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Technical Paper

RANS and LES Study of Lift-Off Physics in Reacting Diesel Jets

2014-04-01
2014-01-1118
Accurate modeling of the transient structure of reacting diesel jets is important as transient features like autoignition, flame propagation, and flame stabilization have been shown to correlate with combustion efficiency and pollutant formation. In this work, results from Reynolds-averaged Navier-Stokes (RANS) simulations of flame lift-off in diesel jets are examined to provide insight into the lift-off physics. The large eddy simulation (LES) technique is also used to computationally model a lifted jet flame at conditions representative of those encountered in diesel engines. An unsteady flamelet progress variable (UFPV) model is used as the turbulent combustion model in both RANS simulations and LES. In the model, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate Xst, and progress variable Cst by solving the unsteady flamelet equations.
Technical Paper

Dependence of Fuel-Air Mixing Characteristics on Injection Timing in an Early-Injection Diesel Engine

2002-03-04
2002-01-0944
In recent years, there has been an interest in early-injection Diesel engines as it has the potential of achieving a more homogeneous and leaner mixture close to top-dead-center (TDC) compared to standard Diesel engines. The more homogeneous mixture may result in reduced NOx and soot emissions and higher efficiency. Diesel engines in which a homogeneous mixture is achieved close to TDC are known as Homogenous Charge Compression Ignition (HCCI) engines. PREmixed lean DIesel Combustion (PREDIC) engines in which the start of fuel injection is considerably advanced in comparison with that of the standard Diesel engine is an attempt to achieve a mode of operation close to HCCI. Earlier studies have shown that in a PREDIC engine, the fuel injection timing affects the mixture formation and hence influences combustion and pollutant formation.
Technical Paper

An Evaluation of a Composite Model for Predicting Drop-Drop Collision Outcomes in Multidimensional Spray Computations

2002-03-04
2002-01-0943
The standard model for predicting the outcome of drop-drop collisions in sprays is one developed based on measurements in rain drops under atmospheric pressure conditions. This model includes the possible outcomes of grazing collisions and coalescence. Recent measurements with hydrocarbon drops and at higher pressure (up to 12 bar) indicate the possibility of additional outcomes: bounce, reflexive separation and drop shattering. The measurements also indicate that the Weber number range over which bounce occurs is dependent on the gas pressure. The probability of a drop-drop collision resulting in bounce increases with gas pressure. A composite model that includes all these outcomes as possibilities is employed to carry out computations in a constant volume chamber and in a Diesel engine. A sub-model for bounce that includes the pressure effects is also part of the composite model.
Technical Paper

A Wall-Modified Flamelet Model for Diesel Combustion

2004-03-08
2004-01-0103
In this paper, a wall-modified interactive flamelet model is developed for improving the modeling of Diesel combustion. The objective is to include the effects of wall heat loss on the transient flame structure. The essential idea is to compute several flamelets with several representative enthalpy defects which account for wall heat loss. Then, the averaged flamelet profile can be obtained through a linear fit between the flamelets according to the enthalpy defect of the local gas which results from the wall heat loss. The enthalpy defect is estimated as the difference between the enthalpy in a flamelet without wall heat loss, which would correspond to the enthalpy in the gas without wall heat loss, and the gas with wall heat loss. The improved model is applied to model combustion in a Diesel engine. In the application, two flamelets, one without wall heat loss and one with wall heat loss, are considered.
Journal Article

Prechamber Hot Jet Ignition of Ultra-Lean H2/Air Mixtures: Effect of Supersonic Jets and Combustion Instability

2016-04-05
2016-01-0795
An experiment has been developed to investigate the ignition characteristics of ultra-lean premixed H2/air mixtures by a supersonic hot jet. The hot jet is generated by combustion of a stoichiometric mixture in a small prechamber. The apparatus adopted a dual-chamber design in which a small-volume (1% of the main chamber by volume) prechamber was installed within a large-volume main chamber. A small orifice (nozzle) connects the two chambers. Spark initiated combustion inside the prechamber causes a pressure rise and pushes the gases though the nozzle, resulting in a hot jet that would ignite the lean mixture in the main chamber. Simultaneous high-speed Schlieren photography and OH* Chemiluminescence were applied to visualize the jet penetration and the ignition processes inside the main chamber. Hot Wire Pyrometry (HWP) was used to measure temperature distribution of the transient hot jet.
Technical Paper

