resolutions with calibrated model constants, excellent agreement with experimental data can be achieved with both models. With these calibrated model parameters held constant, accurate predictions for spray penetrations, droplet diameters and spray angles could be obtained also for different injection configurations and air conditions by both models.

Adaptive collision meshing and satellite droplet formation in spray simulations

Improved numerical methods and physical models have been applied to droplet collision modeling. Numerically, an adaptive collision mesh method is developed to calculate collision rate. This method produces a collision mesh that is independent of the gas phase mesh and adaptively refined according to local parcel number density. An existing model describing the satellite droplet formation during the collision process is improved to reflect the experimental findings that the satellite droplets are much smaller than the parent droplets. The adaptive collision mesh and the improved satellite model have been used to simulate three impinging spray experiments. The model was able to qualitatively predict the occurrence of small satellite drops and bi-modal post-collision drop size distributions. The effect of the collision mesh and the satellite droplet model on a high-speed non-evaporating diesel spray is also assessed.

Initial development of non-evaporating diesel sprays in common-rail injection systems

Transient nature of common-rail diesel sprays was characterized at the initial stage of fuel injection, the effects of injection rate being taken into account. Direct photography and the shadowgraph technique were used to obtain macroscopic spray images at several different experimental conditions. Highly versatile image processing software was developed to analyse the spray images. In the analysis, spray penetration, dispersion and spray angles were defined and accurately measured by the software with consistency. The effects of rail pressure and ambient gas density upon spray behaviours were discussed in detail. Based on the measured injection rates, the asymptotic relations of spray penetration were found at early and late stages of injection. The changes in spray volume were correlated with the measured injection rates. It was found that the initial spray penetration correlates well with the amount of fuel injected. The increase in injection rate during the injection period was observed to induce a smaller spray angle.

A Study of the Air-Entrainment into a Diesel Fuel Spray.

The behavior of air-entrainment into a diesel fuel spray was studied by analyzing the air movement around a free non-evaporating diesel spray in a pressurized vessel. To measure the air movement around the spray, a density difference in the air was made near the surface of the spray as a tracer by heating a SUS wire with large current and the movement of air induced by the spray can be visualized through this density difference. The effects of nozzle hole, injection velocity and ambient air density on the air-entrainment behavior were investigated by this method. The air-entrainment relating to air-fuel ratio and the time dependent change of local air-entrainment were shown. These results indicated that using smaller nozzle hole and increasing ambient air density can increase the air-entrainment into the spray but increasing the injection velocity has little effect on the air-entrainment. Meanwhile, the results of this study were compared with the existing momentum theory, and the possibility of using momentum theory to predict the air-entrainment was described in the paper. Based on comparing the averaged air-fuel ratio inside the spray with both values of measurement and predicted by momentum theory, some discussions were added to help considering the complex phenomena of air-entrainment into a unsteady diesel spray.

Measurement of Droplet Size, Shape and Velocity in Diesel Sprays using a Single and Double Nano-Spark Photography Method.

Fundamental investigation of non-evaporating diesel spray disintegration was carried out using single and double nano-spark back-light photography of diesel sprays injected into a constant volume chamber and relatively clear single and double exposure images of droplets were obtained. Droplet size, shape and velocity as well as the two-dimensional velocity vectors of the droplets have been simultaneously measured by analyzing the spray photographs, using an image processing system and developed analysis method. It was observed that even at the early period of injection, some droplets have a velocity much lower than the penetration velocity of the spray tip. The velocity of droplets around the spray tip is much higher than the velocity of droplets near the nozzle tip. At the early period of injection, droplets flew away from the spray flow around the nozzle tip. However later at the same position, droplets were entrained into the spray flow. No droplet entrains into the spray around the spray tip at the early period of injection.