Physical understanding of unsteady combustion phenomena via high-fidelity numerical simulations

Traditionally, combustion processes were analyzed using experiments. Today, experimental data represent the most reliable insights using e.g. advanced laser techniques for flow visualization and measurements.

At the same time, progress is made in the numerical simulation of combustion processes. While numerical simulations are used e.g. for decades for industrial airfoil design, the combustion community accepted computational techniques as Large Eddy Simulations (LES) only lately. Although computational costs are high, the accessibility to all physical quantities features a major benefit.

In recent years, the AVBP code developed by CERFACS and IFPen has been widely used in the combustion community by several research groups and industry. During the last 5 years, I have extensively used this code for most of my research.

Combustion dynamics of gaseous flames

Longitudinal combustion instabilities

Acoustic perturbations travel in flow direction inside the combustion chamber where they get reflected at walls and turbine stages. The reflected waves travel upstream and  regenerate perturbations of the flame which again excites acoustics to travel downstream. This is continously feeding energy to the acoustic field and establishes a self-excited feedback loop. Watch an example of longitudinal modes impacting the flame of an afterburner.

Transverse combustion instabilities

One of the most famous examples of transverse modes is the F1-engine of the Saturn V. Back in the 1960’s, more than 1200 full-scale tests with varying injector designs have been performed in order to stabilize the combustion process. Still, transverse modes develop in modern combustion devices and little is known. During my thesis I worked on the numerical simulation of transverse instabilities in order to better understand the physical mechansisms.

A. Ghani, T. Poinsot, L. Gicquel & G. Staffelbach
LES of longitudinal and transverse self-excited combustion instabilities in a bluff-body stabilized turbulent premixed flame
Combustion and Flame (2015), vol. 162 (11), pp. 4075-4083
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Here you can watch my numerical simulations of transverse modes during action for an afterburner.

Annular combustion instabilities in a pressurized model combustor

An exciting test rig has been developed by Nicholas Worth and James Dawon at NTNU Trondheim. A full-annular combustion chamber terminated by a choked nozzle is feed by 12 burners with a perfectly-premixed methan/air mixture. Based on the experimental configuration, I have set up the LES computation during my time at IMFT (ERC Advanced Grant of Thierry Poinsot and Laurent Selle).

I continued the collaboration with NTNU and IMFT since my arrival at TU Munich. Currently we are using one LES sector (1/12 from the full geometry) coupled with 11 low-order models of the remaining configuration. The full LES computation serves as a reference case for validation of the low-order tool calles taX. It is an in-house tool of the Thermo-Fluid Dynamics Group guided by Wolfgang Polifke at TUM.

Combustion dynamics of spray flames

Flame Transfer Function of a swirling spray flame

While the concept of the Flame Transfer Function (FTF) has been widely applied to gaseous flames, much less work has been done on spray flames. This is mainly because of the high complexity in e.g. measuring droplet velocities or the robust numerical computation of liquid sprays in turbulent reacting flows.

During the KIAI-Project, I was working together with experimentalists from the ONERA in Toulouse. The test rig contained an aeronautical swirling device (triple staged) with a pilot and a multi-point fuel injector for liquid kerosene. Our objective was to correctly reproduce the Flame Transfer Function of the swirling spray flame with LES. The results from the collaboration can be found in the following publication:

A. Ghani, L. Gicquel & T. Poinsot
Acoustic analysis of a liquid fuel swirl combustor using dynamic mode decomposition
ASME Turbo Expo: Turbine Technical Conference and Exposition (2015), pp. 1-9
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Transverse combustion instabilities in an aeronautical combustor

One of the most famous examples of transverse modes is the F1-engine of the Saturn V. Back in the 1960’s, more than 1200 full-scale tests with varying injector designs have been performed in order to stabilize the combustion process. Still, transverse modes develop in modern combustion devices and little is known. During my thesis I worked on the numerical simulation of transverse instabilities in order to better understand the physical mechansisms. Here you can watch my numerical simulations of these modes during action for an aeronautical system and an afterburner.

A. Ghani, T. Poinsot, L. Gicquel & J.-D. Müller
LES Study of Transverse Acoustic Instabilities in a Swirled Kerosene/Air Combustion Chamber
Flow, Turbulence and Combustion (2016), vol. 96 (1), pp. 207-226
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System Identification in thermoacoustics

Overview

The group of Prof. Polifke from TU Munich has developed an approach using System Identification techniques for fast and robust computation of the flame response. The main idea is to deploy methods from control theory combined with state-of-the-art numerical simulations as shown below. Generally, this is called CFD/SI while the term CFD may be replaced by LES or DNS.

Currently, I am working on this topic and results will come up soon.

Impact of wall heat transfer on flame dynamics

Flame dynamics of a laminar flame

The project of the IMFT group (Thierry Poinsot and me) during the CTR Summer Program 2016 at Stanford University dealt with the impact of wall temperature on the dynamics of laminar and turbulent premixed flames. Previous studies revealed significant changes in the flame response (in particular the response amplitude) when the flame anchoring has been cooled.

D. Mejia, M. Miguel-Brebion, A. Ghani, T. Kaiser, F. Duchaine, L. Selle, T. Poinsot
Combustion and Flame (2018), vol. 188, pp. 5-12
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Watch here the change in flame response for an uncooled (T_w=700K) and a cooled (T_w=285K) cylinder, on which a perfectly-premixed laminar flame is stabilized.

Impact of heat transfer on sceech combustion

Besides the laminar case, we also investigated a turbulent case which featured self-excited combustion instabilities. Conjugate heat transfer simulations allow the coupling of an LES solver with a heat transfer solver for the solid bluff-body. This appraoch allowed us to investiage the dynamic behaviour of the flame to wall temperatures. The high-frequency comsbution instabilities showed to be sensitive to changes in the wall heat transfer. Find out more here:

A. Ghani, M. Miguel-Brebion, L. Selle, F. Duchaine and T. Poinsot
Proceedings of the Summer Program (2016), pp. 133-142
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Flame/wall interaction

Sound generation by flame/wall interactions

Modern combustion chambers are compact in order to reduce size and weight restrictions of the system. In some cases, flow perturbations can cause flame annihilation at the vincinity of (film-cooled) walls. The distruction of the flame surface generates pressure waves which may trigger combustion instabilities.

At IMFT, we observed this sound generation mechanism in experimental campaigns. This motivated us to quantify the pressure level of sound which is produced after flame/wall interaction. Here you can take a look at the pressure field during harmonic excitation of the flow filed resulting in a periodic flame/wall interaction.

A. Ghani & T. Poinsot
Flame Quenching at Walls: A Source of Sound Generation
Flow, Turbulence and Combustion (2017), vol. 99 (1), pp. 173-184
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Consequently, we carried out computations of laminar premixed Head on Quenching (HOQ) and quantified the generated sound level. It turned out, that the pressure fluctuations can be easily modeled analytically, thereby using Cantera for the computation of flow properties.