F. V. Bracco

Mechanism of atomization of a liquid jet

In the atomization regime of a round liquid jet, a diverging spray is observed immediately at the nozzle exit. The mechanism that controls atomization has not yet been determined even though several have been proposed. Experiments are reported with constant liquid pressures from 500 psia (33 atm) to 2500 psia (166 atm) with five different mixtures of water and glycerol into nitrogen, helium, and xenon with gas pressures up to 600 psia (40 atm) at room temperature. Fourteen nozzles were used with length‐to‐diameter ratios ranging from 85 to 0.5 with sharp and rounded inlets, each with an exit diameter of about 340 μm. An evaluation of proposed jet atomization theories shows that aerodynamic effects, liquid turbulence, jet velocity profile rearrangement effects, and liquid supply pressure oscillations each cannot alone explain the experimental results. However, a mechanism that combines liquid–gas aerodynamic interaction with nozzle geometry effects would be compatible with our measurements but the specific...

Comparisons of computed and measured premixed charge engine combustion

Abstract Comparisons are presented of computed and measured cylinder pressure in a reciprocating engine with a pancake combustion chamber and premixed propane/air charges. Engine operating conditions range over volumetric efficiency of 30–60%; equivalence ratio of 0.87–1.1; and rpm of 1000–1500. The computations start from the actual spark times and simulate the growth of the flame kernel into a fully developed turbulent flame by taking into account the increasing influence of turbulent eddies on the growing flame kernel. A k-ϵ submodel is used for turbulence. The species conversion submodel assumes that the species (C3H8, O2, H2O, CO2, CO, H2, and N2) concentrations approach their local thermodynamic equilibrium values with a characteristic conversion time that is a combination of a turbulent mixing time and a chemical conversion time in laminar propaneair flames. In all cases computed and measured cylinder pressure agree well in trends and magnitudes during the entire duration of combustion. The difference in magnitudes generally is much less than 8%. The main conclusion is that laminar flame processes must be explicitly accounted for in order to reproduce certain elements of premixed charge engine combustion.

Stochastic particle dispersion modeling and the tracer‐particle limit

Simulations of spray and particle‐laden flows have commonly relied on random walk models to represent dispersion of liquid or solid particles by turbulent motions in the carrier fluid. Particles respond, through a Lagrangian equation of motion, to the mean fluid velocity, computed simultaneously from an Eulerian solution, and to a random fluctuation velocity. In this paper the performance of available models for the case of tracer particles in dilute concentrations is tested. It is demonstrated that several models in wide use do not preserve the required divergence properties of the imposed mean flow. It is found that the method of sampling the fluctuation velocity in these models leads to a spurious component of mean velocity, causing particles to drift relative to the mean flow. Particles concentrate where the turbulence intensity is minimum (at shear layer edges), and are depleted from regions of high turbulence intensity (near the core of the shear layers). The particle concentration may be up more th...

Fractals and turbulent premixed engine flames

Abstract The fractal nature of premixed turbulent flames in an internal combustion engine is examined. A sheet of laser light, approximately 200 μm thick, is shone through the cylinder of a single-cylinder ported internal combustion engine. The homogeneous charge of propane and air is seeded with submicron TiO 2 particles and the scattered light is collected through a quartz window in the engine head and is imaged on a 100 × 100 diode array camera. The number density of the TiO 2 particles scales with the gas density so that a two-dimensional map of reactants and products is obtained. A field of view 2 × 2 cm in the center of the cylinder is examined and fractal analysis is performed on the front separating reactants from products. Results are presented for two equivalence ratios and three engine speeds, corresponding to different laminar flame speeds S 1 , laminar flame thicknesses δ 1 , turbulent intensities u ′, and Kolmogorov scales η. The examined flames were found to exhibit fractal character within a range of length scales as low as 200 μm and as high as 4.5 mm, which is twice the measured lateral integral length scale in this engine configuration. At stoichiometric conditions, the fractal dimension of the flame surface is found to be statistically different for engine speeds of 300, 1200, and 2400 rpm ( u′ S 1 = 0.5, 2, and 4 ). It increases with increasing u′ S 1 . At lean conditions (Φ = 0.59), when u′ S 1 ⪢ 1 , the fractal dimension does not change with engine speed. For 4 ≤ u′ S 1 and 0.1 η δ 1 ≤ 1 (estimated ranges), the fractal dimension is 2.36 ± 3%. A turbulent flame speed model based on fractal analysis is also briefly examined.

