Study of mass and momentum transfer in diesel sprays based on X-ray mass distribution measurements and on a theoretical derivation (original) (raw)
Related papers
Experimental and numerical momentum flux evaluation of high pressure Diesel spray
Fuel, 2012
In the present paper, a detailed numerical and experimental investigation of high pressure Diesel spray evolution is reported. The analysis is mainly focused on the spray momentum flux measurement, which is increasingly used to characterise both the in-nozzle flow in terms of cavitation intensity and the flow uniformity among the different jets emerging from the same nozzle. In the present study, the potentially critical aspects of the spray momentum flux measurement methodology based on the impact force on a flat target have been investigated comparing CFD-3D results and experimental data pertaining to a wide range of operating conditions in terms of injection pressure, energizing time and back-pressure. The available experimental data, pertaining to a commercial common-rail injector, include the spray momentum flux time-histories and steady values, the injection rate and the spray imaging during momentum flux tests and free-jet evolution. Globally, a good agreement was found among experimental and numerical momentum flux histories; furthermore, CFD allowed to evaluate the significant contribution of the spray gaseous phase to the measured momentum flux, which rapidly tends to become predominant as the measurement station distance from the nozzle exit increases. The obtained results also suggest that some details of the measurement system can significantly affect the spray momentum detection accuracy. Hence, a detailed numerical analysis of the spray impact force was developed to evaluate the target intrusiveness, pointing out the irrelevance in terms of momentum flux time-history of the target presence into the simulated domain with respect to the free jet evolution. A practical strategy to define the optimal combination of target-diameter and nozzle-target distance, for each set of operating conditions, as a compromise between the extreme configurations of the involved parameters, is discussed.
Nowadays Diesel nozzle geometry is a major issue in order to fulfil new emission regulations due to the influence on internal flow, cavitation phenomenon, spray characteristics and therefore atomization behavior, which are very important for engines performance and pollutant formation. The aim of this article is to study the effect of cavitation on Diesel spray behavior. For this purpose, two bi-orifices nozzle geometries, a cylindrical nozzle and a convergent one, are characterized by means of two fundamental spray parameters: mass flux and momentum flux. Five injection pressure values and five discharge pressure levels have been measured in order to change the cavitation regime inside the nozzle flow. It is known from the literature that cavitation brings about a mass flux choke, but there are few studies that investigate its effects on momentum and outlet velocity in real geometries. The key point of this study is the measurement of spray momentum in order to explain the effects of nozzle geometry on spray behavior.
Velocity field analysis of the high density, high pressure diesel spray
International Journal of Multiphase Flow, 2016
In this study, particle image velocimetry (PIV) measurements have been performed extensively on a nonreactive dense diesel spray injected from a single orifice injector, under various injection pressure and steady ambient conditions, in a constant flow chamber. Details of PIV setup for diesel spray measurement without additional seeding are explained first. The measured velocity profiles are compared to those obtained from other similar measurements performed in a different institution, as well as those obtained from a 1D spray model simulation, presenting in both cases a good level of agreement. In addition, the velocity fields under various injection pressures and ambient densities show the dominant effects of these parameters on the behavior of diesel spray. The self-similarity of the transverse cut profiles of axial velocity is evaluated, showing that the measurements are in agreement with the hypothesis of self-similar velocity profiles. Finally, the effect of injection pressure and ambient density on the velocity fluctuations is presented and analyzed as well. While the experimental results presented here could help to understand the complex diesel fuel-air mixing process during injection, they also provide additional spray velocity data for future computational model validation, following the main idea of the Engine Combustion Network.
Experiments in Fluids, 2009
A quantitative and time-resolved X-ray radiography technique has been used for detailed measurements of high-pressure fuel sprays in the near-nozzle region of a diesel engine injector. The technique provides high spatial and temporal resolution, especially in the relatively dense core region. A single spray plume from a hydraulically actuated electronically controlled unit injector model 315B injector with a 6-hole nozzle was isolated and studied at engine-like densities for two different injection pressures. Optical spray imaging was also employed to evaluate the effectiveness of the shield used to isolate a single spray plume. The steady state fuel distributions for both injection pressures are similar and show a dense spray region along the axis of the spray, with the onaxis spray density decreasing as the spray progresses downstream. The higher injection pressure case exhibits a larger cone angle and spray broadening at the exit of the nozzle. For some time periods, the near-nozzle penetration speed is lower for the high injection pressure case than the low injection pressure case, which is unexpected, but can be attributed to the needle and flow dynamics inside the injector causing slower pressure build-up for the former case. Rate of injection testing was performed to further understand near-nozzle behavior. Mass distribution data were obtained and used to find mass-averaged velocity of the spray. Comparisons of the radiography data with that from a common rail single-hole light duty injectors under similar injection conditions show several significant differences. The current data show a larger cone angle and lower penetration speed than that from the light-duty injector. Moreover, these data display a Gaussian mass distribution across the spray near the injector, whereas in previous light-duty injector measurements, the mass distribution had steeper sides and a flatter peak. Measurements are also used to examine the spray models in the STAR-CD software.
