Maozhao Xie

Numerical investigation of soot reduction potentials with diesel homogeneous charge compression ignition combustion by an improved phenomenological soot model

Abstract An improved phenomenological soot model coupled with a reduced n-heptane chemical mechanism was implemented into KIVA-3V code to describe soot formation and oxidation processes in diesel homogeneous charge compression ignition (HCCI) combustion. This model was first validated by the shock tube experiments with a rich n-heptane mixture over wide temperature and pressure ranges. The computational results demonstrate that the phenomenological soot model is capable of predicting the soot yield, particle diameter, and number density with satisfactory accuracy. Then the model was applied to investigate the influence of the orifice diameter and injection pressure on soot emissions in a constant-volume combustion vessel under typical diesel combustion conditions. The predictions showed qualitative agreement with the measurements on the soot volume fraction distribution. The results also indicate that the soot formation can almost be suppressed as the local equivalence ratio is kept lower than 2.0. Finally, the model was used to explore the potentials of soot reduction with HCCI combustion for diesel engines. The overall trend of soot with the variations in the start of injection timing was well reproduced by the model. With the help of an equivalence ratio—temperature map, it was found that nitrogen oxide emissions could be markedly reduced by applying a high exhaust gas recirculation rate and relative low compression ratio for diesel HCCI engines. However, the mixture preparation by using a multi-hole injector with early injection strategy remains a limitation for further reduction in soot emissions.

Evaluation of Spray/Wall Interaction Models under the Conditions Related to Diesel HCCI Engines

Diesel homogeneous charge compression ignition (HCCI) engines with early injection can result in significant spray/wall impingement which seriously affects the fuel efficiency and emissions. In this paper, the spray/wall interaction models which are available in the literatures are reviewed, and the characteristics of modeling including spray impingement regime, splash threshold, mass fraction, size and velocity of the second droplets are summarized. Then three well developed spray/wall interaction models, ORourke and Amsden (OA) model, Bai and Gosman (BG) model and Han, Xu and Trigui (HXT) model, are implemented into KIVA-3V code, and validated by the experimental data from recent literatures under the conditions related to diesel HCCI engines. By comparing the spray pattern, droplet mass, size and velocity after the impingement, the thickness of the wall film and vapor distribution with the experimental data, the performance of these three models are evaluated. The results indicated that the predicted mean droplet diameters by HXT model are in better agreements with measurements due to the consideration of the gas density. However, the film thickness and fuel vapor distribution near the wall region are not significantly affected by the spray/wall interaction models, and all the models present inaccurate predictions relative to the experimental results.

Numerical simulation of homogeneous charge compression ignition combustion using a multi-dimensional model:

Abstract A detailed chemical kinetics model was used in a computational fluid dynamics (CFD) code to study the combustion process of a homogeneous charge compression ignition (HCCI) engine fuelled with isooctane. The detailed chemical kinetics code CHEMKIN was implemented into the multi-dimensional CFD code KIVA-3V so that the chemical reactions and in-cylinder flow were coupled. The results indicate that the grid density and time step have little influence on the accuracy of the computational analysis of the HCCI combustion process; however, the initial temperature of the charge influences the burn point greatly. A comparative study on submodels indicates that the results calculated by the renormalization group k-ε turbulence model, the Han-Reitz heat transfer model, and the crevice flow model agree better with measurements than their counterparts, the standard k-ε turbulence model, the traditional heat transfer model, and the non-crevice flow model respectively. The modified multi-dimensional model predicts with good accuracy the pressure, heat release rate, and emissions under different equivalence ratios. The benefits and limits of four approaches extending the operation range of HCCI engines to higher load, i.e. retarded burn point by decreasing the intake temperature, enhanced thermal stratification by lowering the cylinder wall temperature and increasing the swirl ratio, and increasing the exhaust gas recirculation (EGR) ratio, were analysed by this model. The results indicate that proper combustion phasing retardation can reduce the ringing intensity significantly and maintain the performance and emissions level. Among these four methods, increasing the swirl ratio could reduce the ringing intensity most significantly, while EGR shows an excellent performance in all aspects.

Application of the Optimized Decoupling Methodology for the Construction of a Skeletal Primary Reference Fuel Mechanism Focusing on Engine-Relevant Conditions

For the multi-dimensional simulation of the engines with advanced compression-ignition combustion strategies, a practical and robust chemical kinetic mechanism is highly demanded. Decoupling methodology is effective for the construction of skeletal mechanisms for long-chain alkanes. To improve the performance of the decoupling methodology, further improvements are introduced based on recent theoretical and experimental works. The improvements include: (1) updating the H2/O2 sub-mechanism; (2) refining the rate constants in the HCO/CH3/CH2O sub-mechanism; (3) building a new reduced C2 sub-mechanism; and (4) improving the large-molecule sub-mechanism. With the improved decoupling methodology, a skeletal primary reference fuel (PRF) mechanism is developed. The mechanism is validated against the experimental data in shock tubes, jet-stirred reactors, premixed and counterflow flames for various PRF fuels covering the temperature range of 500–1450 K, the pressure range of 1–55 atm, and the equivalence ratio range of 0.25¬–1.0. Finally, the skeletal mechanism is coupled with a multi-dimensional computational fluid dynamics model to simulate the combustion and emission characteristics of homogeneous charge compression ignition (HCCI) engines fueled with iso-octane and PRF. Overall, the agreements between the experiment and prediction are satisfactory.

