Group-hole Nozzle Research Articles

Effect of Intake Conditions and Nozzle Geometry on Spray Characteristics of Group-Hole Nozzle

The group-hole nozzle concept is proposed to meet the requirement of nozzle hole minimization and reduce the negative impact of poor spatial spray distributions. However, there are limited researches on the effects of intake conditions and nozzle geometry on spray characteristics of the group-hole nozzle. Therefore, in this study, an accurate spray model coupled with the internal cavitating flow was established and computational fluid dynamics (CFD) simulations were done to study the effects of intake conditions and nozzle geometry on spray characteristics of the group-hole nozzle. Experimental data obtained using high-speed digital camera on the high-pressure common rail injection system was used to validate the numerical model. Effects of intake conditions (injection pressure and temperature) and nozzle geometry (orifice entrance curvature radius and nozzle length) on the flow and spray characteristics of the group-hole nozzle were studied numerically. The differences in Sauter mean diameter (SMD), penetration length and fuel evaporation mass between single-hole nozzle and group-hole nozzle under different nozzle geometry were also discussed. It was found that the atomization performance of the group-hole nozzle was better than that of the single-hole nozzle under same intake conditions, and the atomization effect of the short nozzle was better than that of the long nozzle. With increase in the orifice entrance curvature radius, the average velocity and turbulent kinetic energy of the fuel increased, which was conducive to improving the injection rate and flow coefficient of the nozzle. Meanwhile, the penetration length and SMD value rose, while evaporation mass dropped. When the ratio of the orifice entrance curvature radius (R) to the diameter of injection hole (D) was 0.12, the spray characteristics reached a constant state due to elimination of cavitation. Conclusions were made based on these. This study is expected to be a guide for the design of the group-hole nozzle.

Experimental and computational study comparing conventional diesel injectors and diverging group hole nozzle injectors in a high temperature pressure vessel and a heavy-duty diesel engine

Group hole nozzles (GHN) are injector tips with clusters of small holes used in place of a single large hole. In this work, GHN with two holes per cluster and a diverging angle of 15° between the holes were studied using experiments and computational fluid dynamics (CFD) and comparisons were made with conventional diesel injectors. First, experiments were performed in a high temperature pressure vessel (HTPV) to understand the free spray behavior. HTPV experiments revealed that the diverging GHN had slower penetration with a more dispersed spray and a shorter lift-off length relative to the conventional injector. The GHN also exhibited higher natural luminosity compared to the conventional injector, which is indicative of increased soot formation with the GHN. CFD model validated with HTPV experimental data was used to study the sensitivity to the angle between the holes and the hole count per cluster. Results showed that soot formation increased as the divergence angle increased from 0° to 15°. Furthermore, as the divergence angle increased, increasing the hole count per cluster increased the soot formation significantly. Following the HTPV study, the injectors were tested on a single-cylinder heavy-duty diesel engine at a high-load operating condition. Results showed that the diverging GHN injector produced a lower mixing-controlled combustion rate compared to the conventional injector. The slower burn rates resulted in lower NOx and higher soot emissions for the diverging GHN injector. CFD simulations of the engine experiments predicted the slower burn rate seen with the diverging GHN injector. In-cylinder visualization of the CFD results showed that slower penetration with the diverging GHN results in reduced mixing upon surface interaction, and an inability to utilize the fresh air in the outer regions of the combustion chamber which resulted in richer local equivalence ratios and higher soot emissions compared to the conventional injector.

