2D Cavitation simulation

Objective :

To smulate the case for Winkl Hoffer Nozzle to study Cavitation.And compair the result for diffrent pressure differences

• Case 1: Set up the case for Pressure difference of 30 bars
• Case 2: Set up the case for Pressure difference of 70 bars
• Case 3: Set up the case for Pressure difference of 100 bars

Theory:

Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities, in places where the pressure is relatively low.

layman's terms:

Basically Fluids behave diffrently at diffrent pressures .Its state is given by the phase diagram as .

Experienced example: so when we here a ratteling noise from the pipes when the fluid is flowing through them at high speeds its because of cavitation that this bubbles are formed and when they explode they create pressure waves and a ratteling noise is created .

Science behind it:

When the high velocity flowing fluid encounters a sudden change in area which gives to pressure diffrence ,so when this pressure reduces below the static pressure of the fluid ,the fluid changes phase and bubbles are formed .This bubbles occupy occupy almost 5000 times the space needed for a fluid.So in this condition they even collide with each other creating noise.so when the flow of this bubbles goes further again into the high pressure region a microjet due to high pressure surrounding impinges into this bubbles exploding them ,furthermore this micro jets even collides with the walls of our system damaging them .

Effect of cavitation can be seen over the period of time .

Geometry& boundary flagging :

To save up the computation time the sidewalls were set as 2d walls

As the geometry was symmetry only half of it was simulated to save computation time.

Meshing:

Structured mesh was created using face meshing and giving edge sizing for getting accuricy and capturing cavitation effectively mesh sizing at boundaries were kept fine

Boundary name assigned:

Case setup:

1. Setting the multiphase model to mixture & setting participating phases to 2

2. setting the primary and secondary phase to water liquid and water vapour .

3. specifying phase interaction and selecting mass transfer mechanism as cavitation.

4 . selecting viscous model.

• Case 1: Set up the case for Pressure difference of 30 bars

Velocity contour :

Water volume fraction:

massflow rate at outlet:

• Case 2: Set up the case for Pressure difference of 70 bars

Velocity contour :

Water volume fraction:

massflow rate at outlet:

Case 3: Set up the case for Pressure difference of 100 bars

Velocity contour :

Water volume fraction:

massflow rate at outlet:

Mass flow rate of around 84 kg/s was observed

(Note: Due to cavitation and re mixing of bubbles with the flow the outlet massflow rate has fluctuations,which is a major problem faced with design of injectors .So every value specified for all the above cases is averaged ,we can see the behaviour in massflow ratre curve & in animation attached for 100bar difference)

(To capture to varying massflow effect the 3rd case was run in transient condition with time step size of 0.0005s)

(In the animation we can see due to cavitation there is varying velocity effect observed at the ending of injector which is not good for any engine as it me alter the desired air fuel ratio.)

Conclusion:

• As pressure diffrence increases cavitaion zone length increases .
• Due to cavitation we observe fluctuations in massflow rate.
• Mass flow rate at the centre is highest and approaching the walls it decreases .
• More cavitation an shorten the life of injector moreover uneven massflow rates are not desirable for engine performance.