A reduced chemical reaction mechanism is developed and validated in the present study for multi-d... more A reduced chemical reaction mechanism is developed and validated in the present study for multi-dimensional diesel HCCI engine combustion simulations. The motivation for the development of the reduced mechanism is to enhance the computational efficiency of engine ...
In this study we identify components of a typical biodiesel fuel and estimate both their individu... more In this study we identify components of a typical biodiesel fuel and estimate both their individual and mixed thermo-physical and transport properties. We then use the estimated mixture properties in computational simulations to gauge the extent to which combustion is modified when ...
The theoretical basis and numerical implementation of KIVA, a multidimensional computer code for ... more The theoretical basis and numerical implementation of KIVA, a multidimensional computer code for the simulation of atomization and vaporization processes in the injection of a liquid through a round hole into a compressed gas, are described. KIVA is based on the blob-injection model of Reitz and Diwakar (1987), taking into account the effects of liquid inertia, surface tension, and the aerodynamic forces on the jet, as well as drop collision and coalescence and the effect of drops on turbulence in the gas. The predictions of KIVA for different injection regimes are compared with published experimental data in extensive graphs, and good agreement is demonstrated.
ABSTRACT A new spray/wall interaction model was developed with emphasis on the simulation of dies... more ABSTRACT A new spray/wall interaction model was developed with emphasis on the simulation of diesel-like spray impingement generated by high injection pressures, which is widely used in direct-injection gasoline and diesel engines. In contrast to traditional spray/wall interaction models, lift forces were considered on the wall spray droplets in the new model, which generates a vortex at the leading edge of the wall spray instead of droplet rebound or splash. In addition, a wall-jet submodel was adopted to correct the droplet-gas relative velocity in the calculation of drag and lift forces on the droplets. Grid dependency was also significantly reduced in the new model due to the implementation of the wall-jet submodel. The wall-jet submodel was extended to cover both normal and oblique impingements, which are relevant in engines with a side-mounted injector. In the new model a new correlation was also developed to estimate the splashed mass ratio for each individual impact droplet, which provides a more accurate prediction of the overall spray impingement process. Extensive validations of the new model were performed through comparisons against measurements from a variety of diesel spray impingement experiments, instead of just using single drop or drop train impingement experiments. The results indicate that the new model matches experimental measurements very well and can be used for the prediction of spray/wall interaction, especially under the conditions relevant to modern direct-injection diesel and gasoline engines.
Computational and experimental studies on the effects of oxygenated fuels with single and split h... more Computational and experimental studies on the effects of oxygenated fuels with single and split high-pressure fuel injections were conducted at both high and low load engine operating conditions. The oxygenates were a long chained ester and ether blended with conventional diesel fuel. An improved version of the KIVA-II multidimensional engine code was used in the study, and the soot model
The effects of fuel-spray targeting on mixture development, combustion, and pollutant formation w... more The effects of fuel-spray targeting on mixture development, combustion, and pollutant formation were investigated for a late-injection low-temperature combustion (LTC) operating condition in an optically accessible heavy-duty diesel engine. Equivalence-ratio maps, derived from toluene fuel-tracer fluorescence measurements, quantify the effects of fuel-jet targeting on mixture preparation processes under non-combusting conditions. Planar laser-induced fluorescence of formaldehyde (H2CO), hydroxyl radical (OH), and polycyclic aromatic hydrocarbons (PAH) provide complementary measurements during the combustion process from fuel-lean to fuel-rich reaction zones. Three different injector nozzles with included angles of 124°, 152° and 160° yield unique jet–wall and jet–jet interactions. The baseline 152° nozzle directs the fuel jet toward the vertical center of the piston bowl-wall, where the jets impinge on the wall and merge with neighboring jets prior to the peak heat-release rate. Fuel-rich jet–jet interaction regions develop between the jets near the floor of the bowl, which is where the majority of soot and/or PAH formation occurs. These fuel-rich jet–jet interactions can be reduced by either a more narrow injection angle (124°) or a wider angle injection (160°), both of which lead to decreased soot formation. However, jet–wall interactions may play an important role for late-cycle bulk flows. With the 160° injection, less bulk-fluid motion in the piston bowl occurs, leading to extensive quenched regions throughout the center of the combustion chamber. By contrast, the 124° injection enhances large-scale fluid motion, transporting hot second-stage ignition regions into the fuel-lean quenched mixtures at the center of the chamber, potentially improving late-cycle oxidation of unburned hydrocarbons (UHC) and carbon monoxide (CO).
