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Direct injection engine spray processes. mechanisms to improve performance (DIME)

Deliverables

Both of the atomizer techniques considered, pressure swirl and air-assist have sufficient time to achieve complete drop evaporation at full load, i.e. injecting into the intake stroke. However several problems emerge for both pressure swirl and air-assist injectors, though for different reasons, using more restrictive injection strategies. Pressure swirl atomizers have a fundamental problem at part load, i.e. injecting into the compression stroke, in that the primary atomisation mechanism, due to the presence of thinning sheets, generate larger drops at increased gas densities. This dramatically increases the drop momentum, heat up and steady state evaporation timescales and makes it extremely difficult to achieve complete evaporation at ignition. Air assist atomizers possess an engineering problem in that to achieve a constant gauge pressure between driver air supply and cylinder pressure, as a function of load, the compressor must respond as fast as the engine speed cycle changes.
Modification of 2 CV, port with axisymmetric contraction (area ratio 7.5) leading to a short cylindrical section, valve with 45 deg seat angle with rounded edges on both the valve head and the valve seat, exhaust valve similar to intake valve, exhaust port with simple right angle diffusing bend shape; Development of in-cylinder flow field from BDC of induction to TDC of compression at 1000 rpm, investigation of the effect of engine speed on the TDC flow field, speed range of 300 -2000rpm, no induction swirl; Flow at valve exit plane axisymmetric within 3% and nearly uniform along the valve passage, free of tangential velocity component, separation from the valve seat only for lifts greater than 6 mm, separation region to extend for not more than 1.5 mm at maximum lift, flow angle of 45 deg +/- 2 deg, induction generated flow structures decayed soon after IVC resulting in near-zero mean flow pattern at TDC of composition which was not influenced by engine speed, turbulence at TDC of composition was nearly isotropic but not homogeneous at all engine speeds, variations of turbulence intensity of TDC clearance volume, magnitude/spatial distribution determined mainly by intake port, TDC average turbulence intensity increased more than linear with engine speed (300 - 1250 rpm). Rig is ready to run, and optical equipment no being aligned.
The model developed in the frame of the project enables the engineer to quantify the evaporation behaviour / kinetics of multi-component liquid drops under various conditions of convective flow and environmental gas state. The model is based on the approach by Abramzon and Sirignano (1987), which was extended in the frame of the work for computing the behaviour of multi-component liquids. For doing this, the real behaviour of evaporating multi-component liquids must be described realistically. This behaviour consists in influences of the various liquid mixture components on each other, which leads, further to thermal effects, to a change of the vapour pressure in the gaseous phase at equilibrium as compared to the saturation pressure of the pure liquid. Tests of the model results against data from two-component mixtures showed very good agreement. Based on this approach, the model of Abramzon and Sirignano (1987) was extended such that the evaporation behaviour of multi-component liquids can be described realistically. This was shown by comparison with experimental data achieved by the methodology in result # 9. The model describes mixtures with presently up to 10 components. Experiments have shown that it may be possible to reduce the behaviour of the multi-component mixture to that of a mixture with at most five components.
The work performed is aimed at developing an accurate methodology for the analysis of the spray-cooling event, which occurs at the impact of a gasoline spray onto the back surface of intake valves in port-fuel injection systems. The analysis makes use of a Phase Doppler Anemometer to measure droplet characteristics at impact and fast response thermocouples to measure surface temperature in a simplified flow configuration. The practical relevance of such a methodology is that it provides a tool for the optimisation of gasoline injection systems so to avoid the formation of a liquid film and to enhance fuel evaporation and fuel/air mixture preparation. Simultaneous measurements of instantaneous droplets size and velocity together with surface temperature during the period of injection allow deriving a correlation for the Nusselt number for the transient period of engine warm-up. A noteworthy fact in obtained correlations is the use of the Jakob number as a way to account for the heat transfer regimes defined in classical boiling theory. For engines operating at steady state, an integral method is suggested to analyse the thermal behaviour of intake valves upon the fuel spray impact. The methodology defines overall boiling curves dependent on the injection conditions (e g, pressure, duration and frequency). Analysis show that, although the total average heat flux removed from the surface increases with the injection frequency, the opposite occurs for the efficiency of the spray cooling event, which is attributed to the excess of liquid mass remaining at the surface between successive injections.
