HYDRODYNAMIC CAVITATION PHENOMENON IN MULTI-PHASE FLOWS

• Author / Creator

  Shi, Hongbo

• The main objective of the present study is to experimentally and numerical studies of hydrodynamic cavitation phenomenon in the Venturi tubes, in order to validate and further develop numerical multiphase flow models with the obtained data. To achieve this, a visible experimental set-up was designed. Besides, both two-dimensional and three-dimensional numerical simulations were conducted to analyze the characteristics of cavitation that cannot be experimentally evaluated, such as the volume of each phase, the turbulence distribution, and the location of cavitation and mixing zones. The cavitation behavior has been described by means of non-dimensional parameters such as cavitation number, Reynolds number, pressure loss coefficient and vapor/gas volume fraction.

  The commercial computational fluid dynamics (CFD) software, ANSYS FLUENT 16.2 is utilized. For experimental data acquisition, five cavitating Venturi tubes with different geometrical parameters were manufactured and tested at different inlet flow conditions in order to measure their inflow parameters and to obtain their characteristic curves. On the numerical part, based on the two-phase (water-vapor) mixture model and four-phase (water-solid-vapor-air) Eulerian-Eulerian model, a set of global computation model was developed and applied to multi-phase modeling of cavitation process in different cavitating Venturi tubes. Predictions of the pressure drop obtained from the CFD model are generally in good conformance with experimental measurements.

  In the study of two-phase cavitaitng flows, the effect of the convergent angle and divergent angle on the cavitation performance was investigated experimentally and numerically. Both the numerical and experimental studies reveal that the change in the convergent angle and divergent angle has significant effects on flow characteristics and the generation
  of cavitation. It was shown that a 85◦ convergent angle and a 2.5◦ divergent angle are of benefit to cavitation. A scaled-up study of the Venturi geometry has been conducted using CFD-based numerical simulations. Finally, a empirical model enabling the prediction of cavitation in Venturi tubes has been developed and validated.

  In the study of four-phase cavitaitng flows, a new four-phase global model was developed based on a simpler engineering approach and validated against experimental data. Both the
  numerical and experimental studies reveal that the addition of solid particles (Ws=5∼30 wt%) in the cavitating Venturi tube has significant effects on the generation of cavitation. The outcomes show that the higher solid mass concentration is of benefit to cavitation intensity. The proposed CFD model has proved to be an efficient and reliable tool in predicting the cavitation activities and performance characteristics of the cavitating Venturi tube.

  In the micro-scale study, the particle-flow interaction and characteristics of cavitation are investigated numerically using a CFD-based mixture model. The particle size is varied
  between 10 microns and 100 microns. CFD-based simulations are conducted over a wide range of particle Reynolds numbers Re from 1 to 1800 for different particle sizes. The effects
  of the particle shape, the surface roughness, the number of particles, and the particle surface temperature on flow characteristics and occurrence of cavitation were investigated. The results show that particle roughness significantly increases the occurrence of cavitation. Moreover, cavitation development increases as the particle surface temperature increases from 40◦C to 99.9◦C. Finally, an empirical relation enabling the prediction of cavitation in the particular flows has been developed based on CFD results.

  In the end, a computational investigation of the cavitation and mixing characteristics of two miscible (Schmidt number Sc = 103) turbulent water flows with different viscosity ratios is conducted in a Venturi tube. The 2D axis−symmetric RANS and 3D LES turbulence models are used for the prediction of flow characteristics. The numerical results reveal that the RANS solutions underpredict the rate of vapor production and overestimate the mixing rate of two miscible fluids in comparison to LES simulations for the inlet Reynolds number of 19738 (Case−1) and 19286 (Case−2). In particular, there is a larger cavitation zone in the Venturi tube with LES in comparison to RANS calculations.

• Subjects / Keywords

  • Numerical Modeling
  • CFD
  • Cavitation
  • Multiphase flow

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-p35p-7c54

• License

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