Fluid Dynamics of Combustion System for Diesel Engines

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….

Schematic of Combustion in diesel Engine

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, air–fuel 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, air–fuel ratio and NOx concentration.

Fluid Dynamics Factors Geometric Features

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:

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.

Actuation of Injector Nozzle

Mass flow rate through Nozzle

Flow of Fuel through Finite Length Circular Pipe

Coefficient of Discharge Nurick’s Number, K pv is the vapor pressure of the fuel.

Optimal Design of Nozzle Hole

Why Is It Happening?

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.

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