Manolis Gavaises

Vortex flow and cavitation in diesel injector nozzles

Flow visualization as well as three-dimensional cavitating flow simulations have been employed for characterizing the formation of cavitation inside transparent replicas of fuel injector valves used in low-speed two-stroke diesel engines. The designs tested have incorporated five-hole nozzles with cylindrical as well as tapered holes operating at different fixed needle lift positions. High-speed images have revealed the formation of an unsteady vapour structure upstream of the injection holes inside the nozzle volume, which is referred to as ‘string-cavitation’. Computation of the flow distribution and combination with three-dimensional reconstruction of the location of the strings inside the nozzle volume has revealed that strings are found at the core of recirculation zones; they originate either from pre-existing cavitation sites forming at sharp corners inside the nozzle where the pressure falls below the vapour pressure of the flowing liquid, or even from suction of outside air downstream of the hole exit. Processing of the acquired images has allowed estimation of the mean location and probability of appearance of the cavitating strings in the three-dimensional space as a function of needle lift, cavitation and Reynolds number. The frequency of appearance of the strings has been correlated with the Strouhal number of the vortices developing inside the sac volume; the latter has been found to be a function of needle lift and hole shape. The presence of strings has significantly affected the flow conditions at the nozzle exit, influencing the injected spray. The cavitation structures formed inside the injection holes are significantly altered by the presence of cavitation strings and are jointly responsible for up to 10% variation in the instantaneous fuel injection quantity. Extrapolation using model predictions for real-size injectors operating at realistic injection pressures indicates that cavitation strings are expected to appear within the time scales of typical injection events, implying significant hole-to-hole and cycle-to-cycle variations during the corresponding spray development.

Modelling of cavitation in diesel injector nozzles

A computational fluid dynamics cavitation model based on the Eulerian–Lagrangian approach and suitable for hole-type diesel injector nozzles is presented and discussed. The model accounts for a number of primary physical processes pertinent to cavitation bubbles, which are integrated into the stochastic framework of the model. Its predictive capability has been assessed through comparison of the calculated onset and development of cavitation inside diesel nozzle holes against experimental data obtained in real-size and enlarged models of single- and multi-hole nozzles. For the real-size nozzle geometry, high-speed cavitation images obtained under realistic injection pressures are compared against model predictions, whereas for the large-scale nozzle, validation data include images from a charge-coupled device (CCD) camera, computed tomography (CT) measurements of the liquid volume fraction and laser Doppler velocimetry (LDV) measurements of the liquid mean and root mean square (r.m.s.) velocities at different cavitation numbers (CN) and two needle lifts, corresponding to different cavitation regimes inside the injection hole. Overall, and on the basis of this validation exercise, it can be argued that cavitation modelling has reached a stage of maturity, where it can usefully identify many of the cavitation structures present in internal nozzle flows and their dependence on nozzle design and flow conditions.

Evaluation of the Predictive Capability of Diesel Nozzle Cavitation Models

ABSTRACT The predictive capability of Lagrangian and Eulerian multi-dimensional computational fluid dynamics models accounting for the onset and development of cavitation inside Diesel nozzle holes is assessed against experimental data. These include cavitation images available from a real-size six-hole mini-sac nozzle incorporating a transparent window as well as high-speed/CCD images and LDV measurements of the liquid velocity inside an identical large-scale fully transparent nozzle replica. Results are available for different cavitation numbers, which correspond to different cavitation regimes forming inside the injection hole. Discharge coefficient measurements for various real-size nozzles operating under realistic injection pressures are also compared and match well with models’ predictions. The calculations performed have indicated that the two Eulerian models predict a large void zone inside the injection hole and fail to capture the transition from incipient to fully developed cavitation, while the Lagrangian model predicts a more diffused and gradual vapour distribution in agreement with the experimental data. However, all models have predicted similarly the velocity increase inside the injection hole caused by the presence of vapour, and a similar reduction in the nozzle discharge coefficient. Liquid turbulence was significantly underestimated by the Eulerian models in the cavitation zone showing decreasing trends in contradiction with experimental observations while this was better simulated by the Lagrangian model. Following the comparison with experiment, the effect of cavitation model assumptions, numerical implementation, discretisation scheme, model of turbulence grid resolution and cavitation physical sub-models on the predicted results on the nozzle discharge coefficient and hole exit %blockage are evaluated. The latter includes the bubble break-up and its internal initialisation pressure, the influence of the proximity of solid boundaries on the Rayleigh-Plesset equation and the percentage nuclei volume present in the liquid. Overall, it has been found that cavitation modelling has matured to the level where it can usefully identify many of the effects of cavitation on nozzle performance, and so significantly contribute to nozzle design and optimisation.