The structure of first- and second-stage combustion is investigated in a heavy-duty, single-cylin... more The structure of first- and second-stage combustion is investigated in a heavy-duty, single-cylinder optical engine using chemiluminescence imaging, Mie-scatter imaging of liquid-fuel, and OH planar laser-induced fluorescence (OH-PLIF) along with calculations of fluorescence quenching. Three different diesel combustion modes are studied: conventional non-diluted high-temperature combustion (HTC) with either (1) short or (2) long ignition delay, and (3) highly diluted low-temperature combustion (LTC) with early fuel injection. For the short ignition delay HTC condition, the OH fluorescence images show that second-stage combustion occurs mainly on the fuel jet periphery in a thickness of about 1 mm. For the long ignition delay HTC condition, the second-stage combustion zone on the jet periphery is thicker (5–6 mm). For the early-injection LTC condition, the second-stage combustion is even thicker (20–25 mm) and occurs only in the down-stream regions of the jet. The relationship between OH concentration and OH-PLIF intensity over a range of equivalence ratios is estimated from quenching calculations using collider species concentrations predicted by chemical kinetics simulations of combustion. The calculations show that both OH concentration and OH-PLIF intensity peak near stoichiometric mixtures and fall by an order of magnitude or more for equivalence ratios less than 0.2–0.4 and greater than 1.4–1.6. Using the OH fluorescence quenching predictions together with OH-PLIF images, quantitative boundaries for mixing are established for the three engine combustion modes.
A highly diluted, low-flame-temperature diesel engine combustion strategy with two separate fuel-... more A highly diluted, low-flame-temperature diesel engine combustion strategy with two separate fuel-injections per cycle was investigated using simultaneous optical diagnostics at a low-load operating condition. In-cylinder processes were visualized with a suite of laser/imaging diagnostics. The cool flame first-stage ignition reactions occur along the entire length of the jet for the first combustion event. For both injections, the second-stage ignition
A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting... more A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting soot formation and oxidation processes in diesel engines. The model involves nine generic steps, i.e., fuel pyrolysis, precursor species (including acetylene) formation and oxidation, soot particle inception, particle coagulation, surface growth and oxidation. The fuel pyrolysis process leads to acetylene formation and it is described by a single-step reaction. The particle inception occurs via a generic gas-phase precursor species, and the precursor is the product of an irreversible reaction from acetylene. The acetylene addition reaction contributes to soot surface growth. The particle coagulation affects both particle size and number density. The oxidation of soot particles includes two mechanisms—Nagle and Strickland-Constable's O2 oxidation mechanism and Neoh et al.'s OH oxidation mechanism. The quasi-steady state assumption is applied to an H2–O2–CO system for calculating OH concentration. Both acetylene and precursor species have their own consumption paths, each of which is described by a single-step oxidation reaction.Validations of the model have been conducted over a wide range of engine conditions from conventional to PCCI-like combustion. Two engine examples (a heavy-duty diesel engine and a light-duty diesel engine) are presented in this paper. The predictions are compared against measurements, and the applicability of the model to multi-dimensional diesel simulations is assessed. The model's capability of predicting the soot distribution structure in a conventional diesel flame is included in discussion as well. The work reveals that the nine-step model is not only computationally efficient but also fundamentally sound. The model can be applied to diesel engine combustion analysis and, after calibration, is suitable to be integrated with genetic algorithms for system optimization over a controllable range of operations.
