Fluid Dynamics of Combustion System for Diesel Engines

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Fluid Dynamics of Combustion System for Diesel
Engines

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Use of Fluid Dynamics to Design Matured
Combustion System.
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Schematic of Combustion in diesel Engine
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The Design Algorithm to Facilitate Fluid Dynamics

• The basic concept of the design model is as
  follows.
• The fuel injected into a combustion chamber is
  divided into many zones.
• Events in each zone, such as droplet break-up,
  evaporation, airfuel mixing, ignition,
  heatrelease, heat transfer and formation of
  exhaust emis are traced and calculated in order
  to obtain zonal temperature and compositions.
• The mass, internal energy and mole quantity of
  NOx of every zone are calculated to obtain
  cylinder-averaged temperature, airfuel ratio and
  NOx concentration.

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Fluid Dynamics FactorsGeometric Features
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Spray development

• After fuel is injected into the cylinder, spray
  break-up takes place in few crankshaft angles
  followed by wall impingement.
• In these stages, the movement of the fuel jet
  follows different laws.
• Before break-up, the jet follows the energy
  conservation law and the intial velocity of jet
  is as follows

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Diesel fuel injection nozzles
Sac type
VCO-type
The holes in a modern injection system are very
small, typically 50 250 µm, and they are
manufactured using a complicated EDM (Electro
Discharge Machining) process.
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Actuation of Injector Nozzle
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Mass flow rate through Nozzle
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Flow of Fuel through Finite Length Circular Pipe
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Coefficient of Discharge
Nuricks Number, K
pv is the vapor pressure of the fuel.
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Optimal Design of Nozzle Hole
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(No Transcript)
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Why Is It Happening?
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Component of Design Model

• The Global design model contains the following
  submodels
• spray development,
• Air entrainment and mixing,
• Droplet evaporation,
• Heat transfer of a zone,
• Combustion and thermodynamic calculation,
• NOx formation,
• Calculation of gas properties
• Chemical equilibrium compositions.

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Spray Formation

• Spray formation is explained as Breakup
  Mechanism, described as
• Stretching of fuel ligament into sheets or
  streams.
• Appearance of ripples and protuberances.
• Formation of small ligaments or holes in sheets.
• Collapse of ligaments or holes in sheets.
• Further breakup due to vibration of droplets.
• Agglomeration or shedding from large drops.
• The flow parameters of a jet
• Jet Reynolds number
• Jet weber number
• Ohnesorge number