The study performed under this task was aimed at accounting for the complexity introduced by the influence of non-scaled parameters on the description of droplet impact. An experimental installation was built where droplets are generated at the tip of a hypodermic needle and fall by gravity onto a cold and flat surface. The time history of droplet behaviour is recorded with a high-speed camera triggered by the passage of the droplet through a laser beam hitting onto a photodiode. The experiments included the use of several targets made of dissimilar materials, from polished surfaces to strong rough surfaces with different roughness profiles, namely roughness wavelength and shape of the asperities. Surface roughness was characterized with an optical profile meter for opaque surfaces and with a mechanical profile meter for transparent surfaces. Numerous measurements were performed along orthogonal directions in order to form a grid of small target areas with well-defined roughness characteristics. Inhomogenities of the targets, characterised by variations of the mean roughness, were found to be smaller than 10%. The thermodynamic system liquid-vapour-surface was characterized by the equilibrium contact angle, as provided by Young’s law, as a measure of the wetness. Contact angles at equilibrium were measured for several rough surfaces and liquids. The measurements were made with sessile drops inside a thermostat ambient chamber (Ramé-Hart Inc., USA, model 100-07-00), with quartz windows, to avoid optical distortion, previously saturated with the liquid to be studied (water and diesel oil) at a temperature of 20 ±1ºC. After the drop was deposited at the surface, its image is recorded using a colour video camera (JVC Colour TK-1070) mounted on a Wild M3Z microscope, which allows a magnification of 40 times. The video signal was transmitted to an image processor - Video Pix Framegrabber (Sun Microsystems) - and digitized in 640 x 480 pixels images, in a 256 grey level scale. The image acquisition and analysis was performed by a Sun Sparc station IPC, using the Axisymmetric Drop Shape Analysis software (ADSA for SunOS 1.0 Applied Surface Thermodynamics Research Associates, Toronto, Canada). The variation of the contact angle with time was recorded for time intervals of six hundred seconds. At least 8 measurements were taken for each pair liquid-impact surface in order to obtain average values. Furthermore the evolution of the average contact angles with time was obtained for each pair liquid-surface, by curve fitting and the final values were determined by extrapolation. Since the outcomes from droplets impact occur in a much smaller time scale (ms) it was considered that the final value for the contact angle was obtained by fitting the values obtained in the latest period of the measurement and extrapolating them for t = 0s. Experiments included the identification of mechanisms and an attempt to describe the effects of the target surface on them. The results showed that the wetness strongly depends on the mean surface roughness, and may be used as a characteristic parameter of the system, providing that a precursor film is not formed at the target surface during spread. If splash does not occur, the equilibrium contact angle describes the effects of surface roughness on the wetting dynamics. Those effects are expected to alter friction forces at the wall and, therefore, the energy dissipated during spread. It is found that, provided the Reynolds number of the droplet at the impact is large (Re>2000), the energy dissipated at the wall is not greatly affected by the nature of the surface, which appears to be important for low Reynolds numbers, typically Re<1000. However, the nature of the surface significantly alters the onset of splashing. The correlation of Wu (1992) was observed to correlate the dimensionless roughness Ra/Ro with the critical Weber number at the onset of splash for each pair surface material - liquid. The accounted properties of the surface do not allow collapsing all the curves and other characteristics should be taken into account, such as the wavelength of the irregularities.