Diesel engine simulation results using two different combustion models are presented in this stud... more Diesel engine simulation results using two different combustion models are presented in this study, namely the representative interactive flamelet (RIF) model and the direct integration of computational fluid dynamics and CHEMKIN. Both models have been implemented into an improved version of the KIVA code. The KIVA/RIF model uses a single flamelet approach and also considers the effects of vaporization on
It is well known that injection strategies including the injection timing and the injection rate-... more It is well known that injection strategies including the injection timing and the injection rate-shape play an important role in determining engine performance, especially pollutant emissions. But how the injection timing and the injection rate-shape quantitatively affect the performance of diesel low temperature combustion (LTC) is still not well understood. In this paper, the KIVA–CHEMKIN computational fluid dynamics (CFD) code with an improved spray model is used to simulate the spray and combustion processes of diesel LTC with early and late injection timings and seven different injection rate-shapes. The validation of the models is based on comparisons with laser diagnostic and in-cylinder pressure data under a low load operating condition. It is concluded that the use of early injection provides lower soot, HC and CO emissions but higher NOx emissions than the late injection. A rectangular-type (case1) and boot-type (case 4) injection rate-shape displays the potential to reduce the soot, HC and CO emissions compared the other generic rate-shapes considered.
Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA... more Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA-3V code are strongly mesh dependent due to errors in predicting the droplet–gas relative velocity and errors in describing droplet–droplet collision and coalescence processes. To reduce the mesh dependence, gas-jet theory is introduced to predict the droplet–gas relative velocity, and a radius of influence (ROI) of collision methodology is established for each gas phase cell to estimate the collision probability for each parcel in the cell. Spray and combustion processes in a low temperature combustion diesel engine under early and late injection strategies with a fine mesh were predicted using the conventional LDEF model and compared with the measurements of soot, OH, fuel liquid and vapor distributions obtained by laser based diagnostics including, PLIF, LII, and Mie scattering. Then, the KIVA-3V code implemented with the improved spray model based on the gas-jet model and modifications of the spray models was utilized to simulate the processes on a relatively coarse numerical mesh. Comparison of the simulations between the fine and coarse meshes shows that the improved spray model can greatly reduce the mesh dependence for low temperature combustion diesel engine CFD simulations.
ABSTRACT A new spray computational fluid dynamics (CFD) model that comprises gas-jet and radius-o... more ABSTRACT A new spray computational fluid dynamics (CFD) model that comprises gas-jet and radius-of-influence collision, mean collision time, and interpolation method improvements was used to reduce grid-size, hole-location, and time-step dependencies in modeling group-hole-nozzle sprays. The spray model was validated against experimental results obtained from spray visualization and phase Doppler particle analyzer systems. The spray characteristics including spray penetration and droplet sizes of group-hole nozzles were also studied. Standard CFD spray models show significant dependencies of grid size, hole location, and time step for the calculated spray penetration of group-hole nozzles. On the other hand, the new spray model reduced the dependencies successfully for the present nonevaporating spray cases. The calculated results agreed well with the experimental results in terms of both spray penetration and SMD distributions. It was found that the sprays of the group-hole nozzle exhibit similar spray penetrations to those of a single-hole nozzle with the same overall hole area. The computations indicate that the group-hole nozzle has advantages in the near field for reducing spray droplet sizes.
International Journal of Engine Research - INT J ENGINE RES, 2000
... to be tightly coupled to the solution domain. This model [19]. ... On the other hand, global ... more ... to be tightly coupled to the solution domain. This model [19]. ... On the other hand, global search methods, such as the genetic algorithm used in the present work, place teristics to solve the partial differential equations governing quasi-one-dimensional, unsteady, com-...
