Experimental Assessment of Water Hammer-Induced Column Separation in Oil-Hydraulic Pipe Flow

Abstract

Cavitation erosion through water hammer and column separation is a major concern in hydraulic applications such as percussive rock drilling. Cavitation aspects must be considered both in early and late design stages, which require deep knowledge and tools for prediction. In this study, an oil-hydraulic test equipment for water hammer assessment was designed using state-of-the-art simulation tools. Several tests were performed, with and without column separation, showing good repeatability on measured pressures. At higher flow rates, column separation was the dominating feature and several high-pressure peaks with subsequent time delay reduction could be observed. These patterns were affected by the oil temperature, with most substantial changes at lower temperature ranges (<32 °C). Standard transmission line simulations managed to predict the water hammer, but as expected not the column separation, which is the theme of future work using this setup.

High-Speed Imaging of Water Hammer Cavitation in Oil–Hydraulic Pipe Flow

“…The test procedure was very similar, although the pressure level had to be reduced considering the strength of the acrylic glass, which in turn also affected the flow rate. Estimated cavitation numbers G A [28] from one-dimensional simulations were, however, in the same range as our previous work [3] (Table 1).…”

“…In this section, the test equipment and setup will be briefly explained, including the methods for wave-speed calculation and separation-length estimations. More detailed information on the rig can be found in the previous study by Jansson et al [3].…”

“…The test equipment is a modification of that used in previous work by Jansson et al [3]. The steel pipe was replaced by a pipe in transparent acrylic glass (plexiglass) (Figure 1).…”

“…The processes for bubble collapse and cavitation erosion are very short and typically happen on a time scale of microseconds [2]. Repetitive water hammerinduced cavitation may promote erosion in crucial parts of the machinery [3]. Thus, avoiding cavitation or predicting to which extent parts will be subjected to cavitation would increase the lifetime of hydraulic machinery.…”

“…Prediction of column separation and cavitating pipe flow is typically done by reducedorder models [10][11][12][13][14] or sophisticated computational fluid dynamics (CFD) methods [15][16][17][18][19][20]. Modelling predictions of column separation cavitation have mainly been validated against measured pressure characteristics (e.g., [3,12,21,22]), while their ability to estimate local cavitation characteristics is still not well understood. Experimental visualization of these flow regimes has been investigated (e.g., [23][24][25][26][27]), however, not at the small time scales associated with oil-hydraulic applications.…”