Acoustically Enhanced Forced Convection in Compact Heat Exchangers

Towards enhancing the efficiency of gas turbines, most thermodynamic cycles require heat to be either added or dissipated by a heat exchanger, which operates by associating two streams of different thermal potential. Due to form factor limitations of many size restrained applications, the state of the art is advancing towards more compact designs. This forms the need towards higher performance and efficiency heat exchangers – enabling more heat transfer for the same size heat exchanger unit with an unchanged pressure drop. Therefore, we focus on studying the convective heat transfer ramifications of acoustically excited smooth and turbulated walls. Determined by the resistance of the thermal boundary layer, convective heat transfer is undoubtedly a surface phenomenon, only dependent on the near wall region. Therefore, by acoustic streaming of wall bound flow and formation of a coupled Stokes layer, a local influence on the fluid-solid interface can be achieved without simultaneously affecting the mainstream flow motion – increasing heat transfer performance. However, when the net heat exchange of the flat surface is still insufficient, pertubrators are used to promote transport phenomena by improved mixing with the free stream. In order to improve the efficiency of this periodically reattaching flow problem, we use acoustic resonance driven standing waves to trigger a complex instability dynamic. The instability initiates a process of wavelength conversion by Tolmien-Schlichting waves that are later amplified into Kelvin-Helmoltz instability mechanisms in the free shear layer. Globally, considering the closely confined internal air flow inside highly branched heat exchangers, the coupled resonance behavior of interconnected passages and cavities exert a strong influence on the internal convection heat transfer, absent of additional pressure penalty. Neither of these engineering problems has previously received much prior attention.

Heat Exchanger Test Facility