Wall Condensation Modelling in Convective Flow

University essay from KTH/Kraft- och värmeteknologi

Abstract:

Modelling condensation of water vapour is important in a number of engineering applications, such as nuclear reactor containment, rocket engine nozzles and heat exchangers. The current study investigates the possibilities of modelling condensation induced by a cold surface in a flow at high velocity and temperature. A number of non-condensable gases are present in the flow. The possibilities of condensation modelling are investigated in ANSYS CFX and ANSYS Fluent, with focus on ANSYS CFX. A case study is done of a 2D flat plate, with water vapour and non-condensable gases at varying temperatures and velocities. The condensation models in ANSYS CFX are investigated for a few basic flow setups and the model deemed most appropriate for wall condensation is investigated in greater detail.

The wall condensation model in CFX is investigated to a greater extend, and compares well with an analytical solution for laminar flow. The complexity of the flow is gradually increased to determine limitations and best practise settings for flows at high velocity and temperature. For isothermal walls and for a conjugated analysis, using a solid with a specified adiabatic wall temperature and heat flux coefficient to induce condensation, the wall condensation model works well for grids with a y+ above 1. For finer grids, convergence is found to be difficult to achieve.

The choice of material properties for water vapour was found to play an important role in terms of stability. Real gas properties to define the water vapour material properties are deemed important to avoid unphysical results in terms of the temperature. The wall condensation model in ANSYS CFX is deemed to be an appropriate choice for future work with respect to validity and reduced complexity. If the wall condensation model in ANSYS CFX does not prove adequate, it is recommended to investigate an Euler-Euler multiphase model in ANSYS Fluent with condensation and the Eulerian wall film model enabled.

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