群噴孔・斜孔ノズルからの非定常天然ガス噴流の発達過程の可視化観察

群噴孔・斜孔ノズルからの非定常天然ガス噴流の発達過程の可視化観察

3-D numerical consideration of nozzle structure on combustion and emission characteristics of DI diesel injector

A three-dimensional approach was used to capture the main parametric influences of injector geometries on combustion characteristics by utilization of a constant volume vessel. The investigation was performed by means of two case studies. In the first case study, the nozzle inclination angle was varied for group-hole nozzle layout, while the second case study analyses the single dense spray of an injector while its inlet-rounding radius to nozzle diameter ratio was being changed. Results demonstrate that closer included angle (up to 10˚) leads to higher penetration of spray and thus greater LOL (lift-off length) and ID (ignition delay). A decreasing trend as a result for overall emissions of NOx and soot mass fraction was observed. With regard to the second case study, decrement of r/d brings about enhancement of mixing quality of spray that leads to higher evaporation rate and accumulated heat release. Increasing heat release will augment chamber temperature that in turn contributes to higher NOx formation. Higher cavitation caused better liquid jet disintegration and smaller spray droplet that reduces soot mass fraction of late combustion process. The present study is intended to investigate primarily the effect of injector geometry on critical combustion components. The combination of changes in vessel pressure and group-hole nozzle angle is considered to study the combustion behavior of sprays and subsequent emissions content.

Numerical study of the effects of injector needle movement and the nozzle inclination angle on the internal fluid flow and spray structure of a group-hole nozzle layout

In this study, we explored the spray behavior as a result of microscopic variations in the flow characteristics when the nozzle hole inclination angle was varied and the needle reciprocated. The volume fraction and viscosity contours were investigated at three needle modes of 0.01, 0.25, and 0.24mm using three nozzle inclination angles of 10°, 15°, and 30°. We determined the relationship between the two key parameters of liquid viscosity (which developed in the sac-volume and nozzles) and volume vapor fraction in the injector with the spray characteristics and the equivalence ratio by considering different needle positions and inter-nozzle angles as objective variables. It was found that a lower included angle led to higher viscosity development in nozzles with low cavitation formation, whereas increasing the inclination angle increased the viscosity amplitude in the sac-volume section and decreased the viscosity in the nozzles, as well as yielding more cavitation formation along the nozzles. The lowest viscosity was obtained with an inclination angle of 15°, which may facilitate the primary breakup process during spray atomization. The highest spray dispersion was achieved with a nozzle angle of 15°, which was due mostly to higher cavitation and a lower spatial viscosity distribution. Comparisons of the model and experimental data demonstrated the validity of the predicted spray structures based on simulations in terms of penetration and the droplet diameter.

Spray atomization characteristics of a GDI injector equipped with a group-hole nozzle

The main purpose of this paper is to analyze the spray characteristics of a group-hole nozzle in terms of spray behavior and atomization process in comparison to the characteristics of a single-hole nozzle as reference. Spray visualization and PDPA (phase Doppler particle analyzer) experiments were performed using a GDI adjustable injector, which can adopt a different type of nozzle at free spray conditions. By analyzing the spray development behavior and distribution of droplet velocity and diameter based on a time series, and comparing these results with that of a single-hole nozzle, the effects of the group-hole nozzle on the spray characteristics in a GDI injection were elucidated.Experimental results showed that the development processes of spray behavior from a group-hole nozzle were similar to that of the single-hole nozzle. Both the sprays had similar spray tip penetration and dispersion at the same stages of development. However, owing to the constant spray momentum from a spray interaction, the spray behavior from the group-hole nozzle seemed to be more stable than that of the single-hole nozzle. In terms of the averaged droplet size, the group-hole nozzle held an advantage over the single-hole nozzle in decreasing Sauter mean diameter (SMD) by approximately 2μm. In addition, in comparison to the spatial distribution of droplet diameter and velocity between them, it can be confirmed that the group-hole nozzle has strong effects on reduction in diameter as well as rapid dispersion of droplet due to active air entrainment. These atomization characteristics are considered as an important advantages for improving mixture formation in GDI engine.

바이오디젤 연료의 혼합기 형성 및 미립화 증진 방안

In this work, the mixture formation and atomization characteristics of biodiesel fuel were reviewed under various test conditions for the optimization of compression-ignition engine fueled with biodiesel. To achieve these, the effect of nozzle caviting flow, group-hole nozzle geometry and injection strategies on the injection rate, spray evolution and atomization characteristics of biodiesel were studied by using spray characteristics measuring system. At the same time, the fuel heating system was installed to obtain the effect of fuel temperature on the biodiesel fuel atomization. It was revealed that cavitation in the nozzle orifice promoted the atomization performance of biodiesel. The group-hole nozzle geometry and split injection strategies couldn't improve it, however, the different orifice angles which were diverged and converged angle of a group-hole nozzle enhanced the biodiesel atomization. It was also observed that the increase of fuel temperature induced the quick evaporation of biodiesel fuel droplet.

