How to... choose correct heat transfer model in Ansys CFX

 Everyone sholud be aware which heat transfer model should to choose for his own case. You have to choice from three heat transfer models in Ansys CFX - thermal energy, total energy and isothermal. The simplest is isothermal which doesn't include heat transfer  - therefore use it only for easy flows. 

Types of heat transfer models

As for the other two heat transfer models, the choice of one will have a decisive impact on the phenomena you are going to solve. Compared to Fluent, the difference between the models is quite big. Applying these two models to the same problem may result in obtaining different results, especially if we focus on heat transfer in our problem. Thermal Energy is a model strictly for flow analysis, while if we assume the influence of pressure and temperature on the medium which is gas, the combination of Total Energy and Ideal Gas seems to be the right choice.

Below you will find a more detailed comparison of the two analyzed models.

General difference between heat transfer models Ansys CFX (total energy, thermal energy):

Thermal Energy Model  - activates the enthalpy /  temperature enthalpy but doesn't include                      compressibility effects of gas. It's used for low speed thermal models like heat exchagers, free convection, combustion etc. 

Total Energy Model - activates Full Energy Equation with Kinetic Energy Effects. It should be used for gas flows where the Mach number exceeds 0,2 or where compressibility effects (for example high pressure chambers above 2 bar or ***below 0,5 bar goin to vacuum). Sholud be used to low speed flows too where the specific heat is not constant. In that model viscous work term can be included. 

Sources:

https://www.cfd-online.com/Forums/cfx/68982-diff-bet-total-energy-thermal-energy-model.html

https://cfd2012.com/ansys-cfx-compressible-flows.html

Gravity in Ansys CFX

Very important is to choose ideal gas model for the total energy defined equation (orange frame) - that model include impact of viscosity, pressure and temperature. 

In total energy model U have influence in refernece preessure which U define in domains (blue frame). Don't forget to define gravity to make Ur model complete for Total Energy in Ansys CFX. 

This configuration fullly describe forced convetion problems. 

If you want to learn more about fluid mechanics, and in particular about the phenomena of compressibility (incompressibility) of gases, see the YT channel below. This is one of the Ansys channels where you can learn about CFD phenomena from the theoretical side. Lessons are conducted transparently in short training videos. Explanations are formed in a clear way, understandable even for a less experienced engineer. I strongly encourage you to subscribe to this channel because in addition to technical issues related to computer operation, you should also deepen your theoretical knowledge. Why should it be done? Because you need to have an idea of the models and solvers that are defined in CFD programs. It is important to know what influence they have on the general physical model of the analyzed problem.

Compressible Flow Regimes — Lesson 4 - YouTube