Spray Shape Research Articles

LARGE EDDY SIMULATION OF DROPLET STOKES NUMBER EFFECTS ON TURBULENT SPRAY SHAPE

The spatial and temporal development of a spray strongly depends on the local characteristics of turbulence. The turbulence-droplet coupling gives rise to droplet dispersion, which is the underlaying physical phenomenon of interest in this study. Large eddy simulations (LES) provide details of the instantaneous flow field and anisotropy of the larger scales. Hence, LES has the potential of improved spray simulations in flows that are highly nonisotropic/nonstationary. A numerical study on the effect of droplet diameter (d) on spray shape is described by carefully varying d. The droplets are assumed to be non-interacting with each other. They are also assumed to maintain their shape and diameter. The droplet Stokes numbers are within the range 0.07 <= St(p) < 2.56, corresponding to diameters 2 < d < 12 am for a common liquid fuel. In order to emulate a fuel spray, a droplet-laden jet at Re = 10,000 and Ma = 0.3 is considered as a model problem that avoids the dense spray regime. A novel technique to visualize the simulated sprays in a realistic manner is presented, and a qualitative comparison to a diesel spray experiments is made. It is shown that the spray-cloud shape depends strongly on droplet Stokes number. A spray penetration correlation formula is suggested. The nonlinear character of the droplet-eddy interaction and its dependence on droplet size is studied by visualization of droplet trajectories. We show that the spray behavior can be coherently explained by considering the statistical properties of the droplet cloud. The results show that the instantaneous/short-time-averaged probability density functions (PDFs) of droplet statistics explain very coherently the St(p) dependency of the spray shape. The PDFs of the axial and radial components of droplet-gas slip velocity (u(g) - u(p)) are used to explain the visual observations on the spray cloud evolution.

J0503-2-3 ディーゼル噴霧構造の時空間計測への展開([J0503-2]多次元計測技術の展開と応用(2))

For the reduction of diesel engine emissions, it is significant to clarify the spray atomization and formation processes. In this investigation, a detailed observation of fuel droplets characteristic was made by applying a double exposure method using a color film, a light fiber and a wavelength converter to a previous measurement technique. The photographic system is composed of a specialized lens system, and enables to measure the whole spray shape and fuel droplets characteristics at an instant. The light source is the second harmonic, and the third harmonic of Nd:YAG laser. This technique enables the spray measurement of the chronological order. In addition, it is expected new knowledge about a spray atomization mechanism be obtained.

1108 針弁オフセットが拡大ディーゼルノズルの内部流動と噴霧形態に及ぼす影響の解析(エンジンシステムI)

1108 針弁オフセットが拡大ディーゼルノズルの内部流動と噴霧形態に及ぼす影響の解析(エンジンシステムI)

Atomization and spray characteristics of bioethanol and bioethanol blended gasoline fuel injected through a direct injection gasoline injector

The focus of this study was to investigate the spray characteristics and atomization performance of gasoline fuel (G100), bioethanol fuel (E100), and bioethanol blended gasoline fuel (E85) in a direct injection gasoline injector in a gasoline engine. The overall spray and atomization characteristics such as an axial spray tip penetration, spray width, and overall SMD were measured experimentally and predicted by using KIVA-3V code. The development process and the appearance timing of the vortices in the test fuels were very similar. In addition, the numerical results accurately described the experimentally observed spray development pattern and shape, the beginning position of the vortex, and the spray breakup on the spray surface. Moreover, the increased injection pressure induced the occurrence of a clear circular shape in the downstream spray and a uniform mixture between the injected spray droplets and ambient air. The axial spray tip penetrations of the test fuels were similar, while the spray width and spray cone angle of E100 were slightly larger than the other fuels. In terms of atomization performance, the E100 fuel among the tested fuels had the largest droplet size because E100 has a high kinematic viscosity and surface tension.

A comparative analysis of diesel fuel injection parameters in piezoelectric and electromagnetic fuel injectors

The paper presents the results of a comparative analysis of the changes in the injection spray shape and penetration and its vertex angle during the injection through piezoelectric and electromagnetic fuel injectors. The results of the observation of the fuel spray with the use of high speed camera (100 ms frame frequency) have been presented. Moreover, the parameterization and digital processing (including its automation) of the obtained image have too been presented. The influence of the pressure and the timing of the injection on the variable-in-time spray parameters have been shown. A mathematical model describing these relations has been proposed.

