VENTURI STUDIES WITH WATER ON CAVITATIOK INCEPTION CAVITY CHARACTERISTICS AND CAVITATION EROSION BEHIND BLUFF BODIES

Abstract

Cavitation is defined as the formation, growth and collapse of vapour or gas? and vapour?filled bubbles in a liquid subjected to reduced pressures (of the order of the vapour pressure of the liquid) at constant ambient temperature. This phenomenon has been encountered in many diverse fields of engineering and technology and hence has attracted the attention of many researchers. In spite of many investigations, the fundamental mechanics of cavitation and the erosion caused by cavitation are far from being completely understood. The main objective of the present investigations is to achieve a better understanding of this complex phenomenon. The following aspects of cavitating flows past sources mounted in the test sections of two different two?dimensional venturi set?ups are studied under different hydrodynamic conditions: A. The effects of size, shape and surface roughness of the source, and free stream velocity on the incipient and choke cavitation numbers. B. The characteristic dimensions of the steady cavity, viz., the length, maximum width and angle of detachment, and the cavity pressure. C. The mechanism of unsteady cavity and the correlation between Strouhal number and cavitation number. D. Correlation of cavitation erosion with cavitation number and source size and comparison of erosion with the cavity characteristics. The thesis consists of six chapters. Chapter one presents the survey and objectives of the present investigations. The details of the experimental equipment, test procedures and test conditions are given in chapter two. In chapter three the results of the cavitation inception observations are presented, analysed and discussed. The investigations on the characteristics of the fixed cavity and mechanics of the unsteady cavity are covered in chapter four. The results and analysis of cavitation erosion experiments are included in chapter five. The conclusions and recommendations are presented in chapter six. The investigations were conducted in two different two?dimensional venturi set?ups with 101.6 × 39.0 mm and 101.6 × 12.7 mm test sections using water as the test liquid. Circular cylinders of sizes from 10 to 30 mm diameter were employed as cavitation sources for most of the experiments. Additional cylindrical sources up to a maximum diameter of 50 mm and two other sizes of wedge and flat?plate sources were also used in the studies on cavitation inception and cavity characteristics. The ambient pressure was varied in the range 0.50 to 7.88 kg/cm² absolute and the free stream velocity in the range 6 to 25.85 m/sec. A Fastax high?speed motion?picture camera, model W?3, with a maximum picturing rate of 8000 frames per second was used in the investigations of the unsteady cavity. Investigations reveal that the incipient, desinent and choke cavitation numbers increase with the size of the source. These numbers decrease initially with velocity and remain approximately constant thereafter. The origin of incipient cavitation bubbles depends on the source shape. The results indicate that cavitation inception may be delayed by streamlining the boundary of the source. The presence of uniformly distributed roughness over the surface of circular cylinders results in the inception of cavitation in the wake earlier than on the source itself and does not affect the choke cavitation number. The cavitation hysteresis decreases with increase in velocity. The influence of the test?section boundaries on the cavitation numbers is less significant for blockage ratio, d/B ? 0.3, and beyond this value the cavitation numbers increase at an accelerated rate. In the definition of the cavitation number, the use of maximum velocity in the test section instead of free stream velocity results in unified variation of incipient and choke cavitation numbers with velocity for the sources investigated. The normalized length of cavity increases with the decrease in cavitation number. This increase is gradual and linear up to a particular value of cavitation number which increases with source size. Thereafter, the normalized length of cavity increases rapidly tending towards infinitely large values even for small decreases in cavitation number tending to choke conditions. The normalized length of cavity increases with increase in bluffness of the source. The normalized maximum width of cavity decreases with increase in cavitation number. The angle of detachment, ?, increases with cavitation number, K, and decreases with the size of the source. The pressure inside the cavity is above the vapour pressure of water at a given ambient temperature. The variations of the length and maximum width of cavity are influenced by the cavitation number only, irrespective of the variations in ambient pressure and free stream velocity. The use of the effective cavitation number (K – K?) in the analysis of the cavity characteristics results in a unified variation of cavity dimensions. The length of cavity tends to infinity as the effective cavitation number tends to zero. Theoretical models predict that the length of cavity tends to infinity as the cavitation number tends to zero. Hence, the use of the effective cavitation number is a step towards a better analysis of cavity characteristics. The nature of the mechanism of vortex shedding behind sources of circular cylinders, wedges and flat plates appears to be the same. The Strouhal number is a minimum at a certain critical value of cavitation number which increases with source size. The minimum Strouhal number, S???, increases with source size. At the limiting condition of cavity formation, the Strouhal number attains a constant value which increases with source size. The presence of a splitter plate on the downstream side of the circular cylinder affects the frequency of vortex shedding. A sudden jump in the Strouhal number, S?, from 0.29 to 0.70 (approximately) occurs when the normalized length of splitter plate, L??/d, is around 2.50 for the 20 mm diameter source investigated. The variation of cavitation erosion with test time exhibits three zones, namely, incubation zone, accumulation zone and a third zone with a number of peaks which can be approximated to represent the maximum rate zone. The variation of weight loss with free stream velocity indicates peak erosion conditions at a critical velocity. This critical velocity decreases with increase in source size. The occurrence of a peak in the plot of erosion against free stream velocity suggests that it is not possible to express the variation of erosion with velocity by a single power law. The exponent in the power law depends upon the cavitation condition. For the flow conditions corresponding to a cavity length of ?/d = 3, the presence of splitter plate has an inconsistent effect on erosion up to ???/d = 3.0. The erosion is suppressed considerably for ???/d > 5.0.