The experimental database developed in the frame of the project concerns the evaporation behaviour of single liquid droplets under well-defined conditions of convective flow around the droplets, and under well-defined environmental conditions. The droplets consist of pure liquids, binary mixtures, or multi-component mixtures. The liquids involved are - further to water - organic solvents like n-alkanes, alcohols, ketones, and others. The database quantifies the evaporation rate of the pure liquids and of the mixtures. The evaporation rate varies with time during the lifetime of the droplets. For pure liquids, this variation can be described analytically, in agreement with the experimental data. For two- and multi-component liquids, the database quantifies the influence of the liquid composition on the activities of the various components. The database shows that the normalised droplet surface then exhibits a curved shape throughout the lifetime of the droplet, even for constant Sherwood number. For pure liquids, the normalised droplet surface follows the well-known d2 law, i.e. it exhibits a straight line in a plot with linear co-ordinate axes. For two-component liquids, the evolution of the normalised droplet surface approximately consists of two straight lines - one for each liquid component, in the case that the volatilities of the liquids are very different.
This work introduces the origins and estimates the relative timescale magnitudes and identifies the �rate limiting� process for DISI engines as a function of speed and load. The most significant timescales are highly spray and atomizer dependent, therefore to achieve a good overall assessment of the generic processes, and to demonstrate and broad appreciation of the subject two distinct atomizer classes are investigated.
First results of in-cylinder airflow measurements in an optical DISI engine are compared with airflow measurements carried out on a multi-port injected gasoline engine of similar design characteristics. The airflow characterization represents a first and important step to understand the preparation of the ignitable mixture close and around the spark plug in the combustion chamber. A set of sequential in-cylinder fuel spray images is included to allow the spray development and its manipulation by the in-cylinder airflow is identified.
PIV techniques are widely used in fluid mechanics as a precise way to measure bi-dimensional velocity fields of fluid flows. It consists on correlating two consecutive images of a flow seeded with reflective particles. These particles must therefore follow as close as possible the flow and be in a number sufficient to represent the flow with all the complexities (turbulence) but void enough to not perturb the flow itself. For droplets velocity measurements a different approach is necessary as it is the individual velocities of the particles, which are searched, and not the ones of the continuous fluid flow. One will rather talk then of Particle Tracking Velocimetry (PTV) for which different algorithms can be considered. Nevertheless, when the observed particles are in the vicinity of a wall, the specific behaviour of impinging and evaporating droplets must be approached with different algorithms and lighting techniques. Validated algorithms for imaging treatment have been developed and improved for impinging droplets. They are based on tessellation surfaces and tensor deformation estimation, put together into a new mixed algorithm with specific morphology criteria. An additional problem when approaching PTV is particles centre location. For this particular point a wavelet transform based algorithm has been developed.
Number of fluid mechanics applications, flows in engines included, can be characterised through Laser Induced Fluorescence. This technique is based on the specific coupling of a light emitting substance (dopant) and a coherent source of given wavelength. The intensity of the resulting fluorescence depends on a number of factors among which dopant concentration, temperature and pressure are of first order. In present applications, the decorrelation of these influences is not achieved, resulting on only qualitative information of dopant presence without numeral information possible on any of these three physical quantities (concentration, temperature and pressure). The present result is focused on experiments at different temperatures and pressures for known dopant concentrations in order to establish transfer functions of cross influences and allow future measurements to reach quantitative information of dopant concentration when temperature and pressure and known or measured simultaneously. A variety of usual dopants were tested in order to furnish an exploitable database for several applications.
The methodology developed in the frame of the project enables the experimenter to investigate the characteristics of the evaporation behaviour of multi-component liquid drops under well-defined conditions. The experimental means for carrying out the investigations is essentially an acoustic levitator for positioning single liquid droplets. This device is an acoustic resonator in the ultrasound range and enables one to levitate droplets of up to 3µl initial volume in a gaseous environment of controlled humidity and temperature. The secondary airflow induced by the ultrasound simulates the effect of a convective airflow around the evaporating droplet, as if it moved through quiescent air at a certain velocity. The effect is quantified by the Sherwood number. The evaporation behaviour of the droplet is measured by means of an image processing technique, using a CCD video camera and a frame grabber card in a PC. This system evaluates the images to deduce the volume of the droplet as a function of time. For single-component fuels this evaluation enables one to deduce the evaporation rate immediately. For multi-component liquids one needs to use the vertical position of the droplet in the acoustic field as additional information for deducing the evaporation rate from the image data.