Time resolved characteristics of gaseous jet injected by a group-hole nozzle

In this study, we investigate the evolution of a transient gaseous helium jet issuing from a multi-hole fuel injector at pressure ratios of 2, 3, 4 and 5. High speed Schlieren was used to visualize the jet development process and a digital edge detection method applied to determine time resolved jet penetration and tip speed. Both near and far field regions of the jet were investigated. Measurement of jet tip axial penetration and speed revealed that jet formation is comprised of three stages: emergence of individual jets, merging individual jets into the final stage of a merged single jet and penetration of the single merged jet. Penetration of the jet tip was found to be more sensitive to nozzle pressure ratio while the jet spread showed sensitivity to the stage of the jet evolution. Little sensitivity in jet parameters was found for the jet emergence and merge stages where it is likely that nozzle-opening processes play an important role. Scaling of the jet parameters based on those typical of free jets revealed the single merged jet to be similar to that of a free jet, although the form of the scaling depends on the parameters used to define the nozzle diameter and jet speed.

Numerical Simulation on Effect of Nozzle-Hole Layout on Spray Characteristics and Combustion in a DI Diesel Engine

Diesel spray characteristics are closely related to the combustion of the engine where the spray tip penetration and the fuel atomization play a key role especially for direct injection (DI) diesel engine. With different nozzles, the fuel atomization and evaporation will be different thereby affecting the combustion and emission characteristics. A three-dimensional model is built based on the parameters of a DI diesel engine, and its validation is also validated. Three nozzle-hole layouts are designed in this research, including the conventional hole, multi-hole, and group-hole. The spray characteristics and combustion process are studied with three different nozzle-hole layouts by the way of numerical simulation. Further more, the effect of inter-hole spacing of group-hole nozzle on the evaporation rate and combustion process is researched here.

Combustion and emission characteristics of converging group-hole nozzle under lean engine operating conditions

This paper describes the combustion and emission characteristics of converging group-hole nozzles and compares the results to those of a single hole nozzle with the same overall nozzle exit hole area. Engine experiments were performed using a single-cylinder diesel engine operating under overall lean conditions (i.e., equivalence ratio 0.45). The considered nozzle configurations in the experiments included a converging group-hole nozzle (cGHN) with 3° converging angle, 0.090mm hole diameter, and a single hole nozzle (SHN) of 0.128mm hole diameter. The CFD calculations used the KIVA engine simulation code integrated with a Gasjet superposition model. Using the validated calculation models, the test conditions were also expanded to consider wider converging angle cGHNs (up to 12°). The results show that the evaporation of sprays from the cGHN-3° nozzle is more delayed than that of the SHN case and the cGHNs entrain more ambient gas due to smaller droplet sizes in the outer spray periphery. In addition, an increase in the converging angle of the cGHNs promotes fuel evaporation and produces a more homogeneous fuel–air mixture.

Impact of Orifice Angle Configurations on the Droplet Atomization Enhancement of Diesel Fuel in a Group-Hole Nozzle

This paper presents the effect of orifice configuration on the droplet atomization characteristics of diesel fuel injected through different types of group-hole nozzle angles, which are converged (θ=−3 deg), parallel (θ=0 deg), and diverged (θ=+3 deg) orifices in atmospheric pressure and room temperature condition (0.1 MPa, 293 K). It was revealed that the parallel hole nozzle has the largest Sauter mean diameter (SMD) value and both sprays from diverged and converged hole nozzles show better atomization. A comparison of spray tip penetration illustrates that as the orifice angle is converged, spray tip penetration becomes longer, and it must be the reason for the fast spray velocity. These results can confirm the relationship among time, distance, and velocity. Therefore, it can be concluded that the droplet atomization enhancement can be expected in the converged nozzle spray rather than in the parallel and diverged nozzle sprays based on the results of smaller SMD, faster velocity, better air utilization, and higher percentage of small size of droplets in the measuring area.

