Task 5.3 Passive instability control by finned tubes
This task will study rows of parallel finned tubes. Tubes, including finned tubes, in cross-flow can act as sound absorbers (Surendran et al 2017), but they can also act as sound source (they have been observed to whistle) depending on the flow properties. They warrant careful study because inside a combustion chamber, they can suppress a thermoacoustic instability, but they can also aggravate it. A combined experimental and numerical approach will be taken in this task. On the experimental front, a row of finned tubes, will be installed in the flow duct at KTH and excited by an incident acoustic wave. The elements of the transfer matrix will then be measured for several parameters, in particular: incident frequency, flow speed, fin spacing and tube spacing. This will reveal under which conditions the finned tube row acts as a sound absorber, rather than as a sound source. In order to understand the exact physical processes responsible for these observations, the in-house linear Navier-Stokes solver for flow simulations in the frequency domain will be used to calculate and visualise the flow field surrounding the finned tubes.