SAE Technical Paper Series
Diesel spray mixture formation is investigated at target conditions using multiple diagnostics and laboratories. High-speed Particle Image Velocimetry (PIV) is used to measure the velocity field inside and outside the jet simultaneously with a new frame straddling synchronization scheme. The PIV measurements are carried out in the Engine Combustion Network Spray A target conditions, enabling direct comparisons with mixture fraction measurements previously performed in the same conditions, and forming a unique database at diesel conditions. A 1D spray model, based upon mass and momentum exchange between axial control volumes and near-Gaussian velocity and mixture fraction profiles is evaluated against the data. The 1D spray model quantitatively predicts the main spray characteristics (average mixture fraction and velocity fields) within the measurement uncertainty for a wide range of parametric variations, verifying that a Diesel spray becomes momentum controlled and has a Gaussian profile. A required input to the model is the jet angle, which is obtained experimentally. Although an expected result for a gas jet, this is the first time that combined datasets of velocity and mixture fraction have been obtained in vaporizing sprays at Diesel conditions (900 K, 60 bar). Finally, these results show that a consistent database can be built using advanced diagnostics performed by different institutions when the boundary conditions are well known as prescribed by the ECN Spray A framework.
Measurements of Spray Momentum for the Study of Cavitation in Diesel Injection Nozzles
In Diesel injection Systems, cavitation often appears in the injection nozzle holes. This paper analyses how cavitation affects the Diesel spray behavior. For this purpose two spray parameters, mass flux and momentum flux, have been measured at different pressure. We know that cavitation brings about the mass flux choke, but there are few studies about how the cavitation affects the momentum and the outlet velocity. The key of this study is just the measurement of the spray momentum under cavitation conditions.
Development and validation of a theoretical model for diesel spray penetration
A research on the diesel spray injected into stagnant ambient air in a chamber is reported in this paper. The main objective of the investigation is to carry out an in-depth analysis on the influence of injection parameters on the spray internal dynamics and spray macroscopic characteristics. As a result of a theoretical approach based on momentum flux conservation along the sprays' axis, a model which predicts the spray axis velocity and spray tip penetration is obtained. Measurements of momentum flux and spray cone angle are needed in order to predict axis velocity and spray penetration. The chamber density is assumed to be constant and equal to the density of the pressurized air inside the chamber. A Gaussian radial profile is assumed for the axial velocity. Experimental results from a conventional common rail injection system with five axisymmetric nozzles tested in a wide range of injection pressure values and density conditions have been used in order to obtain additional information of the model and also for validation purposes. These experimental results include a large number of momentum flux (impact force), spray tip penetration and spray cone angle measurements.
Atomization and Sprays, 2019
In this work, the effect of high injection pressures and ambient gas properties on diesel spray penetration and spreading angle are studied. To this end a multi-hole piezoelectric injector was used, and MIE-Scattering optical technique to visualize the spray. Injection pressures up to 270 MPa were used throughout the experiments. Additionally, the spray behavior going from subsonic to supersonic state was analyzed by controlling the ambient gas speed of sound, promoting in this way supersonic jets. For this purpose, measurements were done using three different ambient gases (SF 6 , CO 2 , and N 2) at isothermal conditions. The results showed that sprays near transonic or in supersonic state had a higher penetration rate than those in subsonic state. Furthermore, among the sprays near transonic or in supersonic state, those with higher Mach number had faster penetration. Differently, within the sprays at subsonic state, no significant variations in spray penetration rate were found, regardless of the difference in the Mach number. Shock waves appearances were pointed out as a possible explanation for the spray penetration variations observed. Finally, a statistical analysis is presented for the spray penetration under isothermal conditions and for each ambient gas.
Structure of high-pressure diesel sprays
A comprehensive set of computational and experimental results for high-pressure diesel sprays are presented and discussed. The test cases investigated include injection of diesel into air under both atmospheric and high pressure/temperature chamber conditions, injection against pressurized and cross-flowing CF 6 simulating respectively the density and flow conditions of a diesel engine at the time of injection, as well as injection into the piston bowl of both research and production turbo-charged highspeed DI diesel engines. A variety of high-pressure injection systems and injector nozzles have been used including mechanical and electronic high-pressure pumps as well as common-rail systems connected to nozzles incorporating a varying number of holes with diameters ranging from conventional to micro-size. Results from spray imaging, phase Doppler anemometry and CFD calculations of both the internal nozzle flow and the subsequent spray development have been combined to identify the relative importance of the geometric/operating parameters that control the structure of diesel sprays and areas of further research.