Yuri M. Wright

Diesel engine simulations with multi-dimensional conditional moment closure

First-order elliptic Conditional Moment Closure (CMC), coupled with a computational fluid dynamics (CFD) solver, has been employed to simulate combustion in a direct-injection heavy-duty diesel engine. The three-dimensional structured finite difference CMC grid has been interfaced to an unstructured finite volume CFD mesh typical of engine modelling. The implementation of a moving CMC grid to reflect the changes in the domain due to the compression and expansion phases has been achieved using an algorithm for the cell addition/removal and modelling the additional convection term due to the CMC cell movement. Special care has been taken for the boundary conditions and the wall heat transfer. An operator splitting formulation has been used to integrate the CMC equations efficiently. A CMC domain reduction of the three-dimensional problem to two- and zero-dimensions through appropriate volume integration of the CMC equation has been explored in terms of accuracy and computational time. Additional considerations have been reported concerning the initialization of the CMC domain in conserved scalar space during transient calculations where the probability density function of the mixture fraction changes drastically with time as during fuel injection. Predictions compare favourably with the experimental pressure traces for tests at full and half load. A balance of terms in the CMC equation allows conjectures on the structure of the flame and its expansion across the spray after autoignition.

Soot Formation Modeling of n-Heptane Sprays Under Diesel Engine Conditions Using the Conditional Moment Closure Approach

Numerical simulations of soot formation of n-heptane autoigniting spray in a constant-volume vessel under diesel engine conditions with different ambient densities (14.8 and 30 kg/m3) and ambient oxygen concentrations (8–21% O2 mol fraction) were performed using two-dimensional, first-order conditional moment closure (CMC). Soot formation was modeled with a semiempirical two-equation model that considers simultaneous soot particle inception, surface growth, coagulation, and oxidation by O2 and OH. Soot radiation was accounted for with an optical-thin formulation. Results are compared to experimental data by means of ignition delay time, lift-off length, and soot volume fraction distribution. Good predictions of ignition delay and lift-off for all nine cases are achieved. High volume fraction soot location and semi-quantitative distribution have been well described even with this comparatively simple soot model. The findings suggest that the conditional moment closure approach is a promising framework for soot modeling under diesel engine conditions.

Influence of turbulence-chemistry interaction for n-heptane spray combustion under diesel engine conditions with emphasis on soot formation and oxidation

The influence of the turbulence–chemistry interaction (TCI) for n-heptane sprays under diesel engine conditions has been investigated by means of computational fluid dynamics (CFD) simulations. The conditional moment closure approach, which has been previously validated thoroughly for such flows, and the homogeneous reactor (i.e. no turbulent combustion model) approach have been compared, in view of the recent resurgence of the latter approaches for diesel engine CFD. Experimental data available from a constant-volume combustion chamber have been used for model validation purposes for a broad range of conditions including variations in ambient oxygen (8‑21% by vol.), ambient temperature (900 and 1000 K) and ambient density (14.8 and 30 kg/m3). The results from both numerical approaches have been compared to the experimental values of ignition delay (ID), flame lift-off length (LOL), and soot volume fraction distributions. TCI was found to have a weak influence on ignition delay for the conditions simulated, attributed to the low values of the scalar dissipation relative to the critical value above which auto-ignition does not occur. In contrast, the flame LOL was considerably affected, in particular at low oxygen concentrations. Quasi-steady soot formation was similar; however, pronounced differences in soot oxidation behaviour are reported. The differences were further emphasised for a case with short injection duration: in such conditions, TCI was found to play a major role concerning the soot oxidation behaviour because of the importance of soot-oxidiser structure in mixture fraction space. Neglecting TCI leads to a strong over-estimation of soot oxidation after the end of injection. The results suggest that for some engines, and for some phenomena, the neglect of turbulent fluctuations may lead to predictions of acceptable engineering accuracy, but that a proper turbulent combustion model is needed for more reliable results.

Direct numerical simulation of multiple cycles in a valve/piston assembly

The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element solver, adapted for moving geometries using an Arbitrary Lagrange/Eulerian formulation, was employed. Eight cycles were simulated and the ensemble- and azimuthally-averaged data were found to be in good agreement with experimentally determined means and fluctuations at all measured points and times. During the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed a large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a dominant effect on the jet breakup and the vortex ring dynamics below the valve and on the observed significant cyclic variations. Based on statistical averaging, the evolut...

Experimental Study of Ignition and Combustion Characteristics of a Diesel Pilot Spray in a Lean Premixed Methane/Air Charge using a Rapid Compression Expansion Machine

The behaviour of spray auto-ignition and combustion of a diesel spray in a lean premixed methane/air charge was investigated. A rapid compression expansion machine with a free floating piston was employed to reach engine relevant conditions at start of injection of the micro diesel pilot. The methane content in the lean ambient gas mixture was varied by injecting different amounts of methane directly into the combustion chamber, the ambient equivalence ratio for the methane content ranged from 0.0 (pure air) to 0.65. Two different nozzle tips with three and six orifices were employed. The amount of pilot fuel injected ranged between 0.8 and 1.8 percent of the total energy in the combustion chamber. Filtered OH chemiluminescence images of the combustion were taken with a UV intensified high speed camera through the optical access in the piston. Filtered photomultiplier signals of the total emitted light for OH, CH and C2 radicals as well as the pressure signal were simultaneously recorded and the effect of methane content and nozzle geometry on ignition locations, ignition timing and combustion behavior was analyzed. It was seen, that higher values of the equivalence ratio promote propagation of the premixed flame as expected, but delay the auto-ignition of the pilot spray considerably. Furthermore, it was observed that even for very low equivalence ratios, substantial heat release occurs around the spray in the premixed charge, although no flame propagation in the classic sense can be sustained. For the whole range of equivalence ratios, the use of the six hole injector was seen to considerably assist conversion of the entire premixed charge.