Droplet size and spray pattern characteristics of PWM-based continuously variable spray

A Pulse-Width-Modulation-based (PWM-based) continuously variable sprayer was developed using a proportional regulating solenoid valve. Variable flow-rate was obtained by varying the duty cycle of the actuating signal with 24 kHz frequency. Flow-rate regulating ranges of the PWM-based continuously variable spray (i.e. the turndown ratio responding to 100%-40% duty cycle) are 7.14:1, 3.57:1, and 3.70:1 for flat-fan, hollow-cone and solid-cone nozzles, respectively. The purpose of the study was to evaluate the PWM-based continuously variable spray. The method was to quantify the effects of flow-rate control on spray characteristics in terms of droplet size spectra, spray distribution patterns, and spray angle for flat-fan, hollow-cone, solid-cone nozzles. For all nozzles tested, spray distribution concentrated on the center of the spray field with the decrease of flow-rate. But the spray shape is still symmetrical. The sensitivities of the spray angles to flow-rate were 0.83, 0.67, and 0.58 (o)/% respectively for flat-fan, hollow-cone and solid-cone nozzles. Compared with the sensitivities of spray angle for PWM-based intermittent variable spray, they are somewhat larger. As flow-rate was reduced from the maximum (100% flow-rate) to the minimum controllable rate, the observed median diameter of spray droplets decreased by 5.4%, 9.8%, and 9.9 % for flat-fan, hollow-cone and solid-cone nozzles, respectively. This indicates that spray droplet size was affected slightly by flow-rate control. Keywords: Pulse Width Modulation (PWM), continuously variable spray, droplet size, spray distribution pattern, spray angle DOI: 10.3965/j.issn.1934-6344.2009.01.008-018 Citation: Deng Wei, He Xiongkui, Ding Weimin. Droplet size and spray pattern characteristics of PWM-based continuously variable spray. Int J Agric & Biol Eng, 2009; 2(1): 8

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.

Fluctuations of a spray generated by an airblast atomizer

This paper is devoted to the study of the aerodynamic instability of the spray generated by an airblast atomizer. As a result of this instability the spray shape and its velocity fluctuate with a certain frequency, which depends on the operational parameters of the atomizer. The effect of three parameters, namely; chamber pressure, liquid phase flow rate and the gas phase flow rate on the spray fluctuating frequency are investigated. The velocity vector of the drops in the spray and the arrival times to the detection volume are measured using the laser Doppler instrument. The slotting technique is applied to the data of axial velocity and arrival times of the drops in order to estimate the dominating spray frequencies. Additionally, the shape of the spray has been observed using the high-speed video system. The frequencies of the shape fluctuations are estimated using proper orthogonal decomposition of the time-resolved images of the spray. We show that the frequencies of the spray velocity and those exhibited by spray shape coincide over a wide range of spray parameters. Finally, a simple scaling for the spray frequency is proposed and validated by the experimental data.

CFD Analysis of VVT/VVA on the Gas Exchange and Fuel-Air Mixing in a Diesel Engine

A three-dimensional simulation was carried out for investigating effects of negative valve overlap (NVO) on gas exchange and fuel-air mixing processes in a diesel homogeneous charge compression ignition (HCCI) engine with early fuel injection. It was found that the case with longer NVO produced a stronger swirl motion and a more significant vortex below the intake valve due to the high annular jet flow through the valve curtain area during the intake stroke. However, there was not much difference in the values of swirl ratio, tumble ratio and turbulence intensity between different NVOs at the end of compression stroke. It was also seen that enlarged NVO not just increased in-cylinder temperature but also improved the temperature homogeneity. With increased NVO, there is a bigger spray shape and more droplets exist in gaps of sprays. This demonstrates that stronger turbulence intensity and higher temperature homogeneity with higher NVO improve fuel vaporization and air-fuel mixing.

