Erosion by Liquid jet Impingement on Aluminium using plain and cavijets

Abstract

The impact of materials by liquids occurs under various conditions and is common in a variety of engineering equipment such as hydroturbines, pumps, steam turbine blades, diesel engine liners, spillway gates, ship propellers, aircraft and spacecraft surfaces, and liquid-propellant rocket engine machinery. These erosion phenomena have resulted in serious maintenance problems arising from material damage and reduction in efficiency. Considerable attention has been focused in recent years on erosion due to liquid jet and drop impingement and on the similarities between erosion caused by liquid impingement and cavitation. On the positive side, liquid jets are finding several useful applications, such as in cutting, mining, and mixing processes. There have also been developments in the effective utilization of the erosion characteristics of liquid jets, pulsed jets, and cavitationinduced jets. The present study is concerned with erosion due to liquid jet impingement under multiple impacts using plain jets and cavijets, with experiments conducted in a rotating disc apparatus in which four aluminium specimens are mounted on the periphery of the disc. The principal features of the study are:

A wide range of hydrodynamic and geometric parameters are covered in the same setup. Besides studying the variation of cumulative erosion with time, a large number of singletime erosion studies are made, where the duration is sufficiently long to cross the maximum rate of erosion. For all the experiments, four identical specimens are used, and the variations of erosion among these specimens facilitate estimates of uncertainty bands even within the same setup. The jet velocity is also varied, and for several runs, it is comparable to the normal component of impact velocity, thus facilitating the study of the effect of tangential components of impact velocity. Experiments are also conducted by varying the standoff distance between the nozzle outlet and specimen over a wide range. Comparison is made of erosion due to plain jets and cavijets under multipleimpact conditions. In the context of developing general predictive theories of erosion applicable to both liquid impingement and cavitation, the present data can form a useful addition. The applicability of different predictive theories is studied.

Four plain nozzles of diameters 1.75, 2.25, 3.0, and 3.75 mm and four cavijets of nozzle diameter 12 mm with spherical inducers of 5, 6, 7, and 8 mm are used. The jet velocity is varied in the range 5 m/s to 70 m/s, the normal component of impact velocity in the range 39.6 m/s to 60.3 m/s, and the standoff distance in the range 20 mm to 220 mm. The experiments are categorized as totaltime studies and testtime studies. In the former, erosion measurements are made at a single time at the end of a constant test duration. In the latter, variation of cumulative erosion with time is studied by making measurements at specified intervals. A total of 592 experimental runs are made for the totaltime studies on plain jets for a constant test duration of 300 min. These data are presented in terms of the rationalised erosion rate, Rg, defined as the ratio of volume of material eroded per unit exposed area (mean depth of penetration, MDP) to the volume of liquid impinged per unit area (height of impingement, H). Rg may also be viewed as the average erosion rate at the end of the test duration. For testtime studies on plain jets, 25 runs are conducted, and results are presented as variation of MDP with H. For cavijets, experiments are first conducted to determine the critical inducer size, inducer location, and critical standoff distance for maximum erosion. For these conditions, testtime studies are conducted for 6 runs. Experiments are also conducted using a plain jet of suitable diameter to compare erosion due to plain and cavijets. In testtime studies of both plain jets and cavijets, the maximum test duration extends to 600 min. A study of variation of MDP among specimens indicates that variations are relatively high at higher jet velocities. At small times, when MDP values are very low, variability among specimens is high; this gradually reduces to a scatter within ±10% at larger MDP values. Totaltime studies for plain jets show that there is a critical standoff distance for maximum erosion. This distance is not affected by variation in the normal component of impact velocity and is only weakly influenced by jet diameter, increasing gradually with jet diameter. The variation of Rg with jet velocity also indicates a nonmonotonic trend, with erosion increasing up to a critical velocity and decreasing thereafter. The normal component of impact velocity (V) plays the most significant role in erosion. Despite nonmonotonic variation of Rg with standoff distance and jet velocity, an excellent correlation between Rg and V is obtained using all 592 runs, yielding a velocity exponent of 5. The effect of jet velocity is much smaller than that of V and varies with jet diameter. There is no strong influence of jet diameter on the rationalised erosion rate. If Rg is further normalized to separate the effect of V, a 450fold variation in volume loss among all runs reduces to a 6fold variation. In testtime studies, the important parameters examined are: (i) the incubation period, (ii) the maximum instantaneous erosion rate and time to reach it, and (iii) the maximum average erosion rate and time to reach it. Results presented as MDP vs. H follow a typical Scurve for both plain jets and cavijets, with an incubation period, acceleration period, steadystate period of maximum instantaneous erosion rate, and deceleration period. The MDP at the beginning of maximum instantaneous erosion does not vary significantly: the average value is 0.29 mm for plain jets and 0.24 mm for cavijets. Five alternative criteria are examined for determining the incubation period. It is found that Nb, the number of impacts corresponding to the beginning of maximum instantaneous erosion (or the equivalent Hb), is the most suitable criterion. A correlation is obtained between Nb and V for plain jets. Correlations are also obtained between maximum erosion rates and V, and between incubation period and maximum erosion rate for both plain jets and cavijets. Testtime studies for plain jets suggest the possible existence of a secondary nearsteadystate zone following the deceleration zone. For cavijets, there is an optimum inducer size and location for maximum erosion. As with plain jets, a critical standoff distance exists. The effect of jet velocity on cavijet erosion is significant and complex, unlike in plain jets. Jet velocity influences the incubation period, reflecting combined effects of impact pressure and cavitation. Cavijets produce greater erosion than plain jets over long exposure times. The variation of actual exposed erosion area with respect to nominal exposed area is studied for both plain jets and cavijets. It is found that average erosion rate is a better dependent variable than instantaneous rate for developing generalized erosion models. Powerlaw relationships of the form Ra / (Ra) = (MDP / MDP) are obtained for both plain jets and cavijets, for acceleration and deceleration zones. The exponent e is positive in the acceleration zone and negative in the deceleration zone. Testtime studies are used to determine its value.