Spray Characteristics of a Multi-hole Injector for Direct-Injection Gasoline Engines

Abstract The sprays from a high-pressure multi-hole nozzle injected into a constant-volume chamber have been visualized and quantified in terms of droplet velocity and diameter with a two-component phase Doppler anemometry (PDA) system at injection pressures up to 200 bar and chamber pressures varying from atmospheric to 12 bar. The flow characteristics within the injection system were quantified by means of a fuel injection equipment (FIE) one-dimensional model, providing the injection rate and the injection velocity in the presence of hole cavitation, by an in-house three-dimensional computational fluid dynamics (CFD) model providing the detailed flow distribution for various combinations of nozzle hole configurations, and by a fuel atomization model giving estimates of the droplet size very near to the nozzle exit. The overall spray angle relative to the axis of the injector was found to be almost independent of injection and chamber pressure, a significant advantage relative to swirl pressure atomizers. Temporal droplet velocities were found to increase sharply at the start of injection and then to remain unchanged during the main part of injection, before decreasing rapidly towards the end of injection. The spatial droplet velocity profiles were jet-like at all axial locations, with the local velocity maximum found at the centre of the jet. Within the measured range, the effect of injection pressure on droplet size was rather small while the increase in chamber pressure from atmospheric to 12 bar resulted in much smaller droplet velocities, by up to four-fold, and larger droplet sizes by up to 40 per cent.

Cavitation Inside Multi-hole Injectors for Large Diesel Engines and Its Effect on the Near-nozzle Spray Structure

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Characterization of string cavitation in large-scale Diesel nozzles with tapered holes

The cavitation structures formed inside enlarged transparent replicas of tapered Diesel valve covered orifice nozzles have been characterized using high speed imaging visualization. Cavitation images obtained at fixed needle lift and flow rate conditions have revealed that although the conical shape of the converging tapered holes suppresses the formation of geometric cavitation, forming at the entry to the cylindrical injection hole, string cavitation has been found to prevail, particularly at low needle lifts. Computational fluid dynamics simulations have shown that cavitation strings appear in areas where large-scale vortices develop. The vortical structures are mainly formed upstream of the injection holes due to the nonuniform flow distribution and persist also inside them. Cavitation strings have been frequently observed to link adjacent holes while inspection of identical real-size injectors has revealed cavitation erosion sites in the area of string cavitation development. Image postprocessing has...

Evaluation of the Effect of Droplet Collisions on Spray Mixing

A spray model, implemented in a three-dimensional computational fluid dynamics (CFD) code has been used to evaluate the effect of droplet collisions on spray mixing resulting from the overlapping of liquid spray cones produced by two parallel hollow-cone nozzles under the influence of a cross-flow. The computations are compared with experimental results from phase Doppler anemometer (PDA) measurements in mixing steady sprays. The results show that the droplet collisions, which mainly occur in the mixing area of the two different sprays, have great influence on the droplet size and, as a consequence, on the predicted droplet velocities, especially at distances far from the spray nozzles. Information about the collision mechanisms as well as about droplet velocities and droplet dispersion due to collisions is also presented.

