Electrical and transient atomization characteristics of a pulsed charge injection atomizer using electrically insulating liquids

Charge injection atomizers are energy efficient devices that can be used in order to promote the atomization of dielectric liquids, and a potential application of such devices is fine spray delivery in small internal combustion engines. The operation of a pulsed charge injection atomization system operating at practical engine frequencies under a high voltage pulse train has not been well recorded in the literature. This initial investigation defines the electrical and transient global atomization performance of a charge injection atomizer operating under a steady flow regime, but with a typical high voltage pulse train. Results show that voltage-current characteristics follow similar trends to that of a steady flow, steady voltage system, and observation of the data also reveals that output current waveforms depend on the input pulse train frequency. No degradation in charging efficiency was observed at higher frequencies, which suggests that a charge injection atomizer can operate efficiently at practical engine speeds. Photographs also confirmed the high voltage pulse train injects charge that produces sections of primary atomization on the continuous liquid jet.     

Time-resolved processes in a pulsed electrical discharge in argon bubbles in water

A phenomenological picture of a pulsed electrical discharge in gas bubbles in water is produced by combining electrical, spectroscopic, and imaging characterization methods. The discharge is generated by applying 1 m\mu s pulses of 5 to 20 kV between a needle and a disk electrode submerged in water. An Ar gas bubble surrounds the tip of the needle electrode. Imaging, electrical characteristics, and time-resolved optical emission spectroscopic data suggest a fast streamer propagation mechanism and the formation of a plasma channel in the bubble. Comparing the electrical and imaging data for consecutive pulses applied to the bubble at a frequency of 1 Hz indicates that each discharge proceeds as an entirely new process with no memory of the previous discharge aside from the presence of long-lived chemical species, such as ozone and oxygen. Imaging and electrical data show the presence of two discharge events during each applied voltage pulse, a forward discharge near the beginning of the applied pulse depositing charge on the surface of the bubble and a reverse discharge removing the accumulated charge from the water/gas interface when the applied voltage is turned off. The pd value of ~ 300–500 torr cm, the 1 μs long pulse duration, low repetition rate, and unidirectional character of the applied voltage pulses make the discharge process here unique compared to the traditional corona or dielectric barrier discharges.     

Fuel spray visualization and its impingement analysis on in-cylinder surfaces in a direct-injection spark-ignition engine

Abstract  

Experiments are performed to investigate the effects of fuel spray on in-cylinder mixture preparation and its impingement on cylinder walls and piston top inside a direct-injection spark-ignition engine with optical access to the cylinder. Novel image processing algorithms are developed to analyze the fuel impingement quantitatively on in-cylinder surfaces. The technique is useful to optimize the fuel pressure, injection timing and the number of injections to minimize the fuel impingement on in-cylinder surfaces. E85, which represents a blend of 85% ethanol and 15% gasoline (by volume) is used in this study. Two types of fuel injectors are used; (i) low-pressure production-intent injector with fuel pressure of 3 MPa, and (ii) high-pressure production injector with fuel pressures of 5 and 10 MPa. In addition, the effects of split injection are also presented by maintaining the same amount of fuel used in single injection. It is found that the split injection is an effective way to reduce the overall fuel impingement on in-cylinder surfaces while maintaining a reasonably good air–fuel mixture in the cylinder.     

Ballistic imaging of liquid breakup processes in dense sprays

Ballistic imaging is the name applied to a category of optical techniques that were originally developed for medical applications. Recently, ballistic imaging was adapted to acquire instantaneous images of the liquid core inside atomizing sprays; a region that has been heretofore inaccessible to spray researchers. An important difference between spray research and the medical imaging problem is the need for high fidelity single-shot (within 10 μs) imaging in a spray whereas stationary tissue images can be averaged. Transient ballistic imaging diagnostics have been used to reveal details of the primary breakup process in a LOX injector, a turbulent water jet, a water jet in cross-flow, a transient diesel fuel spray, a rocket fuel injector, and an aerated spray. This paper briefly discusses various methods for imaging the liquid core, it introduces ballistic imaging and provides specific examples, it describes detailed studies of photon transmission through dense media, and it then discusses incorporation of those results into a model for a ballistic imaging instrument that can evaluate and optimize various concepts.     

Electrostatic hand pressure knapsack spray system with enhanced performance for small scale farms

Electrostatic force fields have been employed and enhanced in the design of an electrostatic knapsack spray system for increasing the deposition efficiency and reducing the drift of pesticides. The designed induction charge based electrostatic sprayer offers optimum electrode position and electrical conductivity of liquid. The experiments were conducted in ambient conditions for liquid feed rate 340 ml/min at hand pressure of 30 psi. The charge-to-mass ratio was found to be 0.419 mC/kg at 3.25 kV by a spray liquid of conductivity 10.25 mS/cm. There has been 2–3 fold increase of chemical deposition with better uniformity on the target (potted plant).     

