Cavitation

  • Cavitation - Fuel Injectors and Nature

    While the effects of cavitation are generally undesirable with respect to rotating machinery such as propellers, impellers and turbines the opposite is true when applied to fuel injectors.

    When a fuel injector discharges a very high velocity liquid stream under enormous pressure is accelerated through an array of orifices, depending on the design. As the flow streamlines converge through the exit the velocity of the fluid increases and according to the Bernoulli principle this causes a pressure drop. When the internal fluid pressure drops below the vapor pressure of the liquid being injected cavitation bubbles form.

    Injector Nozzle
    Figure 1: typical injector nozzle

    The diagram shown below taken from www.fluid-research.com and generated using CFD (Computational Fluid Dynamic modeling software) depicts an ejection fraction saturated with cavitation bubbles.

    Injector Nozzle Cavitation
    Figure 2:CFD similuted injection event showing cavitation

    In the case of the CCES there are six ports in the modified Ford injector and the fluid being ejected is water rather than diesel fuel. The rationale here is that the bubbles improve fuel dispersion and this leads to more complete combustion. In our case we are not interested in combustion, but we are interested in generating heat and converting all of the water to superheat steam.

    When cavitation bubbles either collapse or are crushed significant energy is released. Under certain circumstances this energy can exceed many thousands of degrees Kelvin. In the CCES device the impact chamber wall is positioned close enough and at such an angle to the ejection port to create a shock wave within the impacting water droplet. This occurs within a time frame such that the water is still in a liquid phase when the collision occurs.

    Impact Chamber Cross Section
    Figure 3: CCES Impact Chamber Cross Section




    12 – Modified Diesel Injector
    13 – Impact Chamber Outer Shell
    14 – Pressure Regulating Valve
    15 – Immersion Thermocouple Probe
    20 – Washer Gasket
    21 – Injector Retention Block
    22 – Ceramic injector insulator block
    25 – Ceramic Insulator Block
    36 – O-ring seal

















    The diagram below depicts the compressing water droplet. The water hammer pressures within the droplet can spike to levels exceeding the injector rail pressure. Some of the water is changing phase due to the ambient heat within the impact chamber and the balance undergoes compression and further heating caused by the energetics of bubble collapse.

    Shockwave Propagation in Water Droplet
    Figure 4: Compression of cavitation bubbles

    If there was no heat generated the impact chamber would be cooled and its temperature would rapidly decrease as the steam is released. This is precisely what happens when liquid water is sprayed onto a hot metal surface. The opposite is observed as evidenced in the 5:03 minute accompanying video.

    In nature the pistol shrimp fires a stream of cavitation bubbles to stun or kill its prey ...view video, whereas the mantis shrimp uses cavitation differently. When it strikes it is found that, as a result of the limb's extraordinary speed, the water cavitates (vaporizes) when the limb hits the prey. When these bubbles collapse they cause a small implosion in the water which produces heat, light and sound. ...view video

    In the case of the CCES the device generates hundreds of thousands of bubbles many time a second for each injector. The resulting heat and hence steam are the essence of the invention.

    Based on our calculations an eight impact chamber steam CCES will use approximately 5 horsepower to produce 250 pounds of steam per hour.

    The following video and explanation of what is going on demonstrates the heating effect of cavitation. This video documents the activity of a single impact chamber where the heater has been turned off. The injector is pulsing at 2 to 3 injections per second and the resulting steam is being released through a 200 psi pressure relief valve. The chamber was initially heated to 375 F and then turned off for the test. The video documents the elevation in impact chamber temperature despite the fact that no additional heat is being added and heat is being released in the form of steam. ...view video

    To view this video with a detailed summary description of the events taking place follow this link ...view video