Search Results

The structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole perpendicular to the nozzle axis has been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. Observations revealed that, at the inception stage, cavitation bubbles are dominantly seen in the vortices at the boundary layer shear flow and outside the separation zone. Cavitation bubbles grow intensively in the shear layer and develop into cloud-like coherent structures when viewed from the side of the nozzle. Shedding of the coherent cloud cavitation was observed. When the flow was increased further the cloud like cavitation bubbles developed into a large-scale coherent structure extending downstream of the hole. Under this condition the cavitation starts as a mainly glassy sheet at the entrance of the hole. Until this stage the spray appeared to be symmetric.

Simulations of a Direct Injection Spark Ignition (DISI) engine have been performed for both early injection with homogeneous charge combustion and for late injection with stratified charge combustion. The purpose has been to study flow characteristics, fuel/air mixing, combustion, and NOx and soot formation. Focus is put on the combustion modeling. Two different full load cases with early injection are simulated, 2000 rpm and 6000 rpm. One load point with late injection is simulated, 2000 rpm and 2.8 bar net MEP. Three different injection timings are simulated at the low load point: 77, 82, and 87 CAD bTDC. The spray simulations are tuned to match measured spray penetrations and droplet size distributions at both atmospheric and elevated pressure. Boundary conditions for the engine simulations are taken from 1-D gas exchange simulations that are tuned to match engine tests.

An expanding cylindrical laminar flame kernel affected by random external strain rates and diffusivity is numerically simulated in order to gain insight into the influence of small-scale turbulence on the combustion variability in engines. In the simulations, the kernel is strained, as a whole, by external velocity gradients randomly generated with either Gaussian or log-normal probability density functions. The influence of small-scale turbulent heat and mass transfer is modeled by turbulent diffusivity, the randomness of which is controlled by the fluctuations in the viscous dissipation averaged over the kernel volume. The computed results show that small-scale phenomena can substantially affect the quenching characteristics of a small flame kernel and the kernel growth history rj(t); the scatter of the computed curves of rf(t) being mainly controlled by the scatter of the duration of the initial stage of kernel development.

This work presents a numerical method for predictions of HC oxidation in the cold turbulent wall jet emerging from the piston top land crevice in an S.I. engine, using a complex chemical reaction model. The method has been applied to an engine model geometry with the aim to predict the HC oxidation rate under engine - relevant conditions. According to the simulation a large amount of HC survives oxidation due to the long ignition delay of the wall jet emitted from the crevice. This ignition delay, in turn depends mainly on chemical composition and temperature of the gas mixture in the crevice and also on the temperature distribution in the cylinder boundary layer.

A complex chemistry model of reduced size (65 species and 268 reaction steps) derived on the basis of n-heptane auto-ignition kinetics, low hydrocarbon oxidation chemistry, poly-aromatic hydrocarbon (PAH) and NOx formation kinetics together with a phenomenological soot model have been integrated with the KIVA code for multidimensional diesel simulations. A partially stirred reactor model is used to handle the turbulence-chemistry interaction. The results obtained from numerical simulations for a direct-injection (DI) diesel spray, which is injected into a constant-volume combustion vessel at engine-like conditions, show that the approach is able to reproduce the transient diesel auto-ignition and combustion processes as observed in many optical imaging studies. The simulated results indicate that the auto-ignition of DI diesel spray occurs at a lean site close to the mean stoichiometric line for the cases tested.

In the first part of the paper, a generalization of the turbulent diffusivity concept is considered and a generalized diffusion coefficient is introduced to account for the development of turbulent diffusivity, pressure-driven countergradient transport, and effects of chemical reactions on turbulent scalar flux. The behavior of the generalized diffusivity is numerically studied in the 1-D statistically planar case and the contributions of the aforementioned processes to the diffusivity are assessed. In the second part of the paper, the generalized diffusivity is incorporated into the Flame Speed Closure (FSC) model of premixed turbulent combustion and the extended FSC model is applied to simulate recent experiments performed using the Leeds fan-stirred bomb. The extended FSC model well predicts the speed, thickness and structure of statistically spherical, premixed, turbulent flames that expand in the bomb after spark ignition.

