Enhancement of Cavitation Intensity in Co-Flow and Ultrasonic Cavitation Peening
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Water cavitation peening is a surface treatment process used to generate beneficial compressive residual stresses while being environmentally sustainable. Compressive residual stresses generated by the collapse of the cavitation cloud at the workpiece surface result in enhanced high cycle fatigue and wear performance. Co-flow water cavitation peening, a variant of cavitation peening involves injection of a high-speed jet into a low-speed jet of water, which makes the process amenable to automation and imparts the variant with the ability to process large structural components. Ultrasonic cavitation peening, another variant of cavitation peening, is used for peening small areas. However, an increase in cavitation intensity is needed to reduce the processing time for practical applications and to enhance process capabilities for a wide range of materials in both these variants. An experimental investigation along with numerical modelling is presented to demonstrate cavitation intensity enhancement through suitable modifications to the inner jet nozzle design in co-flow water cavitation peening. Particularly, the effects of upstream inner jet organ pipe nozzle geometry, inner jet nozzle orifice taper, and inner jet nozzle orifice length are studied to show enhanced cavitation intensity, measured via extended mass loss tests, strip curvature and residual stress measurements, high-speed videography, and impulse pressure measurements. It is found that the optimum inner jet organ pipe nozzle design, which generates enhanced pressure fluctuations through the introduction of a resonating chamber in the upstream section of the inner jet nozzle, generates 61% greater mass loss compared to the unexcited inner jet nozzle. Strip curvature, high speed imaging, and impulse pressure measurements support the mass loss results. Finally, residual stresses generated with the optimum organ pipe nozzle are shown to be deeper and more compressive than those generated with the unexcited nozzle design. The inner jet nozzle variants with diverging, zero and converging tapers are investigated experimentally and numerically to understand their influence on cavitation intensity. It is shown that the converging taper nozzle generates greater cavitation intensity, measured via mass loss and strip curvature measurements, than the zero and diverging taper nozzles. Impulse pressure measurements show the greater frequency of high-intensity events generated by the converging taper nozzle compared to the zero and diverging taper nozzles. Computational fluid dynamics (CFD) simulations help explain the experimental findings. Four nozzle variants with varying inner jet nozzle orifice length to orifice diameter ratios of 1,2,5 and 10 are investigated experimentally and numerically. The inner jet nozzle with an orifice length to orifice diameter ratio of 2 is shown to generate greater cavitation intensity than the other inner jet nozzles. A PEO aqueous solution (cavitation media) with 1000 parts per million by weight (wppm) polymer concentration is shown to enhance cavitation intensity by 69% over cavitation media with only water. High speed videography, impulse force, and surface roughness measurements confirm the greater cavitation activity in the 1000 wppm PEO aqueous solution. This demonstrates that suitable modifications can be engineered in the cavitation media to enhance cavitation intensity in ultrasonic cavitation peening. Thus, this thesis presents experimental and numerical investigations leading to superior inner jet nozzle design in co-flow cavitation peening and an experimental investigation of the role of polymer additives for suitable modification of cavitation media to enhance cavitation intensity in ultrasonic cavitation peening.