Oscillatory flows in jet pumps: towards design guidelines for thermoacoustic applications

Thermoacoustic engines are an interesting alternative to conventional heat engines (such as Stirling engines) due to the absence of moving parts in the hot region and the small temperature difference required to operate. These engines can provide a durable solution in, for example, waste heat recovery applications. Using a traveling wave based configuration, consisting of a toroidal geometry, thermal-to-acoustic efficiencies of up to 30% have been obtained. However, the traveling wave configuration has a major disadvantage: due to the closed looped geometry a time-averaged mass flow, known as "Gedeon streaming", can occur. This type of acoustic streaming can lead to a drastic reduction in efficiency or even prevent the engine from running. Therefore, control of Gedeon streaming is essential in the development of traveling wave thermoacoustic engines. A solution to avoid Gedeon streaming is the application of a jet pump, which is a component with one or more tapered holes. The oscillatory flow through such an asymmetric geometry results in a time-averaged pressure drop across the jet pump. By balancing this time-averaged pressure drop with the pressure drop that exists across the regenerator of the thermoacoustic device, Gedeon streaming can be suppressed. In this thesis, the oscillatory flow in jet pumps is analyzed. The relation between oscillatory flow features and the performance of jet pumps is investigated. Based on this, jet pump design guidelines have been formulated for laminar oscillatory flows. Flow separation is identified as a main source of performance loss in jet pumps and can be avoided by introducing a smooth transition to the tapered inner surface. Compact jet pump designs can be realized by using multiple smaller tapered holes, but this is accompanied by a slight reduction in performance due to the smaller diameter of the individual holes. Identifying and understanding the flow phenomena in jet pumps is shown to be the key to more reliable design calculations for jet pumps in thermoacoustic applications.