materials Article Analysis of the Erosivity of High-Pressure Pulsating Water Jets Produced in the Self-Excited Drill Head Monika Szada-Borzyszkowska , Wojciech Kacalak , Dariusz Lipi ´ nski * and Bla ˙ zej Balasz   Citation: Szada-Borzyszkowska, M.; Kacalak, W.; Lipi´ nski, D.; Balasz, B. Analysis of the Erosivity of High-Pressure Pulsating Water Jets Produced in the Self-Excited Drill Head. Materials 2021, 14, 4165. https://doi.org/10.3390/ma14154165 Academic Editors: Weronika Kruszelnicka and Andrzej Tomporowski Received: 28 May 2021 Accepted: 25 July 2021 Published: 27 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Faculty of Mechanical Engineering, Koszalin University of Technology, Raclawicka 15-17, 75-620 Koszalin, Poland; monika.szada-borzyszkowska@tu.koszalin.pl (M.S.-B.); wojciech.kacalak@tu.koszalin.pl (W.K.); blazej.balasz@tu.koszalin.pl (B.B.) * Correspondence: dariusz.lipinski@tu.koszalin.pl Abstract: The dynamic impact of a water jet with a periodically changing structure can be used in various industries. The paper presents a design solution for a self-excited pulse head. This head can be used in mining for drilling holes and breaking rocks. The design of the head was developed based on computer simulations, which made it possible to learn the mechanism of impulse shaping inside the head. Tests of the water jet produced in the self-excited pulsation head showed the occurrence of periodic changes in its internal structure and pulsation frequency. A significant increase in the dynamic stream pressures was demonstrated for the head working in the water environment compared to the head working in the air environment For example, for nominal medium and highest pressures, this increase is up to 82%, while for the lowest pressures (10 MPa), the pressure force values increase by 46%. It was found that an increase in the nominal water pressure causes a decrease in the frequency of hydrodynamic pulses in the head operating in both the water and air environment. Keywords: water jet; pulsed water jet; FEM; simulations; self-excited pulsating head; hydro jetting-erosion 1. Introduction The use of pulsating water jets in mining is a new issue. A pulsating water jet can be used to intensify rock breaking and in the production of oil and gas. There are many varieties of hole drilling technology using a high-pressure water jet. Among them are the methods using a pulsating water stream. The impulse water jet is used, among others, in drilling technology. Various designs of nozzles shaping a pulsating water stream are known. The results in [1] show that pulsating flows can significantly increase the speed of drilling an oil well. There is still a search for better solutions for the use of a pulsed jet in the drilling technology in oil mining [2,3]. A pulse head with specific geometrical parameters of the vortex chamber is usually used to generate such a stream. The advantage of such heads is the lack of moving parts. The most important technical problems include the strong dependence of the stream properties on the design parameters of the self-excited pulse heads. A pulsed jet of high intensity and powerful water can potentially also be an effective and alternative tool for secondary rock breaking. Effective crushing of rocks by the pulsat- ing stream of water ensures the intensification of the rock cracking energy with a reduced reaction force on the handle of the working device. Rock fracture is also affected by the distance of the water jet from the processed material and the structural structure of the processed material. In the work of Dehkhod [4], the influence of the pulsating stream on the rock fracture process was investigated. The research showed the formation of deep internal damage in the layers of the tested material. The innovative generation of a pulsating water jet was developed by the Vijay team [5]. It consists in generating hydrodynamic pulses formed using a device that segments a water stream with an ultrasonic frequency. Works on the further development of this method Materials 2021, 14, 4165. https://doi.org/10.3390/ma14154165 https://www.mdpi.com/journal/materials