Acoustic Characterization of a Cold Heat Exchanger in Hot Cross-Flow
Motivation
In domestic boilers, the combustion chamber characterized by a cylindrical burner and a radial heat exchanger is a widely employed configuration. Yet, within a certain range of operating conditions, acoustic instabilities have been observed in this configuration. These so-called thermoacoustic instabilities can cause serious structural damages and decrease the overall efficiency of the boiler. Therefore, for future design optimizations it is of high interest to gain a deeper understanding of the mechanisms taking place in such devices.
Our study focuses on the heat exchanger. The coupling between the acoustic field of the combustion chamber and the unsteady heat transfer at the tube bundle can turn the heat exchanger into a source or sink of acoustic energy. The heat exchanger could therefore act as both an enhancer and a damper of the instability.
Objective
The main objective of our research is to characterize the thermo-acoustic behavior of the heat exchanger, in terms of its scattering properties, which describe the heat exchanger as a source of reflection, transmission, production or dissipation of acoustic waves. To this end, analytical and numerical analyses will be carried out.
Numerical Approach
In thermoacoustics modeling, the tube bundle could be considered as an “inverse Rijke tube wire”, which absorbs heat from the hot flow. The numerical simulations aim to resolve the fluid-dynamic interactions between the flow and the tube bundle. One example is the building up of coherent structures which can influence the heat transfer at the tube row. Different profiles of the tube bundle will be taken under consideration, in order to assess the influence of the geometry on the flow conditions and instability enhancement. The time series data will be post-processed by means of SI methods, in order to derive the frequency response of the system.
with a row of HX cylinders in cross-flow
Analytical Approach
Analytical modeling on the scattering properties of the tube bundle will be pursued. The modeling process is challenging task, since, in a heat exchanger, the effects of unsteady heat absorption, mean temperature jump and scattering at geometric elements (e.g., area change), are juxtaposed. A possible strategy is to make use of the superposition of basic acoustical elements. Several analyses will be carried out in order to find the best combination of heat release transfer functions and known basic acoustic elements to which we can approximate the behavior of the cold heat exchanger.
Ackowledgements
The present work is part of the Marie Curie Initial Training Network 'Thermo-acoustic and Aero-acoustic Non-linearities in Green combustors with Orifice structures' (TANGO). We gratefully acknowledge the financial support from the European Commission under call FP7-PEOPLE-ITN-2012.