Search Results

A visualization study using shadowgraphy was performed in an optically-accessible, cylindrical constant-volume combustion chamber to identify the mechanism of flow/flame interaction in spark-ignited, lean propane-air mixtures. The effect of the flow on flame initiation and propagation was examined by varying the pre-ignition mean flow and turbulence within a range typical of modern four-valve spark-ignition (SI) engines, as well as the spark plug orientation relative to the mean flow. The initial flame development was quantified in terms of 2-D images which provided information about the projected flame area and the displacement of the flame center as a function of flow conditions, time from the spark initiation and spark plug orientation. The results showed that high mean flow velocities and turbulence levels can shorten combustion duration in lean mixtures and that the positioning of the ground electrode can have an important effect on the initial kernel formation.

The ability of certain induction systems to enhance turbulence levels at the time of ignition, through formation of long-lived tumbling vortices on the plane of the valve and cylinder axes, has been investigated in a two-valve spark-ignition engine by rotating the intake port at 90° and 45° to the orientation of production directed ports. Detailed measurements of the three velocity components, obtained by laser velocimetry, revealed that the 90° port generated a pure tumble motion, with a maximum tumbling vortex ratio of 1.5 at 295°CA, zero swirl, and 42% turbulence enhancement relative to the standard configuration, while the 45° port gave rise to a combined tumble/swirl structure with a maximum tumbling vortex ratio of 0.5 at 285°CA, swirl ratio of 1.0 at TDC, and turbulence enhancement of 24%. The implications of the two types of flow structures for combustion are discussed.

Ensemble-averaged and in-cycle axial and swirl velocities have been measured by laser Doppler anemometry in the three-dimensional flow field of a four-stroke model engine motored at 200 rpm with a compression ratio of 6.7 and various cylinder head and piston geometries. The inlet configurations comprised an axisymmetric port with a shrouded valve and an off-centre port with two valve and swirl generating vane geometries. The piston configurations comprised flat, cylindrical and re-entrant axisymmetric piston-bowls. The results indicate that with the off-centre port a complex vortical flow pattern is generated during induction, which later either collapses in the absence of induction swirl or is transformed into a single rotating vortex in the transverse plane when swirl is present. The axisymmetric port with the shrouded valve gives rise to a double vortex structure and higher turbulence levels at TDC of compression compared to the off-centre port.

A combined two-phase CFD nozzle model and 1-D fuel injection system model is used to predict the flow development inside the discharge hole of a pressure-swirl atomizer connected to a common-rail based fuel injection system for DISI engines. The fuel injection model accounts for the transient pressure pulses developing inside the common-rail and the injector upstream of the nozzle tip and predicts the fuel injection rate through the nozzle. This is then used as input to a 3-D single-phase CFD model estimating the transient development of the swirl velocity inside the pressure-swirl atomizer, as a function of the geometric characteristics of nozzle.

A dense-particle Eulerian-Lagrangian stochastic methodology, able to resolve the dense spray formed at the nozzle exit has been applied to the simulation of evaporating diesel sprays. Local grid refinement at the area where the spray evolves allows use of cells having sizes from 0.6 down to 0.075mm. Mass, momentum and energy source terms between the two phases are spatially distributed to cells found within a distance from the droplet centre; this has allowed for grid-independent interaction between the Eulerian and the Lagrangian phases to be reached. Additionally, various models simulating the physical processes taking place during the development of sprays are considered. The cavitating nozzle flow is used to estimate the injection velocity of the liquid while its effect on the spray formation is considered through an atomisation model predicting the initial droplet size.

The numerical methodology used to predict the flow inside pressure-swirl atomizers used with gasoline direct injection engines and the subsequent spray development is presented. Validation of the two-phase CFD models used takes place against film thickness measurements obtained from high resolution CCD-based images taken inside the discharge hole of a pressure swirl atomizer modified to incorporate a transparent hole extension. The transient evolution of the film thickness and its mean axial and swirl velocity components as it emerges from the nozzle hole is then used as input to a spray CFD model predicting the development of both non-evaporating and evaporating sprays under a variety of back pressure and temperature conditions. Model predictions are compared with phase Doppler anemometry measurements of the temporal and spatial variation of the droplet size and velocity as well as CCD spray images.