Submicron particles trajectory and collection efficiency in a miniature planar DBD-ESP: Theoretical model and experimental validation

In this paper, the collection efficiency of a plane-to-plane dielectric barrier discharge electrostatic precipitator is investigated experimentally and theoretically using a new model. A parametric study is carried out to evaluate the effects of the applied voltage amplitude and frequency on submicron particles motion and collection within the size range from 0.18 to 0.7 μm. Results show that the amplitude of the particles oscillatory motion increases with the voltage and the particles diameter which increase their collection. The collection efficiency decreases at low frequencies because of the low charge of particles and at high frequencies because of particles fast oscillation.     

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.     

Effect of flash boiling injection on combustion and PN emissions of DISI optical engine fueled with butanol isomers/TPRF blends

Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions.     

The structure of the high-pressure gas jet from a spark plug fuel injector for direct fuel injection

Abstract  

A spark plug fuel injector (SPFI), which is a combination of a fuel injector and a spark plug was developed with the aim to convert any gasoline port injection spark ignition engine to gaseous fuel direct injection (Mohamad in Development of a spark plug fuel injector for direct injection of methane in spark ignition engine. PhD thesis, Cranfield University, 2006). A direct fuel injector is combined with a spark plug using specially fabricated bracket connected to a fuel pipe and a fuel path running along the periphery of a spark plug body to deliver the injected fuel to the combustion chamber. The injection nozzle of SPFI is significantly bigger than normal direct fuel injector nozzles. Therefore, it is important to understand the effect of such a configuration on the injection process and subsequently the air–fuel mixing behaviour inside the combustion chamber. The flow was visualized using the planar laser-induced fluorescent technique. For safety reasons, nitrogen was used as fuel substitute. Nitrogen at 50, 60 and 80 bar pressure was seeded with acetone as a flow tracer and injected into a bomb containing pressurised nitrogen. Bomb pressure was varied to simulate the pressure inside combustion cylinder during the compression stroke where actual injections in engine experiments will take place. The shape and depth of tip penetration of the gas jet were measured. Results show that the gas jet follows the behaviour suggested by vortex ball model (Turner in Mechanics 13:356–369, 1962). The cone angle and the maximum jet width of the fully developed gas jets from the SPFI injection are 23° and 25 mm, respectively regardless of the injection pressures. The penetration lengths of the fully developed jets are between 90 and 100 mm at 8–14 ms after the start of injection, depending on the bomb and injection pressure. Jet penetration is directly proportional to the injection pressure but inversely proportional to the cylinder or bomb pressure. The penetration lengths indicate that sufficient distance should be travelled by the gas jet for satisfactory air–fuel mixing in the engine.     

Interpreting the influence of fuel spray impact on mixture preparation for HCCI combustion with port-fuel injection

This paper addresses the influence of fuel spray impact on fuel/air mixture for combustion in port-fuel injection engines. The experiments include time resolved measurements of surface temperature synchronized with PDA measurements of droplet dynamics at impact and were conducted to quantify the effects of interactions between successive injections on the mixture preparation for combustion in homogeneous charge compression ignition (HCCI) engines. Analysis shows that, during engine warm up, the heat transfer over the entire valve surface occurs within the vaporization-nucleate-boiling regime and the local instantaneous surface temperature correlates with the dynamics of droplets impacting at the same point. A functional relation is found for the heat transfer coefficient, which also describes other experiments reported in the literature. Similarity does not hold after the engine warms up because heat transfer and droplet vaporization at the surface are dominated by multiple interactions between droplets arisen from diverse heat transfer regimes. However, results evidence the existence of a critical surface temperature which sets a transition between overall heat transfer regimes dominated by local nucleate boiling at lower temperatures and by local intermittent transition regimes at higher temperatures. The heat transfer within the overall nucleate boiling regime is shown to be due to a thin film boiling mechanism leading to breakdown of the liquid-film at a nearly constant surface temperature, regardless of injection frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid film. The results further evidence the important role of spray impingement for mixture preparation as required for HCCI.     

