Verification, Validation and Testing of Kinetic Mechanisms of Hydrogen Combustion in Fluid Dynamic Computations

A one-step, a two-step, an abridged, a skeletal and four detailed kinetic schemes of hydrogen oxidation have been tested. A new skeletal kinetic scheme of hydrogen oxidation has been developed. The CFD calculations were carried out using ANSYS CFX software. Ignition delay times and speeds of flames were derived from the computational results. The computational data obtained using ANSYS CFX and CHEMKIN, and experimental data were compared. The precision, reliability, and range of validity of the kinetic schemes in CFD simulations were estimated. The impact of kinetic scheme on the results of computations was discussed. The relationship between grid spacing, timestep, accuracy, and computational cost were analyzed.


Introduction
fast chemical reactions) gives reliable results, so there is no actual need to use the detailed kinetic mechanisms in CFD simulations. However, the assump-27 tion of thin flame is not completely satisfied in rocket combustion chamber 28 where the turbulence is very high. By this reason the model of the chemical In the considered above work Kumaran and Babu used the kinetic mechanism of Stahl and Warnatz [9] published in 1985 as the reference detailed mechanism. This mechanism became a little bit old after publication of works [10,11]  mechanism has been tested in order to see the difference from the updated 89 mechanisms.

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Konnov reported recently about "remaining uncertainties in the kinetic 91 mechanism of hydrogen combustion" in work [13]. He studied the detailed 92 hydrogen combustion mechanism. Konnov found two groups of the uncer-  much computing power it is necessary to have for the fulfilment of a task.

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The work is the first step before the CFD simulations of the experiments 184 carried out at our test facility [29,30].  The new scheme was developed from the skeletal model by Kreutz

Calculations
The CFD calculations have been done with the use of the ANSYS CFX 211 11 solver [1], which utilizes the Finite Volume Element Method (FVEM). of a temperature increase up to 500 K relative to the initial temperature.

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During the 1D tests a freely propagating hydrogen flame has been modeled.

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The computational domain consists of 1604 nodes and 400 rectangular prism  The both tasks were also solved in CHEMKIN II [5]. The results of the simulations with the help of CHEMKIN II were used as a reference data.
CHEMKIN is very widely used for solving chemical kinetic problems, where the computational problem is formulated as solving of a system of ordinary differential equations. Indeed ASYS CFX allows to specify the properties of a system by the different ways, while in CHEMKIN task is set in the one prescribed format. CHEMKIN uses the modified Arrenius form for rate coefficients: The thermodynamic functions: enthalpy, entropy and heat capacity are cal-  The results of the comparisons is depicted in Fig. 3 and Fig. 4, where ANSYS 271 CFX shows the agreement with CHEMKIN.

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Using CHEMKIN the ignition delay times were calculated in the assump- gives us a clear view on the problem: where τ is the chemical time scale in reaction zone, and α is the coefficient where w i is the mass fraction of i-species; X j is the mole fraction of j-species; where    The performed simulations give more information about the evolution of 387 the system than simply ignition delay times. The "classical" behavior was 388 observed without anything unusual in the all "0D" tests (by this reason it is 389 not included in the article). All the kinetic models predict similar temper-390 ature (or pressure) time-resolved profiles, which have the induction period, 391 the following temperature (or pressure) rise, which ends with asymptotic be-392 havior. The gas temperature (or pressure) of the combustion products is 393 predicted correctly by the all kinetic models.

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In the 1D test case the agreement of the simulating data with the ex-395 perimental data is better in sum than in the "ignition" case, see Fig. 5.

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Practically the all models agree with the experimental data. The other distinctive feature of the obtained results is the bad agreement of abridged 398 Jachimowski's model [22] and the good agreement of one-step model [23].

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The results of 1D simulations can be interpret in terms of eq. (1). The sys-400 tem has practically the same physical properties in the all 1D simulations.

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These allow us to conclude that where indexes 1 and 2 designate the attribute to different kinetic models.

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The ignition delay times should be taken at flame temperature. In our case 404 the flame temperature amounts ∼2000 K. Abridged Jachimowski's model 405 [22] has the lowest effective activation energy among the models (see Fig. 2) 406 and predicts the significantly larger τ at high temperatures. As for one-step can be set as a reasonable lower limit for the time step and the grid spacing.

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The upper limit is quite specific to the details of a task. It is necessary to cing can limit the applicability of kinetic model. In Table 3

CFX-Solver
Validation solving the right equations