Neptune CFD : Monophasic simulation

In this phase of study, simulations were carried out for the range of temperature difference mentioned before by imposing a heat flux desity as boundary condition for the heating tube. The simulations were launched in saturation condition with Psat=474.3 kPa. The Boussinesq hypothesis (Equation 2 of BIBLIOGRAPHY RESEARCH - Boussinesq Approximation) was used to predict the variation of density due to ΔT, thus the flow in the studied domain caused by the difference of buoyancy force.

In order to set the properties and conditions, the interface of Neptune CFD, called EDAMOX, was used. It was developed for users to define and select different models as well as boundary conditions to be used during their simulation. The interface can be accessed by the following command in linux terminal:

               edam &

or if the param file already exists

               edam –U param &

Note that one has to be in ‘DATA’ directory to launch Edamox

Figure 11: Edamox interface

The first step of the simulation definition was to select the special module in case of necessity. For single phasic simulation, no specific module has to be selected.

Figure 12: Neptune Special module panel for natural convection

In Fluid & Flow prop windows, the properties of the fluid at the reference temperature are required. In this study, they were fixed at Tsat at Psat=474.3kPa. Since the propone was at static state, no turbulence was expected in the simulation. (see also Rayleigh Calculation in BIBLIOGRAPHIC RESEARCH, Boussinesq approximation)

The mesh file needs to be defined in input-output-control windows. The number of iteration and time of the simulation are required, as well as the output frequency, for post-processing.

Figure 13: Neptune Input-Output-Control panel for natural convection

In order to take into account the variation of density, modifications in USPHYV.F are required to impose the Boussinesq Approximation and, consequently, the option ‘Variable physical properties (call USPHYV)’ has to be selected in the Physical models module of param file. 

Figure 14: Neptune Generalities panel for natural convection

In the Scalars windows, a total enthalpy scalar type was selected in order to be able to fix either a constant heat flux [W/m²] or impose a wall temperature as boundary condition around the tube. The parameter ‘lam.dym.coef’, corresponds in this case to the thermal conductivity of propane [W/m/K] over the specific heat capacity of propane [J/kg/K], as the scalar type is chosen to be total enthalpy.

However, the wall temperature, Tw and wall heat flux, Qwall cannot be accessed if the water/steam special module is not selected, as Neptune CFD does not calculate them for single phasic simulation. Therefore, a wall temperature was imposed, at the first place, as the boundary condition around the tube and the mesh was refined until the temperature of the first mesh around the tube was approximately (with a difference of 0.001) equal to the applied wall temperature. The wall heat flux could be easily accessed from the ‘listing’ file by dividing the energy transfer around the tube by the exchange surface. To impose a wall temperature, a dirichlet boundary condition has to be selected and the value imposed has to be in the same unit as the scalar. One should never considered the 'Timp[K]' option for boundary condition if either the cathare or thesis tables are not selected.

The refined mesh was used for simulations with various heat fluxes imposed, range from 300 W/m2 to 5000 W/m2, as the wall boundary condition. Using ‘Paraview’, a data analysis and visualisation application, the liquid temperature of the first mesh can be exported using the ‘slices’ function (Figure 15). Average wall temperature can, hence, be obtained by an integration of the exported data.

Figure 15: Slice of the geometry, Twall calculation for natural convection