Experimental validation of gap leakage flow models in archimedes screw generators

Abstract

Archimedes screw generators (ASGs) are a form of microhydro power generation that is increasingly being adopted as an alternative renewable energy source. ASGs operate efficiently, even as head approaches zero. A small gap must always exist between the trough and edge of the rotating screw flights to allow screw rotation. This leads to two categories of flow within the screw: the primary flow remains between the helical flights and causes screw rotation, while a secondary leakage flow occurs through the gaps at the screw edges. A high gap leakage flow reduces efficiency because this flow effectively bypasses the working parts of the screw and is lost. An accurate gap leakage model is an essential component of any ASG design model. Current gap leakage models are based on experience with Archimedes screw pumps or are derived from models assuming quasi-static flow through an opening. This experimental study measured the actual leakage flow in an operating laboratory-scale ASG. The fill heights within the buckets were measured using high-speed photography and used to determine bucket volume and therefore non-gap flow. The gap flow is determined based on the difference between the measured total flow through the system and the computed non-gap flow based on the measured fill height and screw speed. The uncertainty of the gap flow is relatively high, since it is calculated based on the difference between two variables with similar magnitude, one of which is calculated based on other measurements. The existing gap flow models did not accurately predict the experimental results. Gap flow was found to be dependent on screw rotation speed, with highest gap flows occurring at intermediate screw rotation speeds. Gap flow reduces to zero as screw rotation speed approaches the maximum possible speed. There is significant uncertainty in the existing gap flow models, and gap flow models constitute one of the largest sources of uncertainty when modeling the performance of ASGs.