Development and Evaporation of Group-Hole Nozzle Sprays under Various Surrounding and Impinging Conditions of Direct-Injection Diesel Engines

In the present study, penetration and evaporation of the diesel sprays injected by a group of two closely spaced orifices (a group-hole nozzle) were investigated and compared with those of conventional single-hole nozzle sprays under various engine loads and wall-impinging conditions of direct-injection (DI) diesel engines. Both free and wall-impinging conditions were considered. The experiments were performed inside a constant-volume vessel under simulated ambient conditions for low and high engine loads of DI diesel engines. To investigate the effect of spray targeting, two impinging conditions (impingement angles of 45° and 90° with the same impingement distance) were applied. Geometry and liquid/vapour mass distributions of the evaporating diesel sprays were analysed using a laser absorption scattering (LAS) technique. Under a free spray condition, fuel evaporation of the group-hole nozzle spray was improved compared with that of the single-hole nozzle spray at low load conditions, while it showed simultaneous deterioration in fuel evaporation and spray tip penetration at high load conditions. Jet axes deflection of the two jets from the group-hole nozzle, which generated a dense liquid region at the central region of the spray, was responsible for this deteriorated evaporation at high load conditions. Under a vertical impingement condition (impingement angle of 90°), the group-hole nozzle spray showed simultaneous improvement in spray tip penetration and fuel evaporation at both low and high load conditions from strong momentum interaction of the two jets on the impingement wall. However, this improvement from the group-hole nozzle did not appear at an inclined wall-impingement condition (impingement angle of 45°) owing to weakened and delayed momentum interaction of the two jets. Both spray tip penetration and fuel evaporation of the group-hole nozzle spray were deteriorated at the inclined wall-impinging condition.

Simulation and analysis of group-hole nozzle sprays using a gas jet superposition model

A gas jet superposition model has been recently developed for computing group-hole nozzle sprays in computational fluid dynamics (CFD) simulations. The objectives of this study are: (1) to perform a systematic validation of the comprehensive spray model for group-hole nozzles using a broad range of experimental data; (2) to analyze the dynamics and physical insight of group-hole nozzle sprays based on the simulation results; and (3) to further clarify the impact of included-angle on spray/mixture properties of group-hole nozzle sprays. An updated version of the KIVA-3V Release 2 code, which employs the Lagrangian-Drop Eulerian-Fluid (LDEF) methodology for numerical calculation of two-phase flows, was used in the simulations. Diverging group-hole nozzles with various included-angles were considered. The test conditions included non-evaporating and evaporating ambient conditions, free sprays and sprays impinging on a flat wall. Detailed comparisons were made between the experiments and computations in terms of spray/mixture characteristics. The results show that numerical parameter dependencies are significantly reduced with the new models, and good levels of agreement are obtained in terms of spray structure, liquid/vapor penetration, overall SMD and cumulative vaporized fuel mass. Both experimental measurements and simulations reveal the importance of included-angle in group-hole nozzle sprays. In particular, some important features of group-hole nozzle spray are captured in the computations by the present models: compared to the equivalent single-hole nozzle, smaller local droplet size can be achieved in the near nozzle field, indicating an enhanced fuel primary atomization; the ambient gas entrainment rate is increased during the injection period, implying the better mixing; the spray axis deflection is identified in the case of group-hole nozzles with smaller angles, which is caused by a negative relative pressure region that exists between the sprays; in addition, the asymmetric structure of wall-impinging group-hole nozzle spray is well predicted by the present models through applying the gas jet superposition model in the entire computational domain.

