Cavitation erosion in waters having different surface tensions

Cavitation damage is affected by physical properties of liquid. In order to study such an effect solely caused by the change of surface tension, cavitation erosion tests were performed using waters with different surface tensions. The value of surface tension was adjusted by adding photographic wetting agent, ranging from 0.0025 to 3.0% (wt.%). The mechanism of surface tension influence on cavitation damage is discussed by relating it to the observed cavitation pattern and bubble behavior. The erosion rate firstly decreases gradually and then drops sharply at about half the value for water (sigma=30 x 10(-3) N/m) when surface tension reduces from that of water. It was also observed that due to the addition of wetting agent, both the maximum of pulse peak height and the number of counts of bubble collapsing pressure (particularly for those strong ones), as well as the total impact energy, were also reduced. This was mainly due to the influences of surface tension on the formation of bubble clusters in terms of their size and number. In particular, the turning point of the size (mean diameter) of bubble clusters coincides with that of the relative erosion rate. This coincidence supports the proposed mechanism. That is, the reduction of surface tension promotes the instability of bubble growth and collapse which breaks up bubbles into smaller bubbles with less or little erosive power. However, there was a discrepancy of the impact energy with bubble size and erosion rate for the high addition concentrations towards and after the turning point of surface tension. This was presumably due to some unknown factors involved. (C) 2002 Elsevier Science B.V. All rights reserved.