The temperature of the chemical reaction is unreasonable

MFiX
#1

Hello,everyone!
I made a simple reaction, H2+O2–>H2O. I have three questions about this.
First, its temperature is not reasonable. It is an exothermic reaction, with half H2 and half O2 in the inlet fluid.
When the inlet temperature is 500K, the maximum temperature in the area after operation is 1000K.
When the inlet temperature is 800K, the maximum temperature in the area after operation is still 1000K.
When the inlet temperature is 1000K, the maximum temperature in the area after operation is 1000K.
When the inlet temperature is 2000K, the maximum temperature in the area after operation is less than 2000K.
Logically speaking, this temperature should be continuously rising and can exceed 1000K or 2000K. I want to know what caused this.
Second, in order to increase the temperature, I noticed that there is a heat of reaction tab.

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I wrote a heat of reaction in it, but it didn’t work. I would like to know that after the reaction heat is written here, is the reaction heat of the reaction based on the reaction heat I filled in the label column or the reaction heat calculated by itself based on the physical properties of the database. Which one has the higher priority?
Third, when I increase the reaction rate to 0.05, the error dt<dtmin will be reported. I have seen some previous cases on the forum, and it seems that the reaction is too fast and the concentration of a certain component is negative? I want to know if this is the root cause, and is there any suggested solution?
Attach my case，thank you very much.
c_2021-11-09T161742.980168.zip (63.5 KB)

#2

Let me answer backward:

  1. You typically do not want to set a uniform reaction rate, and use some kind of limiter to prevent over spending (the rate usually depends on temperature, but I guess you are just doing some testing right now). You should also not test for species mass fractions = zero because a value of 1E-19 would set the rate to your constant value. A value of 0.05 may be too large. Yo may also try to turn of the stiff chemistry, this may help get reasonable time step.
  2. For an exothermic reaction, the heat of reaction should be negative. Now with the rate=5E-6 there is almost no H2O produced and no temperature change. I could see a temperature rise with rate = 0.01 and dh = -100000000 J/kmol.
  3. I don’t see this. How long did you run and do you see a temperature front moving up?
#3

Thank you very much for your answers.
I corrected the setting problem you pointed out. In the end, the temperature problem turned out to be the chemical reaction rate. I set the reaction rate to a reasonable value. As a result, the temperature increased as expected.
Thank you very much for this!