Studies on the valveless scheme to produce high-frequency detonations with different purge methods

Producing high-frequency detonations is an important topic for pulse detonations which has received considerable attentions. The valveless scheme has been verified to be able to obtain high-frequency detonations more than 100 Hz. This work has been conducted to investigate the possibility to achieve a higher detonation frequency and clarify the limits of stable operations preliminarily for the valveless scheme with different purge methods. Oxygen, ethylene, and nitrogen or liquid water are utilized as oxidizer, fuel, and purge medium in the experiments while two injection configurations are employed. The maximum detonation frequencies of 180 Hz and 330 Hz have been achieved in stable operations for two different injection configurations when nitrogen is used as the purge gas. The ceiling frequency for stable detonations is 300 Hz if nitrogen is replaced by liquid water, which indicates that water vapor is capable to create an efficient buffer zone to ensure stable operations. The results imply that the injection configuration also has a great impact on the ceiling stable detonation frequency. Three operating modes have been observed in this study, i.e., a stable detonation mode, an unstable detonation mode, and a deflagration mode. In the unstable mode, failure of detonation initiation occurs frequently and one interesting phenomenon is that the detonation frequency is reduced by half exactly when insufficient filling happens. The supply pressure ratios of oxidizer to fuel and purge to fuel are obtained for different operating modes when the purge method is changed. Furthermore, the equivalence ratios have been also studied for different operating modes which reveals that the range will change when different purge methods and injection configurations are employed. According to the equivalence ratio and the mass flow rates, an equivalent volume fraction of oxygen is defined and its range for the stable detonation mode is clarified.     

Characterization of a Liquid Fuel Injector under Continuous and Modulated Flow Conditions

The purpose of this work was to characterize the spray delivered by a modulated liquid fuel injector designed for active combustion control applications. A novel actuator is used to create a time-varying liquid fuel flow rate upstream of a commercially available injector. In order to be useful in existing burners, the actuator must not degrade the spray, by changing either the size or velocity distributions of the droplets produced by the injector. The amplitude of the induced modulations in flow rate must be strong enough to induce the required periodicity in heat release rate. This paper reports the results obtained from particle imaging velocimetry and phase Doppler anemometry used to characterize the spray, plus hot-film anemometry and pressure transducer measurements used to characterize the response of the fuel line to the induced flow rate fluctuations and to measure the excitation amplitude. It is found that the actuator response time is sufficiently rapid to modulate the liquid flow rate without changing the spray characteristics. Strong modulation of the flow rate is possible at low forcing frequencies, but the time-averaged flow rate is reduced. At higher forcing frequencies, the actuator response time cuts off, leading to a smaller amplitude flow rate modulation, and a relatively unchanged time-averaged fuel flow rate. For these reasons, this actuator is well suited to the control applications envisaged.     

Acoustic monitoring of engine fuel injection based on adaptive filtering techniques

Diesel engines injection process is essential for optimum operation to maintain the design power and torque requirements and to satisfy stricter emissions legislations. In general this process is highly dependent upon the injection pump and fuel injector health. However, extracting such information about the injector condition using needle movements or vibration measurements without affecting its operation is very difficult. It is also very difficult to extract such information using direct air-borne acoustic measurements.In this work adaptive filtering techniques are employed to enhance diesel fuel injector needle impact excitations contained within the air-borne acoustic signals. Those signals are remotely measured by a condenser microphone located 25 cm away from the injector head, band pass filtered and processed in a personal computer using MatLab. Different injection pressures examined were 250, 240, 230, 220 and 210 bars and fuel injector needle opening and closing impacts in each case were thus revealed in the time-frequency domain using the Wigner-Ville distribution (WVD) technique. The energy of 7-15 kHz frequency bands was found to vary according to the injection pressure. The developed enhancement scheme parameters are determined and its consistency in extracting and enhancing signal to noise ratio of injector signatures is examined using simulation and real measured signals; this allows much better condition monitoring information extraction.     

Use of electrostatic precipitation for excess ion trapping in an electrical aerosol detector

In this paper, the technique of electrostatic precipitation was used to remove excess ions from a mixture with charged particles before collection on a filter in a Faraday cup electrometer of an electrical aerosol detector. The ion precipitator part of the detector was designed, constructed, and evaluated. An analytical model was developed to investigate ion and particle transports due to diffusion and space charge effects inside the ion precipitator. Experimental investigations were carried out for positive ions, the positively applied voltage at the wire electrode ranged from 10 to 150 V, ion flow rates ranged from 5 to 15 L/min, and the radial distance of the inlet was 0.15 and 14 mm at a fixed separation between the wire and outer electrodes. The calculation results showed that all charged particles of 10 nm in diameter could pass through the ion precipitator smoothly without precipitation at the outer electrode. For all ion flow rates, an increase in ion trap voltage produced an increase in ion collection efficiency of the precipitator. Experiments confirmed that the efficiency of the ion precipitator could increase to 99% at an ion trap voltage larger than 100 V for all ion flow rates.     

Influence of space charge related to water trees on the breakdown voltage of power cable insulation

A three-dimensional computation of the electric field and breakdown voltage in power cable insulation containing water trees and space charges is presented. The breakdown voltage and the conductivity of cylindrical samples of cable insulation containing water trees were measured. The samples have been aged in wet environment under ac voltages of frequencies comprised between 1 and 5 kHz. Exponential and parabolic spatial variations of permittivity and space charge density and the electrostatic, electro-kinetic and quasi-stationary regimes of the electric field were considered. The best correlation between the experimental breakdown voltage and the calculated one has been obtained in quasi-stationary regime.     

