Abstract
Diaphragm pumps are a sort of leakage-proof reciprocating pumps with low flow rate but high head and better efficiency, and can potentially find their applications in organic Rankine cycle (ORC) systems as the feed-pump of organic fluid to the evaporator. A diaphragm pump in an ORC system may suffer from cavitation in the pump suction chamber inevitably when the pump delivers an organic fluid. However, the cavitation performance of the pump has been a little known for organic fluids so far. In the article, the performance of a specific diaphragm pump was determined based on the existing performance charts provided by the pump manufactory in terms of pump rotating speed and inlet liquid pressure for cold water. The net positive suction head required (NPSHr) was predicted by involving thermodynamic effect in cavitation when the pump feeds the organic liquid R245fa to the evaporator in an ORC system at 480rpm rotational speed. The net positive suction head available (NPSHa) was calculated at 100 kPa and 141 kPa inlet liquid pressures, and the corresponding cavitation safety margins were addressed. The subcooling for the NPSHr and NPSHa as well as the safety margin were figured out. Two one-dimensional (1D) mechanical models for motion of the suction valve were built and solved at 480rpm and 100 kPa and 141 kPa inlet pressures. A preliminary experiment was performed to verify the analytical results. It turned out that the NPSHr is reduced to 2.02m from 3.02m NPSHr of cold water due to the thermodynamic effect in cavitation, and the corresponding subcooling is lowered to 8.28 °C from 12.38 °C. 100 kPa but 141 kPa inlet pressure can result in cavitation in the pump. The 1D mechanical models are subject to a rough spatial resolution for the flow field in the suction chamber, hence three-dimensional(3D) numerical simulations of the flow field are desirable.
Highlights
Cavitation is a phenomenon whereby a proportion of liquid suffers from vaporization when the absolute pressure in a local fluid field is as low as the saturated vapour pressure in a fluid flow system
net positive suction head available (NPSHa) must be higher than net positive suction head required (NPSHr) with a safety margin for a pump
For a positive displacement pump, the counterpart at the pump mean flow rate has dropped off by 3% compared with the flow rate under non-cavitation condition is considered as the NPSHr of that pump
Summary
Cavitation is a phenomenon whereby a proportion of liquid suffers from vaporization when the absolute pressure in a local fluid field is as low as the saturated vapour pressure in a fluid flow system. The hydraulic and cavitation performance of a variable-stroke axial piston swashplate pump was measured in a testing loop with organic fluids R11 and R113 at 1750–3000 rpm rotating speeds and 20–80 ◦C temperatures (Bollina, 1984). A positive displacement pump with sliding vanes was tested with organic fluid R236fa at 700–1200 rpm rotating speeds and 17–24 ◦C temperatures (Bianchi et al, 2016). For organic fluid R245fa, an empirical model was proposed to predict the non-cavitation performance of the diaphragm pump of Hydra-Cell D/G-10X pump in an ORC system (D’Amico et al, 2018). The overall performance of three pumps such as multistage centrifugal pump, diaphragm pump, and roto-jet pump was measured in thermal test rig loop of R245fa in various rotational speeds (Yang et al, 2018). The study will provide useful information for both selection and safe operation of the diaphragm pump in ORC systems
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