Surendran, Aswathy (2017) Passive control of thermoacoustic instabilities in idealised combustion systems using heat exchangers. Doctoral thesis, Keele University.

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Thermoacoustic instabilities pose a great threat to combustion systems, as they could cause severe structural damage, if they are unchecked and uncontrolled. These instabilities are caused due to the existence of a positive feedback loop between the pressure oscillations and heat release rate oscillations. To prevent these instabilities, one can adopt active or passive control strategies.

The aim of the present work is to passively control thermoacoustic instabilities in a domestic boiler system. To this end, the boiler is modelled as a 1D quarter-wave
resonator (open at one end and closed at the other) containing a heat source and a heat exchanger (hex). The heat source follows a simple time-lag law for its heat release rate. The hex is modelled as an array of circular tubes in cross flow, and it is placed near the closed end of the resonator, causing it to behave like a cavity-backed tube row. The hex acts as both heat sink and acoustic scatterer. The heat transfer response is obtained from numerical simulations (transfer function approach) and the acoustic scattering or the aeroacoustic response is modelled through a quasi-steady approach.
The combination of these two responses at the hex along with the cavity backing gives the effective reflection coefficient of the downstream end of the combustor.

Stability maps are constructed for various system parameters. A classical eigenvalue method is used to obtain the complex eigenfrequencies of the first mode of the combustor. From the growth rate (imaginary part of the eigenfrequencies) obtained for different parameter combinations, it is observed that for the eigenfrequency range of interest, an increase in the mean cross flow velocity, in cavity length, or in hex tube diameter, and a decrease in the gap between the hex tubes, all favoured stability.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
Divisions: Faculty of Natural Sciences > School of Computing and Mathematics
Contributors: Heckl, MA (Thesis advisor)
Depositing User: Lisa Bailey
Date Deposited: 06 Nov 2017 10:48
Last Modified: 23 Nov 2020 15:45

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