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Deflagration - to - detonation transition

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Old   January 5, 2012, 03:54
Post Deflagration - to - detonation transition
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Nguyen Phuoc Lam, Eric
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Hi everybody,

My name is Eric and I am working on valveless pulse detonation engine for my final year project. I am trying to simulate the deflagration - to - detonation transition process with FLUENT. The model is a 2D, axisymmetric tube with one close end and the other end is set to ambient condition. The species transport and laminar finite - rate model is used to simulate the combustion process. After initializing the problem with stoichiometric H2/O2 mixture occupying the whole tube, I patch a region of high temperature (1000K) to the close end of the tube to simulate a spark ignition. However, as the simulation is run, the temperature of the flow field ahead of the combustion wave gradually increases up to approximately 1000K when auto-ignition occurs, preventing steady-state CJ detonation wave to be established.

I would be much appreciated if anyone could enlighten me on the possible explanation of the raise in temperature of the flow field ahead of the combustion wave leading to the auto-ignition.

Regards,
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Old   April 12, 2012, 07:40
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hi Eric, I am prateek, I am also modelling DDT but with fully vaporised fuel i.e. kerosene air vapor mixture. Can you tell what boundary conditions you r using. I Hope we can sort out the problem.
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Old   April 16, 2012, 05:27
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hi there,

I apply adiabatic condition for the wall and the pressure outlet for the open end of the tube.

What combustion model did you use?
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Old   April 16, 2012, 07:33
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Hi,
I am using density based solver and species transport model with finite rate/ eddy dissipation model. adiabatic wall conditions and pressure outlet but m not able achieve anything yet. r u using patch for ignition.
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Old   April 16, 2012, 08:36
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hi there,

from my experience, if you are using finite-rate/eddy-dissipation model, the reaction rate will be controlled by the eddy - dissipation rate as it results in lower values for reaction rate.

for me, I managed to simulate direct detonation by patching a region of high pressure and temperature at the closed end of the tube and use the finite rate model

i used the eddy - dissipation model to simulate the initial flame acceleration when passing through the obstacle field, my results indicated that the flame speed asymptote to a constant value but fail to achieve CJ detonation speed. One of my conclusion (if correct) is that detonation can only be modeled with finite-rate model, not eddy-dissipation model
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Old   April 17, 2012, 09:15
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ok...gud! May be that could be the reason i have not achieved it yet.Are you using density based solver ans stiff chemistry solver. Have tried using different solution control method like turbulent kinetic energy and dissipation rate or by changing CFL number. And also the time step size that you are using.
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Old   April 17, 2012, 20:46
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hi there,

I realized that for the eddy - dissipation model, the initial flame initiation and propagation (prior to flame interaction with the first obstacle) is really slow and indeed it depends on the values of k and epsilon that you initialized the flow with (you may want to increase the ratio of epsilon/k to increase the flame speed). I used the density based solver as well, 2nd order implicit time stepping but I did not utilized the stiff chemistry solver.

Just want to check if you have patched a small fraction of steam to the ignition zone to enable the reaction to start?
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Old   May 17, 2012, 12:51
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hi,
I am not patching it with steam, rather m using jetA mixture only i.e. the same stoichiometric composition and initiating with high temperature and pressure coz i felt why to add extra species. In my case, m able to produce combustion wave but its not detonation since velocity is still less than local speed of sound.
Are you getting flame propagation speed of more than mach 1?

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Old   April 24, 2015, 14:46
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Hi

I am trying to do autoignition for mixture fuel using chemkin mechanism.... can u tell me the changes I need to do in my model as I am not getting temperature raise- autoignition.

My initial condition is 14.1bar at 766.1K for nheptane/N2/O2/AR: 0.00562/58/10/30 by mole fraction
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File Type: jpg No Temp rise at Initial values shown.jpg (51.0 KB, 3 views)
File Type: jpg species model for suto ignition at 766 K.jpg (53.4 KB, 4 views)
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