Learning Forum

[naveedullah]

Hi,

Any member knows about the model used for the analysis of the fluid behavior and heat / mass transfer characteristics when water is sprayed inside a pipe with flowing Ammonia, using CFD.

 

[Rob]

Yes. 

However, it's not simple and you need to define the modelling goals before starting to work on the project: the goals will determine the most suitable models. 

[naveedullah]

The main Goals are optimization of the mass flux and heat transfer coefficient, in the ammonia-water absorption process.

 

[Rob]

OK, and as the water is being sprayed can you use DPM? Review the model approach in the User's and Theory Guides. 

[naveedullah]

Thank you Rob, for the hint. I was also of the same opinion based on some literature study. But can we use two models for the complete analysis, DPM for sprayed water through nozzle and Eulerian for Ammonia in the pipe??

I mean, DPM for Injection  and Eulerian for Absorption. 

[Rob]

Euler in the above context is a multiphase model - as opposed to Euler flow which is the physics name for the continuous flow. 

 

[naveedullah]

How can we define chemical reactions in species transport in multiphase modeling?

 

[Rob]

Phase interaction if it's between phases, or species mixture if the reaction is in one or other phases. Don't forget to check the formation enthalpy as that includes latent heat. 

[naveedullah]

Thank you,  I can now use species transport to define the chemical reaction, by making a mixture template of the species. The chemical reaction is NH3​+H2​O↔NH4+​ + OH−

Now the question is how can I add the Ammonium Ion to the fluent database? Should I use an arbitrary material and change its properties as per the Ammonium ion?

 

[Rob]

Yes, just alter something and then click "no" to overwrite if it's a material you want to keep. Check the diffusivity value in the Fluent mixture(s) as the default is usually for gases: liquid diffusivities tend to be 1e-8 to 1e-10 ish. 

[naveedullah]

I am using the Arrhenius reaction for NH3 + H2O>>>NH4OH, with activation energy as 1e+7, the pre-exponential factor of 1e+8, and temperature exponent as 0. 

My question is since the reaction is exothermic with a change in enthalpy of -46.16 KJ/mol, but when I look at the temperature contour of the mixture, the temperature rises up to the water temperature only (280K) and not beyond. Is this due to some latent heat of the water droplets in the discrete phase or something else? Ammonia temperature is 278K and water droplets have 280K.

[Rob]

Is the reaction occurring? Ie how do the contours of products look? 

[naveedullah]

Attached is the mass concentration of NH4OH (the product).

[Rob]

That's the droplet concentration. How does the gas phase look? 

[naveedullah]

This looks the product concentration, which is zero at the inlet. Due to the ammonia-water interaction, it increases as shown in the contour.