Design of Engine Cylinder for Creation of A Selected Turbulent Flow

P M V SubbaraoProfessor

Mechanical Engineering Department

Geometry to create qualitatively and Quantitatively Suitable Flow

Structure …..

Major Steps

• Recognition of Turbulence • Design of cylinder for Creation of Turbulence• Characterization of Generated turbulence

Geometrical Characterization of Turbulent Structure

• Turbulent flows contain a wide range of scales.

• The velocity field contains motions of all 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.

• Hence, at TC the largest eddy will be roughly the clearance height, while at BC it will be roughly the cylinder bore.

Energetic Characterization of Turbulent Structure

• The kinetic energy of these eddies varies:

• The largest eddies are relatively weak; as the size drops, the energy rises rapidly to a peak, and then falls continually down to the smallest eddies.

• The most energetic eddy, at the peak, which is responsible for most of the transport, is about 1/6 the size of the largest eddy ‐ thus,

• 1/6 of the bore, or

• 1/6 of the clearance height

Spectrum of Turbulence

Vortical Description of In-Cylinder Flow

• A general engine flow, at middle third of the intake stroke has an intensity of roughly 10 ×Sp , where Sp ,is the average piston speed.

• The size of largest vortex is found to be roughly B/6 , where B is the bore.

• This means that this vortex has cycle time of (a time scale) of roughly, B/ 60 ×Sp .

• This time scale is 1/60th of time it takes the piston to complete the intake stroke.

• What is the use of this knowledge?

• It tells us that there is a vortex which complete a one full period of mixing, in a time shorter than (about 1/60) the time of the intake stroke.

Turbulent eddy structure in an Engine Cylinder

Multi-scale Turbulent Flames• Multi-scale turbulent flames are essential for operation of high

speed engines.

• Turbulent flames are characterized by rms velocity fluctuation, the turbulence intensity, and the length scales of turbulent flow ahead of flame.

• The integral length scale li is a measure of the size of the large energy containing sturctures of the flow.

• The Kolmogrov scale lk defines the smallest structure of the flow where small-scale kinetic energy is dissipated via molecular viscosity.

• Important dimensionless parameters:

i

T

luReTurbulent Reynolds Number:

u

liT

Eddy turnover time:

Characteristic Chemical Reaction Time:L

LL S

The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.

u

SlDa L

L

i

L

T

Regimes of Turbulent Flame

Da

Re

1

108

10-4 108

Weak Turbulence

Reaction Sheets

Distributed Reactions

Turbulent Burning Velocity

• By analogy with the laminar case, a turbulent burning velocity can be defined as:

A

mu

u

reret

)/()/(

where the distinction has been made between a turbulent ‘engulfment’ velocity ute and a turbulent ‘reacted’ burning velocity utr

based on the rate of production of burned gas.

The Wrinkled Flame Front

• The flame front is now a wrinkled one and thus, the choice of the relevant flame area A is a lot less straightforward than in the laminar case.

• Even in well controlled experiments, e.g. in constant volume combustion bombs, care is needed when picking a flame area.

• It goes without saying that selecting one in engine combustion is a lot more difficult.

Model of the turbulent flame speed

The First Clue…..

Design of Engine Cylinder for Creation of A Selected Turbulent Flow

The Mother of Turbulence in An Engine Cylinder

• The Intake process.

• The jet flow of gas through the inlet valve of an engine during the intake process is generally thought to be responsible for the production of the turbulent field in the engine cylinder.

• To test this hypothesis, Clark performed an experiment where he observed the rapid decrease in the rate of flame propagation in an engine intensity versus crank angle after the intake and exhaust processes were eliminated.

• Semenov repeated this experiment in a motored engine and showed the rapid decay in the instantaneous as the intake and exhaust process were eliminated.

The intake : Mother of Turbulence

The structure of Flow during Intake

r =0 mm

r =10 mm r = 13 mm

r =15 mm

r =23 mmr =28 mm