Quantification of acoustic losses in a bended wave guide using CFD

Thermoacoustic engines are devices that convert heat into work using a physical phenomenon called the thermoacoustic effect. The theoretical efficiency of these devices has the potential to surpass that of regular Stirling piston engines. However, the size of these engines is at present too large to make them attractive for practical applications. Research is currently being performed to make these devices more compact. A large component in the geometry is the flow guide, through which a travelling wave is guided from the stack trough the engine. This research investigates the specific influence of making the bend in the wave guide sharper, for different pipe diameters, and quantify the effect this has on the acoustic intensity decline. Acoustic CFD simulations are performed with ANSYS FLUENT of several bend curvatures, for three different pipe diameters using a boundary condition which induces acoustic waves at 100 Hz and 100 Pa under atmospheric conditions in helium. These simulations are compared to an analytically derived solution for the acoustic field, which is based on the low reduced frequency model. It is found that the loss of acoustic intensity is proportional to the shear wave number for bends with a dimensionless radius higher than 5. In these cases the acoustic intensity loss can be calculated analytically. For flow redirectors with a dimensionless bend radius lower than 5 effects are observed which are not accounted for in the analytical model, leading to a higher acoustic dissipation than is predicted. For extremely sharp flow redirectors, where the inner bend radius is of the same order of the particle displacement amplitude, flow detachment and vortex shedding occur, which introduce extra acoustic losses. A parameter space encompassing the curvature and shear wave number is presented where the three named regimes are designated. The previous can be used for the design optimization of thermoacoutic engines.