The energetic balance of the two different electrostatic processes: Electrostatic liquid spraying and electrostatic induction for electrical field measuring

In the specialized literature, two previous studies [S. Zhao, G.S.P. Castle, K. Adamiac, The effect of space charge on the performance of an electrostatic induction charging spray nozzle, J. Electrostat., 63 (2005) 261–272; S. Zhao, G.S.P. Castle, K. Adamiac, Comparison of conduction and induction charging in liquid spraying, J. Electrostat., 63 (6–10) (2005) 871–876] aim to clarify, by theoretical and experimental means, the following aspects concerning the charge through conduction and induction: charging mechanism, distribution of electrical field, conversion of energy and effect of spatial charge. This paper presents the energetic balance of the two systems, namely induction and conduction charge systems, with the purpose of establishing the quantitative energetic aspects of the phenomena that take place and, most of all, establishing the contribution to the energetic value of the drops that end up on the target and come from the high voltage source supplying power to the system and from the air flow that contributes to spraying and transporting particles to the target. At the same time, the energetic balance of a device which uses electrostatic induction for measuring electrical field intensity and high voltage is presented for exemplification. The device scheme, the way of functioning, the equivalent electrical schemes and the expression of the electrical current generated by the device as a consequence of the electrostatic induction are presented, and it is demonstrated that the electrical energy of the generated current does not result from the energy stored in the electrical field which is being measured, respectively of the one generating the electrostatic induction, but from the mechanical energy, by conversion, consumed by the motor which sets in motion the mobile part of this device.     

Efficacy of electrostatically charged glyphosate on ryegrass

Using a TX-VK3 spray tip attached to an electrostatic sprayer operated at 483 kPa pressure, ryegrass was sprayed with glyphosate at 0.0033 kg ae ha⁻¹. Charge-to-mass ratio (Q/M) for the spray solution was 1.686 mC kg⁻¹ at +10.0 kV charging voltage. Treatment efficacy was assessed using NDVI (Normalized Difference Vegetation Index) spectral reflectance values. Electrostatic charging of glyphosate significantly increased volume median diameter of spray droplets (D_v0.5 = 112.8 μm) compared to uncharged glyphosate (D_v0.5 = 106.5 μm). Ryegrass health declined 80% faster by charging the glyphosate spray solution compared to the uncharged spray.     

Effects of electrode voltage,liquid flow rate,and liquid properties on spray chargeability of an air-assisted electrostatic-induction spray-charging system

Combinations of electrode voltage, liquid flow rate, and properties can enhance chargeability of electrostatic sprays for effective pesticide application, though the combined effects of these parameters are not well understood. Generally, 4 kV voltage and lower (30, 45, and 60 mL min^?1) flow rate of tank water produced greater chargeability compared to ground water sprays. The rate of increase in spray chargeability with decreased liquid flow rate was higher in the lower flow rates. The outcome of the study will be helpful for the more targeted and environmentally safe application of pesticide sprays and development of suitable electrostatic spraying systems.     

高压直流连续波光阴极注入器中电子束流发射度研究

 高平均功率自由电子激光研究中，电子束质量是关键。针对高平均功率自由电子激光目标参数，提出了直流高压连续波光阴极注入器，给出了注入器的束流动力学过程。为了降低输出束流横向发射度，采用特殊结构设计的静电加速腔，加速电压1MV，最大加速梯度10MV/m。用PARMELA程序进行了粒子动力学模拟，电子束束团电荷为0.5nC，束团长度10ps时，注入器输出束流归一化发射度均方根值为5.8mm·mrad。     

Direct numerical simulation of electrohydrodynamic plumes generated by a hyperbolic blade electrode

This paper presents a numerical study of two-dimensional EHD flow occurring between a hyperbolic blade and a plate electrode. The whole set of coupled equations is solved: Navier–Stokes equations, Poisson equation and charge conservation equation. A finite volume approach designed for non-orthogonal structured grid is used to discretize all governing equations. An efficient numerical procedure based on total variation diminishing (TVD) scheme is implemented to compute the distribution of charge density. Two different injection laws are considered: a simple autonomous one and a non autonomous which relates the charge injected by the blade and the local electric field. The flow structure which results in an EHD plume analogous to a thermal plume, has also been successfully characterized numerically by the temporal evolution of the charge density distribution. Preliminary results indicate that the flow is characterized by two different regimes according the value of the applied voltage. The critical Reynolds number for which the transition between the steady and unsteady regimes occurs has been determined to be within the range Re = [1000, 1100].