Single pressure absorption refrigeration cycles, such as the diffusion absorption refrigeration (DAR) cycle, require only thermal input for operation. While such technologies operate without mechanical pumping, they still require a mechanism for fluid circulation. Bubble-pump generators offer a simple and efficient solution for passively driven fluid circulation. In a bubble-pump, vapor bubbles rise through long narrow vertical tubes, lifting liquid to a high reservoir. The liquid can then drain through heat and mass exchangers in a refrigeration system, and return to a lower reservoir.
In this effort, detailed experimental studies were performed of spot-heated bubble-pump generators, designs in which all vapor generation occurs near the base of the component (Figure 1a). High speed photography was performed of the intermittent Taylor (or slug) flow pattern in this component (Figure 1b). Results from this investigation led to the development of a new mechanistic bubble pump model.
Spot-heated bubble-pump generators require high input heat temperatures because all heat is transferred over a small area. An enhanced coupling fluid heated design was developed in which heat transfer is distributed over the entire component length (Figure 2). This enables operation with low source temperatures – as low as 110ÂșC for ambient pressure water operation. This configuration enables DAR operation with low grade thermal sources, such as waste heat or solar heat.
Bubble pumps achieve maximum pumping efficiency at intermediate diameter / Bond number conditions (5 < Bo < 40, 5 < D < 20 mm). This is between the comparatively well characterized capillary / microchannel and large diameter scales. The intermediate scale is particularly challenging to analyze because forces from buoyancy, inertia, surface tension, viscous effects, and turbulence are all significant. Detailed two-phase flows simulations were performed at these conditions to inform model development and bubble pump design (Figure 3).
Related publications
- Rattner, A.S., Garimella, S., 2015, Vertical upward Taylor flow in intermediate diameter tubes: experiments and kinematic closure. International Journal of Multiphase Flow (Accepted).
- Rattner, A.S., Garimella, S., 2015. Coupling-fluid heated bubble pump generators for low-temperature fully thermally activated single pressure absorption systems. Science and Technology for the Built Environment (In Press). DOI: 10.1080/10789669.2015.1004978.