Studies on erosion due to liquid jet impingement

Abstract

The problem of erosion due to liquid jet impingement and cavitation is gaining importance with the rapid technological progress in the recent years. The possible uses of liquid jets in the mixing processes in chemical industry, in mining, tunnelling and rock and metal cutting are being studied in great detail. A detailed survey of the experimental and analytical investigations reported on this problem has been presented in the thesis. The main objective of the present investigations is to achieve a better understanding of the erosion due to the impact of high?velocity liquid jets. The work reported in this thesis pertains to a study of the following aspects:

The growth of erosion and erosion rate with test duration; The influence of jet velocity on erosion; The influence of the distance of nozzles from the test specimens and the jet diameter on erosion; and The variation of erosion with the frequency of impacts of the jet on test specimens.

A liquid jet impingement test rig has been designed and fabricated in the Hydraulic Laboratory. Experiments were conducted by varying the different parameters mentioned above and observations of erosion were made. The test material chosen was Aluminium because considerable damage could be obtained within a reasonable test time in the range of velocities and frequencies of impact covered in the experiments. The volume loss obtained in each case has been studied individually with each one of the parameters. The variation of normalized erosion rate with test duration has been studied for two nozzles of diameters 5 and 6 mm. The study in general indicated four zones of erosion, namely, an initial incubation zone of insignificant volume loss rate followed by an accumulation zone of increasing volume loss rate; an attenuation zone with decreasing volume loss rate and finally, a zone of approximately constant volume loss rate. To obtain a better understanding of the erosion–test time curves, volume loss was observed with two diameters of jets for periods ranging from one hour to a maximum of five hours. It was observed that there was a general increasing trend in the volume loss as jet velocity increased for the 5 mm diameter jet for all the test durations. For the 6 mm diameter jet, a peak in volume loss was observed at intermediate velocities particularly at small test times. The logarithmic plots of volume loss V with velocity U for the three different jets revealed that the erosion pattern followed a power law of the form, V = K U? (A?1) similar to the one obtained in cavitation studies. The exponent n was found to be around 7.5 for all the three jets. Defining a threshold velocity U? and rewriting the power law as, V = (U – U?)? (A?2) the exponent n was found to be around 6. Studies were conducted by varying the distance between the nozzles and test specimens from one to six inches. At a distance of four inches the damage obtained was a maximum. With increase in the diameter of jet, the normalized volume loss increased. The normalized volume loss exhibited a linearly decreasing trend with velocity. The effect of variation of frequency of impacts I on erosion was expressed as a power relationship, V = A? I? (A?3) and the exponent was found to be around 5. Defining a threshold frequency of impacts I? and rewriting the power law as, V = A? (I – I?)? (A?4) the exponent was found to be around 3 and the plot indicated a unified variation of volume