Guest Post/Tutorial by Best Engineering Aids and Consultancies Pvt. Ltd. (BEACON India)
Author – Theophilus Dsouza
Temperature and pressure are interdependent fluid parameters that work together to influence the behaviour of fluid flow within an engineering system. For a Liquid (consisting of a single substance) at a given pressure, the temperature at which it changes its phase to vapour form is called the Saturation Temperature (Tsat). Likewise, for a liquid at a given temperature, the pressure at which it changes its phase to vapour form is called the Saturation Pressure (Psat). If the liquid consists of a single substance the Saturation Pressure and Vapour Pressure (Pvap) are the same and both terms can be used interchangeably.
The Saturation Pressure (Psat) of a liquid decreases at lower temperatures as seen in the table below. A liquid would change phase to its vapour phase once the pressure in the flow field drops below the Saturation Pressure(Psat), at a given temperature. Similarly, the liquid can change its phase to vapour if the temperature in the flow field increases above its Saturation Temperature (Tsat), at a given pressure.
This unplanned vaporization of liquid flow in an Engineering system causes vapour bubbles to form in regions where the Saturation Pressure or Saturation Temperature limits are crossed. As the vapour bubbles are swept to other regions, they collapse, generating intense pressure waves that can erode the nearby solid material. This phenomenon is called Cavitation. It should be avoided as it causes erosion of the components, reduces flow performance and even causes vibrations and noise.
The Solution by Solidworks Flow Simulation
By using SOLIDWORKS Flow Simulation, we can detect cavitation early and prevent it during the design stage through comprehensive CFD analysis. Although, Solidworks Flow Simulation does not have a phase change model incorporated in its solver code, it can still address the issue of cavitation. This is done with the help of Cavitation Models built in the tool, specifically to simulate the phenomena of cavitation in liquids.
The Cavitation Models work under an important assumption that, the characteristic time-scale of the vapor formation process is much less than the characteristic time-scale of the liquid flow. Hence, the process is simulated by applying a thermodynamic equilibrium condition and obtaining the volume and mass fraction of the vapour phase in the flow field. Another key aspect to remember is that cavitation analysis is limited to a single-phase liquid within the computational domain. Mixture of two different liquids cannot be considered to simulate cavitation.
There are two Cavitation Models in Solidworks Flow Simulation:
- Equilibrium Cavitation Model
- Isothermal Cavitation Model
Equilibrium Cavitation Model
- The Equilibrium Cavitation Model is available for Pre-Defined Water (Liquid) only
- The model can describe phase transition caused by pressure reduction in case of hydrodynamic cavitation, and by increasing temperature, in case of boiling
- Cavitation process is simulated by considering a thermodynamic equilibrium of liquid water, vapour (water) and dissolved air
- Water is assumed to be a homogeneous two-phase fluid, the velocities and temperatures of the gaseous (including vapor and non-condensable air) and liquid phase water are assumed to be the same
- The temperature and pressure in the phase transition area should be within the following ranges: 280.15 K < T < 583.15 K and 103 Pa < P < 107 Pa
- The Dissolved air is considered a non-condensable gas. Its mass fraction is calculated for in the analysis. It should be in the range between 10-2 to 10-6
Isothermal Cavitation Model
- This model is only available for user defined liquids.
- The flow is considered isothermal by default and all thermal conditions specified in the project will be ignored. “Conduction in solids” option should therefore be switched off in the Analysis Wizard.
- As the flow is isothermal, the cavitation phenomena due to reduction in pressure below the saturation pressure (at the isothermal temperature) is only considered.
- The Molar mass of the user defined liquid and its saturation pressure at the specific Temperature must be defined in the Engineering Database, while defining the user-defined liquid.
- In addition to the user-defined liquid and its vapor phase, a non-condensable gas can also be considered as dissolved in the liquid phase, with its concentration specified by the mass-fraction.
- Only four gases can be considered: Air (default), Carbon dioxide, methane and helium
- The mass fraction of the dissolved gas is calculated for in the analysis. It should be in the range between 10-2 to 10-6
Key Points to Remember when using Cavitation Model
- Cavitation model is not applicable if you calculate a flow in the model without flow openings (inlet and outlet).
- The fluid region where cavitation occurs should be well resolved by the computational mesh.
- It is recommended to specify ”Global Goal” of ”Average Density” and increase the Analysis interval under Goals Criteria, on the Finishing tab of the Calculation Control Options dialog up to 2.5 travels
- Select “All Satisfied” for the “Criteria to Stop” under the “Finish Conditions” on the “Finishing Tab” of the Calculation Control Options dialog. This ensures that all the goals, along with the global goal of average density is converged until the analysis stops.
- Surface plot and Cut plot of the Mass and Volume fraction of the vapour can be created during post-processing to evaluate the scale of cavitation occurring in critical regions of the computational domain.
Guest Post/Tutorial by Best Engineering Aids and Consultancies Pvt. Ltd. (BEACON India)
Author – Theophilus Dsouza
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