Experimental Study of Cavitation in the Nozzle Hole of Diesel Injectors Using Transparent Nozzles

The flow in nozzles of different sizes is studied by using transparent nozzles of the same size as in diesel injectors and refractive index matching. In a steady flow rig, short exposure video pictures of the flow were made at injection pressures up to 100 MPa. Discharge measurements and a measurement of the flow velocity in the nozzle hole using a modified laser-two-focus-velocimeter completed the picture of the flow. Above an injection pressure threshold that depends on the nozzle geometry and chamber pressure, cavitation appears at the sharp inlet corner of the nozzle. With increasing injection pressure the cavitation reaches the nozzle exit (supercavitation). The discharge coefficient and the spray angle then level off at a value that is almost independent of any further increase of injection pressure. The measured flow velocity is close to the velocity calculated from the pressure drop assuming inviscid incompressible flow.