Swirl-Spray Interactions in a Diesel Engine

2001-03-05
2001-01-0996
Swirl in Diesel engines is known to be an important parameter that affects the mixing of the fuel jets, heat release, emissions, and overall engine performance. The changes may be brought about through interactions of the swirling flow field with the spray and through modifications of the flow field. The purpose of this paper is to investigate the interaction of the swirl with sprays in a Diesel engine through a computational study. A multi-dimensional model for flows, sprays, and combustion in engines is employed. Results from computations are reported with varying levels of swirl and initial turbulence in two typical Diesel engine geometries. It is shown that there is an optimal level of swirl for each geometry that results from a balance between increased jet surface area and, hence, mixing rates and utilization of air in the chamber.
Technical Paper

Comparisons of Computed and Measured Results of Combustion in a Diesel Engine

1998-02-23
980786
Results of computations of flows, sprays and combustion performed in an optically- accessible Diesel engine are presented. These computed results are compared with measured values of chamber pressure, liquid penetration, and soot distribution, deduced from flame luminosity photographs obtained in the engine at Sandia National Laboratories and reported in the literature. The computations were performed for two operating conditions representing low load and high load conditions as reported in the experimental work. The computed and measured peak pressures agree within 5% for both the low load and the high load conditions. The heat release rates derived from the computations are consistent with expectations for Diesel combustion with a premixed phase of heat release and then a diffusion phase. The computed soot distribution shows noticeable differences from the measured one.
Technical Paper

The Computed Structure of a Combusting Transient Jet Under Diesel Conditions

1998-02-23
981071
Numerical computations of combusting transient jets are performed under diesel-like conditions. Discussions of the structure of such jets are presented from global and detailed points of view. From a global point of view, we show that the computed flame heights agree with deductions from theory and that integrated soot mass and heat release rates are consistent with expected trends. We present results of several paramaters which characterise the details of the jet structure. These are fuel mass fractions, temperature, heat release rates, soot and NO. Some of these parameters are compared with the structure of a combusting diesel spray as deduced from measurements and reported in the literature. The heat release rate contours show that the region of chemical reactions is confined to a thin sheet as expected for a diffusion flame. The soot contour plots appear to agree qualitatively with the experimental observations.
Journal Article

Perception of Diesel Engine Gear Rattle Noise

2015-06-15
2015-01-2333
Component sound quality is an important factor in the design of competitive diesel engines. One component noise that causes complaints is the gear rattle that originates in the front-of-engine gear train which drives the fuel pump and other accessories. The rattle is caused by repeated tooth impacts resulting from fluctuations in differential torsional acceleration of the driving gears. These impacts generate a broadband, impulsive noise that is often perceived as annoying. In most previous work, the overall sound quality of diesel engines has been considered without specifically focusing on predicting the perception of gear rattle. Gear rattle level has been quantified based on angular acceleration measurements, but those measurements can be difficult to perform. Here, the emphasis was on developing a metric based on subjective testing of the perception of gear rattle.
Journal Article

A Numerical Investigation of Ignition of Ultra-Lean Premixed H2/Air Mixtures by Pre-Chamber Supersonic Hot Jet

2017-10-05
2017-01-9284
Gas engines often utilize a small-volume pre-chamber in which fuel is injected at near stoichiometric condition to produce a hot turbulent jet which then ignites the lean mixture in the main chamber. Hot jet ignition has several advantages over traditional spark ignition, e.g., more reliable ignition of extra-lean mixtures and more surface area for ignition resulting in faster burning and improved combustion burn time. Our previous experimental results show that supersonic jets could extend the lean flammability limit of fuel/air mixtures in the main chamber in comparison to subsonic jets. The present paper investigated the characteristics of supersonic hot jets generated by combustion of stoichiometric H2/air in a pre-chamber to understand the ignition mechanism of ultra-lean mixtures by supersonic hot jets.
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