Estimation of Turbulent Flame Speeds in SI Engines P M V Subbarao Professor Mechanical Engineering Department Collect the Factors Influencing the Enhanced Combustion in Winkled Flame…..

Flames in SI Engine In Spark Ignition(SI) engines running at very low rpm, laminar flames provides sufficient convective heat transfer to ensure burning of desired amount of fuel-air. At high engine speeds, high burning velocity and enhanced convective transfer is essential. This is possible thru Turbulence. Damkohler (1947) proposed that that this was only due to increase in surface area of the turbulent flame. Turbulence changes the mode of heat transfer and diffusion to include highly convective mixing which also carries active particles into the unburned region.

Transport Phenomena Supporting A flame In general mass, heat and momentum mass may be transported by molecular processes and/or turbulent processes into/out of a flame. Molecular processes are slow and operate on small spatial scales. Turbulent transport depends upon velocity and size of an eddy carrying the transported material. Number of small eddies may be imbedded in a larger eddy. A characteristic of a fully turbulent flow is the existence of a wide range of eddy sizes.

Characterization of Turbulence in Physical Space Mean velocity component Turbulence Level

Spectral Characterization of Turbulent Flow Turbulent flows contain a wide range of eddies of different sizes (scales). An eddy of a given size is the swirling of a fluid with a fixed size and kinetic energy. A turbulent velocity field contains eddies of many sizes, from eddies that are essentially large enough to fill the space available, in our case the engine cylinder, down to eddies often substantially below a millimeter in size. The size of the largest eddy can be guessed by asking for the diameter of the largest sphere that will fit in the available space since turbulent eddies are approximately the same size in all directions.

Energy Transactions among Eddies These eddies pass energy sequentially from the larger eddies gradually to the smaller ones. This process is known as the turbulent energy cascade.

Energy Transactions Vs Size of Eddy In STFs the rate of energy transfer from one scale to the next must be the same for all scales. The rate at which energy is supplied at the largest possible scale (dmax) is equal to that dissipated at the shortest scale (dmin). Let us denote by  this rate of energy supply/dissipation, per unit mass of fluid. d

Kolmogrov’s Quantification of Eddy Energy Kolmogorov, hypothesized that the characteristics of the turbulent eddies of size d depend solely on d itself and on the energy cascade rate . This is to mean that the eddies know; how big they are, at which rate energy is supplied to them and at which rate they must supply it to the next smaller eddies in the cascade. Mathematically, uO depends only on d and . uO = LT−1, [d] = L and [] = L2T−3. The only dimensionally acceptable possibility is:

Size of the Smallest Eddy The shortest eddy scale is set by viscosity, because the shorter the eddy scale, the stronger the velocity shear and the more important the effect of viscosity. Consequently, the shortest eddy scale can be defined as the length scale at which viscosity becomes dominant. Viscosity, denoted by ν, has for dimensions  = L2T-1. It is reasonable to assume that dmin depends only on , the rate at which energy is supplied to that scale and on . Then the only dimensionally acceptable relation is: d This is called the Kolmogorov scale , is typically on the order of a few millimeters or shorter.

Influence of Kolmogorov Eddy Size on Wrinkling of Flame When flame thickness is much smaller than smallest scale of turbulence, turbulent motion can only wrinkle (distort) the thin laminar flame zone. This criterion is also referred to as Williams-Klimov criterion. If all scales of turbulent motion are smaller than the reaction zone thickness, then transport within the zone is no longer governed by molecular processes only, but is controlled/influenced also by turbulence This criterion is called Damköhler criterion.

Clues to Develop a Healthy Turbulent Flow in An Engine Cylinder Eddy turnover time: Characteristic Chemical Reaction Time: The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.

Damköhler Number When chemical reaction rates are fast in comparison to fluid mixing rates(turbulence), Da >> 1 fast chemistry regime Conversely when reactions are slow compared to mixing rates, Da << 1 For a fixed length scale ratio, Da falls as turbulence intensity goes up.

Span of Multi-scale Turbulent Flames Multi-scale turbulent flames are essential for operation of high speed engines. The span of eddy scales:

Identification of A Control (Dimensionless) Parameter for Span Turbulent Reynolds Number: Important controlling dimensionless parameter: Computational Methods will help in simulation of turbulent flames with various combinations of Da & ReT

Regimes of Turbulent Flame 108 Weak Turbulence Reaction Sheets Da 1 Cyclonic Turbulence Flamelets in Eddies Distributed Reactions 10-4 108 ReT

Regime of Fast SI Engine’s Turbulent Combustion

Mechanism of Turbulent Combustion in SI Engines Values of Da and Re for a typical SI-engine lie predominantly in the reaction sheet flame region The size of an energetic eddy must have a turnover time equal to the time needed to diffuse unbrunt mixture into and burnt mixture out of flame zone. During its turnover time an eddy of size L will interact with the advancing flame front. This will be able to transport preheated fluid from a region of thickness in front of the reaction zone over a distance corresponding to its own size.

Model of the turbulent flame speed, ST Turbulence Intensity ST

Effect of Equivalence Ration on Laminar Burning Velocity Sflamelet,Lp,m/s Methane

Methane

Engine Geometry to Control Turbulent Flow

Types of Intake Flows There are two types of structural turbulence that are recognizable in an engine; tumbling and swirl. Both are created during the intake stroke. Tumble is defined as the in-cylinder flow that is rotating around an axis perpendicular with the cylinder axis. Swirl is defined as the charge that rotates concentrically about the axis of the cylinder.

Tumble Motion For most of the modern SI engines, tumble flows are more crucial than the swirl flows. Tumble flow generates proper mixing of air and fuel, and for high flame propagation rate. Also a well defined (single vortex) tumbling flow structure is more stable. TR is defined as the ratio of the mean angular velocity of the vortices on the target plane to the average angular velocity of the crank. The negative or positive magnitudes of TR indicate the direction of the overall in-cylinder tumble flow in a given plane as CW or CCW respectively.

Pentroof Pistons

Variation of tumble ratio with crank angle positions

Tumble in Double Ports

Generation of Swirl during Induction Deflector Wall Port Shallow-Ramp Helical Port Directed port Steep-Ramp Helical Port

Measures of Swirl Two different values are calculated to assess the swirl intensity. Swirl number or swirl coefficient and swirl component or swirl number. The first, the swirl number, is the ratio of angular momentum to the axial momentum: This angular momentum is calculated in the centre of the swirl (not on the cylinder axis). The other is herein called the “swirl component” and is the swirl parameter relevant for experimental tests with a paddle wheel placed in the axis of the cylinder:

Selection of Valve Lift & Valve Geometry Plain Directed Shallow Ramp Helical Steep Ramp Helical

Swirl Generation through Valve Seat

Valve Geometry Vs Turbulence

Control of Turbulence Level

Turbulence Level versus engine speed

Integral Scale Vs Speed

Engineering Correlations for Turbulent Burning Velocity Groff and Matekunas obtained a relation that takes into account the effect of spark timing on the flame speed ratio. They obtained the following relation for flame speed ratio ST/SL = 2.00 + 1.21(u´/SL)(p/pm)0.82s Where p is the pressure, kPa pm is the corresponding motoring pressure, kPa s is the spark advance factor given by s = 1.0 + 0.05 0.4 where  is the spark advance, crank angle degrees before top dead centre.

Symptoms of Normal Combustion in SI Engines

Cyclic Variation of Flame Volume

Unexpected Engine Damage Damage to the engine is caused by a combination of high temperature and high pressure. Piston Piston crown Cylinder head gasket Aluminum cylinder head

Dangerous Accidents