I am trying to simulate a 2 dimensional flameless combustion case of char in ANSYS FLUENT. It involves surface reaction of char with incoming oxygen
what time step should I take?
and secondly is there any option in fluent where I can visualize char left behind after its consumption via oxidation reaction ?
Hi Muhammad
When determining the length of a time step there are two considerations.
1) Computational stability. This can be guaranteed through the Courant–Friedrichs–Lewy or CFL condition. You can look up the whole story online, the Wikipedia article is actually not bad, but what it amounts to is that you have to be sure that the numerical stencil of your calculation has to be at least as large as the physical area of influence. In practice you can guarantee this by finding the maximum velocity in the simulation (vmax). This can be either the speed of actual motion (as in the case of a supersonic flow) or a signal speed (such as the speed of sound). Combine this with the size of your grid cells (dx) and your time step dt is limited to dt < dx/vmax.
2) Physical accuracy. In order to make sure that your solutions are valid, you have to ensure that all physical processes are well resolved in time. If they are not, your calculation can be numerically stable, but the result starts to deviate from what it should be. You will have to determine for every process in your model what the typical timescale is and ensure that your time steps are shorter than the shortest of these timescales. Two such processes in your case would be the rate at which the burning process consumes oxygen and the rate at which new oxygen flows toward the burning area. How much shorter the time step needs to be is not so easy to say. In practice, I usually ensure that each process gets at least 10 time steps per typical time scale, but you may have to experiment to see what you need.
Normally, a code like FLUENT should take care of such things automatically as it runs the simulation, calculating the length of each new time step as it progresses. However, if you have added a new process to the calculation, one that was not part of he original FLUENT package, then you will need to determine a maximum time step for that process yourself.
You may also have to enter a numerical factor that is multiplied with the time step that FLUENT calculates for itself. This is often referred to as the Courant number and has to be less than 1 in order to ensure that all conditions are satisfied.
i hope this helps you.
https://www.sciencedirect.com/science/article/pii/B9780128037713000053
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