Abdulrahman S. Abduljalil

Non-linear phenomena occurring during the start-up process of a travelling-wave looped-tube thermoacoustic engine

This article reports the investigation of non-linear start-up processes in a looped-tube thermoacoustic engine. The engine utilises air as working gas, and has the fundamental frequency of around 111 Hz. It is observed that the mean pressure and the input heat power are the two key parameters controlling the start-up behaviour of the engine. When these are varied, a range of non-linear transient phenomena are observed, which include the ‘on–off’ effect, ‘fishbone-like’ oscillations (i.e. the quasi-periodic pressure amplitude bursts followed by a quasi steady state), and the normal smooth start-up process. The experiments show that the ‘fishbone-like’ bursts are a new mode of pressure amplitude growth. A series of experiments have been conducted to investigate in detail the influence of mean pressure, input heat power and regenerator type on the occurrence of such quasi-periodic amplitude bursts. It is observed that the duration of the pressure amplitude bursts depends on the combination of the heating power and the mean pressure. The observations suggest that there are strong interactions between the acoustic and temperature fields within the regenerator. It is thought that these can lead to the reported quasi-periodic unsteady behaviour of the engine.

Experimental Characterisation of Low-Cost Regenerators for Travelling-Wave Thermoacoustic Devices

This paper presents the construction and testing of three low-cost regenerator designs for travelling wave engines. These include: (i) a regenerator made out of a ceramic substrate of an automotive catalytic converter, with regular square pores, (ii) a steel “scourers” regenerator and (iii) a stainless steel “wool” regenerator. The latter two are examples of materials with random geometries; they are uniformly pressed into a purpose built thinwalled “can” with carefully selected pressure to obtain the required equivalent hydraulic radius. For benchmarking purposes regenerators made out of multiple layers of wire mesh screens are also used as the most representative configuration currently used in travellingwave thermoacoustic systems. To attempt meaningful comparisons, the materials are selected to ensure comparable hydraulic radii. The baseline set of regenerators was designed around the hydraulic radius of 200 μm. This included the ceramic substrate, steel “scourers”, stainless steel “wool” and stacked wire screens (as a benchmark). This set was extended to include steel “scourers” and stacked wire screens (as a benchmark) with hydraulic radius of 120 μm. This resulted in a total of six regenerators. They were initially tested in a steady air flow in order to estimate their pressure drop due to the viscous dissipation. Subsequently, they were installed in a looped-tube travelling-wave thermoacoustic engine to test their relative performance. Testing included the onset temperature difference, i.e. the minimum temperature difference between two regenerator ends required to start the acoustic oscillation, the maximum pressure amplitude and the acoustic power output as a function of mean pressure between 0 and 10 bar above atmospheric. The experimental results indicate that the performance of regenerators made of random materials (scourers and steel wool) is much worse compared to their mesh-screen counterparts of the same hydraulic radius. One of the practical problems is that the fine wire size of the random materials makes it impossible to match the porosity levels of mesh screens when the hydraulic radius is matched. Detailed discussions and performance comparisons are provided.