Spray properties of alternative fuels: A comparative analysis of biodiesel and diesel

Physical properties of biodiesel play an important role in the injection, atomization and combustion performance. An experiment was carried out to investigate the spray properties of biodiesel. The experimental setup was based on an electronic unit-pump (EUP) bench, a constant volume chamber and a high-speed digital camera. The photographs of spray were dealt with by using an image processing procedure. Then the spray tip penetrations and cone angles were obtained and analyzed. The experimental results indicate that the spray tip penetrations and cone angles of biodiesel increase with increasing injection duration. In addition, with decreasing ambient pressure the spray tip penetrations increase while the cone angles decrease. Furthermore, ambient pressure has a stronger effect on the spray properties of biodiesel than injection pressure. On the macroscopically view, the shape of biodiesel spray is similar to that of diesel. The final tip penetrations and cone angles of biodiesel are greater than those of diesel due to its higher viscosity, density and bulk modulus. Copyright © 2008 John Wiley & Sons, Ltd.

The spray characteristics of a pressure-swirl injector with various exit plane tilts

The gasoline spray characteristics of a pressure-swirl injector were investigated with various exit plane tilts. The analysis focused on the correlation between tilt angle and flow angle. Mie-scattering technique and phase Doppler anemometry were employed to analyze the macroscopic spray development and droplet size distribution of the spray. An analytical method for mass flux estimation was applied to understand the velocity distribution at the nozzle exit. The results showed that the spray shape and velocity distribution of the spray were more asymmetrical at high tilt angles. In particular, an opened hollow cone spray was formed when the tilt angle is greater than the complementary flow angle. The pressure drop inside the spray, one of the crucial factors for the swirl spray collapse at various surrounding conditions, was attenuated in this opened hollow cone spray since the pressure inside the spray was assimilated to the surrounding air pressure. The spray collapse at high fuel temperature and back pressure conditions did not appear when the tilt angle is larger than the complementary flow angle due to the reduced pressure drop inside the spray. However, tilt angle should be optimized to fulfill the requirements of spray robustness and avoid the locally rich area. The droplet size of 70° tilted nozzle spray shows a value similar to that of the original swirl spray in the plane that includes nozzle axis and the major axis of exit surface ellipse (Major axis plane) while it shows an increased value in the plane that includes nozzle axis and the minor axis of exit surface ellipse (Minor axis plane).

Application of Drag Reduction Model Accounting for Droplet Number Density to a Diesel Spray

DDM (Discrete Droplet Model) is widely used for the numerical analysis of a fuel spray. In order to achieve highly accurate calculations using DDM, many mathematical sub-models such as breakup, drag force and collision are employed with DDM. In the conventional DDM, an isolated droplet (parcel) is assumed and the influence of surrounding droplets is not directly considered. However, it is well known that the state of the wake flow of droplet or the vortex behind droplet changes by the distance between droplets, and the drag coefficient is influenced. Especially, this effect is important in a dense spray. In this study, a newly developed drag force model, containing the drag reduction effect with the droplet spacing as a parameter is adopted and also, the effect on the spray tip penetration and the spray volume is examined. As a result, the spray shape and Sauter mean diameter were found to depend on the mesh size that defines the droplet spacing. The drag force reduction by the droplet number density tends to increase the spray volume. The calculation of a diesel spray at the injection pressure of 133 and 55 MPa indicated better agreements with experiments compared to the result using a conventional model.

THE EXTENDED PHENOMENOLOGICAL SPRAY MODEL (EPSM): A NEW APPROACH FOR CFD DIESEL FUEL SPRAY SIMULATION UNDER EVAPORATIVE AND NONEVAPORATIVE CONDITIONS

A new approach for the simulation of evaporative and nonevaporative Diesel fuel sprays under steady injections is proposed and tested for several operating conditions and injector geometries. The methodology, called the extended phenomenological spray model (EPSM), which is based on the previously proposed phenomenological spray model method, shows a dramatic reduction in the typical grid sensitivity of commercial Computational Fluid Dynamics (CFD) codes and requires a reduced set of experimental data to set up the boundary conditions. In the EPSM, the spray evolution is controlled by means of a phenomenological relation for the spray tip penetration, while the vaporization spray process is predicted by a scaling law that allows the simulation of particle evaporation and the determination of liquid phase maximum penetration. The method has been extensively validated by comparing the simulation results with experimental data found in literature based on laser sheet techniques for spray tip penetration, spray cone angle, and spray shape determination

