Learning Forum

[Oreste Russo]

I am using Ansys Fluent 21 R2 to solve a hypersonic problem in a nozzle that involves the chemical reaction model (5 species park 93 model already implemented in the software) and thermofluid dynamics.
Starting from the simulation with reactions enabled (volumetric yes) and eulerian flow, I subsequently also enabled the two-temperature model (from the energy box) to evaluate the molecular vibrational effects (O2, N2, NO), in fact, only in this case after a few iterations suddenly a problem occurs convergence which causes a residue explosion

The problem seems related to the activation of two temperature model in the region where the Mach number is greater than 4.5, this causes a small problem in my nozzle simulation because where mach is high the temperature is low and vice versa. Can you confirm that the activation of the two-temperature model to evaluate the vibrational effects is related to the Mach number? How can I change this relationship because I'm interested in activating the two temperature model using temperature?

[C N]

Hello Oreste Russo,

The two temperature model is actually used to measure the degree of the non equillibrium temperature effect in hypersonic flows by the use of Damkohler number. 

The Damkohler number is defined as the ratio of flow time scale / reaction time scale. The flow time is affected by the fluid velocity which is related to the mach number and the reaction time is related to thermochemical properties where the temperature also plays a significant effect.

For the hypersonic flows, the Damkohler number is typical of the order of unity.

This indicates that the fluid element does not reside at one location long enough to bring the local thermodynamic state to equilibrium, and therefore the flow is in thermal non-equilibrium.

To properly address this effect, it is necessary to use a two-temperature model that assumes: the translational and rotational energy modes of species are in equilibrium with one temperature; and the vibrational and electronic energy modes of species are in equilibrium with another temperature.

 A system of conservation equations is solved that includes the Navier-Stokes equations and one additional transport equation that models the conservation of vibrational-electronic energy.

For further detailed information on the exact equations involved in the two temperature model kindly go through the attached user guide link which is shown below

5.2.3. The Two-Temperature Model for Hypersonic Flows (ansys.com)

I hope this solves your issue.

Thanks,

 

[Oreste Russo]

Thank you for being clear and answering my question. Unfortunately I continue to have problems (residual explosion) when I activate the two-temperature model to simulate the hypersonic nozzle exit. How can I solve it? I am interested in considering the effect of translational and rotational energy on species