Air entrainment and air-water separation in hydraulic air compressors

A hydraulic air compressor (HAC) is an isothermal gas compressor that uses hydropower to
\ncompress air, originally developed by Charles Taylor in the 1890s to supply industry with
\ncompressed air. In the modern revival of this technology, the hydropower will be provided by
\npumps rather than natural sources. As such, energy efficiency is an important driver of
\ncomponent design; all of the hydropower is consumed either to overcome irreversibility or to
\ncompress air. The compressor relies on the increasing pressure of water flowing downward in a
\ndowncomer to compress air in the form of bubbles being dragged along with the flow. The air
\nentrainment process at the top of the downcomer is facilitated by a mixing head. At the bottom of
\nthe downcomer, the bubbles are separated from the flow in a separator vessel. The objective of
\nthis thesis is to develop the design methodology for the air entrainment and air-water separation
\ncomponents on either end of the downcomer process.
\nSeveral mixing heads were tested on a small (4.5 m height) prototype HAC. The test without a
\nmixing head successfully entrained air, confirming that air entrainment is a system effect. Two
\nheads with dissimilar geometry were associated with the lowest irreversibility, leading to the
\nconclusion that the best design at that scale is a mixing head incorporating some form of vortex
\nbreaker. Air entrainment is driven by a system energy balance and not exclusively by a local
\nVenturi geometry.
\nThe fraction of the air successfully captured in the plenum of the separator is called the separator
\neffectiveness. Mechanistic models have been created to characterize both the irreversibility and
\nseparator effectiveness of two types of gravity separator (horizontal and vertical orientation) for 
\niv
\nthe design of separators for future commercial-scale compressors. The separator effectiveness
\nmodels require as input the flow field information from computational fluid dynamics analysis
\nand the bubble size distribution at inlet. The bubble size distribution was measured on the small
\nprototype and used to select a bubble size prediction model for testing on a much larger scale (29
\nm height) demonstrator HAC. The displacement model for horizontal separators matched the
\nactual performance at the prototype scale well, particularly at high flow rate. The vertical
\nvelocity model produced a good match for the separator on the demonstrator HAC, but not for
\nthe same bubble size model identified on the small prototype.

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