Lean gas engines have a potential for a significant reduction in both fuel consumption and emission levels. The use of a small pre-chamber with controlled stoichiometric or rich mixture composition is an effective way to deal with ignition problems in such engines. A constant volume vessel equipped with a device for generation of turbulence of known quantities is used to study the operation of a cylindrical pre-chamber of 1% of the main chamber volume. Pressure was measured in the main chamber and Schlieren images of the flame initiation and propagation in the main chamber were recorded for all set-ups. The investigation of the pre-chamber is focused on the position of the spark within the pre-chamber. Spark locations close to the orifice and close to the opposite wall as well as in the middle of the pre-chamber were tested and flame evolution and pressure history were studied.

A model of premixed turbulent combustion is modified for multi-dimensional computations of SI engines. This approach is based on the use of turbulent flame speed in order to suggest a closed balance equation for the mean combustion progress variable. The model includes a single unknown input parameter to be tuned. This model is tested against two sets of experimental data obtained by Bradley et al [17, 18 and 19] and Karpov and Severin [15] in fan-stirred bombs. The model quantitatively predicts the development of the turbulent flame speed, the effects of the initial pressure, temperature, and mixture composition on the turbulent flame speed, and the effects of r.m.s. turbulent velocity and burning mixture composition on the rate of the pressure rise. These results were computed with the same value of the aforementioned unknown input parameter of the model.

In recent years, a free, open source CFD software package called OpenFOAM has been attracting increasing amounts of attention as a promising, inexpensive, and efficient CFD tool for the numerical simulation of processes such as fuel injection and evaporation, turbulent mixing and burning. Here, we describe the further development of OpenFOAM to enable its use in simulating stratified turbulent combustion in DI SI engines. Advanced models of various phenomena relevant to partially premixed turbulent flames were implemented into the code, and the effects of these implementations were investigated by performing unsteady 3D RANS simulations of stratified turbulent burning in a DI SI engine. First, the Flame Speed Closure (FSC) model of premixed turbulent combustion was implemented. Second, a method for evaluating the mean density in premixed turbulent flames that is available in the standard OpenFOAM library was improved.

Over the past years, the so-called Flame Speed Closure (FSC) model was shown to be a very promising tool for multi-dimensional simulations of premixed turbulent combustion in internal combustion and gas turbine engines. The laboratory tests and industrial applications of the model have been mainly limited to moderately turbulent flames. In the paper, three alternative versions of the FSC model, which yield different results at weak turbulence but similar results at moderate one, are discussed and numerically tested against recent experimental data reported by the Leeds [27,34] and Rouen [28] groups for expanding, statistically spherical, premixed, weakly turbulent flames. The computed and measured data on the mean combustion progress variable profiles, mean flame brush thickness development, and observed flame speeds are compared in order to assess and rank the submodels discussed.

Traditionally, the knocking cylinder pressure trace has been characterized by an instant jump followed by a steadily decaying fluctuation. We found many cases where an increase in fluctuation amplitude in time could be observed. Thus, a coherent energy release triggered by the pressure wave typical for the initiation of knock was discovered. Possible mechanisms for the explanation of this phenomenon are discussed: First, the combined pressure and temperature effects on the flame propagation rate in the end-gas, second, a mechanism based on turbulence augmentation by compression. Third, a mechanism of acoustic or shock wave induced flame instability and fourth, a crevice based mechanism. It is shown that only the crevice mechanism is feasible under engine conditions. It is postulated that the very frequent “weak knock” is due to this phenomenon. Experimental evidence is presented for the existence of this new knock mechanism.

Fan-stirred bombs, which are widely used worldwide, offer an unique opportunity to investigate basic features of S.I. engine combustion under well-defined experimental conditions. Extensive data bases on turbulent flame speeds have been generated by various groups utilizing such bombs. However, the use of these data bases is impeded by the fact that the measured flame speeds characterize an inherently transient process, i.e.. the speeds are time-dependent even if the pressure and the unburned gas temperature in the bomb are very close to the initial values; whereas the combustion theory and various models deal commonly with an asymptotically fully developed turbulent flame speed. The goal of this work is to test a method for evaluating the latter quantity by processing the published data on the flame radius growth, measured in expanding, statistically spherical, premixed flames.