There are different turbulence models available in Ansys Fluent, such as Spalart-Allmaras, k-epsilon, k-omega, SST, and GEKO. Each model has its own advantages and disadvantages, depending on the flow conditions and the accuracy required. Some factors that may influence the choice of turbulence model are:
- The Reynolds number of the flow
- The near-wall treatment
- The inlet boundary conditions
- The mesh resolution and quality
- The computational cost and time
For a rotating fan with radial inlet and outlet, you may want to consider using a sliding mesh model with a turbulent boundary layer approach. This can capture the effects of blade tip gap variation, flow recirculation inside blade passage, and other phenomena that depend on the turbulence modeling. You may also want to use a low-Re model such as k-omega or SST for better resolution of boundary layers.
One possible turbulence model that you can try is the GEKO (Generalized K-Omega) model, which offers a flexible and robust approach to RANS turbulence modeling. It has several tunable model constants that can be adjusted to match specific flow conditions. You can watch two videos that introduce this model in Ansys Fluent [here](^3^).
Another possible turbulence model that you can try is the SST-kw (Steady-State Turbulence-kw) model, which is recommended for highly accurate resolution of boundary layers. It has a wall function mesh that simplifies the computation and reduces the computational time. You can read more about this model in [this document](^4^).
Source:
(1) Introducing the GEKO Turbulence Model in Ansys Fluent. https://ansyskm.ansys.com/announcement/introducing-the-geko-turbulence-model-in-ansys-fluent/.
(2) ANSYS FLUENT 12.0 User's Guide - 12. Modeling Turbulence - ENEA. https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node405.htm.
(3) Modeling Turbulent Flows Introductory FLUENT Training. https://www.southampton.ac.uk/~nwb/lectures/GoodPracticeCFD/Articles/Turbulence_Notes_Fluent-v6.3.06.pdf.
(4) Fan Modelling: turbulence model -- CFD Online Discussion Forums. https://www.cfd-online.com/Forums/fluent/45907-fan-modelling-turbulence-model.html.
(5) Lecture 7: Turbulence Modeling Introduction to ANSYS Fluent - Academia.edu. https://www.academia.edu/36090206/Lecture_7_Turbulence_Modeling_Introduction_to_ANSYS_Fluent.
Which turbulence model will be the best for centrifugal fan ?
There is no definitive answer to which turbulence model is the best for a centrifugal fan, as different models may have different advantages and disadvantages depending on the flow conditions and the accuracy required. However, based on the web search results, I can provide you with some information and suggestions that may help you choose a suitable model for your simulation.
One of the factors that may influence the choice of turbulence model is the Reynolds number of the flow, which is a dimensionless parameter that characterizes the ratio of inertial forces to viscous forces in a fluid. The Reynolds number can be calculated from the density, velocity, and viscosity of the fluid, as well as from the geometry and size of the fan components. Generally, higher Reynolds numbers indicate more turbulent flows, which may require more complex models to capture the effects of turbulence.
Another factor that may influence the choice of turbulence model is the near-wall treatment, which is a method to account for the effects of boundary layer separation and recirculation near the wall. The near-wall treatment can be done by using different approaches, such as using a wall function mesh or using a modified k-epsilon model. The wall function mesh simplifies the computation by replacing some terms in the k-epsilon model with wall functions that depend on local velocity gradients. However, this approach may introduce some errors or inaccuracies in certain regions or flow regimes.
A third factor that may influence the choice of turbulence model is the inlet boundary conditions, which are given by various parameters such as inlet pressure, inlet velocity distribution function (VDF), inlet blade angle, and inlet clearance gap. These parameters affect how the flow enters and exits the fan impeller and volute. For example, an inlet VDF can determine whether there is a uniform or non-uniform flow distribution along the impeller passage. An inlet blade angle can affect how much lift or drag force is generated by each blade section. An inlet clearance gap can affect how much turbulence is generated by mixing between air and fluid.
A fourth factor that may influence the choice of turbulence model is the mesh resolution and quality, which are determined by various parameters such as grid size, grid spacing, grid type (sliding or fixed), grid refinement level (global or local), etc. The mesh resolution and quality affect how accurately and efficiently each grid element represents a physical quantity such as velocity or pressure. Higher resolution and quality grids can capture more details and features of complex flows, but they also require more computational resources and time.
A fifth factor that may influence the choice of turbulence model is also computational cost and time, which are affected by various parameters such as number of iterations per time step (NIT), time step size (GST), solver type (finite volume or finite difference), etc. Computational cost and time are important considerations when performing large-scale simulations with many fans or other components. Lower cost and time simulations can reduce operational costs and increase efficiency.
Based on these factors, you may want to consider using one or more models that have been tested and validated for similar flow conditions as your simulation case. Some examples of models that have been used for centrifugal fans are:
- The k-epsilon model (an industry standard for many years), which has two main advantages: it has simple mathematical form; it has good accuracy for low-Reynolds-number flows.
- The SST-kw (Steady-State Turbulence-kw) model (recommended for highly accurate resolution of boundary layers), which has two main advantages: it has a wall function mesh that simplifies computation; it has good accuracy for high-Reynolds-number flows.
- The GEKO (Generalized K-Omega) model (a flexible and robust approach to RANS turbulence modeling), which has several tunable model constants that can be adjusted to match specific flow conditions.
- The SST-reattachment modification (RM) model (a proposed modification to improve SST performance), which has two main advantages: it reduces local inflow in confined areas; it improves agreement with experimental data.
- The RSM-SSG (Speziale et al., Sarkar & Gatski) model (a modified k-epsilon model with improved near-wall treatment), which has two main advantages: it reduces numerical diffusion; it improves agreement with experimental data.
You can find more information about these models in [this document](^4^), [this article](^2^), [this paper](^3^), [this paper](^5^), [this paper](^6^).
Source:
(1) Tonal noise of voluteless centrifugal fan generated by turbulence .... https://pubs.aip.org/aip/pof/article/33/7/075110/1077103/Tonal-noise-of-voluteless-centrifugal-fan.
(2) A Comparative Study on Numerical Flow Simulations of a Centrifugal .... https://www.mdpi.com/1996-1073/16/23/7864/html.
(3) Assessment of Turbulence Model Predictions for an Aero-Engine .... https://asmedigitalcollection.asme.org/turbomachinery/article/133/1/011025/421599/Assessment-of-Turbulence-Model-Predictions-for-an.
(4) Computational turbulent flow characteristics in a centrifugal pump. https://pubs.aip.org/aip/adv/article/12/7/075025/2818977/Computational-turbulent-flow-characteristics-in-a.
(5) http://dx.doi.org/10.5293/IJFMS.2020.13.3.623.
(6) Effect of Blade Profile on the Performance of a Centrifugal Fan with .... https://www.jstage.jst.go.jp/article/ijfms/13/3/13_623/_pdf/-char/en.
(7) https://doi.org/10.3390/en16237864.
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