Spray Impingement & Formation of Films In Ports P M V Subbarao Professor Mechanical Engineering Department Control of wall-film distribution area ….

Instantaneous Rate of Fuel Evaporation in Port In PFI gasoline engines, a large portion of the spray impinges on the walls of the port or the intake valves. Such wall wetting phenomena are found to be major contributors for high unburned hydrocarbon emissions. Instantaneous rate of fuel evaporation in a port is the sum of evaporation rate of fuel droplets and evaporation rate of fuel films

Spray Wall Impingement & Formation of Films The shape, size and location of impingement sites depend on; Fuel spray cone angle. Injection timing, back flow Injection distance, Impingement incidence angle. For every impingement site, there exist; the impingement probability and the passing-by probability. The impingement probability is proportional to spray covered wall area and the downstream flow cross-sectional area.

Basic mechanisms in the spray–wall interactions

Kinematics of Droplet Impingement A droplet impacting on a solid surface first undergoes deformation and spreads out at a certain velocity under the impingement-induced pressure gradients. This spreading flow either remains stable or becomes unstable, leading to different impingement regimes: Stick, spread, rebound, and splash determined by Re & We, characterising the impingement conditions.

Details of Impingement Two basic issues spray impingement processes. Occurrence of type of Impacting regime. Post-impingement characteristics, namely, the rebound velocity magnitude and its direction for the rebound regime, the fraction of the mass deposited on the wall and the size and velocity distributions of the secondary droplets for the splash/breakup regime.

Fluid Dynamic Simulations in Ports

Velocity distribution during Intake Process

Possible Regions of Fuel Film in Port Injected Engiens

List of Impingement Points

Film Dynamics There are two different forces exerted on a fuel wall film. On the gas side, the gas flow tends to drive the film moving along the same direction. On the wall side, the viscous friction tends to resist the film movement. The force balance on the film gives the equation for the film motion:

Deposited Film Mass Fraction The ratio of the reflected to the impinging mass is given by Ohnesorge numbers

Film Evaporation The film vaporization rate is determined by where hf is the heat transfer coefficient, and Yfs the the mass fraction of the fuel vapor near the film surface. Where p and pfs are the total local pressure and the pressure of the saturated fuel vapor near the surface of the wall film. pfs can be calculated from the Clausius–Clapeyron equation presented in the following form

Estimation of Film Temperature For the wall film, the energy equation is described by: Where hfg & kf is the heat transfer coefficient and the liquid fuel thermal conductivity. The terms in right hand of equation are the heat transfer rate from gas to fuel film on the gas side, the heat transfer rate used for vaporization and the heat transfer from wall to the film respectively.

Distribution of Fuel

Film distributions at various Zones

Wall Film Thickness Different Locations

Fuel Film at 1465 Crank Angles

Migration of Fluid Film

Effect of Injection Timing

Effect of Injector Location

Effects of impingement angle

Fuel Evaporation in Prot Injection Engine