Mechanism of Cavitation Erosion at the Exit of a Long Orifice

We carried out experiments to clarify the mechanism of cavitation erosion at the exit of a long orifice equipped at a pressure-reducing line in a pressurized water reactor (PWR). In order to ascertain the mechanism of cavitation erosion at the first stage and progression stage, we used a high-speed video camera. As a result, we observed cavity collapse near the exit of the orifice under oscillating flow conditions, which might be a major factor in the first stage of erosion at the exit of the orifice. To simulate the progression stage, we used an orifice with a cone-shaped flow passage at its exit, corresponding to an orifice diffuser. We observed cavity collapse near the exit, after which cavities that existed upstream in the cone shape collapsed in a manner similar to a chain reaction. The propagation speed varied with the quantity of cavities in the cone-shaped flow passage and cavities collapsed in a concentric circle pattern. Thus, the cavity collapse mechanism was concluded as follows: a pressure wave (shock wave) was generated by cavity collapse near the exit, then propagated upwards, and consequently caused cavity collapse upstream. This mechanism might promote cavitation erosion in an upward direction.