Physical modeling of cavitation using an enthalpy based model

Physical modeling of cavitation using an enthalpy based model

Meijn, G.

Pecnik, R. (mentor)

Mechanical, Maritime and Materials Engineering

Sustainable Process and Energy Technology

Energy Technology

2015-10-05

Cavitation is defined as the process of formation and disappearance of a vapor phase in a liquid when this liquid is subjected to reduced pressures, followed by an increase of pressure. One of the main challenges in the design and application of centrifugal pumps is the ability to control and limit the development of cavitation. It is generally unlikely that a pump will operate across its entire operating range without any cavitation. Computational Fluid Dynamics (CFD) is used extensively to model cavitation in pump impellers. These models are almost always governed by empirical relations, which is no problem for cold water that has ample test data to be validated with, but makes the prediction of cavitation for hydrocarbons or amine solutions impossible. In the present work the cavitation development of water, butane and propane is described using a barotropic model assuming an isenthalpic expansion in the two-phase region. This barotropic relation should only be governed by the fluid properties, no empiricism should be involved. In order to validate the model, it is implemented in both 1-dimensional and 2-dimensional situations. In the 1-dimensional situation the Euler equations are solved for a single dimension in combination with source terms that model the varying area distribution of a Venturi-like nozzle. By forcing all three liquids through the nozzle at different velocities and pressures, insight is gained into the general qualitative performance of the model, both physical and numerical. In order to validate the model quantitatively with test data, the model is implemented into a 2-dimensional situation. A circular rod with a hemispherical head is pointed into the flow to obtain the pressure distribution over the head and part of the rod. This pressure distribution is then compared with data from experiments performed by Rouse \& McNown, in order to perform a simple quantitative validation of the model. Based on the work done in this thesis it is concluded that an isenthalpic barotropic model is a suitable approach to describe cavitation. Due to the independence from empirical relations, the development of cavitation for three different fluids can be modeled, based solely on fluid properties taken from a thermodynamic library. The general results from the 2D implementation are encouraging, but require more work to remedy the numerical instabilities and extremely slow convergence of the solution. The general recommendation for future work is to further develop and improve the numerics to make the solver more efficient and stable, thus viable for bigger simulations.

cavitation

http://resolver.tudelft.nl/uuid:7865d8d5-57c0-4dd0-aea1-4201ca6994f9

Student theses

master thesis

(c) 2015 Meijn, G.