Gas entrainment characteristics of diesel spray injected by a group-hole nozzle

In this study, gas entrainment characteristics of a diesel spray injected by a group of closely spaced two-orifices (group-hole nozzle) were investigated. Both free and wall-impinging sprays were considered. The gas entrainment characteristics of the group-hole nozzle spray were compared to those of single-hole nozzle sprays: one has the same total hole area with the group-hole nozzle, and the other has the same hole diameter. The gas entrainment characteristics of diesel sprays were investigated using a particle image velocitmetry technique coupled with a laser induced fluorescence technique (LIF-PIV technique). The spray tip penetration of the group-hole nozzle was the shortest among the applied nozzles in a free spray condition, while it was the longest in a wall-impinging condition. In the free spray condition, the gas entrainment of the spray was enhanced by the group-hole nozzle due to extensive momentum exchange with surrounding gas and superposed gas entrainment motion of the two-jets injected by the group-hole nozzle. After wall-impingement, the group-hole nozzle spray showed a stronger wall-jet vortex and increased gas entrainment compared to the single-hole nozzle sprays due to enhanced spray/wall interaction caused by the momentum interaction of the two-jets from the group-hole nozzle. Asymmetric shape of the group-hole nozzle spray resulted in an asymmetric gas velocity distribution of the spray both in the free and wall-impinging conditions.

Laser absorption-scattering technique applied to asymmetric evaporating fuel sprays for simultaneous measurement of vapor/liquid mass distributions

This paper describes an Ultraviolet-Visible Laser Absorption-Scattering (UV-Vis LAS) imaging technique applied to asymmetric fuel sprays. Continuing from the previous studies, the detailed measurement principle was derived. It is demonstrated that, by means of this technique, cumulative masses and mass distributions of vapor/liquid phases can be quantitatively measured no matter what shape the spray is. A systematic uncertainty analysis was performed, and the measurement accuracy was also verified through a series of experiments on the completely vaporized fuel spray. The results show that the Molar Absorption Coefficient (MAC) of the test fuel, which is typically pressure and temperature dependent, is the major error source. The measurement error in the vapor determination has been shown to be approximately 18% under the assumption of constant MAC of the test fuel. Two application examples of the extended LAS technique were presented for exploring the dynamics and physical insight of the evaporating fuel sprays: diesel sprays injected by group-hole nozzles and gasoline sprays impinging on an inclined wall.

Improving diesel mixture preparation by optimization of orifice arrangements in a group-hole nozzle

The factors affecting the free and wall-impinging spray characteristics of closely spaced micro-orifices (group-hole nozzles) are investigated to determine the optimal orifice arrangement for improved in-cylinder air utilization. To determine the optimized arrangement of the nozzles, different types of group-hole nozzles with different intervals and angles between the orifices have been studied. The spray tip penetration and fuel evaporation were analysed using a laser absorption scattering technique for non-axisymmetric sprays. In the case of a free spray, the spray tip penetration of the group-hole nozzle increased as the included angle and the distance between orifices decreased. The shortest spray tip penetration was observed when the two jets injected by a group-hole nozzle began to separate, but the periphery of each jet still touched. On the contrary, fuel evaporation deteriorated when the distance and angle between the two orifices decreased since the severe collision/coalescence of the two jets hindered evaporation. In the case of a wall-impinging spray, the spray tip penetration of the group-hole nozzle was strongly related to the distance χ between the arbitrary centres of each jet at the impingement wall. Maximum spray tip penetration of the group-hole nozzle was observed at a specific range of χ. In this range, the strong momentum regions of the jets began to meet just after wall impingement, therefore collision/coalescence before wall impingement was minimized and the interaction between the two wall-impinging jets was maximized. The fuel evaporation in the wall-impinging spray also improved at a distance of χ for maximum tip penetration. The mechanism causing this phenomenon is discussed in terms of near-field spray interaction and wall-impinging jet interaction after wall impingement.

