J. Cent. South Univ. (2015) 22: 3712−3721 DOI: 10.1007/s11771-015-2914-y Impingement capability of high-pressure submerged water jet: Numerical prediction and experimental verification LIU Hai-xia(刘海霞)1, SHAO Qi-ming(邵启明)1, KANG Can(康灿)2, GONG Chen(龚辰)2 1. School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China; 2. School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2015 Abstract: At jet pressures ranging from 80 to 120 MPa, submerged water jets are investigated by numerical simulation and experiment. Numerical simulation enables a systematic analysis of major flow parameters such as jet velocity, turbulent kinetic energy as well as void fraction of cavitation. Experiments facilitate an objective assessment of surface morphology, micro hardness and surface roughness of the impinged samples. A comparison is implemented between submerged and non-submerged water jets. The results show that submerged water jet is characterized by low velocity magnitudes relative to non-submerged water jet at the same jet pressure. Shear effect serves as a key factor underlying the inception of cavitation in submerged water jet stream. Predicted annular shape of cavity zone is substantiated by local height distributions associated with experimentally obtained footprints. As jet pressure increases, joint contribution of jet kinetic energy and cavitation is demonstrated. While for non-submerged water jet, impingement force stems exclusively from flow velocity. Key words: submerged water jet; cavitation; shear effect; impingement test; micro hardness; surface morphology [2]. In this context, cavitating water jet is of great interest 1 Introduction in the investigation of jet impingement, but relevant studies have rarely been reported, particularly as jet Many industrial applications have benefited pressure exceeds 80 MPa [3]. In contrast, the considerably from the utilization of high-pressure water impingement mechanism of non-submerged water jet is jet, a promising material processing tool [1]. The straightforward, and two parameters, namely jet kinetic versatility of high-pressure water jet is being recognized energy and nozzle outlet diameter, are usually and one of the most impressive merits of water jet lies in underlined. the avoidance of excessive heat generation as water jet An in-depth understanding of submerged water jet stream impinges the target work piece. There are two is inseparable from its fluid dynamics features. Typical primary types of water jets, namely water jet and phenomena such as jet breakup, the formation and abrasive water jet (AWJ). For the latter, impingement development of vortices at jet rim, and cavitation have capability of the jet is largely ascribed to abrasive been widely recognized [4]. Weakness of generally used particles with diverse shapes. In comparison, water jet is experimental techniques is evident in measurements of not that powerful. Nevertheless, the participation of these transient and small-scale flow phenomena at high cavitation in submerged water jet opens possibilities for jet pressure. As far as optical flow measurement remarkable impingement or even damage effects. At high technique is concerned, dispersedly distributed bubbles or even ultra-high jet pressures, dual factors of jet kinetic and bubble clusters around the water jet stream hinder energy and cavitation erosion might contribute to the the penetration of laser into the jet stream. In some cases, damage to the impinged work piece. it is even difficult to distinguish the water jet stream In general, with a given nozzle, high jet pressure from ambient fluid. Alternatively, optical profiling leads to high jet velocity and thereby high jet kinetic apparatus facilitates a close examination of surface energy. Jet kinetic energy serves as a key factor morphology features of the impinged solid surface which underlying jet impingement. As for submerged water jet, might reveal flow-rated information, but so far only collapse of cavitation bubbles in high-velocity jet stream some preliminary conclusions have been obtained [5]. In contributes significantly to resultant impingement effects addition, numerical simulation, a vigorous instrument Foundation item: Projects(51205171, 51376081) supported by the National Natural Science Foundation of China; Project(1201026B) supported by the Postdoctoral Science Foundation of Jiangsu Province, China Received date: 2014−09−22; Accepted date: 2015−05−11 Corresponding author: LIU Hai-xia, Associate Professor, PhD; Tel: +86−511−88780072; E-mail: [email protected] J. Cent. South Univ. (2015) 22: 3712−3721 3713 highlighted recently, has been attempted to treat submerged water jet [6]. With respect