The present investigation on air-assisted liquid sheets, for low Weber number conditions, revealed that properly tuned external perturbation may result in structurally different liquid disintegration characteristics, and may also enhance the break-up mechanism as compared with non-imposed flows. This can be attributed to the effective acoustic energy transfer occurring at the optimum excitation frequency, which may reduce the dominance of gravitational and surface tension effects on the liquid sheet. Under acoustic excitations, the interfacial wave development tends to become consistent, so that well-controlled liquid sheet break-up process can be sustained in a low Weber number flow regime. The results of this investigation are of practical relevance in twin-fluid atomizers. The merits of pre-filming-type atomizers strongly depend upon the atomising-air pressure. In other words, at low atomising-air velocities, the quality of primary break-up turns out to be poor. In order to overcome this drawback, properly tuned acoustic excitations that generate resonant conditions inside the air cavity can be effectively utilized. In addition, a bubble-like pattern of critical waves under acoustic perturbations can enhance the radial dispersion of the liquid in the initial stages, which will be useful for direct injection systems.
Within the project, several single and multi-component evaporation models have been developed, tested and implemented into the 3D CFD code FIRE v8. The models show good agreement with single component droplet evaporation and acceptable agreement under certain conditions with multi-component evaporation. The investigations also showed the dependence ofthe single component evaporation on the internal droplet heat conduction. Further different wall interaction models have been set up and implemented into the FIRE code, which also yield considerable improvements compared to the standard models used before. For single component evaporation three different models (Dukowicz, Spalding, Abramzon) have been implemented and tested. Under pure diffusion conditions all models behave similar and follow the typical D2-Law and there is good agreement with measurements. This changes when convection is added. The Dukowicz model without heat transfer correction considerably overestimates evaporation, while the new models agree well with the measurements. Further for multi-component evaporation two approaches have been used, which capture the multi-component behaviour but are still simple enough to be implemented into a 3D CFD code. In the first model it is assumed that mass diffusion is so slow that it can be neglected. The second model is based on the assumption that multi-component evaporation is governed by fuel volatility, which determines the evaporation rate of the fluid at the surface of the droplet and by the diffusion ratio of the components, which controls how fast the species are transported inside of the droplet. For the simulation of the multi-component evaporation the latter model from seems to be suitable for distillation type behaviour. For the diffusion limited case a single component evaporation model seems to be the better choice. Wall interaction of liquid droplets can play a major role for diesel and gasoline engines. Especially for small-bore diesel engines the distance between the injector and the bowl can be very small, so that large parts of the fuel are not yet evaporated or atomised when they hit the wall. This influences the combustion process and consequently the production of emissions, as an incomplete combustion in the vicinity of the wall will result in high HC emissions and soot particles. In the case of DI gasoline engines the wall interaction is most important for wall guided mixture formation concepts, since the wall has to redirect the fuel vapour cloud to the spark plug location. The standard wall interaction model used in the FIRE code was developed for engine applications by Naber and Reitz. This model combines the rebound and the wall jet mode but neglects splashing. The model yields good results when the droplet velocities are not too high at the wall impact and the spray is fully developed. However, in the case of short impingement distances and high impact velocities the radial penetration is usually too long and the spray height is under-predicted. The reason for this is probably that the model does not take into account droplet break up and splashing. Thus more sophisticated wall interaction models, like the one from Bai and Gosman, Mundo and Sommerfeld as well as Amsden and ORourke have been implemented. These models distinguish between dry and wet walls and in the case of splashing they determine the amount of mass that stays in the film and the one that is ripped out by the impingement process. For the calculation of the secondary droplet sizes, velocities and directions mass and energy conservation principles combined with empirical correlations are applied. The final validation and fine-tuning of the models by comparison to the measurements of the other partners of the DIME project has been done for the evaporation models using databases from the literature as well as from the project (hot spray rig data from CMT-UPV). The wall interaction models have been tested using data from the literature. The data prepared within the project are now available and will be used in further validation work. During the implementation of the models special care has been taken to assure numerical stability under all conditions. This resulted in stability far superior to the one in FIRE v7. The improved capabilities of FIRE v8 compared to FIRE v7 will serve a number of automotive manufactures in their development process of more efficient engines that produce lower emissions.