A new method of three‐dimensional coronary artery reconstruction from X‐ray angiography: Validation against a virtual phantom and multislice computed tomography

Objective: To develop and implement a method for three‐dimensional (3D) reconstruction of coronary arteries from conventional monoplane angiograms. Background: 3D reconstruction of conventional coronary angiograms is a promising imaging modality for both diagnostic and interventional purposes. Methods: Our method combines image enhancement, automatic edge detection, an iterative method to reconstruct the centerline of the artery and reconstruction of the diameter of the vessel by taking into consideration foreshortening effects. The X‐Ray‐based 3D coronary trees were compared against phantom data from a virtual arterial tree projected into two planes as well as computed tomography (CT)‐based coronary artery reconstructions in patients subjected to coronary angiography. Results: Comparison against the phantom arterial tree demonstrated perfect agreement with the developed algorithm. Visual comparison against the CT‐based reconstruction was performed in the 3D space, in terms of the direction angle along the centerline length of the left anterior descending and circumflex arteries relative to the main stem, and location and take‐off angle of sample bifurcation branches from the main coronary arteries. Only minimal differences were detected between the two methods. Inter‐ and intraobserver variability of our method was low (intra‐class correlation coefficients > 0.8). Conclusion: The developed method for coronary artery reconstruction from conventional angiography images provides the geometry of coronary arteries in the 3D space.

Effect of Multi-Injection Strategy on Cavitation Development in Diesel Injector Nozzle Holes

The effect of multiple-injection strategy on nozzle hole cavitation has been investigated both experimentally and numerically. A common-rail Diesel injection system, used by Toyota in passenger car engines, has been employed together with a double-shutter CCD camera in order to visualise cavitation inside a submerged and optically accessible (in one out of the six holes) real-size VCO nozzle. Initially the cavitation development was investigated in single injection events followed by flow images obtained during multiple injections consisting of a pilot and a main injection pulse. In order to identify the effect of pilot injection on cavitation development during the main injection, the dwell time between the injection events was varied between 1.5-5ms for different pilot injection quantities. The extensive test matrix included injection pressures of 400 and 800bar and back pressures ranging from 2.4 up to 41 bar. The results have confirmed that cavitation patterns in the pilot are very similar to those of the main injection, while the effect of the dwell time was present through the variation of the actual injection pressure caused by the pressure wave dynamics within the injection system. The flow inside the VCO nozzle was also simulated using a recently developed cavitation CFD model which takes into account the movement of the needle. Simulations have shown that cavitation inception is very fast and synchronous for both the pilot and the main injection events while its intensity in the sense of spatial extent appears to peak at relatively low needle lifts.

Flow in valve covered orifice nozzles with cylindrical and tapered holes and link to cavitation erosion and engine exhaust emissions

Abstract Results from a research programme addressing the development, testing, and production of valve covered orifice (VCO) nozzles operating with current production Tier 3 off-highway diesel engines are reviewed. The common rail injectors operate at pressures exceeding 1300 bar and include pilot and main injection events. Although acceptable engine exhaust emissions can be obtained with conventional VCO nozzles, cavitation erosion may lead to mechanical failure of the nozzle. Redesigning the injector in terms of its durability against surface erosion has been obtained through use of a computational fluid dynamics (CFD) flow solver incorporating a two-phase cavitation model and flow visualization in enlarged transparent nozzle replicas. The model has provided evidence of the flow distribution under realistic pressure and needle lift opening scenarios while at the same time it has been calibrated to indicate the locations where the possibility of cavitation erosion may become significant. The experiments performed in enlarged transparent nozzle replicas have provided evidence of the string cavitation structures formed inside the different nozzle designs. Cross-correlation with engine emission tests indicates that string cavitation may be associated with increased engine exhaust emissions. Proposed injector designs with geometric modification easily implemented in the production series have been proved to be erosion-free while at the same time have improved the engine exhaust emissions.