Multi-dimensional Conditional Moment Closure Modelling Applied to a Heavy-duty Common-rail Diesel Engine

A multi-dimensional combustion code implementing the Conditional Moment Closure turbulent combustion model interfaced with a well-established RANS two- phase flow field solver has been employed to study a broad range of operating conditions for a heavy duty direct-injection common-rail Diesel engine. These conditions include different loads (25%, 50%, 75% and full load) and engine speeds (1250 and 1830 RPM) and, with respect to the fuel path, different injection timings and rail pressures. A total of nine cases have been simulated. Excellent agreement with experimental data has been found for the pressure traces and the heat release rates, without adjusting any model constants. The chemical mechanism used contains a detailed NOx sub-mechanism. The predicted emissions agree reasonably well with the experimental data considering the range of operating points and given no adjustments of any rate constants have been employed. In an effort to identify CPU cost reduction potential, various dimensionality reduction strategies have been assessed. Furthermore, the sensitivity of the predictions with respect to resolution in particular relating to the CMC grid has been investigated. Overall, the results suggest that the presented modelling strategy has considerable predictive capability concerning Diesel engine combustion without requiring model constant calibration based on experimental data. This is true particularly for the heat release rates predictions and, to a lesser extent, for NOx emissions where further progress is still necessary.

Simulations of Diesel Sprays Using the Conditional Moment Closure Model

Numerical simulations of diesel sprays in a constant-volume vessel have been performed with the conditional moment closure (CMC) combustion model for a broad range of conditions. On the oxidizer side these include variations in ambient temperature (800-1100 K), oxygen volume fraction (15-21 %) and density (7.3-58.5 kg/m) and on the fuel side variation in injector orifice diameter (50-363 μm) and fuel pressure (600-1900 bar); in total 22 conditions. Results are compared to experimental data by means of ignition delay and flame lift-off length (LOL). Good agreement for both quantities is reported for the vast majority of conditions without any changes to model constants: the variations relating to the air side are quantitatively accurately predicted; for the fuel side (viz. orifice diameter and injection pressure) the trends are qualitatively well reproduced. For the reference case, three different n-heptane chemical mechanisms (with 22, 29 and 67 species) have further been compared with respect to the ignition process and the subsequent flame stabilization and the flame structure is compared to conceptual models presented in the literature. At those conditions all three mechanisms showed comparable results. Based on the findings reported, CMC is seen as a highly promising approach to model spray combustion for a very broad range of diesel engine relevant conditions.

Predicting In-Cylinder Soot in a Heavy-Duty Diesel Engine for Variations in SOI and TDC Temperature Using the Conditional Moment Closure Model

Numerical simulations of in-cylinder soot evolution in the optically accessible heavy-duty diesel engine of Sandia National Laboratories have been performed with the multidimensional conditional moment closure (CMC) model using a reduced n-heptane chemical mechanism coupled with a two-equation soot model. Simulation results are compared to the high-fidelity experimental data by means of pressure traces, apparent heat release rate (AHRR) and time-resolved in-cylinder soot mass derived from optical soot luminosity and multiple wavelength pyrometry in conjunction with high speed soot cloud imaging. In addition, spatial distributions of soot relevant quantities are given for several operating conditions.

Investigation of wall heat transfer and thermal stratification under engine-relevant conditions using DNS

Unsteady wall heat transfer and thermal stratification during the compression stroke under engine relevant conditions are investigated using direct numerical simulations (DNS). In order to avoid artificial initial and boundary conditions the initial conditions are obtained from a separate DNS of the intake stroke involving thermal and composition mixing. The dynamically changing thermodynamic properties were found to strongly affect turbulence and wall heat transfer during the compression stroke. The increasing pressure results in a strongly reduced kinematic viscosity, and thus in significantly reduced length scales in the flow and temperature fields towards the top dead center (TDC). This has a direct impact on wall heat transfer, since reduced length scales lead to increased temperature gradients at the walls. Hence the heat transfer coefficient, which expresses the hydrodynamic influence on the heat transfer, increases by a factor of approximately five during compression. For the simulated conditions, the heat transfer coefficient extracted from the DNS data is found to agree reasonably well with the global correlation by Hohenberg but deviates strongly from the Woschni correlation. The influence of the boundary layers is not limited to the region close to walls, since close to TDC it affects the temperature distribution in the cylinder core. Vortical structures are identified, which transport cold gases from the boundary layer into the inner cylinder indicating that the assumption of an isentropic core temperature in the inner cylinder is not valid.

Evaporating and non-evaporating diesel spray simulation : comparison between the ETAB and wave breakup model

Evaporating and non-evaporating diesel sprays have been simulated using the Enhanced Taylor Analogy Breakup (ETAB) and the wave atomisation and breakup models implemented in STAR-CD and compared with experiments. The model validation was carried out by means of experimental data of sprays under controlled conditions in a constant-volume bomb. The model constants were calibrated for a specific test case - low injection pressure, non-evaporating conditions and a small nozzle diameter - and subsequently their robustness with respect to changes in boundary conditions was assessed for different injection pressures, evaporating conditions and larger nozzle diameters. The results suggest that, for suitable mesh resolutions with calibrated model constants, excellent agreement with experimental data can be achieved with both models. With these calibrated model parameters held constant, accurate predictions for spray penetrations, droplet diameters and spray angles could be obtained also for different injection configurations and air conditions by both models.