to averaged flow parameters, numerical results are fairly reliable. Nevertheless, unsteady numerical results still endure debates owing to inevitable uncertainties. Finite element method (FEM), another important numerical strategy, has been utilized to simulate stress wave propagation in impacted solid samples, but for submerged water jet, transient energy release due to bubble collapse cannot be Fig. 1 Image (a) and cross-sectional diagram (b) of nozzle embodied with current FEM models [7]. (Unit: mm) Under high pressure conditions, flow characteristics and impingement capability of submerged water jet are cylindrical subdomain is sufficiently spacious to two aspects that call for an in-depth investigation. In line accommodate the development of the water jet in both with this acknowledged viewpoint, the present work streamwise and lateral directions. In particular, the focuses on submerged water jet subjected to jet pressures dimension in streamwise direction exceeds 33d, where d varying from 80 to 120 MPa. Computational fluid is the diameter of the nozzle outlet section, which dynamics (CFD) technique is utilized to virtually ensures a fully developed turbulent water jet. Structured visualize distributions of flow velocity, turbulent kinetic grids were used to discretize the entire computational energy, as well as void fraction of cavitation. A further domain and grid refinement was executed for near-wall step is unfolded as impingement experiment is flow regions. With a grid independence examination, the undertaken to examine the effects of submerged water jet total grid number devoted to the numerical simulation is through impinged solid samples. Ti−6Al−4V samples are 3376780. used as target samples in this respect. Micro hardness and surface morphology are measured for the impinged samples. For comparison, non-submerged water jets are studied under similar jet pressure conditions. It is anticipated to trace flow-related factors underlying the impingement capability of high-pressure submerged water jet and to render support to the design and optimization of water jet devices. 2 Numerical model and procedure 2.1 Geometrical models Fig. 2 Geometrical model of computational domain As shown in Fig. 1, the geometrical model of a nozzle parallel to practical applications is used in the 2.2 Turbulence model present numerical simulation. This nozzle differs from The renormalization group (RNG) k-ε turbulence commonly used convergent nozzles in that the outlet part model, firstly proposed by YAKHOT and ORSZAG [8] is conically divergent rather than a straight pipe. With in 1986, represents an improvement relative to the this nozzle, a tiny water jet stream is entailed. Meanwhile, standard k-ε turbulence model. This model is established the coherent segment of the water jet stream is based upon fuzzy mathematics principles, and the lengthened due to alleviated air disturbance immediately parameters incorporated in this model are deduced from downstream of the nozzle. Therefore, the advantage of related formulae rather than empiricism or experiments. this nozzle in the presence of high and ultra-high jet Along this line, the resultant equation of turbulent kinetic pressures is salient. Nevertheless, small jet stream energy dissipation rate ε differs from its counterpart in diameter elevates the difficulty degree in both the standard k-ε turbulence model. The transport measurement and simulation of the jet stream. equations of turbulent kinetic energy k and ε are: A combination of computational subdomains is illustrated in Fig. 2 where a straight subdomain upstream Dk k t G (1) of the nozzle is not shown for clarity. At initial stage, k Dt xi k xi pure water discharged from the nozzle enters into a cylindrical subdomain filled with water and the D 2 t * streamwise direction is +Z direction. The plane of Z= C1 Gk C 2 (2) Dt xi xi k k 0 mm overlaps with the nozzle outlet section. This 3714 J. Cent. South Univ. (2015) 22: 3712−3721 And the variable C* in Eq. (2) is defined as flow parameters can lend their support to illustrating 2 such a discrepancy, therefore no theoretical exploration C 31 of submerged water jet is incorporated in the present C* C 0 (3) work [11]. 2 2 3 1 0.012 where non-dimensional strain-rate parameter η is 3.1 Velocity distributions obtained from η=Sk/ε, and S denotes mean strain rate. In general, velocity magnitude acts as an indicator of the impingement capability of water jet; thus a high In addition, μ and μt are viscosity and turbulent priority is often granted to cross-sectional velocity viscosity, respectively. Gk is a turbulent kinetic energy production term related to viscous force. distributions in water