INTRODUCTION
Considerable research has been conducted on the use of steady and pulsed jets of water at high pressures to produce slots or holes in rock, and to fracture rock. The primary objectives of prior research have generally been to determine the feasibility of excavating or drilling rock or minerals and to determine the optimum jet parameters for this purpose. The present study was undertaken in order to identify the pertinent dimensionless parameters involving jet and rock properties which characterize the liquid jet penetration process and to develop empirical correlations of the existing test data which would permit engineering prediction by means of scaling relationships. The present paper is in the nature of a progress report, because not all the existing test data have been processed. Also effort has been concentrated on correlation of data on the specific energy (jet kinetic energy per unit volume of rock removed or broken), rather than correlation of jet penetration depth, hole diameter, kerf depth or kerf width. However, it is clear that this method of correlating data on specific energy in terms of dimensionless parameters shows great promise and provides insight into the nature of the physical processes of jet erosion and fracture. It therefore can help to avoid drawing incorrect conclusions from limited experimental data.
DIMENSIONAL ANALYSIS
The method of dimensional analysis is particularly valuable where the mathematical relationships controlling a physical process are unknown or complex. The grouping of experimental parameters into dimensionless combinations aids in interpreting data from experiments where several factors have been varied.
The first case to be considered is for pulsed water jets impinging perpendicular to a large plane surface of rock. Most of the existing test data apply to this case with jet pressures up to 5000 kg/cm2 and jet diameters of 1 to 5 mm. The typical result of one pulse is to form an approximately cylindrical hole in the rock. A useful engineering parameter is the specific energy, which is defined to be the total kinetic energy in the pulsed jet, divided by the volume of the hole.
It is assumed here that the only experimental parameters of the jet which affect the process significantly are:
d = the nozzle diameter
q = the jet stagnation pressure (assumed to equal the nozzle inlet total pressure and also equal to ¿Vj2/2g, where¿ = jet density and vj=jet velocity
L = length of slug of water = vjt, where
t is the pulse duration
S = standoff distance from nozzle to rock
The effects of jet Reynolds number (see Ref. 6), nozzle geometry and nozzle roughness are assumed to be of negligible significance in this study.
It is assumed that the rock resistance to erosion can be characterized by a single strength parameter which is chosen to be the specific energy of fracture: (Mathematical Equation)(Available in full paper), which is the strain energy in uniaxial compression at failure. This parameter is chosen because it is directly related to the Protodyakonov hardness number F by the relation (Ref. 10 & 11).
