Theoretical and experimental investigations on critical multiphase flows in safety relief valves and nozzles

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2016
Thesis identifier
  • T14348
Person Identifier (Local)
  • 201273469
Qualification Level
Qualification Name
Department, School or Faculty
  • Two-phase gas/liquid critical flows are in general complex due to the existence of mass, momentum and heat transfer between phases. In this study these processes have been investigated in a safety relief valve where the complex geometry introduces multiple choking points, shock waves, rapid accelerations involving rapid state changes of velocity, pressure and temperature and flow regime changes. These effects influence to a different degree the valve mass flowrate and the forces acting on the valve and thus the rating and dynamic response when in operation. At present no predictive tools are available for design evaluation of safety valves operating under two phase conditions. In this thesis the ability of the standard two fluid model to predict such processes is investigated.Two fluid (Euler-Euler) multi-dimensional modelling approaches with a mixture k-Ɛ turbulence model to predict such conditions are investigated. The study has been divided into two stages: firstly, for fixed mass fraction conditions air-water two phase critical flows through a conventional spring loaded safety relief valve commonly used in the refrigeration industry have been carried out experimentally and computationally. Secondly, to extend the study to thermal non equilibrium conditions investigations of steam-water two phase flows through a converging-diverging nozzle have been performed computationally. In the first stage, experiments have been carried out for a range of pressures (4.8 - 13.8 bar), a wide range of water fractions (0 - 0.89) and for various different opening positions of the valve disc. Quasi steady flow has been assumed appropriate and valve flow-lift and force-lift characteristics have been obtained, which determine the capacity of the valve to control pressure, the sizing of the spring and the dynamics of the valve during operation. Comparison with the simplified homogenous mixture model will show that this model tends to underestimate mass flowrates for medium to high liquid mass fraction. Force and flow scaling parameters have been explored for use in valve design under various multiphase flow conditions. In the second stage, for thermal non equilibrium conditions the numerical calculations have been made for a range of pressures (1.34 - 1.89 bar), a range of water mass fractions (0 – 0.36). The computational predictions of mass flowrate compare well with experimental data from the literature. In general, the two fluid model can adequately account for mechanical non equilibrium for these complex flow conditions with the use of simplified droplet sizing rules. Thus the CFD Euler-Euler model predictions have been found in good agreement with the experimental data. For thermal non equilibrium where phase changes dominate less progress was achieved and further investigated required.
Advisor / supervisor
  • Dempster, William
Resource Type
Embargo Note
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