Abstract
High-pressure water jets bear a great technological potential to enhance geothermal deep drilling. Compared to existing water cutting technologies, significantly different operation conditions are encountered under deep-drilling conditions, such as high ambient pressures. The fundamental fluid mechanics are significantly affected by those operation conditions. In this work we examine the influence of increasing ambient pressure of up to 12.0 MPa on the water jet characteristics under submerged drilling conditions. PIV measurements of the jet flow field at changing cavitation numbers reveal two characteristic regimes, which are distinguished by a critical cavitation number. In the cavitating regime, the jet decays considerably faster with increasing distance to the nozzle than in the non-cavitating regime. In addition to that, an increasing cavitation intensity shortens the potential core length of the water jet and increases the jet spreading angle and with this has a similar effect on the jet as increasing turbulence intensity in single-phase flows. Related to the decreasing kinetic energy of the jet in the cavitating regime, the resulting impact force of the water jet on the specimen surface decreases with increasing cavitation intensity. Our investigations indicate that a technology transfer from water jet cutting to submerged jet drilling requires adjustments of both nozzle geometries and jet operation conditions.Graphic abstract
Highlights
High-pressure water jets are widely used in industry
We examine the fluid mechanics of the jet drilling process under submerged conditions to quantify the influence on the drilling performance
Our experiments show that an increasing cavitation intensity, i.e., a decreasing cavitation number, has similar effect on the potential core length and jet spreading angle as it is known for jets with an increasing turbulence intensity in singlephase flows (Gauntner et al 1970; Carlomagno and Ianiro 2014)
Summary
High-pressure water jets are widely used in industry. One well-known technology is water jet cutting of various materials, which are processed by a focused liquid jet. Important fluid mechanical parameters for process characterisation are the available kinetic energy of the jet and the stagnation pressure. The kinetic energy of the jet emerging from a nozzle and with this the velocity distribution, the jet spreading angle and the potential core length are quality measures for jet drilling. This mixing results in a widening of the jet and the jet spreading angle α increases In this region of developing flow, the mixing layer surrounds a jet core where the centreline velocity is equal to the nozzle exit velocity. A high jet velocity and a small jet spreading angle are favourable to ensure a high energy impact on the surface Those jet parameters can be used to characterise the emerging jet and the influence of ambient conditions on the jet. Different parameter studies varying the flow rate, stand-off distance between the high-pressure nozzle and the rock specimen, nozzle types and back pressure were conducted, i.e.,
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