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-   -   Reaction rates (http://www.cfd-online.com/Forums/fluent/27587-reaction-rates.html)

Szasz Robert March 23, 2000 12:55

Reaction rates
 
Dear All,

I'm using a methane-air mixture with 2 step reaction mechanism. Trying to modify the reaction rate by a UDF there was no change in the reaction rates (I checked the xy plots), even when I set the reaction rate to 0 in the UDF. I would have two questions: 1. What could be the problem? 2. What does a 'function hook' mean: the parameter (in my case the reaction rate) is calculated only by the UDF, or is calculated by Fluent and after that is modified by the UDF.

Thank you for any ideas, Robi

Jason Norman May 10, 2000 06:06

Re: Reaction rates
 
Altering the rate in a UDF will not show up as a change in reaction rates in the GUI. I presume that you are redifining the *rr and *rr_t rates in the UDF. My understanding is that Fluent calculates the reaction rate according to its data in the finite rate section of problem setup. It then invokes the UDF which adjusts the *rr and *rr_t values to your new chemistry rates etc. However, Fluent uses the initial or old *rr and *rr_t values in the reporting of 'reaction rates'(menu etc.).

The way around this is to define a scalar in the UDF(for your new reaction rate). This scalar can then be plotted etc. (you don't need to solve the scalar since it is updated in the UDF).

As you probably realise, this is quite an inefficient way of adding a reaction rate UDF since the code effectively does the chemistry calculation twice. Fantastic!


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