Relative humidity is a necessary parameter in many engineering applications and accurately simulating its effects can influence HVAC system and building designs, environmental control systems, thermal management, and electronics cooling. SOLIDWORKS Flow Simulation provides options to account for relative humidity in computational fluid dynamics simulations thus enabling more accurate and realistic modeling of thermal and fluid dynamics.
Setting Up Relative Humidity in SOLIDWORKS Flow Simulation
To use the relative humidity option, we can select the “Humidity” option in either the Simulation Wizard or in the General Settings dialog box. This check box is available only if the current fluid type is set to gases and will not be useable if simulating liquids or other fluids. Once the option is selected, water vapor is added to the specified mixture of gases.
SOLIDWORKS Flow study General Settings
The value for relative humidity can be specified under the Initial Conditions in the study’s General Settings alongside the reference temperature and pressure. When setting up boundary conditions and initial conditions, relative humidity can be defined as well.
Humidity Properties for boundary condition
For heat transfer problems, the material properties defined in SOLIDWORKS Flow Simulation should account for the effects of humidity. For example, the thermal conductivity of materials like fiberglass insulation can vary with humidity levels. Additionally, the relative humidity value can vary with certain conditions such as altitude and we will want to make sure to account for that in the fluid study.
Defining a Varying Value
To define variation in humidity we must first create a new parameter for the simulation to use. Creating parameters is accessed through Tools>Flow Simulation>Parameters and adding a new parameter from the dialog that appears. The newly defined parameter can then be used in the dependency definition to specify the variation of humidity. As shown in the image below, a parameter, “Altitude”, is defined so that humidity can change based on the altitude defined.
Altitude parameter with defined dependency
In the Initial Conditions dialog of the study’s General Settings, the Dependency option is used to vary humidity with the altitude parameter defined above. This process can be repeated for other study settings that need to vary based on a specific parameter.
Varying Relative Humidity based on “Altitude” parameter
Relative Humidity Assumptions
Some of the assumptions and limitations to be considered when using relative humidity in SOLIDWORKS Flow Simulation studies are listed below:
- The humidity option is incompatible with the High Mach number flow option.
- Humidity is only available with Gases (both in individual gases and in mixtures).
- The volume fraction of condensed water does not exceed 5%.
- Surface condensation such as dew formation on solid surfaces is not considered.
- The state of condensed steam at a given point is governed solely by the local conditions at this point.
Additional Considerations
Per the SOLIDWORKS Flow Simulation User Guide, if we analyze gas flow containing an amount of water vapor in conditions that are likely to get over the dew point, it is recommended to consider humidity in the calculations. To avoid the risk of finishing calculations before the condensation develops completely it is also recommended to specify one or more goals that are strongly dependent on condensation. It is critical to ensure the calculation does not stop before this goal is converged.
Relative Humidity Cut Plot
Are you designing HVAC systems, cooling electronics, or environmental control systems and want to start incorporating relative humidity to improve your design accuracy? Click here to read more about SOLIDWORKS Flow Simulation.