A reduced chemical reaction mechanism is developed and validated in the present study for multi-d... more A reduced chemical reaction mechanism is developed and validated in the present study for multi-dimensional diesel HCCI engine combustion simulations. The motivation for the development of the reduced mechanism is to enhance the computational efficiency of engine ...
In this study we identify components of a typical biodiesel fuel and estimate both their individu... more In this study we identify components of a typical biodiesel fuel and estimate both their individual and mixed thermo-physical and transport properties. We then use the estimated mixture properties in computational simulations to gauge the extent to which combustion is modified when ...
The theoretical basis and numerical implementation of KIVA, a multidimensional computer code for ... more The theoretical basis and numerical implementation of KIVA, a multidimensional computer code for the simulation of atomization and vaporization processes in the injection of a liquid through a round hole into a compressed gas, are described. KIVA is based on the blob-injection model of Reitz and Diwakar (1987), taking into account the effects of liquid inertia, surface tension, and the aerodynamic forces on the jet, as well as drop collision and coalescence and the effect of drops on turbulence in the gas. The predictions of KIVA for different injection regimes are compared with published experimental data in extensive graphs, and good agreement is demonstrated.
ABSTRACT A new spray/wall interaction model was developed with emphasis on the simulation of dies... more ABSTRACT A new spray/wall interaction model was developed with emphasis on the simulation of diesel-like spray impingement generated by high injection pressures, which is widely used in direct-injection gasoline and diesel engines. In contrast to traditional spray/wall interaction models, lift forces were considered on the wall spray droplets in the new model, which generates a vortex at the leading edge of the wall spray instead of droplet rebound or splash. In addition, a wall-jet submodel was adopted to correct the droplet-gas relative velocity in the calculation of drag and lift forces on the droplets. Grid dependency was also significantly reduced in the new model due to the implementation of the wall-jet submodel. The wall-jet submodel was extended to cover both normal and oblique impingements, which are relevant in engines with a side-mounted injector. In the new model a new correlation was also developed to estimate the splashed mass ratio for each individual impact droplet, which provides a more accurate prediction of the overall spray impingement process. Extensive validations of the new model were performed through comparisons against measurements from a variety of diesel spray impingement experiments, instead of just using single drop or drop train impingement experiments. The results indicate that the new model matches experimental measurements very well and can be used for the prediction of spray/wall interaction, especially under the conditions relevant to modern direct-injection diesel and gasoline engines.
Computational and experimental studies on the effects of oxygenated fuels with single and split h... more Computational and experimental studies on the effects of oxygenated fuels with single and split high-pressure fuel injections were conducted at both high and low load engine operating conditions. The oxygenates were a long chained ester and ether blended with conventional diesel fuel. An improved version of the KIVA-II multidimensional engine code was used in the study, and the soot model
The effects of fuel-spray targeting on mixture development, combustion, and pollutant formation w... more The effects of fuel-spray targeting on mixture development, combustion, and pollutant formation were investigated for a late-injection low-temperature combustion (LTC) operating condition in an optically accessible heavy-duty diesel engine. Equivalence-ratio maps, derived from toluene fuel-tracer fluorescence measurements, quantify the effects of fuel-jet targeting on mixture preparation processes under non-combusting conditions. Planar laser-induced fluorescence of formaldehyde (H2CO), hydroxyl radical (OH), and polycyclic aromatic hydrocarbons (PAH) provide complementary measurements during the combustion process from fuel-lean to fuel-rich reaction zones. Three different injector nozzles with included angles of 124°, 152° and 160° yield unique jet–wall and jet–jet interactions. The baseline 152° nozzle directs the fuel jet toward the vertical center of the piston bowl-wall, where the jets impinge on the wall and merge with neighboring jets prior to the peak heat-release rate. Fuel-rich jet–jet interaction regions develop between the jets near the floor of the bowl, which is where the majority of soot and/or PAH formation occurs. These fuel-rich jet–jet interactions can be reduced by either a more narrow injection angle (124°) or a wider angle injection (160°), both of which lead to decreased soot formation. However, jet–wall interactions may play an important role for late-cycle bulk flows. With the 160° injection, less bulk-fluid motion in the piston bowl occurs, leading to extensive quenched regions throughout the center of the combustion chamber. By contrast, the 124° injection enhances large-scale fluid motion, transporting hot second-stage ignition regions into the fuel-lean quenched mixtures at the center of the chamber, potentially improving late-cycle oxidation of unburned hydrocarbons (UHC) and carbon monoxide (CO).