A gas jet superposition model for CFD modeling of group-hole nozzle sprays

In the present study, a jet superposition modeling approach is explored to model group-hole nozzle sprays, in which multiple spray jets interact with each other. An equation to estimate the merged jet velocity from each of the individual jets was derived based on momentum conservation for equivalent gas jets. Diverging and converging group-hole nozzles were also considered. The model was implemented as a sub-grid-scale submodel in a Lagrangian Drop–Eulerian Gas CFD model for spray predictions. Spray tip penetration predicted using the present superposition model was validated against experimental results for parallel, diverging and converging group-hole nozzles as a function of the angle between the two holes at various injection and ambient pressures. The results show that spray tip penetration decreases as the group hole diverging or converging angle increases. However, the spray penetration of the converging group-hole nozzle arrangement is more sensitive to the angle between the two holes compared to diverging nozzle because the radial momentum component is converted to axial momentum during the jet–jet impingement process in the converging group-hole nozzle case. The modeling results also indicate that for converging group-hole nozzles the merged sprays become ellipsoidal in cross-section far downstream of the nozzle exit with larger converging angles, indicating increased air entrainment.

Experimental Investigation of Intake Condition and Group-Hole Nozzle Effects on Fuel Economy and Combustion Noise for Stoichiometric Diesel Combustion in an HSDI Diesel Engine

Experimental Investigation of Intake Condition and Group-Hole Nozzle Effects on Fuel Economy and Combustion Noise for Stoichiometric Diesel Combustion in an HSDI Diesel Engine

Flame structure of wall-impinging diesel fuel sprays injected by group-hole nozzles

This paper describes an investigation of the flame structure of wall-impinging diesel sprays injected by group-hole nozzles in a constant-volume combustion vessel at experimental conditions typical of a diesel engine. The particular emphasis was on the effect of the included angle between two orifices (0–15 deg. in current study) on the flame structure and combustion characteristics under various simulated engine load conditions. The laser absorption scattering (LAS) technique was applied to analyze the spray and mixture properties. Direct flame imaging and OH chemiluminescence imaging were utilized to quantify the ignition delay, flame geometrical parameters, and OH chemiluminescence intensity. The images show that the asymmetric flame structure emerges in wall-impinging group-hole nozzle sprays as larger included angle and higher engine load conditions are applied, which is consistent with the spray shape observed by LAS. Compared to the base nozzle, group-hole nozzles with large included angles yield higher overall OH chemiluminescence intensity, wider flame area, and greater proportion of high OH intensity, implying the better fuel/air mixing and improved combustion characteristics. The advantages of group-hole nozzle are more pronounced under high load conditions. Based on the results, the feasibility of group-hole nozzle for practical direct injection diesel engines is also discussed. It is concluded that the asymmetric flame structure of a group-hole nozzle spray is favorable to reduce soot formation over wide engine loads. However, the hole configuration of the group-hole nozzle should be carefully considered so as to achieve proper air utilization in the combustion chamber. Stoichiometric diesel combustion is another promising application of group-hole nozzle.

An investigation of mixture formation and in-cylinder combustion processes in direct injection diesel engines using group-hole nozzles

Mixture properties of the transient fuel spray injected by group-hole nozzles were quantitatively studied in a constant volume chamber via the laser absorption scattering (LAS) technique, and compared with conventional single-hole nozzles. Specific areas investigated included non-evaporating and evaporating ambient conditions, and the conditions of free spray and spray impinging on a flat wall. The particular emphasis was on the effect of one of the key parameters of the group-hole nozzle structure, namely, the interval between orifices. Subsequently, in-cylinder ignition and combustion processes were investigated by direct flame visualization in a single-cylinder optical research engine using the group-hole nozzle and the standard multi-hole nozzle, together with heat release analysis. Results show that the group-hole nozzle can produce a smaller Sauter mean diameter (SMD) of droplets under the non-evaporating condition. Under the evaporating condition, for the free spray cases, there exists a trade-off between the fuel evaporation and the spray penetration in the application of the group-hole nozzle. With the increase in interval between orifices, the ratio of evaporation increases, but the spray tip penetration decreases. For the spray impinging on a flat wall, the group-hole nozzle can enhance the spray tip penetrations to some extent compared with the single-hole nozzles. Flame images taken in a direct injection optical engine show that the enhancement of the fuel atomization and evaporation by the group-hole nozzle may contribute to a decrease in overall flame luminosity level, suggesting reduced soot formation in diesel engines.