SPRAY EVOLUTION IN A TWIN-FLUID SWIRL ATOMIZER

The paper describes an experimental study of the evolution of spray structure in an internally mixed, twin- fluid swirl atomizer. A laser sheet visualization technique was used to study the primary spray formation process. The order of magnitude of the ALR (Air-liquid ratio) for which the study was made ranged from 10-3 to 10-1. It was observed that at low ALR a fully developed air core inside the hollow conical spray is present suggesting the assistance provided by the atomizing air to the atomization process by de-stabilizing the liquid internally. At high ALR conditions of around 0.1, a solid cone spray shape is observed. At moderate ALR`s, a mixed cone is formed, having an outer hollow structure and a solid core, with a gap between the inner and outer cone. This can be attributed to the combined effect of swirl (producing the hollow cone) and the increase in the axial momentum of the liquid inside the atomizer due to squeezing by the surrounding atomizing air (forming the inner core). Also, the phenomenon of liquid break up in terms of breaking distance at various ALR conditions was analyzed in this study. It was observed that the structure of the spray, e.g., cone angle, solidity and breaking distance, do not change much with the increase in ALR beyond a critical value of 0.08. The variation from hollow to solid cones demonstrated by the atomizer in this study makes it flexible and efficient enough to be used for commercial applications, as the atomizer is capable of providing a wide range of spray patterns depending upon the application requirement.

Shape, microstructure and wear of spray formed hypoeutectic Al–Si alloys

The manufacturing process through spray forming leads to give near-net-shape and fine grain microstructure. In this process Si particles, which are not distributed uniformly in conventional casting process, are distributed uniformly throughout the casting. In the present study disc shape spray form castings were made of Al–6.91Si and Al–10.1Si alloys, and then their shape, microstructure and wearing properties were studied. The shape of the deposit was observed to be the most uniform at 30° inclination angle of the substrate. Microstructure of spray formed alloys showed equiaxed grains morphology in contrast to as cast alloys. The wearing properties of Al–10.1Si alloy were found to be better than that of the Al–6.91Si alloy.

Macroscopic diesel fuel spray shadowgraphy using high speed digital imaging in a high pressure cell

Abstract Spray formation from diesel fuel injection through a realistic heavy-duty multi-hole common rail injector is studied in a newly developed high pressure, high temperature cell, using digital high speed shadowgraphy at 4500 frames per second. Care is taken to establish accurate synchronisation between camera and injection system and because of the relatively large exposure time, an effective camera image time is calculated for every frame. Further emphasis is given to determining the actual start of fuel mass injection by comparing (for each injection) a predetermined, rail pressure dependent needle relaxation distance to the actual needle lift signal. The spatiotemporal evolution of the spray is found to reproduce well in general, but often sprays suffer from short-lived, small, laterally moving anomalies, which influence axial motion and the spray cone angle. High speed shadowgraphy allows this to be observed and taken into account. After an overview of methods found in the literature, an algorithm for geometrical analysis is presented, which is based on an extension of a combination of those methods. In this algorithm, a local spray angle ϑi(x) is determined from lateral cross-sections at 80% of the shadow level in order to encompass most of the spray without being too sensitive to background noise. The macroscopic cone angle ϑcone is derived from the approximate constancy of ϑi(x) over a relatively long axial distance. Spray penetration is obtained by lateral integration of the spray shadow. A procedure for accurate correlation of spray growth with time shows that the growth is proportional to tb with b = 0.57 ± 0.02 for a common rail pressure of 150 MPa and a gas density 33 kg/m3 (N2 at room temperature). The exact value of b is very sensitive to uncertainties in synchronisation and the start of injection determination. The spray cone angle ϑcone is not constant, but varies with time during an injection, mainly as a result of spray shape changes.

Modeling weight variability in a pan coating process using Monte Carlo simulations

The primary objective of the current study was to investigate process variables affecting weight gain mass coating variability (CV(m) ) in pan coating devices using novel video-imaging techniques and Monte Carlo simulations. Experimental information such as the tablet location, circulation time distribution, velocity distribution, projected surface area, and spray dynamics was the main input to the simulations. The data on the dynamics of tablet movement were obtained using novel video-imaging methods. The effects of pan speed, pan loading, tablet size, coating time, spray flux distribution, and spray area and shape were investigated. CV(m) was found to be inversely proportional to the square root of coating time. The spray shape was not found to affect the CV(m) of the process significantly, but an increase in the spray area led to lower CV(m) s. Coating experiments were conducted to verify the predictions from the Monte Carlo simulations, and the trends predicted from the model were in good agreement. It was observed that the Monte Carlo simulations underpredicted CV(m) s in comparison to the experiments. The model developed can provide a basis for adjustments in process parameters required during scale-up operations and can be useful in predicting the process changes that are needed to achieve the same CV(m) when a variable is altered.

