Session: 04-44 Combustion dynamics - modeling II

Paper Number: 128594

128594 - Intrinsic Thermoacoustic Feedback in Auto-Ignition Flames Stabilized in a Converging-Diverging Nozzle 

In a previous paper presented at the ASME Turbo Expo 2023 in Boston [1], we have investigated, using  an analytical model, the thermo-acoustics of auto-ignition flames in a converging diverging nozzle with Mach number at the throat in the range of  M=0.2-0.6 . This concept, which has been named "Mach Stabilized Combustion",  represents a novel design for the second stage of a sequential combustion system.     
 The sequential combustion system is characterized  by two combustors. In the  first combustor   a portion of the fuel is premixed with air and   the resulting mixture of reactants is consumed by a conventional lean premixed flame. 
        The remaining part of the fuel is injected  in the hot gases from the first combustor (whose temperature is reduced by dilution with part of the air). Due to its high temperature in the range of 1300K,  the resulting mixture of reactants typically burn  in auto-ignition mode. 
The previous thermo-acoustics analysis of Mach Stabilized Combustion [1] has shown very intense flame response to  inlet temperature fluctuations even in cases where the     upstream and downstream boundaries are fully non-reflective. 
The frequency response to temperature fluctuations, despite the application of thermo-acoustically non-reflecting inlet and outlet conditions, shows peaks at discrete values of the frequency.
It has been speculated that this amplitude peaks are due to internal feedback mechanism between the flame and the upstream acoustic field.  The basis of this speculation is that the  flame dynamic displacement produces upstream and downstream propagating acoustic waves. The upstream propagating acoustic waves carry a temperature fluctuation which affect the free radicals reactions in the cold reactants with an impact on the auto-ignition delay time and on the position of the flame. This represents a closed feedback loop which under some circumstances, can lead to strong amplification of the overall response 
even when the acoustic waves are free to leave the domain from the two boundaries without reflection. 

  The problem of intrinsic thermocoustic instability modes has been already and extensively addressed in case of conventional lean premixed propagating flames 
   by several authors. For example in  [2]  this problem has been studied   using the balance of acoustic energy fluxes across the flame. In [3] it is instead shown that the flame-acoustic coupling can be unstable in the limit of no acoustic reflection from the boundaries. 
   
   The scope of the present work is to analysis the presence of intrinsic instability modes in the context of Mach stabilized flames whose properties are significantly different than in the case of conventional propagating flames. This will be carried out here in a rigorous fashion analysing the 
   problem of the closed feedback loop in the limit of boundary reflection coefficient going to zero. The final part of the work will be dedicated to identify a phenomenological interpretation of the observed intrinsic modes.

[1]  Biagioli, F. Freitag, E. and Syed, K.J., "Analytical modelling of flame-dynamics within auto-ignition hydrogen and methane flames in Mach stabilized combustion",  Prooc. of the ASME Turbo Expo 2023, June 26-30, 2023, Boston, Paper GT2023-102235
[2]    Emmert, T.,   Bomberg, S., Polifke, W.  "Intrinsic thermoacoustic instability of premixed flames" , Combustion and Flame,
  162,  pp. 75–85, 2015
[3] Hoeijmakers, M.,  Kornilov V., Arteaga, I. L.,  Philip de Goey, P. and  Nijmeijer, H. "Intrinsic instability of flame–acoustic coupling", Combustion and Flame, 161,  pp. 2860–2867, 2014

Presenting Author: Fernando Biagioli Infosys

Presenting Author Biography: Principal Engineer at Infosys

Authors:

Fernando Biagioli Infosys
Khawar Syed Infosys Ltd