modelMixedConvection

Information

This tutorial gives step by step instructions on building and simulating a mixed convection model. The model tests the coupled simulation of Buildings.ThermalZones.Detailed.CFD with the FFD program by simulating ventilation with mixed convection in an empty room.

Case Description

The temperature of the floor is fixed at 30°C and the temperature of the walls and the ceiling are fixed at 10°C. The supply air temperature is fixed at 10°C.

Figure (a) shows the schematic of the FFD simulation and Figure (b) shows the velocity vectors and temperatures on the X-Z plane at Y = 0.5 m as simulated by the FFD.

Figure (a)

Figure (b)

Step by Step Guide

This section describes step by step how to build and simulate the model.

1. Add the following model components into the MixedConvection model:

   • Buildings.ThermalZones.Detailed.CFD. This model is used to implement data exchange between Modelica and FFD. Name it as roo.
   • Buildings.BoundaryConditions.WeatherData.ReaderTMY3. Use weather data from OHare Intl. Airport, Chicago, Illinoi, U.S.A. Name it as weaDat.
   • Modelica.Blocks.Sources.Constant. Three models are needed to specify that internal radiation, internal convective heat gain and internal latent heat gain are zero. Name these models as qRadGai_flow, qConGai_flow and qLatGai_flow, respectively.
   • Modelica.Blocks.Routing.Multiplex3. This block is used to convert three numbers into a vector. Name it as multiple_x3.
   • Buildings.HeatTransfer.Sources.FixedTemperature. Two models are needed to specify the temperature on the floor and other walls. Name them as TFlo and TOthWal respectively. Please note that it is necessary to declare TOthWal as a vector of 5 elements.
   • Buildings.Fluid.Sources.MassFlowSource_T. This model provides inlet air for the roo. Name it as bouIn.
   • Buildings.Fluid.Sources.Boundary_pT. This model is the outdoor environment to which the outlet of roo is connected. Name it as bouOut.

2. In the textual editor mode, add the medium and the number of surfaces as below:

   package MediumA = Buildings.Media.GasesConstantDensity.MoistAirUnsaturated (T_default=283.15);
     parameter Integer nConExtWin=0;
     parameter Integer nConBou=0;
     parameter Integer nSurBou=6;
     parameter Integer nConExt=0;
     parameter Integer nConPar=0;
   

3. Edit roo as below:

   Buildings.ThermalZones.Detailed.CFD roo(
     redeclare package Medium = MediumA,
     surBou(
       name={"East Wall","West Wall","North Wall","South Wall","Ceiling","Floor"},
       A={0.9,0.9,1,1,1,1},
       til={Buildings.Types.Tilt.Wall,
           Buildings.Types.Tilt.Wall,
           Buildings.Types.Tilt.Wall,
           Buildings.Types.Tilt.Wall,
           Buildings.Types.Tilt.Ceiling,
           Buildings.Types.Tilt.Floor},
       each absIR=1e-5,
       each absSol=1e-5,
       each boundaryCondition=Buildings.ThermalZones.Detailed.Types.CFDBoundaryConditions.Temperature),
       AFlo = 1*1,
       hRoo = 1,
       linearizeRadiation = false,
       useCFD = true,
       sensorName = {"Occupied zone air temperature", "Velocity"},
       cfdFilNam = "resources/Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial/MixedConvection.ffd",
       nConExt = nConExt,
       nConExtWin = nConExtWin,
       nConPar = nConPar,
       nConBou = nConBou,
       nSurBou = nSurBou,
       nPorts = 2,
       portName={"Inlet","Outlet"},
       samplePeriod = 6);
   

4. Set the parameters for the following components:

   • Set qRadGai_flow, qConGai_flow and qLatGai_flow to 0.
   • Set TFlo to 303.15 Kelvin.
   • Set TOthWal to 283.15 Kelvin.

5. Set the values for the parameters of bouIn and bouOut as below:

   Fluid.Sources.MassFlowSource_T bouIn(
     redeclare package Medium = MediumA,
     nPorts=1,
     m_flow=0.1,
     T=283.15);
   

   Fluid.Sources.FixedBoundary bouOut(
     redeclare package Medium = MediumA,
     nPorts=1);
   

6. Connect the components as shown in the figure below.

   

7. Confirm in the textual editor that the connections to roo.ports are as follows:

   connect(bouIn.ports[1], roo.ports[1]);
   connect(bouOut.ports[1], roo.ports[2]);
   

8. Use the Simplified CFD Interface (SCI) to generate the input file for the FFD.

   • Use a 20 X 20 X 20 stretched grid.
   • Set the time step size of the FFD to 0.1 seconds.
   • Generate the input files, which have by default the names input.cfd (mesh file) and zeroone.dat (obstacles file).
   • Rename the files as MixedConvection.cfd and MixedConvection.dat, respectively.

9. Revise the FFD parameter input file MixedConvection.ffd (an example file is available in Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial/):

     inpu.parameter_file_format SCI
     inpu.parameter_file_name MixedConvection.cfd
     inpu.block_file_name MixedConvection.dat
     prob.nu 0.000015 // Kinematic viscosity
     prob.rho 1.205 // Density
     prob.gravx 0 // Gravity in x direction
     prob.gravy 0 // Gravity in y direction
     prob.gravz -9.81 // Gravity in z direction
     prob.cond 0.0257 // Conductivity
     prob.Cp 1006.0 // Specific heat capacity
     prob.beta 0.00343 // Thermal expansion coefficient
     prob.diff 0.00001 // Diffusivity for contaminants
     prob.coeff_h 0.0004 // Convective heat transfer coefficient near the wall
     prob.Temp_Buoyancy 10.0 // Reference temperature for calculating buoyance  force
     init.T 10.0 // Initial condition for Temperature
     init.u 0.0 // Initial condition for velocity u
     init.v 0.0 // Initial condition for velocity v
     init.w 0.0 // Initial condition for velocity w
   

10. Put the files MixedConvection.ffd, MixedConvection.dat, and MixedConvection.cfd in the directory Buildings/Resources/Data/ThermalZones/Detailed/Examples/FFD/Tutorial/.
11. Set the simulation stop time of the Modelica model to 180 seconds and choose, for example, the CVode solver.
12. Translate the model and start the simulation.
13. Post-process: click the Tecplot macro script Buildings/Resources/Image/Rooms/Examples/FFD/Tutorial/MixedConvection.mcr that will generate the temperature contour and velocity vectors shown in the Figure (b). Note: Tecplot is needed for this.

Parameters

Components

Contents

Name	Description
MediumA	Medium model

Revisions

• September 16, 2021, by Michael Wetter:
  Removed assignment of parameter lat as this is now obtained from the weather data reader.
  This is for IBPSA, #1477.
• September 07, 2017, by Thierry Nouidui:
  Refactored the FFD C-code and revised the documentation. This is for issue 612.
• July 25, 2014, by Michael Wetter:
  Revised documentation.
• June 27, 2014, by Wei Tian, Thomas Sevilla, Wangda Zuo:
  First implementation.