The structure of first- and second-stage combustion is investigated in a heavy-duty, single-cylin... more The structure of first- and second-stage combustion is investigated in a heavy-duty, single-cylinder optical engine using chemiluminescence imaging, Mie-scatter imaging of liquid-fuel, and OH planar laser-induced fluorescence (OH-PLIF) along with calculations of fluorescence quenching. Three different diesel combustion modes are studied: conventional non-diluted high-temperature combustion (HTC) with either (1) short or (2) long ignition delay, and (3) highly diluted low-temperature combustion (LTC) with early fuel injection. For the short ignition delay HTC condition, the OH fluorescence images show that second-stage combustion occurs mainly on the fuel jet periphery in a thickness of about 1 mm. For the long ignition delay HTC condition, the second-stage combustion zone on the jet periphery is thicker (5–6 mm). For the early-injection LTC condition, the second-stage combustion is even thicker (20–25 mm) and occurs only in the down-stream regions of the jet. The relationship between OH concentration and OH-PLIF intensity over a range of equivalence ratios is estimated from quenching calculations using collider species concentrations predicted by chemical kinetics simulations of combustion. The calculations show that both OH concentration and OH-PLIF intensity peak near stoichiometric mixtures and fall by an order of magnitude or more for equivalence ratios less than 0.2–0.4 and greater than 1.4–1.6. Using the OH fluorescence quenching predictions together with OH-PLIF images, quantitative boundaries for mixing are established for the three engine combustion modes.
A highly diluted, low-flame-temperature diesel engine combustion strategy with two separate fuel-... more A highly diluted, low-flame-temperature diesel engine combustion strategy with two separate fuel-injections per cycle was investigated using simultaneous optical diagnostics at a low-load operating condition. In-cylinder processes were visualized with a suite of laser/imaging diagnostics. The cool flame first-stage ignition reactions occur along the entire length of the jet for the first combustion event. For both injections, the second-stage ignition
A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting... more A nine-step phenomenological soot model has been implemented into the KIVA-3V code for predicting soot formation and oxidation processes in diesel engines. The model involves nine generic steps, i.e., fuel pyrolysis, precursor species (including acetylene) formation and oxidation, soot particle inception, particle coagulation, surface growth and oxidation. The fuel pyrolysis process leads to acetylene formation and it is described by a single-step reaction. The particle inception occurs via a generic gas-phase precursor species, and the precursor is the product of an irreversible reaction from acetylene. The acetylene addition reaction contributes to soot surface growth. The particle coagulation affects both particle size and number density. The oxidation of soot particles includes two mechanisms—Nagle and Strickland-Constable's O2 oxidation mechanism and Neoh et al.'s OH oxidation mechanism. The quasi-steady state assumption is applied to an H2–O2–CO system for calculating OH concentration. Both acetylene and precursor species have their own consumption paths, each of which is described by a single-step oxidation reaction.Validations of the model have been conducted over a wide range of engine conditions from conventional to PCCI-like combustion. Two engine examples (a heavy-duty diesel engine and a light-duty diesel engine) are presented in this paper. The predictions are compared against measurements, and the applicability of the model to multi-dimensional diesel simulations is assessed. The model's capability of predicting the soot distribution structure in a conventional diesel flame is included in discussion as well. The work reveals that the nine-step model is not only computationally efficient but also fundamentally sound. The model can be applied to diesel engine combustion analysis and, after calibration, is suitable to be integrated with genetic algorithms for system optimization over a controllable range of operations.
