CFD INVESTIGATION OF TURBULENT WATER FLOW IN A PIPE ELBOW: ASSESSMENT OF CAVITATION RISK

Cavitation represents the formation and subsequent collapse of vapor or gas bubbles in the vicinity of solid surfaces. Depending on the mechanism of initiation, cavitation can be classified as hydrodynamic, acoustic, optical, or particle cavitation. Broadly, these phenomena may be grouped into two categories: cavitation induced by stress in liquids (hydrodynamic and acoustic) and cavitation induced by localised energy deposition (optical and particle). In mechanical systems, particularly in fluid transport applications, hydrodynamic cavitation is the most common type and is associated with adverse effects such as noise, vibration, and a reduction in the load-carrying capacity and durability of machine elements (gears and bearings). A combined mechanism, referred to as hydrodynamic–acoustic cavitation (HAC), arises when structural vibrations at specific natural frequencies, or other excitation sources, produce pressure fluctuations that promote bubble formation under the influence of ultrasound. In such cases, acoustic cavitation act synergistically with hydrodynamic cavitation, resulting in more severe material degradation within the system. Both hydrodynamic and acoustic cavitation processes generally progress through four characteristic stages: incubation, acceleration, deceleration, and terminal stage. Potentially, a fifth phase may also appear, which refers to the complete degradation of the work and the consequences of which can be catastrophic. In fluid transport piping systems, cavitation arises in regions where the local pressure decreases below the saturation pressure. Such conditions may occur along straight pipe sections due to frictional losses, at abrupt changes in flow direction (e.g., elbows), within various fittings, or in pump impellers. In addition, entrained gas molecules within the liquid often act as nucleation sites for vapour–gas bubble formation, thereby promoting the initiation of cavitation [8–10]. Therefore, one of the key parameters when sizing and determining the geometry of pipelines is protection against cavitation. In this study, computational fluid dynamics (CFD) simulations using OpenFOAM were conducted to investigate water flow through a DN65 pipe elbow at five different average inlet velocities: 5, 10, 15, 20, and 25 m/s. Although such elbows are not typically subjected to these flow rates in practical applications, the primary objective of this research was to identify the threshold average inlet velocity at which the system pressure falls below the saturation pressure.