Characteristics, correlations similarities and prediction of erosion due to cavitation and liquid impingment, Ph.D. Thesis

Abstract

Erosion due to cavitation and liquid impingement is a common type of wear that occurs in a variety of engineering equipment, structures, and devices, such as:

Hydro turbines Pumps Steam turbine blades Diesel engine cylinder liners Spillway gates Stilling basin piers Ship propellers Aircraft and spacecraft surfaces Liquid-propellant rocket engine machinery

Both cavitation and liquid impingement exert hydrodynamic forces on surfaces, and the nature of damage and material qualities required to resist erosion are similar in both cases. Cavitation and impingement erosion tests have therefore been used almost interchangeably for many years. Despite numerous investigations using different equipment and procedures for evaluating materials, generalized correlations and quantitative prediction of erosion, irrespective of equipment type and erosion form, are not readily available for design engineers. Furthermore, the characteristics of cavities and cavitation erosion in rotating components are not thoroughly understood to apply state-of-the-art design solutions or suggest suitable materials and overlays.

Objectives The objectives of the investigations presented in this thesis are:

Achieve a better understanding of erosion characterization and resistance, as well as the mechanism of erosion. Formulate a set of generalized equations to predict erosion of materials irrespective of equipment type, test liquid, and erosion form.

For these purposes, the following aspects of cavitation erosion and cavity characteristics using a rotating disk device have been studied under different hydrodynamic conditions:

Scope of Study

Effect of hydrodynamic and cavity parameters on erosion:

Influence of test duration on erosion rate Eroded particle distribution Area and growth of erosion Surface profiles

Mechanism and development of erosion on metallic and non-metallic materials from macroscopic and microscopic studies.

Effect of material properties on incubation period and erosion:

Correlations of erosion rate at different stages with single, multiple, and combined properties Role of true and dynamic properties in correlations

Similarities between different forms of erosion

Prediction of erosion of materials tested in various laboratories using generalized equations formulated in this study.

Experimental Details

Experimental data were obtained using a rotating disk device with water as the test liquid. Inducers of 25.4 mm diameter and 3 mm height were arranged on disks to vary velocity in the range 35.0 – 37.3 m/s. Pressure in the rotating disk chamber varied from 1.05 to 2 kg/cm² at 20 ± 3°C.

Key Observations

Erosion rate-time curves for metallic and non-metallic materials tested at different pressures and velocities exhibit several peaks and do not conform to standard patterns reported earlier, except in a few cases. At higher pressures where erosion is less, the number of particles increases, and as material strength increases, the average particle size decreases. Growth of erosion toward upstream is more pronounced, and erosion rate at an edge on a specimen exposed to sufficient non-cavitation period increases. Studies indicate that Thiruvengadam’s theory of erosion is applicable to rotating and stationary components. However, the modified intensity of erosion proposed in this investigation predicts erosion rates more accurately. The exponential relation of erosion with flow velocity varies considerably at different stages of erosion.

Maximum Erosion Conditions

Occur when transverse oscillations dominate over longitudinal oscillations and when cavity length is maximum in the rotating disk device. Correlations of cavity length, width, and area with erosion indicate that cavity parameters can be predicted with knowledge of flow conditions, enabling protective techniques to reduce erosion.

Microscopic Studies

Detailed microphotographic studies show that plastic deformation starts at weak spots such as inclusions and unfavorably oriented grains, transforming into pits surrounded by slip lines. Development of damage and erosion in brass, stainless steel, and mild steel is similar to that in copper, although flow patterns, slip lines, and pit morphology vary.Observations on Micro-Hardness and Failure

Both increase and decrease in micro-hardness were observed on damaged specimens, indicating the role of shear and compression during erosion. Failure of Perspex appears to be primarily brittle in nature.

Incubation Period and Material Properties

Correlations between incubation period and various material properties show:

At low intensities, energy properties primarily influence the duration. As intensity increases, elastic and strength properties become more significant.

Analysis of data from various laboratories reveals that strength properties predominate over energy properties. Results also show a good correlation between erosion rate and material properties.

Influence of Properties at Different Stages

Extensive correlations indicate that different properties influence different stages of erosion, depending on erosion intensity:

Initial phases: Hardness, shock compression rates, acoustic impedance, and strain energy play significant roles. Progressive stages: Modified resilience, ultimate resilience, and tensile strength become more important.

Combination of properties in series provides better correlations than single or multiple properties alone. Both energy and strength properties are highly significant in such correlations. Analysis of data from other laboratories supports these conclusions.

Role of True Material Properties

Use of true material properties improves correlations. It is possible to qualitatively evaluate resistance to cavitation erosion with knowledge of true stress-strain curves. Overall analysis of correlations for different forms of erosion (tested in this and other laboratories) reveals that:

Materials can be evaluated and erosion predicted even in entirely different devices. This supports the similarities between different forms of erosion, irrespective of erosion stage.

Best Predictors for Erosion

For prediction purposes:

Single properties: Tensile strength (Ts) and ultimate resilience (UE) are best. Multiple properties: Hardness (H) and elastic modulus (E) combinations are most effective.

Prediction equations formulated include:

e=Tsa,e=A1Tsb+A2,e=a2Uc,e=MUd,e=(SeH)x,e=A1(SeH)y+A2(UH)ze = Ts^{a}, \quad e = A_1 Ts^{b} + A_2, \quad e = a_2 U^{c}, \quad e = M U^{d}, \quad e = (SeH)^{x}, \quad e = A_1 (SeH)^{y} + A_2 (UH)^{z}e=Tsa,e=A1?Tsb+A2?,e=a2?Uc,e=MUd,e=(SeH)x,e=A1?(SeH)y+A2?(UH)z where:

eee = erosion rate PPP = erosion resistance SeSeSe = strain energy Subscripts indicate normalized quantities

These equations predict erosion rates of materials tested in other laboratories reasonably well.