A new simulation approach to the modeling of the whole fuel injection process within a common-rail fuel injection system for direct-injection gasoline engines, including the pressure-swirl atomizer and the conical hollow-cone spray formed at the nozzle exit, is presented. The flow development in the common-rail fuel injection system is simulated using an 1-D model which accounts for the wave dynamics within the system and predicts the actual injection pressure and injection rate throughout the nozzle. The details of the flow inside its various flow passages and the discharge hole of the pressure-swirl atomizer are investigated using a two-phase CFD model which calculates the location of the liquid-gas interface using the VOF method and estimates the transient formation of the liquid film developing on the walls of the discharge hole due to the centrifugal forces acting on the swirling fluid.

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 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 41bar.

The flow structure in a four-stroke model engine motored at 200 rpm with a compression ratio of 3.5 has been investigated. Ensemble-averaged axial and swirl mean and rms velocities have been obtained by laser-Doppler anemometry downstream of an axisymmetrically located single valve with 30 and 60 degree seat angles and various lifts, with and without induction swirl. In all cases, the intake-generated flow structure in the axial plane disappears by the time the inlet valve closes and results in nearly homogeneous turbulence during compression with levels of 0.5–0.7 times the mean piston speed. The swirling flow, however, which is induced by means of vanes, persists through the compression stroke, evolving from a spiralling motion early during intake into solid body type of rotation near TDC of compression, with associated swirl ratios increasing with valve lift.

Local measurements of the mean and rms velocities have been obtained by laser Doppler velocimetry in the coolant passages of a transparent model of a Ford 2.5L diesel cylinder head and block at a steady flowrate of 6.83 × 10-4 M3/s. The simulation of the coolant fluid by a mixture of hydrocarbon fluids at a predetermined constant temperature allowed accurate matching of the refractive index to that of the acrylic model, thus providing optical access for LDV measurements of the internal flow in sensitive areas where cooling is essential to prevent metal-fatigue failure. The results were obtained in sufficient detail to allow further validation of CFD coolant flow models.

The initiation and development of cavitation in enlarged transparent acrylic models of six-hole nozzles for direct injection Diesel engines has been visualised by a high-speed digital video camera in a purpose-built refractive index matching test rig. The obtained high temporal resolution images have allowed improved understanding of the origin of the cavitation structures in Diesel injector nozzles and clarification of the effect of sac geometry (conical mini-sac vs. VCO) on cavitation initiation and development in the nozzle holes. The link between cavitation and flow turbulence in the sac volume and, more importantly, in the injection holes has been quantified through measurements of the flow by laser Doppler velocimetry (LDV) at a number of planes as a function of the Reynolds and cavitation numbers.

An enlarged transparent model of a six-hole vertical diesel injector has been manufactured in order to allow flow measurements inside the sac volume and the injection holes to be obtained using a combination of laser Doppler velocimetry (LDV) and the refractive index matching technique under steady state conditions. The measurement points were concentrated in the sac volume close to the entrance of the injection holes as well as inside them on a vertical plane passing through the axis of two injection holes for two different needle lifts. The velocity flow field was characterized in terms of the mean velocity and the turbulent intensity. The results revealed that, under certain conditions, cavitation may occur in the recirculation zone formed at the entrance to the hole since the pressure in this region can reach the value of the vapor pressure of the flowing liquid; this was found to strongly depend on the needle lift and eccentricity.

A constant-volume combustion chamber was used to examine injection of a small quantity of slightly rich fuel/air mixture towards the spark plug around the time of ignition, in an overall very lean mixture rotating at velocities representative of modern spark-ignition engines. The results show that it is possible to achieve 100% ignitability with overall air-fuel ratios in excess of 50 and much faster burn rates than those with initially homogenous mixtures of the same equivalence ratio with high swirl and turbulence. The advantages of this method of local charge stratification have been demonstrated in terms of both pressure measurements and shadowgraphs of the early flame development while the transient characteristics of the injected rich mixture at the spark plug gap were monitored by a fast flame ionization detector.