A study of abrasive waterjet characteristics by CFD simulation

Abstract

Computational fluid dynamics (CFD) models for ultrahigh velocity waterjets and abrasive waterjets (AWJs) are established using the Fluent6 flow solver. Jet dynamic characteristics for the flow downstream from a very fine nozzle are then simulated under steady state, turbulent, two-phase and three-phase flow conditions. Water and particle velocities in a jet are obtained under different input and boundary conditions to provide an insight into the jet characteristics and a fundamental understanding of the kerf formation process in AWJ cutting. For the range of downstream distances considered, the results indicate that a jet is characterised by an initial rapid decay of the axial velocity at the jet centre while the cross-sectional flow evolves towards a top-hat profile downstream.

Introduction

Abrasive waterjet (AWJ) technology is a state-of-the art cutting tool used to machine a wide range of metals and non-metals, particularly ‘difficult-to-cut’ materials such as ceramics and marbles [1], [2], [3], and layered composites [3], [4], [5], [6]. AWJ machining includes AWJ slotting, turning, drilling and milling [7]. Compared with the traditional and other non-traditional machining methods, the AWJ cutting technology has a number of distinct advantages, such as no thermal distortion, high machining versatility, ability to produce contours, good surface quality, easy integration with mechanical manipulators, and minimal burrs [8].

Typically, an abrasive waterjet system includes the following components: a special high-pressure pump or intensifier, a water catching unit, a nozzle positioning system, an abrasive delivery system, and a mixing unit made of an orifice, a mixing chamber and a focus nozzle. The commonly used or conventional AWJ machines are entrainment abrasive waterjet systems in which water is pumped to a very high pressure by using an intensifier technology. This high-pressure water then flows through an orifice to form a very high velocity jet of water. As the water jet passes through the mixing chamber, abrasive particles are sucked into the mixing chamber through a separate inlet due to the vacuum created by the water jet. The turbulent process in the mixing chamber causes the water and particles to mix and form a very powerful abrasive waterjet. By transferring the momentum between water and abrasive particles in the narrow focus nozzle, high velocity streams of abrasives are formed with great cutting capabilities.

Since the introduction of AWJ cutting technology, a large amount of research and development effort has been made to explore its applications and associated science [9]. However, this technology is still under flux and development. Its many aspects are yet to be fully understood. Specifically, an understanding of the hydrodynamic characteristics (e.g. velocity and pressure distributions) of an abrasive waterjet is essential for improving nozzle design, as well as for modelling, evaluating and improving AWJ cutting performance. However, this work has proved to be complicated. For example, the water–particle interaction in the mixing unit is extremely intricate while the ultrahigh velocity and small nozzle and particle dimensions make the investigation of the jet and particle behaviour difficult. Nevertheless, some important investigations have been reported on understanding the AWJ dynamic characteristics for relatively low velocity AWJs and for particular jet cutting status through theoretical [10], [11] and experimental [12], [13] studies as well as CFD simulation [14], [15]. However, research on ultrahigh pressure waterjets and abrasive waterjets to arrive at a comprehensive understanding of the jet properties has received little attention [3], [16].

The present work is to gain a fundamental knowledge of the ultrahigh velocity jet dynamic characteristics such as the velocity distribution. This knowledge is essential for enhancing the AWJ cutting technology, understanding the kerf formation or cutting process and modelling the various cutting performance measures that are required for process control and optimisation. For this purpose, CFD analysis is found to be a viable approach because direct measurement of particle velocities and visualisation of particle trajectories are very difficult for the ultrahigh speed and small dimensions involved. In this paper, CFD models for ultrahigh velocity waterjets and abrasive waterjets are established using the Fluent6 flow solver [17]. Jet dynamic characteristics such as the water and particle velocities for the flow downstream from a very fine nozzle are then simulated under steady state, turbulent, two-phase and three-phase flow conditions and a range of inlet conditions and input parameters. The results from the CFD study are then analysed to gain an insight into the jet characteristics and a fundamental understanding of the kerf formation process in ultrahigh velocity AWJ cutting.