Feasibility of “intermittent” active control of combustion instabilities in liquid fueled combustors using a “smart” fuel injector

This paper describes an experimental investigation of the feasibility of an “intermittent” active control approach for suppressing combustion instabilities in liquid fueled combustors. The developed controller employs a “smart” fuel injector that can modify the spray properties in response to changes in combustor operating conditions. This action weakens or breaks up the coupling between the combustion process and combustor acoustic modes oscillations, thus preventing the excitation of large amplitude instabilities. This approach differs significantly from previously proposed active control methods, both in concept and implementation, as it requires only “intermittent” modification of the combustion process by a single control action as opposed to the continuous action required by most other active control methods. The “smart” fuel injector used in this study consisted of a double-staged, air-assisted atomizer in which counter swirling, primary (inner stage) and secondary (outer stage) air streams were supplied to the injector through separate sets of tangentially oriented orifices. Control of the ratio of air mass flow rates supplied to these two stages, by use of a diverter valve, resulted in significant changes in the spray shape and its axial, tangential, and radial velocity components. This variation in spray properties of the “smart” injector was characterized for different values of the inner to outer air flow rate ratio in cold flow tests with a PDPA system. These results were then correlated with the characteristics of the “intermittently” controlled combustor. Measured quantities included the instability amplitudes, axial dependence of the mean and oscillatory heat release amplitudes, and the characteristics of the recirculation zones, which were all shown to depend on the fuel spray properties. The results of this study demonstrate the feasibility of using “smart” fuel injectors with capabilities for varying the combustion process characteristics to reduce the amplitudes of detrimental combustion instabilities in real engines to acceptable levels.

The structure of a break-up zone in the transient diesel spray of a valve-covered orifice nozzle

A break-up zone in the diesel spray of a valve-covered orifice nozzle was investigated to observe the effect of the injection rate on the spray structure and to obtain physical insight into the development of a transient diesel spray. The surface shape and internal structure of the diesel spray from a common-rail injection system were visualized with high spatial and temporal resolution under atmospheric ambient conditions. During the injection period, common-rail pressures of 39.5 MPa and 112 MPa were used to obtain highly magnified spray images from the nozzle exit to about 260 nozzle diameters downstream. Before the atomization break-up regime appeared, a short transition period was observed, while ligaments were formed on the disturbed liquid column surface. During atomization, the spray was surrounded by these short fine ligaments, which were arranged and bent towards the direction of the spray penetration. The internal structure of the break-up zone consisted of complex entangled ligaments and dispersed liquid drops. The break-up process occurred simultaneously at the spray surface and the core. The entrained ambient air that penetrated through the crevices of the densely packed ligaments seemed to stretch the coherent structure and to carry the small droplets from the surface of the spray. The collapse of a cavitation bubble might have caused the ligaments to form near the nozzle exit at the core of the spray, although details of the process remain to be identified. Using image-processing techniques, the sizes of the liquid drops downstream from the spray were measured. The average size of the drops decreased as the injection rate decreased and, as expected, a higher common-rail pressure produced smaller droplets.

Monte Carlo simulations to determine coating uniformity in a Wurster fluidized bed coating process

This paper presents a coating model to predict the mass coating uniformity in a Wurster fluid bed coater using a Monte Carlo method. The velocity and voidage data obtained using imaging techniques on the same Wurster coater are used as inputs to the model. The semi-circular Wurster fluid bed used in this work was 22.9 cm in diameter. A batch of 3.6 kg tablets was used to conduct coating experiments and the coating weight gain distribution results were compared to predictions from the simulation. The model rigorously considers the sheltering effect of particles as they move in the spray zone. Good agreement was obtained when comparing the results with an analytical model. Spray shape and orientation of discretization were found to play an important role in predicting the coating uniformity. A simple spray experiment in a particle-free bed showed that the direction of spray material, in general, was vertically upward. Simulation results confirmed that an upward cylindrical spray model gives better agreement with experimental results compared to a solid cone spray model. Finally, the model was used to predict the changes in coating uniformity with bed operating conditions such as gas velocity and gap height. A wider coating distribution was found for the case with the lower gas velocity and gap height.