In Phase Doppler Anemometry, the measurement of the particle diameter is directly a function of the value of the index of refraction. The measurement of the value of the index of refraction is easy only when the product (liquid) under study is at room temperature and stable. In other cases, the value of the refractive index is dependent on the temperature and composition, as in spray evaporation. We prove (analytically and numerically) that the value of the refractive index could be extracted from the phase recorded by commercial Dual Mode PDA with accuracy on the second digit. A post processing software has been written that automatically extract the refractive index value from a Dual Mode PDA phase measurements, in addition of velocity and size, without extra hardware or adjustment. The accuracy has been experimentally validated on calibration sprays.
The impact of fuel sprays on solid surfaces inside internal combustion engines is a common process during fuel injection. The surfaces are usually curved and have an elevated temperature. It is important for the fuels spray development to know to what extent and in what sense the spray-surface interaction influences the spray characteristics - e.g. the drop size spectra. This result has provided a methodology to quantify the influence of splashing, i.e. of disintegration of liquid droplets upon impact on the surface, on the spray by evaluating the splashing potential of droplets as a function of their Ohnesorge number and their Reynolds number, the latter formed with the velocity component normal to the solid surface. The splash PDF derived from this work may be implemented in CFD codes of the partners, e.g. FIRE, for estimating the importance of splashing in different regions of the solid surface interacting with the spray. This methodology enables a higher degree of detail in the simulation of fuel injection and therefore a higher certainty in the prediction of fuel spray behaviour inside internal combustion engines. The economic benefit resulting from this improvement is difficult to estimate, and much of the benefit may be a competitive advantage on the market, which depends equally on product quality as on customer behaviour.
A methodology to obtain liquid and vapour phases' distribution in Diesel sprays in engine representative conditions using the Laser-Induced Fluorescence (LIF) technique. Among LIF techniques, the Exciplex technique has been selected because it allows simultaneous imaging of both liquid and vapour phases thanks to the spectral separation between liquid and vapor fluorescence spectra. The technique developed makes use of two tracers, mixed with a reference fuel. A first part of the work has focused on the refinement of tracers' proportions in order to separate in the best possible way emission spectra of liquid and vapour phases. Software has been developed for simultaneous grabbing images coming from two intensified cameras. Image post-processing allows for extraction of liquid and vapour phase contours. The technique has shown its capability to obtain qualitative analysis of spray behaviour. Fuel concentration quantification has shown to be possible in the pure liquid or pure vapour phases, but not in the regions where liquid and vapour phases coexist. For quantification of fuel concentration in either liquid or vapour phase, a theoretical model of the temperature field in the spray is used, as well as some correlations available in the literature.
A complete methodology to obtain bi-dimensional fuel-air concentration fields in Diesel sprays without evaporation using the Planar Laser-Induced Fluorescence (PLIF) technique with multi-component commercial Diesel fuel. In fuel-air concentrations LIF measurements, dopants with known characteristics are usually added to reference non-fluorescent fuels. By such means, it is possible to know exactly the fluorescence parameters of the blend and its dependency on pressure and temperature, which can be especially important in close-to-engine conditions. The drawbacks of using reference fuels are that measurements are not performed under real conditions because of the change in the properties of the fuel itself: important effects such as cavity in the nozzle and changes in the penetration length and cone angle can depend on fuel properties. Also the fact that the measurements are related solely to the dopant concentration makes additional calculations necessary to obtain the fuel-air ratios. In our case, commercial fuels are used instead of seeding reference fuels, so that measurements can be performed under real conditions and they are directly related to fuel-air concentrations. The methodology developed considers all the correction and calibration procedures to perform accurate measurements on the basis of the images taken, which have been implemented into a purpose-made software.

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