Numerical investigation of the influence of intake valve lift profile on a diesel premixed charge compression ignition engine with a variable valve actuation system at moderate loads and speeds

Variable valve actuation is attracting increasing attention for the control of diesel premixed charge compression ignition engines due to its fast-response characteristics. In this study, a three-dimensional computational fluid dynamics model, coupled with detailed chemical kinetics, is used to evaluate the influence of the intake valve lift profile on combustion and emissions of a diesel premixed charge compression ignition engine at moderate loads and speeds. The results indicate that, among all the tested variable valve actuation strategies, late intake valve closing shows the most potential for control of ignition timing and reduction of nitrogen oxide emissions, while maintaining low soot emissions and fuel consumption. A moderate decrease of intake valve lift is beneficial for the reduction of soot emissions without significant impacts on fuel consumption due to the enhanced intake flow. The enhancement of turbulence kinetic energy of the in-cylinder mixture is helpful for fuel/air mixing and soot reduction. However, the strategies for increasing turbulence kinetic energy by using intake valve reopening around the start of injection timing and one intake valve deactivation lead to deterioration in fuel efficiency due to the increased pumping work. In general, the increase of swirl ratio with various variable valve actuation strategies impedes the fuel/air mixing, resulting in rapid increases of soot emissions, and a lower swirl ratio is more beneficial for mixing and combustion in diesel premixed charge compression ignition engines with early injection timing.

Large eddy simulation of fluid injection under transcritical and supercritical conditions

ABSTRACT When a cryogenic fluid initially at a subcritical temperature is injected into a supercritical environment, it will experience a process across a pseudo-boiling point, at which the specific heat reaches its maximum value under the corresponding pressure. Large eddy simulation (LES) is conducted to explore the effects of pseudo-vaporization phenomenon around the pseudo-critical temperature on fluid jet evolution. To highlight the pseudo-vaporization effect, a cryogenic nitrogen jet with different injection temperatures, which correspond to transcritical and supercritical conditions, respectively, is injected into a chamber with same supercritical conditions. All of the thermophysical and transport properties are determined directly from fundamental theories combined with a real fluid equation of state. It is found that when the fluid transits through the pseudo-boiling point, the constant-pressure specific heat reaches a local maximum, while the thermal conductivity and viscosity become minimum. The condition-averaged constant-pressure specific heat suggests that the pseudo-boiling point has the effect of increasing the density gradients. Vorticity and Q-criterion analysis reveals that high-temperature injection facilitates the mixing of jet fluid with ambient gas. Also, the high-temperature injection of supercritical fluid can earlier transit into the full developed region.

Implementation and Improvement of ISAT in HCCI Multidimensional Modeling with Detailed Chemical Kinetics

In Situ Adaptive Tabulation(ISAT) has been implemented into HCCI multidimensional modeling with detailed chemical kinetics,and the performance of ISAT was discussed.The results showed that ISAT could reduce the computational time remarkably,and control the global error efficiently.Taking into account of the characteristics of chemical reactions during HCCI combustion process,an enhanced approach,the partial ISAT(PaISAT) is presented,and it can significantly improve the accuracy and speed-up factor.Moreover,the required memory of ISAT is reduced due to adoption of dynamic trimming technique.

Interactions Between Surface Reactions and Gas-phase Reactions in Catalytic Combustion and Their Influence on Ignition of HCCI Engine1

Abstract The catalytic combustion of methane in a microchannel whose surface was coated with platinum (Pt) catalyst was studied by numerical-simulation. The effects of gas-phase reactions on the whole catalytic combustion process were analyzed at a high inlet pressure. A sensitivity analysis of the detailed mechanisms of the surface reaction of methane on Pt revealed that the most sensitive reactions affecting the heterogeneous ignition are oxygen adsorption/desorption and methane adsorption, and the most sensitive reactions affecting the homogeneous ignition are OH and H 2 O adsorption/desorption. The combustion process of the homogeneous charge compression ignition (HCCI) engine whose piston face was coated with Pt catalyst was simulated. The effects of catalysis and the most sensitive reactions on the ignition timing and the concentration of the main intermediate species during the HCCI engine combustion are discussed. The results show that the ignition timing of the HCCI engine can be increased by, catalysis, and the most sensitive reactions affecting the ignition timing of the HCCI engine are OH and H 2 O adsorption/desorption.

Three-dimensional numerical investigation on wall film formation and evaporation in port fuel injection engines

ABSTRACT Wall film formation and evaporation were studied on a flat wall inside a constant-volume vessel using a three-dimensional numerical method. The computation was based on the discrete phase model (DPM) of spray dispersion, a spray–wall interaction model coupled with an enhanced wall film evaporation sub-model, in which the operating conditions of cold wall are considered for port fuel injection (PFI) engines. The influence of impacting parameters including injection pressure, the impingement distance from the injector and the impinged wall, injection duration, impingement angle, and wall temperature was discussed.

Numerical Investigation on Mass Dispersion in Turbulent Flows through Porous Media with High Porosity

With porous combustor as engineering background, the mass dispersion process of methane and air in highly porous media is studied numerically based on a simplified model for the periodical structure of the porous media. The computational results obtained from the microscopic flow field within a unit cell, i.e., a representative elementary volume, into which a low Reynolds number turbulent model is incorporated, are employed to compute the longitudinal and transverse dispersion coefficients through a volume average approach. The results are compared with experimental data and empirical formulas available in the literature, and a reasonable agreement is achieved.