Sustainable design and durability of domestic micro combined heat and power scroll expander systems.

Abstract

Research to understand the mechanisms of wear within the main components of three different micro-CHP scroll expander systems was conducted. This was performed in order to identify the possible tribo-mechanical effects (abrasion, adhesion, cavitation, fatigue) which occur on the substrate of these components during the operation of the scroll which can seriously affect the lifecycle of the micro-CHP unit. Three-dimensional interferometer, surface scanning and scanning electron rnicroscopy (SEM) were used for surface analyses. The critical components for durability were identified on the tip seal and the steel plate of the scroll expander. Abrasive wear derived from a two-body contact on the interface of the tip seal and the steel plate. Three-body wear was found across the steel plate of the scroll. Finally, cavitation pits were revealed. Interestingly, cavitation was generated by the increment of pressure. It was found that sufficiently high pressure can liquefy instantaneously part of the refrigerant close to the bottom boundary, creating conditions for the generation of cavitation bubbles within the liquefied refrigerant. This finding resolves the puzzle of how the refrigerant which enters the scroll in gas phase produces cavitation. The wear mechanisms identified can significantly reduce the performance of the scroll. Specimens made from the steel plate (high carbon steel) and the tip seal (high performance reinforced fluoroelastomer) of the scroll expander were used for bench tests. The parts were used to perform sliding tribological tests using a special purpose-built modified micro-friction machine TE 57 in order to clearly identify the sliding wear and friction mechanisms. These tests were performed under a specific load and lubrication regime. The experimental conditions were adjusted to those of the industrial applications. Furthermore an experimental study using an ultrasonic transducer (submerged into the fluids) was utilised to produce cavitation bubbles. Using high-speed camera techniques the bubbles were observed within the working fluids. A thorough investigation of the dynamic behaviour of the bubbles and their cavitation mechanisms was conducted using the two scroll fluids (lubricant/refrigerant). The experimental results were effectively correlated with the computational ones. Additionally, the impact of the scroll fluid cavities across the surface of various commercial steel grades, including the actual steel plate of the scroll, was determined. Finally, their cavitation performance and durability, over a prolonged period of time was investigated.