On the intact core of full-cone sprays

A voltage was applied between the nozzle unit and fine needles, rods, and screens inserted at various axial and radial positions into atomizing full-cone water sprays and the corresponding electrical resistance was measured in an attempt to determine the shape and length of the intact liquid core. The parameters of the experiment were: room temperature; air compressed at 0.1, 1.0, and 2.9 MPa; injection ..delta..p = 13.7 MPa; and five straight-hole nozzles with diameters of 127, 178, 305, 343, and 508 ..mu..m, and the same length-to-diameter ratio of 4. The results show that current is carried not only by intact liquid cores but also by atomized unconnected sprays and even across such sprays. Thus the shape of the intact core could be deduced only in the vicinity of the nozzle exit. In the atomization regime, the length of the intact core is found to be proportional to the nozzle diameter and to increase as the square root of the liquid-to-gas density ratio, i.e. x/sub 1/=Rhod(p/sub lpg/)/sup 12/ where Rhoapprox. =7.

A DISCUSSION OF TURBULENT FLAME STRUCTURE IN PREMIXED CHARGES

Propagation of turbulent flames in spark-ignition engines is considered from the viewpoint of the different possible regimes of premixed turbulent combustion. Nondimensional parameters defining known combustion regimes are reviewed, and numerical values of these parameters are estimated for both research and production engines. The reaction-sheet regime is inferred to apply at least for some operating conditions, and therefore literature on turbulent flame propagation in the reaction-sheet regime is reviewed. Implications of these results on interpretations of existing experimental observations of combustion in engine cylinders and on modeling of turbulent flame propagation in engines are discussed.

Measurements of drop size at the spray edge near the nozzle in atomizing liquid jets

The drop size distribution was measured from back‐lighted spark photographs at the edge of steady sprays in the immediate vicinity of the nozzle exit. The conditions of these liquid‐into‐gas sprays were such that the outer surface of the liquid jets broke up into small drops at the nozzle exit. The objective was to elucidate the mechanism of breakup. At room temperature, n‐hexane and n‐tetradecane at pressures from 2.86 to 9.76 MPa were injected into gaseous nitrogen at 1.48 to 2.86 MPa through three straight cylindrical nozzles of different diameters, 127 and 335 μm, and length‐to‐diameter ratios, 4 and 10. In all cases, the drop sizes could be fitted satisfactorily with a chi‐square distribution with degree of freedom equal to 28. The Sauter mean drop diameter and other average diameters were found to decrease with increasing injection velocity and decreasing liquid surface tension, to be insensitive to nozzle diameter and length, and to increase slightly with increasing gas density. The trends and magn...

A study of velocities and turbulence intensities measured in firing and motored engines

Laser Doppler velocimetry was used to make cycle-resolved velocity and turbulence measurements under motoring and firing conditions in a ported homogeneous charge S.I. engine. The engine had a flat pancake chamber with a compression ratio of 7.5. In one study, the effect of the intake velocity on TDC turbulence intensity was measured at 600, 1200, and 1800 rpm with three different intake flow rates at each speed. The TDC swirl ratio ranged from 2 to 6. The TDC turbulence intensities were found to be relatively insensitive to the intake velocity, and tended to scale more strongly with engine speed. For the combustion measurements, the engine was operated at 600, 1200, and 2400 rpm on stoichiometric and lean propane-air mixtures. Velocity measurements were made in swirling and non-swirling flows at several spatial locations on the midplane of the clearance height.

Cycle-Resolved LDV Integral Length Scale Measurements In an I.C. Engine

Lateral integral length scales of the tangential velocity component were measured directly using a two-point, single probe-volume, Laser Doppler Velocimetry system in a motored, ported, single-cylinder I.C. engine with a pancake-shaped chamber

Three-dimensional computations of flows in a stratified-charge rotary engine

The first three-dimensional rotary-engine computations are reported of exhaust, intake (with side and peripheral ports, and with different intake turbulence intensities and length scales), compression, homogeneous-charge combustion, dual liquid fuel injection, and dual liquid fuel injection and combustion. The model includes a k-epsilon submodel for turbulence, a stochastic treatment of the fuel drops and a hybrid laminar and mixing-controlled submodel for the conversion of reactant to products. The code is an extensively modified version of KIVA. The latter was developed at the Los Alamos National Laboratory for reciprocating engines. The modifications include: dynamic rezoning of the grid in both the x-y and the x-z planes; adoption of a cartesian coordinate system fixed to the housing with analytical grid generation and grid velocities related to the rotor velocity; inclusion of radial and tangential ports for both intake and exhaust.