Diesel engine simulation results using two different combustion models are presented in this stud... more Diesel engine simulation results using two different combustion models are presented in this study, namely the representative interactive flamelet (RIF) model and the direct integration of computational fluid dynamics and CHEMKIN. Both models have been implemented into an improved version of the KIVA code. The KIVA/RIF model uses a single flamelet approach and also considers the effects of vaporization on
It is well known that injection strategies including the injection timing and the injection rate-... more It is well known that injection strategies including the injection timing and the injection rate-shape play an important role in determining engine performance, especially pollutant emissions. But how the injection timing and the injection rate-shape quantitatively affect the performance of diesel low temperature combustion (LTC) is still not well understood. In this paper, the KIVA–CHEMKIN computational fluid dynamics (CFD) code with an improved spray model is used to simulate the spray and combustion processes of diesel LTC with early and late injection timings and seven different injection rate-shapes. The validation of the models is based on comparisons with laser diagnostic and in-cylinder pressure data under a low load operating condition. It is concluded that the use of early injection provides lower soot, HC and CO emissions but higher NOx emissions than the late injection. A rectangular-type (case1) and boot-type (case 4) injection rate-shape displays the potential to reduce the soot, HC and CO emissions compared the other generic rate-shapes considered.
Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA... more Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA-3V code are strongly mesh dependent due to errors in predicting the droplet–gas relative velocity and errors in describing droplet–droplet collision and coalescence processes. To reduce the mesh dependence, gas-jet theory is introduced to predict the droplet–gas relative velocity, and a radius of influence (ROI) of collision methodology is established for each gas phase cell to estimate the collision probability for each parcel in the cell. Spray and combustion processes in a low temperature combustion diesel engine under early and late injection strategies with a fine mesh were predicted using the conventional LDEF model and compared with the measurements of soot, OH, fuel liquid and vapor distributions obtained by laser based diagnostics including, PLIF, LII, and Mie scattering. Then, the KIVA-3V code implemented with the improved spray model based on the gas-jet model and modifications of the spray models was utilized to simulate the processes on a relatively coarse numerical mesh. Comparison of the simulations between the fine and coarse meshes shows that the improved spray model can greatly reduce the mesh dependence for low temperature combustion diesel engine CFD simulations.
ABSTRACT A new spray computational fluid dynamics (CFD) model that comprises gas-jet and radius-o... more ABSTRACT A new spray computational fluid dynamics (CFD) model that comprises gas-jet and radius-of-influence collision, mean collision time, and interpolation method improvements was used to reduce grid-size, hole-location, and time-step dependencies in modeling group-hole-nozzle sprays. The spray model was validated against experimental results obtained from spray visualization and phase Doppler particle analyzer systems. The spray characteristics including spray penetration and droplet sizes of group-hole nozzles were also studied. Standard CFD spray models show significant dependencies of grid size, hole location, and time step for the calculated spray penetration of group-hole nozzles. On the other hand, the new spray model reduced the dependencies successfully for the present nonevaporating spray cases. The calculated results agreed well with the experimental results in terms of both spray penetration and SMD distributions. It was found that the sprays of the group-hole nozzle exhibit similar spray penetrations to those of a single-hole nozzle with the same overall hole area. The computations indicate that the group-hole nozzle has advantages in the near field for reducing spray droplet sizes.
International Journal of Engine Research - INT J ENGINE RES, 2000
... to be tightly coupled to the solution domain. This model [19]. ... On the other hand, global ... more ... to be tightly coupled to the solution domain. This model [19]. ... On the other hand, global search methods, such as the genetic algorithm used in the present work, place teristics to solve the partial differential equations governing quasi-one-dimensional, unsteady, com-...
Uploads
Papers by Rolf Reitz