Two-phase flow modelling and application to high performance heat pumps

Description

Reverse cycles allow environmental and waste heat to be converted into process heat, providing energy in an efficient and environmentally sustainable manner. Systems that perform such cycles are called heat pumps or chillers, depending on whether the useful effect sought is heating or cooling, respectively. Only the term "heat pump" will be used in this thesis, including all the useful effects they can provide. The uses of such a technology range from buildings to the process industry, and in recent years they have become technologically mature and ready for intensive use. Despite this, there are many technological difficulties in using heat pumps, especially given the stringent demands associated with reducing pollution and global warming. Modern heat pumps must be efficient, durable, use environmentally friendly fluids, and be able to integrate flexibly into complex systems, such as power plants or process industries. There is also a trend in recent years to increase the operating temperatures of heat pumps to replace the use of fossil fuels, thus electrifying thermal users. In such a framework, this thesis specifically addresses high speed two-phase flows, which may occur in heat pump cycles and which, if properly managed, may significantly contribute to enhance their performance. In particular, this thesis will mainly present modelling and experimental work about two-phase flow expansion, to recover power in place of lamination valves. Secondly, this thesis also starts the investigation of heat pump compressor surge, potentially subject to two-phase flow if spray intercooling is employed: research on this latter topic is started here but deems further investigations in the near future. In the introductory section, fundamentals of reverse cycle operation are recalled, and then the state of the art of contemporary research is reviewed. In the first part of this thesis modeling and experimental analyses regarding the use of high-speed two-phase flows within heat pumps are shown. Considering replacing common expansion valves with turbines or ejectors, it is necessary to have designer friendly models that can predict the physical characteristics of high-speed two phase flows. To begin with, a model for the analysis of two-phase supersonic nozzles is shown, demonstrating its potential for low-quality initial conditions of the refrigerant, which are not adequately studied in the open literature and are representative of the pressurized liquid downstream the heat pump condenser. After that, a novel statistical model for the analysis of maximum flow rates in supersonic nozzles is introduced, limited to CO2 as refrigerant. Maximum two-phase flow rates under sonic conditions represent a technical problem of great complexity, which is addressed here with a different approach from what is commonly shown in physical models in the open literature, since here it is devoted specifically to the preliminary design of nozzles. In the second part, experimental measurements made on a static bladeless turbine test-rig expanding two-phase flow are reported. Bladeless turbines can be a viable substitute for throttling valves, as they suffer less than conventional turbines from erosion problems due to high-speed two-phase flows. The measurements provided both qualitative results, through optical measurements, and quantitative results, through pressure measurements, with the goal of providing a validated basis for CFD models. The third part of the thesis is devoted to the experimental analysis of the stable and unstable operation of centrifugal compressors within inverse closed loops, potentially subject to two-phase flow, in terms of droplet ingestion at compressor intake, either due to spray intercooling or to liquid entrainment from the evaporator. This topic is sparsely covered in the literature, and it is of definite interest for improving the performance of heat pumps. This section is closed with a brief exposition of the ultimate future goal, which is that of two-phase compression. Finally, an example of experimental integration of a heat pump within a combined cycle is shown, this being one of the possible new applications of high performance heat pumps. The coupling between the two systems required a long work of interfacing between technologies that are well known but have quite different features and operational requirements. As concluding remark, this thesis aims to contribute to the future enhancement of heat pump performance providing the scientists with new tools and evidences for dealing with two-phase flows, both for energy harvesting in expansion as well as for compressor work reduction in compression.

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