Use of heat exchanger for passive control of combustion instabilities

One of the major concerns in the operability of power generation systems is their susceptibility to combustion instabilities. In this work, we aim to examine the effective use of heat exchangers, an integral component in any power generation system, to passively control combustion instability. The combustor is modelled as a quarter-wave resonator (1-D, open at one end, closed at the other) with a compact heat source within, which follows time lag law for heat release. The heat exchanger (hex) is modelled as an array of tubes with bias flow and is placed near the closed end of the resonator, causing it to behave like a cavity-backed slit plate: an effective acoustic absorber. For simplicity and ease of analysis, we treat the physical processes of heat transfer and acoustic scattering occurring at the hex as two individual processes separated by an infinitesimal distance. The aeroacoustic response of the tube array is modelled using a quasi-steady approach and the heat transfer across the hex is modelled by assuming it to be a heat sink. Unsteady numerical simulations were carried out to obtain the heat exchanger transfer function (HTF), which is the response of the heat transfer at hex to upstream velocity perturbations. Combining the aeroacoustic response and the HTF, in the limit of the infinitesimal distance between these processes tending to zero, gives the net influence of the hex. Other parameters of interest are the heat source location and the cavity length (the distance between the tube array and the closed end). We then construct stability maps for the first resonant mode of the aforementioned combustor configuration, for various parameter combinations. Preliminary observations show that stability can be achieved for a wide range of parameters

Aeroacoustic response of an array of tubes with bias-flow

Heat exchanger tube bundles, consisting of tube arrays in cross flows, are vital in the efficient working of power generation systems. If sound propagates through these bundles, it can lead to resonance or acoustic attenuation, and thereby affecting the working of the power generation unit. Therefore, it is important to study the aeroacoustics of tube rows. The aim of the present work is to experimentally validate the quasi-steady compressible model developed to study the aeroa-coustic response of an array of tubes with bias flow. In order to accomplish this, the array of tubes is approximated by a geometry consisting of two half cylinders separated by a gap and having a bias flow through the gap. Firstly, the case with no flow is considered and the experimental results for the reflection and transmission coefficients are compared against the analytical expressions developed by Huang and Heckl (Huang and Heckl, 1993, Acustica 78, 191-200). Then the cases with flow are considered. A quasi-steady subsonic compressible model is developed to predict the reflection and transmission coefficients, valid for low Strouhal numbers, with the additional assumption of small Helmholtz number and low Mach numbers. This model is validated against the experimental results for the transmission and reflection coefficients. A two-port multi-microphone measurement technique is used to obtain the pressure data and a subsequent wave decomposition is utilised to extract the transmission and reflection coefficients. The results show good agreement with theory for various Mach numbers, in the low Strouhal number regime